The contribution of a serum component(s) modified by B cells to the mechanism for macrophage activation by liposomes.
The effects of liposomes on the activation of mouse peritoneal macrophages were investigated in vitro by measuring the phagocytosis of opsonized sheep red blood cells (SRBC) via Fc receptors on the surface of macrophages. The addition of liposomes to mouse peritoneal exudate cells in RPMI-1640 medium with 10% fetal calf serum (FCS) caused an increase in the ingestion activity. In the absence of FCS, this increase was not observed, suggesting that some component(s) present in FCS is the causative factor(s) for this activation. When liposomes were added to the macrophage monolayer, the ingestion activity did not increase, showing that liposomes did not activate peritoneal macrophages directly, and that non-adherent cells may be involved. Following addition of the culture medium (conditioned medium), which was prepared by incubation of FCS with liposome-treated non-adherent cells or cell ghosts, to the macrophage monolayer, the ingestion activity of macrophages increased in either case. When the conditioned medium prepared with liposome-treated B cells was used for cultivation of liposome-untreated macrophages, a markedly enhanced ingestion activity was observed. However, the conditioned medium prepared with liposome-treated T cells did not affect the ingestion activity. These findings demonstrate that liposome-activated B cells modify some component(s) in FCS, and that the modified component(s) subsequently activates the phagocytosis of opsonized SRBC via Fc receptors on macrophages.