We have investigated the expression of Ia antigens on accessory cells which function in two murine response systems: the proliferative response to Con A and the in vitro primary antibody response to soluble TNP-conjugated protein antigens. Pretreatment of murine lymphoid cells with anti-Ia and complement (C) abrogated their ability to respond to Con A. Reconstruction experiments have shown that glass adherent spleen cells treated with a T cell-specific rabbit anti-mouse brain serum and C and irradiated with 1000R (spleen-adherent cells, SAC) were able to restore responsiveness to Con A. In a second assay system, T cell-dependent IgM responses of normal spleen cells to soluble TNP-conjugated protein antigens were abrogated by Sephadex G-10 passage. Responses in this system were similarly reconstituted by the addition of non-T, non-B, radioresistant SAC as accessory cells.

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