CCL2 and Parkinson disease: Among the top enriched genes (Fig. 3; Table S2), we found some immune-related genes such as IL10, FOXP3, DEFB4A, CCL2 and CCR2. This suggests DA microbes in PD can potentially affect the immune system by modulating those genes involved in various types of immune responses, which is consistent with the finding in the original study that gut microbiota impact neuroimmune responses in a mouse model of PD.