Recent advances in the prevention of graft-vs-host disease (GVHD) have allowed the use of haploidentical bone marrow cells for correction of lethal genetic defects of the immune system. Sequential analyses of blood lymphocyte phenotypes and functions were done before and after transplantation of haploidentical marrow stem cells into 17 infants with severe primary T cell deficiencies. The marrow was depleted of post-thymic T cells and most other mature marrow cells by soy lectin agglutination and sheep erythrocyte rosetting. The studies were performed to define the time course and extent of appearance of immune function, and to identify factors leading to resistance to engraftment. No pretransplant immunosuppression was used. T cell function was detected between 34 and 287 days after transplantation, but a sharp rise usually occurred between 84 and 115 days, and normal function was reached between 113 and 210 days. Fifteen of the patients are alive from 6 to 41 mo post-transplantation, 12 have improved or have normal T lymphocyte function, and nine have proven T cell chimerism. Increased immunoglobulins of several isotypes have been noted in 11 patients and specific antibodies in seven patients, although B cell chimerism has been detected in only one patient. B cell function required 2 to 2.5 yr for normalization. No GVHD occurred in 14 patients, and the other three had only transient mild skin rashes. Two patients died of viral infections. Failure to engraft was correlated with some pre-transplant lymphocyte responses to mitogens and allogeneic cells (three cases), but not with the presence of pre-transplant natural killer cell function (five cases) nor with the presence of purine salvage pathway enzyme deficiencies (four cases). The latter, however, was associated with poor lymphoid function in two patients. These studies indicate that the thymic microenvironment of most infants with severe combined immunodeficiency disease is capable of differentiating donor stem cells to mature and functioning T lymphocytes which can cooperate with apparently normal host B cells for antibody production.

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