An altered percentage of Th17 cells has been described in the peripheral blood and synovial fluid of RA patients, but to date conflicting data have been reported. Our objective was to examine the frequency and phenotype of Th17 cells in the peripheral blood of early RA patients, and in the synovial fluid of patients with established RA. We were also interested in determining which cell type is the main producer of IL-17 in eRA peripheral blood and RA synovial fluid. Our early arthritis clinic allowed the study of T cells from early RA patients who have not received disease modifying drugs or steroids, thereby minimizing interference of drugs with ex vivo T cell responses. Several previous studies indicate that Th17 cells may play an important role in the pathogenesis of RA; therefore we hypothesized that their numbers might be augmented in eRA patients. We surprisingly detected a significantly decreased frequency of circulating Th17 cells when analyzing all of our patients with eRA as a single group, whereas established RA patients showed circulating Th17 frequencies that were not different from controls. Remarkably, after dividing eRA patients according to the presence or absence of anti-CCP antibodies, it was evident that only anti-CCP positive eRA patients demonstrated a decreased circulating Th17 population; in contrast, the frequency of circulating Th17 cells in anti-CCP negative eRA was comparable to the one observed in healthy controls. That is, despite the fact that anti-CCP+ patients represent 45% of our eRA population sample, their markedly decreased Th17 frequency is able to bring down the final numbers in the total eRA group, low enough to result in a significant difference in comparison with healthy controls. At the same time, the frequency of total peripheral blood CD4+ T cells, the frequency of circulating Th1 cells, and the secretion of CD4-derived IFN-c, TNF-a and IL10, were not different between eRA patients and healthy controls. Of note, the frequency of circulating Th17/Th1 cells was decreased in the peripheral blood of anti-CCP+ but not anti-CCP- eRA patients, consistent with recent observations indicating that IL-17+/ IFN-c+ double producers arise from Th17 and not from Th1 cells. Interestingly, among anti-CCP+ eRA patients, both the Th17 and the Th17/Th1 frequency were negatively correlated with the titre of anti-CCP antibodies which further reinforces the link between low Th17 counts and CCP seropositivity. In AZD2281 763113-22-0 addition, a significantly lower frequency of circulating Th17 and Th17/Th1 cells was observed in patients who presented with erosive versus patients presenting with non-erosive eRA; however, after adjusting for anti-CCP antibody status it was evident that this was linked to the strong relation between anti-CCP antibodies and erosions. We chose to analyze cells from the synovial fluid of RA as representative cells from the RA inflammatory site: RA synovial fluid T lymphocytes represent T cells that have reached the joint through the peripheral blood, and have acquired an activated phenotype by locally interacting with the inflamed synovial tissue, where hyperplastic synovial fibroblasts and activated synovial macrophages are abundant. We observed that, when compared with the peripheral blood of healthy subjects and of patients with early or established RA, an increased frequency of Th17 cells was present in RA synovial fluid, together with an increased frequency of Th17/Th1 cells. This is consistent with a set of previous studies on RASF Th17 frequencies but discordant with others.