Host defense against Neospora caninum infection via IL-12p40 production through TLR2/TLR3-AKT-ERK signaling pathway in C57BL/6 mice.

Fig. 9. The role of TLR2 and TLR3 in controlling N. caninum infection. When N. caninum invaded, TLR2 and TLR3 signals were activated to induce the phosphorylation of AKT signal, and in turn regulated the phosphorylation of ERK signal, which induced the secretion of IL-12p40 to achieve the purpose of resistance to N. caninum infection. Besides, IL-12p40 exerted an inhibitory effect on N. caninum by promoting IFN-γ secretion to induce NO and GBP2 production during N. caninum infection. We speculated that IL-12p40 could induce the activation of NK and CTL cells and the differentiation of Th1-type lymphocytes via IFN-γ to eliminate N. caninum. [Display omitted] • TLR2/TLR3-AKT-ERK signal axis regulated N. caninum induced IL-12p40 production in mouse macrophages. • IL-12p40 played a crucial role in anti- N. caninum via inducing IFN-γ, NO, and GBP2 production in mouse macrophages. • LTLR2 was more predominant than TLR3 in regulating IL-12p40 production in mice during N. caninum infection. TLR2/3-IL-12p40 axis controlled the N. caninum proliferation in bovine macrophages. Neospora caninum is an intracellular parasite which can cause neosporosis and significant economic losses in both dairy and beef industries worldwide. A better understanding of the immune response by host cells against N. caninum could help to design better strategies for the prevention and treatment of neosporosis. Although previous studies have shown TLR2/TLR3 were involved in controlling N. caninum infection in mice, the precise mechanisms of the AKT and MAPK pathways controlled by TLR2/TLR3 to regulate N. caninum -induced IL-12p40 production and the role of TLR2/TLR3 in anti- N. caninum infection in bovine macrophages remain unclear. In the present study, TLR2−/− mice displayed more parasite burden and lower level of IL-12p40 production compared to TLR3−/− mice. N. caninum could activate AKT and ERK signaling pathways in WT mouse macrophages, which were inhibited in TLR2−/− and TLR3−/− mouse macrophages. In N. caninum- infected WT mouse macrophages, AKT inhibitor or AKT siRNA could decrease the phosphorylation of ERK. AKT or ERK inhibitors reduced the production of IL-12p40 and increased the number of parasites. The productions of ROS, NO, and GBP2 were significantly reduced in TLR2−/− and TLR3−/− mouse macrophages. Supplementation of rIL-12p40 inhibited N. caninum proliferation and rescued the productions of IFN-γ, NO, and GBP2 in WT, TLR2−/−, and TLR3−/− mouse macrophages. In bovine macrophages, the expressions of TLR2, TLR3, and IL-12p40 mRNA were significantly enhanced by N. caninum , and N. caninum proliferation was inhibited by TLR2/TLR3 agonists. Taken together, the proliferation of N. caninum in mouse macrophages was controlled by the TLR2/TLR3-AKT-ERK signal pathway via increased IL-12p40 production, which in turn lead to the productions of NO, GBP2, and IFN-γ during N. caninum infection. And in bovine macrophages, TLR2 and TLR3 contributed to inhibiting N. caninum proliferation via increased IL-12p40 production. [ABSTRACT FROM AUTHOR]