T-cell cross-reactivity may explain the large variation in how cancer patients respond to checkpoint inhibitors.

The therapeutic use of the immune system to specifically attack tumours has been a long-standing vision among tumour immunologists. Recently, the use of checkpoint inhibitors to turn-off immunosuppressive signals has proven to be effective in enhancing T-cell reactivity against patient-specific neoantigens, resulting from somatic mutations. Several of the identified T-cell epitopes share similarity with common bacterial and viral antigens, suggesting the involvement of pre-existing microbial cross-reactive T cells in rapid and durable tumour regression seen in some patients. This notion of T-cell cross-reactivity is further supported by the findings that intestinal bacteria can influence checkpoint-blockade therapy. Moreover, early data indicate the presence of such T cells in long-term survival breast cancer patients. This review highlights the main challenges for cancer immunotherapy and discusses the potential contribution of T-cell cross-reactivity in cancer immunotherapy and whether it can be used as a biomarker to predict the responsiveness to checkpoint inhibitors.