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 expression as well as general features of the immune system change profoundly at metamorphosis. In this study gene expression of class Ia, class Ib, and the immune proteasome component lmp7 was investigated by Northern blotting at all stages of development. Class Ia genes are expressed in most adult tissues, with highest levels in intestine. Class Ib genes are expressed at lower levels, and their tissue distribution is somewhat more restricted than that of class Ia. Consistent with the idea that particular class Ib isotypes can perform distinct functions, preferential 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 are first present in tadpole lung, gill, and intestine, organs with epithelial surfaces in contact with the environment. In every tissue except colon and muscle, class Ia expression increases markedly after metamorphosis. Interestingly, expression of the MHC-linked proteasome component lmp7 mirrored class Ia expression, while the constitutive lmp7 homologue X was expressed ubiquitously at all stages. Class Ib transcripts were never detected before metamorphosis, suggesting that the Xenopus class Ib proteins identified to date do not subserve class Ia functions in tadpole life.