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Circulating maternal T lymphocytes were noted in the peripheral blood of six patients with severe combined immunodeficiency. Phenotypical analyses revealed the presence of both CD4 and CD8 subsets in some but not all cases. The maternal T cells could be stimulated by anti-TCR/CD3 mAb +/- rIL-2, but were virtually silent in the MLR and against the recall Ag purified protein derivative of tuberculin and tetanus toxoid, even in immunized patients engrafted with T cells from a responding mother. Using a panel of mAb against TCR V region gene encoded epitopes including V beta 5, V beta 6, V beta 8, V beta 12, and V alpha 2, we show that maternal T cells displayed a profoundly reduced TCR diversity, characterized by a lack of one or even several TCR V subsets in all six cases and a dramatic (5- to 25-fold) expansion of other TCR V subsets in three cases. In one patient analyzed, limited TCR diversity was also seen in T cells cultured from bone marrow and skin; restimulation experiments of these cells against cells expressing host MHC Ag were unsuccessful, as were attempts to exclusively allocate anti-host proliferative responses of maternal control T cells to the TCR V subsets that had undergone expansion in vivo. We conclude that a severely reduced TCR diversity is a common feature of maternal T cells engrafted in severe combined immunodeficiency patients. These novel findings provide a structural basis to understand the failure of these cells to protect the host from infections and may also help to understand their relative inefficiency to induce lethal, multi-organ, graft vs host disease. Moreover, as an experiment of nature, the reported phenomenon clearly illustrates the functional consequences in vivo of an insufficient TCR diversity.