Murine CD93 (C1qRp) Contributes to the Removal of Apoptotic Cells In Vivo but Is Not Required for C1q-Mediated Enhancement of Phagocytosis

Human CD93 (known as C1qRp) has been shown to be a phagocytic receptor involved in the in vitro C1q-dependent enhancement of phagocytosis. However, binding of CD93 to C1q and its function remain controversial. In this study, we have generated CD93-deficient mice ( CD93 −/− ) to investigate its biological role(s). The CD93 −/− mice were viable and showed no gross abnormalities in their development. Thioglycolate-elicited peritoneal macrophages deficient in CD93 showed a similar enhancement in complement- and FcγR-dependent uptake of RBC to the wild-type macrophages when plated on C1q-coated surfaces suggesting that the lack of this receptor had no effect on these C1q-mediated events. There was no impairment in either complement- or FcγR-dependent phagocytic assays in vivo. By contrast, the CD93 −/− mice had a significant phagocytic defect in the clearance of apoptotic cells in vivo (human Jurkat T cells and murine thymocytes: p = 0.0006 and p = 0.0079, respectively) compared with strain-matched controls. However, in vitro, the CD93 −/− macrophages showed similar engulfment of apoptotic cells to wild-type macrophages. Furthermore, no supporting evidence for a role of CD93 as an adhesion molecule was found using intravital microscopy or analyzing peritoneal cell recruitment in response to three different inflammatory stimuli (thioglycolate, zymosan A, and IL-1β). Thus, our findings indicate that murine CD93 is expressed on the peritoneal macrophage, especially on thioglycolate-elicited cells, but does not appear to play a key role in C1q-mediated enhancement of phagocytosis or in the intercellular adhesion events tested. However, our results suggest that it may contribute to the in vivo clearance of dying cells.