Single-cell RNA sequencing reveals peripheral immunological responses and cell-specific biomarker in patients with Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease characterized by bradycardia, static tremor, rigidity and postural instability. At present, the current knowledge remains inadequate for the connection between the disease severity and peripheral immune response. In the present study, using single-cell RNA sequencing (scRNA-seq), we profiled peripheral blood mononuclear cells (PBMCs) from clinical cases, including 2 early PD, 2 advance PD and 2 matched controls, representing five major immune cell subsets: T cells (CD4+, CD8+), monocytes, B cells, natural killer (NK) cells and hematopoietic stem cells (HSCs). The characteristic alterations of cell subset proportions were disentangled, in which T cells and NK cells were declined in the progression of PD. We further enlarged the sample size and collected peripheral blood samples of 14 normal individuals, 21 early and 14 advance PD patients. T cells and NK cells showed an obvious reduction in the blood samples as the disease proceeded. Moreover, NK cells infiltration into cerebral motor cortex indicated by immunostaining of human brain sections evidenced the close communication existing between peripheral immune response and central nervous system. Importantly, NK cells participated in numerous common immune-related biological processes. Strikingly, NK cell-specific XCL2 that was specifically expressed in cluster 6 was further identified and positively correlated with the PD severity. All these findings delineated the critical role of peripheral adaptive immune response, especially mediated by NK cells, in the pathogenesis of PD, providing clues of immune-related biomarkers or therapeutic targets during PD progression in future investigations. Overall design: In this study, we presented 5 major immune cell subsets, T cells (CD4+, CD8+), B cells, NK cells, monocytes and HSCs, and displayed the distinct proportion of cell subsets and different gene expression patterns in the progression of PD by scRNA-seq analysis. Among these cell subsets, T cells and NK cells were found decreased as the disease proceeded, which was evidenced by the decreased percentage of T cells and NK cells in PD patients with the increase of severity with the clinical sample size enlarged. Moreover, the NK cell type-specific key genes were proved to be involved in PD related signal transduction and immune regulation. Correlation analysis revealed the number of NK cells was negatively correlated with the severity of PD. Strikingly, NK cells were found expressed in the motor cortex of human brain sections from PD patients. Importantly, NK cell-specific crucial gene was identified in the present study (XCL2), which might present great potentials as PD-associated biomarkers.