Non-classical major histocompatibility complex proteins as determinants of tumour immunosurveillance

Tumours develop in vertebrate organisms endowed with immune systems that are potentially able to eradicate them. Nevertheless, our ever- increasing understanding of the complex interactions between lymphocytes and tumour cells fuels the long- standing hope of developing efficient immunotherapies against cancer. This review focuses on a versatile family of proteins, the major histocompatibility complex class Ib, which has been recently implicated in both the establishment of anti- tumour immune responses and in tumour immune response evasion. We focus on a subset of class Ib proteins, human leukocyte antigen ( HLA)- G, Qa- 2, CD1d and NKG2D ligands, which bind to either stimulatory or inhibitory receptors expressed on T, natural killer ( NK) and NKT lymphocytes, and thereby modulate their anti- tumour activity.