The role of Axl/Mertk/Tyro3 receptor tyrosine kinases in the clearance of apoptotic cells

The phagocytosis of apoptotic cells is critical for maintaining normal development, tissue homeostasis, and lymphocyte maturation. While this phenomenon is well documented, a limited number of receptors have been identified and mechanisms have only been partly elaborated. We hypothesized that the Axl/Mertk/Tyro3 family of receptor tyrosine kinases are involved in the clearance of apoptotic cells by immune cells. Here we demonstrate that in addition to Mertk, Axl and Tyro3 are required for efficient phagocytosis of apoptotic cells in macrophages and dendritic cells in vitro. Macrophages rely primarily on Mertk and less on Axl and Tyro3; whereas in contrast, dendritic cells rely on Axl and Tyro3 rather than Mertk for engulfment of dying cells. The difference between macrophage and dendritic cell phagocytosis can be partly explained by ligand and receptor expression levels. In vivo, Mertk is critical for clearance of apoptotic cells in the thymus and retina, whereas in the spleen Axl and Tyro3 are most important for engulfment. Support for cooperation of the receptors in signaling can be found in data showing no detectable phosphorylation of Mertk in axl/tyro3-/- macrophages. This phosphorylation is thought to be important for downstream signaling of Vav1 and FAK to rearrange actin cytoskeleton or phagocytosis and migration. Indeed in the absence of tyro3 or axl/tyro3, there is lower association of Mertk with Vav1, and lower phosphorylation of FAK in the absence of tyro3. Although our data suggests an interaction between the three receptors in this family, no interaction was detectable in primary cells or transiently transfected cell lines. The phagocytosis of apoptotic cells is thought to down regulate proinflammatory cytokines; however, in the absence of Mertk, Axl, and/or Tyro3, no defect in the down regulation of inflammatory cytokines in macrophages was observed when compared with wild type. Therefore, we conclude that usage of Axl, Mertk, and Tyro3 is dependent on cell type and is critical for efficient phagocytosis both in vitro and in vivo, however the downstream signaling initiated by these receptors is limited to engulfment, and does not include the down regulation of inflammatory cytokines.