Particle digestibility is required for induction of the phosphatidylserine recognition mechanism used by murine macrophages to phagocytose apoptotic cells.

One of the characteristic features of programmed cell death in vivo is the rapid recognition and removal of apoptotic cells by macrophages. Although there are several potential mechanisms by which the macrophage can identify a cell as apoptotic, it has been shown recently that murine-elicited macrophages stereospecifically recognize phosphatidylserine (PS) exposed on the surface of apoptotic cells. The particulate stimulus, beta-1, 3-glucan, stimulates bone marrow-derived macrophages to express several characteristics of inflammatory macrophages, and induced these cells to recognize PS on apoptotic cells; this activity was correlated with the ability to form rosettes with PS-expressing RBC. Induction of PS recognition in bone marrow-derived macrophages was associated with digestibility of the stimulus, because L, but not D amino acid particles or latex, were able to stimulate macrophage recognition of PS. The requirement for digestibility could be bypassed by the addition of exogenous TGF-beta, which induced macrophage recognition of PS after stimulation with either latex or D amino acid particles. That endogenously produced TGF-beta played a role in the glucan-stimulated response was indicated by the ability of anti-TGF-beta antibodies to inhibit digestible particle-induced recognition of PS. The induction of the PS recognition mechanism correlated well with the expression of other markers for the inflammatory phenotype. These studies indicate that the PS receptor may be a marker for the inflammatory phenotype, which appears to be induced by the phagocytosis of particulate digestible stimuli. Endogenously produced TGF-beta is suggested to play an autocrine or paracrine priming role in the induction of the PS receptor.