Early TCR Αβ and Γδ Repertoire Dynamics after Myeloablative Allo-HCT with Post-Transplant Cyclophosphamide

Allogeneic hematopoietic cell transplantation (allo-HCT) with post-transplant cyclophosphamide (PTCy) is effective in mitigating graft-versus-host disease (GVHD), but the dynamics of T-cell receptor (TCR) repertoire reconstitution under this platform remain unexplored. We examined TCR α (TRA), β (TRB), γ (TRG), and δ (TRD) receptor diversity and clonality both pre- and early post-HCT in 35 patients with hematologic malignancies who underwent myeloablative allo-HCT with PTCy from matched unrelated (MUD n=25), mismatched unrelated (MMUD n=4), or haploidentical donors (n=6). TCR repertoire metrics including, Shannon diversity, Simpson evenness, clonality indices, and unique clone counts were assessed in peripheral blood at recipient baseline (pre-conditioning), and post-HCT days 30 and 60. The TCR repertoire of the donor product was also assessed. Across all TCR, we observed an expected early contraction of repertoire diversity after transplant. For example, median TRB Shannon diversity declined sharply from baseline to day 30 (p < 0.001) and remained significantly lower than baseline by day 60 (p < 0.01). Simpson evenness also decreased post-HCT, indicating a less even (more oligoclonal) repertoire, while clonality measures (e.g., Gini–Simpson index and largest clone frequency) increased significantly at day 30 (p < 0.001). Unique TCR clone counts were markedly reduced after transplant (day 30, p < 0.001) with partial recovery by day 60. Notably, donor age influenced γδ T-cell reconstitution at these early transplant time points recipients of grafts from younger donors exhibited higher TRG and TRD diversity than those with older donors (MWU, p = 0.03 and 0.04). Clinical factors shape post-transplant trajectories. Patients who reactivated CMV developed oligoclonal expansions with rising largest-clone fractions (Fig.1). aGVHD was similarly linked to expanding dominant T-cell clones and falling repertoire evenness (Fig.1) consistent with antigen-driven proliferation. These findings indicate that PTCy-based allo-HCT induces a profound yet partially reversible narrowing of the T cell receptor (TCR) repertoire during the early post-transplant period. Donor age is a factor associated with greater early γδ TCR diversity. These insights underscore the importance of further investigating early post-transplant TCR reconstitution kinetics to elucidate the mechanisms underlying transplant-related complications and to inform interventions that improve patient outcomes.