Outcomes of ERK Signaling Differ in Macrophages and Dendritic Cells

Abstract Stimulation of dendritic cells and macrophages by pathogen-related products activates a variety of immune responses. Pathogen-induced cell signaling and associated outcome research is abundant in macrophages, but minimal in dendritic cells. We investigated the role of ERK signaling in the induction of cytokines such as IL-12p40 and IL-10 following activation by Pam3Cys or Mycobacterium tuberculosis (Mtb) H37Ra. ERK ablation was accomplished pharmacologically with MERK1/2 inhibitor U0126 or genetically by using cells from Tpl2−/− mice (TPL2 connects TLR to ERK). We utilized western blotting to examine ERK activation, ELISA to assess cytokine production, and qRT-PCR to investigate mRNA levels. In macrophages, blockade of ERK signaling inhibited IL-10 production and increased IL-12p40 production (6–10 fold increase in IL-12p40 mRNA and 3–4 fold increase in protein). In dendritic cells, ERK blockade similarly inhibited IL-10 production, but produced a very different change in IL-12p40 expression: ERK blockade decreased IL-12p40 production (2-fold decrease in IL-12p40 mRNA and protein). This result suggests that the impact of ERK signaling in response to these stimuli differs between macrophages and dendritic cells for a subset of ERK-regulated genes. ERK-mediated suppression of IL-12p40 in macrophages may prevent excess inflammation and associated tissue damage, while ERK-mediated induction of IL-12p40 in dendritic cells may promote priming of Th1 responses. Future studies will examine other genes and proteins involved in infection responses. Greater understanding of the role that ERK signaling plays in different cell types may inform the development of host-directed therapy for infections such as tuberculosis.