text: Numerous cytokines, both proinflammatory and anti-inflammatory, have been detected in the ST of RA, and the balance between these opposing cytokine activities regulates disease severity [10]. Endogenous IL-10, produced mainly by macrophages and T cells, inhibits proinflammatory cytokine production by ST cells [12]. However, this regulatory activity seems to be restricted during chronic inflammation. The activation of both the extracellular stimulus-regulated kinase and p38 kinase pathways, induced by TNF-alpha and IL-1, inhibits the Jak1-STAT3 signaling pathway shared by IL-10 and IL-6 in adhered macrophages [13]. More importantly, IL-10-mediated STAT3 activation is mostly undetectable in RA synovial macrophages. This impaired IL-10 signaling is probably induced by chronic exposure to immune complexes in vivo, because both cell surface IL-10R1 expression and IL-10-induced Jak1 activation are suppressed in IFN-gamma-primed macrophages by a protein kinase C-dependent pathway following ligation of the IgG Fc gamma receptor [14]. Furthermore, dendritic cells from RA synovial fluids are resistant to the immunoregulatory effect of IL-10 due to decreased transport of intracellular IL-10R1 in the presence of proinflammatory cytokine stimuli such as TNF-alpha, IL-1, and granulocyte-macrophage colony-stimulating factor [15]. We have demonstrated that the resistance of RA CD4+ T cells to IL-10 may be associated with defective IL-10-dependent STAT3 activation, but not with IL-10R1 expression. Inhibitory effects of IL-10 on these inflammatory cell types are therefore differentially modulated at the signal transduction level under the inflammatory environment in RA. 
  In association with impaired IL-10-mediated STAT3 activation, STAT3 was found to be tyrosine phosphorylated persistently (up to 6 hours) in freshly isolated PB and ST CD4+ T cells from RA patients. STAT3 is activated by a variety of cytokines, notably the IL-6 family of cytokines (e.g. IL-6, IL-11, leukemia inhibitory factor, and oncostatin M) and growth factors, in addition to IL-10 [4]. Of these cytokines, IL-6 plays a predominant role in eliciting a systemic reaction such as the acute phase response in active RA, due mainly to its abundance in the blood circulation [27]. Consistent with this notion, IL-6 was the major STAT3-activating factor contained in the serum of active RA patients, and the responsiveness to IL-10 was suppressed in normal CD4+ T cells after 36 hours of incubation with IL-6. These results suggest that both the sustained STAT3 activation and the resistance to IL-10 inhibition found in RA CD4+ T cells may be induced after chronic exposure in vivo to high concentrations of serum IL-6. However, it is also possible that STAT3 activity could be constitutively induced in CD4+ T cells by their own IL-10 secretion, leading to the loss of sensitivity to exogenous IL-10, because RA CD4+ T cells in the ST are capable of producing significant levels of IL-10 [34]. 
  CD4+ T cells isolated from the ST of RA also showed a defect in the IL-10-induced STAT3 signaling pathway. It is most probable that the resistance of CD4+ T cells to IL-10 can be even augmented after migration into the inflamed ST, because IL-6 is highly concentrated compared with the blood level [27]. In addition, the involvement of other essential proinflammatory cytokines in this process was suggested by our preliminary experiments demonstrating that IL-10-mediated IFN-gamma inhibition in CD4+ T cells was reduced by pretreatment with IL-1beta and TNF-alpha, although less effectively than by IL-6 (data not shown). Furthermore, IFN-gamma and IL-10 produced by CD4+ T cells themselves could be responsible for impaired IL-10 signaling in the ST, because T-cell infiltrates produce both cytokines [34,35]. In an autocrine fashion, IL-10 may persistently stimulate STAT3 activation and IFN-gamma can induce SOCS1 protein as a crosstalk inhibitor of IL-10 signaling [32]. The T-cell-inhibitory effect of IL-10 may therefore be modulated complicatedly upon exposure to an inflammatory environment in RA joints, where many cytokines are present substantially [10]. 
  