Effects of post-transplant cyclophosphamide on immune reconstitution after haploidentical and HLA-matched transplant

Post-transplant cyclophosphamide (PTCy) is increasingly used for graft-versus-host disease (GVHD) prophylaxis in both HLA-matched (matched-PTCy) and haplotype-mismatched (haplo-PTCy) hematopoietic stem cell transplantation (HCT), but the effects of PTCy on immune reconstitution in different donor settings remain incompletely characterized. We analyzed immune reconstitution in 210 adult patients undergoing haplo-PTCy, matched-PTCy, or HLA-matched HCT using tacrolimus/methotrexate (matched-CNI) for GVHD prophylaxis. Both PTCy cohorts also received tacrolimus/mycophenolate mofetil (Tac/MMF). Reconstitution of T, B, NK and dendritic-cell subsets in serial blood samples was monitored using flow cytometry and a panel of 37 cell surface markers. T cell receptor (TCRVb) sequencing was used to assess repertoire diversity in purified CD4Treg, CD4Tcon, and CD8 T cell populations. Early recovery of T, B, NK and plasmacytoid dendritic cells, was delayed in both PTCy cohorts compared with matched-CNI with more pronounced delays after haplo-PTCy. Delayed recovery after PTCy primarily involved naïve CD4 and CD8 T-cell subsets and naïve B-cell populations. Recovery of NK-T cells and γδT cells was delayed for more than 1 year in both PTCy groups. T cell receptor (TCR) sequencing revealed delayed recovery of repertoire diversity after haplo-PTCy compared to matched-CNI but not after matched-PTCy. CD8 clonality remained elevated at later time points after haplo-PTCy, suggesting more persistent repertoire restriction in this setting. Overall, these findings identify multiple effects of GVHD prophylaxis with PTCy on immune reconstitution. However, immune recovery is more rapid and T cell receptor repertoire diversity better preserved when recipient and donor are HLA-matched.