Immunotherapy is the stimulation of the immune system to treat cancer, hence enhancing the immune system’s inherent ability to fight the illness.
It is an application of cancer immunology’s foundational research and a burgeoning subspecialty of oncology.
Immunotherapy for cancer takes advantage of the fact that cancer cells frequently include tumour antigens, chemicals that can be identified by the immune system’s antibodyproteins by adhering to them.
Antigens of tumours are frequently proteins or other macromolecules (e.g., carbohydrates). Normal antibodies bind to external pathogens, whereas immunotherapy antibodies bind to tumour antigens, so labelling and identifying cancer cells for the immune system to suppress or destroy.
The clinical efficacy of cancer immunotherapy is very diverse between cancer types; for instance, certain subtypes of gastric cancer respond well to immunotherapy, whilst other subtypes do not.
James P. Allison and Tasuku Honjo were awarded the Nobel Prize in Physiology or Medicine in 2018 for their discovery of cancer therapy based on the inhibition of negative immune regulation.
Despite the fact that numerous immune responses can be created in response to tumour cells, these responses are typically insufficient to prevent tumour progression. These natural defence mechanisms may be augmented or supplemented as an approach to cancer treatment. In this final section, some types of cancer immunotherapy now in use or in development are described.
Manipulation of Co-Stimulatory Signals Can Enhance Immunity
Multiple research groups have proven that tumour immunity can be improved by delivering the co-stimulatory signal required for CTL precursor activation (CTL-Ps).
Antigen recognition occurs when mouse CTL-Ps are treated in vitro with melanoma cells, but in the absence of a costimulatory signal, the CTL-Ps do not multiply and develop into effector CTLs.
When the melanoma cells are transfected with the gene encoding the B7 ligand, however, the CTL-Ps develop into effector CTLs.
These results suggest that B7-transfected tumour cells could be utilised to produce a CTL response in vivo.
When P. Linsley, L. Chen, and their colleagues transplanted B7+ melanoma cells into melanoma-bearing mice, the melanomas fully regressed in almost 40% of the mice. S. Townsend and J. Allison vaccinated mice against malignant melanoma using a similar method.
First, normal mice were immunised with irradiated, B7-transfected melanoma cells, and subsequently they were challenged with unmodified malignant melanoma cells. It was discovered that the “vaccine” protected a high percentage of the mice.
It is envisaged that a similar vaccine could prevent metastases in human patients following surgical excision of a primary melanoma.
Due to the fact that human melanoma antigens are shared by a variety of human malignancies, it may be able to construct a panel of B7-transfected melanoma cell lines that are typed for tumor-antigen expression and HLA expression.
In this method, the tumour antigen(s) expressed by a patient’s tumour would be identified, and the patient would subsequently be immunised with an irradiated B7-transfected cell line that expresses a similar tumour antigen (s).
Enhancement of APC Activity Can Modulate Tumor Immunity
It has been demonstrated that mouse dendritic cells cultivated in GM-CSF and treated with tumour fragments, then reinfused into mice, activate TH cells and CTLs specific for the tumour antigens. When the mice were subsequently exposed to living tumour cells, they exhibited tumour immunity.
These investigations have led to a variety of strategies designed to increase the number of antigen-presenting cells, such that these cells can activate TH cells or CTLs specific to tumour antigens.
Attempts have been made to transfect tumour cells with the GM-CSF-encoding gene.
When reinfused into the patient, these modified tumour cells will release GMCSF, promoting the development and activation of host antigen-presenting cells, particularly dendritic cells.
As these dendritic cells accumulate near tumour cells, the GMCSF released by tumour cells will boost the presentation of tumour antigens to TH cells and CTLs by the dendritic cells.
Dendritic cells derived from peripheral-blood progenitor cells can be cultured in the presence of GM-CSF, TNF-alpha, and IL-4 to expand their number. These three cytokines stimulate the development of many dendritic cells.
There is a possibility that if these dendritic cells are stimulated with tumour fragments and subsequently reintroduced into the patient, they will activate TH and TC cells specific for the tumour antigens.
The validity of these hopes will be determined by additional inquiry. Several adjuvants, such as the attenuated strains of Mycobacterium bovis known as bacillus Calmette-Guerin (BCG) and Corynebacterium parvum, have been employed to enhance tumour immunity.
These adjuvants stimulate macrophages, elevating their expression of several cytokines, class II MHC molecules, and the B7 co-stimulatory protein.
These activated macrophages are superior activators of T-helper (TH) cells, resulting in an increase in both humoral and cell-mediated immune responses. Thus far, adjuvants have shown relatively limited therapeutic benefits.
Use of transfected tumor cells for cancer immunotherapy. (a) Tumor cells transfected with the B7 gene express the co-stimulatory B7 molecule, enabling them to provide both activating signal (1) and co-stimulatory signal (2) to CTL-Ps. As a result of the combined signals, the CTL-Ps differentiate into effector CTLs, which can mediate tumor destruction. In effect, the transfected tumor cell acts as an antigen-presenting cell. (b) Transfection of tumor cells with the gene encoding GM-CSF allows the tumor cells to secrete high levels of GM-CSF. This cytokine will activate dendritic cells in the vicinity of the tumor, enabling the dendritic cells to present tumor antigens to both TH cells and CTL-Ps.
Cytokine Therapy Can Augment Immune Responses to Tumors
The isolation and cloning of the various cytokine genes has facilitated their large-scale production. A variety of experimental and clinical approaches have been developed to use recombinant cytokines, either singly or in combination, to augment the immune response against cancer.
Among the cytokines that have been evaluated in cancer immunotherapy are IFN-alpha, beta, Gamma and ; IL-1, IL-2, IL-4, IL-5, and IL-12; GM-CSF; and TNF. Although these trials have produced occasional encouraging results, many obstacles remain to the successful use of this type of cancer immunotherapy.
The most notable obstacle is the complexity of the cytokine network itself. This complexity makes it very difficult to know precisely how intervention with a given recombinant cytokine will affect the production of other cytokines.
And since some cytokines act antagonistically, it is possible that intervention with a recombinant cytokine designed to enhance a particular branch of the immune response may actually lead to suppression.
In addition, cytokine immunotherapy is plagued by the difficulty of administering the cytokines locally.
In some cases, systemic administration of high levels of a given cytokine has been shown to lead to serious and even life-threatening consequences.
Although the results of several experimental and clinical trials of cytokine therapy for cancer are discussed here, it is important to keep in mind that this therapeutic approach is still in its infancy.
Interferons
Large quantities of purified recombinant preparations of the interferons, IFN-alpha, IFN-beta, and IFN-gamma, are now available, each of which has shown some promise in the treatment of human cancer.
To date, most of the clinical trials have utilised IFN-alpha. Daily injections of recombinant IFN-alpha have been proven to elicit partial or total tumour regression in certain patients with hematologic malignancies such as leukemias, lymphomas, and myelomas and with solid tumours such as melanoma, Kaposi’s sarcoma, kidney cancer, and breast cancer. Interferon-mediated anticancer action may entail multiple pathways.
All three kinds of interferon have been found to boost class I MHC expression on tumour cells; IFN-gamma has also been shown to promote class II MHC expression on macrophages.
Given the evidence for lower amounts of class I MHC molecules on malignant tumours, the interferons may work by restoring MHC expression, hence enhancing CTL activity against malignancies.
In addition, the interferons have been demonstrated to limit cell division of both normal and malignantly altered cells in vitro. It is probable that part of the anticancer effects of the interferons are attributable to their ability to directly impede tumor-cell proliferation.
Finally, IFN-gamma directly or indirectly boosts the activity of TC cells, macrophages, and NK cells, all of which play a role in the immune response to tumour cells.
Tumor Necrosis Factors
In rare circumstances, the tumour necrosis factors TNF-alpha and TNF-beta have been demonstrated to have direct anticancer activity, killing some tumour cells and lowering the rate of growth of others while sparing normal cells.
In the presence of TNF-alpha or TNF-beta, a tumour suffers apparent hemorrhagic necrosis and regression. TNF-alpha has also been found to block tumor-induced vascularization (angiogenesis) by harming the vascular endothelial cells in the vicinity of a tumour, hence limiting the flow of blood and oxygen that is necessary for progressive tumour growth.
In Vitro–Activated LAK And TIL Cells
Animal experiments have demonstrated that lymphocytes can be stimulated towards tumour antigens in vitro by cultivating them with x-irradiated tumour cells in the presence of IL-2 and additional tumour antigens.
These activated lymphocytes mediate more efficient tumour killing than untreated lymphocytes when they are reinjected into the original tumor-bearing animal.
It is difficult, however, to activate in vitro enough lymphocytes with antitumor specificity to be beneficial in cancer therapy.
While sensitising lymphocytes to tumour antigens by this method, S. Rosenberg discovered that, in the presence of high concentrations of cloned IL-2 but without the addition of tumour antigens, large numbers of activated lymphoid cells were generated that could kill fresh tumour cells but not normal cells.
He named these cells lymphokine-activated killer (LAK) cells. In one study, for example, Rosenberg observed that injection of LAK cells and recombinant IL-2 into tumorbearing rats induced efficient tumor-cell killing.
LAK-cell populations are often >90% activated NK cells. However, limited numbers of TCR-bearing cells are found in LAK populations and it is plausible that they may potentially contribute to their tumoricidal activity.
Because vast numbers of LAK cells can be created in vitro and because these cells are active against a wide variety of malignancies, their usefulness in human tumour immunotherapy has been studied in multiple clinical trials.
In these trials, peripheral-blood lymphocytes were taken from patients with various stage metastatic malignancies and were stimulated in vitro to create LAK cells.
In an early trial, patients were then infused with their autologous LAK cells combined with IL-2. In this trial, which covered 25 patients, cancer regression was seen in some participants.
Subsequently, a more larger trial with 222 individuals resulted in total regression in 16 people. However, a variety of negative side effects are connected with the high amounts of IL-2 required for LAKcell activation.
The most significant is vascular leak syndrome, in which lymphoid cells and plasma move from the peripheral circulation into the tissues, resulting to shock.
Tumors contain lymphocytes that have entered the tumour and probably are taking part in an anticancer response.
By obtaining small biopsy samples of tumours, one can obtain a population of these cells and increase it in vitro using IL-2. These activated tumor-infiltrating lymphocytes are called TILs.
Many TILs show a wide range of antitumor activity and appear to be indistinguishable from LAK cells. However, certain TILs cells show particular cytolytic activity against their autologous tumour.
These tumor-specific TILs are of interest since they exhibit higher anticancer activity and require 100-fold lower doses of IL-2 for their activity than LAK cells do. In one study, TIL populations were expanded in vitro from biopsy samples acquired from individuals with malignant melanoma, renal-cell carcinoma, and small-cell lung cancer.
The increased populations of TILs were reinjected into autologous patients coupled with continuous infusions of recombinant IL-2. Renal-cell carcinomas and malignant melanomas demonstrated partial regression in 29% and 23% of the patients, respectively.
Treatment with Monoclonal Antibodies
Monoclonal antibodies have been employed in various ways as experimental immunotherapeutic treatments for cancer. For example, anti-idiotype monoclonal antibodies have been employed with some effectiveness in treating human B-cell lymphomas and T-cell leukemias.
In one extraordinary trial, R. Levy and his colleagues successfully treated a 64-year-old man with terminal B-cell lymphoma. At the time of treatment, the lymphoma had metastasized to the liver, spleen, bone marrow, and peripheral blood.
Because this was a B-cell malignancy, the membrane-bound antibody on all the malignant cells had the same idiotype. By the approach indicated in Figure, these researchers developed mouse monoclonal antibody specific for the B-lymphoma idiotype.
When this mouse monoclonal anti-idiotype antibody was introduced into the patient, it attached selectively to the B-lymphoma cells, because these cells produced that particular idiotype.
Since B-lymphoma cells are sensitive to complement-mediated lysis, the monoclonal antibody triggered the complement system and destroyed the lymphoma cells without affecting other cells.
After four injections with this anti-idiotype monoclonal antibody, the tumours began to decrease, and this patient entered an exceptionally long period of total remission. However,this technique demands that a custom monoclonal antibody be created for each lymphoma patient.
This is extremely expensive and cannot be utilised as a general therapy approach for the hundreds of people diagnosed each year with B lymphoma. Recently, Levy and his colleagues have used direct immunisation to engage the immune systems of patients to an attack against their B lymphoma.
In a clinical experiment with 41 B-cell lymphoma patients, the genes encoding the rearranged immunoglobulin genes of the lymphomas of each patient were extracted and utilised to encode the production of recombinant immunoglobulin that carried the idiotype typical of the patient’s malignancy.
Each of these Igs was attached to keyhole limpet hemocyanin (KLH), a mollusk protein that is widely utilised as a carrier protein because of its rapid recruitment of T-cell assistance.
The patients were inoculated with their own tumor-specific antigens, the idiotypically distinct immunoglobulins produced by their own lymphomas.
About 50% of the patients developed anti-idiotype antibodies against their malignancies. Significantly, improved clinical outcomes were reported in the 20 individuals with anti-idiotype reactions, but not in the others. In fact, 2 of these 20 experienced total remission.
Despite its promise, the anti-idiotypic strategy is by its very nature patient-specific. A more universal monoclonalantibody therapy for B-cell lymphoma is predicated on the fact that most B cells, whether normal or malignant, express lineagedistinctive antigens.
One such determinant, CD20, has been the object of considerable efforts; a monoclonal antibody to it, generated in mice and modified to contain largely human sequences, has been beneficial in the treatment of B-cell lymphoma.
Aside from CD20, a variety of tumor-associated antigens are being evaluated in clinical trials for their appropriateness as targets for antibody-mediated anti-tumor therapy. A variety of cancers display greatly elevated amounts of growth-factor receptors, which are interesting targets for anti-tumor monoclonal antibodies.
For example, in 25 to 30 percent of women with metastatic breast cancer, a genetic change of the tumour cells results in the increased production of HER2, an epidermal-growth-factor–like receptor.
An anti-HER2 monoclonal antibody was produced in mice and the genes encoding it were extracted. Except for the regions encoding the antibody’s CDRs, the mouse Ig sequences were substituted with human Ig counterparts.
This avoids the formation of human anti-mouse antibodies (HAMAs) and allows the patient to receive repeated doses of the “humanised” anti-HER2 in substantial amounts (100 milligrammes or more) (100 milligrammes or more).
Preparations of this antibody, named Herceptin, are now commercially available for the treatment of HER2-receptor– containing breast tumours. Monoclonal antibodies also have been employed to prepare tumour specific anti-tumor medicines.
In this method, antibodies to tumor-specific or tumor-associated antigens are combined with radioactive isotopes, chemotherapeutic medicines, or strong toxins of biological origin. In such “guided missile” therapies, the poisonous substances are directed selectively to tumour cells.
This focuses the damaging effects on the tumour and spares normal tissues. Reagents known as immunotoxins have been produced by connecting the inhibitor chain of a toxin (e.g., diphtheria toxin) to an antibody against a tumor-specific or tumor-associated antigen.
In vitro experiments have revealed that these “magic bullets” can kill tumour cells without damaging normal cells. Immunotoxins specific for tumour antigens in a range of cancers (e.g., melanoma, colorectal carcinoma, metastatic breast carcinoma, and several lymphomas and leukemias) have been examined in phase I or phase II clinical trials.
In a number of trials, significant numbers of leukaemia and lymphoma patients achieved partial or complete remission.
However in a handful of cases, the clinical responses in patients with greater tumour sizes were disappointing. In some of these patients, the sheer size of the tumour may render most of its cells inaccessible to the immunotoxin.
Treatment of B-cell lymphoma with monoclonal antibody specific for idiotypic determinants on the cancer cells. Because all the lymphoma cells are derived from a single transformed B cell, they all express membrane-bound antibody (Ab-1) with the same idiotype (i.e., the same antigenic specificity). In the procedure illustrated, monoclonal anti-idiotype antibody (Ab-2) against the B-lymphoma membranebound antibody was produced (steps 1–4). When this anti-idiotype antibody was injected into the patient (step 5), it bound selectively to B-lymphoma cells, which then were susceptible to complement-plusantibody lysis.
Key Facts
Tumor cells differ from normal cells in several ways. In example, abnormalities in the regulation of proliferation of tumour cells allow them to proliferate endlessly, then penetrate the underlying tissue, and eventually metastasis to other tissues. Normal cells can be converted in vitro by chemical and physical carcinogens and by transforming viruses. Transformed cells have changed growth properties and are occasionally capable of producing cancer when they are put into animals.
Proto-oncogenes encode proteins involved in control of proper cellular growth. The conversion of proto-oncogenes to oncogenes is one of the fundamental steps in the development of most human malignancies. This conversion may happen from mutation in an oncogene, its translocation, or its amplification.
A number of B- and T-cell leukemias and lymphomas are associated with translocated proto-oncogenes. In its new place, the translocated gene may come under the influence of enhancers or promoters that trigger its transcription at higher levels than typical.
Tumor cells show tumor-specific antigens and the more common tumor-associated antigens. Among the latter are oncofetal antigens, and higher quantities of normal oncogene products. In contrast to tumour antigens created by chemicals or radiation, virally encoded tumour antigens are shared by all cancers induced by the same virus.
The tumour antigens recognised by T cells fall into one of four major categories: antigens encoded by genes with tumor-specific expression; antigens encoded by variant forms of normal genes that have been altered by mutation; certain antigens normally expressed only at certain stages of differentiation or differentiation lineages; antigens that are overexpressed in particular tumours.
The use of a range of genetic, biochemical, and immunological methods has allowed the identification of many tumor-associated antigens. In many cases the antigen is expressed on more than one type of tumour. Common tumour antigens give hope for the design of better therapeutics, diagnosis, and monitoring, and have crucial implications for the prospect of anti-tumor immunisation.
The immune response to cancers includes CTL-mediated lysis, NK-cell activity, macrophage-mediated tumour killing, and destruction mediated by ADCC. Several cytotoxic factors, including TNF-alpha and TNF-beta, help to mediate tumor-cell death. Tumors may elude the immune response by modifying their tumour antigens, by lowering their production of class I MHC molecules, and by antibodymediated or immune complex-mediated suppression of CTL function.
Experimental cancer immunotherapy is exploring a range of approaches. Some of these are the enhancement of the co-stimulatory signal required for T-cell activation, genetically engineering tumour cells to secrete cytokines that may increase the intensity of the immune response against them, the therapeutic use of cytokines, and ways of increasing the activity of antigen-presenting cells.
A number of encouraging clinical findings have been established with therapy using monoclonal antibodies against tumor-associated and (in a few cases) tumor-specific antigens. Coupling of antibodies against tumour antigens with poisons, chemotherapeutic drugs, or radioactive elements is being explored. The expectation is that such tactics will focus the harmful effects of these drugs on the tumour and spare normal cells their damaging effects.
Key elements in the design of strategies for vaccination against cancer are the identification of significant tumour antigens by genetic or biochemical approaches; the development of strategies for the effective presentation of tumour antigens; and the generation of activated populations of helper or cytotoxic T cells.
