B7-H1 is an immunoglobulin-like immune suppressive molecule broadly detectable on the majority of human and rodent cancers, and its functions have been attributed to delivering an inhibitory signal to its counter-receptor programmed death-1 (PD-1) on T cells. Here we report that B7-H1 on cancer cells receives a signal from PD-1 to rapidly induce resistance against T cell-mediated killing because crippling signaling capacity of B7-H1 but not PD-1 ablates this resistance. Importantly, loss of B7-H1 signaling is accompanied by increased susceptibility to immune-mediated tumoricidal activity. In addition to resistance against T-cell destruction, B7-H1 ؉ cancer cells also become refractory to apoptosis in- IntroductionCancer cells display altered surface molecular signatures that distinguish them quantitatively and qualitatively from their normal derivatives. These modifications in receptor and ligand expression commonly facilitate tumor growth and progression or to evasion of host defense mechanisms. 1,2 For example, some tumor cells down-regulate their cell surface major histocompatibility complex (MHC), which is required for recognition by tumor antigen-specific T lymphocytes. 3 As a result, these tumor cells become less recognizable by the immune system and more resistant to immunemediated destruction. Another example is that during progression, cancer cells frequently overexpress proteases and modify glycosylation of cell surface proteins that are normally involved in tissue repair, remodeling, and homeostasis to facilitate invasion and metastasis. 4,5 In general, these modifications in cell membrane ligands and receptors regulate interactions between tumor cells and nontransformed cells in the microenvironment in a fashion that enhances tumor growth, invasion, and immune resistance.We previously identified an immunoglobulin (Ig)-like molecule termed B7-H1, 6 which is either constitutively or inducibly expressed by the majority of human and rodent cancer cells. 7,8 Ample evidence demonstrates that B7-H1 acts as a ligand for the receptor programmed death-1 (PD-1) to deliver an inhibitory signal to T cells, leading to inhibition of immune responses. 9 The mechanisms underlying B7-H1/PD-1-mediated suppression include induction of apoptosis, anergy, unresponsiveness, and exhaustion of T cells. 7,[10][11][12][13][14] Interaction between B7-H1 and PD-1 is also shown to participate in the suppression of autoimmune diseases and transplantation rejection in animal models. 15-18. A recent study suggests that B7-H1, in addition to PD-1, also binds B7-1 (CD80) on T cells to inhibit their activation. 19 We and others have observed that B7-H1 ϩ tumor cells are much more resistant to CD8 ϩ cytolytic T cell (CTL)-mediated destruction in vitro than their B7-H1-negative parental cells, and this resistance is correlated with decreased efficacy of immunotherapy in mouse tumor models. [20][21][22] Ablation of B7-H1 and PD-1 interaction by neutralizing antibodies could restore CTL-mediated lysis of tumor cells in vitro, suggesting ...

IntroductionInteraction of receptors and ligands between T cells and antigenpresenting cells (APCs) is a very early event for transmitting signals to control T-cell growth, differentiation, tolerance, and death. 1 For example, cytotoxic T lymphocyte (CTL)-associated antigen 4 (CTLA-4), which is rapidly up-regulated on activated T cells and delivers an inhibitory signal to T cells upon binding to CD80/CD86, antagonizes CD28 costimulation and maintains T-cell homeostasis and selftolerance. 2 CTLA-4 signaling, however, is not required for T-cell anergy. 3 Recent studies have established a critical role in T-cell regulation of a new molecular pathway involving B7-H1, B7-DC, and their receptor PD-1. 1,3 PD-1 is inducible on activated T, B, and myeloid cells. 4 PD-1 knockout (KO) mice develop systemic and organ-specific autoimmune diseases. 5,6 Therefore, PD-1 may play an essential role in suppression of autoimmunity and inflammation. B7-H1 is constitutively expressed in a fraction of dendritic cells (DCs) and macrophages, and cell-surface B7-H1 could be up-regulated in virtually all nucleated cells by inflammatory cytokines, including interferons. 7,8 In contrast, the expression of B7-DC is restricted to DCs and activated macrophages. 9,10 The exact role of B7-DC in the regulation of adaptive immunity and inflammation has yet to be clarified because it displays both positive and negative effects on T-cell activation, depending on the systems used. 1,3 There is ample evidence supporting the idea that, in peripheral tissues, B7-H1 is a dominant ligand for PD-1-mediated inhibition of effector T-cell function. 1,3 Several independent mechanisms underlying suppression of T-cell responses by B7-H1/PD-1 include deletion/apoptosis, 8,11 resistance to lysis, 12,13 growth inhibition, 14 and generation and maintenance of T-cell exhaustion. 15 Several studies also reveal an association of elevated expression of B7-H1 and PD-1 with disease progression, T-cell suppression, exhaustion, and death during HIV infection. [16][17][18][19] While the suppressive effect of PD-1 on execution of effector T-cell functions in peripheral organs is well documented, its role in making early decisions related to tolerance induction among naive or recently primed T cells is unknown. This requires early analysis of T-cell phenotype and function after exposure to tolerogen prior to exit from lymphoid organs. We used a classic in vivo system of T-cell anergy whereby naive CD8 ϩ OT-1 T-cell receptor (TCR) transgenic T cells are transferred into B6 mice. 20 OT-1 T cells specifically recognize a peptide encoding an H-2K b -restricted epitope of chicken ovalbumin (OVA). Virtually all transferred OT-1 T cells are engaged by antigen after intravenous administration of excess amounts of soluble OVA peptide without adjuvant as a tolerogen (0.5 mg/mouse). As a result, OT-1 T cells expand vigorously and reach up to 20% of total CD8 ϩ cells in the spleen within 3 days. After reaching a peak at day 4 or 5, T-cell numbers contract dramatically due to apoptosis. Wh...

IntroductionAmong the numerous molecules that regulate T-cell function, the inhibitory receptor PD-1 and its 2 B7 family ligands, B7-H1(PD-L1) and B7-DC (PD-L2), seem to play important roles in microbial immunity, tumor immunity, and autoimmunity. 1-6 PD-1 contains a classic immunoreceptor tyrosine-based inhibitory motif (ITIM) and binds the phosphatase SHP-2. 7 PD-1 engagement has been shown to induce T cell apoptosis and to inhibit proliferative responses and cytokine release in vitro in response to TCR engagement. 8,9 Definitive evidence that PD-1 represents an inhibitory receptor comes from analysis of PD-1 knockout mice, which develop strain-specific autoimmune syndromes later in life. 10,11 The more delayed and tissue-specific autoimmune phenotypes of PD-1 knockout mice contrast markedly with the multiorgan autoimmunity observed within the first few weeks of birth for CTLA-4 knockout mice. [12][13][14] These findings support the notion that PD-1 is part of a system that fine-tunes immune responses, in contrast to the "on-off switch" mediated by the B7-1/B7-2-CD28/CTLA-4 system. Most of the inhibitory roles of PD-1 on T-cell responses have been attributed to its interaction with B7-H1. In vivo, B7-H1:PD-1 interactions have been documented to inhibit T-cell-effector responses to both tumors as well as to normal peripheral tissues. It is highly likely that the up-regulation of B7-H1 by tumors that serves to protect them from immune attack is a reflection of the normal expression of B7-H1 within peripheral tissues that represents a natural mechanism for regulating tissue injury by effector T cells. B7-H1 is expressed constitutively on subsets of macrophages, B cells, and thymocytes as well as certain nonhematopoietic cells within many organs. 1,15 This PD1 ligand is inducible on dendritic cells and all lymphocytes, as well as endothelial cells, epithelial cells, myocytes, and mesenchymal cells. 1,15 B7-H1 plays a well-documented role in down-modulating T-cell responsesparticularly CD8 responses-within the liver, where it is expressed on cells lining the sinusoids. 16 B7-H1 knockout mice display decreased apoptosis of intrahepatic CD8 cells as well as enhanced CD8 responses within the liver after infections with microbes such as adenovirus and Listeria monocytogenes. 16,17 The role of B7-H1: PD-1 interactions extends beyond the liver. For example, autoimmune diabetes is exacerbated in diabetes-prone mouse strains crossed onto a B7-H1-null background. 18 Further evidence for the role of the B7-H1:PD-1 interaction in effector T-cell responses has come from recent studies demonstrating that "exhausted" CD8 cells from mice chronically infected with the clone 13 strain of LCMV express high levels of PD-1. 6 This inactivated functional phenotype can be reversed in mice treated with blocking antibodies to B7-H1, emphasizing the important role of the B7-H1:PD-1 interaction in down-modulating effector immune responses. To date, however, the earliest point at which a role for B7-H1/PD-1 in the CD8 T cell response has b...

B7-H3 is a B7 family molecule with T cell costimulatory function in vitro. The in vivo role of B7-H3 in the stimulation of tumor immunity is unclear. We report here that expression of B7-H3 by transfection of the mouse P815 tumor line enhances its immunogenicity, leading to the regression of tumors and amplification of a tumor-specific CD8+ CTL response in syngeneic mice. Tumor cells engineered to express B7-H3 elicit a rapid clonal expansion of P1A tumor Ag-specific CD8+ CTL in lymphoid organs in vivo and acquire the ability to directly stimulate T cell growth, division, and development of cytolytic activity in vitro. Our results thus establish a role for B7-H3 in the costimulation of T cell immune responses in vivo.