How can the adaptive immune system be targeted as a therapeutic approach for PD?

Parkinson’s disease (PD) is mainly characterized by a loss of the dompaniergic cells located in the midbrain’s substantia nigra pars compacta. In addition to this neuronal loss, PD is also defined by the presence of proteinaceous inclusion bodies called Lewy bodies (composed of a number of proteins, including aggregated alpha-synuclein). Evidence also shows that neuroinflammation occurs in PD animal models as well as in human samples of patients (serum, plasma, cerebrospinal fluid or post-mortem brain tissue) by means of activated microglia and the presence of several pro-inflammatory cytokines in the substantia nigra. The paper recently published in PLoS One by Marina Romero-Ramos’ group emphasizes the importance of understanding the role of neuroinflammation and microglial activation in the genesis of PD. According to this article, alpha-synuclein could be the antigen that induces the immune response observed in PD. These authors describe a model of progressive neurodegeneration based on over-expression of human wild type alpha-synuclein mediated by injection of recombinant adeno-associated virus directly into the substantia nigra. Their data indicate that alpha-synuclein expression not only leads to persistent microglia activation, but that depending on the degree of induced neuropathology distinct microglial responses will occur. They describe the existence of two distinct microglial responses dependent on the stage of alpha-synuclein induced neuropathology: alpha-synuclein expression levels capable of inducing dopaminergic cell death correlate with the long-term induction of macrophagic microglia; however, at levels where only neurodegeneration is taking place, they just observe microglia with antigen presenting capabilities. This work brings to light better understanding on the cellular events that could take place in the brain during the chronic development of PD.

The therapeutic approach is based on the notion that the inciting event that underlies the pathogenesis of Parkinson’s disease (PD) is the accumulation of aggregated proteins within neuronal cell bodies and microglia is associated with microglial activation and neuronal death. Deposition of such misfolded and nitrated alpha-synuclein (N-a-syn) occurs into Lewy bodies within nigral dopaminergic neurons of the substantia nigra pars compacta (Spillantini et al., 1997; Giasson et al., 2000).  However, it is its subsequent release into extracellular spaces and draining cervical lymph nodes that can affect neuronal loss by engaging innate and adaptive immune responses (Zhang et al., 2005; Thomas et al., 2007; Benner et al., 2008; Lee et al., 2008; Reynolds et al., 2008; Reynolds et al., 2008; Theodore et al., 2008). This, we and others believe, leads to oxidative stress, microglial and APC activation, and neuronal degeneration (Zhang et al., 2005; Thomas et al., 2007; Reynolds et al., 2008b). It is now accepted that a-syn immunization can readily generate a humoral response for clearing protein aggregates (Masliah et al., 2005). There are consequences to such clearance, however.  Indeed, immunization with nitrated forms of a-syn (N-4YSyn) used as an immunogen elicits profound effector T cell responses and exacerbates neuroinflammation and neurodegeneration (Benner et al., 2008). This finding is analogous to the untoward T cell-mediated meningoencephalitic responses observed by amyloid beta immunization (Morgan et al., 2009). Adding credence to this notion is the fact that reported that T cell responses elicited during the course of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication led to accelerated neurodegeneration (Theodore et al., 2008). 

Interestingly, components of adaptive immunity can also affect neural repair and protection. Indeed, regulatory T cells (Treg) protect against MPTP-induced dopaminergic degeneration (Reynolds et al., 2007). This raised the specter of opposing effects for CD4+ T cell subsets on brain disease where auto-aggressive effector T cell responses speed the tempo of disease Treg would attenuate neurodegeneration (Reynolds et al., 2007; Banerjee et al., 2008; Gee et al., 2008; Liu et al., 2009).  This is also in keeping with known anti-inflammatory and neurotrophic function of Treg, their role in controlling immune-mediated inflammation and regulation of mononuclear phagocyte phenotype (Cederbom et al., 2000; Thornton and Shevach, 2000; Reynolds et al., 2007; Tiemessen et al., 2007; Liu et al., 2009; Reynolds et al., 2009). Thus in our report we investigated whether an adjuvant to promote specific adaptive immune responses could be used with N-a-syn as a vaccine for PD.  Using vasoactive intestinal peptide (VIP), a neuropeptide known to induce Treg responses (Delgado et al., 2005; Gonzalez-Rey et al., 2006), we were able to show that replacement of functional Treg within the N-a-syn splenocyte mixture results in neuroprotection and is modulated through Th17 mechanisms.  

The importance of this work rests in the possibility that N-a-syn vaccination can be utilized for treatment of PD when an untoward adaptive neurodegenerative immune response is corrected by adjuvant therapy.   

The recent paper published by Reynolds and coworkers opens a fascinating possibility to modulate neuroinflammatory processes to slow down the progression of Parkinson’s disease.  The study published shows that a manipulation of adaptive immunity may represent a good target for neuroprotection in Parkinson’s disease. The concept of an involvement of the adaptive arm of the immune system in PD has been suggested initially by Mc Geer and coworkers who reported in one case of PD the presence of infiltrated CD8+ lymphocytes within the substantia nigra (McGeer et al. Ann. Neurol 1988). Recently, in a study funded by the MJ Fox foundation, we have not only shown that CD4+ T cells invade the PD substantia nigra as well, but also that in MPTP intoxicated mice, CD4+ lymphocytes are instrumental in the degenerative process. This deleterious effect is likely mediated by the Fas/Fas ligand pathway but not IFN-g (Brochard et al. J Clin Invest, 2009). What the study of Reynolds et al. adds to the role of the adaptive arm of immunity is that nitrated alpha-synuclein can elicit an immune response involving the subpopulation of pro-inflammatory lymphocytes T_H17 which is shown to be detrimental when transferred to MPTP-intoxicated animals. Furthermore, they show that the regulatory T cell  (T_Reg) response is altered under such circumstances but that T_Regs from vasoactive intestinal peptide (VIP)-treated animals can circumvent this defect thereby conferring efficient neuroprotection when co-administrated with nitrated a-synuclein splenocytes. This opens new avenues for manipulating with an unprecedented specificity the inflammatory processes in PD by  manipulating a subclass of lymphocyte. Yet, before such a conclusion can be firmly drawn, one has to know whether these different T cell subclass responses are equally involved in the human pathology and whether their regulatory efficiency will be identical in the absence of immunization to nitrated alpha-synuclein.

Involvement of the adaptive immune system in PD has been observed in human postmortem PD brains, where T cell infiltration is present (McGeer et al., Ann. Neurol, 1988 & Brochard et al., J Clin Invest, 2009). There has also been observed a correlation between dopaminergic neuron survival and HLA+ cells (Orr et al., Brain, 2005), where HLA expression correlates with a-syn deposition in SN PD brains (Croisier et al., J Neuroinflammation, 2005). Together, suggesting a role for microglia with antigen presenting capacity (HLA+/MHCII+) in the disease process.

We have observed, in an AAV a-syn rat model, that the type of T cell response varies depending on the level of neuropathology, but it always correlated with the peak of microglia proliferation. Indeed microglia seem to be directly implicated in the type of immune response elicited, as the presence of predominant CD68+ microglia correlated with strong CD8 and CD4 T cell infiltration, whereas MHC II+ microglia recruited mainly CD4 T cells  (Sanchez-Guajardo et al., PLoSONE, 2010).  The first scenario was found associated with dopaminergic cell death, whereas the second one was independent of cell death but related to the presence of neurons expressing a-syn. In both cases striatal neurodegeneration continued, being more pronounced in the group with a strong T cell/CD68+ microglia response in SN.

One could thus envisage that depending on the type of microglia presenting antigen, which may be determined by the state of a-syn modification and aggregation that varies throughout pathology, different cytokines and co-stimulatory molecules will be elicited and these will direct the T cell response along different pathways. Indeed eliciting a CD4 T cell response can have many outcomes, as CD4 T cells can differentiate along many pathways and in turn specifically activate CD8 and B cells. The importance of CD4 regulation has been evidenced by Brochard and colleagues where deletion of CD4 T cells ameliorated pathology and reduced CD8 infiltration (Brochard et al., J Clin Invest, 2009) and by Reynolds et al. who have shown N-asyn elicits a detrimental CD4 effector T cell response and the abrogation of function of Tregs (Reynolds et al., J Immunology, 2010). Both groups have used the MPTP model where a-syn is not mishandled in the host's brain, so it is very interesting to determine the antigen priming the immune response and which cells are doing so, as it is unlikely that T cells will reach the brain without being previously primed. Indeed the fact that in postmortem PD brains has been observed IgG deposition but no B cell infiltration, points to lymphocyte priming taking place outside the brain (Orr et al., Brain, 2005).

Answering this questions is crucial to develop an immunoneurotherapy approach, as a functional T cell response will only be induced and maintained if the T cells receives simultaneously 3 signals from the APC: TCR-MHC complex interaction, co-stimulation and the appropriated survival/differentiating cytokines.  These T cells will then elicit immune responses specific only to the antigen they were primed for. Reynolds et al. have open up the possibility of manipulating this response, but it still remains unclear if it is mediated by the T cell population or by changing the immunological environment. Does the modulation happen in the periphery or do these cells migrate to the brain?  What induces the APC to elicit opposite immunological outcomes? These remain essential questions in order to specifically modulate an immune response which target is found in an organ to which the immune components normally do not have access.