Differential expression of two distinct T-cell receptors during thymocyte development

The product of the T-cell receptor (TCR) γ-gene¹ has recently been found to be expressed on a subset of both peripheral cells ² and thymocytes^3,4. As an initial approach to understanding the role of this γ-chain of TCR (TCRγ) in T-cell development, we have studied the ontogeny of TCR expression at the protein level in the developing murine thymus. We show here that the first T3-associated TCR to be expressed in the developing thymus is a disulphide-linked heterodimer composed of a γ-chain of relative molecular mass 35,000 (M_r 35K) and a 45K partner (termed TCR δ). This TCR γδ is first detected approximately two days before the appearance of cell-surface TCR αβ heterodimers. We report that N-glycosidase digestions reveal that all of the γ-protein expressed on fetal thymocytes, as in adult CD4 ^-8^- (L3T4^-, Lyt2^-) thymocytes ⁴, bear N-linked carbohydrate side chains. The major γ-gene transcribed in mature, αβ-bearing T cells ( V_{γ 1.2} C_γ2) encodes no N-linked glycosylation site ¹ so these results suggest that the fetal γδ receptor defines a distinct T-cell lineage whose development in the thymus precedes classical αβ -bearing cells.