Role of mitogen-activated protein kinases in CpG DNA-mediated IL-10 and IL-12 production: central role of extracellular signal-regulated kinase in the negative feedback loop of the CpG DNA-mediated Th1 response

The mitogen-activated protein kinases, extracellular signal-regulated kinase (ERK), and p38, are activated in response to infectious agents and innate immune stimulators such as CpG DNA, and regulate the subsequent initiation and termination of immune responses. CpG DNA activates p38 and ERK with slightly different kinetics in monocytic cells. The present studies investigated the roles of these two key mitogen-activated protein kinases in regulating the CpG DNA-induced production of pro- and anti-inflammatory cytokines in the macrophage-like cell line RAW264.7. p38 activity was essential for the induction of both IL-10 and IL-12 expression by CpG DNA. In contrast, CpG DNA-mediated ERK activation was shown to suppress IL-12 production, but to be essential for the CpG DNA-induced IL-10 production. Studies using rIL-10 and IL-10 gene-deficient mice demonstrated that the inhibitory effect of ERK on CpG DNA-mediated IL-12 production is indirect, due to the role of ERK in mediating IL-10 production. These results demonstrate that ERK and p38 differentially regulate the production of pro- and anti-inflammatory cytokines in APCs that have been activated by CpG DNA. CpG DNA-induced p38 activity is required for the resulting innate immune activation. In contrast, ERK plays a central negative regulatory role in the CpG DNA-mediated Th1 type response by promoting production of the Th2 type cytokine, IL-10.