Requirement for accessory cells in the antibody response to T cell‐independent antigens in vitro

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Abstract
The antibody responses of mouse spleen cells in vitro to three thymus‐independent (TI) antigens namely, polymeric flagellin (POL) of Salmonella adelaide, DNP‐Ficoll and “soluble” sheep erythrocyte (SRBC) antigen, were found to be dependent on adherent accessory cells (A cells) but to a lesser degree than the response to intact SRBC. Evidence for this comes from selective depletion of A cells from spleen cells and reconstitution with A cell‐rich populations. Thus, depletion of A cells by adherence on glass resulted in abolition of the response to SRBC leaving the response to POL intact. More thorough removal of A cells by treatment with carbonyl iron powder was required for appreciable reduction of the responses to POL, DNP‐Ficoll and “soluble” SRBC. This reduction in responsiveness was not due to poor cell survival after A cell depletion or to the loss of immunocompetent cells since 1) the recoveries of viable cells in all cultures were similar; 2) the contents of Θ‐ and Ig‐bearing cells and tritium‐labeled POL‐binding cells were unaltered after carbonyl iron treatment, and 3) responsiveness was fully restored by the addition of irradiated and anti‐Θ‐treated A cells from the peritoneal cavity or the spleen. Hence, the hitherto A cell independence of TI antigens on which some theories of B cell activation are based is a result of inadequate depletion procedures, and the minimal model for B cell activation must take into account two cell types: B cells and A cells.