TLR2 signaling subpathways regulate TLR9 signaling for the effective induction of IL-12 upon stimulation by heat-killed Brucella abortus.
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.
Department of Immunology, Medical School, Nankai University, Tianjin, China.
Chun-Yan
CY
Zhang
Nan
N
Bai
Zhu-Hong
ZH
Zhang
Ning
N
Liang
Lan
L
Dong
Rong
R
Xiang
Cheng-Hu
CH
Liu
D000818:Animals; D000942:Antigens, Bacterial / Q000737:chemistry / Q000276:immunology*; D002003:Brucella abortus / Q000276:immunology*; D016207:Cytokines / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism; D003713:Dendritic Cells / Q000276:immunology*; D005786:Gene Expression Regulation / Q000276:immunology; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism; D006358:Hot Temperature; D007113:Immunity, Innate; D053762:Interleukin-12 Subunit p40 / Q000378:metabolism*; D051379:Mice; D008809:Mice, Inbred C3H; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D020239:Receptor Cross-Talk / Q000276:immunology; D015398:Signal Transduction / Q000276:immunology; D018417:Th1 Cells / Q000276:immunology; D051195:Toll-Like Receptor 2 / Q000235:genetics / Q000378:metabolism*; D051217:Toll-Like Receptor 9 / Q000378:metabolism*; D048051:p38 Mitogen-Activated Protein Kinases / Q000378:metabolism
D000942:Antigens, Bacterial; D016207:Cytokines; D004268:DNA-Binding Proteins; D020662:Guanine Nucleotide Exchange Factors; D053762:Interleukin-12 Subunit p40; C491790:RASGRP1 protein, human; D051195:Toll-Like Receptor 2; D051217:Toll-Like Receptor 9; D048051:p38 Mitogen-Activated Protein Kinases