Characterization of the stimulator cells in the murine primary mixed leukocyte response

Macrophage-enriched and depleted spleen cell populations were examined for their ability to stimulate a murine 1 degree mixed leukocyte response. A quantitative comparison of unfractionated spleen cells, Sephadex g-10 passed spleen cells, 4-hr glass adherent and 24-hr glass adherent spleen cells revealed that a potent 1 degree MLR stimulator cell was phagocytic and that T and B lymphocytes failed to stimulate a response. By using several inb;ed and recombinant mouse strains congenic for different subregions of the MHC, it was found that macrophage-enriched cells were capable of inducing an MLR against K, D, I-A, and I-E differences and against K and D mutants. Both macrophage-enriched and depleted spleen cells failed to stimulate responses when the stimulator and responder differed at I-J, I-C, S, or G. Moreover, MLR stimulation induced by K or D region differences alone was dependent upon stimulator cells that expressed Ia antigens. Therefore, it would appear that only a minor subpopulation of the cells that express the relevant MHC alloantigens is immunogenic in the murine 1 degree MLR.