IL-6-mediated STAT3 phosphorylation and inhibition of IL-10 effect in normal CD4+ T cells
STAT3 is activated by many cytokines and growth factors such as the IL-6 family of cytokines (IL-6, IL-11, leukemia inhibitory factor, and oncostatin M), platelet-derived growth factor, and epidermal growth factor, in addition to IL-10 [4], but previous studies have demonstrated that IL-6 is the major factor in RA synovial fluid that induces constitutive activation of STAT3 in mononuclear cells [31]. Since IL-6 is also abundant in sera of active RA patients, frequently detected at > 1 ng/ml [27], we examined whether persistent exposure of CD4+ T cells to high concentrations of IL-6 in the blood circulation was responsible for their sustained STAT3 activation and resistance to IL-10 inhibition in active RA. Both STAT1 and STAT3 phosphorylation was activated by IL-6 in normal CD4+ T cells (data not shown), in agreement with previous observations [4]. Normal CD4+ T cells were thus incubated for 20 min with culture medium containing 30% serum from active RA patients and neutralizing anti-IL-6 antibody or control antibody, and STAT phosphorylation was examined by western blot analysis. RA serum was able to induce tyrosine phosphorylation of STAT3 but not STAT1, and this STAT3 activation was mostly abolished by neutralization of IL-6 activity (Fig. 5a). These results indicate that IL-6 is the dominant STAT3-activating factor contained in sera of active RA patients. The lack of STAT1 activation by RA serum suggests that much higher concentrations of IL-6 may be required for STAT1 activation as compared with STAT3 activation, or that inhibitors of STAT1 signaling may be present in RA serum.
We next examined whether IL-6 could suppress the effect of IL-10 to inhibit IFN-γ production by CD4+ T cells. After preincubation with or without 10 ng/ml IL-6 for 36 hours, normal CD4+ T cells were stimulated by CD3 and CD28 costimulation in the presence or absence of 1 ng/ml IL-10 for 36 hours, and the IFN-γ production was measured by ELISA. IL-6 pretreatment of normal cells reduced IL-10-mediated inhibition of IFN-γ production (Fig. 5b), indicating that high concentrations of IL-6 could modulate T-cell responsiveness to IL-10. Taken together, these findings suggest that persistent exposure to serum IL-6 may have a role in both the induction of STAT3 activation and the resistance to the inhibitory effect of IL-10 in RA CD4+ T cells.