Heat shock proteins as mediators of aggregation-induced 'danger' signals: implications of the slow evolutionary fine-tuning of sequences for the antigenicity of cancer cells.

Organisms 'tune' to their environment through adaptations which confer a selective advantage. However, in complex systems, a primary change of positive adaptive value might have multiple minor secondary effects, usually of negative adaptive value, which could invoke further counter-adaptations. This 'fine-tuning', a 'debugging', mainly at the intracellular level, would appear an evolutionary burden detracting from the positive nature of the primary change. However, if the primary mutation is in a potential oncogene, secondary, short-term effects may include the recruitment, in an apparently random manner, of unmutated non-oncogene products into the antigenic repertoire of the cancer cell. This 'danger' signal, provided by the co-aggregation of oncogene and non-oncogene products, would be mediated by inducible heat-shock proteins (Hsps), and lead to display of corresponding MHC-peptide complexes. It was argued previously that T cells specific for peptides from most 'self' intracellular antigens are not eliminated during T cell 'education', and so would be available for subsequent immune activation by the corresponding peptides. These considerations might explain why cancer specific antigens have been so elusive, why cancer antigenicity is often individual specific, and why therapeutic approaches involving complexes of peptides with Hsps may be successful.