Understanding of the antitumor immunity role of CD4+ T cells has grown substantially since the late 1990s. CD4+ T cells (mature T-helper cells) play an important role in modulating immune responses to pathogens and tumor cells, and are important in orchestrating overall immune responses.
Immunosurveillance and immunoediting This discovery furthered the development of a previously hypothesized theory, the immunosurveillance theory. The immunosurveillance theory suggests that the immune system routinely patrols the cells of the body, and, upon recognition of a cell, or group of cells, that has become cancerous, it will attempt to destroy them, thus preventing the growth of some tumors. (Burnet, 1970) More recent evidence has suggested that immunosurveillance is only part of a larger role the immune system plays in fighting cancer. Remodeling of this theory has led to the progression of the immunoediting theory, in which there are 3 phases, Elimination, Equilibrium and Escape.
Elimination phase As mentioned, the elimination phase is synonymous with the classic immunosurveillance theory.
In 2001, it was shown that mice deficient in RAG-2 (Recombinase Activator Gene 2) were far less capable of preventing MCA induced tumours than were wild type mice. (Shankaran et al., 2001, Bui and Schreiber, 2007) RAG proteins are necessary for the recombination events necessary to produce TCRs and Igs, and as such RAG-2 deficient mice are incapable of producing functional T, B or NK cells. RAG-2 deficient mice were chosen over other methods of inducting immunodeficiency (such as SCID mice) as an absence of these proteins does not affect DNA repair mechanisms, which becomes important when dealing with cancer, as DNA repair problems can lead to cancers themselves. This experiment provides clear evidence that the immune system does, in fact, play a role in eradication of tumor cells.
Further knock out experiments showed important roles of αβ T cells, γδ T cells and NK cells in tumour immunity (Girardi et al. 2001, Smyth et al., 2001)
Another experiment involving interferon gamma (IFNγ−/−) showed that these mice are more likely to develop certain types of cancers as well, and suggests a role of CD4+ T cells in tumor immunity, which produce large amounts of IFNγ (Street et al., 2002)
Perforin deficient mice were also shown to have a reduced ability to ward off MCA induced cancers, suggesting an important role of CD8+ T cells. (Street et al. 2001) Perforin is a protein produced by CD8+ T cells, which plays a central role in the cytotoxic killing mechanisms by providing entry of degradative granzymes into an infected cell. (Abbas and Lichtman, 2005)
Finally, the innate immune system has also been associated with immunosurveillance (Dunn et al., 2004).
Equilibrium phase The equilibrium phase of the immunoediting theory is characterized by the continued existence of the tumour, but little growth. Due to the extremely high rate of mutation of cancer cells, it is probable that many will escape the elimination phase, and progress into the equilibrium phase. There is currently little evidence to support the existence of an equilibrium phase, aside from the observation that cancers have been shown to lie dormant, i.e. to go into remission, in a person's body for years before re-emerging again in the final escape phase. It has been noted that tumors that persist in the equilibrium phase show reduced immunogenicity when compared to tumors which have been grown in immunodeficient mice (Shankaran et al., 2001) Three possible outcomes for tumors managing to evade the immune system, and reach the equilibrium phase have been proposed: 1) eventual elimination by the immune system 2) a prolonged or indefinite period of dormancy, or 3) progression into the final escape phase.
Escape phase As the name implies, the escape phase is characterized by a reduced immunogenicity of the cancer cells, their subsequent evasion of the immune system and their ability to be clinically detected. A number of theories have been proposed to explain this phase of the theory.
Cancer cells, through mutation, may actually have mutations in some of the proteins involved in antigen presentation, and as such, evade an immune response. (Dunn et al., 2004) Tumor cells may, through mutations, often begin producing large quantities of inhibitory cytokines IL-10, or transforming growth factor β (TGF-β) (Khong and Restifo, 2002) thereby suppressing the immune system, allowing for large-scale proliferation (Salazar-Onfray et al., 2007). Also, it has been observed that some cancer patients exhibit higher than normal levels of CD4+/CD25+ T cells, a subset of T cells often called regulatory T cells, for their known immunosuppressive actions. These T cells produce high levels of IL-10 and TGF-β, thereby suppressing the immune system and allowing for evasion by the tumor (Shimizu et al., 1999).
Tumour antigens Tumour antigens are those expressed by tumor cells, and recognizable as being different from self cells. Most currently classified tumor antigens are endogenously synthesized, and as such are presented on MHC class I molecules to CD8+ T cells. Such antigens include products of oncogenes or tumor suppressor genes, mutants of other cellular genes, products of genes that are normally silenced, over-expressed gene products, products of oncogenic viruses, oncofetal antigens (proteins normally expressed only during development of the fetus) glycolipids and glycoproteins. Detailed explanations of these tumor antigens can be found in Abbas and Lichtman, 2005. MHC class II restricted antigens currently remain somewhat obscure. Development of new techniques has been successful in identifying some of these antigens, however, additional research is required. (Wang, 2003)
Antitumour immunity Historically, much more attention and funding has been devoted to the role of CD8+ T cells in antitumor immunity, rather than to CD4+ T cells. This can be attributed to a number of things; CD4+ T cells respond only to presentation of antigens by MHC class II, however, most cells express only MHC class I; second, CD8+ T cells, upon being presented with antigen by MHC class I, can directly kill the cancerous cell, through mechanisms which will not be discussed in this article, but which have been well categorized; (See Abbas and Lichtman, 2005) finally, there is simply a more widespread understanding and knowledge of MHC class I tumor antigens, while MHC class II antigens remain somewhat obscure.(Pardol and Toplain, 1998).
It was believed that CD4+ T cells were not involved directly in antitumour immunity, but rather functioned simply in the priming of CD8+ T cells, through activation of antigen-presenting cells (APCs) and increased antigen presentation on MHC class I, as well as secretion of excitatory cytokines such as IL-2 (Pardol and Toplain, 1998, Kalams and Walker, 1998, Wang 2001).
Controversial role in antitumor immunity The role of CD4+ T cells in antitumor immunity is controversial. It was suggested that CD4+ T cells can have a direct role in antitumor immunity through direct recognition of tumor antigens presented on the surface of tumor cells in association with MHC class II molecules. Of note, results from recent reports suggest that direct recognition of tumors from tumor-antigen specific CD4+ T cells might not be always beneficial. For example, it was recently shown that CD4+ T cells primarily produce TNF after recognition of tumor-antigens in melanoma. TNF may in turn increase local immunosuppression and impair the effector functions of CD8 T cells (Donia M. et al., 2015).Donia M. et al., Cancer Res 2015 available at http://cancerres.aacrjournals.org/content/75/18/3747
Th1 and Th2 CD4+T cells The same series of experiments, examining the role of CD4+ cells, showed that high levels of IL-4 and IFNγ were present at the site of the tumor, following vaccination, and subsequent tumour challenge. (Hung, 1998) IL-4 is the predominant cytokine produced by Th2 cells, while IFNγ is the predominant Th1 cytokine. Earlier work has shown that these two cytokines inhibit the production of each other by inhibiting differentiation down the opposite Th pathway, in normal microbial infections (Abbas and Lichtman, 2005), yet here they were seen at nearly equal levels. Even more interesting was that both these cytokines were required for maximal tumor immunity, and that mice deficient in either showed greatly reduced antitumor immunity. IFN-γ null mice showed virtually no immunity, while IL-4 null mice showed a 50% reduction when compared to immunised wild type mice.
The reduction of immunity in IL-4 deficient mice, has been attributed to a decrease in eosinophil production. In mice deficient in IL-5, the cytokine responsible for differentiation of myeloid progenitor cells into eosinophils, less eosinophils are seen at the site of tumour challenge, which is to be expected. (Hung, 1998) These mice also show reduced antitumor immunity, suggesting that IL-4 deficient mice, which would produce less IL-5, and subsequently have reduced eosinophil levels, elicit their effect through eosinophils.
Th-1 activity in tumor immunity Th1 cells are one of the two main Th cell polarizations first identified and involved in type 1 immunity. Th1 differentiation is IL-12 dependent, and IFN-γ is the signature cytokine of cells of a Th1 lineage.
Th1 cell anti-tumor activity is complex and includes many mechanisms. Th1 cells are indirectly responsible for activating tumor-suppressing CTLs by activating the antigen-presenting cells which then present antigen to and activate the CTL.
IFN-γ produced by Th1 cells activates macrophages, increasing phagocytosis of pathogen and tumor cells. Activated macrophages produce IL-12, and since IL-12 promotes Th1 cell differentiation, this forms a tumor-suppressing feedback loop.
Th1 and NK cells both contribute to killing of tumor cells via the TNF-related apoptosis-inducing ligand (TRAIL) pathway. NK cells produce IFN-γ and are also activated by IL-12, creating another tumor-suppressing feedback loop.
Th-2 activity in tumor immunity Th2 cells are the other Th cell polarization initially defined. Th2 differentiation is dependent on the presence of IL-4 and the absence of IL-12, and signature cytokines (type 2 cytokines) of Th2 cells include IL-4, IL-5, and IL-13. Th2 cells are involved in type 2 immunity. Although it has traditionally been associated with tumor promotion, emerging evidence indicates a potential tumor-suppressive function highlighting a paradigm shift in our understanding about type 2 immunity in cancer.
Th2 mediated anti-tumor activity primarily involves recruitment of eosinophils to the tumor environment type 2 cytokines, including IL-5. Anti-tumor eosinophil activity includes attraction of tumor-specific CTLs, activation of macrophages, and vascularization of the tumor stroma.
Th-17 activity in tumor immunity Th17 are a recently identified subset of Th cells that are primarily involved in promoting inflammatory responses. Th17 differentiation is induced by TGF-β and IL-6, and signature cytokines of Th17 cells include IL-17A and IL-17F.
The mechanisms of Th17 cell activity in the tumor microenvironment are not well understood. Th17 cells can orchestrate chronic inflammatory responses, which tend to promote tumor growth and survival. In addition, some tumors have been shown to express high levels of IL-6 & TGF-β, which would reinforce a Th17 polarization, creating a tumor-promoting feedback loop.
Th17 cells have also been found to have the capacity to differentiate into IFN-γ secreting cells, thus suppressing tumor growth via IFN-γ-related pathways.
Treg activity in tumor immunity