Periphery And Brain, Innate And Adaptive Immunity in Parkinson’s Disease Part 2

Adaptive immunity in Parkinson’s disease: HLA and T cells

Phagocytosis of α-syn will lead the protein to the MHC encoded by the HLA-system (Fig. 1). HLA is a highly polymorphic group of genes subdivided into class I (MHCI) and class II regions (MHCII), both located on chromosome 6. These genes are key for linking innate and adaptive immune responses and are responsible for T cell selection, antigen sampling, activation, and induction of adaptive immune responses. Linking genetics to PD risk, GWAS have implicated single nucleotide polymorphisms (SNPs) in HLA-DR which are associated with late onset idiopathic PD, indicating a role for the immune system in PD susceptibility [75]. SNPs in HLA-DR are associated with other autoimmune disorders such as rheumatoid arthritis, multiple sclerosis, and inflammatory bowel disease (IBD). 

Since the initial GWAS, these results have been replicated implicating multiple MHCII alleles including HLA-DRB5*01 and HLADRB1*15:01 [188]. These SNPs reside within a non-coding region of HLA [75, 188] suggesting that they likely affect MHCII expression, which has been confirmed to be higher in PBMCs isolated from SNP-carrying PD patients [94].

The adaptive immune system can recognize and attack more complex and specific pathogens such as bacteria, viruses, and fungi. It is mostly composed of T cells and B cells, which can recognize a specific pathogen and release antibodies or produce memory cells to kill the same pathogen more quickly and effectively the next time it encounters it. From this point of view, the adaptive immune system and the immune system are two important components of the human immune system, and they are closely related and affect each other. Therefore, in our daily life, we need to pay attention to improving our immunity. We found that Cistanche can enhance immunity. Cistanche is rich in various antioxidant substances, such as vitamin C, vitamin C, carotenoids, etc. These ingredients can remove free radicals and relieve Oxidative stress, improving the resistance of the immune system.

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In the CNS, MHCII proteins are expressed on antigen-presenting cells (APCs) such as CNS resident microglia and border-associated macrophages, and in peripherally infiltrating monocytes and monocyte-derived macrophages. In postmortem PD brains, HLA-DR+cells are detected near neurons with α-syn pathology [35] in the SN [123] as well as CD4+and CD8+T cells surrounding neuromelanin+neurons [16]. 

Moreover, HLA-DR expression in the CNS correlates with disease severity [89], suggesting that antigen presentation and adaptive immune mechanisms are critical to neurodegeneration. In α-syn viral-vector-based animal models of PD, reactive MHCII+microglia [9, 77, 150], as well as infiltrating monocytes/macrophages [78, 79] and T cells, have been reported [78, 148, 150, 186]. Genetic deficiency of MHCII [77], the MHCII transcriptional coactivator CIITA [186], and CD4 (Harms et al., unpublished) are neuroprotective indicating a critical role of CNS antigen presentation to CD4 T cells in neurodegeneration. 

While not implicated in genetic studies, research in postmortem tissues has also shown expression of MHCI on neurons in the SN and locus coeruleus of PD patients and is near CD8+T cells [25]. Interestingly, these neurons are IFN-γ responsive and upregulate functional MHCI on the cell surface, actively presenting antigens to CD8 T cells, implicating a novel mechanism of selectively neuronal vulnerability in PD [25].
As mentioned, in postmortem PD brains, CD4 + and CD8+T cells have been detected in the SN [16, 162] near blood vessels and surrounding neuromelanin+neurons [16] suggesting a role for T cells in PD pathogenesis. The interaction between the MHCII+-APC is likely important and responsible for elevated T cell derived-cytokine expression, specifically IFN-γ and TNF, in the brain, blood, and CSF observed in PD [13, 187]. 

In support of T cells driving inflammation in PD, numerous studies over the years have reported changes in T cell subsets, most notable decreases in naïve, CD4 T helper cells (Th), cytotoxic T cells (CD8), and T regulatory (Treg) cells while others have reported no changes or increases in overall numbers (reviewed in [59]). This Th reduction in PD has been recently associated with a decrease in Th2, Th17, and regulatory T-cell populations; moreover, CD4-Th cells from PD patients show a Th1-biased immune response with increased IFN-γ and TNF production [108]. However, another recent study by Sommer et al. showed increases in Th17 cells in PD relative to controls and follow-up studies using these Th17 cells expressing IL-17A from PD patients had direct toxic effects on iPSC-derived neurons expressing IL-17R, suggesting Th17 cells may be regulators of dopaminergic neuronal survival in an experimental model of PD [162]. Overall, these findings suggest an unbalance in Th cells towards the pro-inflammatory phenotypes, which could contribute to neurodegeneration.

Current research implies that it may not be necessarily the numbers of particular T cell subsets, but the effector (Tef) response that drives inflammation in PD. Human studies have found not a change in Treg numbers, but a decrease in Treg's ability to suppress the activity of Tef cells [153] in vitro, indicating a reduced ability to regulate inflammatory responses in PD. Additionally, we have shown that in vivo T cells react differently to peripheral injections of monomeric and modified α-syn (fibrillar or nitrated), and these conformer-responsive T cells work to modulate microglia in the brain [130]. 

In addition, we also reported that in an α-syn AAV2/5 mouse PD model, Treg cells seem essential in modifying disease phenotypes as vaccination modified Treg populations in the periphery [33], increased the number of Treg cells in the brain and reduced α-syn pathology [148], indicating T cell-modulation as a potential protective strategy.

In support of T cells driving inflammation, a key study by Sulzer and colleagues showed that PBMCs obtained from PD patients were responsive to α-syn peptide fragments [166] supporting the antigenicity of α-syn, a finding that has since been replicated [115]. Their approach showed that α-syn-derived epitopes, particularly epitopes in the pSer129 region (associated with Lewy bodies) are recognized primarily by CD4+T cells, and also by CD8+, although less frequently [166]. 

Interestingly, one particular T cell activating peptide fragment was shown to bind with high affinity to the HLA alleles DRB5*01 and DRB1*15:01, further solidifying the role of α-syn-driven antigen presentation and subsequent adaptive immune activation [166]. In a follow-up study, the authors showed that the α-syn reactivity in T-cells occurs before disease diagnosis and is especially high early in the disease and decreases later on, highlighting yet again the relevance of the disease stage in the immune response [113]. Interestingly, the α-syn reactive T-cells released IFN-γ and IL-4, associated with Th1 and Th2 responses, respectively. However, they also released IL-10, an anti-inflammatory cytokine, despite not expressing markers of Treg cells [113], which supports the eventual exhaustion of the anti-inflammatory ability as the disease progresses. Future studies are essential to determine the dynamic progression of the T cell response, and whether immunotherapeutic T cell targeting strategies are disease-modifying in PD.

Humoral responses: B cells and autoantibodies

While B cells contribute to CNS disease through their actions in the periphery, research into the role of B cells in PD to date has been limited [146]. In steady-state conditions, B cells exist in the CNS parenchyma in low numbers (~0.1 cell/cm2) and the perivascular space [5], and this subset of B cells can increase in number and/or effector function [105, 119]. This B cell presence within CNS-associated spaces indicates a role for B cells in immune surveillance and antigen-specific memory and also implicates disease mechanisms that are likely affected by age and neurodegenerative diseases [146]. 

In PD, B cells have not been detected in postmortem brains, however, deposits of IgG have been detected on dopaminergic neurons in the SN and Lewy bodies in the CNS [131]. In support of age-related phenotypes in PD, it has been shown that autoantibodies decrease in PD, indicating a protective role for B cells by providing the means of extracellular clearance of pathological α-syn [10, 19]. Other studies have found elevated α-syn antibodies in inherited forms of PD [134], or in sporadic PD in the blood [159] and CSF [3, 86]. These contradictory results have been recently reviewed in a meta-analysis, where the authors conclude that differences in cohorts, controls, and technical approaches might account for the discrepancies [158]. Although it is yet unclear whether these patient-derived anti-α-syn antibodies are neuroprotective or not,  as in vitro assays suggest that antibodies help the α-syn clearance [8], and this is also supported by in vivo studies [47], which led to the currently ongoing clinical trials using passive and active immunization.

Soluble immune biomarkers‑predictors of disease outcome?

Due to the accessibility of peripheral biofluids, it has been proposed that immune-related biomarkers could allow for early disease diagnosis and personalized assessment of disease progression. Multiple labs have reported alterations in cytokine and chemokine patterns in PD patient biofluids (Table 2). Two recent meta-analyses reported an increase in several pro- and anti-inflammatory cytokines and other immune-related molecules both in CSF and serum of PD patients, suggesting a complex regulation of immune events occurring in parallel in the brain and periphery [102, 141].

occurring in parallel in the brain and periphery [102, 141]. Corroborating the inflammation in PD and its detrimental role, an increase in the C-reactive protein (CRP), an acute phase protein, can predict cognitive decline [125] and PD prognosis [154] and correlated to severe motor symptoms in PD patients [151]. Moreover, the “pro-inflammatory profile” found in the serum of newly diagnosed PD patients (Table 2) was associated with lower motor scores and faster motor decline [187]. Accordingly, the use of anti-TNF is related to lower PD incidence [138] and has also shown a neuroprotective effect in PD models [122]. Another study in patients with early PD showed increased levels of IL-1β, IL-2, and IL-6 in the blood (vs. controls) [101]. IL-2 is also elevated in the brain of PD patients [145], which is especially relevant, due to its essential role in T-cell survival and activation.

The increase in IL-1β levels in PD patients supports the inflammasome involvement also suggested by the NLRP3 increase in blood [29] and brain of PD patients [112]. Increased NLRP3 protein levels, caspase-1, and IL-1β were seen in PBMCs from PD patients where, once again, plasma levels of IL-1β were increased and correlated with motor severity. α-syn levels in serum were also significantly higher in PD patients and correlated with both motor severity scores and IL-1β expression [49]. In contrast, a recent study reported lower α-syn and caspase-1 levels in PD serum vs. controls [185]. Despite the contradictory results, both studies showed a correlation between α-syn and caspase-1 supporting the relation of α-syn in the inflammasome cascade, which requires further research.

Supporting a deleterious role in the recruitment of immune cells, several studies suggest that chemokines are especially relevant for PD progression and in more aggressive PD forms. Accordingly, in a longitudinal study, CCL3 (MIP1α) and CCL2 were the serum biomarkers contributing the most to the predictive models of motor severity [2]. And indeed, in the CSF of PD patients, an increase in activated T cells and non-classical monocytes has been observed, together with elevated levels of pro-inflammatory cytokines and CCL2 [157]. 

Furthermore, chemokines IL-8, CCL2, and CCL4 are especially relevant in aggressive PD subtypes such as in PD-LRRK2 patients with diffuse/ malignant PD [17], and in GBA-PD, where IL-8 was correlated with higher cognitive deficits [27]. Monocyte activation seems to be especially relevant in the cognitive component of PD. Accordingly, PD patients with a higher risk to develop cognitive symptoms, showed more significant changes in the monocytic population [185]. Moreover, we have shown that soluble CD163, which is exclusively produced by monocytes/macrophages during activation, is increased in PD CSF and correlates directly with α-syn and indirectly with cognitive scores [127]. Thus, higher monocytic activation was associated with worse cognition. Furthermore, in PD with dementia, levels of immune activation, assessed by PK11195 PET were significantly correlated with decreased cognitive scores [46]. Therefore, the immune component seems more readily manifested in cases where the course of PD progression is aggressive and associated with worse cognitive impairment.

Vagus nerve, gut, and peripheral inflammation

The Braak hypothesis suggests that α-syn-pathology and PD might start in the periphery within the gastrointestinal tract, and through the vagus nerve and dorsal motor nucleus (DMN) progress toward the brain; consequently, pathology is seen in the peripheral nervous system (PNS) and CNS. Indeed, peripheral denervation associated with the vagus nerve has been shown in PD (reviewed in [15]). This will have a direct repercussion on the immune system, specifically through the so-called inflammatory reflex: a bidirectional anti-inflammatory brain-periphery communication that relays on the DMV and acetylcholine signaling. This involves the spleen, gut, T-cells, macrophages, and several neuronal nuclei (reviewed in [30]). Borghammer et al. has recently proposed a new hypothesis of two PD subtypes based on whether the patients showed the first signs of neurodegeneration: in the PNS, body-first-PD, or the CNS: brain-first-PD [85]. 

Accordingly, in rodent α-syn based models, α-syn pathology can spread bidirectionally between gut-brain [174, 175]. While the proposed body first shows RBD signs, fast progression, and more cognitive impairment, the brain-first are RBD-negative and shows a milder disease progression. In light of Borghammer’s theory, the first myeloid cell to encounter aggregated α-syn would be peripheral monocytes and macrophages in the body-first PD type, vs. microglia in the brain-first-PD subtype. Moreover, the loss of immune control exerted by the DMN will occur early in the body-first-PD, which might contribute to the faster and more severe progression of this subtype. Accordingly, RBD patients (putative body-first-PD) show a decrease in the anti-inflammatory cytokine IL-10, rather than a pro-inflammatory profile, which might be related to the DMN affection [100].

As a direct consequence of the findings of PNS degeneration in PD, studies regarding gut-brain axis and microbiota influence in neurodegeneration have been of interest. Interestingly, in the rAAV- α-syn rodent model, overexpression of α-syn in the SN led to enteric nervous system changes and altered microbiota [129], while in the transgenic α-syn PD model, the microbiota influenced the neurodegeneration process [147]. Indeed, changes in the microbiota have been related to PD [167], but also to RBD, suggesting that this might be a factor of very early relevance [81]. The influence of microbiota on shaping the immune system has long been known, but this is a novel exciting concept within the PD field (reviewed in [88]). The relevance of the gastrointestinal tract has also been investigated in epidemiological studies suggesting that the risk and incidence of PD are lower in those persons that underwent vagotomy or appendectomy earlier in life [168, 169]. Within this context, inflammatory events in the digestive tube (like in the appendix) seem of high relevance, particularly due to the enrichment of α-syn of the myenteric plexus of the appendix and the presence of macrophages with engulfed α-syn in the area [66]. This is also supported by the relation of PD with IBD [18]. Inflammation and dysbiosis will result in a leaky gut wall that may cause immune activation that promotes neurodegeneration (Fig. 1).

A recent study found increased levels of endotoxin in PD patients’ blood, especially those with a higher risk for dementia, suggesting an active role of bacterial infection in the outcome of the disease [185]. This is in agreement with the synergistic neurotoxic effect of chronic (peripheral) LPS and α-syn shown by Hong’s lab [56, 193]. Interestingly, a study in WT mice showed that intraperitoneal LPS injection, before α-syn peripheral intravenous administration, led to α-syn internalization by inflammatory monocytes that in turn can infiltrate the brain, suggesting that the peripheral activated monocytes can act as a Trojan horse in PD, promoting the entrance of peripheral (modified) α-syn into the CNS [137]. Altogether this has contributed to the double-hit hypothesis of PD and further corroborates the complex and multisystem nature of PD (see [92]).

Other factors related to α‑synuclein and the immune response in Parkinson’s disease

Lysosomal dysfunction seems to be at the center of α-syn pathology, a process of special relevance in glia (see review [52]); in that regard, we would shortly discuss two proteins genetically related to PD: LRRK2 and glucocerebrosidase (Gcase). LRRK2 is expressed in immune cells, but considerably higher in monocytes and microglia than in T cells [58], suggesting LRRK2 is a key player in innate immunity. Genomic studies implicate LRRK2 mutations not only in PD but also in other inflammatory disorders, specifically IBD, further supporting the functional role of LRRK2 in immune cells [179]. Within the cell, LRRK2 has been implicated in phagocytosis through the autophagy/lysosomal degradation pathway [155], and LRKK2 mutation leads to abnormal chaperone-mediated autophagy and α-syn accumulation [82]. Rab proteins have been identified as substrates for LRRK2 kinase activity [114, 143], implicating a role for LRRK2 in membrane trafficking and regulation of immune cell functions such as phagocytosis, exocytosis, and antigen presentation. LRRK2 is also implicated in the modulation of cell-surface markers in monocytes and microglia [172] and regulation of cytokine production [124], and pathogenic mutations are associated with enhanced neuroinflammation and neurodegeneration upon systemic inflammation [106].

As mentioned, immune cells will respond to α-syn promoting inflammation and protein clearance/degradation. These two processes might be mediated by LRRK2, due to common receptor pathways (to both α-syn and LRRK2), or by LRRK2-mediated direction to autophagy degradation [37]. Pathogenic mutations of LRRK2 seem to compromise microglia's ability to internalize and degrade α-syn [99]. In addition, LRRK2 expression and phosphorylation increase in microglia and monocytes upon TLR2 or TLR4 stimulation [124, 155], both α-syn interactors. Although LRRK2 response was slightly different in monocytes vs. microglia cell lines, both showed autophagic deficits upon LRRK2 knock-down [155], further implicating LRRK2 in the regulation of lysosomal degradation in myeloid cells. In a human study, both asymptomatic and PD patient carriers of LRRK2 G2019S mutation showed increased levels of peripheral inflammatory cytokines [43], suggesting a pathological contribution of LRRK2 mutations in the mediating peripheral immune response. Altogether, this suggests that dysregulation of these and other LRRK2-associated signaling pathways might relate to α-syn accumulation and consequent neuroinflammation. For relevant additional reading on the role of LRRK2 in immune system modulation, we refer the reader to [22, 179].

Mutations in the GBA1 gene, encoding the lysosomal enzyme Gcase are responsible for causing the autosomal lipid storage disorder, Gaucher disease (GD), characterized by the deposition of glucocerebrosides in monocytes-macrophages. GBA1 mutations are the most important genetic risk factors for PD [161]. Postmortem analysis of brain tissue from patients with GBA-PD showed increased levels of α-syn in the SN and a significant correlation between the reduction in the Gcase protein levels and an increase in p129/total α-syn [69]. Indeed, a biochemical connection between GBA and α-syn has been reported, with GBA mutations leading to the accumulation of α-syn in human cells [36] and α-synuclein pathology in mouse models, associated with autophagy failure [152]. Additionally, α-syn pathology itself can lead to lysosomal dysfunction [121]. Thus, the additive effect of GBA mutations on lysosomal failure and lipid accumulation might explain the higher PD risk observed. Remarkably, GBA carriers without manifested PD show immune activation by PK1195 PET in SN [126], supporting an early role in the immune system.

Interestingly LRRK2 and GCase seem to converge since LRRK2 mutation led to decreased GCase in patient-derived cells, and inhibition of LRRK2 kinase activity increased GCase activity in neurons with either LRRK2 or GBA1 mutations.[191]. Accordingly, activation of GCase can rescue neuronal health in iPSC models of genetic GBA and LRRK2 PD [20]. Gcase was also protective in a model of peripheral synucleinopathy based on injections of α-syn PFF in the gut, supporting the importance of the integrity of lysosomal function in the peripheral pathology in PD [28]. Accordingly, Gcase activity was found reduced in monocytes from idiopathic PD patients [6] further supporting lysosomal dysfunction in peripheral myeloid cells. In monocyte-derived macrophages from patients with type 1 GD, inflammasome activation showed to be the result of impaired lysosomal autophagy [1]. In these cells, the increase in p62 led to activation of the p65-NFκB pathway, which per se promoted the expression of inflammatory cytokines and increased IL-1β secretion [1], providing a link between inflammation, lysosome storage, and autophagy impairment, three major processes with possible relevant implications for α-syn clearance and the PD pathogenesis. More work is needed to elucidate the relevance of such mutations in immune cells in the α-syn neurodegenerative process. For further reading see: [126, 163].

Conclusions

It is increasingly clear that the immune system is a relevant component of the disease pathogenesis in PD, as there is strong evidence for innate and adaptive immune mechanisms in both human disease and α-syn based animal models. As research progresses and methodologies for detection evolve, it is evident that these changes in the immune component in PD occur early and change dynamically with disease progression. Although previously thought to involve CNS-specific immune mechanisms, research has now shown that both brain, as well as peripheral immune cells, are involved in this inflammatory event providing strong evidence for innate and adaptive immune system crosstalk in the CNS and periphery.

At the core of disease pathogenesis, α-syn has proven to be a key player as it not only contributes to the hallmark pathology observed in PD postmortem tissue but is also key in activating and driving inflammation and neurodegeneration in human PD. These central pathways of α-syn-driven innate and adaptive immune activation have been recently dissected in α-syn-based animal models, which have been instrumental in modeling human disease. Using these animal models in parallel with human-based studies allowed us to identify novel pathways potentially driving neurodegeneration in the PNS and CNS that involve not only the immune system, but also implicate the gut microbiome, genetic predisposition, and environmental immune challenges. We believe that analysis of longitudinal changes in the inflammatory profile in patients, in combination with peripheral immune profiling, gut microbiome testing, α-syn blood and CSF analysis, and PET imaging may provide a unique opportunity for the discovery or detection of unique immune-based biomarkers to predict disease outcomes and progression. Early detection and a clear understanding of the progressive immune system involvement in PD may lead to novel therapeutics that not only target CNS-specific components but also target the periphery, offering neuroprotection and halting disease progression.

Acknowledgments

We are grateful to Prof Dr. Malú Tansey for her mentoring, and rewarding and insightful scientific discussions regarding the focus of this review.

Funding

The authors declare no conflict of interest. Funding support for the research covered in this article was provided by the Michael J. Fox Foundation, the Danish Parkinson Foundation, and the Aarhus University Forskningsfond AU IDEAS Center NEURODIN. SAF is funded by a Ph.D. fellowship from the Ph.D. School at the Health Faculty, Aarhus University.

Open Access

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References

1. Afaki E, Moaven N, Borger DK, Lopez G, Westbroek W, Chae JJ et al (2016) Lysosomal storage and impaired autophagy lead to inflammasome activation in Gaucher macrophages. Aging Cell 15:77–88. https://doi.org/10.1111/acel.12409 

2. Ahmadi Rastegar D, Ho N, Halliday GM, Dzamko N (2019) Parkinson’s progression prediction using machine learning and serum cytokines. NPJ Parkinsons Dis 5:14. https://doi. org/10.1038/s41531-019-0086-4 

3. Akhtar RS, Licata JP, Luk KC, Shaw LM, Trojanowski JQ, Lee VM (2018) Measurements of auto-antibodies to alpha-synuclein in the serum and cerebral spinal fluid of patients with Parkinson’s disease. J Neurochem 145:489–503. https://doi. org/10.1111/jnc.14330 

4. Al-Qassabi A, Tsao TS, Racolta A, Kremer T, Canamero M, Belousov A, et al (2020) Immunohistochemical detection of synuclein pathology in the skin in idiopathic rapid eye movement sleep behavior disorder and Parkinsonism. Mov Disord. https://doi. org/10.1002/mds.28399 

5. Anthony IC, Crawford DH, Bell JE (2003) B lymphocytes in the normal brain: contrasts with HIV-associated lymphoid infiltrates and lymphomas. Brain 126:1058–1067. https://doi.org/10.1093/ brain/awg118 

6. Atashrazm F, Hammond D, Perera G, Dobson-Stone C, Mueller N, Pickford R et al (2018) Reduced glucocerebrosidase activity in monocytes from patients with Parkinson’s disease. Sci Rep 8:15446. https://doi.org/10.1038/s41598-018-33921-x 

7. Aufray C, Fogg D, Garfa M, Elain G, Join-Lambert O, Kayal S, et al (2007) Monitoring of blood vessels and tissues by a population of monocytes with patrolling behavior. Sci (New York, NY) 317:666–670. https://doi.org/10.1126/science.1142883 

8. Bae EJ, Lee HJ, Rockenstein E, Ho DH, Park EB, Yang NY, et al (2012) Antibody-aided clearance of extracellular alpha-synuclein prevents cell-to-cell aggregate transmission. J Neurosci 32:13454–13469. https://doi.org/10.1523/JNEUR OSCI.1292-12.2012 

9. Barkholt P, Sanchez-Guajardo V, Kirik D, Romero-Ramos M (2012) Long-term polarization of microglia upon alpha-synuclein overexpression in nonhuman primates. Neuroscience. https://doi. org/10.1016/j.neuroscience.2012.02.004 

10. Besong-Agbo D, Wolf E, Jessen F, Oechsner M, Hametner E, Poewe W et al (2013) Naturally occurring alpha-synuclein autoantibody levels are lower in patients with Parkinson's disease. Neurology 80:169–175. https://doi.org/10.1212/WNL.0b013 e31827b90d1 

11. Biber K, Neumann H, Inoue K, Boddeke HW (2007) Neuronal “On” and “Off” signals control microglia. Trends Neurosci 30:596–602. https://doi.org/10.1016/j.tins.2007.08.007 

12. Bliederhaeuser C, Grozdanov V, Speidel A, Zondler L, Ruf WP, Bayer H, et al (2016) Age-dependent defects of alpha-synuclein oligomer uptake in microglia and monocytes. Acta Neuropathol 131:379–391. https://doi.org/10.1007/s00401-015-1504-2 

13. Blum-Degen D, Muller T, Kuhn W, Gerlach M, Przuntek H, Riederer P (1995) Interleukin-1 beta and interleukin-6 are elevated in the cerebrospinal fluid of Alzheimer’s and de novo Parkinson’s disease patients. Neurosci Lett 202:17–20 

14. Bohnen NI, Hu MTM (2019) Sleep disturbance as a potential risk and progression factor for Parkinson’s disease. J Parkinsons Dis 9:603–614. https://doi.org/10.3233/jpd-191627 

15. Borghammer P, Van Den Berge N (2019) Brain-first versus gutfrst Parkinson’s disease: a hypothesis. J Parkinsons Dis 9:S281– S295. https://doi.org/10.3233/JPD-191721 

16. Brochard V, Combadiere B, Prigent A, Laouar Y, Perrin A, Beray-Berthat V et al (2009) Infiltration of CD4+ lymphocytes into the brain contributes to neurodegeneration in a mouse model of Parkinson's disease. J Clin Invest 119:182–192. https://doi. org/10.1172/JCI36470 (36470[pii]) 

17. Brockmann K, Schulte C, Schneiderhan-Marra N, Apel A, PontSunyer C, Vilas D et al (2017) Inflammatory profile discriminates clinical subtypes in LRRK2-associated Parkinson’s disease. Eur J Neurol 24:427-e426. https://doi.org/10.1111/ene.13223 

18. Brudek T (2019) Inflammatory bowel diseases and Parkinson’s disease. J Parkinsons Dis 9:S331–S344. https://doi.org/10.3233/ JPD-191729 

19. Brudek T, Winge K, Folke J, Christensen S, Fog K, Pakkenberg B et al (2017) Autoimmune antibody decline in Parkinson’s disease and multiple system atrophy; a step towards immunotherapeutic strategies. Mol Neurodegener 12:44. https://doi. org/10.1186/s13024-017-0187-7 

20. Burbulla LF, Jeon S, Zheng J, Song P, Silverman RB, Krainc D (2019) A modulator of wild-type glucocerebrosidase improves pathogenic phenotypes in dopaminergic neuronal models of Parkinson’s disease. Sci Transl Med. https://doi.org/10.1126/scitr assumed.aau6870 

21. Bussi C, Peralta Ramos JM, Arroyo DS, Gallea JI, Ronchi P, Kolovou A, et al (2018) Alpha-synuclein fibrils recruit TBK1 and OPTN to lysosomal damage sites and induce autophagy in microglial cells. J Cell Sci. https://doi.org/10.1242/jcs.226241 

22. Cabezudo D, Baekelandt V, Lobbestael E (2020) Multiple-hit hypothesis in Parkinson’s disease: LRRK2 and inflammation. Front Neurosci 14:376. https://doi.org/10.3389/fnins.2020.00376 

23. Cao S, Standaert DG, Harms AS (2012) The gamma chain subunit of Fc receptors is required for alpha-synuclein-induced proinflammatory signaling in microglia. J Neuroinflammation 9:259.https://doi.org/10.1186/1742-2094-9-259 

24. Cao S, Theodore S, Standaert DG (2010) Fcgamma receptors are required for NF-kappaB signaling, microglial activation, and dopaminergic neurodegeneration in an AAV-synuclein mouse model of Parkinson’s disease. Mol Neurodegener 5:42. https:// doi.org/10.1186/1750-1326-5-42 

25. Cebrian C, Zucca FA, Mauri P, Steinbeck JA, Studer L, Scherzer CR, et al (2014) MHC-I expression renders catecholaminergic neurons susceptible to T-cell-mediated degeneration. Nat Commun 5:3633. 

26. Chahine LM, Beach TG, Brumm MC, Adler CH, Coffey CS, Mosovsky S, et al (2020) In vivo distribution of alpha-synuclein in multiple tissues and biofluids in Parkinson's disease. Neurology 95:e1267–e1284. https://doi.org/10.1212/WNL.0000000000 010404 

27. Chahine LM, Qiang J, Ashbridge E, Minger J, Yearout D, Horn S, et al (2013) Clinical and biochemical differences in patients having Parkinson's disease with vs without GBA mutations. JAMA Neurol 70:852–858. https://doi.org/10.1001/jamaneurol .2013.1274 

28. Challis C, Hori A, Sampson TR, Yoo BB, Challis RC, Hamilton AM et al (2020) Gut-seeded alpha-synuclein fibrils promote gut dysfunction and brain pathology specifically in aged mice. Nat Neurosci 23:327–336. https://doi.org/10.1038/s4159 3-020-0589-7 

29. Chatterjee K, Roy A, Banerjee R, Choudhury S, Mondal B, Halder S, et al (2020) Inflammasome and alpha-synuclein in Parkinson’s disease: a cross-sectional study. J Neuroimmunol 338:577089. https://doi.org/10.1016/j.jneuroim.2019.577089 

30. Chavan SS, Tracey KJ (2017) Essential neuroscience in immunology. J Immunol 198:3389–3397. https://doi.org/10.4049/ Immunol.1601613 

31. Chen Z, Li G, Liu J (2020) Autonomic dysfunction in Parkinson’s disease: Implications for pathophysiology, diagnosis, and treatment. Neurobiol Dis 134:104700. https://doi.org/10.1016/j. nbd.2019.104700 

32. Choi YR, Kang SJ, Kim JM, Lee SJ, Jou I, Joe EH et al (2015) FcgammaRIIB mediates the inhibitory effect of aggregated alpha-synuclein on microglial phagocytosis. Neurobiol Dis 83:90–99. https://doi.org/10.1016/j.nbd.2015.08.025 

33. Christiansen JR, Olesen MN, Otzen DE, Romero-Ramos M, Sanchez-Guajardo V (2016) alpha-Synuclein vaccination modulates regulatory T cell activation and microglia in the absence of brain pathology. J Neuroinflammation 13:74. https ://doi.org/10.1186/s12974-016-0532-8 

34. Codolo G, Plotegher N, Pozzobon T, Brucale M, Tessari I, Bubacco L, et al (2013) Triggering of the inflammasome by aggregated alpha-synuclein, an inflammatory response in synucleinopathies. PLoS ONE 8:e55375. https://doi.org/10.1371/journ al.pone.0055375 

35. Croisier E, Moran LB, Dexter DT, Pearce RK, Graeber MB (2005) Microglial inflammation in the Parkinsonian substantia nigra: relationship to alpha-synuclein deposition. J Neuroinflammation 2:14 

36. Cullen V, Sardi SP, Ng J, Xu YH, Sun Y, Tomlinson JJ et al (2011) Acid beta-glucosidase mutants linked to Gaucher disease, Parkinson disease, and Lewy body dementia alter alpha-synuclein processing. Ann Neurol 69:940–953. https://doi. org/10.1002/ana.22400 

37. Daher JP (2017) Interaction of LRRK2 and alpha-synuclein in Parkinson’s disease. Adv Neurobiol 14:209–226. https://doi. org/10.1007/978-3-319-49969-7_11 

38. Daniele SG, Beraud D, Davenport C, Cheng K, Yin H, Maguire-Zeiss KA (2015) Activation of MyD88-dependent TLR1/2 signaling by misfolded alpha-synuclein, a protein linked to neurodegenerative disorders. Sci Signal 8:ra45. https://doi. org/10.1126/scisignal.2005965 

39. Dhillon JS, Trejo-Lopez JA, Rife C, Levites Y, Sacino AN, Borchelt DR, et al (2019) Comparative analyses of the in vivo induction and transmission of alpha-synuclein pathology in transgenic mice by MSA brain lysate and recombinant alpha-synuclein fibrils. Acta Neuropathol Commun 7:80. https://doi. org/10.1186/s40478-019-0733-3 

40. Doorn KJ, Moors T, Drukarch B, van de Berg W, Lucassen PJ, van Dam AM (2014) Microglial phenotypes and toll-like receptor 2 in the substantia nigra and hippocampus of incidental Lewy body disease cases and Parkinson’s disease patients. Acta Neuropathol Commun 2:90. https://doi. org/10.1186/s40478-014-0090-1 

41. Drouin-Ouellet J, St-Amour I, Saint-Pierre M, LamontagneProulx J, Kriz J, et al (2015) Toll-like receptor expression in the blood and brain of patients and a mouse model of Parkinson’s disease. Int J Neuropsychopharmacol. https://doi.org/10.1093/ ijnp/pyu103 

42. Dufy MF, Collier TJ, Patterson JR, Kemp CJ, Luk KC, Tansey MG, et al (2018) Lewy body-like alpha-synuclein inclusions trigger reactive microgliosis before nigral degeneration. J Neuroinflammation 15:129. https://doi.org/10.1186/s1297 4-018-1171-z 

43. Dzamko N, Rowe DB, Halliday GM (2016) Increased peripheral inflammation in asymptomatic leucine-rich repeat kinase 2 mutation carriers. Mov Disord 31:889–897. https://doi.org/10.1002/ mds.26529 

44. Earls RH, Menees KB, Chung J, Barber J, Gutekunst CA, Hazim MG, et al (2019) Intrastriatal injection of preformed alpha-synuclein fibrils alters central and peripheral immune cell profiles in non-transgenic mice. J Neuroinflammation 16:250. https://doi. org/10.1186/s12974-019-1636-8 

45. Earls RH, Menees KB, Chung J, Gutekunst CA, Lee HJ, Hazim MG, Rada B, Wood LB, Lee JK (2020) NK cells clear alpha-synuclein and the depletion of NK cells exacerbates synuclein pathology in a mouse model of alpha-synucleinopathy. Proc Natl Acad Sci USA 117:1762–1771. https://doi.org/10.1073/ pnas.1909110117 

46. Edison P, Ahmed I, Fan Z, Hinz R, Gelosa G, Ray Chaudhuri K, et al (2013) Microglia, amyloid, and glucose metabolism in Parkinson’s disease with and without dementia. Neuropsychopharmacology 38:938–949. https://doi.org/10.1038/npp.2012.255 

47. El-Agnaf O, Overk C, Rockenstein E, Mante M, Florio J, Adame A, et al (2017) Differential effects of immunotherapy with antibodies targeting alpha-synuclein oligomers and fibrils in a transgenic model of synucleinopathy. Neurobiol Dis 104:85–96. https ://doi.org/10.1016/j.nbd.2017.05.002 

48. Emmer KL, Waxman EA, Covy JP, Giasson BI (2011) E46K human alpha-synuclein transgenic mice develop Lewy-like and tau pathology associated with age-dependent, detrimental motor impairment. J Biol Chem 286:35104–35118. https://doi. org/10.1074/jbc.M111.247965 (M111.247965[pii]) 

49. Fan Z, Pan YT, Zhang ZY, Yang H, Yu SY, Zheng Y et al (2020) Systemic activation of NLRP3 inflammasome and plasma alpha-synuclein levels are correlated with motor severity and progression in Parkinson’s disease. J Neuroinflammation 17:11. https:// doi.org/10.1186/s12974-019-1670-6 

50. Fellner L, Irschick R, Schanda K, Reindl M, Klimaschewski L, Poewe W et al (2013) Toll-like receptor 4 is required for alpha-synuclein dependent activation of microglia and astroglia. Glia 61:349–360. https://doi.org/10.1002/glia.22437 

51. Ferreira SA, Romero-Ramos M (2018) Microglia response during Parkinson’s disease: alpha-synuclein intervention. Front Cell Neurosci 12:247. https://doi.org/10.3389/fncel.2018.00247 

52. Filippini A, Gennarelli M, Russo I (2019) Alpha-synuclein and glia in Parkinson’s disease: a beneficial or a detrimental duet for the endo-lysosomal system? Cell Mol Neurobiol 39:161–168.https://doi.org/10.1007/s10571-019-00649-9 

53. Freeman D, Cedillos R, Choyke S, Lukic Z, McGuire K, Marvin S et al (2013) Alpha-synuclein induces lysosomal rupture and cathepsin-dependent reactive oxygen species following endocytosis. PLoS ONE 8:e62143. https://doi.org/10.1371/journ al.pone.0062143 

54. Funk N, Wieghofer P, Grimm S, Schaefer R, Buhring HJ, Gasser T et al (2013) Characterization of peripheral hematopoietic stem cells and monocytes in Parkinson’s disease. Mov Disord 28:392–395. https://doi.org/10.1002/mds.25300 

55. Gao HM, Kotzbauer PT, Uryu K, Leight S, Trojanowski JQ, Lee VM (2008) Neuroinflammation and oxidation/nitration of alpha-synuclein linked to dopaminergic neurodegeneration. J Neurosci 28:7687–7698. https://doi.org/10.1523/JNEUROSCI.0143- 07.2008 (28/30/7687[pii]) 

56. Gao HM, Zhang F, Zhou H, Kam W, Wilson B, Hong JS (2011) Neuroinflammation and alpha-synuclein dysfunction potentiate each other, driving the chronic progression of neurodegeneration in a mouse model of Parkinson’s disease. Environ Health Perspect 119:807–814. https://doi.org/10.1289/ehp.1003013 

57. Gardai SJ, Mao W, Schule B, Babcock M, Schoebel S, Lorenzana C, et al (2013) Elevated alpha-synuclein impairs innate immune cell function and provides a potential peripheral biomarker for Parkinson’s disease. PLoS ONE 8:e71634. https:// doi.org/10.1371/journal.pone.0071634 

58. Gardet A, Benita Y, Li C, Sands BE, Ballester I, Stevens C et al (2010) LRRK2 is involved in the IFN-gamma response and host response to pathogens. J Immunol 185:5577–5585. https://doi. org/10.4049/Immunol.1000548 

59. Garretti F, Agalliu D, Lindestam Arlehamn CS, Sette A, Sulzer D (2019) Autoimmunity in Parkinson’s disease: the role of alphasynuclein-specifc T cells. Front Immunol 10:303. https://doi. org/10.3389/fmmu.2019.00303 

60. George S, Rey NL, Tyson T, Esquibel C, Meyerdirk L, Schulz E, et al (2019) Microglia affect alpha-synuclein cell-to-cell transfer in a mouse model of Parkinson’s disease. Mol Neurodegener 14:34. https://doi.org/10.1186/s13024-019-0335-3 

61. Gerhard A, Pavese N, Hotton G, Turkheimer F, Es M, Hammers A, et al (2006) In vivo imaging of microglial activation with [11C] (R)-PK11195 PET in idiopathic Parkinson’s disease. Neurobiol Dis 21:404–412. https://doi.org/10.1016/j.nbd.2005.08.002 

62. Giasson BI, Duda JE, Quinn SM, Zhang B, Trojanowski JQ, Lee VM (2002) Neuronal alpha-synucleinopathy with severe movement disorder in mice expressing A53T human alpha-synuclein. Neuron 34:521–533 

63. Gliem M, Schwaninger M, Jander S (2016) Protective features of peripheral monocytes/macrophages in stroke. Biochim Biophys Acta 1862:329–338. https://doi.org/10.1016/j.bbadi s.2015.11.004 

64. Gomez-Isla T, Irizarry MC, Mariash A, Cheung B, Soto O, Schrump S et al (2003) Motor dysfunction and gliosis with preserved dopaminergic markers in human alpha-synuclein A30P transgenic mice. Neurobiol Aging 24:245–258

65. Gordon R, Albornoz EA, Christie DC, Langley MR, Kumar V, Mantovani S et al (2018) Inflammasome inhibition prevents alpha-synuclein pathology and dopaminergic neurodegeneration in mice. Sci Transl Med. https://doi.org/10.1126/scitranslm ed.aah4066 

66. Gray MT, Munoz DG, Gray DA, Schlossmacher MG, Woulfe JM (2014) Alpha-synuclein in the appendiceal mucosa of neurologically intact subjects. Mov Disord 29:991–998. https://doi. org/10.1002/mds.25779 

67. Grozdanov V, Bliederhaeuser C, Ruf WP, Roth V, FundelClemens K et al (2014) Inflammatory dysregulation of blood monocytes in Parkinson’s disease patients. Acta Neuropathol 128:651–663. https://doi.org/10.1007/s00401-014-1345-4 

68. Grozdanov V, Bousset L, Hofmeister M, Bliederhaeuser C, Meier C, Madiona K et al (2019) Increased immune activation by pathologic alpha-synuclein in Parkinson’s disease. Ann Neurol 86:593–606. https://doi.org/10.1002/ana.25557 

69. Gundner AL, Duran-Pacheco G, Zimmermann S, Ruf I, Moors T, Baumann K et al (2019) Path mediation analysis reveals GBA impacts Lewy body disease status by increasing alpha-synuclein levels. Neurobiol Dis 121:205–213. https://doi.org/10.1016/j. nbd.2018.09.015 

70. Guo M, Wang J, Zhao Y, Feng Y, Han S, Dong Q et al (2020) Microglial exosomes facilitate alpha-synuclein transmission in Parkinson’s disease. Brain. https://doi.org/10.1093/brain/awaa0 90 

71. Gustafsson G, Eriksson F, Moller C, da Fonseca TL, Outeiro TF, Lannfelt L et al (2017) Cellular uptake of alpha-synuclein oligomer-selective antibodies is enhanced by the extracellular presence of alpha-synuclein and mediated via Fc gamma receptors. Cell Mol Neurobiol 37:121–131. https://doi.org/10.1007/ s10571-016-0352-5 

72. Gustot A, Gallea JI, Sarroukh R, Celej MS, Ruysschaert JM, Raussens V (2015) Amyloid fibrils are the molecular trigger of inflammation in Parkinson’s disease. Biochem J 471:323–333.https://doi.org/10.1042/BJ20150617 

73. Hall S, Janelidze S, Surova Y, Widner H, Zetterberg H, Hansson O (2018) Cerebrospinal fluid concentrations of inflammatory markers in Parkinson’s disease and atypical parkinsonian disorders. Sci Rep 8:13276. https://doi.org/10.1038/s41598-018- 31517-z 

74. Halliday GM, Stevens CH (2011) Glia: initiators and progressors of pathology in Parkinson’s disease. Mov Disord 26:6–17. https ://doi.org/10.1002/mds.23455 

75. Hamza TH, Zabetian CP, Tenesa A, Laederach A, Montimurro J, Yearout D et al (2010) Common genetic variation in the HLA region is associated with late-onset sporadic Parkinson’s disease. Nat Genet 42:781–785. https://doi.org/10.1038/ng.642 

76. Hansson O, Hall S, Ohrfelt A, Zetterberg H, Blennow K, Minthon L et al (2014) Levels of cerebrospinal fluid alpha-synuclein oligomers are increased in Parkinson’s disease with dementia and dementia with Lewy bodies compared to Alzheimer’s disease. Alzheimer's Res Ther 6:25. https://doi.org/10.1186/alzrt 255 

77. Harms AS, Cao S, Rowse AL, Thome AD, Li X, Mangieri LR, et al (2013) MHCII is required for alpha-synuclein-induced activation of microglia, CD4 T cell proliferation, and dopaminergic neurodegeneration. J Neurosci 33:9592–9600. https://doi. org/10.1523/JNEUROSCI.5610-12.2013 

78. Harms AS, Delic V, Thome AD, Bryant N, Liu Z, Chandra S, et al (2017) alpha-Synuclein fibrils recruit peripheral immune cells in the rat brain before neurodegeneration. Acta Neuropathol Commun 5:85. https://doi.org/10.1186/s40478-017-0494-9 

79. Harms AS, Standaert DG (2014) Monocytes and Parkinson’s disease: invaders from outside? Mov Disord 29:1242. https:// doi.org/10.1002/mds.25993 

80. Harms AS, Thome AD, Yan Z, Schonhof AM, Williams GP, Li X et al (2018) Peripheral monocyte entry is required for alpha-synuclein-induced inflammation and Neurodegeneration in a model of Parkinson's disease. Exp Neurol 300:179–187. https:// doi.org/10.1016/j.expneurol.2017.11.010 

81. Heintz-Buschart A, Pandey U, Wicke T, Sixel-Doring F, Janzen A, Sittig-Wiegand E et al (2018) The nasal and gut microbiome in Parkinson’s disease and idiopathic rapid eye movement sleep behavior disorder. Mov Disord 33:88–98. https://doi. org/10.1002/mds.27105 

82. Ho PW, Leung CT, Liu H, Pang SY, Lam CS, Xian J, et al (2020) Age-dependent accumulation of oligomeric SNCA/ alpha-synuclein from impaired degradation in mutant LRRK2 knock-in mouse model of Parkinson disease: a role for therapeutic activation of chaperone-mediated autophagy (CMA). Autophagy 16:347–370.

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