Making immunotherapy ‘cold’ tumours ‘hot’ by chemotherapy-induced mutations—a misconception
Authors: T. Helleday
Journal: Annals of Oncology (2019)

Introducing neo-antigens in order to prime cancers to respond to immunotherapy is an area receiving increasing attention. Some argue this can be done through chemotherapy-induced mutations. While DNA-damaging agents potentially may increase immunogenicity through, for instance, the cGAS and STING pathway, I here lay down the arguments why chemotherapy-induced mutations are likely not to generate targetable neo-antigens. Traditionally, cancers with a high mutation burden and clonal heterogeneity have been difficult to treat using chemotherapy, such as malignant melanoma or mismatch repair defective colorectal cancers. The high mutational burden is the same reason of these cancers to respond well to immunotherapy, which has revolutionized treatment of mismatch repair defective cancers [1.Le D.T. Uram J.N. Wang H. et al.PD-1 blockade in tumors with mismatch-repair deficiency.N Engl J Med. 2015; 372: 2509-2520Crossref PubMed Scopus (6107) Google Scholar]. The high load of mutations in these tumours generate neo-antigens that are presented by major histocompatibility complex class I on the cell surface to be recognised by cytotoxic CD4+ T cells. The programmed death-1/programmed death ligand-1 regulatory mechanism prevents the T cells from killing neo-antigen-presenting cancer cells. Anti-programmed death ligand-1 immunotherapy prevents the negative feedback mechanism and stimulates T-cell-mediated killing of cancer. Intra-tumour heterogeneity has been proposed to be a main barrier to chemotherapy treatment [2.Gerlinger M. Rowan A.J. Horswell S. et al.Intratumor heterogeneity and branched evolution revealed by multiregion sequencing.N Engl J Med. 2012; 366: 883-892Crossref PubMed Scopus (5730) Google Scholar], which may very well also present a limitation to immunotherapy as clonal neo-antigens presenting early in disease will be present in all cancer cells, which are also those cancers that are also likely to respond best to treatment [3.McGranahan N. Furness A.J. Rosenthal R. et al.Clonal neoantigens elicit T cell immunoreactivity and sensitivity to immune checkpoint blockade.Science. 2016; 351: 1463-1469Crossref PubMed Scopus (1944) Google Scholar]. With several examples of exceptional responders, the idea is to extend immunotherapy to all cancers by making immune 'cold' tumours 'hot' by priming the cancers in combination treatments. Chemotherapy-induced DNA lesions often trigger the cGAS and STING pathway, which potentially can increase immunogenicity of the cancer [4.Chen Q. Sun L. Chen ZJ. Regulation and function of the cGAS-STING pathway of cytosolic DNA sensing.Nat Immunol. 2016; 17: 1142-1149Crossref PubMed Scopus (952) Google Scholar]. This or other ways of chemotherapy-induced immune responses is a plausible hypothesis to explore. However, the hypothesis of increasing the number of neo-antigens by chemotherapy-induced mutations is poorly founded for the following reasons: Chemotherapy treatments are mostly cytotoxic and poorly induce mutations. Many DNA-damaging chemotherapy drugs induce DNA lesions that obstruct replication forks in the S phase of the cell cycle. Obstructed forks are either repaired and the cancer cell survive, often at the expense of a mutation being generated, or the obstructed forks kill the cancer cell. Chemotherapy treatments have been selected to kill cancer cells and often do not increase mutations or rearrangements more than 10-fold above spontaneous levels [5.Parris C.N. Walker M.C. Masters J.R. Arlett CF. Inherent sensitivity and induced resistance to chemotherapeutic drugs and irradiation in human cancer cell lines: relationship to mutation frequencies.Cancer Res. 1990; 50: 7513-7518PubMed Google Scholar, 6.Helleday T. Arnaudeau C. Jenssen D. Effects of carcinogenic agents upon different mechanisms for intragenic recombination in mammalian cells.Carcinogenesis. 1998; 19: 973-978Crossref PubMed Scopus (34) Google Scholar, 7.Arnaudeau C. Tenorio Miranda E. Jenssen D. Helleday T. Inhibition of DNA synthesis is a potent mechanism by which cytostatic drugs induce homologous recombination in mammalian cells.Mutat Res. 2000; 461: 221-228Crossref PubMed Scopus (50) Google Scholar].Chemotherapy-induced mutations will be too few as treatment is added to an already established cancer. In order for DNA lesions to be turned into a mutation they need to be fixed through replication. Thus, a mutation occurring late in cancer development will only generate mutations in single daughter cells. This is in sharp contrast to high mutational rates caused by loss of mismatch repair or repeated exposure to UV or carcinogens at an early stage that increases mutation rates through hundreds rounds of replication (Figure 1). For example, a mismatch repair defect increases the mutation rate >100 fold [8.Bhattacharyya N.P. Ganesh A. Phear G. et al.Molecular analysis of mutations in mutator colorectal carcinoma cell lines.Hum Mol Genet. 1995; 4: 2057-2064Crossref PubMed Scopus (86) Google Scholar], providing maybe ∼1000 mutations instead of ∼10 in each round of replication in the entire genome. This would, with over ∼100 rounds of replication over many years, generate hundreds of thousands of mutations in each cancer cell as is the case of a mismatch repair defective cancer (Figure 1). A 10-fold increase in mutations in one round of replication is going to result in only a marginal increase in overall mutations (Figure 1).Non-dividing cancer cells do not mutate. Most cells in established tumours are non-dividing and chemotherapy treatments trigger cell cycle checkpoints, lowering the amount of cycling cells. As new mutations require replication, the vast majority of cancer cells will not mutate following chemotherapy.The immune system primarily recognises clonal neo-antigens. It has been demonstrated that cancers harbouring clonal neo-antigens (present in all cells) demonstrate highest survival and are likely to respond best to immunotherapy [3.McGranahan N. Furness A.J. Rosenthal R. et al.Clonal neoantigens elicit T cell immunoreactivity and sensitivity to immune checkpoint blockade.Science. 2016; 351: 1463-1469Crossref PubMed Scopus (1944) Google Scholar]. Furthermore, it has been demonstrated that cancer cells with new antigens escape and remain unrecognised if the cancer cells carrying the neo-antigen only make up a small proportion of the tumour [9.Gejman R.S. Chang A.Y. Jones H.F. et al.Rejection of immunogenic tumor clones is limited by clonal fraction.Elife. 2018; 7: e41090Crossref PubMed Scopus (62) Google Scholar]. For clonal neo-antigens to be generated, the mutations need to occur early in cancer evolution (Figure 1). Following chemotherapy, any mutation would be introduced in a single cancer cell and hence would likely go undetected by the immune system. Furthermore, even if such a cell could be recognised this is unlikely to generate any significant overall response, as there is no evidence of abscopal effects in this setting. In conclusion, chemotherapy may potentially boost immunotherapy responses through the cGAS, STING or other pathways. However, for the above reasons it is unlikely that chemotherapy will introduce novel targetable neo-antigens in tumours. The TH lab is primarily funded by the University of Sheffield, Swedish Research Council, ERC (695376-TAROX), and the Swedish Cancer Society.