Tim-4 internalizes apoptotic cells through Axl-mediated signal transduction

Abstract A pivotal step in phagocytosis of apoptotic cells, called efferocytosis, is phagocytic recognition of phosphatidylserine (PS) on dying cells, primarily via PS receptors. Among them, Tim-4 has been studied to elucidate signaling since its identification. However, how apoptotic cells secured by Tim-4 are internalized remains still unclear. Here, we reveal that Axl-mediated signal transduction is necessary for Tim-4 to ingest apoptotic cells. We found correlated expression patterns for Tim-4 and Axl across various tissues, and biochemical and cell biological analyses showed that Tim-4 interacted most strongly with Axl among the TAM receptor family. Mechanistically, the immunoglobulin variable (IgV) domain of Tim-4 interacted with two distinct domains within the extracellular region of Axl. Co-expression of Tim-4 and Axl synergistically promoted Tim-4-mediated efferocytosis. Crucially, disrupting the interaction or pharmacologically or genetically blocking Axl signaling abolished the synergistic effect of Axl on Tim-4-mediated efferocytosis. These findings collectively demonstrate that during Tim-4-mediated efferocytosis, Axl acts as a key signaling relay, biochemically interacting with Tim-4 to transmit apoptotic cell recognition signals into phagocytes, thereby enabling efficient apoptotic cell ingestion.