Phospholipid flippases enable precursor B cells to flee engulfment by macrophages.

Abstract

ATP11A and ATP11C, members of the P4-ATPases, are flippases that translocate phosphatidylserine (PtdSer) from the outer to inner leaflet of the plasma membrane. Using the W3 T lymphoma cell line, we found that Ca²⁺ ionophore-induced phospholipid scrambling caused prolonged PtdSer exposure in cells lacking both the <i>ATP11A</i> and <i>ATP11C</i> genes. <i>ATP11C</i>-null (<i>ATP11C</i>^<i>-/y</i> ) mutant mice exhibit severe B-cell deficiency. In wild-type mice, ATP11C was expressed at all B-cell developmental stages, while ATP11A was not expressed after pro-B-cell stages, indicating that <i>ATP11C</i>^<i>-/y</i> early B-cell progenitors lacked plasma membrane flippases. The receptor kinases MerTK and Axl are known to be essential for the PtdSer-mediated engulfment of apoptotic cells by macrophages. <i>MerTK</i>^<i>-/-</i> and <i>Axl</i>^<i>-/-</i> double deficiency fully rescued the lymphopenia in the <i>ATP11C</i>^<i>-/y</i> bone marrow. Many of the rescued <i>ATP11C</i>^<i>-/y</i> pre-B and immature B cells exposed PtdSer, and these cells were engulfed alive by wild-type peritoneal macrophages, in a PtdSer-dependent manner. These results indicate that ATP11A and ATP11C in precursor B cells are essential for rapidly internalizing PtdSer from the cell surface to prevent the cells' engulfment by macrophages.