HLA-G expression by tumors

PROBLEM: It has been proposed that the expression of the non-classical MHC class I antigen, HLA-G, by trophoblast is one mechanism by which the placenta evades attack by maternal uterine NK cells. A similar mechanism is thought to be operative in the escape from immunosurveillance by tumor cells. However, data on the expression of HLA-G by tumor cells are highly conflicting. METHOD OF STUDY: In the present study, we have examined tissue sections from a wide variety of tumors by immunohistology and also several cell lines by flow cytometry and RT-PCR. Whilst very faint bands were detected in three cell lines (hepG2, Mead, CaSki) by RT-PCR, no tumors or cell lines were observed to express HLA-G protein. Furthermore, we found that tumor deposits are not usually infiltrated by NK cells. CONCLUSION: Our observations, therefore, do not support the proposal that tumor cells express HLA-G in order to evade host NK cell cytolysis.