The presumptive phosphatidylserine receptor is dispensable for innate anti-inflammatory recognition and clearance of apoptotic cells

The role of the presumptive phosphatidylserine receptor (PSR) in the recognition and engulfment of apoptotic cells, and the antiinflammatory response they exert, has been of great interest. Genetic deficiency of PSR in the mouse is lethal perinatally, and results to date have been ambiguous with regard to the phagocytic and inflammatory phenotypes associated with that deficiency. Recently, we found that the specific functional recognition of apoptotic cells is a ubiquitous property of virtually all cell types, including mouse embryo fibroblasts, and reflects an innate immunity that discriminates live from effete cells. Taking advantage of this property of fibroblasts, we generated PSR+/+, PSR+/-, and PSR-/- fibroblast cell lines to examine definitively the involvement of PSR in apoptotic recognition and inflammatory modulation. Our data demonstrate that PSR-deficient cells are fully competent to recognize, engulf, and respond to apoptotic cells. Signal transduction in the responder cells, including the activation of Akt and Rac1, is unimpaired in the absence of PSR. We confirm as well that PSR is localized predominantly to the nucleus. However, it does not play a role in pro-inflammatory transcription or in the anti-inflammatory modulation of that transcriptional response triggered by apoptotic cells. We conclude that PSR is not involved generally in either specific innate recognition or engulfment of apoptotic cells.