Abstract
Neuromyelitis optica spectrum disorder (NMOSD) and myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD) are autoimmune diseases characterized by immune-mediated damage to the central nervous system. Current treatments primarily focus on chronic immunosuppression. Immune tolerance induction offers a novel approach to restoring immune balance while minimizing systemic side effects. Central and peripheral immune tolerance mechanisms regulate autoreactive lymphocytes, ensuring immune homeostasis. Dysregulation of these pathways underpins NMOSD and MOGAD pathogenesis. Antigen-specific therapies targeting aquaporin-4 (AQP4) or myelin oligodendrocyte glycoprotein (MOG) autoantigens include peptide-based vaccines and nanoparticle delivery systems, promoting T cell anergy and regulatory T cell (Treg) expansion. Cell-based therapies utilizing ex vivo-expanded Tregs or regulatory B cells (Bregs) have shown promise in preclinical models but face challenges in clinical translation due to scalability and safety concerns. Gene-editing technologies such as CRISPR/Cas9 present opportunities to modulate immune pathways and restore tolerance, although delivery and off-target effects remain obstacles. Additionally, strategies addressing double-seronegative NMOSD, which lacks detectable autoantibodies, emphasize broad immune modulation rather than antigen specificity. While significant progress has been achieved, the transition to clinical application requires overcoming hurdles such as optimizing antigen delivery, ensuring long-term efficacy, and identifying reliable biomarkers. Advances in personalized medicine hold promise for achieving sustained remission, reducing dependency on immunosuppression, and improving patient outcomes in NMOSD and MOGAD. This review explores advancements in tolerance strategies, highlighting their potential in NMOSD and MOGAD.
Introduction
Neuromyelitis optica spectrum disorder (NMOSD) and myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD) are autoimmune disorders characterized by inflammatory damage to the central nervous system (CNS).1,2 While advances in understanding their pathophysiology have illuminated the roles of antibodies such as immunoglobulin G autoantibody targeting aquaporin-4 (AQP4-IgG) and myelin oligodendrocyte glycoprotein (MOG-IgG), therapeutic strategies to induce immune tolerance to these autoantigens remain underexplored. 3 Although MOGAD is classically defined by the presence of pathogenic anti-MOG IgG1 antibodies, a T cell-dependent immunoglobulin subclass, accumulating evidence indicates that MOG-specific CD4⁺ T cells play a central role in disease pathogenesis. 4 Experimental autoimmune encephalomyelitis (EAE) models induced by MOG peptide immunization have shown that MOG-reactive CD4⁺ T cells alone are sufficient to drive CNS demyelination, even in the absence of B cells or antibody production. 5 In more complex models utilizing full-length human MOG protein, both MOG-specific T and B cells are required; however, B cells primarily serve as antigen-presenting cells (APC) rather than producers of pathogenic antibodies. 6 Notably, in transgenic mice expressing both MOG-specific T and B cells but lacking the ability to secrete antibodies, opticospinal inflammation still develops. 7 These findings collectively support a model in which T cell-mediated mechanisms can independently initiate and propagate CNS autoimmunity, uncoupled from classical humoral effector functions. Such preclinical observations suggest that a subset of patients currently categorized as double-seronegative NMOSD may, in fact represent a T cell-driven, seronegative form of MOG-directed autoimmunity. 4
Immune tolerance is fundamental to preventing autoreactivity, involving central and peripheral tolerance mechanisms to maintain immune equilibrium. 8 Dysregulation of these processes underpins the pathogenesis of NMOSD and MOGAD, where autoreactive lymphocytes drive chronic inflammation and tissue injury.3,8
The immunopathogenesis of NMOSD, primarily mediated by AQP4-IgG, involves astrocyte damage through complement activation and inflammatory cell infiltration. 9 In contrast, MOGAD is associated with MOG-IgG targeting oligodendrocytes. 10 Despite overlapping features, their distinct immune mechanisms necessitate tailored approaches for treatment. 3 For double seronegative (DN) NMOSD cases, where neither AQP4-IgG nor MOG-IgG are detected, a clear autoantigen target further complicates therapeutic strategies, implicating cellular mechanisms such as dysregulated T cells. 11
Current treatments for NMOSD and MOGAD rely heavily on immunosuppression, which could potentially dampen immune responses carrying the risks of infections and malignancies.3,12 In contrast, antigen-specific tolerance induction represents a promising alternative. Strategies such as T- and B-cell modulation, the use of tolerogenic dendritic cells (DCs), peptide-based vaccines, and nanoparticle (NP)-mediated antigen delivery, aim to restore immune homeostasis while preserving protective immunity. 13 Advances in these areas, alongside robust biomarkers for monitoring therapeutic efficacy and disease activity, are critical for optimizing tolerance-based approaches. 8
This review explores emerging immunological insights and therapeutic strategies for inducing tolerance in NMOSD and MOGAD, emphasizing the potential of precision medicine to transform management and improve patient outcomes.
Overview of NMOSD and MOGAD
AQP4-IgG-seropositive NMOSD and MOGAD are two recently well-recognized antibody-mediated autoimmune diseases of the CNS.1,2 For many years, clinical and radiological overlaps between these two disorders led to confusion, often classifying them as variants of multiple sclerosis (MS) due to their shared involvement of the optic nerve and spinal cord. 14 The discovery of AQP4-IgG and later MOG-IgG clarified this distinction. However, misdiagnosis of these disorders due to misinterpretation and misapplication of diagnostic criteria is still observed in clinical practice.15,16
A fundamental understanding of antibody testing methods, including the optimal specimen, technique, and potential challenges, is important for diagnosing AQP4-IgG-seropositive NMOSD and MOGAD. The cell-based assay (CBA) in serum is recommended for detecting AQP4-IgG and MOG-IgG, with live CBA presenting certain advantages over the fixed technique.17–19 In 2007, it was demonstrated that the three-dimensional structure of the MOG protein influences MOG-IgG recognition. Since both glycosylation and the conformation of MOG are critical for accurate recognition, the surface expression of full-length human MOG (typically the α-1 isoform, consisting of 218 amino acids) on HEK293 cells is used to detect MOG-IgG reliably. This method has been instrumental in confirming the presence of MOG-IgG in non-MS.20,21 While CBA have high specificity, testing a broad and unselected population for MOG-IgG may yield a false-positive rate as high as 28%.22,23 Moreover, MOG-IgG positivity is rare in adults experiencing their first demyelinating event that resembles MS. 24 Timing of MOG-IgG testing is important,25,26 as antibody titers fluctuate and may decrease over months from presentation, with some patients subsequently testing negative.27,28 Recent observational studies suggest that MOG-IgG in cerebrospinal fluid (CSF) may have diagnostic and prognostic utility.29–33 Close to 40% of patients suspected of having NMOSD who test negative for AQP4-IgG in serum are positive for MOG-IgG using advanced CBA. 34 It became evident that these patients actually have MOGAD rather than NMOSD; thus, enhancing the understanding of various disorders and clarifying the MOGAD phenotype. MOG-IgG was first recognized in children with acute disseminated encephalomyelitis (ADEM), but it is now observed in both adults and children with optic neuritis (ON), transverse myelitis (TM), and brain or brainstem syndromes. 34
MOGAD is primarily associated with demyelination caused by an oligodendrocytopathy, whereas NMOSD involves astrocyte damage due to AQP4-IgG. 3 Cases of concurrent MOG-IgG and AQP4-IgG have been rarely reported, indicating that these antibodies involve distinct immunopathogenic mechanisms.35–37 A relapsing course is observed in 90% of NMOSD patients and in 43% of MOGAD patients after 8 years. 38 While some patients with MOG-IgG may initially meet NMOSD diagnostic criteria due to overlapping clinical and neuroimaging features, it is now well-recognized that MOGAD is a distinct condition with its own pathophysiology. 2 Likewise, it is clear that MOGAD and NMOSD have different risk factors, clinical and radiological findings, and prognostic outcomes, leading to the establishment of separate diagnostic criteria.1,2
NMOSD typically affects adult women and is classically characterized by severe ON, typically unilateral, with limited visual recovery. However, bilateral involvement may occur, either simultaneously or in a sequential pattern. Optic nerve lesions are usually longitudinally extensive, frequently involving more than 50% of the nerve, and demonstrate a predilection for the posterior segments, including the optic chiasm and optic tract.
Beyond the optic pathway, NMOSD is defined by the presence of longitudinally extensive transverse myelitis, affecting the central gray matter of the spinal cord, and by core clinical syndromes such as area postrema syndrome (manifested by intractable nausea, vomiting, and hiccups), diencephalic syndrome, and symptomatic narcolepsy. 39
In contrast, MOGAD presents in both children and adults, frequently with bilateral or sequential ON, anterior optic nerve involvement with optic disc swelling, and a generally favorable visual prognosis when compared with NMOSD.1–3,8,9 TM in MOGAD may be longitudinal or short-segment but often shows patchy distribution and preferential involvement of the gray matter, sometimes producing a “H-sign” on imaging. In children, MOGAD is commonly associated with ADEM-like presentations, while in adults, it may manifest with encephalitis, seizures, or brainstem syndromes. While NMOSD is almost invariably relapsing without treatment, MOGAD may follow a monophasic course in a substantial proportion of patients (~50%), although relapses do occur, particularly in those with persistent clinical activity. Recovery from relapses is generally better in MOGAD compared to NMOSD, where attacks are more disabling and cumulative neurological impairment is common.1,2 The 2023 MOGAD criteria 2 outlined three key aspects for diagnosis: (1) core clinical demyelinating events and supporting clinical or magnetic resonance imaging (MRI) features, (2) MOG-IgG and their titers, and (3) exclusion of alternative diagnoses. The panel emphasized that while MOG-IgG serostatus is a critical biomarker, the revised criteria were specifically developed to ensure that the diagnosis of MOGAD is not based solely on antibody status but also includes core clinical and imaging features, thereby minimizing the risk of misclassifying MOGAD patients as seronegative NMOSD or MS. Distinguishing these relapsing neuroinflammatory autoimmune disorders is crucial in clinical practice, as therapeutic strategies differ between these conditions. 3
NMOSD is a spectrum of diseases characterized by recurrent episodes of neuroinflammation and most NMOSD patients test positive for specific AQP4-IgG. 39 NMOSD was previously known as NMO, but the term NMOSD now encompasses the full spectrum of the CNS manifestations, affecting not only the optic nerves and spinal cord but also the brain and brainstem. 1
The diagnosis of NMOSD was initially based on the presence of AQP4-IgG in serum, with the discovery of these antibodies by Lennon et al. 40 in 2004 being pivotal in differentiating NMO from MS. In 2015, NMOSD was defined and stratified according to AQP4-IgG serological status. Additionally, six core clinical characteristics were well-described, and brain and spinal cord magnetic resonance (MRI) findings suggestive or typical of NMOSD were better and well-defined. 1 However, 20%–30% of patients meeting the 2015 NMOSD diagnostic criteria do not have detectable AQP4-IgG in serum, raising concerns about whether these cases represent the same condition or a distinct entity. 41
While NMOSD and MOGAD share certain clinical similarities, they are distinct disorders with different immunopathological mechanisms and targets.1,2 Advances in biomarker identification and diagnostic criteria have enabled the differentiation of these disorders, resulting in more accurate diagnoses and treatment strategies. Ongoing research is important to further understand the pathophysiology of MOGAD and NMOSD, and to enhance therapeutic options for both conditions. Recently, several medications have been developed as molecularly targeted therapies for NMOSD, including complement inhibition, B cell depletion, and interleukin-6 (IL-6) receptor blockade. 12 These treatments have demonstrated efficacy and safety for NMOSD, and their potential application in MOGAD is also being explored, since there are no approved treatments for this condition.
Underlying immune mechanisms
In NMOSD, IgG1 antibodies targeting AQP4 (a transmembrane protein), called AQP4-IgG, enter from the periphery to the CNS through regions of increased blood–brain barrier (BBB) permeability or injury, or by transcytosis across endothelial cells. 3 AQP4-IgG selectively binds to astrocytic orthogonal arrays of particles (OAPs) of the AQP4 antigen on astrocyte feet, initiating complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC), which leads to astrocytic damage, BBB disruption, and subsequent neuroinflammation. 42 NMOSD lesions occur where the density of OAPs is highest.43,44 The first mechanism involves complement cytotoxicity through C5b-9 complex formation (MAC), while the second relies on the sublytic effect of C5b-9 and the dysregulation of astrocyte glutamate transport by C5a-preactivated neutrophils.45–47 Consequently, inflammatory responses through the recruitment of other innate immune cells such as eosinophils and macrophages, perpetuate the inflammatory process by releasing toxic enzymes and pro-inflammatory cytokines, resulting in oligodendrocyte loss, demyelination, and finally neuron loss. 3 Therefore, AQP4-IgG induces damage based on two major effector functions: First, the binding of AQP4-IgG to OAPs is essential for initiating classical complement pathway activation. This high-avidity interaction is facilitated by the preferential recognition of OAPs over individual AQP4 tetramers. Second, AQP4-IgG engagement induces selective internalization of the M1 isoform of AQP4, a process that disrupts its association with key membrane-bound proteins such as EAAT2, leading to impaired glutamate homeostasis and contributing to excitotoxic damage. 48 The involvement of AQP4-specific CD4+ T cells is pivotal for activating B cells and sustaining antibody production, driven in part by HLA-DRB1*03:01-associated immune predisposition. 49 Additionally, IL-6, which is a multifunctional cytokine, was found to be elevated in the serum and CSF of NMOSD patients,50,51 especially during relapses. It contributes to NMOSD pathophysiology by inducing inflammation (supporting plasmablast survival and polarizing CD4+ T cells into Th17 cells), promoting AQP4-IgG production, disrupting BBB integrity, and enhancing proinflammatory T-cell activation.52,53 Consequently, IL-6 induces astrocyte injury and promotes demyelination. 54 It is important to highlight that in NMOSD, the primary target of autoimmune attack is the astrocyte, with demyelination occurring as a consequence of the highly destructive pathogenic process.55,56
Conversely, MOGAD is characterized by IgG1 autoantibodies targeting the extracellular domain of MOG, which is accessible on the outer surface of oligodendrocytes. 57 These antibodies can mediate CDC, ADCC, and antibody-dependent cellular phagocytosis (ADCP). However, current evidence suggests that their pathogenic potential is contingent upon a concomitant pro-inflammatory milieu. In particular, factors such as surface density or conformational presentation that influence antibody binding and immune activation remain incompletely understood. In contrast to NMOSD, complement activation is less prominent in MOGAD lesions, 56 and MOG-IgG1 binding appears to occur in a bivalent manner that is suboptimal for C1q recruitment and classical pathway activation,57,58 though it may enhance Fc receptor-mediated mechanisms such as ADCC and ADCP. 58 While complement inhibition is a validated therapeutic strategy in NMOSD,59,60 its relevance in MOGAD remains uncertain.
Experimental models further support a cooperative mechanism involving both B and T cells. Knock-in mice producing high titers of MOG-specific antibodies fail to develop demyelination in the absence of T cell-mediated inflammation, highlighting the insufficiency of autoantibodies alone to initiate CNS injury.61,62 This implies that autoreactive B cells may persist in the immune repertoire through clonal ignorance, remaining non-pathogenic under physiological conditions but contributing to disease when inflammatory cues are present. These insights emphasize the need for therapeutic strategies in MOGAD that address both humoral and cellular components of the immune response to effectively mitigate CNS tissue damage. However, MOG-specific T cells are capable of inducing EAE even in the absence of B cells or antibodies.4,6,63 The heterogeneity of MOGAD, which includes a range of clinical phenotypes like ON, TM, or ADEM, reflects the diverse immunological triggers and mechanisms involved. 34 Current data supporting the pathogenicity of MOG-IgG in vivo is indirect: IgG staining is observed in some MOGAD lesions, and MOG-IgG is found in the CSF of many patients, with titers frequently elevated during attacks. 64 Thus, these antibodies may either contribute secondarily to the disease or merely act as a marker of ongoing inflammatory pathology. 4
Induction of immune tolerance in NMOSD and MOGAD requires addressing both humoral and cellular immune pathways. In NMOSD, strategies targeting B cells, such as anti-CD20 or anti-CD19 therapies, and complement inhibitors have shown success in reducing relapses by directly mitigating antibody-mediated damage. 12 However, these therapies do not address the underlying loss of immune tolerance to AQP4, which remains an ongoing challenge. In MOGAD, the focus has shifted toward modulating cellular immunity, particularly targeting MOG-specific T cells that drive inflammation. Tolerance strategies in this context may include antigen-specific approaches, such as tolerogenic DCs or peptide-based immunotherapies, which aim to re-establish immune homeostasis by selectively suppressing pathogenic T and B cell responses.65,66
Key barriers to achieving immune tolerance include the heterogeneity of immune responses and the lack of robust biomarkers for patients’ stratification.6,64 In NMOSD, as previously mentioned, the strong association with HLA-DRB1*03:01 suggests that genetic predisposition may play a critical role in tolerance breakdown.49,67 Identifying linear peptides that comprise an major histocompatibility complex (MHC) class II-restricted AQP4 T-cell epitope sequence linked to a thiol reductase has been shown to generate antigen-specific cytolytic CD4+ T cells capable of selectively eliminating APCs that present this epitope. Furthermore, these cytolytic CD4+ T cells effectively eliminate autoreactive pathogenic T cells targeting the same epitope. 68 This approach represents a potential therapeutic strategy for restoring immune tolerance in NMOSD (NCT04629274). In contrast, MOGAD lacks a consistent HLA association, complicating efforts to design targeted tolerance strategies. Moreover, the transient nature of MOG-IgG seropositivity in some patients and the occurrence of relapses despite seroconversion highlight the need for strategies addressing cellular immunity independently of humoral factors. 69
Overall, inducing immune tolerance in NMOSD and MOGAD remains an unmet clinical need, requiring innovative approaches that address these disorders’ distinct and overlapping immune mechanisms. Advancing our understanding of the cellular and humoral drivers of autoimmunity will be important for developing targeted therapies that restore immune homeostasis and improve patients’ outcomes.
Current immunotherapeutic landscape in NMOSD and MOGAD
NMOSD and MOGAD are considered heterogeneous disorders, presenting challenges in clinical trial design and efficacy analysis. 70 The management goal is to achieve timely diagnosis and early treatment to reduce the frequency and severity of attacks, 12 as disability is primarily associated with relapses rather than disease progression. 71
Acute management is crucial in NMOSD and MOGAD, as exacerbations often lead to significant residual disability in NMOSD and a considerable proportion of MOGAD patients may suffer permanent disability because of incomplete recovery from relapses.34,72
Prompt and intensive treatment is critical in both NMOSD and MOGAD. 73 Although no evidence-based acute management protocols exist, high-dose intravenous methylprednisolone (IVMP, 1 g/day for 3–7 days) is standard. 74 Complete recovery after the first attack occurs in 35%–60% of patients treated with IVMP.74–77 In MOGAD, a tapering course of oral corticosteroids for 3–6 months is recommended to prevent early relapses, with suggested dose reductions to ⩾12.5 78 or ⩾20 mg, 79 as reported in Australian and UK cohorts.27,78,79 While generally well tolerated, the long-term use of corticosteroids remains controversial. 80
For NMOSD patients unresponsive to IVMP, plasmapheresis (PLEX), ideally initiated within 5 days, is recommended (5–7 sessions, 1.5 L exchange every other day),74,81 and similar benefits have been reported in MOGAD. 82 PLEX may also be used as a first-line option in severe cases. 74 Its therapeutic effect stems from the removal of pathogenic plasma components (immunoglobulins, complement, cytokines) and modulation of both humoral and cellular immune responses.83–85
Intravenous immunoglobulin (IVIgG) may be considered in NMOSD patients with poor response to IVMP, with retrospective data showing a 50% response rate 86 ; some evidence also supports combining IVIgG with IVMP. 87 In MOGAD, IVIgG is useful for incomplete recovery or as a long-term option, particularly in children.81,88
While long-term relapse prevention is standard in NMOSD, the optimal approach in MOGAD remains under debate. Recent consensus guidelines inform NMOSD treatment and switching strategies,89–91 but similar guidance for MOGAD is still evolving.
Long-term immunosuppressive treatments can be categorized into distinct therapeutic classes:
- B-cell-targeting therapies: Rituximab (anti-CD20) has been extensively used off-label in both NMOSD and MOGAD. A phase II/III, multicenter, double-blind, randomized controlled trial (RIN-1) conducted in Japanese patients with NMOSD demonstrated favorable efficacy outcomes with this monoclonal antibody. 92 Inebilizumab (anti-CD19) is an FDA- and EMA-approved therapy for AQP4-IgG-seropositive NMOSD since 2020. 93 Additional anti-CD20 agents, including BAT4406F (NCT04146285, NCT06044350) and MIL62 (NCT05314010), are currently undergoing early-phase clinical trials.
- IL-6 receptor inhibitors: Satralizumab (FDA- and EMA-approved in 2020 for AQP4-IgG-seropositive NMOSD) is also being investigated for MOGAD in an ongoing phase III clinical trial (NCT05271409).94,95 Tocilizumab, although not formally approved, has been used off-label in AQP4-IgG-seropositive NMOSD patients demonstrating superior efficacy compared to azathioprine (AZA) in the TANGO trial. 96
- Complement inhibitors: Eculizumab and Ravulizumab, both targeting complement component C5, are approved for AQP4-IgG-seropositive NMOSD. Eculizumab received FDA approval in 2019, followed by Ravulizumab in 2024.59,60
- Neonatal Fc receptor (FcRn) antagonists: Rozanolixizumab is currently under investigation for MOGAD in an ongoing phase III clinical trial (NCT05063162).
- Broad-spectrum immunosuppressants: AZA and Mycophenolate mofetil (MMF) are widely used off-label for both NMOSD and MOGAD.97–100 AZA is being evaluated for MOGAD in an ongoing phase III clinical trial (NCT05349006). Long-term oral corticosteroids are also commonly employed, particularly in relapsing disease courses. Maintenance IVIgG has shown benefit in preventing relapses in MOGAD, especially in patients with frequent relapses.
As of 2025, four therapies, Eculizumab, Ravulizumab, Inebilizumab, and Satralizumab have received regulatory approval for the treatment of AQP4-IgG-seropositive NMOSD. In contrast, no therapies have yet been approved for MOGAD. Nevertheless, several agents (e.g., corticosteroids, MMF, AZA, IVIgG, tocilizumab, rituximab) are employed in clinical practice based on observational data and expert consensus.38,80,100–102
In all clinical trials conducted to date, the primary endpoint has been the efficacy and safety of delaying the time to a new relapse. These therapies have shown significant benefits a preventing subsequent relapses and have demonstrated favorable safety profiles. 12 However, certain limitations remain, including high treatment costs and increased susceptibility to infections, which may limit their widespread applicability, particularly in resource-limited settings.12,103 Moreover, these immunosuppressive therapies are not antigen-specific, do not fully suppress underlying inflammatory activity, and fail to eliminate autoreactive immune cell populations, particularly long-lived memory cells. Their efficacy largely relies on sustained immunosuppression, which may compromise protective immune responses against infections and malignancies. 8
Consequently, there is growing interest in the development of immune reprogramming strategies aimed at restoring antigen-specific immune tolerance. These emerging approaches seek to promote long-lasting disease remission without impairing the host’s defense mechanisms against pathogens.
Definition and mechanisms of immune tolerance
Immune tolerance is a fundamental physiological process that safeguards the host by preventing inappropriate immune responses against self-antigens, thereby maintaining immune homeostasis and averting autoimmune pathology. 104 It represents a finely regulated balance between immune activation and inhibition, orchestrated through a multilayered network of cellular and molecular mechanisms. Immune tolerance is broadly classified into central and peripheral tolerance, each operating at distinct stages of immune cell development and activation.
Central tolerance is established during lymphocyte maturation in the primary lymphoid organs, the thymus for T cells and the bone marrow for B cells, where immature lymphocytes expressing high-affinity receptors for self-antigens undergo deletion (clonal deletion) or are rendered non-functional through receptor editing or clonal diversion.105,106 In the thymus, medullary thymic epithelial cells (mTECs) and DCs present a diverse array of self-antigens, a process governed in part by the autoimmune regulator protein, which facilitates negative selection of autoreactive T cells.107,108 Similarly, developing B cells in the bone marrow are subjected to receptor editing, deletion, or anergy if they strongly recognize self-antigens.
Despite these stringent mechanisms, central tolerance is not absolute, and autoreactive lymphocytes can escape into the periphery. Peripheral tolerance serves as a critical second checkpoint that regulates mature T and B lymphocytes outside primary lymphoid organs. This is mediated by several non-redundant mechanisms that collectively prevent the activation of autoreactive lymphocytes and maintain immune homeostasis:
- Anergy: In the absence of appropriate co-stimulatory signals, T or B cells that recognize antigens become functionally inactivated rather than activated. This state of non-responsiveness prevents these potentially autoreactive cells from initiating immune responses. 109
- Regulatory T and B cells: These specialized subsets exert immunosuppressive functions through the secretion of anti-inflammatory cytokines, such as IL-10 and transforming growth factor-beta (TGF-β), and via direct cell–cell contact. Their presence is essential in downregulating excessive immune activity and preserving tolerance.110–112
- Immune checkpoints: Co-inhibitory molecules such as cytotoxic T lymphocyte-associated protein 4 (CTLA-4) and programmed cell death protein 1 play a pivotal role in modulating T cell activation. By interfering with APC interactions, these checkpoints suppress T cell proliferation and are vital in preventing chronic immune activation and autoimmunity. 109
- Clonal deletion: Persistent exposure to high concentrations of self-antigens in peripheral tissues can induce apoptosis in autoreactive T or B cells. This mechanism eliminates potentially harmful clones from the repertoire, thereby reducing the likelihood of autoimmune responses. 113
In NMOSD, B cells upregulate AQP4 expression after CD40 engagement and present it in an MHC-II context to eliminate AQP4-specific thymocytes. Because AQP4-IgG is an isotype-switched antibody and its production is T cell-dependent, the failure of immune tolerance likely also involves T helper cells. 114 Thymic B cells, rather than mTECs, play a crucial role in tolerating the T cell repertoire against AQP4. These findings support the concept that thymic negative selection of CD40-induced self-antigens in B cells serves as a mechanism to regulate self-destructive T cell–B cell interactions within the systemic immune compartment. Failure of this tolerance mechanism may lead to enhanced T-cell-dependent autoantibody production and the development of overt autoimmune disease. 115
Tregs are present at a reduced frequency in NMOSD patients. 116 A functionally compromised Treg compartment, lacking AQP4-specific T cell clones, may contribute to defective peripheral tolerance, producing pathogenic AQP4-IgG autoantibodies due to insufficient Treg-mediated suppression. 117 Similarly, some studies have reported a reduced proportion of Bregs and decreased IL-10 expression. During acute NMOSD relapses, CD19+CD24hiCD38hi and CD19+CD5+CD38hi Breg cells exhibit significant impairment, accompanied by lower IL-10 levels. 118 The coordinated action of central and peripheral tolerance is essential for maintaining immune homeostasis. Dysregulation of these mechanisms can lead to chronic inflammation and tissue destruction. Understanding these processes is pivotal for developing therapeutic strategies aimed at restoring immune tolerance, which holds significant potential for treating autoimmune and inflammatory disorders while preserving overall immune function. Figure 1 summarizes the main mechanisms involved in the induction of immune tolerance.
Mechanisms of immunotolerance. Immune tolerance to Self-Ag is primarily established during development, occurring in the thymus for T cells and in the bone marrow for B cells. This process eliminates self-reactive T cells through apoptosis in response to excessive reactivity to self-antigens presented by major histocompatibility complex molecules. Subsequently, positive selection ensures the survival of functionally competent T cells released into the periphery. A similar selection process occurs in the bone marrow to limit B cell autoreactivity. Central tolerance is maintained through receptor editing and apoptosis, significantly reducing the potential for autoimmunity. Receptor editing involves ongoing immunoglobulin light chain gene recombination, leading to secondary rearrangements that modify antigen receptor specificity by replacing one light chain with another. Despite these central tolerance mechanisms, some self-reactive mature T and B cells evade deletion and enter the peripheral lymphoid compartment. Multiple peripheral tolerance mechanisms further regulate immune homeostasis and prevent self-reactive immune cells from mediating tissue damage. These mechanisms include Treg and Breg cells, activation-induced cell death, and the absence of co-stimulatory signals that induce anergy. For clarity, some steps involved in tolerance induction are not depicted in the accompanying schematic.
Importance of immune tolerance in autoimmune diseases
Disruption of either central or peripheral tolerance mechanisms is a hallmark of autoimmune diseases, including MS, systemic lupus erythematosus (SLE), and type 1 diabetes. When immune tolerance fails, autoreactive lymphocytes escape regulatory control, leading to chronic inflammation and progressive damage to specific organs or tissues.118–120
In disease management, restoring immune tolerance is a cornerstone of addressing the root cause of autoimmune conditions. Unlike traditional immunosuppressive therapies that broadly suppress the immune system, tolerance-inducing approaches aim to target pathogenic immune cells with high specificity while preserving protective immune responses. This precision reduces the risks associated with generalized immunosuppression, such as infections and malignancies, making tolerance-based interventions safer and more sustainable for long-term use. These include the use of tolerogenic DCs, antigen-specific therapies, and biologics that modulate immune checkpoints. 104 Advances in biotechnology, such as gene editing 121 and the engineering of regulatory T cells (Treg), 122 offer novel tools to precisely control immune responses. Additionally, the development of nanoparticles for targeted antigen delivery 123 and the exploration of mucosal immunotherapy, such as intranasal antigen administration, 124 provide innovative methods to induce localized immune tolerance with minimal systemic side effects. To illustrate the clinical translation of these approaches, a phase Ib clinical trial (NCT02283671) evaluating autologous tolerogenic DCs loaded with myelin peptides was conducted in patients with MS, and seropositive and seronegative NMOSD. The trial demonstrated good safety and tolerability, with no serious adverse events reported. Immunological outcomes included significantly increased IL-10 production by peripheral blood mononuclear cells (PBMCs) and trends toward elevated regulatory T cells type 1 (Tr1) cells and reduced T cell proliferation. These findings support the feasibility of antigen-specific tolerance induction in humans, complementing earlier preclinical evidence. By focusing on the root cause of immune dysregulation, these strategies aim to achieve sustained remission and potentially prevent disease activity. Furthermore, these approaches have the potential to transform disease management, offering durable remission, reduced dependence on chronic medications, and improved outcomes and quality of life for patients.
Role of Tregs and Bregs in autoimmune diseases
Tregs and regulatory B cells (Bregs) play critical roles in maintaining immune tolerance and preventing autoimmunity. These cells suppress excessive or aberrant immune responses and promote homeostasis by modulating the activity of effector T cells, B cells, and innate immune cells.110–112,125
Tregs are a subset of CD4+ T cells, distinguished by the expression of the transcription factor FOXP3, which is essential for their development, stability, and suppressive function. Tregs exert their immunosuppressive effects through various mechanisms, including secretion of anti-inflammatory cytokines such as IL-10, TGF-β, and IL-35.126,127 Additionally, Tregs inhibit immune activation through direct cell-to-cell interactions mediated by molecules like CTLA-4. 125
In addition, Tregs can be categorized into different subsets based on their origin and functional properties.110–112,128,129
- Natural Tregs: Derived in the thymus, these cells are crucial for maintaining tolerance to self-antigens and preventing autoimmunity.
- Induced Tregs: Generated in the periphery from conventional CD4+ T cells in response to specific antigenic stimulation in a tolerogenic environment. Induced Tregs (iTregs) play a significant role in maintaining tolerance to environmental and dietary antigens.
- Tr1 cells: A subset of iTregs characterized by high IL-10 production but lacking FOXP3 expression. They are particularly important in regulating inflammation in mucosal tissues.
- Th3 cells: Another subset of Tregs known for their secretion of TGF-β, primarily involved in gut immune homeostasis.
Bregs suppress inflammatory responses through the secretion of IL-10, IL-35, and TGF-β, as well as through direct interactions with effector cells. 130 They modulate T cell activation, suppress DCs maturation, and regulate macrophage polarization toward anti-inflammatory phenotypes. They can be classified into several subsets based on surface markers and cytokine production:
- CD19+CD24hiCD38hi Bregs: These are found in the peripheral blood and are the most extensively studied human Breg population. They are potent producers of IL-10 and play a key role in maintaining peripheral tolerance. 131
- CD5+ B1a cells: This subset of Bregs is primarily studied in mice and has been shown to suppress inflammation through IL-10 production. 132
- Marginal zone Bregs: Found in the spleen, these cells contribute to systemic tolerance by producing IL-10 and interacting with other immune cells. 133
- Plasmablast-derived Bregs: These cells arise during chronic inflammation and contribute to the regulation of autoimmune responses, especially in diseases such as SLE and RA. 134
Both Tregs and Bregs represent promising therapeutic targets. Strategies to expand or enhance the functionality of these regulatory cells, such as cell-based therapies or cytokine modulation, are under investigation for their potential to restore immune balance and achieve durable remission in autoimmune diseases.
Limitations in inducing long-term tolerance
Inducing long-term tolerance in autoimmune and inflammatory diseases presents several limitations that require careful consideration. One major challenge is the difficulty in achieving a stable and sustained immune regulatory environment, as immune tolerance induction strategies often face issues of tolerance breakdown over time. This may occur due to the persistence of autoantigens, chronic inflammation, or the activation of compensatory immune responses that counteract the tolerogenic effects. Furthermore, the complexity of the immune system, including the heterogeneous nature of immune responses across individuals, complicates the development of universally effective tolerance-inducing therapies. 135
Another limitation is the risk of inducing immune suppression rather than tolerance. 120 Tolerance-inducing strategies, particularly those involving immune-modulatory agents such as tolerogenic DCs or biologics, may lead to excessive immune suppression, thereby increasing susceptibility to infections or malignancies. Additionally, the manipulation of immune cells in ways that promote tolerance could inadvertently result in off-target effects or altered immune cell function, potentially worsening disease outcomes or leading to the development of other autoimmune disorders. 136
The timing and mode of delivery of tolerance-inducing agents also pose significant challenges. Immune tolerance is often highly dependent on the developmental stage of the immune system, meaning that strategies that are effective at certain stages of immune response may not be applicable later in life or in patients with established disease. 137 Moreover, the BBB further complicates treatment by restricting access of therapeutic agents to CNS-resident immune cells and inflamed tissues. Advanced delivery systems, such as nanoparticles or intranasal formulations, are required to effectively transport antigens or tolerogenic agents across the BBB without causing systemic side effects.138,139
Finally, the heterogeneity of NMOSD and MOGAD, including variations in disease pathogenesis, clinical presentations, and genetic factors, necessitates personalized approaches to tolerance induction. Tailoring therapies to individual patient profiles adds another layer of complexity, requiring a better understanding of the molecular mechanisms underlying disease pathogenesis and immune tolerance. Precision medicine, utilizing biomarkers such as AQP4-IgG, MOG-IgG, or Treg functionality, is essential to tailor treatments and optimize therapeutic outcomes.
Novel approaches targeting B cells, complement pathways, and cytokines
B cell targeting
B cells are integral to both humoral and cellular immunity, and their dysregulation is a hallmark of many autoimmune diseases, such as SLE, RA, MS, NMOSD, and MOGAD. B cells contribute to the autoimmune process through the production of pathogenic autoantibodies, antigen presentation to T cells, and the secretion of pro-inflammatory cytokines. 140 Due to their pivotal role in disease pathogenesis, targeting B cells has become a cornerstone of therapy for several autoimmune diseases.
B cell depletion therapy
The depletion of B cells remains one of the most well-established therapeutic strategies for autoimmune diseases. Anti-CD20 monoclonal antibodies, have been widely used to deplete mature B cells in diseases such as RA, SLE, MS, NMOSD, and MOGAD. 141 CD20 is expressed on most B cells, excluding early progenitor and plasma cells. The depletion of CD20+ B cells reduces the number of circulating autoreactive B cells and limits the production of pathogenic autoantibodies. 142 However, B cell depletion therapy has its limitations. Although B cell depletion significantly reduces autoantibody production, it does not completely eliminate the presence of long-lived plasma cells that may continue to produce autoantibodies for extended periods. Therefore, the relapse of disease after cessation of therapy is common. 143 Furthermore, while B cell depletion can provide clinical benefits, it can also increase the risk of infections, particularly with encapsulated bacteria (namely, Neisseria meningitidis, Streptococcus pneumoniae, Haemophilus influenzae type b), due to the reduction in B cell-mediated humoral immunity. 141
Targeting B cell subsets
The therapeutic targeting of specific B cell subsets—such as memory B cells, plasmablasts, and plasma cells—represents an emerging strategy aimed at improving the specificity and efficacy of B cell-directed therapies.144,145 Plasmablasts, which serve as precursors to antibody-producing plasma cells, are frequently implicated in the generation of pathogenic autoantibodies during disease relapses. Targeting this subset using monoclonal antibodies such as blinatumomab, or agents directed against surface markers like CD138, holds promise for reducing the production of pathogenic antibodies while preserving protective immune responses against infectious agents. 93
Modulating B cell activation and signaling
Another emerging strategy involves targeting intracellular signaling pathways essential for B cell activation, survival, and differentiation. In particular, inhibition of signaling molecules such as spleen tyrosine kinase and phosphoinositide 3-kinase, both of which are integral to B cell receptor signaling, is being explored as a means to suppress B cell-mediated immune responses.144,145 These approaches aim to reduce pathogenic antibody production and modulate B cell function and are currently under investigation in clinical trials for various autoimmune disorders. 146
Targeting CD40/CD40L pathway
The CD40/CD40L (CD40 ligand) interaction is essential for B cell survival, activation, and class-switch recombination. Targeting this pathway has been proposed as a way to selectively modulate B cell function. Inhibitors of CD40L or agonists of CD40 have shown promise in preclinical studies and are being tested in clinical trials to modulate autoantibody production without inducing global B cell depletion. 147
Complement pathway modulation
The complement system, a critical component of the innate immune response, is composed of over 30 proteins that work together to enhance immune surveillance, mediate inflammation, and facilitate pathogen elimination. Dysregulation of the complement system, however, is increasingly recognized as a driver of autoimmune and inflammatory diseases. In particular, inappropriate activation of the complement cascade can contribute to tissue injury, inflammation, and the propagation of disease in conditions such as SLE, vasculitis, age-related macular degeneration (AMD), and NMOSD. 3 The therapeutic targeting of the complement system has become an area of intense research, with several innovative strategies emerging in recent years. 3
Inhibition of complement activation
One of the key therapeutic strategies involves the inhibition of central complement components, such as C3 and C5, which are integral to the initiation and amplification of the complement cascade.59,60 C3 plays a pivotal role in both the classical and alternative pathways of complement activation, and its inhibition effectively blocks downstream effector functions including opsonization, immune complex clearance, and the generation of pro-inflammatory mediators such as C5a. C3 inhibitors, such as APL-2 and CR2-Crry, are currently under investigation in clinical trials for diseases such as SLE and dry AMD, as well as in preclinical models of NMOSD. These agents have demonstrated promising efficacy in reducing complement-mediated tissue injury and inflammation. 148
Targeting complement receptors and regulators
Beyond directly inhibiting complement proteins, targeting complement regulators and receptors represents an additional strategy to modulate complement activity. Complement regulators such as factor H and complement receptor 1 (CR1) play crucial roles in limiting complement activation and preventing excessive inflammation. 149 Dysfunction of these regulators can lead to uncontrolled complement activation and tissue damage. Strategies to enhance the activity of these regulators or mimic their function are being explored as potential therapeutic interventions. 150
Blocking complement-C5a receptor pathway
C5a, a potent anaphylatoxin generated during complement activation, mediates a variety of inflammatory responses by binding to its receptor, C5a receptor (C5aR). C5aR signaling induces the recruitment of neutrophils and other immune cells to sites of tissue damage, contributing to chronic inflammation. 146 Selective C5aR inhibitors, such as avacopan, are being tested for their ability to prevent C5a-induced inflammation without inhibiting the entire complement system. These inhibitors have demonstrated efficacy in treating diseases with a strong complement-driven inflammatory component, such as granulomatosis with polyangiitis and microscopic polyangiitis.151,152
Targeting the alternative pathway
The alternative complement pathway can drive inflammation even in the absence of immune complexes or antibodies, making it a key player in diseases like AMD and certain forms of SLE. The use of selective inhibitors targeting proteins involved in the alternative pathway, such as factor B, represents a novel approach to controlling complement activation while avoiding the broad immunosuppressive effects of global complement inhibition. 153
Cytokines inhibition
Cytokines are critical mediators of immune responses, orchestrating the activation and differentiation of immune cells. In autoimmune diseases, dysregulated cytokine production contributes to inflammation, tissue damage, and disease progression. Recent therapeutic advances have focused on targeting specific cytokines or their receptors to attenuate pathogenic immune responses.
Inhibitors of pro-inflammatory cytokines such as tumor necrosis factor (TNF)-α, IL-1, and IL-6 have been successfully used in clinical practice for diseases like RA, inflammatory bowel disease, psoriasis, and NMOSD.95,154,155 However, TNF-α blockers have been linked to serious CNS side effects in demyelinating disorders. 156 The use of Janus kinase inhibitors, which disrupt intracellular cytokine signaling, has also demonstrated success in treating a variety of autoimmune disorders by inhibiting the downstream effects of multiple cytokines. 157
Beyond the classical inflammatory cytokines, emerging research has focused on targeting cytokines involved in T cell differentiation and function. For instance, IL-17 and IL-23 inhibitors have proven effective in treating conditions like psoriasis and ankylosing spondylitis, where Th17-driven inflammation plays a prominent role. 158
Antigen-specific therapies: AQP4 and MOG antigen-based tolerance induction
The induction of immune tolerance against AQP4 and MOG represents a promising therapeutic strategy to prevent or reverse autoimmune-mediated damage in the CNS. Among the various approaches being investigated, peptide-based therapies aimed at inducing antigen-specific tolerance have garnered significant interest due to their potential to selectively target autoimmune responses while minimizing global immune suppression.
Peptide-based therapies for antigen-specific tolerance induction
Peptide-based tolerance induction involves the administration of synthetic peptides that correspond to the immunodominant epitopes of target autoantigens like AQP4 or MOG. The goal of these therapies is to specifically promote immune tolerance by modulating the immune response to these antigens without broadly suppressing immune function. This strategy can induce tolerance through several mechanisms, including T cell anergy, Treg cells expansion, and B cells tolerance. 159
Peptide vaccine approaches
Peptide vaccines are designed to deliver specific sequences derived from AQP4 or MOG to the immune system in a controlled manner. The peptides are often conjugated with adjuvants to enhance their immunogenicity and ensure sufficient activation of immune responses. The goal is to induce immune tolerance through the presentation of these peptides in a manner that prevents autoimmune activation, without causing generalized immune suppression. In clinical trials, peptide vaccines targeting MOG have shown promise in reducing autoimmune reactivity, preventing relapse, and improving clinical outcomes in animal models of MS. 160
While the preclinical evidence for AQP4 and MOG antigen-based tolerance induction is compelling, clinical translation of these strategies presents several challenges. One critical issue is the identification of optimal peptide epitopes capable of inducing robust tolerance without triggering immune escape mechanisms. Furthermore, the development of safe and effective delivery methods for peptide vaccines, such as adjuvants and nanoparticle-based formulations, remains a key area of research.
Clinical trials will need to establish the most appropriate dosing schedules and long-term safety profiles for peptide-based therapies. Moreover, combining peptide-based therapies with other immunomodulatory agents or utilizing strategies such as gene editing or immune checkpoint modulation could further enhance the efficacy of tolerance induction. The integration of biomarker-driven approaches to personalize treatment regimens may also improve patient outcomes and minimize side effects.
Tolerance induction through tolerogenic DCs
Peptide-based therapies may be enhanced by combining them with tolerogenic mesenchymal cells, which are specialized in inducing antigen-specific immune tolerance. Tolerogenic DCs loaded with AQP4 peptides have demonstrated the capacity to promote the deletion or anergy of autoreactive T cells and to expand regulatory Tregs, thereby modulating pathogenic immune responses. This strategy capitalizes on the ability of DCs to direct antigen-specific responses toward tolerance rather than inflammation.161–163 A phase Ib clinical trial investigating autologous tolerogenic DCs loaded with peptides derived from various myelin proteins and AQP4 showed clinical stability in treated patients, with no increase in relapse rates, disability progression, or radiological activity. Importantly, an increase in IL-10 production by peptide-stimulated PBMCs and an elevated frequency of Tr1 cells was observed, supporting the tolerogenic potential of this approach. 161 However, there is currently no clear evidence that tolerogenic DCs loaded with MOG peptides can induce comparable immunological tolerance. This may reflect fundamental differences in disease mechanisms: while the pathogenic role of AQP4-IgG in NMOSD is well established, mediated through astrocyte injury and complement activation, the role of MOG-IgG in MOGAD remains less clearly defined.
Despite promising preclinical results, several challenges remain in the development of peptide-based therapies for tolerance induction. One major limitation is the need to identify the optimal peptide epitopes that can effectively induce tolerance in a manner that avoids immune escape or exacerbation of the disease. Additionally, the long-term safety and efficacy of peptide-based therapies must be thoroughly assessed in clinical trials to ensure that these approaches do not inadvertently trigger immune activation or promote immune-mediated tissue damage. Advances in antigen delivery systems, such as nanoparticle-based vaccines or peptide-conjugated nanoparticles, may improve the precision and effectiveness of peptide-based therapies. 164
Cell-based therapies
Regulatory T and B cells
Cell-based therapies targeting Tregs and Bregs represent promising strategies for the treatment of autoimmune diseases, including NMOSD and MOGAD. However, the clinical translation of these therapies faces significant challenges, including issues related to cell expansion, safety, and long-term efficacy.
Regulatory T cells
In the context of NMOSD and MOGAD, Tregs have been shown to be dysfunctional or present in insufficient numbers, contributing to the development and progression of the autoimmune response against AQP4 and MOG. Thus, enhancing the number and function of Tregs through adoptive cell therapy has emerged as a potential therapeutic approach for these disorders.165–167
In animal models of NMOSD and MOGAD, the infusion of ex vivo-expanded Tregs has demonstrated promising results in controlling autoimmune inflammation and preventing CNS demyelination. For instance, in experimental autoimmune NMOSD models, the transfer of AQP4-specific Tregs has been shown to attenuate clinical symptoms, reduce inflammatory infiltrates in the CNS, and prevent the destruction of astrocytes. 117 Similarly, in MOG-induced EAE models, MOG-specific Tregs have been shown to suppress the autoimmune response, reduce the frequency of relapses, and mitigate the severity of demyelination. 168
The mechanism of action of Treg-based therapies involves the suppression of autoreactive T cells and the promotion of immune tolerance through the production of immunosuppressive cytokines such as IL-10, TGF-β, and IL-35. Tregs can also exert their effects through direct cell-to-cell contact, modulating DCs function and promoting the generation of other regulatory immune cell subsets. 169
Although preclinical studies have demonstrated the therapeutic potential of Treg-based therapies in autoimmune CNS disorders, several challenges must be overcome for successful clinical translation in NMOSD and MOGAD. One of the primary challenges is the isolation and expansion of Tregs in vitro. While the expansion of Tregs from peripheral blood or tissue samples has been achieved in preclinical studies, the process remains complex and resource intensive. Furthermore, maintaining the stability and suppressive function of Tregs during ex vivo expansion is critical, as expanded Tregs may lose their regulatory properties if cultured under suboptimal conditions or for prolonged periods.170,171
Another challenge lies in ensuring the specificity of Tregs against AQP4 and MOG without affecting the broader immune system. Strategies such as the use of peptide-loaded DCs or genetic modification to enhance the specificity of Tregs are under investigation but are not yet standardized in clinical practice. 172
Moreover, the long-term safety and efficacy of Treg-based therapies remain uncertain. The risk of inducing immune suppression that could lead to opportunistic infections or malignancies must be carefully considered. Additionally, the potential for Tregs to suppress beneficial immune responses, such as anti-tumor immunity, needs to be addressed in clinical trials. 171
Regulatory B cells
In the context of NMOSD and MOGAD, Bregs have been shown to be impaired, contributing to the breakdown of immune tolerance and the exacerbation of the autoimmune response.173,174 Consequently, Breg-based therapies have been proposed as a means to restore immune balance and reduce autoimmune activity in these diseases.
The mechanism by which Bregs exert their regulatory effects involves the secretion of anti-inflammatory cytokines, which directly inhibit the activation of autoreactive T cells and dampen the pro-inflammatory milieu. Additionally, Bregs can influence the differentiation and function of other immune cells, including DCs and macrophages, to further promote immune tolerance. 133
Despite some promising preclinical data, the clinical application of Breg-based therapies in NMOSD and MOGAD faces several challenges. One of the main issues is the identification and isolation of Bregs from peripheral blood, as these cells represent a rare population and are difficult to distinguish from other B cell subsets. In addition, the expansion of Bregs in vitro is technically challenging, as these cells require specific conditions for optimal growth and function. Furthermore, maintaining the regulatory function of Bregs during ex vivo expansion is a critical consideration, as prolonged culture may alter their phenotype and cytokine production.
A significant challenge in Breg-based therapies is the lack of well-defined markers for identifying functional Bregs. While several markers, including CD19, CD24, and CD38, have been proposed for Breg identification, there is no consensus on the precise phenotype of functional Bregs. This lack of standardization complicates the development of reliable protocols for isolating and expanding Bregs for therapeutic use. 112
Moreover, the clinical safety and efficacy of Breg-based therapies remain uncertain. The risk of inducing immunosuppressive effects that could result in infections or malignancies is a concern, particularly given the role of B cells in humoral immunity. Additionally, there is the potential for Bregs to inadvertently suppress beneficial immune responses, including anti-infectious or anti-tumor immunity, which must be carefully monitored in clinical trials.
Table 1 includes additional cell-based trials involving mesenchymal stem cells (MSCs) that warrant discussion. In one trial (NCT02249676), the administration of autologous MSCs to refractory NMOSD patients resulted in a significant reduction in disability (EDSS decreased from 4.9 to 4.3, p = 0.02). Similarly, umbilical cord-derived MSCs delivered intravenously and intrathecally (NCT01364246) led to a marked improvement in clinical scores (EDSS from 3.2 to 1.4, p < 0.05). Furthermore, MSCs exhibit paracrine activity through the secretion of immunomodulatory cytokines and trophic factors, and are capable of migrating to sites of injury where they actively participate in tissue repair and regeneration. 175 These findings suggest that MSCs may exert therapeutic benefits via immunomodulatory, reparative, and neuroprotective mechanisms in both seropositive and seronegative NMOSD cases.
Immune tolerance therapies in clinical development for NMOSD and MOGAD.
AAR, annualized relapse rate; AE, adverse effects; ANCA, antineutrophil cytoplasmic antibodies; AQP4, Aquoporin4; BAFFR, B cell activating factor-receptor; BCMA, B cell maturation antigen; CART, chimeric antigen receptor T cells; DC, dendritic cell; EDSS, expanded disability status scale; Gd, gadolinium; hUC-MSCs, human umbilical cord-mesenchymal stem cells; IL-10, interleukin-10; IV, intravenous; MG, myasthenia gravis; MOGAD, myelin oligodendrocyte glycoprotein antibody-associated disease; MRI, magnetic resonance imaging; MS, multiple sclerosis; MSC, mesenchymal stem cells; NMOSD, neuromyelitis optica spectrum disorder; N/R, not reported; PBMC, peripheral blood mononuclear cells; QoL, health-related quality of life; SLE, systemic lupus erythematosus; Tr1, regulatory T cells type 1; VEP, visual evoked potential.
Chimeric antigen receptor T cell therapy
Chimeric antigen receptor T (CAR-T) cell therapy has emerged as a promising approach in the treatment of various autoimmune and neuroinflammatory conditions, by engineering T cells to express a receptor that targets a specific antigen on the surface of pathogenic cells. 176 In NMOSD, CAR-T cells could be designed to specifically recognize and eliminate autoreactive T cells that target AQP4 or autoreactive B cells that produce AQP4-specific antibodies (NCT03605238; NCT04561557). 177 This targeted approach could offer a more precise and long-lasting solution compared to traditional immunosuppressive treatments, which broadly suppress immune function and may result in significant side effects. By targeting AQP4-specific immune cells, CAR-T therapy has the potential to reduce the frequency and severity of relapses in NMOSD, while promoting sustained remission and potentially modifying the disease course.
Similarly, in MOGAD, CAR-T cells could be engineered to target the immune cells that produce anti-MOG antibodies or to modulate the T cell response that drives the inflammatory process. 178 Given that MOG is a crucial component of the myelin sheath, targeting the immune cells responsible for its destruction could prevent further demyelination and preserve neurological function. The ability to selectively target these immune components at the cellular level offers the promise of a more effective therapy for patients who are refractory to current immunomodulatory treatments. Despite the promising theoretical benefits, the use of CAR-T cell therapy in MOGAD remains in the early stages of clinical investigation. Although, to our knowledge, no such translational research is currently being performed in the field of MOGAD, this may form an interesting research avenue.
Table 1 provides additional clinical details for CAR-T therapies in NMOSD and MOGAD. An open-label phase I trial of CT103A, targeting B cell maturation antigen in refractory NMOSD patients, reported that while all participants experienced adverse events of grade ⩾3, most showed functional improvements and absence of relapses. Two ongoing trials (NCT04561557 and NCT05828212) are actively recruiting patients with seropositive NMOSD or MOGAD using CD19- or dual-targeted CAR-T cells, with endpoints including safety, relapse rates, and immunological responses. In addition, an open-label clinical study is currently underway to evaluate the safety, tolerability, pharmacokinetics, and pharmacodynamics of CAR-T cell therapy in adult subjects with relapsed or refractory neurological autoimmune diseases, including NMOSD and MOGAD (NCT06869278). These studies reflect the growing clinical interest in highly targeted cellular therapies as tools for durable immune tolerance induction.
The long-term benefits of CAR-T cell therapy in NMOSD and MOGAD could be substantial. If successful, CAR-T cells could provide a more durable solution than current treatments, offering sustained remission with fewer relapses and reduced disability progression. This could significantly improve the quality of life for patients, as well as reduce the need for frequent, invasive treatments like plasma exchange and IVIgG therapy. Additionally, CAR-T therapy could be tailored to individual patients, allowing for a personalized approach to treatment that targets the specific immune components responsible for disease activity.
Hematopoietic stem cell transplantation
The role of hematopoietic stem cell transplantation (HSCT) in NMOSD has been explored in only a few studies, and outcomes have been mixed.179,180 A recent workshop organized by the European Committee for Treatment and Research in Multiple Sclerosis in partnership with the European Society for Blood and Marrow Transplantation recommends that autologous HSCT (AHSCT) could be considered as a rescue therapy for NMOSD that does not respond to treatment, or as an induction therapy for aggressive disease, especially with the use of conditioning regimens that include anti-CD20 or antibody-depleting strategies. Allogeneic HSCT should only be considered for individuals in whom AHSCT has failed, and no other treatment options are available. 180
Gene therapy
The application of gene-editing technologies, such as CRISPR/Cas9, has revolutionized the field of immunology and autoimmune disease treatment by offering the potential to directly modify immune cell function and restore immune tolerance. 181
Mechanisms of Gene Editing for Immune Tolerance Gene-editing technologies, including CRISPR/Cas9, TALENs (Transcription Activator-Like Effector Nucleases), and Zinc Finger Nucleases (ZFNs), enable precise modification of the genome by inducing double-strand breaks at specific genomic loci followed by repair processes, either through error-prone non-homologous end joining or homology-directed repair. These technologies can be applied to immune cells to induce tolerance by several mechanisms, including 182 :
Targeted deletion or modulation of autoimmune genes: Gene-editing can be used to selectively disrupt genes involved in the activation or survival of autoreactive immune cells. For example, the targeted deletion of key molecules involved in the activation of pathogenic T cells or B cells could potentially prevent the development of autoimmune responses.183,184
Enhancing regulatory immune cells: CRISPR/Cas9 and other gene-editing techniques can also be employed to enhance the function of regulatory Tregs and Bregs, which are crucial for maintaining immune tolerance. Gene-editing may be used to introduce or upregulate tolerance-promoting molecules or pathways within these cells, increasing their ability to suppress autoimmunity. 185 For example, CRISPR/Cas9 has been used to enhance Treg expansion and function in preclinical models by editing key genes involved in Treg development, such as Foxp3, which is essential for Treg differentiation. 186
Restoration of self-tolerance in autoreactive immune cells: Another approach involves the correction of autoreactive T cells and B cells through gene editing to restore their tolerance mechanisms. 187 CRISPR/Cas9 has been applied to induce tolerance to AQP4-specific T cells in an animal model of NMOSD, with promising results in reducing clinical severity and inflammatory lesions. 188
Gene-editing technologies hold promise for restoring immune tolerance, yet several challenges must be addressed before clinical translation. Efficient delivery to immune cells, minimizing off-target effects, and ensuring functional integrity post-editing remain critical hurdles. Ex vivo manipulation and large-scale expansion introduce variability, while immune rejection risks persist even with autologous cells. Long-term efficacy and safety require extensive evaluation, particularly regarding unintended immune dysregulation. Ethical and regulatory frameworks must ensure oversight, especially concerning germline modifications. Advances in precision editing and immune modulation strategies are essential to overcoming these barriers and achieving safe, effective, and durable therapeutic applications.189,190
Nanoparticles and drug delivery systems
NPs are promising for targeted delivery of antigens and immunomodulatory agents in autoimmune diseases like NMOSD and MOGAD. They overcome challenges of traditional drug delivery, such as poor bioavailability and off-target effects. Engineered from lipids, polymers, or metals, NPs offer precise therapeutic delivery with reduced toxicity, improving treatment outcomes for these conditions. 191 Their applications could include:
Targeted antigen delivery: Nanoparticles can be functionalized to deliver antigens (e.g., AQP4 or MOG peptides) to immune cells like dendritic or B cells, promoting immune tolerance rather than activation. For instance, MOG peptide-loaded nanoparticles could modulate MOG-specific T cells, mitigating autoimmune responses in EAE. 192 In an animal model of NMOSD, administration of liposome-encapsulated AQP4 peptide201–220 targeted the lymph nodes, where it was presented to DCs, effectively suppressing immune responses to this epitope. 193
Immunomodulatory therapy: Nanoparticles can enhance the stability and bioavailability of cytokines (e.g., IL-10, TGF-β194,195) or immunosuppressive agents (e.g., glucocorticoids, methotrexate), enabling controlled drug release to sustain therapeutic effects while minimizing systemic toxicity.196,197
Gene therapy and editing: Nanoparticles facilitate the delivery of nucleic acids (DNA, RNA, CRISPR/Cas9) to immune cells, potentially restoring immune homeostasis by enhancing Treg cells function and inducing antigen-specific tolerance.198,199
Controlled and sustained release: The release kinetics of encapsulated antigens or immunomodulatory agents can be controlled to ensure that the therapeutic agents are delivered at the optimal dose over an extended period, rather than causing peak concentrations that may lead to toxicity. 200
Figure 2 summarizes the principal emerging strategies for inducing immune tolerance in NMOSD and MOGAD, while Table 1 provides an overview of the leading immunotolerance therapies currently in clinical development for these disorders.
Pathophysiological mechanisms and induction of immune tolerance in NMOSD and MOGAD. (1) Peptide-based tolerance induction: Peptide vaccines containing self-antigen epitopes promote immune tolerance by interacting with APCs, thereby modulating T-cell differentiation and regulating the secretion of inflammatory cytokines. (2) Role of tolerogenic DCs: Tolerogenic DCs are crucial for maintaining central and peripheral immune tolerance. They achieve this through mechanisms such as T cell clonal deletion, induction of T cell anergy, and the generation and activation of Treg cells. (3) Breg cell exert immunoregulatory functions through multiple pathways, including the secretion of immunomodulatory cytokines, the release of cytotoxic granzyme B, which induces T cell apoptosis, and the upregulation of FasL, PD-1, and PD-L1. These molecules suppress CD4+ and CD8+ T cell activity and promote the differentiation of Tr1 cells. (4) Targeted elimination of autoreactive B cells: CAR T cells targeting B cell markers such as CD19 facilitate the broad depletion of autoreactive B cells. Alternatively, CAR-modified Treg cells can suppress autoreactive responses without complete B cell depletion. (5) Gene editing technologies, such as CRISPR-Cas9, offer a strategy to restore antigen-specific immune tolerance by engineering DCs into a tolerogenic phenotype, expanding autoantigen-specific Treg and Breg populations and optimizing CAR-T cell function. (6) Synthetic nanoparticles—including polymeric, lipid-based, metallic, and peptide-polymer formulations—can be engineered to co-deliver antigens, antibodies, nucleic acids (DNA/RNA), and immunomodulatory agents in tailored combinations to modulate immune responses.
Conclusion and future perspectives
While substantial progress has been made in understanding the pathophysiology of NMOSD and MOGAD, the mechanisms underlying the induction of tolerance to AQP4 and MOG autoantigens remain unclear. Effective tolerance induction strategies must address autoreactive B cell depletion and the regulation of T cell responses. Regulatory T cells, essential for maintaining immune homeostasis and preventing autoimmunity, are particularly interesting. Antigen-specific Tregs have demonstrated potential in preclinical autoimmune disease models, but selective expansion or activation of Tregs without impairing global immunity remains a challenge.
Emerging strategies, such as peptide-based vaccines or nanoparticle-based antigen delivery systems, aim to induce peripheral tolerance to AQP4 or MOG. These approaches require optimization for efficient antigen presentation, minimal systemic effects, and durable outcomes. Additionally, progress depends on identifying optimal dosing regimens and delivery methods while advancing knowledge of immune pathways regulating tolerance.
A significant hurdle to clinical translation is the absence of reliable biomarkers to monitor treatment efficacy and predict outcomes. Diagnostic markers like AQP4-IgG and MOG-IgG are insufficient for tracking disease activity or therapeutic response. Biomarkers reflecting immune tolerance, such as Treg frequency and function, CNS inflammation resolution, or suppression of autoreactive immune cells, are urgently needed for patient selection, regimen optimization, and trial monitoring.
There are obvious theoretical and practical challenges with these approaches related to their biotechnological feasibility, scarcity of human research data, heterogeneity of NMOSD and MOGAD, prior exposure to immunotherapy, and level of neurological disability.
An important limitation in NMOSD management is the clinical and immunological heterogeneity between seropositive and DN cases. DN-NMOSD patients lack detectable AQP4-IgG or MOG-IgG antibodies, suggesting the involvement of alternative antibodies or non-immune-mediated mechanisms in disease pathogenesis. The absence of defined antigenic targets complicates the development of antigen-specific tolerance strategies. Consequently, treatment remains empirical and phenotype-driven, with acute attacks typically managed as in NMOSD seropositive patients. 11 DN-NMOSD patients may demonstrate lower corticosteroid responsiveness and reduced relapse rates following tapering compared to MOGAD. 201 Currently, no relapse-prevention therapies are approved, and conventional immunosuppressants are used off-label. Furthermore, recent clinical trials of eculizumab excluded DN-NMOSD patients, and two randomized controlled trials of satralizumab showed no efficacy in DN-NMOSD individuals, leaving the role of IL-6 and other pathways in this subgroup of patients unresolved.94,95 However, a critical caveat in interpreting these findings is the limited representation of DN-NMOSD patients within these studies, precluding definitive conclusions regarding the pathogenic relevance of complement and IL-6 pathways in this population.
Developing tolerance-inducing therapies requires robust preclinical and clinical models. While the AQP4-IgG passive transfer model provides insights, it lacks full fidelity to human disease. Advanced models with humanized immune systems or 3D organoids could better simulate CNS pathology, enabling personalized, antigen-specific treatments to restore immune tolerance and reduce CNS damage in NMOSD and MOGAD.
