HLA-G: Facts and Fictions

Human leukocyte antigen (HLA)-G is a nonclassical MHC class I molecule with modulatory effects on NK and T cells. Unlike classical HLA class I molecules, HLA-G has seven isoforms, three of which are soluble. Soluble HLA-G molecules are reportedly able to transduce negative signals to immune cells after interacting with their corresponding receptors. The expression of these molecules plays significant roles in maternal tolerance against semi-allogenic fetuses. Overexpression of HLA-G in tumors and increased serum levels of soluble HLA-G have been reported in different malignancies, and these changes may be involved in tumoral immune evasion and cancer progression. To improve immune responses against tumor cells, the downmodulation of HLA-G by siRNA or blocking monoclonal antibodies can be helpful in cancer immunotherapy. Additionally, HLA-G can be considered a potential biomarker for the diagnosis and/or prognosis of certain cancers. Although polymorphism of the HLA-G gene-coding region is more limited than in classical HLA class I, some genetic variations in regulatory regions of the gene control the expression level of this molecule. Furthermore, epigenetic factors such as infections may affect the expression of HLA-G in infection-related cancers.