Thymic epithelial antigen, acquired during ontogeny and defined by the anti-p19 monoclonal antibody, is lost in thymomas.

The protein defined by the anti-p19 monoclonal antibody (raised against a 19-kilodalton protein from human T leukemia virus) and normally acquired during the ontogenesis of the human thymic epithelium was studied by indirect immunofluorescence in thymomas and thymic hyperplasias. Hyperplastic (and involuted) thymuses from patients with myasthenia gravis exhibited a clear-cut expression of this differentiation antigen exclusively in epithelial cells, thus qualitatively similar to what was observed in the normal thymus. In thymoma epithelial cells, however, this expression was strongly altered: in the 12 malignant thymomas studied, no anti-p19 reactivity was found. In the four benign cases, a variable number of epithelial cells expressed this antigen. These results suggest that the differentiation antigen of the human thymic epithelium defined by the anti-p19 monoclonal antibody is lost during the malignant transformation occurring in thymomas. Furthermore, this monoclonal antibody provides an additional tool for the study of the degree of dedifferentiation of thymomas.