Normal mouse serum immunosuppressive activity: action on adherent cells.

The immunosuppressive activity of normal mouse serum (NMS) was evaluated by enumeration of primary in vitro plaque-forming cell response produced by normal spleen cell suspensions in response to sheep erythrocytes. The site of action of the inhibitor(s) was shown to be at the level of adherent cells since incubation of this cell type with NMS could reproducibly inhibit responses, whereas, in appropriate experiments, incubation of nonadherent cells with NMS had no effect. Furthermore, addition to normal spleen cells of excess spleen-adherent cells, normal peritoneal adherent cells, or of 2-mercaptoethanol could abrogate the inhibitory ability of NMS. Addition of excess nonadherent cells under the same conditions had no influence on inhibition. NMS was also shown to decrease the number of cell clusters, hemolytic clusters, and the number of nucleated cells recovered upon culture termination. It is suggested that NMS, acting on the adherent cell may hinder cell-to-cell contact and interactions.