Helper T Cell-Replacing Factors Secreted by Thymus-Derived Cells and Macrophages: Cellular Requirements for B Cell Activation and Synergistic Properties

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Abstract
The biologic activities of helper T cell-replacing factors derived from concanavalin A-stimulated murine T cells (TRF-T) and from lipopolysaccharide-activated macrophages (TRF-M) have been compared. TRF-T stimulates immune responses to heterologous erythrocyte antigens (SRBC and BRBC) in T cell-depleted spleen cultures but not in macrophage-depleted spleen cultures. TRF-M stimulates immune responses in both T cell-depleted and macrophage-depleted spleen cultures. Under conditions where LPS stimulates the release of TRF-M from cultures of activated macrophages, TRF-T has no effect on TRF-M production. Thus, TRF-T does not appear to function by stimulating the release of TRF-M from macrophages. In macrophage-depleted spleen cultures, saturating concentrations of TRF-T and TRF-M when mixed together exhibit striking synergistic effects on the induction of immune responses to erythrocyte antigens. The kinetics of the synergistic effects of TRF-M and TRF-T are consistent with an effect of TRF-M on the production of TRF-T sensitive B cells.