Class II gene products of the major histocompatibility complex (MHC) are not expressed usually in abundance on normal epithelium. The cell surface visibility of such proteins for the immune system is thought to be limited protectively in order to minimize inflammation consequent to the recognition of self-antigens in parenchymal structures by T lymphocytes. In the current experiments we investigated whether the previously recognized sparseness of Aβ on the surface of tubular epithelial cells might be accounted for by a protein coding difference deduced from the primary structure of its transcript compared with sequence from lymphoid cells that normally express Aβ in generous amounts. We demonstrate, however, using clones obtained from a cDNA library prepared from tubular epithelium harvested from H-2S (Aβ/α+; Eβ/α-) mice susceptible to autoimmune interestitial nephritis, that the nucleotide sequence encoding the class II Aβ chain in cells from both compartments is essentially identical. Our findings suggest that there is no primary structural aberrancy in the coding region of parenchymal Aβ that would contribute to its low expression. The protective tolerance afforded by reduced numbers of class II molecules in normal tissues is, therefore, more likely the result of repressive regulatory processes.
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