Defects in antigen processing and presentation: mechanisms, immune evasion and implications for cancer vaccine development.

Human tumour cells express mutated and non-mutated proteins that can be processed and presented by these cells as peptides bound to human leukocyte antigen (HLA). Some of these peptides are recognized by cognate T cell receptors as 'non-self', leading to specific killing of tumour cells by T cells. This process is fundamental to the success of cancer immunotherapy, which exploits the ability of the immune system to eliminate transformed cells. Mutated antigens (neoantigens) have been implicated in the remarkable therapeutic efficacy of immune checkpoint inhibitors (ICIs), which boost endogenous antitumour immune responses. In recent years, the combination of ICIs with personalized cancer vaccines that target neoantigens and other tumour-specific antigens has emerged as a new therapeutic strategy. However, the robust immune pressure that ICIs exert on cancer cells inevitably amplifies the phenomenon of immune editing, which can allow cancer cells to develop resistance mechanisms that subvert surveillance by the immune system. Diminished antigenicity can be due to defects in the antigen processing and presentation machinery, such as HLA-I/II loss of heterozygosity and loss of functional β2-microglobulin. This poses a considerable challenge for combination therapies that include ICIs and for the design of cancer-specific vaccines.