A functional T3 molecule associated with a novel heterodimer on the surface of immature human thymocytes

The known T-cell receptors (TCRs) involved in the recognition of antigen and major histocompatibility complex (MHC) molecules are glycoproteins comprised of polymorphic disulphide-linked α-and β-chains^1-5. The genes encoding these chains are homologous to immunoglobulin genes and consist of V (variable), J(joining) and C (constant) regions that rearrange during development^6-13. TCRs are expressed relatively late in thymocyte development and only in association with an invariant molecular complex of proteins termed T3 (refs 3, 14). Immature thymocytes do not express the TCR-T3 complex but do express messenger RNA encoding a third rearranging T-cell receptor-like gene, termed Tγ (refs 13,15-20). Here we report a clone of normal immature T4^-T8^- human thymocytes, designated CII, which does not express mature mRNA for Tα or Tβ genes, but does express high levels of Tγ mRNA. This clone also expresses high levels of surface T3, and antibodies to T3 induce immunologically relevant functions in CII cells. Immunoprecipitation of CII surface-labelled proteins with anti-T3 co-precipitates a T3 molecular complex together with two additional and novel peptides of relative molecular mass (M_r) 44,000 (44K) and 62,000 (62K).