Tissue Parenchymal Cell Expression of B7-H1 Inhibits Infiltrating T Cell Expansion and Prevents Persistence of Graft-Versus-Host Disease

Abstract 2974

Alloreactive donor CD8⁺ T cells facilitate engraftment and mediate graft versus leukemia (GVL) effects but also cause graft versus host disease (GVHD) in murine and human recipients after allogeneic hematopoietic cell transplantation (HCT). B7-H1 (PD-L1) expression by antigen-presenting cells has an important role in tolerizing activated T cells by binding to PD-1. We and others previously reported that disruption of binding between B7-H1 and PD-1 augments acute GVHD. Parenchymal cells do not usually express B7-H1 but can be induced by inflammatory cytokines (i.e. IFN-g) to express B7-H1. The role of B7-H1 expression by parenchymal tissue cells in regulating the expansion and persistence of donor CD8⁺ cells in tissues of mice with GVHD has not yet been evaluated. In the current studies, we evaluated the role of B7-H1 expression by GVHD target tissues in regulating donor CD8⁺ T cell function in 3 different experimental GVHD systems, using in vivo bioluminescent imaging (BLI), in vivo BrdU-labeling, and in vitro proliferation assays. The first system evaluated the role of B7-H1 expression in TBI-conditioned recipients. In these recipients, injected donor CD8⁺ T cells showed two waves of expansion that correlated with two phases of clinical GVHD. The first wave of donor CD8⁺ T cell expansion was associated with upregulated expression of B7-H1 in GVHD target tissues and only weak clinical GVHD. The second wave of donor CD8⁺ T cell expansion was associated with loss of B7-H1 expression, vigorous donor CD8⁺ T proliferation and expansion in the GVHD target tissues, and lethal GVHD. In a gain-of-function experiment, B7-H1 expression was induced in hepatocytes by hydrodynamic injection of B7-H1 cDNA during the second wave of T cell expansion in mice with GVHD; this subsequently decreased T cell expansion in the liver and ameliorated GVHD. The second system evaluated the role of B7-H1 expression in anti-CD3-conditioned recipients. In wild-type recipients, injected donor CD8⁺ T cells had only a single wave of expansion, and the mice had no signs of GVHD. B7-H1 expression by tissue cells (i.e. hepatocytes) was up-regulated, and the tissue infiltrating donor CD8⁺ T cells were anergic. In B7-H1^−/− recipients, injected donor CD8⁺ T cells proliferated vigorously in GVHD target tissues and caused lethal GVHD.The third system evaluated the role of B7-H1 in unconditioned Rag-2^−/− recipients after administration of blocking anti-B7-H1 and in the B7-H1^−/−Rag-2^−/− chimeras with B7-H1 sufficient Rag-2^−/− bone marrow cells, in which B7-H1 deficiency was only in tissue parenchymal cells. Both blockade of B7-H1 and B7-H1 deficiency in parenchymal cells resulted in vigorous donor CD8⁺ T proliferation in GVHD target tissues and caused lethal GVHD. Taken together, these results show that expression of B7-H1 in GVHD target tissue parenchymal cells plays an important role in regulating the proliferation of infiltrating donor CD8⁺ T cells and preventing the persistence of GVHD. Our studies also indicate that TBI but not anti-CD3 conditioning can lead to loss of GVHD target tissue cell expression of B7-H1 and persistence of GVHD.