text: Introduction
  Interleukin-10 (IL-10) is a cytokine with broad anti-inflammatory properties that inhibits macrophage and dendritic cell (DC) function (Moore etal., 2001). IL-10 limits the immune and inflammatory responses to pathogens and gut flora and prevents damage to the host (Moore etal., 2001; O'Garra and Vieira, 2004), but when dysregulated may result in chronic infection (Brooks etal., 2006; Ejrnaes etal., 2006; Moore etal., 2001). IL-10 is expressed by T helper 2 (Th2) cells, B cells, DCs, and macrophages (Moore etal., 2001), and also by Th1 cells (Anderson etal., 2007; Assenmacher etal., 1994; Del Prete etal., 1993; Gerosa etal., 1996; Jankovic etal., 2007; Pohl-Koppe etal., 1998) and (reviewed in O'Garra and Vieira, 2007; Trinchieri, 2007), certain regulatory (Treg) Tcells (Moore etal., 2001; O'Garra and Vieira, 2004; Roncarolo etal., 2006), and Th17 cells (Awasthi etal., 2007; Fitzgerald etal., 2007; McGeachy etal., 2007; Stumhofer etal., 2007). 
  In vitro human CD4+ and CD8+ Tcell clones, or mouse CD4+ Tcells that produce both interferon-gamma (IFN-gamma) and IL-10, can be differentiated by Tcell receptor (TCR)-stimulation in the presence of IL-12 (Chang etal., 2007; Gerosa etal., 1996; Jeannin etal., 1996; Meyaard etal., 1996; Windhagen etal., 1996). Furthermore, Th1 cell clones coproducing IFN-gamma and IL-10 have been isolated from bronchoalveolar lavage (BAL) of active pulmonary tuberculosis (TB) patients (Gerosa etal., 1999). IL-10 production by Th1 cells was also reported in animals infected with Toxoplasma gondii (Jankovic etal., 2002; Shaw etal., 2006) or with Leishmania major (Anderson etal., 2007) and shown to be required for regulation of the immune response in these infections (Anderson etal., 2007; Jankovic etal., 2007). The relative amounts of IL-10 and IFN-gamma produced by Th1 cells may influence the balance between clearance and persistent infection with certain pathogens (Moore etal., 2001; Trinchieri, 2007), thus determining whether chronic infection or immunopathology ensues. 
  Th1, Th2, and Th17 cell responses differentiate along distinct signaling pathways (Glimcher and Murphy, 2000; Ivanov etal., 2007; Stockinger and Veldhoen, 2007). Th1 cell development requires signal transducer and activator of transcription (STAT)1activation, induced by type I IFN or IFN-gamma, the transcription factor T-box 21 (T-bet), and IL-12-induced STAT4 signaling, which can couple with IL-18-induced IRAK and NF-kappaB transcription factors to drive the high amounts of IFN-gamma required to eradicate intracellular pathogens (Glimcher and Murphy, 2000). Th2 cell development, with expression of IL-4, IL-5, and IL-13, requires IL-4, STAT6, and the transcription factor GATA binding protein (GATA)-3 (Glimcher and Murphy, 2000). The development of Th17 cells requires IL-6, TGF-beta, and the STAT3-dependent expression of the transcription factor RORgammat (Ivanov etal., 2007; Stockinger and Veldhoen, 2007). 
  Th1 and Th2 cell responses can also be induced by varying the dose of antigen presented to the naive Tcell by the antigen-presenting cell (APC). Whereas high doses of antigen, with sustained TCR signaling and extracellular-signal regulated (ERK) mitogen-activated protein kinase (MAPK) phosphorylation, result in Th1 cells producing IFN-gamma via an IL-12-independent mechanism, low doses of antigen, with transient ERK1 and ERK2 activation, favor Th2 responses and IL-4 secretion (Constant etal., 1995; Hosken etal., 1995; Jorritsma etal., 2003; Yamane etal., 2005). 
  Because Th1 and Th2 cells cross regulate each other's development and function and can suppress Th17 cell responses, and all differentiate along distinct signaling pathways (Glimcher and Murphy, 2000; Stockinger and Veldhoen, 2007), IL-10 produced by all these Th cells may thus act as a feedback regulator to control the pathology associated with an overexuberant, albeit efficacious, inflammatory response. Whether IL-10 production by these different Th cell subsets is induced by independent and/or common mechanisms is unknown. 
  Here, we showed that invitro differentiation of IL-10-producing Th1 cells from naive CD4+ Tcells required IL-12-induced STAT4 signaling, strong TCR activation (high antigen dose), and sustained ERK1 and ERK2 phosphorylation. Furthermore, we showed that activation of ERK1 and ERK2 is a requirement for production of IL-10 by Th1, Th2, and Th17 cell subsets. This common but highly regulated pathway for IL-10 induction and maintenance ensures its function as a feedback loop to control damage to the host and also allows a protective response to ensue as opposed to chronic infection. 
  