Immune responses against malignant tumors are targeted to various types of molecules that cancer cells express and may be recognized as foreign by the immune system. Protein antigens that stimulate T-cell responses are the most important types of antigens for protective antitumor immunity. In the past, the term tumor antigen has been used to encompass many different molecules expressed by tumor cells that are detected by the binding of antitumor antibodies, whether or not they stimulate protective immune responses. The tumor antigens that elicit T-cell immune responses can be classified into several groups (Fig. 1).
Fig1. Tumor antigens. Tumor neoantigens are most often encoded by random somatic mutations in genes unrelated to the cancer phenotype, but the encoded mutated peptides have new T-cell receptor con tact residues and are recognized as foreign by the patient’s T cells. Proteins that are not mutated but are abnormally expressed by tumors may induce T-cell responses in their hosts. Many of these tumor antigens include proteins, such as cancer-testis antigens, encoded by genes that are normally not expressed in most cells of adults, because of epigenetic suppression, but are depressed in tumor cells. Some tumor antigens may be overexpressed because of gene amplification, such as the HER2/NEU protein, which is abundant in many breast carcinomas. Tissue-specific antigens are proteins expressed by both cancer cells and the normal cell types from which tumors are derived, such as tyrosinase made by both melanocytes and malignant melanoma cells. Because of either gene deregulation or the abundance of the tumor cells, the amount of these proteins is high in the tumors, leading to T-cell responses. Tumors caused by oncogenic viruses produce viral proteins that stimulate CD8+ T cells specific for the infected cancer cells. EBV, Epstein-Barr virus.
Neoantigens Encoded by Mutated Genes
Tumor neoantigens are proteins encoded by mutated genes, which appear foreign to the immune system because they do not exist in normal cells and newly arise as a cancer develops. Most often, these neoantigens are encoded by genes carrying passen ger mutations, which are point mutations or deletions that are unrelated to the development or malignant phenotype of the tumors. The occurrence of passenger mutations, often abundant in tumors, reflects the genomic instability of cancer cells, and the mutant proteins encoded by passenger mutations are the only truly tumor-specific antigens in that they are unlikely to occur in any normal tissue or other tumors. Alternatively, a smaller number of neoantigens may be encoded by driver mutations in tumor-promoting oncogenes or tumor suppressor genes, and these may be shared by different tumors that develop in part because of these oncogenic mutations. Because T cells recognize only peptides bound to major histocompatibility complex (MHC) molecules, tumor neoantigens can be recognized only if peptides carrying the mutated amino acid sequences can bind to the patient’s MHC alleles. Exome sequencing of many cancers has revealed large numbers of passenger mutations, and computer algorithms have been used to predict which of these mutations occur within peptide sequences that are likely to bind to the cancer patient’s MHC alleles. Studies of cancer patients’ T cells indicate that the tumor neoantigen peptides predicted to bind to MHC molecules in individual patients do, in fact, stimulate T-cell responses in those patients, and that the number of different tumor antigen-specific T-cell clones that are activated correlates with the number of mutations in the cancer. An example of this is a subset of cancer patients whose tumors have mutations in genes encoding DNA mismatch repair proteins, which result in a very high mutational burden. These patients have strong T-cell responses to their tumors and are most likely to benefit from therapies designed to activate T cells.
Abnormally Expressed Unmutated Cellular Proteins
Some tumor antigens are the products of genes that are silenced in normal cells and derepressed in tumor cells or are proteins made by normal cells but produced in excessive amounts by tumors. These antigens are not inherently foreign for the host, but nevertheless they stimulate immune responses. There are several possible explanations for their immunogenicity. Normally, the antigens may be expressed for a limited time or at a particular location—for example, only during embryonic development or only in tissues that are not accessible to the immune system—so there is no long-lived immunologic tolerance to these proteins. Expression in a tumor later in life or in locations that are not protected from immune cells may be enough to stimulate immune responses. The amount of anti gen produced in a patient with cancer may be abnormally high because of overexpression in each tumor cell or an abundance of tumor cells, and this, too, may be enough to elicit an active immune response in certain conditions, such as in the setting of innate responses to dying host or tumor cells.
Major categories of unmutated tumor antigens that are more abundant in tumors than in normal tissues include cancer-testis antigens, proteins encoded by amplified genes, and tissue differentiation antigens. The expression of only some of these structurally unaltered tumor antigens is sufficiently different from expression in normal cells to stimulate protective immunity in patients. However, many of these tumor antigens are targets for antibody therapy and potential candidates for tumor vaccines.
• Cancer-testis antigens are proteins expressed in gametes and trophoblasts and in many types of cancers but not in normal somatic tissues. The first cancer-testis antigens identified were melanoma-associated antigens (MAGEs). They are expressed in melanomas and many other types of tumors and in normal testis. Subsequently, several other unrelated gene families have been identified that encode antigens expressed by melanoma cells and are recognized by CTL clones derived from melanoma patients. The MAGE proteins and these other melanoma antigens are silent in most normal tissues, except the testis and placental trophoblast, but they are expressed in a variety of malignant tumors. More than 200 cancer-testis genes in over 40 different gene families have been identified. About half are encoded by genes on the X chromosome, and the rest are distributed on the other chromosomes. It has been postulated that in most somatic cells, the genes encoding these proteins are silenced by epigenetic mechanisms, such as methylation of the promoter regions, but the loci are demethylated in cancer cells, allowing the genes to be expressed.
• Some proteins are expressed at abnormally high levels in tumor cells because the genes encoding these proteins are amplified. Although the products of these amplified genes typically do not induce protective immune responses, presumably because they are expressed in normal cells and induce tolerance, the proteins can be targeted by therapeutic approaches. One example of such a protein is the epidermal growth factor variant called HER2/NEU, which is overexpressed in some breast cancers. A monoclonal antibody specific for HER2 is used to treat patients whose tumors show high HER2 expression.
• Differentiation antigens are found on tumor cells and on the cell types of origin of the tumors but not on cells from other tissues. Two examples of such differentiation antigens in melanomas are tyrosinase, an enzyme involved in melanin biosynthesis, and MART-1, a protein required for melanosome function. Both CD8+ CTLs and CD4+ helper T-cell responses specific for tyrosinase and MART-1 peptides are found in patients with melanoma, perhaps because these antigens are expressed at high levels due to the large number of tumor cells. However, in many cases, differentiation antigens do not induce immune responses because they are normal self antigens. Even in these situations, differentiation antigens are important in oncology because they aid in accurate diagnosis of tumor types and serve as targets for passive immunotherapy. For example, some lymphomas and leukemias arise from B cells and express surface markers characteristic of this lineage, such as CD19 and CD20. Antibody and T-cell therapies targeting these proteins are used to treat the cancers.
Antigens of Oncogenic Viruses
The products of oncogenic viruses function as tumor antigens and elicit specific T-cell responses that may serve to eradicate virus-induced tumors. Viruses are implicated in the development of a variety of tumors in humans and experimental animals. Examples in humans include the Epstein-Barr virus (EBV), which is associated with B-cell lymphomas and nasopharyngeal carcinoma, and human papillomavirus (HPV), which is associated with carcinomas of the uterine cervix, oropharynx, and other sites. In most of these DNA virus-induced tumors, viral DNA in extrachromosomal episomes or integrated into host DNA encodes protein antigens that are found in the nucleus, cytoplasm, or plasma membrane of the tumor cells. These endogenously synthesized viral proteins can be processed and presented by MHC molecules on the tumor cell surface. Some viruses, such as hepatitis B and C, are associated with cancer but are not directly oncogenic. It is thought they promote tumors by inducing chronic inflammatory reactions in which tumor promoting growth factors and other signals are generated.
The ability of adaptive immunity to prevent the growth of DNA virus-induced tumors has been established by many observations. For instance, EBV-associated lymphomas and HPV-associated cervical cancers arise more frequently in immunosuppressed individuals, such as allograft recipients receiving immunosuppressive therapy and patients with AIDS. The efficacy of virus-specific adaptive immunity to prevent tumors may be due in large part to preventing infection and eliminating infected cells before cancers develop. Vaccination to prevent infection by these viruses also decreases the incidence of cancers caused by these viruses. A vaccine against HPV has reduced the incidence of cervical cancer and other HPV-associated lesions. The vaccine is composed of recombinant HPV capsid proteins from the most common oncogenic strains of HPV, which form virus-like particles free of viral genome. Vaccination against hepatitis B virus has reduced the incidence of chronic HBV infection and therefore HBV associated liver cancer.
Other Antigens of Tumors
Many attempts have been made to detect antigens in tumor cells and in the plasma of patients with cancer by producing antibodies against tumors and using these as screening reagents. Several classes of tumor antigens have been identified by this approach. It is, however, now clear that most of these antigens are produced even in normal cells, especially under conditions of tissue injury and inflammation. Therefore, the role of these antigens in tumor immunity is uncertain.
Oncofetal Antigens
Oncofetal antigens are proteins thought to be expressed at high levels in cancer cells and in fetal, but not adult, tissues. However, their expression in adults is not limited to tumors, but is increased in tissues and in the circulation in various inflammatory conditions, and the antigens are found in small quantities even in normal adult tissues. There is also no evidence that oncofetal antigens are inducers of antitumor immunity. Thus, their usefulness as tumor markers, targets of antibodies, or vac cine candidates is limited. The two most-studied oncofetal anti gens are carcinoembryonic antigen (CEA) and α-fetoprotein (AFP).
CEA (CD66) is a highly glycosylated membrane protein that functions as an intercellular adhesion molecule. High CEA expression is normally restricted to cells in the intestines, pancreas, and liver during the first two trimesters of gestation. Its expression is increased in many carcinomas of the colon, pancreas, stomach, and breast, and serum levels are also increased in these patients. Serum CEA can, however, be elevated in the setting of nonneoplastic diseases, such as chronic inflammatory conditions of the intestines or liver, so it is of limited clinical utility.
AFP is a circulating glycoprotein normally synthesized and secreted by the yolk sac and liver in fetal life. Fetal serum concentrations can be as high as 2 to 3 mg/mL, but serum concentrations in adults are low. Serum levels of AFP can be elevated in patients with hepatocellular carcinoma, germ cell tumors, and occasionally gastric and pancreatic cancers. An elevated serum AFP level is sometimes used as an indicator of advanced liver or germ cell tumors or of recurrence of these tumors after treatment.
Altered Glycolipid and Glycoprotein Antigens
Most human and experimental tumors express higher-than normal levels or abnormal forms of surface glycoproteins and glycolipids, including gangliosides, blood group antigens, and mucins. Tumors often have dysregulated expression of the enzymes that synthesize the carbohydrate side chains of mucins, which leads to the appearance of tumor-specific epitopes on the carbohydrate side chains or on the abnormally exposed poly peptide core. Several mucins have been the focus of diagnostic and therapeutic studies. One of these, a mucin called MUC1, is an integral membrane protein that is normally expressed only on the apical surface of breast ductal epithelium, a site that is relatively sequestered from the immune system. In some carcinomas, however, MUC1 is expressed in a nonpolarized fashion and contains new, tumor-specific carbohydrate and peptide epitopes detectable by mouse monoclonal antibodies. Whether effective vaccines can be developed with these epitopes remains an open question.
