Effect of increase in macrophage Ia expression on subsequent immune response in vivo

By the use of a culture supernatant containing a lymphokine capable of inducing Ia-rich peritoneal macrophages in vivo, we investigated the effect of increased macrophages Ia expression on the induction of antibody response to sheep erythrocytes (SRBC) in mice. Peritoneal macrophages from mice injected intraperitoneally (i.p.) with lymphokine 3 and 4 days earlier contained a high proportion of Ia-bearing macrophages without any significant increase in their phagocytic activity for SRBC. In lymphokine-treated mice, slight enhancement was observed in the primary IgG plaque-forming cell (PFC) response only at an early stage after i.p. immunization with SRBC. When primed mice were injected with lymphokine before secondary i.p. immunization with SRBC, the secondary IgG PFC response was greatly enhanced. On the other hand, lymphokine treatment did not affect either the primary or the secondary IgM PFC response. This effect was dependent on both timing of lymphokine injection and route of secondary immunization with SRBC. Secondary immunization via several routes other than the i.p. route never resulted in such an enhanced secondary IgG PFC response. These results suggest that lymphokine-induced Ia-rich peritoneal macrophages, which first encounter the antigen in the peritoneal cavity, play an important role in the modification of the subsequent antibody response.