The nature of the cells which proliferate in response to allogeneic histoincompatible lymphocytes in human mixed leukocyte cultures (MLC) was investigated. Direct information on the differentiation of proliferating cells was obtained in an in vitro chimeric system with T- and B-cell sex chromosome markers. Highly purified T and B cells were prepared from the peripheral blood of three pairs of HL-A genotype identical, MLC-negative siblings of opposite sex by E-rosette sedimentation techniques. Recombined T and B cells of the siblings were stimulated with allogeneic cells, and karyotype analysis of proliferating cells performed. 96% of the proliferating cells were found to carry the sex chromosome marker of the respective T-cell population. These results indicate that proliferation in human MLCs is a feature of T cells.

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