Species‐Nonspecific Action of Soluble Immunosuppressive Factor Produced by Murine Immature Erythroid Cells

Abstract

Immature murine erythroid cells have natural suppressor cell activity suppressing in vitro mouse plaque-forming cell response and LPS-stimulated proliferative response of mouse spleen cells. A cell population enriched by erythroid precursors can produce a soluble activity with similar suppressive abilities. This erythroid suppressor activity (ErSF) has a low Mr (1 to 10 kDa). It can suppress both IgM and IgG secretion of PWM-stimulated human B cells and inhibit the proliferative response of PWM-stimulated human B lymphocytes, thereby indicating a species-nonspecific type of action. Antigen-induced mouse PFC response and mitogen-stimulated mouse and human B cell proliferation were inhibited dose dependently by the addition of ErSF, which did not significantly reduce cell viability, however. Kinetic studies showed that ErSF suppressed Ig secretion and proliferation of mitogen-stimulated B lymphocytes in all culture periods. The suppressive effect of ErSF on B cell proliferation was indirect, requiring the presence of IL-2 (but not of IL-1 alpha, IL-1 beta, or BCGF). In the presence of IL-2, the proliferative response of SAC-stimulated B cells was suppressed by ErSF when added at all time intervals studied but was maximal when ErSF was added on the third day of culture. The role of suppressor factor produced by immature murine erythroid cells in the phenomenon of "natural suppression" has been discussed.