Peripheral inflammation and neuroinflammation can result in neurodegeneration and parkinsonism. There is recent evidence that immune responses – especially monocytic cytokine secretion patterns – in Parkinson’s Disease (PD) patients are dysregulated. It is known that serum extracellular vesicles (EVs) can induce proinflammatory reactions of immune cells. Moreover, PD EVs seem to carry an altered cargo – specifically containing higher levels of α-synuclein. Interestingly, α-synuclein pathology seems to act synergistically with monocytic dysregulation in PD to trigger excessive inflammatory responses. As it is known that monocytes can contribute to PD neurodegeneration and immune dysregulation plays an early event in PD, we investigated the predisposition of primary human monocytes of healthy donors to pathologic PD serum EVs by assessing release of proinflammatory interleukin-6 (IL-6) upon EV exposure. We isolated serum EVs of healthy controls (HCs) and PD patients by differential ultracentrifugation (UC) and checked the quality and purity of our EVs by Nanoparticle Tracking Analysis (NTA) and Western Blot (WB). NTA as well as WB proved our EV isolation protocol to be not only effective but also providing high purity. Prior to treatment, we isolated human monocytes of young as well as aged healthy donors by positive, CD14-based selection of monocytes from peripheral blood mononuclear cells. Monocytes were then stimulated with lipopolysaccharide (LPS) as a positive control and with HC EVs as well as with PD EVs. To rule out any unspecific activation of monocytes during isolation or cultivation and to quantify basal IL-6 release, negative controls with only unconditioned media were performed. We could show that treatment with pooled HC and PD EVs results in an inflammatory activation of young as well as aged human monocytes. The activation shows widespread interindividual differences. We also found that young monocytes seem to be more suitable for detecting slight changes in cytokine secretion as they show a more distinct cytokine secretion pattern. Young monocytes react to triggers such as LPS with a higher increase in IL-6 secretion but show a lower basal cytokine secretion rate than aged monocytes. Most importantly, we could demonstrate that there is a predisposition of some healthy donors’ monocytes to react excessively to PD EVs. Our findings indicate that this predisposition is more likely to be enriched in young healthy donors’ monocytes than in those of aged healthy individuals. On average, no differences in monocytic activation upon stimulation with HC and PD EVs could be observed – neither in young nor in aged monocytes. Furthermore, we could show that monocytic activation by HC as well as PD EVs is not only influenced by monocytic vulnerability but also by the composition of EVs. Stimulating young healthy donors’ monocytes with individual EVs showed that EVs that can increase cytokine secretion in one donor do not necessarily boost another donors’ immune response. The opposite may be even the case as we could show that some donors’ EVs do not influence or even inhibit monocytic IL-6 release. Taken together, our data suggest that there are indeed ‘responders’ among healthy individuals to pathologic PD EVs as some healthy donors’ monocytes seemingly reacted to PD EVs with an increased IL-6 release. Additionally, our data prompt that pathologic PD EVs and dysregulation of PD monocytes could potentiate each other and could play a detrimental role in initiating and maintaining neuroinflammatory processes in PD
