[Analysis of the murine autologous mixed lymphocyte reaction. I. Characterization of the stimulator cells and their recognition sites].

A stimulating activity in the SMLR was found in the mice of a week after birth at low but significant level and it became to be an adult level within only two to three weeks. A responding activity however remained very low until three to four weeks after birth. But this discrepant phenomenon could not be due to immaturation of the responder cells, since some T cell population was detected in the splenocytes of young mice which suppressed SMLR of adult mice. The responder cells of SMLR were T cells. On the other hand, stimulator populations of this reaction consisted of macrophages and B lymphocytes. Spleen adherent cells and peritoneal exudate cells showed a strong stimulating activity. But in the SMLR, highly purified B cells did not have a stimulating capacity, since removal of adherent populations from surface Ig(+) spleen cells entailed a loss of stimulating activity for responder T lymphocytes. With a help of macrophages, B lymphocytes acquired a stimulating activity which was easily blocked with anti-Ia antisera. Responder T lymphocytes recognized B lymphocytes across the MHC barrier in the secondary responses, whereas they were able to respond only to the histocompatible macrophages. Specific stimulation in the SMLR therefore was determined not by B cells, but by macrophages. Recognition sites of stimulators in the SMLR were presumed to be Ia antigens, especially products of I-A(B) subregions from the results of blocking assay with anti-Ia antisera and of secondary responses of the SMLR primed cells.