Background
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The T-cell receptor zeta (TCR-ζ) chain plays an important role not only in the assembly of the TCR/CD3 complex but also in coupling cell surface receptors to downstream intracellular signalling pathways. We have recently identified a subset of T cells in the peripheral blood that expresses low levels of TCR zeta (TCR-ζ-dim). Since several groups have documented reduced expression of TCR-ζ chain in synovial-joint T cells from patients with rheumatoid arthritis, we set out to test the hypothesis that peripheral blood TCR-ζ-dim T cells represent a subset of circulating memory effector T cells.

Objective
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To characterise the cell surface markers and cytokine-producing capacity of the TCR-ζ-dim T cells.

Methods
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Three-colour staining and flow cytometry (FACS) were performed on fixed and permeabilised cells to study cell-surface markers and the expression of intracellular TCR-ζ. Intracellular TNF-α and IFN-γ expression were also detected by FACS after stimulation with phorbol ester and ionomycin in the presence of monensin, and the expression in TCR-ζ-bright and TCR-ζ-dim populations was compared.

Results
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Engagement of TCR on peripheral blood T cells *in vitro* led to down-regulation of TCR-ζ. Furthermore, levels of TCR-ζ from inflamed joints were also dramatically reduced in comparison with that from the peripheral blood. As expected, synovial-fluid and membrane T cells expressed more CD45RO, CD45RB-dull and CD62L-neg than peripheral blood T cells. In comparison with the CD3^+^ TCR-ζ-bright population, peripheral blood TCR-ζ-dim T cells were also enriched for CD45RO expression, and expressed the CD45RB-dull and CD62L-neg markers of memory T cells. A significant proportion of TCR-ζ-dim T cells were found to be CD28-negative. After stimulation *in vitro*, TCR-ζ-dim cells were found to be enriched for TNF-α and IFN-γ producers in comparison with the TCR-ζ-bright T-cell subset from the sample individual. Preliminary data suggest that TCR-ζ-bright and TCR-ζ-dim T-cell subsets can be distinguished on the basis of their relative expression of CD52.

Conclusion
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CD3^+^ TCR-ζ-dim cells are characteristic of T cells activated through the TCR and are enriched for cells capable of expressing TNF-α and IFN-γ. We postulate that this T-cell subset may represent circulating effector cells that may play a role in promoting the chronic inflammatory process.
