Enhancing effect of radioresistant spleen cells on the primary immune response against sheep RBC by mouse spleen cells in vitro.

Irradiated spleen cells cultured for 3 days, caused a stimulation of the primary in vitro immune response by normal spleen cells. These irradiated spleen cells were fractionated by velocity sedimentation and the fractions were tested for their stimulating activity. Only the macrophage enriched fractions were found to cause stimulation. The macrophages in these fractions were stuffed with erythrocytes and dead cells. The fractions enriched in thymus derived cells, had no effect on the immune response. Irradiated spleen cells cultured for 24 hours caused inhibition. It has not yet been determined whether this inhibition was due to some transient change in the macrophage population during incubation. The stimulating effect by the irradiated spleen cells from SPF mice was strongly reduced, which at least partly could be ascribed to the naturally occurring low number of macrophages in the spleens of these mice.