Peripheral blood CD8+ T-cell profiles in patients with psoriatic arthritis: a cross-sectional case-control study

OBJECTIVE: While CD4+ T-cells are traditionally regarded as the main pathogenic T-cell subpopulation in psoriatic arthritis (PsA), the role of circulating CD8+ T-cells remains poorly characterized. We evaluated the differential representation of CD8+ T-cell subpopulations in peripheral blood (PB) of PsA patients. PATIENTS AND METHODS: CD8+IL-17+, CD8+IFNγ+ and CD8+IL-17-IL-22+ T-cells were evaluated by flow-cytometry in 25 consecutive PsA patients, 7 rheumatoid arthritis (RA) patients, 16 patients with psoriasis, and 26 healthy controls (HC). RESULTS: We observed a significant expansion of circulating IFN-γ producing CD8+ T-cells in PsA when compared to psoriasis [21.2 (6.9- 55.8)% vs. 3.8 (0.7-11.8)%, p < 0.0001] and HC samples [21.2 (6.9-55.8)% vs. 4.05 (0.44-19.8)%, p < 0.0001]. A frequency of circulating IFN-γ producing CD8+T-cells ≥ 9% distinguished PsA from psoriasis patients with a specificity of 84% and a sensitivity of 87.5% [AUC = 0.9 (0.80-0.99), p < 0.0001]. In addition, we found a significant expansion of circulating IL-17 producing CD8+ T cells in RA patients when compared to PsA, psoriasis and HC samples. By contrast, there were no significant between-group differences in the prevalence of circulating IL-22 producing CD8+ T-cells. In PsA patients there was a significant correlation between number of swollen joints and frequency of circulating IFN-γ producing CD8+ T-cells, and between extent and severity of psoriasis and frequency of circulating IL-17 producing CD8+ T-cells. CONCLUSIONS: Circulating IFNγ-producing CD8+ T-cells are raised in PsA when compared to psoriasis, suggesting a potential pathogenetic involvement of CD8+ T-cells and IFNγ production in chronic joint inflammation and damage. The significant enrichment of circulating IL-17 producing CD8+ T-cells in RA when compared to PsA warrants functional characterization and confirmation in larger studies. We found no significant enrichment of circulating IL-22 producing CD8+ T-cells in PsA, RA and psoriasis.