Prolonged stimulation by TNF uncouples T cell receptor (TCR) signalling pathways at many levels

CD4+ cells at sites of inflammation such as the rheumatoid joint exhibit profound proliferative hyporesponsiveness. We recently demonstrated that expression of the zeta chain of the TCR (TCR zeta) was down-regulated in TNF-treated T cells. Reduced expression of TCR zeta, together with impaired assembly and stability of the cell surface TCR/CD3 complex, may uncouple proximal signal transduction pathways from the TCR. We have proposed that signalling pathways downstream of the TCR may be attenuated as a result, and that this may contribute to the hyporesponsive phenotype. A T cell hybridoma cell line was generated which expressed a single-chain Fv/TCR zeta chimaeric receptor (C2-zeta) under control of retroviral LTR: TNF was not expected to affect expression of C2-zeta. We now report that the receptor-proximal tyrosine phosphorylation responses of these cells were reduced after prolonged TNF-treatment, even though C2-zeta expression was unchanged. Furthermore, while the amplitude and duration of calcium release from intracellular stores were slightly lower in TNF-treated T cells, there was a marked attenuation of capacitative calcium entry in these cells. In addition, PMA- and ionomycin-stimulated IL-2 production was also suppressed in cells that had undergone prolonged culture in TNF. These data indicate that chronic exposure to TNF attenuates a number of signal transduction events downstream of the TCR, independently of TNF-induced suppression of TCR zeta expression. Such changes may account, in part, for the profound hyporesponsiveness of T cells at sites of inflammation.