Selective defect of precursor T cells associated with apparently normal B lymphocytes in severe combined immunodeficiency disease.

Two patients, one with an autosomal and the other a sex-linked form of severe combined immunodeficiency, had more than 95% B cells in their peripheral blood. Despite an increased absolute number of B lymphocytes, the patients were unable to produce serum antibodies. In each patient, geno- or pheno-identical bone marrow transplantation was followed by the visualization of a thymus shadow and the appearance of both cellular and humoral functions. Chromosome of allotype studies showed that the T cell originated from the donor whereas serum immunoglobulins were synthesized by host B cells. In these patients the pathogenesis appears to be a selective defect of bone marrow precursor T cells without concomitant intrinsic B cell defect. The successful outcome of the graft in these two patients, who are now, respectively, 5 years and 11 months of age and free of infections, indicates that the preferred form of therapy in such patients is transplantation of bone marrow stem cells, which populate the thymus and mature slowly into T cells that cooperate fully with host B cells in synthesis of antibody.