Antigen processing and immune regulation in the response to tumours.

The MHC class I and II antigen processing and presentation pathways display peptides to circulating CD8+ cytotoxic and CD4+ helper T cells respectively to enable pathogens and transformed cells to be identified. Once detected, T cells become activated and either directly kill the infected / transformed cells (CD8+ cytotoxic T lymphocytes) or orchestrate the activation of the adaptive immune response (CD4+ T cells). The immune surveillance of transformed/tumour cells drives alteration of the antigen processing and presentation pathways to evade detection and hence the immune response. Evasion of the immune response is a significant event tumour development and considered one of the hallmarks of cancer. To avoid immune recognition, tumours employ a multitude of strategies with most resulting in a down-regulation of the MHC class I expression at the cell surface, significantly impairing the ability of CD8+ cytotoxic T lymphocytes to recognize the tumour. Alteration of the expression of key players in antigen processing not only affects MHC class I expression but also significantly alters the repertoire of peptides being presented. These modified peptide repertoires may serve to further reduce the presentation of tumour-specific/associated antigenic epitopes to aid immune evasion and tumour progression. Here we review the modifications to the antigen processing and presentation pathway in tumours and how it affects the anti-tumour immune response, considering the role of tumour-infiltrating cell populations and highlighting possible future therapeutic targets.