Co-signaling molecules are surface glycoproteins that positively or negatively regulate the
Co-signaling molecules are surface glycoproteins that positively or negatively regulate the T cell response to antigen. functions and therapeutic implications and to introduce newly identified B7 members such as B7-H5, B7-H6, and B7-H7. T-T conversation for T cell homeostasis (17). Another reverse signaling through B7.1 and B7.2 was reported in T cell-APC conversation (18-20). B7 molecules expressed on DCs can transmit a suppressive signal into DCs through CTLA-4 on Treg cells or CTLA-4.Ig by enhancing IFN- production of DCs, which, in turn, induces indoleamine 2,3-dioxygenase (IDO), PIK-293 an enzyme metabolizing tryptophan into kynurenine, an immunosuppressive metabolite, in autocrine or paracrine mode (18). Thus, the bidirectional B7-CTLA-4 pathway appears to be critical for the downregulation of T cell response and PIK-293 induction of T cell tolerance. B7-H1 (CD274, PD-L1) AND B7-DC (CD273, PD-L2) B7-H1 is usually constitutively expressed not only on APCs but also in a wide range of normal somatic tissues including the heart, lung, placenta and liver (21,22). In contrast, B7-DC expression is largely restricted to APCs such as DCs and macrophages. Both molecules share the PD-1 receptor that is expressed on T cells, Treg cells, B cells, activated monocytes, DCs, NK cells, and NKT cells (23,24). The cytoplasmic tail of the PD-1 receptor contains ITIM and the immunoreceptor switch motif (ITSM) that binds to SHP-1 and SHP-2, transmitting a co-inhibitory signal into T cells and down-regulating Bcl-xL expression, which leads to T cell functional impairment and apoptosis (21). Due to its ectopic expression on non-hematopoietic tissues including normal peripheral organs and cancer tissues, B7-H1 is believed to limit T cell activation in peripheral organs, leading to peripheral tolerance. Thus, the B7-H1-PD-1 pathway in tumor microenvironment can be a crucial immune escape mechanism (25-27), indicating that B7-H1 and PD-1 can act as key immune checkpoint proteins. Many clinical observations further support the immune checkpoint activities of the B7-H1-PD-1 pathway by demonstrating the correlation between B7-H1 expression levels in cancer tissues and patients’ survival. Specifically, high levels of cancer B7-H1 expression is associated with poor prognosis in patients with kidney, lung, pancreas and urothelial cancers (28-32). In addition, chronic exposure to antigens that are derived from cancers and chronic viral infections can PIK-293 upregulate the PD-1 expression on antigen-specific effector T cells, which leads Rac-1 to a state of functional exhaustion or anergy, one of the crucial immune evasion mechanisms for cancers and persistently infecting viruses (33-35). In PIK-293 an effort to discover another possible receptor for B7-H1, recent studies revealed that B7-H1 binds to B7.1 that acts as a counter receptor delivering a co-inhibitory signal. Interestingly, B7-H1 can also act as a counter receptor for B7.1 and transmit a co-inhibitory signal, indicating that the B7-H1-B7.1 pathway regulates immune functions through co-inhibitory bidirectional conversation (22). Furthermore, there are some and evidence indicating that both B7-H1 and B7-DC bind to a yet-unknown co-stimulatory receptor that is involved in T cell activation (36,37). Similar to the B7-CTLA-4 pathway, B7-H1-PD-1 pathway is also implicated in growth and suppressive activity of Treg cells (38). It has been well known that IFN- is usually a key cytokine to induce B7-H1 in non-hematopoietic cells such as cancer cells. Therefore, B7-H1 induction in tumor microenvironment by PD-1+ CTLs that infiltrated and produced IFN- may PIK-293 represent an adaptive immune resistance, leading to immune escape of cancer cells in the presence of anti-tumor responses (3,27,39). Many preclinical and clinical trials have been underway using B-H1 or PD-1 blocking antibodies. Early clinical studies have shown the promise in the treatment of patients with advanced cancers such as colon, renal, and lung cancers (40). Preclinical models also demonstrate a powerful synergy between tumor vaccines and blockade of the B7-H1-PD-1 pathway (41,42). B7-H2 (ICOSL, CD275) B7-H2 is usually a co-stimulatory ligand that binds only to ICOS,.