Lack of neuroprotection after systemic administration of the soluble TNF inhibitor XPro1595 in an rAAV6-α-Syn + PFFs-induced rat model for Parkinson's disease

Parkinson's disease (PD) is characterized by dopaminergic neurodegeneration, α-Synuclein (α-Syn) pathology, and inflammation. Microglia in the substantia nigra pars compacta (SNpc) upregulate major histocompatibility complex class II (MHCII), and variants in genes encoding MHCII affect PD risk. Additionally, elevated TNF levels and α-Syn-reactive T cells in circulation suggest a strong link between innate and adaptive immune responses in PD. We have previously reported that reduced levels of the class II transactivator, the master regulator of MHCII expression, increases susceptibility to α-Syn-induced PD-like pathology in rats and are associated with higher serum levels of soluble TNF (sTNF). Here, we demonstrate that inhibiting sTNF with a dominant-negative TNF variant, XPro1595, known to be neuroprotective in endotoxin- and toxin-induced neurodegeneration models, fails to protect against robust α-Syn-induced PD-like pathology in rats. We used a model combining rAAV-mediated α-Syn overexpression in SNpc with striatal injection of α-Syn preformed fibrils two weeks later. Systemic XPro1595 treatment was initiated one-week post-rAAV-α-Syn. We observed up to 70 % loss of striatal dopaminergic fibers without treatment, and no protective effects on dopaminergic neurodegeneration after XPro1595 administration. Pathological α-Syn levels as well as microglial and astrocytic activation were not reduced in SNpc or striatum following XPro1595 treatment. An increase in IL-6 and IL-1β levels in CSF was observed in rats treated with XPro1595, possibly explaining a lack of protective effects following treatment. Our results highlight the need to determine the importance of timing of treatment initiation, which is crucial for future applications of sTNF therapies in PD patients.