Preferential requirement of CD3ζ-mediated signals for development of immature rather than mature thymocytes

Antigen recognition signals by the TCR are transduced through activation motifs present in the cytoplasmic region of CD3 chains, in vitro analysis has suggested that the CD3ζ chain mediates different signals from other CD3 chains. To analyze the in vivo function of CD3ζ-mediated signals for T cell development, mice expressing a mutant CD3ζ chain lacking all the activation motifs were generated by introducing the transgene into ζ-knockout mice. Mature CD4⁺ single-positive (SP) thymocytes in these mice were greater in number than in ζ-deficient mice, and the promoted differentiation was indicated by the changes of CD69 and HSA phenotypes. We found that even in the absence of activation motifs in CD3ζ, these mature cells became functional, being able to induce Ca²⁺ mobilization and proliferation upon stimulation. On the other hand, CD4^-CD8^- double-negative (DN) thymocytes, most of which were arrested at the CD44^-CD25⁺ stage similarly to those in ζ-deficient mice, could not be promoted for differentiation into CD4⁺CD8⁺ double-positive thymocytes in these mice in spite of the fact that the expression of the transgene in DN thymocytes was higher than that of ζ in wild-type mice. These results demonstrate the preferential dependence of the promotion of development and/or expansion of DN thymocytes rather than mature thymocytes upon the activation signals through the ζ chain and suggest differential requirements of TCR signaling for mature SP and immature DN thymocyte developments in vivo.