Early Immune Reconstitution Is Similar in Patients with and without Acute GvHD Undergoing Haploidentical HCT with Post-Transplant Cyclophosphamide As Gvhd Prophylaxis.

Haploidentical hematopoietic stem cell transplantation (haplo-HCT) has emerged as a promising treatment option for patients with hematological malignancies and non-malignant disorders lacking suitable HLA-matched donors. Post-transplant cyclophosphamide (PTCy) has revolutionized the field by mitigating the risk of severe graft-versus-host disease (GvHD) and enhancing graft-versus-leukemia (GvL) effects. Studies have demonstrated that PTCy reduces proliferative capacity and causes functional impairment of surviving alloreactive T cells. Early immune reconstitution (eIR) after haplo-HCT represents a crucial factor for determining the success of HCT. We evaluated the eIR and its correlation with HCT outcomes in haplo-HCT patients receiving PTCy-based GvHD prophylaxis.Sixty-four patients who underwent haplo-HCT with PTCy-based GvHD prophylaxis between October 2021 and August 2023 were included in the study (Table1). Peripheral blood samples were collected at pre-determined time points- pre-conditioning, D+21, D+28, D+60 and D+90 post-HCT. Multicolor flow cytometry-based enumeration of the lymphoid lineage, highlighting the sequential reconstitution of its subsets - T cells, B cells, and NK cells was performed. Helper T cells (CD4+, Th) and cytotoxic T cells (CD8+, Tc) and their subsets [Central memory (CM), Effector memory (EM), Naïve, and Terminally differentiated (TD)] were also ascertained. Clinical outcomes post-HCT, such as engraftment, chimerism, rejection, and acute GvHD, were assessed prospectively.The absolute lymphocyte count (ALC) normalized at D+60 for 12 out of 27 patients (44.4%) Ref range ≥1000 cells/ul). T cell subsets recovered gradually, but their levels remained well below those of the pre-conditioning. Tc cells recovered by D+60 (514.96(9.72-4549), Ref range 135-852 cells/ul), while Th cells did not recover until D+90 (77.7(1.27-401.1), Ref range 464-1721 cells/ul). CM and EM cells are the predominant subsets, which recovered faster among both Th and Tc cells, and there was a reduction in both naïve (Th - 3.2 (0-66.3) and TD (Th - 1.3 (0-45.7) at D+90 when compared to pre-conditioning (Th naive: 36.96 (0.15- 361.8) Th TD: 2.6 (0-134.3)). B and NK cell recovery was slower at early time points and increased from D+60 (B cells:18.2(0-759.2), NK cells: 97.85(1.572-1526)). Further analysis revealed no differences in eIR pattern between patients with and without acute GvHD at D+28 [T cells {62.26 (17.64-759.11) vs 124.2 (6-1391.4)} NK cells {14.4(0.78-163.2 vs 14.96(0.25-523.5)}, Th cells {20.7 (1.86-277.2) vs 13.09(0.5-355.6}, Tc cells {61.4 (5.9-461.7) vs 77.4 (4.1-1203)}].The present study highlights that the eIR pattern did not influence the development of acute GvHD. We are currently exploring the role of proliferation and activation markers in the interplay of cellular, genetic, and metabolomic markers involved in GvHD pathogenesis.