Immuno-oncology is the interaction between the immune system and cancerous cells (tumors). Immuno-oncology research is not only investigating how the immune system reacts to cancerous cells and how the cancerous cells respond, but also how the immune system can be manipulated to effectively target tumors and eradicate them. For detailed information on immuno-oncology read our mini-review entitled “Understanding immuno-oncology – the interplay between the immune system and cancer”.
It is not fully understood exactly how the immune system and the cancerous cells interact with each other within the tumor microenvironment. To aid understanding of this complex process, Dunn et al. (2002) proposed that there are three distinct phases to eventual tumor survival called the three “E” of immunoediting.
Three Stages of Immunoediting
Elimination phase
The immune system is constantly on the lookout for foreign antigen, termed immune surveillance. Upon transformation of normal healthy cells to cancerous cells, specific tumor antigens and cytokines are released which are recognized and responded to as foreign by the immune system. This initial immune response to cancerous cells would typically involve recruitment of cells from the innate immune system such as dendritic cells, macrophages, monocytes and NK cells. Further to this, it is thought that tumor associated neutrophils (TAN) may have an anti-tumor role. The secretion of various cytokines, chemokines and perforin such as interferon gamma (IFN-γ), tumor necrosis factor alpha (TNF-α) related apoptosis inducing ligand (TRAIL), interleukin (IL)-2 and CXCL10 aid cytotoxicity, resulting in apoptosis and tumor cell death. The adaptive immune response, including both T and B cells will then also respond along with associated signaling molecules and together with the innate immune cells launch a co-ordinated fight against the tumor cells, resulting in eradication.
Equilibrium phase
During this phase the surviving cancerous cells are held in a steady state. T cells and associated cytokines such as IL-12 and IFN-γ eliminate cancerous cells at the same rate of cancer cell proliferation, thereby limiting tumor growth. This is the longest phase and can last for several years.
Throughout this equilibrium phase, selective pressures on these cancerous cells are at work, resulting in the progression to the escape phase and an increase in tumor heterogeneity.
Escape phase
The development of non-immunogenic tumor cell variants means that these new type of cancerous cells are able to grow, avoiding the immune response and therefore elimination. The cancerous cells can avoid the immune response in a number of ways, examples include:
No longer recognized by the immune system, where antigenicity is not recognized as foreign
Mimicking MHC class I molecules
Reducing expression of major histocompatibility antigen on cell surface
Expressing surface markers preventing NK cell or T cell recognition
Releasing proteins that mimic antibodies
No longer susceptible to effector mechanisms, such as CD8+ T cells
Induce immunosuppression
Producing immunosuppressive cytokines like tumor growth factor beta preventing NK cell binding
Activating immune check points whose role is to prevent inflammation and ensure homeostasis such as PD-L1 (CD274), CTLA-4 (CD152) and TIM3 (CD366)
Signaling to incite the influx of myeloid derived suppressor cells, including TANs and tumor associated macrophages (TAMs), which have been linked with tumor progression. Read our macrophage polarization mini-review for further information on TAMs and the markers associated with different cancer types
This phase results in escape from the immune response and cancer cell growth.
