Posttransplant Cyclophosphamide Promote Naïve-Subset Dominant B Cell Recovery Coordinated with Early Treg Expansion; Implication of Reduced Risk of Pathogenesis into Chronic Gvhd

Posttransplant cyclophosphamide (PTCy) is an effective prophylaxis for both acute and chronic graft-versus-host disease (GVHD) after allogeneic hematopoietic stem cell transplantation (HSCT). We recently studied the immune reconstitution dynamics of each lymphocyte subset after PTCy-based transplant using murine haploidentical BMT model and reported that PTCy strongly promoted Treg-dominant T-cell reconstitution and stem cell-derived mature B-cell generation with broad BCR-diversity. We also found that the early reconstitution of Treg could contribute to promote naïve B cell emergence from bone marrow, indicating the T and B cell recovery might be mutually coordinated after PTCy-based transplant (Iwamoto et al, ASH2017). However, the detailed process of immune reconstitution in patients after haploidentical HSCT with PTCy has not been well studied. To address this issue, we here investigated the early dynamics of donor-lymphocyte subset chimerisms in patient after clinical PTCy-based haploidentical HSCT with comparing those in patients after low-dose ATG-based haploidentical HSCT and patients after cord blood transplantation. Laboratory studies were undertaken in 13 adult patients who received HLA-mismatched allogeneic graft; unrelated cord blood (n=5), and haploidentical related peripheral blood after ATG-based conditioning (n=5) and haploidentical related peripheral blood after PTCy-based conditioning (n=5). Blood samples were obtained before and at 1, 2, 4, 6 and 8 weeks after HSCT. Peripheral blood mononuclear cells (PBMCs) were isolated from blood samples by density gradient centrifugation and cryopreserved before being analyzed. After thawing, to analyze the subset-specific chimerism, PBMCs were stained with anti-HLA monoclonal antibodies and other subset-specific antibodies as follows: Pacific Blue conjugated anti-CD4, eFluor450 conjugated anti-CD3, PE-Cy7 conjugated anti-CD25, anti-CD14, APC conjugated anti-CD127, anti-CD56, and APC-eFluor780 conjugated anti-CD8a, anti-CD19. Gated lymphotes (CD4⁺Tcons, CD4⁺Tregs, CD8⁺T cells, B cells, NK cells, Monocytes) were analyzed their chimerism by flowcytometry. To examine the detailed phenotype of B cells, the expression of CD27, CD24, CD38 and IgD were tested. Flowcytometry-based method enables us to analyze the lymphocyte subset chemerism in the very early phase after HSCT. At 2 weeks after HSCT, our analysis revealed that CD4⁺Tcons, CD4⁺Tregs and CD8⁺T cells had already achieved complete donor chimerisms (>95% in all subsets) in patients after ATG-based SCT and had been approaching complete donor chimerisms (85.8%, 75.4% and 87.2%, respectively) in patients after CBT. In contrast, percentage of donor chimerisms of CD4⁺Tcons, CD4⁺Tregs and CD8⁺T cells after PTCy-based haplo-SCT was 73.5%, 59.6% and 59.2%, respectively, and those remained to be in the lower levels than other 2 groups. However, at 4 weeks after HSCT, all examined patients achieved complete donor chimerism of T cells, NK cells and Monocytes (>90%). At 8 weeks after HSCT, the number of B cells in PTCy-based haplo-group was higher than in ATG-based haplo-group (3494 vs 1901/mm3). Of note, B cell population in PTCy-based haplo-group at 8 weeks contained the significantly higher percentage of CD24+CD27-IgD+CD38+ transitional/naïve subset and the significantly lower percentage of CD24+CD27+IgD-CD38neg/dim activated/switched-memory subset when compared to B cell population in ATG-based haplo-group (59.9% vs 10.2%, 2.6% vs 21.5%, P<0.02 respectively), suggesting PTCy treatment might be associated with the favorable B cell reconstitution with naïve-subset dominant composition. Moreover, in patients after PTCy-based haplo-group, the percentage of activated/switched-memory subsets in B cell population at 8 weeks was inversely correlated with percentage of Treg in CD4 T cells at 4 weeks (P<0.05, r2=0.77). Taken together, consistently with our murine study, the current data from clinical samples again suggest that PTCy-based immune-modulation lead to coordinated T and B cell recovery, especially promoting naïve-subset dominant B cell recovery with help of the early expansion of Treg, which might reduce the risk of subsequent chronic GVHD. These data provide the important information for understanding the immunological reconstitution after PTCy-based haploidentical HSCT.