Radiation-Free Anti-CD3-Conditioning Regimen Maintains Tissue Protection Mechanisms and Prevents GVHD: Role of Tissue Expression of B7H1

We reported that donor CD8⁺ T cells mediated graft versus leukemia (GVL) activity without causing graft versus host disease (GVHD) in anti-CD3-conditioned recipients, although the same dose of donor CD8⁺ T cells caused lethal GVHD in recipients conditioned with sublethal total body irradiation (TBI) (J. Immunol. 2007). We recently observed that donor CD8⁺ T cells from the liver of anti-CD3-conditioned recipients showed a marked reduction in proliferation in response to anti-CD3/CD28 stimulation, as compared to those from TBI-conditioned recipients, indicating that donor CD8⁺ T cells are tolerized in the tissues of anti-CD3-conditioned recipients. B7H1, a co-inhibitory molecule, is constitutively expressed by hematopoietic cells and expressed by parenchymal cells after IFN-γ induction. B7H1 tolerizes T cells by interaction with its ligand PD-1 on activated T cells. To explore the role of B7H1 in GVHD prevention in anti-CD3-conditioned recipients, donor C57BL/6 CD8⁺ T cells (20×10⁶) and bone marrow cells (100×10⁶) were transplanted into anti-CD3-conditioned wild-type or B7H1^−/− BALB/c recipients. While the wild-type recipients all survived for more than 100 days without signs of GVHD, the B7H1^−/− recipients developed severe GVHD with diarrhea, weight-loss, and hair-loss, and 70% of them died 60 days after transplantation. Similarly, while donor CD8⁺ T and BM cells induced little GVHD in unconditioned Rag-2^−/− BALB/c recipients, they induced severe lethal GVHD in B7H1^−/− Rag-2^−/− BALB/c recipients. Furthermore, donor CD8⁺ T and BM cells still induced lethal GVHD in unconditioned chimeric B7H1^−/−Rag-2^−/− recipients reconstituted with B7H1^+/+Rag-2^−/− BM. In addition, we observed upregulation of B7H1 expression by hepatocytes and intestine epithelial cells of anti-CD3-conditioned BALB/c or unconditioned Rag-2^−/− recipients after donor cells infusion, as judged by RT-PCR, flow cytometry analysis, and histoimmunostaining of B7H1. In vivo bioluminescent imagine showed much more severe tissue infiltration of donor T cells in B7H1^−/− recipients as compared to B7H1^+/+ recipients, and the in vitro proliferation of donor CD8⁺ T cells from the liver of the former recipients was much more vigorous than that of the latter recipients. These results demonstrate that B7H1 expression by tissue parenchymal cells rather than hematopoietic cells tolerizes infiltrating donor T cells in GVHD target tissues and prevents GVHD; and that the radiation-free anti-CD3-conditioning regimen can maintain this tissue protection mechanism.

(This study is surpported by Marcus Foundation and NIH R01 AI 066008).