The Presumptive Phosphatidylserine Receptor Is Dispensable for Innate Anti-inflammatory Recognition and Clearance of Apoptotic Cells

Justin E Mitchell,Marija Cvetanovic,Nitu Tibrewal,Vimal Patel,Oscar R Colamonici,Ming O Li,Richard A Flavell,Jerrold S Levine,Raymond B Birge,David S Ucker

doi:10.1074/jbc.m509775200

Abstract

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.

Highlights

Numerous cellular alterations associated with apoptotic cell death have been described, including plasma membrane reorganization asso
We have found that the specific functional recognition of apoptotic cells is a ubiquitous property of virtually all cell types, including non-phagocytic lymphocytes, and reflects an innate immunity that discriminates live from effete cells without regard to self (2).[5]
Taking advantage of the ability of non-professional phagocytes to recognize, engulf, and respond to apoptotic cells,5 we established immortalized 3T3 fibroblast cells lines from PSRϩ/ϩ, PSRϩ/Ϫ, and PSRϪ/Ϫ mouse embryo fibroblasts to test the role of PS-specific receptor (PSR) in these processes

Summary

PSR Is Dispensable for Apoptotic Cell Recognition

Growth retardation, defects in embryonic lung, kidney, gut, and erythroid development, and, at low frequency, aberrant eye and brain development. Kunisaki et al (14) suggested that phagocytosis of apoptotic cells by macrophages was diminished during embryogenesis, based on immunohistochemical staining of fetal liver and thymus sections of their PSRϪ/Ϫ mice Their data are confounded, by an apparent reduction in the number of tissue-resident macrophages (stained with F4/80 antibody; Ref. 16), hinting at another possible developmental impairment related to PSR deficiency. In marked contrast to the results of Li et al (13) and Kunisaki et al (14), extensive histological analysis of numerous tissues by Bose et al (15) revealed no defects in the phagocytosis of apoptotic (TUNELϩ) cells Their in vitro phagocytosis studies with fetal liverderived macrophages (differentiated in culture) showed no engulfment defect of PSR-deficient macrophages. Our data, which demonstrate that PSRdeficient cells are fully competent to recognize apoptotic cells, definitively exclude a general role for PSR in apoptotic recognition and inflammatory modulation

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION