C1q-CD44 interactions regulate microglial phagocytosis, proliferation, and migration.

Microglia, the immune cells of the central nervous system (CNS), quickly respond to neurodegeneration by proliferating and migrating to areas of disease, phagocytosing debris, and releasing cytokines to initiate inflammation. Critically, the mechanisms underlying these microglial functions remain only partly understood. One molecular regulator of interest is complement protein C1q, the initiator molecule of the complement cascade that increases 300-fold in healthy aging and accumulates with neurodegeneration. We have previously reported that exogenous C1q treatment alters inflammatory gene expression and cell function in human induced pluripotent stem cell-derived microglia (iMG). Here, we test the hypothesis that C1q induced cell changes are modulated by novel C1q receptor, CD44. We first confirmed expression of five novel C1q receptors at the RNA and protein levels, and then validated C1q-receptor binding on the iMG cell surface using proximity ligation assay. Based on these results, we selected CD44 as an initial target and generated CD44 knockout iMG to test the role of CD44 in the iMG response to C1q. We demonstrate that C1q-CD44 interactions regulate changes in microglial phagocytosis, proliferation, and migration. These data suggest C1q interacts with CD44 to modulate microglial functions that are critical to health and disease, thus informing future directions to test whether these interactions are altered in neurodegenerative disease.