ACCUMULATION OF IMMATURE CD3(-)CD4(-) SINGLE-POSITIVE CELLS THAT LACKCD69 IN EPITHELIAL-CELL TUMORS OF THE HUMAN THYMUS()CD8()

Human thymomas are epithelial tumors that are associated with a large number of nonneoplastic T cells of mainly immature phenotype, suggesti ng that epithelial cells of thymomas retain the function of thymic cor tical epithelium. We report here that a large proportion (16-85%, mean +/- SD 58 +/- 22%, n = 17) of CD4(+) single-positive T cells within t he thymoma lack cell surface expression of CD3 and also of CD69, an ea rly activation antigen that is expressed by positively selected thymoc ytes. Normal thymus had this CD4(+)CD8(-)CD3(-) population at much low er levels (12 +/- 7%, n = 11). It has been reported that in the human the CD4(+)CD8(-)CD3(-) cells are the predominant population that is in termediate between the CD4(-)CD8(-) double-negative and CD4(+)CD8(+) d ouble-positive stages. In contrast to CD4(+) single-positive cells, mo st of the CD8(+) single-positive cells in the thymoma as well as in th e normal thymus expressed CD69 and a high level of CD3, A partial expl anation of these observations is that the epithelial cells of thymoma were unable to positively select all the immature thymocytes generated in the thymoma. This relative inefficiency of positive selection coul d not be attributed solely to the paucity of MHC class II expression i n the thymoma. (C) 1995 Academic Press, Inc.