Subunit composition of pre-T cell receptor complexes expressed by primary thymocytes: CD3δ is physically associated but not functionally required

Maturation of immature CD4^-CD8^- (DN) thymocytes to the CD4⁺CD8⁺ (DP) stage of development is driven by signals transduced through a pre-T cell receptor (TCR) complex, whose hallmark is a novel subunit termed pre-Tα (pTα). However, the precise role of pre-TCRs in mediating the DN to DP transition remains unclear. Moreover, progress in understanding pre-TCR function has been hampered thus far because previous attempts to demonstrate expression of pTα-containing pre-TCRs on the surface of normal thymocytes have been unsuccessful. In this report, we demonstrate for the first time that pTα-containing pre-TCR complexes are expressed at low levels on the surface of primary thymocytes and that these pre-TCR complexes comprise a disulfide-linked pTα-TCR-β heterodimer associated not only with CD3-γ and -ε, as previously reported, but also with ζ and δ. Interestingly, while CD3-δ is associated with the pre-TCR complex, it is not required for pre- TCR function, as evidenced by the generation of normal numbers of DP thymocytes in CD3-δ-deficient mice. The fact that any of the signaling components of the pre-TCR are dispensable for pre-TCR function is indeed surprising, given that few pre-TCR complexes are actually expressed on the surface of primary thymocytes in vivo. Thus, pre-TCRs do not require the full array of TCR-associated signaling subunits (γ, δ, ε, and ζ), possibly because pTα itself possesses signaling capabilities.