CD31 Acts as a Checkpoint Molecule and Is Modulated by FcγR-Mediated Signaling in Monocytes

Monocytes and macrophages express FcgammaR that engage IgG immune complexes such as Ab-opsonized pathogens or cancer cells to destroy them by various mechanisms, including phagocytosis. FcgammaR-mediated phagocytosis is regulated by the concerted actions of activating FcgammaR and inhibitory receptors, such as FcgammaRIIb and SIRPalpha. In this study, we report that another ITIM-containing receptor, PECAM1/CD31, regulates FcgammaR function and is itself regulated by FcgammaR activation. First, quantitative RT-PCR and flow cytometry analyses revealed that human monocyte FcgammaR activation leads to a significant downregulation of CD31 expression, both at the message level and at surface expression, mainly mediated through FcgammaRIIa. Interestingly, the kinetics of downregulation between the two varied, with surface expression reducing earlier than the message. Experiments to analyze the mechanism behind this discrepancy revealed that the loss of surface expression was because of internalization, which depended predominantly on the PI3 kinase pathway and was independent of FcgammaR internalization. Finally, functional analyses showed that the downregulation of CD31 expression in monocytes by small interfering RNA enhanced FcgammaR-mediated phagocytic ability but have little effect on cytokine production. Together, these results suggest that CD31 acts as a checkpoint receptor that could be targeted to enhance FcgammaR functions in Ab-mediated therapies.