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 used validated expression of five recently identified C1q receptors at the RNA and protein levels, and then we used proximity ligation assay to validate C1q-receptor binding on the iMG cell surface. CD44 was selected as an initial target and thus CD44 knockout iMG were generated to test whether the C1q response is dependent on CD44. While the C1q-induced inflammatory response was not dependent on CD44, we demonstrate that C1q-CD44 interactions regulate changes in microglial phagocytosis, proliferation, and migration. These data suggest C1q interacts with CD44 on iMG to modulate microglial functions that are critical to health and disease. This data informs future work which will test how C1q-CD44 interactions are altered in neurodegenerative disease and if these interactions could be modulated as a therapeutic target.