There is now ample evidence that T-macrophage interactions in the induction of various immune responses are genetically restricted. We found that the in vitro generation of T helper cells with soluble antigens requires I-A compatible macrophages, or GRF (genetically related macrophage factor) a complex of Ia products and antigenic fragments. However, it is unclear whether genetic restriction of T-macrophage interactions is imposed by priming or already expressed prior to priming. Using F1 T cells and parental macrophages we were able to generate helper cells that cooperated preferentially with B cells and macrophages of the parental haplotype used for initial priming, indicating that genetic restriction may be induced by priming. However, results obtained by using T cells from chimeric mice instead of F1 hybrids suggested that the restriction of T-macrophage interaction may be already present prior to priming. This conclusion is supported by absorption experiments — unprimed splenic T cells or thymocytes preferentially adsorb I-A compatible GRF.

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