Spleen explants from mice tolerant to rabbit serum albumin (RSA) failed to react in vitro to dinitrophenyl (DNP)-RSA; antibodies to DNP were, however, produced by such spleens, when stimulated with α-DNP-poly(Lys). To study the function of T and B cells in recognition of carrier determinants, spleen explants from X-irradiated mice, which had been inoculated with combinations of thymus and bone marrow cells from normal and from RSA tolerant donors, were tested for their reactivity in vitro to the DNP-RSA conjugate. A significant response was obtained only by spleens of mice containing bone marrow and thymus from normal donors. Spleens of mice treated with thymus from tolerant and bone marrow from normal or with thymus from normal and bone marrow from tolerant donors did not respond to DNP-RSA. The absence of the response to DNP-RSA by tolerant B cells combined with normal T cells was unexpected. It could not be attributed to binding of the tolerogen to B cells which would have prevented the interaction with T-cells. Neither could the result be attributed to an inhibition of normal cells by RSA-tolerant B-cells. θ-positive cells in the bone marrow are not the cells controlling the recognition of carrier determinants in the B population, since elimination of θ-positive cells did not affect the reactivity of spleens repopulated with B and T cells. Nor are bone marrow macrophages responsible for the lack of reactivity in spleens containing tolerant B cells, since normal macrophages did not restore reactivity. Hence, the production of antibodies to DNP is based on the recognition of carrier determinants not only by T cells, as previously established, but also by B cells. Whether this indicates a B-B in addition to the T-B cell cooperation is an inviting possibility.
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