Further studies of antigen stimulation of deoxyribonucleic acid synthesis in rabbit spleen cell cultures. II. The effects of specific antibody.

Insoluble immune complexes, prepared with heterologous protein antigens and homologous antibodies, affected the incorporation of [H]thymidine into spleen cell suspension from rabbits immunized to these proteins. Low concentrations of such complexes stimulated thymidine uptake, this being antigen specific, while high concentrations impaired the response to antigen. The inhibitory effect of high concentrations of immune complexes was found to be non-specific as far as the responses to antigens were concerned, but the PHA-responsiveness of rabbit spleen cells was not significantly impaired by the presence of such complexes. When macrophages from the peritoneal cavities of normal rabbits were incubated with insoluble immune complexes and then washed, such cells were able to stimulate an antigen-specific response by spleen cells from immunized rabbits. It was concluded that, in these experiments, the failure of spleen cells from immunized rabbits to respond to antigen in the presence of immune complexes was the result of a direct inhibition of the mechanism of proliferation by these complexes. Inhibition of the response to antigen was also produced by the presence of free specific antibodies in the culture medium, even in excess of antigen. It was postulated that, in this situation, antibody reacted with antigen present on the surfaces of cells, particularly macrophages, thus blocking the stimulatory mechanism.