Human Pro-T-Cell Manufacturing in Vitro Is a Safe Procedure for Hematopoietic Stem Cell Transplantation with Delayed T-Cell Reconstitution: Interim Results from 2 Different Clinical Trials

Introduction: Alternative donor Hematopoietic stem cell transplantation (HSCT) strategies are followed by a prolonged period of immune deficiency due to delayed T-cell reconstitution. Either Umbilical Cord (CB) blood transplant or CD34+ selected haplo-transplant provided similar T-cell lymphocytes (CD3+ and CD4+) recoveries at any time point, and recipients had significantly fewer CD4+ T-cells during the first six months compared to HLA-matched donors. The GMP generation of Human T lymphoid Progenitors (HTLP) that are competent to rapidly become in vivo differentiated functional and thymic-educated T-cells may overcome the post-transplant delayed immune reconstitution.

Method: HTLP safety and efficacy to accelerate immune reconstitution is currently evaluated in 2 multicentric and single-arm clinical trials: after umbilical cord blood in adult hematologic malignancy (Eudract 2019-004883-23) and after partially HLA compatible CD34+ immune-selected allogeneic hematopoietic stem cell transplantation in Severe Combined Immune Deficiency (SCID) patients (Eudract 2018-001029-14).

Results: We enrolled and treated six patients, 4 SCID and 2 adults with malignancy, with an average follow-up of 8.00 (range, 1.60 to 37.7) months. HTLP cell product was infused between D7 to D14 post-HSCT for SCID patients after ensuring the decrease of the ATG level while meaning for the adult patient; the expanded pro-T cells were injected at D0. The Pro-T manufacturing process allows to robustly expand CD34+ cells to HTLP with a median of 79.7% (range 54.6; 98.9) of differentiation (CD7+ CD3-) and a median fold of proliferation of 14.7 (range 8.80; 18.5). No chromosome instability was induced during the expansion. Patients received increasing doses ranging from 0.1 to 0.5 10 ⁶ HTLP/kg. The amount of CD3+/kg was less than 11.2 (range 0; 588)/ kg. No early or mid-term toxicity due to the administration of HTLP has been reported for evaluable patients (6/6). One SCID patient died on D53 because of severe VOD. Primary engraftment was confirmed in 4 out of 5 patients. Interestingly, despite the gradual increase of doses, there was no aGVHD over grade >2. Preliminary efficacy results suggest that circulating CD3+ CD4+ TCRαβ+ T cells > 50/μl before M4 depended on the increased infused dose of HTLP and an intrathymic paracrine effect. Notably, there was no viral infection disease or relapse during the outcome.

Discussion conclusion: First infusions of HTLP in both clinical trials confirmed the safety and reproducible GMP manufacturing. HLTP offers an exciting perspective for improving immune reconstitution in alternative hematopoietic transplants.