Microglia in the crosstalk between peripheral and central nervous systems in Parkinson's disease

Parkinson's disease (PD) is increasingly recognized as a multisystem disorder involving pathological α-synuclein (α-syn) accumulation and widespread neuroimmune dysregulation. Microglia, the resident immune cells in the central nervous system (CNS), are pivotal mediators of the bidirectional communication between the CNS and peripheral systems. In addition to sensing neuronal injury and α-syn pathology, microglia dynamically respond to peripheral immune signals, including circulating cytokines, immune cell infiltration, and microbial metabolites, through pattern recognition receptors such as Toll-like and NOD-like receptors. Furthermore, microglia regulate blood-brain barrier integrity, modulate peripheral immune cell recruitment, interact with meningeal lymphatic vessels, and contribute to the propagation of α-syn within the CNS and along the gut-brain axis. However, a comprehensive framework encompassing their diverse roles in peripheral-central immune crosstalk remains underdeveloped. This review synthesizes recent advances elucidating how microglia link the CNS to peripheral immune and metabolic signals in PD. We further highlight microglial contributions to α-syn propagation along the gut-brain axis and discuss how their functional states influence disease progression. A deeper understanding of microglial involvement in this complex neuroimmune interface may inform the development of effective and system-level therapeutic strategies for PD.