Ex vivo T-lymphopoiesis assays assisting corrective treatment choice for genetically undefined T-lymphocytopenia

Persistent selective T-lymphocytopenia is found both in SCID and congenital athymia. Without molecular diagnosis, it is challenging to determine whether HCT or thymus transplantation ought to be performed. Ex vivo T-lymphopoiesis assays have been proposed to assist clinical decision-making for genetically undefined patients. We investigated 20 T-lymphocytopenic patients, including 13 patients awaiting first-line treatment and 7 patients with failed immune reconstitution after previous HCT or thymus transplantation. Whilst developmental blocks in ex vivo T-lymphopoiesis indicated hematopoietic cell-intrinsic defects, successful T-lymphocyte differentiation required careful interpretation, in conjunction with clinical status, immunophenotyping, and genetic investigations. Of the 20 patients, 13 proceeded to treatment, with successful immune reconstitution observed in 4 of the 6 patients post-HCT and 4 of the 7 patients after thymus transplantation, the latter including two patients who had previously undergone HCT. Whilst further validation and standardization are required, we conclude that assessing ex vivo T-lymphopoiesis during the diagnostic pathway for genetically undefined T-lymphocytopenia improves patient outcomes by facilitating corrective treatment choice.