The macrophage in the antibody-mediated suppression of the humoral response.

Abstract

Inbred Balb/c mice were used throughout the experiments. Highly purified IgM-type and IgG-type anti-sheep red blood cell (SRBC) antibodies were used. <sup>51</sup>Cr-labelled SRBC were incubated with antibodies at different antigen-antibody ratios. Thioglycollate-induced peritoneal macrophages were incubated with SRBC or antibody-complexed SRBC and injected into syngeneic animals. The number of macrophages varied within groups, the radioactivity being, however, constant. The antibody response was quantitated by the Jerne plaque technique at different intervals after transfer. Macrophage-associated antigen triggers a significant immune response. IgG-complexed SRBC given in macrophages evoke only a marginal response even when only a minority of available antigenic determinants are antibody-complexed. The immune response after transfer of IgM-complexed macrophage-associated antigen is suppressed when tested on the 4th day; but on the 7th day, a moderate reaction is discernible. Suppression was observed also when antigen was given in macrophages and antibody intravenously. Suppression was dependent on the antibody dose injected or used for complexing with antigen, particularly in the IgM system. The degradation of engulfed Ag-Ab complexes proceeds in macrophages more efficiently than in free antigen. Two possible mechanisms of antibody-mediated suppression are discussed, i.e., blocking of antigen in macrophages by antibody or increased decay of Ab-Ag complexes in the lysosomal system.