EXPRESSION OF MHC CLASS IA AND CLASS IB DURING ONTOGENY - HIGH EXPRESSION IN EPITHELIA AND COREGULATION OF CLASS IA AND LMP7 GENES

The amphibian Xenopus permits the examination of immune responses in a species that progresses through two distinct lives, tadpole and adult , in which animals are free-living and immunocompetent. MHC gene expre ssion as well as general features of the immune system change profound ly at metamorphosis, In this study gene expression of class Ia, class Ib, and the immune proteasome component lmp7 was investigated by North ern blotting at all stages of development, Class Ia genes are expresse d in most adult tissues, with highest levels in intestine, Class Ib ge nes are expressed at lower levels, and their tissue distribution is so mewhat more restricted than that of class Ia, Consistent with the idea that particular class Ib isotypes can perform distinct functions, pre ferential expression of class Ib genes is found in some tissues, with one family being expressed exclusively in epithelia, The onset of MHC expression is not simultaneous in all tissues: class Ia transcripts ar e first present in tadpole lung, gill, and intestine, organs with epit helial surfaces in contact with the environment, In every tissue excep t colon and muscle, class Ia expression increases markedly after metam orphosis, Interestingly, expression of the MHC-linked proteasome compo nent lmp7 mirrored class Ia expression, while the constitutive lmp7 ho mologue X was expressed ubiquitously at all stages. Class Ib transcrip ts were never detected before metamorphosis, suggesting that the Xenop us class Ib proteins identified to date do not subserve class Ia funct ions in tadpole life.