TRAF6-Dependent Mitogen-Activated Protein Kinase Activation Differentially Regulates the Production of Interleukin-12 by Macrophages in Response toToxoplasma gondii

Abstract

The production of interleukin-12 (IL-12) is critical to the development of innate and adaptive immune responses required for the control of intracellular pathogens. Many microbial products signal through Toll-like receptors (TLR) and activate NF-κB family members that are required for the production of IL-12. Recent studies suggest that components of the TLR pathway are required for the production of IL-12 in response to the parasite Toxoplasma gondii; however, the production of IL-12 in response to this parasite is independent of NF-κB activation. The adaptor molecule TRAF6 is involved in TLR signaling pathways and associates with serine/threonine kinases involved in the activation of both NF-κB and mitogen-activated protein kinase (MAPK). To elucidate the intracellular signaling pathways involved in the production of IL-12 in response to soluble toxoplasma antigen (STAg), wild-type and TRAF6−/− mice were inoculated with STAg, and the production of IL-12(p40) was determined. TRAF6−/− mice failed to produce IL-12(p40) in response to STAg, and TRAF6−/− macrophages stimulated with STAg also failed to produce IL-12(p40). Studies using Western blot analysis of wild-type and TRAF6−/− macrophages revealed that stimulation with STAg resulted in the rapid TRAF6-dependent phosphorylation of p38 and extracellular signal-related kinase, which differentially regulated the production of IL-12(p40). The studies presented here demonstrate for the first time that the production of IL-12(p40) in response to toxoplasma is dependent upon TRAF6 and p38 MAPK

TNF Receptor-Associated Factor 6-Dependent CD40 Signaling Primes Macrophages to Acquire Antimicrobial Activity in Response to TNF-α

“…These findings as well as the demonstration that TRAF6 regulates maturation of dendritic cells and IL-12 production by these cells (34,35,73) suggest that this adapter protein plays an important role in resistance against intracellular pathogens.…”

Toxoplasma gondiiGenotype Determines MyD88-Dependent Signaling in Infected Macrophages

“…For infection with RH tachyzoites, representative of virulent type I parasite strains, IL-12 production did not require TLR/IL-1R adaptor MyD88. Activation of p38 MAPK, necessary for parasite-induced IL-12 (34,35), was also normal in MyD88 Ϫ/Ϫ macrophages infected with RH tachyzoites. p38 MAPK phosphorylation occurred in the absence of detectable MKK3, MKK4, and MKK6 activation in MKK3…”

“…We and others recently found a requirement for p38 MAPK signaling in macrophage IL-12 induction in response to soluble tachyzoite extracts and live parasites of the virulent type I RH strain (34,35). p38 MAPK activation in response to T. gondii involves parasite-induced autophosphorylation rather than a more conventional pathway of phosphorylation mediated by upstream MAPK kinases (MKK) 3 such as MKK3, MKK4, or MKK6 (34).…”

Host cell Ca2+ and protein kinase C regulate innate recognition ofToxoplasma gondii

“…For example, the parasite inhibits transcription factors associated with host defense such as NF-B (Butcher et al, 2001;Shapira et al, 2002) and STAT1␣ (Luder et al, 2001), while promoting activation of STAT3, which suppresses synthesis of IL-12 . Nevertheless, live parasites and parasite-derived products induce pro-inflammatory signaling in macrophages, including activation of the IKK complex (Shapira et al, 2005), as well as phosphorylation of the mitogen-activated protein kinases (MAPK) p38 and ERK1/2, which have been shown in several studies to be important regulators of the macrophage IL-12 response Mason et al, 2004). Previous work has demonstrated that induction of IL-12 synthesis by T. gondii requires the Toll-like receptor (TLR) adaptor molecules MyD88 (Scanga et al, 2002) and TRAF6 (Mason et al, 2004), and there is evidence that parasite recognition is mediated by one or more TLRs (Hitziger et al, 2005;Mun et al, 2003;Scanga et al, 2002;Yarovinsky et al, 2005).…”