Synuclein vaccination modulates regulatory T cell activation and microglia in the absence of brain pathology

Background Passive and active immunization with [alpha]-synuclein has been shown to be neuroprotective in animal models of Parkinson's disease. We have previously shown that vaccination with [alpha]-synuclein, long before [alpha]-synuclein-induced brain pathology, prevents striatal degeneration by inducing regulatory T cell infiltration in parenchyma and antibody deposition on [alpha]-synuclein overexpressing neurons. However, the effect of peripheral [alpha]-synuclein on the immune system is unknown, as are the mechanistic changes induced in the CD4 T cell population during successful neuroprotective animal studies. We have studied the changes induced by vaccination with [alpha]-synuclein in the CD4 T cell pool and its impact on brain microglia to understand the immune mechanisms behind successful vaccination strategies in Parkinson's disease animal models. Methods Mice were immunized with WT or nitrated [alpha]-synuclein at a dose equivalent to the one used in our previous successful vaccination strategy and at a higher dose to determine potential dose-dependent effects. Animals were re-vaccinated 4 weeks after and sacrificed 5 days later. These studies were conducted in naive animals in the absence of human [alpha]-synuclein expression. Results The CD4 T cell response was modulated by [alpha]-synuclein in a dose-dependent manner, in particular the regulatory T cell population. Low-dose [alpha]-synuclein induced expansion of naive (Foxp3 + CCR6-CD127lo/neg) and dopamine receptor type D3+ regulatory T cells, as well as an increase in Stat5 protein levels. On the other hand, high dose promoted activation of regulatory T cells (Foxp3CCR6 + CD127lo/neg), which were dopamine receptor D2+D3-, and induced up-regulation of Stat5 and production of anti-[alpha]-synuclein antibodies. These effects were specific to the variant of [alpha]-synuclein used as the pathology-associated nitrated form induced distinct effects at both doses. The changes observed in the periphery after vaccination with low-dose [alpha]-synuclein correlated with an increase in CD154+, CD103+, and CD54+ microglia and the reduction of CD200R+ microglia. This resulted in the induction of a polarized tolerogenic microglia population that was CD200R-CD54CD103CD172a+ (82 % of total microglia). Conclusions We have shown for the first time the mechanisms behind [alpha]-synuclein vaccination and, importantly, how we can modulate microglia's phenotype by regulating the CD4 T cell pool, thus shedding inv