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 antigen s in normal, acanthotic and malignantly transformed ovine skin was inv estigated using monoclonal antibodies and an immunoperoxidase techniqu e. The subjects were sheep that had been exposed to high levels of sun light for more than 6 years. The expression of MHC class II antigens i n the plasma membrane of cells within the normal epidermis was restric ted to basally located dendritic and mononuclear cells. Normal keratin ocytes did not express MHC class II antigens. However, we observed low levels of intracellular MHC class II expression in both acanthotic an d neoplastic keratinocytes: Expression of MHC class I antigens was var iable 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 origin ating 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.