Phase I trial of autologous regulatory T cells for immune aplastic anemia.

Immune-mediated aplastic anemia (AA) is a bone marrow failure syndrome characterized by cytotoxic CD8-mediated autoimmune suppression of hematopoietic stem/progenitor cells resulting in varying degrees of peripheral blood cytopenias. Treatment with immunosuppressive therapy and hematopoietic stem cell transplantation are not applicable to all patients and effective responses occur in only a proportion of patients, highlighting the unmet need for alternative treatments. We have previously shown a reduction in the number and function of regulatory T cells (Treg) in AA patients and their functional restoration following in vitro expansion that laid the foundations for this phase I trial. Required numbers of Treg were collected from leukapheresis and expanded under Good Manufacturing Practice conditions from all six patients in the trial who were resistant/refractory to standard forms of treatment. The trial design included two doses of autologous Treg (5×106/ kg) administered 2 weeks apart. Mass cytometry, single-cell sequencing and cytokine profiling were performed on blood samples collected at various timepoints. Treg were successfully expanded to required doses from all six patients with no adverse or immune-related events in any of the patients. Hematologic responses were observed in three patients. In addition, we were able to track the persistence of the expanded Treg in vivo, correlate clinical efficacy with the accumulation of clusters of post-infusion Treg, and identify phenotypic markers in the infusion product that correlate with in vivo expansion. Treg from AA patients are expandable, safe for infusion, and may help to modulate the abnormal immune milieu associated with AA, with induction of clinical response. (Clinical-Trials.gov identifier: NCT05386264; EudraCT number: 2021- 000082-33).