Research Article: Toll-Like Receptor 2 Impairs Host Defense in Gram-Negative Sepsis Caused by Burkholderiapseudomallei (Melioidosis)

Date Published: July 31, 2007

Publisher: Public Library of Science

Author(s): W. Joost Wiersinga, Catharina W Wieland, Mark C Dessing, Narisara Chantratita, Allen C Cheng, Direk Limmathurotsakul, Wirongrong Chierakul, Masja Leendertse, Sandrine Florquin, Alex F de Vos, Nicholas White, Arjen M Dondorp, Nicholas P Day, Sharon J Peacock, Tom van der Poll, Steven M Opal

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.

Partial Text: The recently discovered family of Toll-like receptors (TLRs), which are conserved across many species, has emerged as an important first line of defense against invading pathogens. They detect host invasion by pathogens, initiate immune responses, and form the crucial link between the innate and adaptive immune systems [1,2]. Upon the first encounter with a pathogen, TLRs recognize conserved motifs termed pathogen-associated molecular patterns (PAMPs) [1,2]. In general, the immune activation that follows the interaction between TLRs and PAMPs is sufficient to eliminate the wide variety of pathogens that daily invade the human body. However, in the case of sepsis, these TLR-mediated inflammatory responses may exceed the threshold below which immune system homeostasis is maintained and thereby cause harm.

In the present study we aimed to characterize the expression and function of the TLRs in septic melioidosis, linking observational studies in patients with culture-proven disease with functional studies in TLR-deficient mice. We made the following key observations. (1) Patients with septic melioidosis have increased expression of CD14, TLR1, TLR2, and TLR4 on the cell surfaces of circulating monocytes and granulocytes and increased levels of mRNAs encoding CD14, LY96, TLR1, TLR2, TLR3, TLR4, TLR5, TLR8, and TLR10 in peripheral blood cells. (2) Both TLR2 and TLR4 contribute to cellular responsiveness to Burkholderia in vitro, as reflected by reduced TNFα release by alveolar macrophages and whole blood from TLR2 KO and TLR4 KO mice and by activation of HEK293 cells stably transfected with either TLR2 or TLR4 upon incubation with B. pseudomallei. (3) The LPS of B. pseudomallei signals through TLR2 and not through TLR4. And (4) TLR4 is not important for protective immunity against experimentally induced melioidosis, whereas TLR2 is associated with the growth and dissemination of the infection, significantly contributing to distant organ injury and lethality.



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