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A cDNA clone encoding the a chain of the human T-cell antigen receptor was isolated by screening a library from the human T-cell line Jurkat with a mouse a-chain cDNA clone. This human a-chain clone, together with a human antigen receptor 3-chain cDNA clone, was used to determine the stage of T-cell development at which antigen receptor mRNAs first appear. Blot-hybridization analysis of mRNA isolated from a panel of human thymic tumor lines clearly demonstrated that 13-chain transcripts could be detected in all T-lineage cells. However, a-chain transcripts were only found in the most phenotypically mature lines, which express the antigen receptor-associated molecule T3. Furthermore, 13-chain transcripts were abundant in RNA prepared from purified T3-negative thymocytes, whereas a-chain transcripts were virtually absent. From these results we conclude that a-chain expression occurs later in thymic ontogeny than that of the 18 chain and propose that it controls surface expression of the antigen receptor-T3 complex.

A number of cDNA clones homologous to the human T-celi receptor ,3-chain gene have been isolated from a library constructed from the human leukaemic cell line Jurkat. The nucleotide sequences of two of these clones demonstrate thatJurkat synthesises an RNA, probably derived from an aberrantly rearranged CO1 gene, which cannot encode a fnctional /-chain. This transcript contains a novel joining region and similar transcripts appear to be relatively abundant in several T-celi lines.

The cultured human B lyniphoblastoid cell line Maja synthesises two forms of the -y heavy chain of immunoglobulin G (IgG) that differ in apparent molecular weight. The lower-molecular-m eight form is secreted into the culture medium as a water-soluble product in association with light chains and coniigrates on dodecyl sulphate polyacrylamide gels with serum IgG y chains. The higher-molecular-weight form is not detected in culture supernatants. In distinction to the secreted form, the higher-molecular-weight form is labelled by a lipophilic, photoactivatable nitrene and is inserted asymmetrically in a transmembrane orientation into rough microsomes. It is concluded that Maja cells synthesise secretory (ys) and membrane-associated (rm) forrns o f IgG heavy chains.

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