T-cell costimulation and coinhibition generated by engagement of the B7 family and their receptor CD28 family are of central importance in regulating the T-cell response, making these pathways very attractive therapeutic targets. Here we describe HERV-H LTRassociating protein 2 (HHLA2) as a member of the B7 family that shares 10-18% amino acid identity and 23-33% similarity to other human B7 proteins and phylogenetically forms a subfamily with B7x and B7-H3 within the family. HHLA2 is expressed in humans but not in mice, which is unique within the B7 and CD28 families. HHLA2 protein is constitutively expressed on the surface of human monocytes and is induced on B cells after stimulation with LPS and IFN-γ. HHLA2 does not interact with other known members of the CD28 family or the B7 family, but does bind a putative receptor that is constitutively expressed not only on resting and activated CD4 and CD8 T cells but also on antigen-presenting cells. HHLA2 inhibits proliferation of both CD4 and CD8 T cells in the presence of T-cell receptor signaling. In addition, HHLA2 significantly reduces cytokine production by T cells including IFN-γ, TNF-α, IL-5, IL-10, IL-13, IL-17A, and IL-22. Thus, we have identified a unique B7 pathway that is able to inhibit human CD4 and CD8 T-cell proliferation and cytokine production. This unique human T-cell coinhibitory pathway may afford unique strategies for the treatment of human cancers, autoimmune disorders, infection, and transplant rejection and may help to design better vaccines. Interactions between members of the B7 ligand and CD28 receptor families generate positive costimulation and negative coinhibition, which are of central importance in regulating T-cell responses (1-3). B7-1/B7-2/CD28/CTLA-4 is the most extensively characterized of these pathways. Ligands B7-1 (CD80) and B7-2 (CD86) on antigen-presenting cells (APCs) bind to CD28 on naïve T cells and provide a major costimulatory signal to activate naïve T cells. After the initial activation, coinhibitory molecule cytotoxic T lymphocyte antigen-4 (CTLA-4, CD152) is induced on T cells and engages the same B7-1 and B7-2 ligands to restrain T-cell function. In contrast to the costimulatory activity of CD28, the interaction of B7-1 or B7-2 with CTLA-4 is essential for limiting the proliferative response of recently activated T cells to antigen and CD28-mediated costimulation.During the past decade, several new pathways in the B7 and CD28 families have been identified, including B7h/ICOS, PD-L1/PD-L2/PD-1, B7-H3/receptor, and B7x/receptor. B7h (4) (also called ICOS-L, B7RP-1 (5), GL50 (6), B7H2 (7), LCOS (8), and CD275) binds to the inducible costimulator (ICOS, CD278) on activated T cells (9), which induces strong phosphatidylinositol 3-kinase activity (10, 11) and leads to the expression of transcription factors involved in follicular helper CD4 T (Tfh) differentiation (12). Therefore, the B7h/ICOS pathway provides critical T-cell help to B cells. Deficiencies in this pathway result in substantially reduced numbers of mem...

Purpose HHLA2 (B7H7/B7-H5/B7y) is a newly identified B7 family member that regulates human T cell functions. However, its protein expression in human organs and significance in human diseases are unknown. The objective of this study was to analyze HHLA2 protein expression in normal human tissues and cancers, its prognostic significance, to explore mechanisms regulating HHLA2 expression, and to identify candidate HHLA2 receptors. Experimental Design An immunohistochemistry protocol and a flow cytometry assay with newly generated monoclonal antibodies were developed to examine HHLA2 protein. HHLA2 gene copy number variation was analyzed from cancer genomic data. The combination of bioinformatics analysis and immunological approaches was established to explore HHLA2 receptors. Results HHLA2 protein was detected in trophoblastic cells of the placenta and the epithelium of gut, kidney, gallbladder and breast, but not in most other organs. In contrast, HHLA2 protein was widely expressed in human cancers from the breast, lung, thyroid, melanoma, pancreas, ovary, liver, bladder, colon, prostate, kidney, and esophagus. In a cohort of 50 patients with stage I–III triple negative breast cancer, 56% of patients had aberrant expression of HHLA2 on their tumors, and high HHLA2 expression was significantly associated with regional lymph node metastasis and stage. The Cancer Genome Atlas revealed that HHLA2 copy number gains were present in 29% of basal breast cancers, providing a potential mechanism for increased HHLA2 protein expression in breast cancer. Finally, Transmembrane and Immunoglobulin Domain Containing 2 (TMIGD2) was identified as one of the receptors for HHLA2. Conclusion Wide expression of HHLA2 in human malignancies, association with poor prognostic factors and its T cell coinhibitory capability, suggests that the HHLA2 pathway represents a novel immunosuppressive mechanism within the tumor microenvironment and an attractive target for human cancer therapy.

Cytotoxic CD8 T lymphocytes (CTLs) play a pivotal role in the control of infection. Activated CTLs, however, often lose effector function during chronic infection. PD-1 receptor and its ligand PD-L1 of the B7/CD28 family function as a T cell coinhibitory pathway and are emerging as major regulators converting effector CTLs into exhausted CTLs during chronic infection with human immunodeficiency virus, hepatitis B virus, hepatitis C virus, and other pathogens capable of establishing chronic infections. Importantly, blockade of the PD-1/PD-L1 pathway is able to restore functional capabilities to exhausted CTLs and early clinical trials have shown promise. Further research will reveal how chronic infection induces upregulation of PD-1 on CTLs and PD-L1 on antigen-presenting cells and other tissue cells and how the PD-1/PD-L1 interaction promotes CTLs exhaustion, which is crucial for developing effective prophylactic and therapeutic vaccination against chronic infections.

SUMMARY 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 expressed B7-H1 and that B7-H1-deficient mice were highly susceptible to dextran sodium sulfate- or trinitrobenzenesulfonic acid-induced gut injury. B7-H1 deficiency during intestinal inflammation led to high mortality and morbidity, which were associated with severe pathological manifestations in the colon, including loss of epithelial integrity and overgrowth of commensal bacteria. Results from bone marrow chimeric and knock-out mice showed B7-H1 expressed on intestinal parenchyma, but not on hematopoietic cells, controlled intestinal inflammation in an adaptive immunity-independent fashion. Finally, we demonstrated that B7-H1 dampened intestinal inflammation by inhibiting TNF-α production and by stimulating IL-22 from CD11c+CD11b+ lamina propria cells. Thus, our data uncover a new mechanism by which intestinal tissue-expressed B7-H1 functions as an essential ligand for innate immune cells to prevent gut inflammation.

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 over-expressed in a variety of human cancers with negative clinical associations including metastasis. However, the function of B7x in the cancer context, whether expressed on cancer cells or on surrounding “host” tissues, has not been elucidated in vivo. We utilized 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 wildtype 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 (TANs), macrophages, and regulatory T cells into tumor-bearing lungs. Importantly, the TANs strongly bound B7x protein and inhibited proliferation of both CD4 and CD8 T cells. These results suggest that host B7x may enable metastasizing cancer cells to escape local anti-tumor immune responses through interactions with the innate as well as the adaptive immune systems. Targeting the B7x pathway thus holds much promise for improving the efficacy of immunotherapy for metastatic cancer.