Treatment of Secondary Graft Failure after Hematopoietic Stem Cell Transplantation with Alpha/Beta T-Cell Depleted Grafts As Booster Infusions

Allogeneic hematopoietic stem cell transplantation (HSCT) is commonly used as a treatment for hematological malignancies, immunodeficiencies, and inborn errors of metabolism. HSCT is associated with several complications and risk factors, for example graft versus host disease (GVHD), viral infections, relapse, and graft rejection. Infusions of extra lymphocytes from the original stem cell donor can be used as a treatment after transplantation for, e.g., relapse, poor immune reconstitution or secondary graft failure. The term booster applies for infusions of donor cells without conditioning. Secondary graft failure was in this study defined as initial signs of engraftment with subsequent development of bone marrow hyperplasia requiring frequent transfusions beyond day 60 as well as prolonged neutropenia and thrombocytopenia. While high levels of CD3+ cells in grafts can increase the risk for GVHD they can also promote the graft versus leukemia (GVL) effect. In peripheral blood, 95% of T-cells express the αβ T-cell receptor and the remaining T-cells express the γδ T-cell receptor, which are not dependent upon human leukocyte antigen (HLA) molecules for activation. As αβ T-cells are the primary mediators of GVHD, depleting them from the graft should reduce this risk, leaving γδ T-cells as primary T‑cell type in the booster infusion.

In this pilot study, five patients transplanted with HLA-matched related and unrelated donors were treated with αβ T-cell depleted stem cell boosts, with secondary graft failure as primary indication. All patients also suffered from infectious complications prior to the infusion, which is likely to have been associated with the poor graft function and development of secondary graft failure. The αβ T-cell depletion was performed using a CliniMACS system (Miltenyi Biotech). Analysis of the booster infusion fraction as well as blood samples from the patients was performed; cell characterization by using flow cytometry and evaluation of chimerism as well spectratyping of γδ-chains by using PCR. Monitoring of white blood cells, platelets, and granulocytes was carried out for 30 days post infusion. The majority of γδ T-cells in the grafts expressed Vδ2 and/or Vδ9. Median log depletion of γδ T-cells in the products was 3.72, and the mean yield of γδ T‑cells was 84.4%. Most patients receiving αβ-depleted stem cell boosts increased their levels of white blood cells, platelets, and/or granulocytes 30 days post infusion. No signs of GVHD or other side effects were detected. A larger pool of patients with longer follow-up time is needed to confirm the data in this study and compare the treatment modality to other therapy options.