Heterogeneity of splenic natural suppressor cells induced in mice by treatment with cyclophosphamide

Administration of high dose cyclophosphamide (CY, 200 mg/kg body weight) to adult mice induces transient, nonspecific suppressor activity in the spleen of treated animals. Characterization of the CY-induced natural suppressor (NS) cells which inhibit mixed lymphocyte reactions revealed a heterogenous population of lymphocytes expressing the CD8 T cell marker and the B220 B cell marker, as well as cells bearing the granulocyte-monocyte marker CD11b. On a cell per cell basis the most potent of these suppressors were found to be positive for CD11b. Inhibitory activity was also detected in the CD8^-, CD11b^-, B220^- compartment of CY-spleen, suggesting the presence of null NS cells. The fact that several phenotypically distinct cell populations contribute to the overall inhibitory effect of CY-spleen cells indicates that natural suppression defines an activity rather than a specific cell type. Interestingly, NS activity was observed to reside solely within the fraction of CY-spleen that is agglutinable with soybean agglutinin or wheat germ agglutinin, suggesting that expression of receptors for these plant lectins is a universal characteristic of CY-induced NS cells, regardless of their lineage. CY-spleen cell-mediated suppression of lymphoproliferative responses was found to be partially dependent on DNA synthesis and totally dependent on protein synthesis, but did not require cell-cell contact, indicating the production of soluble suppressor factor(s).