Non-co-ordinate expression of HLA-DR antigens and invariant chain

The expression of HLA-DR and MHC class II antigen-associated invariant chain (Ii) was studied in normal colorectal mucosa, adenomas and carcinomas using monoclonal antibodies and a sensitive immunoperoxidase technique on frozen sections. In six of 15 specimens of normal mucosa, epithelial cells expressed Ii, but were unreactive for HLA-DR antigens. In 15/20 adenomas and 51/70 carcinomas, Ii-positive tumour cells clearly outnumbered HLA-DR-positive tumour cells. Expression of Ii in non-neoplastic epithelium adjacent to carcinoma was much stronger than expression of HLA-DR. The results indicate that in certain tissues expression of these two antigens is not closely associated.