INVOLVEMENT OF THYROID-HORMONES IN THE EXPRESSION OF MHC CLASS-I ANTIGENS DURING ONTOGENY IN XENOPUS

The major histocompatibility complex (MHC) is a cluster of genes encod ing products central to all major functions of the vertebrate immune s ystem. Evidence for an MHC can be found in all vertebrate groups that have been examined except the jawless fishes, Expression of MHC class I and class II antigens early in ontogeny is critically important for development of T lymphocytes capable of discriminating self from nonse lf. Because of this essential role in T-cell development, the ontogeny of MHC expression in the South African clawed frog, Xenopus laevis, w as studied. Previous studies of MHC class I expression in Xenopus laev is suggested that class I antigens are virtually absent from tadpole t issues until climax of metamorphosis. We therefore examined the possib le role of thyroid hormones (TH) in the induction of class I. By flow cytometry, a small amount of class I expression was detectable on sple nocytes and erythrocytes in untreated frogs at prometamorphic stages 5 5-58, and the amount increased significantly at the conclusion of meta morphic climax. Thus, metamorphosis is associated with increased inten sity of class I expression. Neither inhibition nor acceleration of met amorphosis altered the timing of onset of class I expression. However, inhibition of metamorphosis prevented the increase in class I express ion characteristic of adult cell populations. Because expression was n ot accelerated in TH-treated frogs or delayed in metamorphosis-inhibit ed frogs, it is unlikely that TH are the direct developmental cues tha t induce expression, although they seem to be required for the upregul ation of class I expression occurring at metamorphosis. Differences in the pattern of expression in different subpopulations of cells sugges t a complex pattern of regulation of expression of class I antigens du ring ontogeny.