Structure and cancer immunotherapy of the B7 family member B7x.

B7x (B7-H4 or B7S1) is a member of the B7 family that can inhibit T cell function. B7x protein is absent in most normal human tissues and immune cells, but it is overexpressed in human cancers and often correlates with negative clinical outcome. The expression pattern and function of B7x suggest that it may be a potent immunosuppressive pathway in human cancers. Here, we determined the crystal structure of the human B7x immunoglobulin variable (IgV) domain at 1.59 Å resolution and mapped the epitopes recognized by monoclonal antibodies. We developed an in vivo system to screen therapeutic monoclonal antibodies against B7x and found that the clone 1H3 significantly inhibited growth of B7x-expressing tumors in vivo via multiple mechanisms. Furthermore, the surviving mice given 1H3 treatment were resistant to tumor rechallenge. Our data suggest that targeting B7x on tumors is a promising cancer immunotherapy and humanized 1H3 may be efficacious for immunotherapy of human cancers.

Tissue-expressed B7-H1 critically controls intestinal inflammation.

B7-H1 (PD-L1) on immune cells plays an important role in T cell coinhibition by binding its receptor PD-1. Here, we show that both human and mouse intestinal epithelium express B7-H1 and that B7-H1-deficient mice are highly susceptible to dextran sodium sulfate (DSS)- or trinitrobenzenesulfonic acid (TNBS)-induced gut injury. B7-H1 deficiency during intestinal inflammation leads to high mortality and morbidity, which are associated with severe pathological manifestations in the colon, including loss of epithelial integrity and overgrowth of commensal bacteria. Results from bone marrow chimeric and knockout mice show that B7-H1 expressed on intestinal parenchyma, but not on hematopoietic cells, controls intestinal inflammation in an adaptive immunity-independent fashion. Finally, we demonstrate that B7-H1 dampened intestinal inflammation by inhibiting tumor necrosis factor α (TNF-α) production and by stimulating interleukin 22 secretion from CD11c(+)CD11b(+) lamina propria cells. Thus, our data uncover a mechanism through which intestinal tissue-expressed B7-H1 functions as an essential ligand for innate immune cells to prevent gut inflammation.

B7x and myeloid-derived suppressor cells in the tumor microenvironment: A tale of two cities.

A new study demonstrates the tumorigenic functions of B7x and reveals a link between B7x and myeloid-derived suppressor cells (MDSCs) within the tumor microenvironment. We propose that the binding of B7x to a hitherto unidentified receptor on MDSCs may stimulate their proliferation and/or immunosuppressive functions, hence promoting tumor growth.

Host b7x promotes pulmonary metastasis of breast cancer.

B7x (B7-H4 or B7S1) is an inhibitory member of the B7 family of T cell costimulation. It is expressed in low levels in healthy peripheral tissues, such as the lung epithelium, but is overexpressed in a variety of human cancers with negative clinical associations, including metastasis. However, the function of B7x in the context of cancer, whether expressed on cancer cells or on surrounding "host" tissues, has not been elucidated in vivo. We used the 4T1 metastatic breast cancer model and B7x knockout (B7x (-/-)) mice to investigate the effect of host tissue-expressed B7x on cancer. We found that 4T1 cells were B7x negative in vitro and in vivo, and B7x(-/-) mice had significantly fewer lung 4T1 tumor nodules than did wild-type mice. Furthermore, B7x(-/-) mice showed significantly enhanced survival and a memory response to tumor rechallenge. Mechanistic studies revealed that the presence of B7x correlated with reduced general and tumor-specific T cell cytokine responses, as well as with an increased infiltration of immunosuppressive cells, including tumor-associated neutrophils, macrophages, and regulatory T cells, into tumor-bearing lungs. Importantly, tumor-associated neutrophils strongly bound B7x protein and inhibited the proliferation of both CD4 and CD8 T cells. These results suggest that host B7x may enable metastasizing cancer cells to escape local antitumor immune responses through interactions with the innate and adaptive immune systems. Thus, targeting the B7x pathway holds much promise for improving the efficacy of immunotherapy for metastatic cancer.

T cell coinhibition and immunotherapy in human breast cancer.

Costimulation and coinhibition generated by the B7 family and their receptor CD28 family have key roles in regulating T lymphocyte activation and tolerance. These pathways are very attractive therapeutic targets for human cancers including breast cancer. Gene polymorphisms of B7x (B7-H4/B7S1), PD-1 (CD279), and CTLA-4 (CD152) are associated with increased risk of developing breast cancer although the underlying mechanisms are unclear. In human breast cancer microenvironment, up-regulation of coinhibitory B7/CD28 members B7x, B7-H3 (CD276), and PD-L1 (B7-H1/CD274) on tumor cells as well as PD-1 and PD-L1 on tumor-infiltrating immune cells are emerging as immune evasion pathways. Chemotherapy can affect the expression of these molecules, and therefore may dampen the immune response against breast cancer. Immunotherapy targeting T cell coinhibition as monotherapy or combined with standard therapies are in early stages of clinical development, but hold great promise for treatment of human breast cancer.

B7x in the periphery abrogates pancreas-specific damage mediated by self-reactive CD8 T cells.

B7x (B7-H4 or B7S1) is the seventh member of the B7 family, and its in vivo function remains largely unknown. Despite new genetic data linking the B7x gene with autoimmune diseases, how exactly it contributes to peripheral tolerance and autoimmunity is unclear. In this study, we showed that B7x protein was not detected on APCs or T cells in both human and mice, which is unique in the B7 family. Because B7x protein is expressed in some peripheral cells such as pancreatic ß cells, we used a CD8 T cell-mediated diabetes model (AI4αß) in which CD8 T cells recognize an endogenous self-Ag, and found that mice lacking B7x developed more severe diabetes than control AI4αß mice. Conversely, mice overexpressing B7x in the ß cells (Rip-B7xAI4αß) were diabetes free. Furthermore, adoptive transfer of effector AI4αß CD8 T cells induced diabetes in control mice, but not in Rip-B7xAI4αß mice. Mechanistic studies revealed that pathogenic effector CD8 T cells were capable of migrating to the pancreas but failed to robustly destroy tissue when encountering local B7x in Rip-B7xAI4αß mice. Although AI4αß CD8 T cells in Rip-B7xAI4αß and AI4αß mice showed similar cytotoxic function, cell death, and global gene expression profiles, these cells had greater proliferation in AI4αß mice than in RIP-B7xAI4αß mice. These results suggest that B7x in nonlymphoid organs prevents peripheral autoimmunity partially through inhibiting proliferation of tissue-specific CD8 T cells, and that local overexpression of B7x on pancreatic ß cells is sufficient to abolish CD8 T cell-induced diabetes.