Interaction between thymocytes and thymus-derived macrophages. II. Engulfment of thymocytes by macrophages

A high percentage (80-90%) of immature thymocytes were engulfed by syngeneic thymus-derived macrophages (TDMφ) following cocultivation for 3 days. Elimination occurred via internalization of thymocytes by the macrophages. We unequivocally demonstrated the presence of many live thymocytes inside the TDMφ by means of specific staining. Mature PNA^- thymocytes were phagocytized to a lower degree than immature thymocytes, and T splenocytes were not eliminated at all. Bone marrow-derived macrophages internalized immature thymocytes to a degree similar to TDMφ. Since thymocyte survival was not at all affected by Mφ culture supernatants alone, we conclude that cell to cell contact is necessary for thymocyte elimination. To identify the surface molecules which participate in internalization of thymocytes by the macrophages, both cell types were pretreated with a variety of agents. Treatment of thymocytes with tunicamycin (N-glycosylation inhibitor) and anti-Lyt-2 mAb decreased their elimination by Mφ. Similarly, treatment of Mφ with neuraminidase, trypsin, and anti-Ia mAb markedly suppressed their capacity to engulf thymocytes. On the other hand, thymocyte elimination was unaffected by (1) cell cultivation in syngeneic serum rather than heterologous serum; (2) use of allogeneic rather than syngeneic thymocytes; and (3) use of X-irradiated Mφ and LPS-activated Mφ rather than nontreated Mφ.