Donor HLA mismatch promotes full donor T-cell chimerism in the allogeneic stem cell transplant with reduced-intensity conditioning and post-transplant cyclophosphamide GVHD prophylaxis.

Full donor T-cell chimerism (FDTCC) after allogeneic stem cell transplant (allo-SCT) has been associated with improved outcomes in hematologic malignancy. We studied if donor human leukocyte antigen (HLA) mismatch improves achievement of FDTCC because mismatched HLA promotes donor T-cell proliferation where recipient T-cells had been impaired by previous treatment. Patients (N = 138) received allo-SCT with reduced-intensity conditioning (RIC) from 39 HLA mismatched donors (16 unrelated; 23 haploidentical) with post-transplant cyclophosphamide (PTCy) or 99 matched donors (21 siblings; 78 unrelated) with PTCy (N = 18) or non-PTCy (N = 81). Achievement of FDTCC by day 100 was higher with HLA mismatched donors than matched donors (82.1% vs. 27.3%, p < 00,001), which was further improved with 200 cGy total body irradiation (87.9%) or lymphoid (versus myeloid) malignancy (93.8%). Since all mismatched transplants used PTCy, FDTCC was higher with PTCy than non-PTCy (68.4% vs. 25.7%, p < 0.00001), but not in the matched transplant with PTCy (38.9%), negating PTCy as the primary driver. Lymphocyte recovery was delayed with PTCy than without (median on day + 30: 100 vs. 630/µL, p < 0.0001). The benefit of FDTCC was not translated into survival outcomes, especially in myeloid malignancies, possibly due to the insufficient graft-versus-tumor effects from the delayed lymphocyte recovery. Further studies are necessary to improve lymphocyte count recovery in PTCy transplants.