Mechanism of removal of senescent cells by human macrophages in situ.

Mechanisms by which macrophages discriminate between "mature-self" and "senescent-self" were investigated using the human red blood cell (RBC) system as a model. Conditions simulating those encountered in situ were adhered to as closely as possible by using short term culture techniques and incubating with autologous macrophages and immunoglobulins (Ig). It was found that RBC aged in vitro were phagocytized when they were incubated in pooled, normal human IgG, allogeneic Ig, autologous IgG or Ig, washed with medium, and then incubated with autologous macrophages. RBC treated in the same way but incubated in IgM or Iga, Ig-depleter serum, or Medium 199 alone were not phagocytized. This indicates that Ig is required for phagocytosis and suggests that the Ig which attaches to RBC is IgG. When freshly drawn RBC were separated into young (Y) and old (O) RBC according to density and incubated with autologous macrophages, less than 5% of the Y-RBC were phagocytized, whereas greater than 30% of the O-RBC were phagocytized, independent of whether the final incubations were performed in medium without serum (Y-RBC, 5 +/- 2% phagocytized; O-RBC, 33 +/- 1.5%), or autologous Ig-depleted serum (Y-RBC, 2 +/- 2.5%; O-RBC, 51 +/- 17%), or whole serum (Y-RBC, 0%; O-RBC, 43 +/- 5%). This indicates that (1) the Ig is attached in situ to the old RBC, and (2) that phagocytic recognition is not inhibited by other serum components. Scanning electron microscopy, employing labeled anti-IgG, IgM, and IgA reagents, revealed that Y-RBC had essentially no Ig on their surface, whereas old RBC had IgG on their surface. These findings indicate that IgG attaches in situ to senescent human RBC, making them vulnerable to phagocytosis by macrophages.