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TLR2 signaling subpathways regulate TLR9 signaling for the effective induction of IL-12 upon stimulation by heat-killed Brucella abortus

Cellular & Molecular Immunology volume 9, pages 324–333 (2012)Cite this article

Abstract

Brucella abortus is a Gram-negative intracellular bacterium that induces MyD88-dependent IL-12 production in dentritic cells (DCs) and a subsequent protective Th1 immune response. Previous studies have shown that the Toll-like receptor 2 (TLR2) is required for tumor-necrosis factor (TNF) production, whereas TLR9 is responsible for IL-12 induction in DCs after exposure to heat-killed Brucella abortus (HKBA). TLR2 is located on the cell surface and is required for optimal microorganism-induced phagocytosis by innate immune cells; thus, phagocytosis is an indispensable preliminary step for bacterial genomic DNA recognition by TLR9 in late-endosomal compartments. Here, we hypothesized that TLR2-triggered signals after HKBA stimulation might cross-regulate TLR9 signaling through the indirect modulation of the phagocytic function of DCs or the direct modulation of cytokine gene expression. Our results indicate that HKBA phagocytosis was TLR2-dependent and an essential step for IL-12p40 induction. In addition, HKBA exposure triggered the TLR2-mediated activation of both p38 and extracellular signal-regulated kinase 1/2 (ERK1/2). Interestingly, although p38 was required for HKBA phagocytosis and phagosome maturation, ERK1/2 did not affect these processes but negatively regulated IL-12 production. Although p38 inhibitors tempered both TNF and IL-12 responses to HKBA, pre-treatment with an ERK1/2 inhibitor significantly increased IL-12p40 and abrogated TNF production in HKBA-stimulated DCs. Further experiments showed that the signaling events that mediated ERK1/2 activation after TLR2 triggering also required HKBA-induced Ras activation. Furthermore, Ras-guanine nucleotide-releasing protein 1 (RasGRP1) mediated the TLR2-induced ERK1/2 activation and inhibition of IL-12p40 production. Taken together, our results demonstrated that HKBA-mediated TLR2-triggering activates both the p38 and ERK1/2 signaling subpathways, which divergently regulate TLR9 activation at several levels to induce an appropriate protective IL-12 response.

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Acknowledgements

This work was supported through funding from a Tianjin Municipal Science and Technology commission grant (No. 08ZCGYSH04700).

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Author notes

  1. Chun-Yan Zhang and Nan Bai: These authors contributed equally to this work.

  2. CH Liu, Department of Immunology, Medical School, Nankai University, Tianjin 30071, China. E-mail: cl45@nankai.edu.cn

  3. R Xiang, Department of Immunology, Medical School, Nankai University, Tianjin 30071, China. E-mail: rxiang@nankai.edu.cn

Authors and Affiliations

  1. Department of Immunology, Medical School, Nankai University, Tianjin, China

    Chun-Yan Zhang, Nan Bai, Zhu-Hong Zhang, Ning Liang, Lan Dong, Rong Xiang & Cheng-Hu Liu

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Zhang, CY., Bai, N., Zhang, ZH. et al. TLR2 signaling subpathways regulate TLR9 signaling for the effective induction of IL-12 upon stimulation by heat-killed Brucella abortus. Cell Mol Immunol 9, 324–333 (2012). https://doi.org/10.1038/cmi.2012.11

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