Inflammatory responses after intracerebral hemorrhage: From cellular function to therapeutic targets

Abstract

Inflammatory responses occur rapidly after intracerebral hemorrhage and participate in both short-term toxicity and long-term recovery. Microglia/macrophages react to hemorrhagic injury and exhibit dynamic phenotypes and phagocytic capability. Astrocytes secrete cytokines, chemokines, and gliotransmitters that can regulate neuroinflammation. In addition, infiltrating neutrophils and T-lymphocytes modulate immunoreactions, which further cross-talk with microglia/macrophages. Thus, the search for effective immunotherapy to target specific cell type-mediated inflammation might represent a new direction for intracerebral hemorrhage treatment, separate from traditional anti-inflammatory drug discovery.

Intracerebral hemorrhage (ICH) is a devastating disease in which brain injury results from neuroinflammation, oxidative stress, and cell death. Recently, to explore ICH therapies, scientists have investigated a variety of brain cells with a focus on their specific functions, molecular mechanisms, and effects on neuroinflammation.

Microglia/macrophages

Microglia respond rapidly to ICH damage and secrete immunomodulators, including cytokines, chemokines, and ferrous iron. Microglia and infiltrating macrophages (M/Mφ) engulf red blood cells within the hematoma. M2-like M/Mφs are beneficial after ICH because of their phagocytic ability and anti-inflammatory function. ¹ For example, brain-permeable iron chelator VK28 accelerates iron clearance by enhancing M2-like M/Mφs. ² Additionally, molecules and compounds such as lipocalin-2, TGF-β1, and the natural compound pinocembrin have been shown to modulate M/Mφ phenotype and function. These studies imply that promoting an M/Mφ shift to an M2-like phenotype improves outcomes after ICH. ¹

CD163, a hemoglobin scavenger receptor, is recognized as an M2-like M/Mφ marker. Its expression was elevated from days 1 to 7 after ICH. ³ Interestingly, deferoxamine was able to suppress CD163 expression and improve ICH outcomes. ³ In another ICH study, CD163 depletion showed early beneficial properties but delayed injurious effects. ⁴ Although it is unclear why CD163 deletion has a biphasic influence on ICH outcome, the late deleterious effects are consistent with an anti-inflammatory role of CD163 during the recovery stage. ⁴ Whether and how CD163 mediates M/Mφ phagocytosis, neuroinflammation, and hematoma clearance needs further study.

Nucleotide oligomerization domain-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome is expressed in microglia after a variety of brain injuries. One ICH study ⁵ showed that NLRP3 inflammasome enhanced neuroinflammation by releasing interleukin (IL)-1β and recruiting myeloid cell infiltration, whereas inhibition of NLRP3 decreased inflammation and improved outcomes. These results imply critical roles for NLRP3 inflammasome in microglia-mediated inflammatory response.

After ICH, monocyte-derived macrophages (MDMs) infiltrate into the brain. Chang et al. ⁶ demonstrated that enhancing phagocytic ability of the infiltrating MDMs promotes hematoma resolution, which is associated with increases in an MDM M2-like phenotype switch. Despite recent progress, our knowledge about the different effects of microglia and MDMs on phagocytosis, neuroinflammation, and hematoma clearance after ICH remains limited.

Astrocytes

Astrocytes, which maintain brain hemostasis in response to physical stimuli, are the most abundant glia cells in the brain. After ICH, astrocytes are involved in synthesis of intermediate filaments. This process, known as astrogliosis, limits expansion of the injury area. However, astrocytes are also vital for regulating inflammatory response after ICH because they release gliotransmitters, cytokines, and lipid molecules and engage in cross-talk with microglia through chemokines. ¹

Heme oxygenase-1 (HO-1) converts heme to carbon monoxide, ferrous iron, and biliverdin. Microglial HO-1 mediates a proinflammatory process that produces toxic levels of heme metabolites early after ICH. However, in transgenic mice that overexpress GFAP-promoter-driven human HO-1, astrocyte HO-1 preserved neuronal viability and blood–brain barrier integrity and produced anti-inflammatory effects after ICH. ⁷ Thus, the cell-specific function of HO-1, along with brain iron deposition, needs to be considered when developing therapies to target HO-1.

Inflammation is associated with upregulation of hepcidin, the central regulator of systemic iron homeostasis. By binding to ferroportin, hepcidin inhibits cellular iron efflux. Decreased hepcidin leads to tissue iron overload. Nonetheless, whether hepcidin–ferroportin interaction determines the direction of iron flow in the brain must be established. Xiong et al. ⁸ demonstrated that TLR4/MyD88 signaling leads to an increase in astrocyte-derived hepcidin, which aggravates iron-induced brain injury by preventing brain iron efflux into circulation. Astrocyte and M/Mφ interaction might represent a new research direction in the ICH field.

Other blood cell types

Neutrophils

Neutrophils are the first leukocytes that infiltrate into the ICH brain and contribute to the neuroinflammatory response. Recently, Zhao et al. ⁹ showed that IL-27 accelerated hematoma clearance and improved outcomes after ICH. These results were explained by the ability of microglia-derived IL-27 to promote neutrophil maturation in the bone marrow after ICH and to encourage polymorphonuclear neutrophil polarization to a less neurotoxic phenotype. Interestingly, a recent clinical study reported a strong inverse association between neutrophil counts and hematoma expansion. Therefore, targeting neutrophil function and differentiation might limit ICH injury.

T-lymphocytes

Lymphocytes, T-lymphocytes in particular, play a crucial role in sustained inflammation after ICH. A lower lymphocyte count and higher neutrophil-to-lymphocyte ratio is an independent indicator of poor outcome in ICH patients. Importantly, regulatory T-lymphocytes (Tregs) are recognized to exert anti-inflammatory effects after stroke injury. Specifically, inhibiting the Akt/mTOR signaling pathway with programmed death ligand 1 increases the percentages of brain-infiltrating Tregs and has yielded beneficial effects after ICH. Using Fox3^DTR mice, Zhou et al. ¹⁰ found that Tregs have a protective function after ICH. Moreover, Tregs were able to promote M/Mφ transition to an M2-like phenotype through the IL-10/GSK3β/PTEN axis. With strong immunoregulatory properties, Tregs and Treg-M/Mφ interactions might represent a promising line of ICH investigation.

Conclusion

For decades, researchers have attempted to develop anti-inflammatory agents for ICH treatment. Unfortunately, effective clinical therapies are still lacking. Recent studies have elucidated the underlying mechanisms of different cell types that contribute to neuroinflammation after ICH, including M/Mφs, astrocytes, neutrophils, and T cells. Additionally, studies of cell–cell interactions have identified key cerebroprotective immunomodulators and/or signaling pathways that can regulate inflammatory responses after ICH. Traditionally, neuroinflammation research has focused on global (non-cell-type-specific) inflammatory response, specifically, pro- and/or anti-inflammatory cytokines in brain tissue composed of mixed cell types and immune cell infiltration. However, translational studies that target “global inflammation” after ICH have met with poor success. Therefore, investigating cell-type specific functions, such as microglial phagocytosis, cell-type-specific inflammatory response, and cell-cell interactions, might be better able to identify new potential therapeutic targets for treating ICH.