Inducer T lymphocytes activate other cells to divide and express new function. Known target cells include other lymphocytes and haematopoietic stem cells1. We now provide evidence that the inducer T cell acts on another important target population: mast cells. Mast cells have a central role in the expression of immediate hypersensitivity and are also prominent in T-cell mediated reactions of the delayed type2-7. Because the proliferation of differentiated cells is often regulated by soluble growth factors, we examined an inducer T-cell clone for its ability to stimulate mast cell proliferation. We report here that cloned Ly1+2- inducer T cells produce a factor that selectively induces morphologically and karyotypically normal mouse mast cell clones to proliferate. We therefore suggest that inducer T cells may regulate mast cell numbers by releasing a soluble growth factor that stimulates them to divide. Because mast cell products also affect certain T-cell functions8-10, mast cell-T cell interactions may comprise part of an immunoregulatory circuit.
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