Expression of human T cell receptor-γδ structural forms

The human TCR-γδ occurs in three biochemically distinct forms (forms 1, 2bc, and 2abc). A 40-kDa TCR γ-chain is disulfide-linked to the TCR δ-chain in form 1, whereas 40-kDa or 55-kDa TCR-γ polypeptides are noncovalently associated with the TCR δ-chain in forms 2bc and 2abc, respectively. Sequence analysis of TCR-γ cDNA clones indicates that form 1 utilizes the Cγ1 gene segment, whereas forms 2bc and 2abc appear to use allelic Cγ2 gene segments containing either two copies (b and c) or three copies (a, b, and c) of the CII exon, respectively. We transfected TCR-γ cDNA encoding form 1 or form 2abc into the MOLT-13 cell line that expresses form 2bc. The transfected TCR γ-chains associate with the resident MOLT-13 TCR-δ, normally part of form 2bc, to yield CD3-associated TCR-γδ heterodimers identical to those seen on the donor cell lines (form 1 or 2abc). These transfection experiments show directly that, 1) when a single TCR-δ subunit is available, the presence or absence of disulfide linkage between TCR γ- and TCR δ-chains is controlled by the TCR γ-chain, and 2) the difference in the amount of N-linked carbohydrate attached to the transfected TCR-γ proteins of form 2bc vs form 2abc is influenced by the presence or absence of CII exon copy 'a' which appears to alter the secondary and/or tertiary structure of these TCR γ-chain constant regions, thereby affecting the attachment of N-linked glycans. In contrast to the similar structure and usage of Cβ1 and Cβ2, TCR-γδ forms show striking differences in structure and are not equally represented in peripheral blood. Although the role of each form is unknown, it is possible that variable or joining-gene segment selection events or functional differences account for their unequal usage.