Essential Role of Regulatory T Cells on Early B Cell Reconstitution after Haploidentical BMT with Posttransplant Cyclophosphamide

Posttransplant cyclophosphamide (PTCy) is an effective prophylaxis for both acute and chronic graft-versus-host disease (GVHD) after allogeneic hematopoietic stem cell transplantation (HSCT). Recent studies reported that PTCy has been associated with low incidence of viral infections or EBV-related posttransplantation lymphoproliferative disease (EB-LPD), suggesting PTCy-based immune modulation leads the favorable immune reconstitution after transplant. Recently, we studied the immune reconstitution dynamics of each lymphocyte subset after transplant using PTCy and reported that PTCy-based immune-modulation strongly promoted Treg-dominant T-cell reconstitution and stem cell-derived mature B-cell generation by using murine model of haploidentical HSCT with PTCy. TCR- and BCR- repertoire analysis demonstrated that PTCy resulted in the broader diversity of both T cell and B cell repertoire than non-PTCy controls. However, the mechanism of coordination of increased Tregs and B cells after PTCy-based HSCT has not well studied. To address this issue, we here investgated the impact and role of PTCy on Tregs and B cell recovery and the mutual influence between the two important subsets by using murine BMT model. Irradiated B6D2F1 mice were transplanted with 5x10⁶ spleen cells from the CD45.1 B6 mice together with 5x10⁶ TCD-BM from CD45.2 B6 donors. Cyclophosphamide 100mg/kg or control vehicle was administered at day 3 after transplant. Peripheral blood mononuclear cells (PBMCs) and splenic cells were sequentially obtained at day21. The chimeric balances among host-residual H-2kd+ cells, donor graft-derived cells and donor BM-derived cells in Tregs and B cells were monitored separately. To evaluate the homeostatic stability, proliferation marker Ki-67 and anti-apoptotic BCL-2 were also quantitatively examined respectively. To examine the detailed phenotype of Tcells, CD4, CD8, CD25, Foxp3, ICOS, CXCR5 were tested. Also, to examine the detailed phenotype of B cells, the expression of B220, CD21, CD23, CD24 were tested. At day21 after HSCT, the number of Treg cell in spleen was significantly higher on PTCy-treated recipients than that of non-PTCy control (P < 0.05). We also checked the emergence of donor-derived follicular helper T cells (Tfh) in lymph node because recent studies have shown that Tfh could contribute B cell differentiation after HSCT. In our analysis at day21 in this PTCy model, Tfh cells were still few in both PTCy group and non-PTCy group, suggesting the early T cell recovery after PTCy did not promote B cell differentiation into pathological plasmablast. The number of B cell was markedly higher in PTCy-treated recipients than that of non-PTCy control (P < 0.01). The increased B cell population was mainly transitional-1 phenotype suggesting these B cells were recent emigrants from bone marrow. In consistent with low recovery of Tfh cells, plasmablast-like phenotype of B cells were not observed. Next, to evaluate the impact of graft-derived Treg recovery on B cell reconstitution, we depleted Treg from spleen cells of B6 donors by using CD25+ magnetic beads. After the depletion, residual Treg cell in the graft was under 1% of total CD4 T cells. At day21, splenic Treg cells is under 3% of total CD4 T cells in Treg-depleted group, that is much lower than Treg in Treg-repleted group. Interestingly, the number of B cells at day21 in PTCy-treated recipients was equivalent to that of non-PTCy recipients when we depleted Treg from donor graft, suggesting that well-balaced B cell recovery promoted by PTCy was completely abolished by Treg-depletion. Taken together, our data suggested that PTCy could induce the expansion of Treg recovery and naïve B cell recovery without urging early emergence of Tfh cells in lymph node, resulting in well-diversed T and B cell reconstitution. Treg appears to contribute to promote naïve B cell emergence from bone marrow, indicating the T and B cell recovery might be mutually influenced. Moreover, slow recovery of Tfh cells in lymph node could be advantage for preventing B cell from forced differentiation into plasmablast causing GVHD. These data provide the important information for understanding the immunological reconstitution after PTCy-based haploidentical HSCT.