Toll-Like Receptor 2 Impairs Host Defense in Gram-Negative Sepsis Caused by Burkholderia pseudomallei (Melioidosis)

Abstract

BackgroundToll-like receptors (TLRs) are essential in host defense against pathogens by virtue of their capacity to detect microbes and initiate the immune response. TLR2 is seen as the most important receptor for gram-positive bacteria, while TLR4 is regarded as the gram-negative TLR. Melioidosis is a severe infection caused by the gram-negative bacterium, Burkholderia pseudomallei, that is endemic in Southeast Asia. We aimed to characterize the expression and function of TLRs in septic melioidosis.Methods and FindingsPatient studies: 34 patients with melioidosis demonstrated increased expression of CD14, TLR1, TLR2, and TLR4 on the cell surfaces of monocytes and granulocytes, and increased CD14, TLR1, TLR2, TLR4, LY96 (also known as MD-2), TLR5, and TLR10 mRNA levels in purified monocytes and granulocytes when compared with healthy controls. In vitro experiments: Whole-blood and alveolar macrophages obtained from TLR2 and TLR4 knockout (KO) mice were less responsive to B. pseudomallei in vitro, whereas in the reverse experiment, transfection of HEK293 cells with either TLR2 or TLR4 rendered these cells responsive to this bacterium. In addition, the lipopolysaccharide (LPS) of B. pseudomallei signals through TLR2 and not through TLR4. Mouse studies: Surprisingly, TLR4 KO mice were indistinguishable from wild-type mice with respect to bacterial outgrowth and survival in experimentally induced melioidosis. In contrast, TLR2 KO mice displayed a markedly improved host defenses as reflected by a strong survival advantage together with decreased bacterial loads, reduced lung inflammation, and less distant-organ injury.ConclusionsPatients with melioidosis displayed an up-regulation of multiple TLRs in peripheral blood monocytes and granulocytes. Although both TLR2 and TLR4 contribute to cellular responsiveness to B. pseudomallei in vitro, TLR2 detects the LPS of B. pseudomallei, and only TLR2 impacts on the immune response of the intact host in vivo. Inhibition of TLR2 may be a novel treatment strategy in melioidosis.

Heme Oxygenase-1 and Carbon Monoxide PromoteBurkholderia pseudomalleiInfection

“…Wang et al (59) observed that the HO-1/CO pathway is able to suppress LPS-induced TLR4 signaling, including the production of TNF-a and IL-6 in murine macrophages, by regulating the interaction of TLR4 with caveolin-1. Whereas both TLR4 and TLR2 have been found to contribute to cellular responsiveness to B. pseudomallei in vitro, only TLR2 might have an impact on the immune response of the intact host in vivo (60).…”

Burkholderia pseudomallei-Induced Expression of a Negative Regulator, Sterile-α and Armadillo Motif-Containing Protein, in Mouse Macrophages: a Possible Mechanism for Suppression of the MyD88-Independent Pathway

“…Although the roles of TLRs have been extensively investigated in B. pseudomallei infection, it is still controversial whether TLR2 or TLR4 or the combination of both is involved in triggering host cells. Wiersinga et al used HEK293 cells stably transfected with TLR2/ CD14 or TLR4/CD14 to show that the LPS isolated from B. pseudomallei signals only through TLR2 (35). Moreover, TLR2-deficient mice infected with B. pseudomallei display decreased bacterial loads and prolonged survival, suggesting that TLR2 is essential in B. pseudomallei infection (35).…”

“…Wiersinga et al used HEK293 cells stably transfected with TLR2/ CD14 or TLR4/CD14 to show that the LPS isolated from B. pseudomallei signals only through TLR2 (35). Moreover, TLR2-deficient mice infected with B. pseudomallei display decreased bacterial loads and prolonged survival, suggesting that TLR2 is essential in B. pseudomallei infection (35). In contrast, West et al showed that tumor necrosis factor alpha (TNF-␣) production was decreased in LPS-treated TLR4 Ϫ/Ϫ primary macrophages, indicating that TLR4 is involved in proinflammatory cytokine production (34).…”

Interleukin-1 Receptor-Associated Kinase M-Deficient Mice Demonstrate an Improved Host Defense during Gram-negative Pneumonia

“…Alveolar macrophages were harvested from IRAK-M -/-and WT mice by bronchoalveolar lavage (BAL) (n = 8 per strain) as described (15,16 (19). Labeled bacteria were incubated with macrophages for 1 h at 37°C (MOI 100:1) and cells were analyzed using FACSCalibur (Becton Dickinson, Breda, the Netherlands).…”