Immunology (Lippincott Illustrated Reviews Series) 2nd Edition
Chapter 19: Tumor Immunity
Cell growth and cell death are normally balanced so that a stable number of cells are maintained in a given tissue. Occasionally, however, cells arise that no longer respond to the usual checks and balances for division and death. These are tumor cells. Development from a normal cell to a cancerous one requires several transformation steps. Transformed tumor cells express characteristic cell surface antigens, and these antigens often initiate immune responses. Therapeutic approaches, which attempt to exploit these normal immune responses to tumors, continue to be investigated. However, tumors also evade recognition by the immune system, and at times, tumor growth appears to be enhanced by immune mediators produced against that very tumor.
A tumor, or neoplasm, is a collection of the clonal descendants of a cell whose growth has gone unchecked. When a tumor continues to grow and to invade healthy tissue, it is considered to be a cancer.
A. Terminology and definitions
Malignant tumors are distinguished from benign tumors by their progressive growth and invasiveness. Metastasis is a characteristic of many malignant tumors (cancers). Metastatic cells become dislodged from the main tumor, invade blood or lymphatic vessels, and travel to other tissues, where they continue to grow and to invade. In this way, tumors at one site can give rise to secondary tumors at other sites within the body (Fig. 19.1).
Metastasis. Tumor cells can detach from the primary tumor and travel through the vasculature to establish metastatic tumors at other sites.
Classification of tumors is based on the embryonic origin of the tissue from which the malignant cells are derived. Carcinomas develop from endodermal or ectodermal tissues (e.g., skin, glands) and constitute most malignant tumors, including cancers of the breast, colon, and lung. Sarcomas develop from bone and cartilage and have a much lower incidence than carcinomas. Leukemias are malignant cells of hematopoietic lineage that proliferate as individual cells, whereas lymphomas arise from malignant hematopoietic cells but grow as solid tumors.
B. Malignant transformation
Experiments with cultured cells have allowed researchers to trace the development of tumors. Cells that are infected with certain viruses (e.g., SV40 or Rous sarcoma virus), irradiated (ultraviolet light or ionizing radiation), or treated with certain DNA-altering chemicals show altered growth properties and often induce tumors when injected into animals (Fig. 19.2). Such transformed cells can be grown in culture almost indefinitely. In some cases when retroviruses (RNA viruses) induce such growth change in cells, the process is related to the presence of oncogenes (cancer-producing genes) of the virus. Change from a normal cell to a tumor cell is known as malignant transformation. The process of malignant transformation requires at least two distinct phases. The first phase is initiation, which changes the genome of the cell; the second is promotion, which results in stimulation of cell division.
Malignant transformation. Transfection or irradiation of normal cultured cells alters them in such a way that they will induce the formation of tumors when injected into experimental animals.
C. Tumors of the immune system
Lymphomas and leukemias are tumors of immune cells. Lymphomas are solid tumors within lymphoid tissues such as bone marrow and lymph nodes. Hodgkin and non-Hodgkin lymphomas are examples. Leukemias are composed of dispersed single cells that arise from the bone marrow and may involve cells from either lymphoid or myeloid lineages. Acute leukemias arise from less mature cells and are found in both children and adults. Chronic leukemias are tumors of more mature cells that develop slowly and are seen only in adults.
D. Oncogenes and cell growth
In some cases, malignant transformation induced by retroviruses (or RNA viruses) has been linked to the presence of cancer-causing genes called oncogenes within the retrovirus. The viral oncogene Src (v-Src) from the Rous sarcoma virus is an example of this type of gene. Inserting this virus into normal cells in culture results in malignant transformation. Cells have genes, referred to as proto-oncogenes or cellular oncogenes, that are counterparts of retroviral oncogenes. Conversion of a cellular proto-oncogene (e.g., c-Src) into a cancer-promoting oncogene (e.g., v-Src) can occur by mutation. This change is generally accompanied by a change in cellular growth because the cellular oncogenes normally code for growth-controlling proteins.
1. Stimulators of cell division: Oncogenes that function as stimulators of cell division include those that encode growth factors and growth factor receptors. Oncogenes may also code for proteins involved in signaling pathways, particularly via tyrosine phosphorylation, and those that function as transcription factors. Increased activity of proteins encoded by oncogenes in this category can result in uncontrolled cellular proliferation. Examples include sis, which encodes a chain of platelet-derived growth factor, and erb-b, which encodes epidermal growth factor receptor (Table 19.1). Src and Abl in their proto-oncogenic (cellular) forms encode tyrosine kinases that regulate cell division. In their oncogenic forms, the regulatory function of these proteins has been lost, and the affected cells will have unregulated proliferation. Ras codes for a GTP-binding protein; continued stimulation of division occurs when the oncogene form of ras remains active. Transcription factors are encoded by the fos, jun, and abl oncogenes.
2. Tumor suppressor genes: Oncogenes that are inhibitors of cell division and are sometimes referred to as anti-oncogenes function as tumor suppressor genes. When a tumor suppressor is inactivated through mutation, the ability to suppress cell growth is lost, and uncontrollable cell proliferation can result. Mutated forms of the tumor suppressor p53 have been found in many human tumor cells. Mutation of the tumor suppressor Rb can lead to development of the malignant retinal tumors in children with hereditary retinoblastoma.
3. Regulators of apoptosis: A third category of cancer-related genes are those that regulate apoptosis. Some members of this group prevent programmed cell death (apoptosis), whereas others induce it. Bcl-2, an antiapoptotic oncogene discovered in a B-cell follicular lymphoma, normally regulates cell survival of selected lymphocytes during development. When Bcl-2 is inappropriately expressed, a cell that would normally die via apoptosis instead survives, resulting in unregulated cell proliferation. One of several proteins related to the prosurvival Bcl-2 is Bax, which is pro-apoptotic. The ratio of Bcl-2 to Bax proteins within a cell determines whether that cell will survive or undergo programmed cell death.
E. Tumor antigens
Tumor cells express antigens on their surfaces that are often the targets of immune responses. Many tumor antigens are cellular peptides presented by MHC molecules that stimulate antigen-specific T-cell proliferation (Table 19.2). Some antigenic molecules on tumor cells are variant forms of normal proteins that result from mutation of the gene encoding the protein. Others are normally found only on cells of certain developmental stages or lineages and are antigenic when expressed out of their usual context. Still, other tumor antigens are simply molecules found at higher than normal concentration on tumor cells, whereas a few others are proteins encoded by genes unique to tumors.
1. Tumor-specific transplantation antigens (TSTAs): TSTAs are not found on normal somatic cells but result from mutations of genes and the resulting altered proteins that are expressed by the tumor cells. Identification of TSTAs on naturally occurring tumors has proved difficult, most likely because the immune response generally eliminates cells that express TSTAs at levels great enough to be antigenic. However, TSTAs have been identified on tumors induced in culture by viral transformation or treatment with carcinogenic chemicals. When introduced into syngeneic mice, TSTAs induce cell-mediated immune responses that attack the tumor cells (Fig. 19.3).
2. Tumor-associated transplantation antigens (TATAs): TATAs are not unique to tumor cells; rather, their expression on tumor cells is altered. For example, the tumor antigen may be found in excessive amounts or may be expressed on a cell type where it would not normally exist. Human breast cancer cells often have high levels of the growth factor receptor HER2/neu, which is found in very low concentrations on normal cells but is overexpressed on approximately 20% to 30% of primary breast tumors. HER2+ tumors are aggressive with high chance of recurrence (see Table 19.2). MAGE-1, BAGE, and GAGE-2 are examples of oncofetal antigens because they are expressed on tumors and on normal fetal cells. After the fetal stage of development, normal differentiated cells do not express these oncofetal antigens, except for germline cells of the testis. However, oncofetal antigens are also displayed on human melanomas, gliomas, and breast carcinomas. Another oncofetal antigen, alpha-fetoprotein, is found in fetal liver cells and liver carcinoma cells (and serum of individuals with liver cancer). Other tumor cells may express greater than normal levels of tissue-specific molecules (e.g., MART-1 and gp75 are overexpressed by melanoma cells), whereas still other tumor cells express aberrant forms of such molecules. An example is MUC-1, a glycosylated (carbohydrate-containing) mucin that is found with decreased glycosylation on pancreatic tumors. Decreased levels of carbohydrates may reveal hidden MUC-1 epitopes.
Identification of TSTAs. Transformed cells injected into syngeneic mice sometimes induce tumor formation but sometimes do not. When a nontumorigenic line is generated (tumor rejected) and CD8+ CTLs are harvested from that animal, those CTL cells can recognize TSTA-bearing tumor cells.
III. IMMUNE SURVEILLANCE
The immune surveillance theory suggests that cancer cells frequently arise within the body but are normally eliminated before they multiply sufficiently to become clinically detectable. Accordingly, through the workings of an effective immune system that patrols the body and mounts responses against abnormal cells, most transformed cells never become true cancers. Tumors arise only if they are able to escape immune surveillance (Fig. 19.4). Evidence supporting the immune surveillance theory comes from immunosuppressed and immunodeficient individuals who have increased tumor incidence.
Immune surveillance. The immune system is on patrol for abnormal cells, often halting malignant cell growth before tumors arise. Only those malignant cells that escape immune detection become clinical tumors.
The first line of immune defense against tumors comes from the less specific component of the immune response, the innate immune system. These mechanisms prevent spread of malignant disease and are not specific to particular tumor antigens but recognize broad characteristics of tumor cells.
1. NK cells: NK cells have a limited ability to discriminate between tumor cells and normal cells. Recall that NK recognition of targets occurs via killer activation receptors (KARs) and killer inhibitory receptors (KIRs) (see Chapter 5). KIRs recognize human MHC class I molecules: HLA-B and HLA-C. Another inhibitory NK receptor, CD94, recognizes another class I molecule called HLA-E. When a KAR is engaged by binding to its carbohydrate ligands on target cells, the “kill” signal to the NK cell is activated (Fig. 19.5). However, if the KIR receptors are engaged by binding of ligands on the surface of a target cell, then the “do not kill” signal is received by the NK cell, and the target cell survives. Failure to engage the KIR will result in NK-induced lysis of the target cell. When expression of MHC I molecules on the cell surface is abnormally low, as is the case in some malignant cells, KIRs might not recognize ligands on the target (malignant) cell and might proceed to kill it. In some cases, Fc receptors on NK cells can bind to antibody present on tumor cells (produced as part of the adaptive response against the tumor cell), leading to antibody-dependent cellular cytotoxicity.
NK recognition of tumor cell targets. Transformed cells may have fewer MHC I molecules per cell and express stress molecules that are recognized by KARs on NK cells, allowing the NK cell to kill that target cell. Decreased MHC I expression decreases NK-KIR binding, permitting killing of that target cell.
NK cells that attack malignant cells are sometimes referred to as lymphokine-activated killer cells (LAKs). These cells are generated in the presence of high concentrations of interleukin-2 and are able to kill fresh tumor cells. Tumor-infiltrating lymphocytes (TILs) are T lymphocytes, often CD8+ CTLs. They may also include some CD4+ T cells and NKT cells. A therapeutic strategy against malignant melanoma involves obtaining tumor-specific TILs from tumor biopsies and expanding the cells by stimulating with interleukin-2. These cells are then injected back into the patient. In some cases, partial regression of the tumors has been observed.
2. Cytokines: Cytokines with antitumor activity are secreted by macrophages, which are often found in the vicinity of tumors (Fig. 19.6). Tumor necrosis factor (TNF) is one such antitumor cytokine. When injected into animals with tumors, TNF-α and TNF-β can stimulate necrosis of the tumor cells. TNF-α also inhibits angiogenesis, the growth of new blood vessels by decreasing blood flow to the tumor.Interferons are another group of cytokines with antitumor activity. IFN-α, IFN-β, and IFN-γ have all been shown to increase MHC I expression on tumor cells (which often downregulate MHC I expression to evade the immune response). Increasing the MHC I expression can increase susceptibility of the tumor cells to CTLs. IFN-γ may also directly inhibit proliferation of tumor cells.
Cytokines with antitumor activity. Activated macrophages release TNF-α and TNF-β, which induce tumor cell necrosis and also release IFN-α, IFN-β, and IFN-γ, which increase tumor cell MHC I molecules on tumor cells, allowing them to become targets of CTL killing.
Specific antigen-dependent immune responses can develop to antigens on tumor cells. Although they are not always effective in halting progression of a tumor, evidence exists that both humoral and cell-mediated immune responses can be induced in response to the presence of malignant cells (Fig. 19.7).
• Antibodies are known to be generated against certain tumor-specific antigens present on the surface of malignant cells.
• CTLs can sometimes kill tumor cells by direct contact.
• DTH reactions involve Th1 cells recruiting and activating macrophages, which attack and kill tumor cells.
IV. IMMUNE EVASION
Although both innate and adaptive immune responses are evoked by malignant cells, tumor cells often escape the immune system and go on to produce tumors and diseases that are often fatal. Several mechanisms that facilitate evasion of the immune response by tumor cells have been identified.
Adaptive immune responses against tumor cells. Humoral as well as cell-mediated immune responses are mounted against tumor cells.
A. Antibody enhancement of tumor growth
Because attempts to immunize cancer patients by injecting specific antibodies that were developed in culture against their tumor cells often resulted in enhanced tumor growth, studies have been initiated to explore the mechanisms of antibody-induced tumor cell growth. The antitumor antibodies may bind to the antigens on the tumor cells, masking the antigens and blocking the ability of CTL cells to bind and kill the tumor cell. Antibody bound to tumor antigen may inhibit binding of Fc receptors on macrophages, dendritic cells, and NK cells.
B. Antibody modulation of tumor antigens
In the presence of antibodies directed against tumor antigens, downregulation of expression of certain tumor-specific antigens has been demonstrated. In a process known as antigenic modulation, the antigens disappear for a time and then reappear when the antibody is eliminated. Cells that do not express the antigen are no longer targets of other adaptive immune responses (Fig. 19.8).
Antigenic modulation. Antibodies to tumor antigens may cause the tumor to downregulate antigen expression.
Decreased expression of tumor cell MHC I may impair CTL recognition, but increase recognition by and vulnerability to NK cells.
C. Modulation of MHC I expression
Tumor cells often express reduced levels of MHC I molecules. Malignant transformation may result in a reduction or total loss of MHC I molecules by the transformed cells. If tumor cells express decreased amounts of MHC I, NK cell responses to them may be enhanced, whereas CTL-mediated responses against those tumor cells are decreased (Fig. 19.9).
V. CANCER IMMUNOTHERAPY
Cancer immunotherapy is based on enhancement of the natural immune responses that the body mounts against malignant cells.
A. Cytokine therapy
Interleukins and interferons have been used to enhance the immune response to tumors. Because systemic administration of these cytokines can be dangerous, local application is required, complicating the treatment protocols. Although therapeutic benefit is sometimes obtained with cytokine therapy, more research and refinement of protocols are likely necessary before more widespread use and more benefit will be obtained from this approach (Fig. 19.10A).
Cancer immunotherapies. A. Application of exogenous cytokines can heighten immune responses to tumor cells. B. Monoclonal antibodies generated against TSTAs of the patient’s tumor cells can be “tagged” with toxins or radioactive materials to deliver a therapeutic “magic bullet” to a tumor.
B. Monoclonal antibodies
Anti-idiotypic monoclonal antibodies have been used to treat B-cell lymphomas (Fig. 19.10B), but the approach is complicated, requires custom antibodies for each patient’s tumors, and is quite expensive; therefore it is not currently practical or efficacious for general use. More general approaches to produce monoclonal antibodies against determinants that are shared by all B-cell lymphomas are being investigated. Use of monoclonal antibodies to deliver a toxin or radioisotope directly to the tumor cells, sparing healthy cells, is another promising approach. Monoclonal antibodies are also being developed against certain growth factor receptors commonly expressed by certain tumors.
C. Cancer vaccines
Development of vaccines to protect against the future development of cancer has many obvious benefits. Identification of the viruses responsible for malignant transformation, such as human papillomavirus in cervical cancer, has facilitated development of an immunization protocol to prevent infection by the virus, thus preventing development of cervical cancer. Vaccines are also being developed in attempt to prevent cancers from recurring in individuals who have been diagnosed with conditions including melanoma, a life-threatening skin cancer, and renal carcinoma. For melanoma, TSTAs have been shown to be quite similar from person to person, and vaccines now being developed are based on the common TSTAs.
Herceptin and HER2 positive breast cancer
Trastuzumab (Herceptin) is a drug therapy used to treat HER2+ breast cancer. Herceptin blocks binding to the HER2/neu receptor that is overexpressed on the tumor cells. The HER2 protein is a single chain growth factor receptor that normally functions by dimerizing with other receptor chains and signaling via phosphorylation of tyrosine residues. The normal biological response to HER signaling is stimulation of cell division. Tumors that express HER2 are overstimulated to divide. When binding and signaling via HER2 is blocked by the monoclonal antibody drug therapy, the cells expressing HER2 are arrested in the G1 phase of the cell cycle and their division is halted.
Customized vaccines are also being made using a patient’s own tumor cells and are given to patients after surgical removal of their tumors. Such vaccines are designed to stimulate an immune response against any malignant cells remaining in their bodies. Promising results have been obtained in some clinical trials.
• Cancer cells have unregulated rates of cell growth and invade healthy tissue.
• Metastasis is a characteristic of many malignant cells as they become dislodged from the main tumor and travel to distant sites in the body.
• Lymphomas and leukemias are tumors of immune cells that are derived from hematopoietic cells. Lymphomas are solid tumors, whereas leukemias grow as dispersed, single malignant cells.
• Malignant transformation is the process by which a normal cell becomes a cancerous cell.
• Oncogenes are sometimes linked to malignant transformation. Mutations of cellular oncogenes often results in a change in cellular growth.
• Tumor antigens include tumor-specific transplantation antigens (TSTAs) that result from altered proteins expressed as a consequence of gene mutations within tumor cells and tumor-associated transplantation antigens (TATAs) that are not unique to tumor cells but have unusual expression on tumor cells.
• The immune surveillance theory suggests that cancer cells frequently arise within the body but are normally eliminated by the immune system before a tumor develops.
• Innate immune responses against tumors include NK cell killing of tumors and macrophage production of antitumor cytokines, including tumor necrosis factor and the interferons.
• Adaptive immune responses against tumors include generation of antitumor antibodies, CTL killing of tumor cells, and DTH reactions.
• Immune evasion by tumor cells facilitates survival of malignant cells. Antitumor antibodies may actually enhance the growth of some tumors and may result in decreased detection of some tumor antigens. In addition, tumor cells often have lower than normal levels of MHC I molecules, helping the tumors to evade immune detection.
• Cancer immunotherapy is designed to increase the immune response against cancer cells. Cytokines and monoclonal antibodies have proven to have some limited effects in treating certain cancers. Vaccination, either to prevent development of a type of cancer or to inhibit recurrence of a tumor within a patient, continues to be explored.
19.1. Which of the following may be expected in cells over expressing Src?
A. Enhanced rate of apoptosis
B. Death by necrosis
C. Increased expression of MHC I molecules
D. Senescence (loss of ability to divide)
E. Unregulated cell division
The correct answer is E. A mutation in the oncogene Src results in loss of regulatory function of a tyrosine kinase that normally regulates cell division. Src does not regulate cell death by apoptosis or by necrosis nor would a mutant Src induce senescence. Increased MHC I expression would not be linked to a tumor cell with a mutated Src; tumor cells often have decreased levels of MHC I expression.
19.2. A bone marrow biopsy from a patient with acute lymphocytic leukemia reveals the presence of a mutated form of p53 within leukemic cells. This mutation is likely responsible for which of the following?
A. An increase in the Bax-to-Bcl-2 ratio
B. Decreased activity of NK cell KIR function
C. Excess activity of a GTP-binding protein
D. Growth of malignant cells as a solid tumor
E. Loss of suppression of cell growth
The correct answer is E. P53 is a tumor suppressor gene. When it is mutated, the suppressor action is lost, resulting in unregulated cell growth. An increase in the Bax-to-Bcl-2 ratio would favor apoptosis and not tumor growth that is seen with mutant p53. P53 mutants do not mediate NK cell KIR function. P53 is a tumor suppressor gene and not a GTP-binding protein such as ras. Leukemias grow not as solid tumors but as dispersed, single malignant cells.
19.3. Which of the following is correct regarding tumor-specific transplantation antigens (TSTAs)?
A. Also present in high concentration on normal somatic cells
B. Often found on normal fetal cells as well as on tumor cells
C. Readily identified on most naturally occurring tumors
D. Result from mutant proteins expressed by tumor cells
E. Stimulate apoptosis on cells that express them
The correct answer is D. TSTAs result from mutant proteins expressed by tumor cells. Mutations of genes within the tumor cells lead to altered proteins on the surfaces of the tumor cells. TSTAs are not found on normal somatic cells or on normal fetal cells but are unique to tumors. Although they have been demonstrated on experimentally induced tumors, identification of TSTAs on naturally occurring tumors has proved to be very difficult. Stimulation of apoptosis of tumor cells would result in elimination of tumor cells and would be beneficial to the patient with the tumor. Expression of TSTAs does not appear to stimulate apoptosis.
19.4. According to the immune surveillance theory,
A. antibodies arise during fetal development that can destroy tumors.
B. cancer cells rarely arise within a normal individual.
C. innate immune responses eliminate specific tumor cell antigens.
D. tumors arise only if malignant cells escape immune detection.
E. tumor-infiltrating lymphocytes prevent malignant transformations.
The correct answer is D. According to the immune surveillance theory, tumors arise only if malignant cells escape detection by the immune system. This theory suggests that cancer cells frequently arise within the body but are normally eliminated before becoming clinically detectable. This theory does not suggest that germline-encoded antibodies develop to destroy tumors. Innate immune responses against tumors are based on broad characteristics of tumors, not on specific tumor cell antigens. Tumor-infiltrating lymphocytes may induce tumor regression and do not induce a normal cell to become transformed into a cancer cell.
19.5. Which of the following is a cytokine known to have antitumor activity?
A. Epidermal growth factor
E. Platelet-derived growth factor
The correct answer is B. Interferons-α, -β, and -γ have all been shown to increase MHC I expression on tumor cells, and IFN-γ also appears to inhibit tumor cell proliferation. The growth factors and other cytokines listed have growth stimulatory actions.
19.6. Lymphokine-activated killer (LAK) cells are indistinguishable from
A. B lymphocytes.
C. malignant somatic cells.
D. NK cells.
E. T lymphocytes.
The correct answer is D. LAK cells are NK cells that are generated in the presence of high concentrations of interleukin-2 and are able to kill fresh tumor cells. LAK cells are not B or T lymphocytes nor are they macrophages. LAK cells can kill tumor cells and are not themselves malignant somatic cells.
19.7. Which of the following provides evidence of immune evasion by tumor cells?
A. Downregulation of MHC I molecules by tumor cells
B. Enhanced production of tumor necrosis factor by macrophages
C. IFN-γ-mediated inhibition of tumor cell proliferation
D. Generation of antibodies against tumor-specific antigens
E. Stimulation of tumor cell apoptosis by increased Bax expression
The correct answer is A. Downregulation of MHC I molecules is a defense mechanism used by many tumor cells to evade recognition by the immune system. The other mechanisms listed all describe immune responses initiated against tumor cells that have the potential to stop tumor growth. TNF and IFN-γ are produced by macrophages and inhibit tumor cell proliferation. Antibodies directed against tumor-specific antigens are part of the humoral immune response aimed at halting tumor progression. Stimulation of apoptosis by increased Bax expression would serve to eliminate tumor cells and is therefore not a mechanism to evade the immune response.
19.8. A new method to reduce the incidence of cervical cancer involves
A. administration of tumor necrosis factor to the cervix.
B. injection of antibodies against other patients’ cervical tumors.
C. stimulation of antibody-mediated cell lysis of cervical tumor cells.
D. use of patient’s tumor cells to develop an individualized vaccine.
E. vaccination against human papillomavirus.
The correct answer is E. Vaccination against human papillomavirus, the causative agent in cervical cancer, may reduce the future incidence of cervical cancer. Neither administration of TNF nor injection of antibodies against other patients’ cervical tumors is being done to prevent occurrence of cervical cancer. For certain other cancers, individualized vaccines are being used in clinical trials. However, such a vaccine requires that the patient have a tumor and would therefore not reduce the incidence of a type of cancer.