Major histocompatibility complex antigens in normal, acanthotic and neoplastic ovine skin: an association been tumor invasiveness and low level MHC class I expression.

The distribution and density of ovine MHC class I and class II antigens in normal, acanthotic and malignantly transformed ovine skin was investigated using monoclonal antibodies and an immunoperoxidase technique. The subjects were sheep that had been exposed to high levels of sunlight for more than 6 years. The expression of MHC class II antigens in the plasma membrane of cells within the normal epidermis was restricted to basally located dendritic and mononuclear cells. Normal keratinocytes did not express MHC class II antigens. However, we observed low levels of intracellular MHC class II expression in both acanthotic and neoplastic keratinocytes. Expression of MHC class I antigens was variable in normal and acanthotic epithelium; it was usually present, but of low intensity in very early ovine squamous cell carcinoma and was increased in small, but morphologically typical, tumors. Tumors originating on the nose, which are more invasive than those on the ear, were found to express significantly less MHC class I (P < 0.05). Thus, an association between tumor invasiveness and low level expression of MHC class I was apparent. This may have diagnostic value and highlights a mechanism by which neoplastic cells may evade immune surveillance by T cells.