Department of Microbiology and Immunology, University of Texas Health Science Center at San Antonio, San Antonio, TX 78229, USA.
J Immunol. 2010 Dec 15;185(12):7562-72. doi: 10.4049/jimmunol.0903790. Epub 2010 Nov 22.
An inadequate innate immune response appears to contribute to the virulence of Francisella tularensis following pulmonary infection. Studies in mice suggest that this poor response results from suppression of proinflammatory cytokine production early during infection, but the mechanisms involved are not understood. PI3K is known to regulate proinflammatory cytokine expression, but its exact role (positive versus negative) is controversial. We sought to clarify the role of PI3K in regulating proinflammatory signaling and cytokine production during infection with F. tularensis live vaccine strain (LVS). In this study, we demonstrate that the induction of TNF and IL-6 expression by LVS in mouse bone marrow-derived macrophages was markedly enhanced when PI3K activity was inhibited by either of the well-known chemical inhibitors, wortmannin or LY294002. The enhanced cytokine expression was accompanied by enhanced activation of p38 MAPK and ERK1/2, both of which were critical for LVS-induced expression of TNF and IL-6. LVS-induced MAPK activation and cytokine production were TLR2- and MyD88- dependent. PI3K/Akt activation was MyD88-dependent, but was surprisingly TLR2-independent. LVS infection also rapidly induced MAPK phosphatase-1 (MKP-1) expression; PI3K and TLR2 signaling were required. Peak levels of MKP-1 correlated closely with the decline in p38 MAPK and ERK1/2 phosphorylation. These data suggest that infection by LVS restrains the TLR2-triggered proinflammatory response via parallel activation of PI3K, leading to enhanced MKP-1 expression, accelerated deactivation of MAPKs, and suppression of proinflammatory cytokine production. This TLR2-independent inhibitory pathway may be an important mechanism by which Francisella suppresses the host's innate immune response.
先天免疫应答不足似乎导致了弗氏耶尔森菌感染肺部后的毒力。小鼠研究表明，这种不良反应是由于感染早期促炎细胞因子产生受到抑制所致，但具体的机制尚不清楚。PI3K 已知可调节促炎细胞因子的表达，但它的确切作用（正调控还是负调控）存在争议。我们试图阐明 PI3K 在调控弗氏耶尔森菌活疫苗株（LVS）感染期间促炎信号和细胞因子产生中的作用。在这项研究中，我们证明 LVS 在小鼠骨髓来源的巨噬细胞中诱导 TNF 和 IL-6 的表达，当 PI3K 活性被两种著名的化学抑制剂wortmannin 或 LY294002 抑制时，明显增强。增强的细胞因子表达伴随着 p38 MAPK 和 ERK1/2 的激活增强，这两者对 TNF 和 IL-6 的表达都是至关重要的。LVS 诱导的 MAPK 激活和细胞因子产生依赖于 TLR2 和 MyD88。PI3K/Akt 的激活依赖于 MyD88，但令人惊讶的是，PI3K/Akt 的激活不依赖于 TLR2。LVS 感染还迅速诱导了 MAPK 磷酸酶-1（MKP-1）的表达；PI3K 和 TLR2 信号均为必需。MKP-1 的峰值水平与 p38 MAPK 和 ERK1/2 磷酸化的下降密切相关。这些数据表明，LVS 感染通过平行激活 PI3K 来抑制 TLR2 触发的促炎反应，导致 MKP-1 的表达增强、MAPK 的快速失活以及促炎细胞因子产生的抑制。这种不依赖于 TLR2 的抑制途径可能是弗朗西斯菌抑制宿主先天免疫反应的一个重要机制。
