Generation and Function of αδ T Cells after Allogeneic Bone Marrow Transplantation in Humans: Comparison in Absence or Presence of HLA-Matched or Mismatched Thymus

We have observed two patients who exhibited an exclusive increase of δ TCS1₊ subset of αδ T cells in the peripheral blood after bone marrow transplantation (BMT). In one case with severe combined immunodeficiency (SCID) who received haploidentical BMT from his father, αδ T cells appeared only after thymus transplantation. However, his T cell-mediated immunity remained severely defective despite the generation of T cells of donor origin. In the other case with aplastic anemia, δ T CS1_-αδ T cells began to increase in the peripheral blood later. This indicates that the thymus is necessary for the generation of αδ T cells and that the δ TCS1₊ subset is dominant in the early stages of their ontogeny. δ TCS1₊ T cell lines were established from both patients, and allo-reactivity was investigated. The cell line from the latter case reacted to recipient cells in a mixed lymphocyte reaction, but did not show cytotoxity to the allogeneic cells including recipient cells. The other cell line, from the former case, did not react to either donor or recipient cells. This indicates that an intact thymus is needed for αδ T cells to acquire allo-reactivity. Both cell lines showed MHC non-restricted cytotoxity against NK-sensitive target cells.