Contribution of the PD–1–PD–L Pathway to Chronic Graft–Versus–Host Disease

Chronic graft-versus-host disease (cGVHD) remains a major cause of late phase mortality and morbidity after allogeneic hematopoietic stem cell transplantation. We investigated the roles of the PD-1 pathway in cGVHD using a well-defined mouse model (B10.D2 into BALB/c). First, we confirmed that CD4 and CD8 cells from the spleen and peripheral lymph nodes (PLN) of allogeneic recipients expressed significantly more PD-1 compared to those of syngeneic recipients on days 14, 21, 28, and 56 after transplantation (p < 0.05). Upon transferring PD-1 KO donor T cells of the B10.D2 background into allogeneic bone marrow transplantation (BMT) models, there was severe weight loss and 100% of the recipients died by day 23. To avoid early death and to examine the roles of the PD-1 pathway in cGVHD, we next administered six antibody doses to recipients of wild-type (WT) donors from day 14 of BMT, just prior to the development of cGVHD. Mice treated with anti-PD-1 showed 70% mortality by day 35 in contrast to only 10% mortality in those administered anti-B7H1 Ab or anti-B7DC Ab. All groups treated with anti-PD-1 Ab, anti-B7H1 Ab, or anti-B7DC Ab had significantly higher cGVHD scores than controls (anti-PD-1 Ab: 2.51±0.31, anti-B7H1 Ab: 2.31±0.15, anti-B7DC Ab: 2.01±0.42, vs. control: 1.18±0.27, p<0.05). To further examine the roles of the PD-1 pathway in cGVHD, we next used B7H1 KO mice as recipients. Allogeneic B7H1 KO BMT recipients showed significantly more severe skin cGVHD than WT controls (p < 0.05). Histopathological examination of the skin showed significantly increased cGVHD damage in recipients of B7H1 KO donors (5.86 ± 0.85 vs. 8.38 ± 0.38, P < 0.05). Since B7H1 expression of hematopoietic cells is critical for donor Treg cell expansion, we then harvested cells from PLNs 14 and 28 days after BMT and analyzed cytokine expression levels. Recipients of B7H1 KO showed fewer Foxp3+ regulatory T cells in the early phase (day 14), whereas there was no difference in the frequency of Foxp3+ regulatory T cells in the late phase (day 28). We previously reported that Th1 and Th17 cells contribute to the pathogenesis of cGVHD using this mouse model. On day 28, IL-17+ IFNg+ T cells were detected significantly more frequently in recipients of B7H1 KO donors than those of WT recipients (p < 0.05). These results suggested that B7H1 expression in the recipient regulates the frequency of IL-17+ IFNg+ T cells and Foxp3+ regulatory T cells. Finally, we established chimeras that expressed B7H1 in hematopoietic cells, but not in parenchymal tissue by transferring WT BM cells into B7H1KO recipients. Chimeric mice had significantly poorer cGVHD scores than control mice (WT BM cells into WT recipients), suggesting the role of host parenchymal tissue cell expression of B7H1 in cGVHD. Taken together, the PD-1 axis, especially B7H1 expression on recipients, regulates the frequency of IL-17+ IFNg+ T cells and contributes to the pathogenesis of cGVHD.