Abstract
The CD3ɛ and ζ chains of the TCR have been shown to possess independent signaling capabilities. Studies with chimeric molecules containing the cytoplasmic domains of either ζ or ɛ have suggested that these two structurally distinct members of the TCR-CD3 complex are able to function autonomously and have redundant features in the context of TCR-signal transduction in mature T cells. Expression of a chimeric human IL-2-receptor-ζ-chain molecule in the CD4+8+ T-cell line, DPK, has enabled us to directly analyze responses initiated by the ζ-chain-signaling module alone within the context of immature T-cell differentiation. In this paper, we show that antibody crosslinking of the chimeric ζ chain delivers only a limited activation signal as measured by Ca[2+] flux, induction of low-level CD5 expression, and minimal differentiation as assessed by loss of cell-surface CD8 expression. TCR-induced activation through antibody crosslinking of the endogenous CD3ɛ receptor in the absence of costimulation was also relatively inefficient in initiating activation and differentiation. However, co-crosslinking of the CD4 coreceptor with CD3 resulted in a synergistic response, where as there was little effect of co-crosslinking of CD4 and the ζ-chain chimera. Striking differences were also observed in the substrate pattern of tyrosine phosphorylation, as well as lymphokine secretion following triggering through the intact TCR versus the ζ chain alone. These results indicate that although the ζ-chain may possess some signaling capacities similar to that of the intact TCR, it appears to have limited function as an autonomous subunit in initiating CD4+8+ T-cell differentiation.