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Differentiation of lymphoid cells: enhancement of the induction of immunoglobulin production by thymus cells and a product secreted by thymus cells.


 The induction of immunoglobulin M production, as a consequence of the incubation of rabbit spleen cells for 72 hr in cell culture in the absence of added antigen, was markedly enhanced when thymus cells were cultured along with spleen cells. In contrast, immunoglobulin production by lymph node cells was only slightly stimulated by the presence of thymus cells during culture. Immunoglobulin production by spleen cells was equally enhanced, regardless of whether the thymus cells were derived from unimmunized animals or from animals immunized with a dinitrophenyl-antigen. Thymus cells added to cultures of spleen cells at 48 hr were still markedly stimulatory even though the induction process in the absence of thymus cells appears to be largely complete at 48 hr. Ficoll density gradient separation of spleen cells revealed some fractions that were slightly enhanced and others that were markedly enhanced by thymus cells. Finally, a cellfree mediator secreted into the culture medium by thymus cells also enhanced immunoglobulin production to about the same extent as thymus cells.