Immunology of West Nile Virus Infection and the Role of Alpha-Synuclein as a Viral Restriction Factor

Abstract

West Nile virus (WNV) is a single-stranded RNA flavivirus and is a major cause of viral encephalitis worldwide. Experimental models of WNV infection in mice are commonly used to define acute neuroinflammatory responses in the brain. Alpha-synuclein (Asyn) is a protein of primarily neuronal origin and is a major cause of Parkinson's disease (PD), a disorder characterized by loss of dopaminergic neurons. Both WNV and PD pathologies are largely mediated by inflammation of the central nervous system (neuroinflammation) and have overlapping inflammatory pathways. In this review, we highlight the roles of the immune system in both diseases while comparing and contrasting both protective and pathogenic roles of immune cells and their effector proteins. Additionally, we review the current literature showing that Asyn is an important mediator of the immune response with diverging roles in PD (pathogenic) and WNV disease (neuroprotective).

West Nile Virus

West Nile virus (WNV) is a positive sense, single-stranded RNA virus of the Flaviviridae family and is the principal cause of viral encephalitis in North America. WNV is sustained in an enzootic cycle in birds and mosquitoes, with humans being incidental hosts. The virus was originally isolated in Uganda in 1937 and is endemic in several regions throughout the world, including the United States (33,90,124). WNV is most commonly spread to humans through the bite of an infected mosquito, but infections through blood transfusions, organ transplantation, intrauterine transmission, and breastfeeding have occurred (56).

WNV infections in humans vary in severity: around ∼80% of those infected are asymptomatic, whereas others develop fever and systemic illness. Another subset of these patients develop potentially lethal encephalitis or meningitis as the virus spreads to the central nervous system (CNS)—referred to as neuroinvasion. Symptoms of WNV encephalitis can include bradykinesia, cogwheel rigidity, acute flaccid paralysis, ataxia, and wide-based gait. Long-term movement disorders, cognitive disorders, and other long-term disabilities can occur after WNV infection (113,114). Individuals who are elderly or immunocompromised are at greater risk of developing severe WNV disease, and genetic variations found in chemokine or interferon-related genes also confer greater risk (12,47,86). There are currently no vaccines available against WNV, and only supportive care can be provided.

Early immune responses to WNV

Following a mosquito bite, WNV initially enters the keratinocytes and Langerhans cells (LCs) of the skin (75). In mouse models, WNV infection results in migration of the LCs from the epidermis into the draining lymph nodes, a process mediated by interleukin 1-beta (IL-1β) and Toll-like receptor 7 (TLR7) (22,147). Further amplification of the virus then occurs in the lymph nodes and spleen (131).

Early immune responses to WNV are mediated by pattern recognition receptors, including retinoic acid-inducible gene-I (RIG-I)-like receptors in the cytoplasm and TLRs. Upon recognition of viral RNA, RIG-I receptors drive downstream interferon responses, particularly IFNβ and interferon regulatory factor 3 (IRF3) target genes (77,104). Recognition of viral RNA in endosomes by TLR3 and TLR7 triggers the activation of NFκB-, IRF3-, and IRF7-dependent gene expression (62,104). Type I interferon responses are critical for controlling WNV infection, as mice lacking the IFNα/β receptor succumb more readily to WNV infection and have increased viral burden in the CNS and peripheral tissues (109).

Mechanisms of neuroinvasion and neuronal injury

The mechanisms by which WNV gains access to the CNS are not completely understood. WNV may cross the blood–brain barrier (BBB) through a few different mechanisms: transmigration of the virus through the BBB (137), transneuronal spread from peripheral nerves into the CNS (59,85,111), or through infection of immune cells in the periphery, which then act as Trojan horses to deliver virus to the brain (139). Proinflammatory cytokines produced during peripheral WNV infection may alter the permeability of the BBB, allowing the virus to cross. Tumor necrosis factor alpha (TNFα) is well known to regulate the BBB permeability (34,43,80). Signaling through TNFα receptor 1, downstream of TLR3, promotes the WNV entry into the brain by increasing the “leakiness” of the BBB (142). Additionally, other proinflammatory cytokines, such as IL-1β and macrophage migration inhibitory factor, can increase the BBB permeability, whereas others, including type I (IFNα and IFNβ) and type III (IFNλ) interferons, appear to stabilize the BBB and decrease virus entry into the brain (5,32,70). Both peripheral immune cells and brain-resident microglia, as well as neuronal cells, contribute to the production of these cytokines (148). Once inside the brain, WNV can infect and replicate inside several resident cell types, including microglia, astrocytes, and neurons (103,120).

WNV encephalitis results from infection and injury of neurons, which can be induced directly by the virus or by responding immune cells (26). Several studies have shown that apoptosis is induced in WNV-infected neuronal cells (29,36). Mice deficient in caspase 3, a key mediator of apoptosis, show decreased neuronal cell death after intracranial WNV infection and are more resistant to lethal infection (110). Death receptor-mediated apoptosis pathways play a role in this process as well, as genes associated with these pathways are upregulated in the brains of WNV-infected mice compared, and the caspase 8 activity is increased (28). These results suggest that apoptosis is a major contributor to WNV-induced neuronal damage.

Immune Responses in the Brain Contribute to Viral Clearance and WNV Pathology

Early immune cell entry into the brain

Studies of WNV pathogenesis in murine models of disease have provided extensive data on immune response pathways in the brain. Entry of WNV into the brain results in activation of and increased numbers of microglia, and infiltration of immune cells from the periphery. Microglia are brain-resident innate immune cells that function as macrophages and can mediate either proinflammatory or pro-regenerative responses (67). Various signals can activate microglia, including cytokines and microbial products, and increasing evidence suggests that activated microglia can alter the BBB permeability (30). While proliferation of microglia has been shown in various inflammatory models (31,78), studies using WNV and other CNS-invading pathogens suggest that the increase in microglia numbers during infection is mostly due to infiltration of inflammatory monocytes, which are microglial precursors (37,46).

Infiltration of immune cells into the brain is critical to control WNV infection. In addition to the impacts on the BBB by cytokines described in the previous section, numerous other cytokines or chemokines may facilitate immune cell entry into the brain (74). Humans with a genetic deficiency impacting the function of the chemokine receptor CCR5 have a greater risk of WNV infection (47). Likewise, studies in mice have shown that CCR5 helps to control viremia and leukocyte infiltration into the cortex of infected mice, and it has been shown to help recruit regulatory T cells (Tregs) into the CNS during infection with Japanese encephalitis virus (JEV), another flavivirus related to WNV (39,63). Similarly, the chemokines CCL2 and CCL7 assist in recruiting immune cells into the CNS, but only CCL7 impacts viral clearance and survival of WNV infection (8). In contrast, chemokine receptor CCR7 is required for immune cell infiltration into the lymph nodes but seems to play a role in restricting their entry into the brain. CCR7-deficient mice, despite having more immune cells in the brain, have higher WNV viremia, increased mortality, and increased production of IFNγ and TNFα (7). This suggests that CCR7 restricts lethal WNV infection, likely by decreasing the pathological effects of the immune cells and cytokine production.

While some cytokines and chemokines are beneficial in restricting infection, others play more pathogenic roles. Mice deficient in osteopontin display greater survival after WNV infection compared with their wild-type counterparts and have less infiltration of polymononuclear neutrophils in their brains, suggesting that osteopontin may recruit immune cells to the brain in a deleterious manner (99). Similarly, blockade of chemokine receptors CCR2 and CXCR3 in infected mice lowers immune cell recruitment to the brain, and when blocked simultaneously, infected mice display enhanced survival (83). Further research is needed to examine the protective versus pathogenic functions of chemokines and their receptors, to understand how the balance is tipped from protective to pathogenic during infection, and to determine what role chemokines and receptors may play in determining susceptibility to severe WNV infection in humans.

Presentation of viral antigen by antigen-presenting cells expressing major histocompatibility complex (MHC) class I or II molecules to T lymphocytes is a critical step in mounting a successful immune response. While many studies have demonstrated that immune cells infiltrate the brain during WNV infection, it is unclear what degree of antigen processing occurs in the brain or in the periphery before cells infiltrate the CNS. Several studies have demonstrated that MHC molecules are expressed on CNS-resident cells: MHC-I expression has been shown in microglia, substantia nigra dopaminergic and locus coeruleus norepinephrinergic neurons, and hippocampal endothelium; MHC-II expression has been shown in microglia; and WNV infection results in the upregulation of MHC class I and II on infected astrocytes in vitro (25,76,81,135). These results suggest that antigen presentation in the brain is certainly possible; however, direct presentation of WNV antigens in the brain has not been shown and further studies are needed.

Natural killer cells (NK cells) are another important cellular mediator of the early immune response against viral infections and serve as a link between innate and adaptive immunity. NK cells can directly eliminate cells infected with virus, a process mediated by perforin and granzymes, or indirectly through cytokine secretion (95). Studies have shown that NK cells are first activated in the spleen and are then able to cross into the CNS (16,136,143). However, the role of NK cells in limiting WNV infection is controversial. Some studies indicate that NK cells can effectively clear WNV, whereas others have found that NK cells do not contribute to WNV clearance or survival from disease (66,130,153). A more recent study has shown that peripheral blood mononuclear cells (PBMCs) from humans with or without a history of WNV infection infected in vitro with WNV mount a robust NK cell response, and these NK cells have cytotoxic potential and are able to secrete cytokines (151). The discrepancies seen in these studies may be explained by differing organ-specific functions of NK cells, a role of NK cells other than direct cytotoxic killing, or evasion of NK cell responses by the virus itself (116,143). Future studies should compare NK cell functions in restricting WNV viremia in different organs, functions of different NK cell subsets during infection, and the ability of different virus strains to evade NK cell responses.

Adaptive immune responses in the brain: friend or foe?

Cells of the adaptive immune system, including T and B lymphocytes, are crucial for controlling viral infections. T and B cells infiltrate the brain after WNV infection and can persist there for several weeks (127). Several studies show that CD4⁺ and CD8⁺ T cells play an important role in controlling WNV infection (92). Rag-deficient mice (Rag1 ^−/−), which lack T cells, are highly susceptible to severe WNV disease and death; and this phenotype can be mostly reversed by adoptive transfer of CD8⁺ T cells (18,41). Likewise, WNV-infected mice deficient in the CD8 α-chain or MHC class Ia exhibit greater mortality, higher WNV titer in the brain, and have a greater persistence of the virus in the CNS (119). CD8⁺ T cells kill virally infected cells through several different mechanisms: (a) exocytosis of secretory granules containing perforin and granzymes, (b) activation of death receptor pathways through TRAIL or FasL, and (c) indirectly through secretion of cytokines such as TNFα or IFNγ (9,65). All these pathways seem to play a role in controlling WNV infection, as mice deficient in perforin, TRAIL, or FasL display increased mortality after infection and greater viral burden and persistence in the CNS (17,118,121,145). Interestingly, the difference in viral burden in these studies was limited to the CNS and was not seen in the periphery, suggesting that CD8⁺ T cells are particularly important in controlling infection in the CNS.

Like CD8⁺ T cells, CD4⁺ T cells appear to regulate the immune response to WNV in the CNS without affecting the kinetics of viral replication in the periphery. In mice with a genetic deficiency of CD4⁺ T cells or CD4⁺ T cells depleted by antibody, serum IgG levels at 10 and 15 days postinfection were lower and CD8⁺ T cell responses were inhibited compared with their wild-type counterparts, suggesting that a major function of CD4⁺ cells during WNV infection is to provide help for antibody-mediated responses and CD8⁺ T cell responses (123). However, CD4⁺ T cells can limit WNV infection directly as well: when CD4⁺ T cells were transferred into Rag1^-/- mice, the survival of lethally infected mice improved, despite the mice lacking CD8⁺ T cells and B cells. The WNV-specific CD4⁺ T cells in these mice also produced IFNγ, IL-2, and granzyme B and were capable of cytotoxicity, suggesting that CD4⁺ T cells themselves can mount a robust immune response (19).

While T cells seem to enable the clearance of WNV, several studies also indicate that they play a damaging pathogenic role. CD8-deficient C57BL/6J mice infected with 10³ plaque-forming units (PFU) of the Sarafend strain of WNV showed increased mortality compared with wild-type mice, but when these same mice were given a higher dose of 10⁸ PFU, they exhibited increased survival (144). Another study using mice deficient in Ifit1, an innate immune effector protein, found that antibody-mediated depletion of CD8⁺ T cells resulted in greater survival times. These data indicate that T cells can have pathogenic roles and contribute to mortality, at least under certain conditions. It is possible that these differences are partially due to virus dose, virus strain, and varying inoculation routes. Particularly, it is possible that a high viral burden in the CNS may tilt the balance from a protective CD8⁺ T cell response to a more pathogenic response, as a result of overly sustained T cell responses.

The pathogenicity of WNV may also be limited by T regulatory cells (Tregs), which dampen responses from CD4 and CD8 T cells. Studies have shown an increased frequency of CD4⁺ Tregs in the blood of patients with WNV, and symptomatic individuals generally had fewer Tregs than asymptomatic individuals (68,69). Similar results were seen in a mouse model, in which lack of Treg expansion after WNV infection resulted in uncontrolled inflammation (132). Tregs may therefore limit severe WNV disease by preventing overactivation or prolonged activation of the immune response.

γδ T cells comprise a small proportion of T cells in blood and lymphoid tissue but are much more prominent in mucosal sites and epithelium. In contrast to αβ T cells, γδ T cells lack MHC restriction and do not require conventional antigen processing. γδ T cells can rapidly secrete cytokines such as IFNγ and TNFα and may have direct cytotoxic potential similar to that of αβ CD8⁺ T cells and NK cells (143). Mice lacking the T cell receptors (TCR) δ chain are more susceptible to WNV infection and severe encephalitis, suggesting that γδ T cells are important for controlling WNV infection (141). However, different subsets of γδ T cells may have distinct pathogenic or protective functions. One study found that IFNγ-producing Vγ1+ γδ T cells were more protective, as mice depleted of these cells had higher viremia and mortality during WNV infection, whereas depletion of TNFα-producing Vγ4⁺ cells resulted in a decreased viral load in the brain and lower mortality (146). Therefore, the overall effect of γδ T cells on WNV infection may depend on which subsets are predominant in each individual, and furthermore, the proportion of these γδ cell subsets present may differ between older and younger individuals (140).

Alpha-synuclein

Alpha-synuclein (Asyn) is a 14 kDa protein encoded by the gene SNCA1 that is involved in the pathogenesis of Parkinson's disease (PD) and other neurodegenerative disorders known collectively as synucleinopathies. PD is classically characterized by a progressive loss of dopaminergic neurons and the presence of Lewy bodies composed of misfolded Asyn aggregates (125). Missense mutations or multiplications of the Asyn gene are causative of PD, and aberrations at the Asyn locus are associated with susceptibility to sporadic PD (27,101,108,122,152). While the exact functions of Asyn are unknown, it is present on synaptic vesicles and is thought to be involved in neurotransmission and vesicle transport (21).

The factors that result in the aggregation of Asyn are also not completely understood. One prevailing theory is that an environmental trigger, such as a viral infection, results in post-translational modifications and misfolding of Asyn, and consequent Asyn fibrils can spread to the CNS via the peripheral nervous system or the olfactory bulb in a prion-like manner (35,58,73,105,106). The gastrointestinal (GI) tract and exposed olfactory neurons are likely entry sites for such viruses (79). Studies have found correlations between the development of PD symptoms and several viral infections, including influenza, hepatitis C, JEV, St. Louis encephalitis virus, coxsackievirus, and WNV (53,60,79,102,107,134,150). Acute infection with viruses like WNV can also cause transient PD-like symptoms likely due to direct viral injury of dopaminergic neurons. Infection with JEV also induces post-encephalitis Parkinsonian symptoms in rats, and this has been used as an animal model of PD (50,94). However, many of these viruses are currently associated with the development of transient symptoms, and it is less likely that viruses directly cause sporadic PD. Impaired clearance of Asyn aggregates through autophagy or ubiquitin–proteasome pathways can also contribute to PD (40,49,91,98). Although Asyn is mainly expressed in neurons, its expression has been shown in other cell types as well, including erythroid precursors, megakaryocytes, T lymphocytes, B lymphocytes, NK cells, and monocytes (55,64,88,117). Asyn is present in both secreted and membrane-bound forms.

Inflammation in PD

Neuroinflammation is a hallmark of synucleinopathies. Several studies have shown that inflammation plays a critical role in PD pathogenesis and that much of this inflammation is mediated in one way or another by Asyn. Several studies have indicated roles for microglia and T lymphocytes in PD-associated inflammation. Reactive microglia, which upregulate human leukocyte antigen-DR (HLA-DR), are found in the substantia nigra of PD patients (81). Neuronal damage in PD is also accompanied by changes in microglial function, resulting in the secretion of reactive oxygen species (ROS) and proinflammatory cytokines such as TNFα, IL-6, and IL-1β (4,71,133,149,155). Dopaminergic neurons are particularly sensitive to the effects of TNFα and IFNγ, and lipopolysaccharide-induced inflammation in rats results in decreases in dopamine levels and neuron numbers in the substantia nigra (13,24,61,82,87). TNFα has been detected in high levels in the cerebral spinal fluid (CSF) and postmortem brains of PD patients and its secretion can further potentiate the activation of microglia and the production of ROS (15,133). Infiltrating CD4⁺ and CD8⁺ T lymphocytes are also present in the brains from a PD mouse model (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced) and in postmortem brain samples from PD patients. Additionally, genome-wide association studies have also linked single-nucleotide polymorphisms in the HLA-DR gene to PD (1,51,89). However, it is unclear whether the inflammation triggers PD or is instead a consequence of it. Additionally, peripheral immune cell frequencies are altered in PD patients. Studies have found reduced numbers of CD4⁺ T cells and CD19⁺ B cells and increased numbers of NK cells in PD patients compared with healthy controls (23,93,126). Again, it is unclear whether alterations in peripheral cells promote PD pathogenesis or result from it, and further studies are needed.

Emerging roles for Asyn in immunity and inflammation

Although Asyn is predominately expressed in the nervous system, recent studies have shown that it plays an active role in the immune system as well. Secreted Asyn can activate microglia to varying degrees, which results in the production of the proinflammatory cytokines TNFα, IL-6, IL-1β, and IFNγ, as well as the upregulation of MHC-II and MHC-I (25,38,154). Correspondingly, these cytokines are present in the CSF and postmortem brains of PD patients (14,84). Similar to CNS viral infections, various chemokines and cytokines promote immune cell infiltration into the brain throughout the course of PD. In a mouse model of Asyn overexpression, CCR2⁺ monocytes entered the CNS from the periphery, and deletion of CCR2 was neuroprotective (52). Recent studies indicate that Asyn itself has chemotactic properties. One study showed that Asyn is expressed in GI neurons in children with mucosal inflammation and that both monomeric and oligomeric Asyn are chemotactic for CD11b+ neutrophils and monocytes (128). Additionally, Asyn monomers and oligomers promote dendritic cell maturation, as evidenced by the upregulation of CD80, CD83, CD86, and HLA-DR (128). Another study found that Asyn aggregates acted as chemoattractants to direct microglia toward damaged neurons, another process dependent on CD11b (138).

CD4 and CD8 T cells also enter the brain during PD in both humans and mouse models, and cell death is attenuated in the absence of T cells (20). In addition to activating factors from monocytes or dendritic cells, T cells may be activated directly by Asyn. Two antigenic peptides derived from Asyn have been found to stimulate CD4⁺ T cells, which mainly produced IFNγ or IL-4 (129). Asyn may also play a role in early T cell development: in one study of Asyn-knockout (KO) mice, the T lymphocytes showed a significant defect in development, resulting in a lower proportion of peripheral T cells, hyperactivated T cells that were deficient in Th2 polarization, and increased proportions of CD4/CD8 double-negative T cells in the thymus (115). However, the exact mechanisms by which Asyn may impact lymphocyte development have not yet been elucidated. It is possible that Asyn has a synaptic function in lymphocytes similar to its function in neurons, in which case the SNARE interactions needed for TCR to localize to the immunological synapse could be impacted. It may also impact the affinity of MHC-TCR interactions, which would affect the downstream differentiation of T lymphocytes.

Contrary to its role in PD, multiple studies have indicated that native Asyn has protective roles in other CNS diseases. In the murine model of experimental autoimmune encephalitis (EAE), a model which is used to study multiple sclerosis, Asyn seems to be neuroprotective: disease onset occurs earlier and pathogenic T helper 1 cells are increased in Asyn-KO mice during EAE (42). More studies are needed to determine how Asyn can be proinflammatory in one setting while being anti-inflammatory in another. It is possible that post-translational modifications or oligomerization of Asyn may produce different responses or the differences may simply correlate with Asyn expression levels in the CNS.

Asyn is subject to several post-translational modifications, and the majority of Asyn present in Lewy bodies is post-translationally modified (11,96). Around 90% of Lewy body Asyn is phosphorylated at serine 87 (S87) and serine 129 (S129) compared with less than 5% of Asyn being phosphorylated at S129 in healthy brains (3,44,97). Whether phosphorylated Asyn contributes significantly to inflammation in PD is not completely understood, and the studies conducted so far have produced conflicting results (6,72,112). Nitrated forms of Asyn are reported to more readily form aggregates and fibrils (57,100). It is possible that microglia-derived nitric oxide and ROS may nitrate or oxidize Asyn, contributing to PD pathology (45). Ubiquitinated forms of Asyn exist as well and are present in Lewy bodies (48,54). Further studies are needed to understand how post-translational modifications of Asyn contribute to inflammation associated with PD.

Asyn as a novel inhibitor of viral infection

The aforementioned studies show that Asyn has multiple immunomodulatory functions, which can promote disease pathogenicity but can also offer protection from proinflammatory responses in other scenarios. Multiple studies have shown that Asyn expression is upregulated during viral infections in both humans and mice, including WNV and norovirus infections (10,128). After crossing the BBB, neuroinvasive viruses such as WNV infect neurons and induce apoptotic cell death. However, only select neurons are infected, suggesting that certain neurons exhibit an innate ability to restrict viral infection. Studies in our laboratory were the first to show that Asyn expression inhibits WNV viral growth and consequent neuronal injury (10). Mice with a genetic deletion of Asyn (Asyn KO) show decreased survival during WNV infection and have increased WNV titers in the brain, whereas viral titers in the spleen are consistent with those of wild-type mice (10). Similar results were seen when these experiments were repeated using a peripheral inoculation of Venezuelan equine encephalitis virus TC83, which is an attenuated virus that does not normally cause CNS infection or disease in mice. However, in the absence of Asyn, TC83 caused neuroinvasive disease, further suggesting that Asyn specifically inhibits the neuroinvasive capability of neurotropic viruses. Cortical neurons are normally particularly resistant to infection and WNV-induced cell death, but in Asyn-KO mice, these neurons show increased apoptosis compared with controls. Asyn appears to modulate endoplasmic reticulum stress pathways, which are linked to neuronal cell death in the KO mice (10). Additionally, postmortem human brain samples from WNV-infected patients were examined, and these brains had higher expression of Asyn compared with those of uninfected individuals who died of nonviral causes. These results identify Asyn as a novel inhibitor of viral infection in the CNS that promotes survival from WNV and other neuroinvasive viral disease by preventing neuron damage and death (79). Further studies are needed to define the exact mechanism by which Asyn does this. It is possible that Asyn prevents viral entry by modulating the BBB junctions or it may promote the antiviral immune response through chemotaxis of immune cells into the CNS since Asyn is secreted from neurons.

Conclusion

Several studies in humans and mice have shed light on the numerous immune responses that impact both PD and WNV disease, and many cell types can play pathogenic roles in both diseases (Fig. 1). We have identified Asyn, aberrations of which can be causative of PD, as a novel inhibitor of severe neuroinvasive viral disease. Further studies are necessary to define the mechanisms by which Asyn protects the CNS from damage and which factors tilt the balance between its protective and pathogenic functions.

FIG. 1.

A comparison of protective and pathogenic immune responses in WNV infection and PD and the role of Asyn. During WNV infection (left side), the entry of virus into the brain induces the activation of microglia and infiltration of several types of immune cells, the latter process mediated by chemokines and receptors such as CCL5, CCL7, CCR5, and CCR7. These cells control WNV viremia through the secretion of cytokines, promotion of antibody-mediated responses, and direct killing of infected cells (top left). However, certain immune responses and prolonged immune activation can result in severe inflammation, BBB breakdown, and neuron death (bottom left). Tregs may limit pathogenic damage by dampening responses from other immune cells. Asyn is neuroprotective during WNV infection, but the mechanisms underlying this process are not completely understood. It may modulate the immune response through chemotaxis of protective immune cells or may otherwise dampen or activate appropriate responses. In contrast, PD occurs when Asyn oligomers or fibrils form Lewy bodies, inducing inflammation and nerve damage. The healthy brain (top right), containing no deposits of Asyn, is shown for comparison. Phagocytosis of Asyn may help maintain the healthy brain, and PD disease may be exacerbated when this process does not occur. PD, Parkinson's disease; Asyn, alpha-synuclein; WNV, West Nile virus; BBB, blood–brain barrier; Tregs, regulatory T cells.

Footnotes

Acknowledgment

J.D.B. is supported by VA Merit I01BX003863 and DOD PRMRP PR160117 Medical Investigator Initiated Research Award.

Author Disclosure Statement

No competing financial interests exist.

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