The immunocompetent subpopulation by mouse thymus cell (TH-2) was isolated by buoyant density centrifugation and by hydrocortisone pretreatment. TH-2 cells undergo a proliferative one-way or two-way mixed lymphocyte culture (MLC) response only when cultured with allogeneic or congenic peripheral lymphoid cells. However, mixtures of allogeneic TH-2 cells alone do not proliferate in either one-way or two-way MLC reactions. Such MLC mixtures are proliferative only if mitomycin-blocked peripheral lymphoid cells are also present in the mixture. The peripheral helper cell has been found to be of low net density, non-adherent, insensitive to anti-thy-1 serum cytotoxicity, but sensitive to the cytotoxic effets of anti-immunoglobulin serume plus complement. The helper effect does not depend on proliferation nor does it appear to involve demonstrable soluble mediators. The nature of failure of MLC between TH-2 subpopulations appears to be dependent on the exppression of some product of the K, I regions of the H-2 locus. Possible mechanisms by which a B-cell-like helper cell triggers TH-2 proliferation are discussed terms of the present knowledge of specific alloantigen receptor on T and B cells, and the immunoglobulin Fc region receptors on T cells.

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