G-CSF as an Adjunctive Therapy with Umbilical Cord Blood Cell Transplantation for Traumatic Brain Injury

Abstract

Traumatic brain injury (TBI), a major contributor to deaths and permanent disability worldwide, has been recently described as a progressive cell death process rather than an acute event. TBI pathophysiology is complicated and can be distinguished by the initial primary injury and the subsequent secondary injury that ensues days after the trauma. Therapeutic opportunities for TBI remain very limited with patients subjected to surgery or rehabilitation therapy. The efficacy of stem cell-based interventions, as well as neuroprotective agents in other neurological disorders of which pathologies overlap with TBI, indicates their potential as alternative TBI treatments. Furthermore, their therapeutic limitations may be augmented when combination therapy is pursued instead of using a single agent. Indeed, we demonstrated remarkable combined efficacy of human umbilical cord blood (hUCB) cell therapy and granulocyte-colony-stimulating factor (G-CSF) treatment in TBI models, providing essential evidence for the translation of this approach to treat TBI. Further studies are warranted to determine the mechanisms underlying therapeutic benefits exerted by hUCB + G-CSF in order to enhance its safety and efficacy in the clinic.

Introduction

Traumatic brain injury (TBI), characterized by damage to the brain as a result of a violent impact, blow, or jolt to the head or a penetrating head injury, is a major health problem worldwide contributing to a significant number of deaths and cases of permanent disability (18,30). In the US, the annual incidence of TBI has been estimated at 1.7 million, for which 1.1 million are treated in emergency, 275,000 are hospitalized for nonfatal TBI, and 52,000 die (30). Moreover, armed troops deployed in Afghanistan and Iraq are exposed to TBI caused by explosive devices, falls, and vehicle or motorcycle accidents (29). This type of TBI has been referred to as a “signature wound,” which continuously increases among the military populations (29). The impact of brain injuries can range in scope from mild (a brief change in mental status or consciousness) to severe (an extended period of unconsciousness or amnesia after the injury) TBI (National Institute of Neurological Disorders and Stroke, National Institutes of Health). The mortality indices vary from 1% in mild TBI, to 2–5% in moderate TBI and up to 30–50% after suffering a severe head injury (113). Nevertheless, compared with other TBI cases, mild TBI comprises 70–80% of all such injuries [for review see (3)].

The pathophysiology of brain injury after head trauma is complicated and can be characterized by the initial primary injury and the subsequent secondary injury that ensues days after the trauma (32,65). Primary injury, which occurs at the time of trauma, can result from either direct physical impact (focal TBI) or from inertial forces due to rapid acceleration–deceleration of the brain (diffuse TBI) (9,40,82,124). On the other hand, secondary injury results from processes initiated by the trauma and has been thought to underlie prolonged activation of many molecular cascades, such as neuroinflammation, oxidative stress, excitotoxicity, mitochondrial dysfunction, hypoxia–ischemia, cerebral edema, and others (34,42,61,65,97,121,124,133), which cause delayed secondary brain damage to the already damaged brain tissue. Secondary injury has also been considered as the most destructive component of TBI (36,65,66), responsible for brain damage and death observed within a few days or weeks after TBI in human studies and experimental TBI models. Moreover, the development of secondary brain damage is a major factor in determining the patient's clinical outcome (82). Therefore, from a drug development perspective, an appealing therapeutic target is to retard the development of secondary brain damage.

In addition to the pathologic outcomes of primary and secondary brain injuries, human TBI is sometimes coupled with other health complications, such as seizures, hydrocephalus or posttraumatic ventricular enlargement, vascular injuries, cranial nerve injuries, and polytrauma (National Institute of Neurological Disorders and Stroke, National Institutes of Health). Higher function disabilities have also been observed in some chronic TBI patients, which include cognitive impairments, sensory–motor deficits, communication problems, and psychiatric disorders, for example, depression, anxiety, personality changes, aggression, and others (4,126,127). Furthermore, TBI survivors are known to suffer from neurodegenerative disorder-like symptoms in the long term, including Parkinson's disease and other movement disorders, Alzheimer's disease (AD), dementia pugilistica, and posttraumatic dementia (50,73,126,127).

Therapeutic opportunities for TBI remain very limited (48). Severe TBI patients are mostly subjected to surgery (acute treatment) to remove or repair ruptured blood vessels and bruised brain tissue, as well as other complications due to brain trauma, or prescribed with medications to mitigate symptoms such as headaches, chronic pain, behavioral problems, depression, and seizures (13,120,121). A majority of TBI patients are relegated to rehabilitation therapy (14,33,59,117), although obviously this type of intervention does not prevent or reverse much of the brain damage resulting from the injury. Indeed, there is a substantial need for clinically efficacious therapies for TBI, especially those that prevent and/or reduce secondary injury and facilitate long-term functional recovery after TBI. Moreover, compounding the limited therapeutic options for TBI, general public awareness is also inadequate (30). For instance, individuals who experienced concussions (mild head injury or mild TBI) usually forego seeking medical attention (as deficits produced by mild TBI are usually subtle and less often recognized) and will only do so when symptoms become severe (3). This practice, however, creates difficulties not only in the diagnosis but also in treating TBI. Thus, increased public awareness will not only aid in the identification of concussions but will also lead to early initiation of treatment, thereby halting their progression toward more severe TBI symptoms.

Stem Cell Transplantation in Tbi: Potential of Umbilical Cord Blood Cells

A specific goal with the use of neuroprotective agents has been to reduce the development of secondary injuries by reducing the direct toxic cell damage and the cytotoxic brain edema (89). However, the clinical trials that have been performed to test efficacy of several neuroprotective substances that showed beneficial effects in animal studies (e.g., as nimodipine, glutamate inhibitors, the competitive N-methyl-d-aspartate receptor antagonists, magnesium sulfate, and scavenging agents) failed to show any significant efficacy in the treatment of patients with clinical TBI (59,61,64,72,112). This indicates not only complex TBI pathophysiology but also the need for other treatment modalities.

Considering the massive secondary cell death mediating the progression of TBI, novel treatments have been fashioned to target the delayed therapeutic time window post-TBI referred to as “neuroregeneration” in contrast to the narrow “neuroprotection” window relegated to the acute TBI phase (71,109). A major component of regenerative medicine is stem cell-based therapeutics, which have shown promising therapeutic potential for various types of neurological disorders, including TBI (e.g., 57,58,62,109,130), and have reached limited clinical trials [for review see (43)]. Translating cell therapy for treatment of brain diseases to the clinic has involved testing the safety, efficacy, and mechanisms of action of a variety of transplantable donor cells, including fetal stem cells, cancer-derived neuron-like cells, embryonic stem cells, induced pluripotent stem cells, and adult stem cells, such as umbilical cord blood, bone marrow stromal cells, amnion cells, among others (16,57,58,62,63,74,75,84,87,90,109,130). However, among these various types of stem cells, adult stem cells are of interest, as they circumvent ethical and moral problems and also teratogenic and oncogenic risks usually associated with transplantation of embryonal or fetal-derived stem cells (85).

Our group and several others have assessed the clinical utility of human umbilical cord blood (hUCB)-derived cells for various intractable disorders such as stroke, Parkinson's disease, Huntington's disease, and cerebral palsy [(1); for review see (86)]. Limited clinical trials of hUCB cells are being explored in cerebral palsy, inborn metabolic disorders, and stroke [for reviews see (17,45,86,119)]. Numerous studies have described several advantages of hUCB in cell transplantation therapy, such as providing an unlimited supply of cells in culture, thereby circumventing ethical and logistical issues, availability in significant quantities and producing higher yields of hematopoietic progenitor cells, retained capacity of stem or progenitor cell from cord blood to proliferate and differentiate despite years of cryopreservation, low incidence of graft-versushost disease when compared to that of the adult bone marrow, and long-standing and successful clinical history in the hematopoietic field [for review see (131)]. hUCB stem cells have also been used experimentally in animal models of TBI. In some studies, transplanted cells derived from the mononuclear fraction of hUCB conferred neuroprotection by decreasing inflammation and brain tissue loss, promoting neurogenesis, and rescue of neurological dysfunctions (e.g., 11,41,78). Additionally, mesenchymal stem cells (MSCs) derived from hUCB promoted functional benefits by increasing angiogenesis and vasculogenesis (1,53,55). However, in order to initiate clinical trials of hUCB cell therapy for TBI, rigorous translational research is required to determine optimal transplantation regimen and to provide important information on the regenerative mechanisms of transplanted stem cells. To this end, demonstrating a well-defined stem cell source is crucial for quality assurance and quality control of graft origin and also for validity and reproducibility of experimental outcomes (23). In addition to identifying the optimal transplantable hUCB cell phenotype, low graft survival has been documented in the TBI brain, which may be due to the host tissue microenvironment that does not promote cell survival, likely created by the secondary neuroinflammatory response (52,121,128). While robust graft survival may not be necessary to induce behavioral recovery, the alternative mechanism of graft-induced bystander effects still requires for the hostile microenvironment to be abrogated in order to facilitate improved clinical outcomes (23). Therefore, the concept that stem cell therapy can be enhanced by rendering a receptive microenvironment (i.e., less neuroinflammation) appeals to advancing regenerative medicine for treating the injured brain.

G-Csf as Stand-Alone and Adjunctive Therapy: Implications for the Treatment of Tbi

G-CSF, Biology, and Mechanisms of Action

Granulocyte-colony-stimulating factor (G-CSF, molecular weight about 20,000) is a myeloid growth factor produced by activated macrophages, endothelial cells, and fibroblasts. It was first discovered in the mid-1960s as a hematopoietic glycoprotein that regulates the survival, proliferation, differentiation, and function of neurotrophil granulocyte progenitor cells and mature neutrophils (67,123). The first murine and human recombinant G-CSF became available in the late 1980s. Recently, G-CSF can be produced in large quantities in Escherichia coli and mammalian cells using recombinant DNA technology [for review see (123)].

G-CSF has been documented to exert a range of actions. Accordingly, G-CSF induced growth of mainly neutrophilic granulocyte colonies in a CFU-GM assay, and it also acted on the function of mature neutrophils, such as enhancement of chemotactic peptide N-formylmethionyl-leucyl-phenylalanine binding on mature neutrophils (79,123). G-CSF knockout studies have clearly delineated the specific role of G-CSF in the proliferation and differentiation of granulocyte progenitor and precursor cells (56). Moreover, mice lacking endogenous G-CSF displayed chronic neutropenia and impaired neutrophil mobilization, implying the crucial role of G-CSF in maintaining the normal balance of neutrophil production during steady-state myelopoiesis (56,123).

G-CSF, produced in the bone marrow in response to cellular stimuli, binds to specific receptors (i.e., G-CSF receptor) found in hematopoietic progenitor cells, monocytes, platelets, neurons, endothelial cells, and small-cell lung cancer cells (24,39,70,101). Activation of these receptors by G-CSF is followed by induction of signaling cascades like the Janus kinase/signal transducer and transcription activator, Ras/mitogen-activated protein kinase and phosphotidyl inositol 3-kinase/Protein kinase B/Akt pathways, which have been shown to influence cellular proliferation, antiinflammatory, and antiapoptotic processes (26,44,83,95,101, 102,108,114,122) and also in mobilizing stem cells to target sites (e.g., sites of injury, see below). Recent studies have also revealed important roles of G-CSF in the CNS (94). G-CSF passes the blood–brain barrier (BBB) and acts on neurons for recovery (25,69). Therefore, G-CSF is a novel neurotrophic factor and a highly attractive candidate for the treatment of neurodegenerative conditions (6).

Therapeutic Uses of G-CSF

Approved Indications

G-CSF is approved by the Food and Drug Administration for treating cancer patients receiving myelosuppressive or myeloablative chemotherapy followed by bone marrow transplantation. Strong chemotherapy may cause severe neutropenia or febrile neutropenia in cancer patients. In clinical trials in small-cell lung cancer patients, G-CSF was shown to prevent infection as manifested by febrile neutropenia, decreased hospitalization, intravenous antibiotic usage, and incidence of microbiologically documented infections and also caused 50% reduction in the number of patients experiencing febrile neutropenia (15,19). In two separate randomized, controlled trials conducted in patients with Hodgkin's disease and non-Hodgkin's lymphoma and subjected to myeloablative chemotherapy and autologous bone marrow transplantation, G-CSF treatment caused a reduction in the duration of severe neutropenia and hospitalization and also frequency of antibiotic therapy [(54), for review see (123)].

Prolonged neutropenia has been shown to occur after stem cell transplantation (123). A previous study by which G-CSF was injected 24 h after autologous marrow infusion in patients with Hodgkin's disease reported earlier recovery of neutrophils compared with that in control patients (111). Other studies have also reported earlier increase in absolute neutrophil counts in G-CSF-treated patients with relapsed Hodgkin's disease, non-Hodgkin's lymphoma, amyeloid lymphoblastic leukemia, and germ cell tumors than in historical control patients (35,99,123).

Patients with acute myeloid leukemia receiving induction or consolidation therapy have also benefited from the above-mentioned ability of G-CSF to reduce the time for neutrophil recovery and duration of fever, as well as the incidence of infections. In a prospective, randomized study conducted to determine the efficacy and safety of G-CSF treatment after standard intensive chemotherapy in patients with relapsed or refractory acute leukemia, investigators reported that G-CSF accelerated the recovery of neutrophils and reduced incidence of infections in both groups (34). Along this line, the efficacy of G-CSF to reduce frequency and duration of infection-related events in patients with congenital, idiopathic, and cyclical neutropenia and recurrent infections has also been confirmed in a multicenter randomized study (7,20). These studies detail clinical use of this drug in patients with severe chronic neutropenia.

In earlier clinical trials, cancer patients treated with G-CSF showed a 100-fold increase in the number of colonyforming progenitor cells in the peripheral blood (27). Since then, studies have been performed to investigate whether G-CSF-mobilized peripheral blood progenitor cells (PBPCs) could reconstitute hematopoiesis (22), or whether G-CSF could mobilize granulocytes in normal donors. Sheridan et al. (98) demonstrated that platelet recovery was accelerated in G-CSF-mobilized PBPC-treated patients with poorprognosis nonmyeloid malignancies. Moreover, Bensinger et al. (8) showed safety and feasibility of PBPCs collected after G-CSF treatment. Subsequent studies have since employed PBPCs instead of bone marrow stem cells in allogeneic stem cell transplantation. These studies marked the beginnings of the clinical use of G-CSF for mobilization and collection of PBPCs for transplantation.

Other Indications/Potential Applications

Recently, several investigators have examined the use of G-CSF to mobilize stem cells from the bone marrow into the peripheral blood for their possible regenerative effect in myocardial infarction [for review see (100)]. A number of clinical trials have been performed to determine the efficacy of G-CSF in cardiac repair. Although these studies reported variable outcomes in terms of effectiveness of G-CSF in modulating functional recovery, they did provide important findings on the mechanisms and roles of signaling pathways in regulating homing and engraftment of bone marrow stem cells to the infarcted heart (100).

As mentioned above, G-CSF not only acts on the hematopoietic system but also on the CNS. Several studies have reported ability of G-CSF to activate several neuroprotective pathways, thereby exerting antiapoptotic effects, inducing mobilization of hematopoietic stem cells, stimulating neuronal differentiation of adult neuronal stem cells, angiogenesis, and anti-inflammation (5,68,105–107), indicating its potential applicability in neurological diseases such as cerebral ischemia and AD. Of note, the efficacy of G-CSF in animal models of stroke in different species has been demonstrated (92,94,95). Administration of optimal doses of G-CSF increased CD34⁺ cells in peripheral blood, which have been shown to decrease infarct volumes in animal models of stroke (51). In some studies, G-CSF reduced glutamate-induced neurotoxicity (38), influenced apoptotic pathways (105,106), attenuated edema formation and interleukin-1β expression (104), and induced the cerebral G-CSF receptor to reduce infarct volume in stroke animals (94,95). Moreover, G-CSF stimulated endogenous neurogenesis and vascularization (48,104). Recently, clinical studies are under way to determine the safety and efficacy of G-CSF for acute ischemic stroke (93,103,107). G-CSF has also been shown to enhance memory and neurobehavioral function in AD animal models (81). Pilot clinical trials reported that administration of G-CSF in a dosage regimen commonly used for bone marrow donors was well tolerated and safe, and it provided positive changes in a hippocampal-dependent task of cognitive performance in AD patients (87).

The capacity of G-CSF to induce repair and regeneration of neuronal tissues, including the spinal cord, has also been suggested. For example, G-CSF promoted neuronal survival, oligodendrocyte protection, and functional recovery in animal models of spinal cord injury (SCI) (47,76). In phase I/IIa clinical trials, the safety and feasibility of G-CSF as a neuroprotective therapy in patients with acute SCI were described, indicating the possibility for G-CSF to improve neurological and functional outcomes in patients with acute SCI (110).

Recent studies have also tested the efficacy of G-CSF treatment in animal models of TBI. However, discrepant results were obtained from these studies in that while some investigators reported improvement of TBI-associated behavioral and histological impairments, others found minimal effects of G-CSF on functional and neurological outcomes in TBI animals (e.g., 83,129). Currently, a limited clinical trial is under way for testing G-CSF in TBI patients (personal communication with Dr. Juan Sanchez-Ramos, USF/VA). Despite inconsistent efficacy results, the translation of G-CSF for TBI in the clinic is based on the drug's safety profiles for its indication in acute ischemic stroke and AD (28,87,88,103,104).

Adjunctive Use of G-CSF

The potential uses of G-CSF as an adjunctive agent in the treatment of neutropenia are well known. In this article, we limit description of adjunctive uses of G-CSF in neurological conditions (e.g., stroke), including spinal cord injuries, as well as in myocardial infarctions. Moreover, the studies described are those that were conducted in experimental models. An exhaustive review on the synergistic effects of adjunctive therapy of G-CSF with other agents in various neurological disorders and other medical conditions will be pursued in the future.

As different mobilizing agents may release bone marrow stem cells with different phenotypic characteristics and biological behaviors, G-CSF was also used in conjunction with other mobilizing agents, for example, stem cell factor (SCF), with the primary goal of enhancing stem cell mobilization [(49), for review see (21)]. Consistent with the known synergism with colony-stimulating factors, administration of G-CSF and SCF highly increased by 250-fold the number of circulating pluripotent hematopoietic stem cells (10). Indeed, some studies found that such combination therapy reduced mortality, infarct size, and improved left ventricular function, accompanied by increased homing of bone marrow cells to the infarcted myocardium and formation of new cardiomyocytes and ventricular structures (21). In chronic stroke studies, G-CSF + SCF induced more stable and long-lasting functional improvement compared with treatment with SCF or G-CSF alone, attributable to increased angiogenesis and neurogenesis through bone marrow-derived cells and the direct effects on stimulating neurons to form new neuronal networks (126).

The therapeutic effects of G-CSF and coadministration with the cytokine fms-like tyrosine kinase 3 (Flt3) ligand in a model of acute myocardial infarction (AMI) in rats (20,88), as well as in rat models of SCI were described (118). Both studies reported optimal results when animals were subjected to combined rather than single treatment of these drugs (118). The observed improvement in behavioral and morphologic parameters and tissue regeneration in animals treated with a combination of both factors has been associated with a prolonged time course of mobilization of bone marrow cells (91,118).

Moreover, G-CSF combined with stem cell (e.g., bone marrow mononuclear cell) transplantation produced synergistic beneficial effects in an experimental mouse model of cerebral ischemia (132). These effects were hypothesized to involve, at least partially, enhancement of proliferation and differentiation of bone marrow stem cells resulting in improved host brain regeneration and functional recovery (132). Recently, effects of G-CSF treatment alone or in combination with bone marrow mesenchymal stem cells (BM-MSCs) after stroke were evaluated in aged rats (5). Although the combination therapy produced remarkable angiogenesis in the formerly infarct core and beyond in the “islet of regeneration,” it did not afford better neuroprotection and recovery poststroke compared with G-CSF treatment alone (5). That effects of G-CSF were more robust than those exerted by combination treatment with BM-MSCs is consistent with the previous observation on survival-enhancing capacity and beneficial effects on functional outcomes of G-CSF treatment in aged rats (80). Combined treatment of bone marrow stromal cell transplantation with G-CSF also promoted functional recovery after spinal cord transection (60). In the former study, enhanced efficacy of the combination therapy has been partially attributed to increased proliferation and differentiation of bone marrow stem cells (BMSCs) and enhanced host brain regeneration and functional recovery (132). Synergistic effects between BMSCs and G-CSF may have also been due to extrinsic and endogenous neurogenesis in the traverse SCI (132).

Combining G-CSF with other agents has also been pursued, for example, hematopoietic growth factors (erythropoietin) (21), pharmacotherapies (e.g., simvastatin) (37), amniotic membrane wrappings (31), and others in experimental models of stroke, SCI, and AMI. A majority of these studies reported enhanced effects of combination therapy, or synergism, when compared with treatment with either drug alone. In conclusion, the above-mentioned studies support improved therapeutic benefits when G-CSF was combined with other drugs or agents for treatment of various medical conditions.

Combination Therapy of G-Csf and Transplantation of Umbilical Cord Blood Cells for Tbi

In view of the successful outcomes when G-CSF was used as an adjunctive treatment for other disorders and the discordant findings on efficacy of G-CSF in TBI animal models, we tested the putative benefits of combination therapy in TBI by investigating behavioral and histological improvement after transplantation of hUCB and coadministration of G-CSF, in a controlled cortical impact model of moderate TBI in adult rats (2). In a recent study, we showed that a combination therapy with hUCB + G-CSF produced functional improvement to a greater extent than that exerted by monotherapy with hUCB or G-CSF (2). Notably, beneficial effects of combination therapy were more longer lasting than those exerted by hUCB transplantation or G-CSF administration alone. These results indicate that complementary brain repair processes distinctly or mutually afforded by these two therapies could have mediated improved functional recovery exerted by combination treatment of hUCB + G-CSF. Regenerative mechanisms such as those accomplished by G-CSF-mobilized endogenous stem cells, the growth factors secreted by hUCB grafts, as well as the potential graft–host integration that facilitates reconstruction of synaptic circuitry (125), may have altogether caused more profound functional improvement in TBI rats subjected to a combination therapy than in rats that underwent hUCB or G-CSF monotherapy (2,23).

To determine the extent to which combination therapy of hUCB + G-CSF ameliorated TBI-induced neuroinflammation in gray and white matter areas, we conducted immunohistochemical staining with OX-6, which labels MHCII⁺ cells (putatively activated microglia) (2). We found that concomitant with improvement in behavioral functions, combination therapy of hUCB + G-CSF resulted in more profound reduction of TBI-induced upregulation of MHCII⁺ cells in the cortex, striatum, thalamus, subventricular zone (SVZ), and the dentate gyrus (DG) of the hippocampus, compared with hUCB and/or G-CSF monotherapy. Furthermore, we also observed that combined therapy of hUCB + G-CSF decreased the number of MHCII⁺ cells not only in the corpus callosum and fornix, but also in the cerebral peduncle. In animal models of TBI as well as in preclinical stroke and aging studies, hUCB treatment has been shown to decrease inflammation and facilitate neurogenesis and angiogenesis (46,96,134). Moreover, treatment with G-CSF modulated neurogenesis in TBI and in other neurodegenerative disorders (e.g., hypoxic injury, AD, etc.) (88). We investigated whether combination therapy exerted synergistic effects in attenuating TBI-induced impairment in endogenous neurogenesis and hippocampal cell loss, and found reduction in neuroinflammation, which coincided with elevated neurogenesis in DG and SVZ while increasing the survival of CA3 neurons in TBI rats. Together, the above results show that combined therapy of hUCB + G-CSF synergistically dampened TBI-induced neuroinflammation and also enhanced endogenous neurogenesis and reduced hippocampal cell loss (2,23).

The widespread effects of hUCB + G-CSF combination therapy in diverse brain regions can be explained by the complementary interactions between hUCB and G-CSF. Previous studies found that G-CSF-mobilized stem cells infiltrate injured tissues to promote neural repair (28,88,104). As mentioned above, G-CSF displayed capacity to cross the BBB to act upon neurons and glial cells through the G-CSF receptor (134), of which their activation has been demonstrated to downregulate expression of proinflammatory cytokines and to enhance neurogenesis (95,115). The addition of G-CSF to hUCB may promote stemness maintenance and, under appropriate conditions (such as in combination with SCF), guide neural lineage commitment of hUCB (108,116). In a previous study, combined treatment of G-CSF and SCF promoted cell cycle exit and coaxed hematopoietic stem cells toward neural lineage differentiation possibly via enhancement of neurogenin 1 activity (77). Furthermore, mobilized bone marrow cells and hUCB cells may also exert their therapeutic benefits via a paracrine mechanism, that is, transplanted cells secrete trophic factors, growth factors, chemokines, and immune-modulating cytokines to the injured milieu, in line with the concept of “bystander effects” of transplanted stem cells (11,12,129,130). In summary, a receptor-mediated transport mechanism coupled with paracrine effects of transplanted cells may underlie extensive influence of combination therapy of hUCB + G-CSF in diverse brain regions as evidenced by enhanced anti-inflammatory, improved neurogenesis, and increased cell survival in TBI rats, which were given hUCB + G-CSF.

Conclusion and Future Perspective

TBI has been recently described as a progressive cell death process rather than an acute event, characterized by a worsening histopathology. Therapeutic opportunities for TBI remain very limited with patients subjected to rehabilitation therapy and a few other experimental treatments. Based on the efficacy of stem cell-based interventions in other neurological disorders, they could be used as alternative treatment options for TBI. However, variable functional outcomes of stem cell therapy suggest the need for adjunctive pharmacological treatments to enhance their therapeutic benefits. Our recent studies on the effects of combination therapy of hUCB + G-CSF in TBI models support the potential usefulness of this approach and demonstrate how stand-alone therapies (i.e., hUCB and G-SCF) could overcome their therapeutic limitations when synergy is accomplished through combination therapy. Further studies are required to demonstrate the safety and efficacy of this approach in the clinic and, ultimately, to determine the mechanism(s) of action.