Causal relationship between immune mediators and parkinson's disease: A Mendelian randomization analysis

Numerous studies have revealed a correlation between immune system dysfunction and Parkinson's disease (PD), yet the causal link between them remains unclear. To investigate the causal relationship between immune mediators and Parkinson's disease (PD), we conducted two independent Mendelian Randomization (MR) analyse using genetic variants associated with 731 immune cell phenotypes and 91 circulating inflammatory proteins as instrumental variables. The genetic variant data for immune cell phenotypes were derived from a genome-wide association study (GWAS) involving 3,757 individuals, while the genomic protein quantitative trait loci (pQTL) data for circulating inflammatory proteins were sourced from a GWAS dataset comprising 14,824 individuals of European descent. Additionally, we utilized PD risk data from a large meta-analysis of GWAS, which included 33,674 PD cases and 449,056 controls. Our primary analysis was conducted using the inverse-variance weighted (IVW) method, complemented by various Mendelian randomization (MR) approaches and sensitivity analyses to robustly assess the potential causal relationships between immune cell traits, inflammatory proteins, and PD risk. MR analysis reveals a significant causal link between 26 immune cell traits and the risk of PD. Of these, an increase in 15 traits, such as the proportion of resting CD4 regulatory T cells (OR = 1.039 [95% CI: 1.005-1.074], P = 0.024), is associated with a higher risk of PD. Conversely, an increase in 11 traits, such as the FSC-A of HLA DR + CD4 + T cells (OR = 0.934 [95% CI: 0.872-1.000], P = 0.0244), indicates a reduced risk of PD, a relationship that remains significant after false discovery rate (FDR) correction. Additionally, a significant causal relationship exists between four inflammatory proteins and PD risk, with two positively correlated and two negatively correlated with PD risk, all maintaining significance after FDR correction. This study reveals complex causal relationships between immune cells and inflammation-related proteins and the risk of Parkinson's disease (PD), emphasizing that even the same type of immune cell may play different roles at various stages of PD development.