Mapping the peripheral immune landscape of Parkinson's disease patients with single-cell sequencing

Parkinson's Disease (PD) is widely recognized for its impact on the central nervous system. Recent breakthroughs underscore the crucial role of interactions between central and peripheral systems in PD's pathogenesis, highlighting the need for a paradigm shift in PD research. The spotlight is now shifting as we explore beyond the central nervous system, discovering that peripheral changes such as inflammatory dysfunctions may predict the rate of disease progression and severity. However, the cellular mechanisms driving these immunity changes remain largely unknown. Despite over 200 years of research on PD, robust diagnostic or progression biomarkers and disease altering therapeutics are still lacking. Thus, understanding peripheral immune signatures could lead to earlier diagnosis and more effective treatments for PD. Here, we sought to define the transcriptomic alterations of the complete peripheral immune cell compartment by single-cell RNA- and T-cell receptor-sequencing with hopes of uncovering PD signatures and potential peripheral blood biomarkers. Following transcriptional profiling of 78 876 cells from 10 healthy controls and 14 PD donors, we observed five major classes of immune cell types; myeloid (monocytes, dendritic cells) and lymphoid (T, B, natural killer) cells from which we identified 38 cellular subtypes following bioinformatic re-clustering. Comparing immune cell subtypes and phenotypes between PD patients and healthy controls revealed notable features of PD: 1) a significant shift of classical CD14+ monocytes towards an activated CD14+/CD83+ state, 2) changes in lymphocyte subtypes abundance, including a significant decrease in CD4+ naive and mucosal-associated invariant T-cells subtypes, along with an increase in CD56+ natural killer cells, 3) the identification by T-cell receptor sequencing of several PD specific T-cell clones shared between multiple patients, suggesting the implication of common epitopes in PD pathogenesis, 4) a notable increase in the expression of activation signature genes, including the AP-1 stress-response transcription factor complex, across all PD cell types. This signal was not present in atypical Parkinsonism patients with multiple systems atrophy or progressive supranuclear palsy. Overall, we present a comprehensive atlas of peripheral blood mononuclear cells from control and PD patients which should serve as a tool to improve our understanding of the role the immune cell landscape plays in PD pathogenesis.