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

Abstract Newborn screening for severe combined immunodeficiency promotes early diagnosis and timely treatment, improving clinical outcomes. Selective T-lymphocytopaenia is found both in haematopoietic cell-intrinsic and thymic stromal cell-intrinsic defects, including congenital athymia which is associated with a T-B + NK + immunophenotype. Without a molecular diagnosis, it is challenging to determine whether haematopoietic cell transplantation (HCT) or thymus transplantation ought to be performed. Ex vivo T-lymphocyte differentiation assays have been proposed to assist clinical decision-making for genetically undefined T-lymphocytopaenic patients by assessing the intrinsic potential of their haematopoietic progenitors to differentiate into mature T-lymphocytes. We investigated 18 T-lymphocytopaenic patients, including 12 patients awaiting first-line treatment and 6 patients with failed immune reconstitution after previous HCT or thymus transplantation. Whilst early developmental blocks in ex vivo T-lymphopoiesis indicated haematopoietic cell-intrinsic defects, successful differentiation of mature T-lymphocytes required careful interpretation, in conjugation with clinical status and presentation, immunophenotyping, and available genetic investigations. 5 patients were referred for HCT and 12 for thymus transplantation. 12/18 patients proceeded to treatment with successful immune reconstitution in 4/5 patients after HCT and 4/7 after thymus transplantation, the latter including two patients previously treated with HCT. Two treated patients died, either after HCT or after thymus transplantation, due to pre-existing complications, and two patients have yet to show immune reconstitution seven months and one year after thymus transplantation respectively. Overall, we conclude that including ex vivo T-lymphocyte differentiation assays in the diagnostic pathway for genetically undefined T-lymphocytopaenia improves patient outcomes by facilitating corrective treatment choice between HCT and thymus transplantation.