Immune Escape Mechanisms As a Guide for Cancer Immunotherapy Gregory L
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Published OnlineFirst December 12, 2014; DOI: 10.1158/1078-0432.CCR-14-1860 Perspectives Clinical Cancer Research Immune Escape Mechanisms as a Guide for Cancer Immunotherapy Gregory L. Beatty and Whitney L. Gladney Abstract Immunotherapy has demonstrated impressive outcomes for exploited by cancer and present strategies for applying this some patients with cancer. However, selecting patients who are knowledge to improving the efficacy of cancer immunotherapy. most likely to respond to immunotherapy remains a clinical Clin Cancer Res; 21(4); 1–6. Ó2014 AACR. challenge. Here, we discuss immune escape mechanisms Introduction fore, promote tumor outgrowth. In this process termed "cancer immunoediting," cancer clones evolve to avoid immune-medi- The immune system is a critical regulator of tumor biology with ated elimination by leukocytes that have antitumor properties the capacity to support or inhibit tumor development, growth, (6). However, some tumors may also escape elimination by invasion, and metastasis. Strategies designed to harness the recruiting immunosuppressive leukocytes, which orchestrate a immune system are the focus of several recent promising thera- microenvironment that spoils the productivity of an antitumor peutic approaches for patients with cancer. For example, adoptive immune response (7). Thus, although the immune system can be T-cell therapy has produced impressive remissions in patients harnessed, in some cases, for its antitumor potential, clinically with advanced malignancies (1). In addition, therapeutic mono- relevant tumors appear to be marked by an immune system that clonal antibodies designed to disrupt inhibitory signals received actively selects for poorly immunogenic tumor clones and/or by T cells through the cytotoxic T-lymphocyte–associated antigen establishes a microenvironment that suppresses productive anti- 4 (CTLA-4; also known as CD152) and programmed cell death-1 tumor immunity (Fig. 1). (PD-1; also known as CD279) molecules are demonstrating long- Immune signatures for cancer immunotherapy are actively term survival benefits for some patients with metastatic melano- being explored across many clinical studies (8). However, unlike ma (2, 3). However, not all tumors appear to respond to these small-molecule inhibitors, for which the presence or absence of a immunomodulatory maneuvers. This observation emphasizes target mutation may predict response, the efficacy of immuno- the heterogeneity of cancer and suggests the existence of addi- therapy relies on the functional competence of multiple immu- tional immunoregulatory mechanisms in many patients. A major nologic elements. For example, strategies designed to harness the challenge for cancer immunotherapy, therefore, lies in under- antitumor potential of endogenous T cells rely on the proper standing these resistance mechanisms for selecting patients who execution of a series of steps that have been described in the cancer are most likely to benefit. immunity cycle (9). In this cycle, tumor cells must release immu- A central tenet of cancer immunotherapy is that the immune nogenic tumor antigens for the priming and activation of tumor- system actively surveys for malignant transformation and can be specific T cells. Tumor-reactive T cells must then infiltrate tumor induced to recognize and eliminate malignant cells. This premise tissue and recognize cancer cells in the context of a peptide–MHC was initially proposed by Thomas and Burnet in 1957 in the complex to induce cancer cell death. To evade immune-mediated immunosurveillance hypothesis, which postulated a role for the elimination, tumors must then develop strategies that disrupt this immune system in controlling the development and outgrowth of cycle. Therefore, predicting the clinical benefit of T-cell immuno- nascent transformed cells (4, 5). This hypothesis has since been therapy is likely to require an understanding of each of these steps refined based on knowledge that the immune system cannot only as it relates to a patient's individual tumor. Here, we discuss tumor protect against tumor development but can also select for tumors antigenicity, tumor immunogenicity and the tumor microenvi- with decreased antigenicity and/or immunogenicity and there- ronment as key elements of this cycle that may be used to predict clinical benefit with T-cell immunotherapy and guide the devel- opment of rational combinations. Abramson Cancer Center, Department of Medicine, Division of Hema- tology-Oncology, University of Pennsylvania Perelman School of Med- icine, Philadelphia, Pennsylvania. Antigenicity Corresponding Author: Gregory L. Beatty, University of Pennsylvania Perelman The ability of the immune system to distinguish between School of Medicine, Department of Medicine, Division of Hematology/Oncology normal and malignant cells is fundamental to cancer immuno- Smilow Center for Translational Research, Room 8-112, 3400 Civic Center therapy and relies, in part, on malignant cells retaining sufficient Boulevard Building, 421 Philadelphia, PA 19105-5156. Phone: 215-746-7764; Fax: antigenicity. Tumors can express a variety of nonmutated and 215-573-8590; E-mail: [email protected] mutated antigens that have the potential to elicit tumor-specific doi: 10.1158/1078-0432.CCR-14-1860 immune responses (10). However, to avoid immune-mediated Ó2014 American Association for Cancer Research. elimination, cancer cells may lose their antigenicity. Loss of www.aacrjournals.org OF1 Downloaded from clincancerres.aacrjournals.org on September 29, 2021. © 2014 American Association for Cancer Research. Published OnlineFirst December 12, 2014; DOI: 10.1158/1078-0432.CCR-14-1860 Beatty and Gladney cells may inform the potential susceptibility of a cancer to Translational Relevance immune elimination by endogenous T cells. Immunotherapy is a promising treatment modality for Tumor antigens can be derived from viral proteins, proteins cancer. However, predicting which patients will benefit encoded by cancer-germline genes, differentiation antigens, and remains a challenge. Fundamental to the outgrowth of malig- proteins arising from somatic mutations or gene rearrangements nant cells is their ability to evade immune elimination, and (10). However, because malignant cells across patients and even therefore understanding the mechanisms exploited by a cancer within the same patient can be quite different, it remains unclear may help guide the application of immunotherapy. Here, we how to effectively quantify the antigenicity of a cancer. One propose that the tailoring of immunotherapy to patients may approach might be to determine the frequency of somatic muta- require knowledge of tumor antigenicity and immunogenicity tions within a cancer as a predictor of neoantigens. Until recently as well as the composition of the tumor microenvironment. In though, it was unclear whether mutated proteins that produce our discussion, we describe challenges and future directions in "neoantigens" could even serve as effective targets for cancer applying this knowledge to enhancing the efficacy of immu- immunotherapy. Recent studies have now demonstrated that T notherapy by improving patient selection and by informing cells recognizing neoantigens infiltrate tumor tissue and can the development of rational combination therapies. mediate strong tumor regressions after ex vivo expansion and adoptive transfer (11–13). These findings raise the possibility that tumor mutational frequency might correlate with respon- siveness to immunotherapy (14). antigenicity can arise due to the immune selection of cancer cells Mutational analysis of solid tumor malignancies has recently fi that lack or mutate immunogenic tumor antigens as well as revealed signi cant variability with some immunogenic cancers, through the acquisition of defects or deficiencies in antigen such as melanoma, displaying a high mutation rate and other presentation [e.g., loss of major histocompatibility (MHC) nonimmunogenic cancers, such as PDAC, showing a low muta- fi expression or dysregulation of antigen processing machinery; tion rate (15, 16). Although this nding has sometimes been used ref. 6]. As a result, knowledge of the antigenicity of malignant to explain the poor immunogenicity of cancers such as pancreatic Tumor (i) Loss of antigenicity (ii) Loss of immunogenicity (iii) Immunosuppressive microenvironment Immunosuppressive leukocytes (e.g., macrophages) Angenic tumor cell Antumor leukocytes (e.g., effector T cells) Poorly angenic tumor cell Immunosuppressive molecule (e.g., PD-L1) Figure 1. Immune escape mechanisms in cancer. Clinically apparent tumors must evolve mechanisms to evade immune elimination. During this process, nascent transformed cells may be selected for based on (i) a loss of antigenicity and/or (ii) a loss of immunogenicity. Loss of antigenicity may be achieved through the acquisition of defects in antigen processing and presentation or through the loss of immunogenic tumor antigens leading to a lack of immunogenic peptides presented in the context of a peptide–MHC complex. Although a loss of antigenicity is also associated with a loss of immunogenicity, malignant cells can gain additional immunosuppressive properties, such as expression of PD-L1 or secretion of suppressive cytokines (e.g., IL10 and TGFb), which further reduces their immunogenicity. (iii) Tumors may also escape immune elimination by orchestrating an immunosuppressive microenvironment. Malignant transformation induced by alterations in oncogenes and tumor-suppressor genes can