Human effector T cells have been difficult to isolate and characterize due to their phenotypic and functional similarity to the memory subset. In this study, a biochemical approach was used to analyze human effector CD4 T cells generated in vitro by activation with anti-CD3 and autologous monocytes for 3 to 5 days. The resultant effector cells expressed the appropriate activation/differentiation markers and secreted high levels of interferon γ (IFN-γ) when restimulated. Biochemically, effector CD4 T cells exhibited increases in total intracellular tyrosine phosphorylation and effector-associated phosphorylated species. Paradoxically, these alterations in tyrosine phosphorylation were concomitant with greatly reduced expression of CD3ζ and CD3ε signaling subunits coincident with a reduction in surface T-cell receptor (TCR) expression. Because loss of CD3ζ has also been detected in T cells isolated ex vivo from individuals with cancer, chronic viral infection, and autoimmune diseases, the requirements and kinetics of CD3ζ down-regulation were examined. The loss of CD3ζ expression persisted throughout the course of effector T-cell differentiation, was reversible on removal from the activating stimulus, and was modulated by activation conditions. These biochemical changes occurred in effector T cells generated from naive or memory CD4 T-cell precursors and distinguished effector from memory T cells. The results suggest that human effector T-cell differentiation is accompanied by alterations in the TCR signal transduction and that loss of CD3ζ expression may be a feature of chronic T-cell activation and effector generation in vivo.
Increased density of class I major histocompatibility complex antigens and decreased density of T-cell differentiation antigens in the early stages of T-cell activation
