Protect the killer: CTLs need defenses against the tumor
Gordon J. Freeman1, Arlene H. Sharpe2
& Vijay K. Kuchroo3
1 Department of Adult Oncology, Dana-Farber Cancer Institute, Harvard Medical School Boston, Massachusetts, USA
2 Department of Pathology, Harvard Medical School Boston, Massachusetts, USA
3 Center for Neurologic Diseases, Brigham and Women's Hospital, Harvard Medical School Boston, Massachusetts, USA kuchroo@rics.bwh.harvard.edu
Evasion of the immune system is an all too common feature of cancer. A new study suggests one evasion mechanism: induction of T-cell apoptosis through B7-H1, a molecule expressed on the surface of many tumor cells (pages 793−800).
Suppression of immunological reactivity is one of the major mechanisms by which neoplastic cells are thought to escape host defenses. Tumor-reactive CD4+ and CD8+ T cells can be found in the peripheral blood, lymph nodes and tumors of some tumor-bearing patients, and in some cases, elevated numbers of tumor-infiltrating T lymphocytes (TILs) correlate with better prognosis1. But unfortunately, in most cases tumors continue to grow, suppressing anti-tumor immune responses through mechanisms that are, as yet, not wholly clear. However, it is known that tumors can evade immune attack by downregulating MHC expression on their surface or by producing inhibitory cytokines such as transforming growth factor- (TGF-) or interleukin-10 (IL-10).
In this issue, Dong et al. show that tumors can actively inhibit immune responses by expression of B7 homolog 1 (B7-H1), also known as programmed death ligand 1 (PD-L1)2. B7-H1 is a B7 family member and ligand for programmed death-1 (PD-1), a member of the CD28 family3,
4,
5,
6. Receptor−ligand pairs of the B7-CD28 superfamily have key roles in regulating T-cell activation and tolerance7. Dong et al. built on recent findings that B7-H1 is expressed on many tumors5 and examine the consequences of this expression. They report that B7-H1 on tumors can interact with receptors on cytotoxic T lymphocytes (CTLs) and promote CTL death. They then go on to provide a potential avenue for therapy by showing that antibodies against B7-H1 enhance killing of tumor cells by protecting CTLs from B7-H1-induced death.
Some pathways within the B7-CD28 superfamily provide a critical, positive costimulatory signal that augments and sustains T-cell responses. This costimulatory signal works in conjunction with an antigen-specific signal provided by the interaction of the peptide antigen−major histocompatibility complex (MHC) with the T-cell receptor (TCR). Other B7-CD28 superfamily pathways contribute key negative signals that limit, terminate and/or attenuate T-cell responses.
The B7-1/B7-2−CD28/CTLA-4 (cytotoxic T-lymphocyte antigen-4) costimulatory pathway is the best characterized. However, this pathway is complex in that the B7 molecules B7-1 (CD80) and B7-2 (CD86) can each engage two receptors, the stimulatory CD28 receptor and the inhibitory CTLA-4 (CD152) receptor. The components of the pathway described by Dong et al.—the PD-1 receptor and its two ligands, B7-H1 (PD-L1) and PD-L2 (B7-DC)—belong to the most recently defined pathway within the B7-CD28 superfamily. The functions of this new pathway are just beginning to be studied3,
4,
5,
6,
8. In contrast to other CD28 family members, PD-1 expression is not restricted to T cells. PD-1 is expressed on activated, but not resting, CD4+ and CD8+ T cells, B cells and myeloid cells5,
8,
9. PD-1-deficient mice have an autoimmune phenotype, providing direct in vivo evidence that PD-1 acts as a negative regulator of immune responses10,
11.
Dong et al. and others5 have shown that many human tumors, including carcinomas of the breast, lung, ovary and colon, express B7-H1, whereas the corresponding normal tissues do not. Dong et al. show that this expression has functional significance. They find that the presence of B7-H1 on tumor cells can actively inhibit immune responses by promoting the apoptosis of effector CTLs via induction of Fas ligand (FasL) and IL-10, thereby providing a novel means for evading immune destruction by CTLs (Fig. 1a). Monoclonal antibody against B7-H1 reduced this apoptosis and led to greater tumor-cell killing. Notably, IFN-, a cytokine produced by CTLs, was found to induce expression of B7-H1 on many tumor cell lines. Thus, effector CTLs may hasten their own demise by production of IFN- and subsequent upregulation of B7-H1 expression on the tumor3,
4,
5.
a, B7-H1 and T-cell apoptosis. Tumors express B7-H1/PD-L1, which can potentially interact with at least two different receptors on activated cytotoxic T lymphocytes (CTLs). When the T-cell receptor (TCR) is activated by binding to MHC−peptide, the interaction of B7-H1 with its receptors provides second signals that downregulate the effector functions and survival of CTLs. Ligation of the second receptor may upregulate Fas-Ligand and IL-10 production, resulting in T-cell apoptosis. Interaction of B7-H1 with PD-1 recruits SHP-2, resulting in dephosphorylation of TCR proximal kinases, reduced TCR signal and decreased cytokine production, and ultimately cell-cycle arrest or death by growth-factor deprivation. b, Hypothetical model for temporal expression of B7-H1 receptors on T cells. Naive T cells do not express B7-H1 receptors. Activated T cells may rapidly express the second receptor for B7-H1, whose ligation upregulates IL-10 production and FasL expression. PD-1, a known receptor for B7-H1, is expressed later after activation on effector T cells. Tumors may engage either receptor, resulting in inhibition of the CTL function by IL-10 production and/or induce cell death by FasL upregulation or growth-factor deprivation.
Given that other pathways within the B7-C28 superfamily also regulate anti-tumor immunity, Dong et al. investigated the functional interactions between B7-H1 and another family member, B7-1. In this experiment, they used the P815 mastocytoma tumor cell line, which can grow progressively in mice and kill them. B7-1-transfected cells stimulate a potent antitumor immune response that leads to tumor rejection. However, B7-H1- and B7-1-cotransfected cells grew and killed the host, indicating that B7-H1-mediated inhibition of the anti-tumor immune response is strong enough to overcome potent costimulation mediated by the B7-1 receptor, CD28. These results are consistent with previous studies8, showing that B7-H1-mediated inhibition of CD8+ T-cell activation cannot be overcome by CD28-mediated costimulation but can be overcome by exogenous IL-2. Together, these studies suggest that the level of IL-2 will determine whether B7-H1 can lead to CD8+ T-cell apoptosis. B7-H1 seems to be a particularly potent inhibitor of CD8+ T-cell responses5,
8. In contrast, in CD4+ T cells, strong CD28-mediated costimulation can overcome B7-H1-mediated inhibition, probably because of higher production of IL-2 by CD4+ T cells5,
8. In lymphoid tissue, B7-1-CD28 and B7-2-CD28 interactions may overcome the immunoinhibitory effects of B7-H1 on CD4+ T cells, but in the absence of B7-1 and B7-2 on peripheral target tissues and tumors, the immunoinhibitory effects of B7-H1 may predominate. Further in vivo work is needed to determine the contribution of B7-H1 to tumor progression, but its in situ expression on a broad range of solid tumors is striking.
The current and previous studies by Dong et al. using resting T cells and B7-H1−immunoglobulin (Ig) fusion protein suggest that B7-H1 could provide a positive costimulatory signal, stimulating T-cell proliferation and inducing IL-10 production3,
6. In contrast, other studies have suggested that the interaction of B7-H1 with PD-1 results in inhibition of T-cell proliferation, reduced cytokine production and cell-cycle arrest4,
5,
8. The current study begins to reconcile these seemingly opposite results by providing evidence for a second receptor for B7-H1. A CTL clone that does not express PD-1 still bound B7-H1−Ig, indicating the presence of a second receptor for B7-H1. Incubation of the CTL clone with a B7-H1+ breast-tumor cell line resulted in increased apoptosis of CTL. Antibodies against B7-H1 but not PD-1−Ig fusion protein reduced the apoptosis of the CTL clone. These data suggest that engagement of the second receptor for B7-H1 functions in T- cell apoptosis, perhaps by inducing FasL.
The divergent data on B7-H1 function would make even more sense if PD-1 and the second receptor are expressed with different kinetics and on different subpopulations of T cells (Fig. 1b). When human primary T cells were stimulated with anti-CD3 and plate-bound B7-H1−Ig, there was an early increase in proliferation with increased IL-10 production, and a later decrease in proliferation with induction of apoptosis. Unstimulated T cells do not express any B7-H1 receptors. But if the second receptor is expressed earlier than PD-1, then B7-H1 engagement of this receptor could lead to early T-cell expansion and IL-10 production. IL-10 in turn could be responsible for the reduction in proliferation and apoptosis; IL-10 is a pleiotropic inhibitor of immune responses, reducing MHC and B7-2 expression on antigen-presenting cells, inducing anergy (unresponsiveness to antigen), regulatory T cells and increasing T-cell apoptosis. At later times, the activated effector T cell may express PD-1, and respond to B7-H1 with a combination of reduced cytokine production, cell-cycle arrest and apoptosis. Engagement of PD-1 by B7-H1 leads to recruitment of SHP-2, which can dephosphorylate TCR proximal kinases, leading to a reduced TCR signal5. Thus, the outcome of B7-H1 engagement of either receptor is inhibition of immune responses, in contrast to the opposing outcomes of B7-1 and B7-2 engagement of CD28 versus CTLA-4.
Many current attempts at harnessing the immune system to attack tumors focus on the generation of vaccines that can efficiently produce tumor-specific CTLs. In addition, efforts are underway to adoptively transfer effector T cells into cancer patients to promote tumor regression. The results of Dong et al. indicate that in either approach, CTLs will need defenses against the tumor. They suggest a new therapeutic approach by showing that blockade of the B7-H1 pathway results in increased survival of the T cells and decreased growth and expansion of tumor cells. Sustained CTL-effector responses may require blockade of B7-H1 on the target tumor tissue. Blockade of CTLA-4, another immunoinhibitory molecule, strongly stimulates anti-tumor immunity12.
Like any compelling study, this one leads the way to the next set of experiments. In this case, the next step is to address whether blockade of B7-H1 will stimulate anti-tumor immunity against naturally occurring B7-H1+ tumors in vivo and whether blockade of both CTLA-4 and B7-H1 pathways will have synergistic effects in promoting anti-tumor immunity and inducing tumor regression.
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(TGF-
, a cytokine produced by CTLs, was found to induce expression of B7-H1 on many tumor cell lines. Thus, effector CTLs may hasten their own demise by production of IFN-
