Increased expression of class II major histocompatibility complex antigens in untreated and cyclosporine-treated rat lung allografts.

Expression of class II antigens was investigated in the normal rat lung and in lung allografts during acute rejection and after cyclosporine treatment. Therefore cryostat sections were stained with an immunoperoxidase technique that used anti-class II antigen monoclonal antibodies. In normal lungs only cells of the lymphoid tissue expressed class II antigens. During acute rejection, class II antigens were expressed by infiltrating recipient lymphocytes and were also induced on the graft's bronchial epithelium. After cyclosporine treatment, signs of rejection in the lung were restricted to the bronchial epithelium, again expressing class II antigens. These results suggest that the class II antigen positivity of bronchial epithelium may be used as a specific index for pulmonary rejection when infection as an inducing factor can be excluded. Furthermore, the epithelium, once it has become class II antigen positive, is likely to be a target for acute or low-grade rejection, resulting in the development of bronchiolitis obliterans.