Lack of In Vitro and In Vivo Recognition of Francisella tularensis Subspecies Lipopolysaccharide by Toll-Like Receptors

Abstract

Francisella tularensis is an intracellular gram-negative bacterium that is highly infectious and potentially lethal. Several subspecies exist of varying pathogenicity. Infection by only a few organisms is sufficient to cause disease depending on the model system. Lipopolysaccharide (LPS) of gram-negative bacteria is generally recognized by Toll-like receptor 4 (TLR4)/MD-2 and induces a strong proinflammatory response. Examination of human clinical F. tularensis isolates revealed that human virulent type A and type B strains produced lipid A of similar structure to the nonhuman model pathogen of mice, Francisella novicida. F. novicida LPS or lipid A is neither stimulatory nor an antagonist for human and murine cells through TLR4 or TLR2. It does not appear to interact with TLR4 or MD-2, as it is not an antagonist to other stimulatory LPS. Consistent with these observations, aerosolization of F. novicida LPS or whole bacteria induced no inflammatory response in mice. These results suggest that poor innate recognition of F. tularensis allows the bacterium to evade early recognition by the host innate immune system to promote its pathogenesis for mammals.

LPS remodeling is an evolved survival strategy for bacteria

“…The importance of the innate immune response in control of Francisella infection has been previously demonstrated (14). Lipid A modification enzymes, such as LpxF (30) and FlmK (15) have been demonstrated to play important roles in virulence but not in recognition by components of the host innate immune system, similar to WT Fn LPS (15).The lpxF and flmK deletion mutants displayed increased sensitivity to components of the host innate immune system and attenuation in murine infection models.…”

“…Finally, LPS isolated from the ΔlpxD1 and ΔlpxD2 mutants was not recognized by components of the host innate immune system, similar to WT Fn LPS (14), suggesting that lipid A molecules with long chain acyl chains have altered binding properties to the LPS receptor, TLR4 complex (Fig S6 A and B). Therefore, even small changes to the outer membrane composition, such as shortening/ lengthening specific acyl chain components, play an important role in modulating antibiotic susceptibility, membrane remodeling, and interactions with the host innate immune system.…”

“…Induction of the innate immune system by modifying the tetraacylated lipid A structure by overexpression of the late acyltransferase, LpxL in Y. pestis results in a complete loss of virulence (13). In contrast, it has been demonstrated that Francisella expresses a unique tetraacylated LPS (14,15) that fails to activate Toll-like receptor 4 (TLR4) at all growth temperatures due to the lack of the 1-position phosphate moiety and the length and position of the acyl groups attached to the diglucosamine backbone, enabling this Gram-negative organism to evade host detection (16).…”

Subversion of Host Recognition and Defense Systems by Francisella spp

“…Most reports support this conclusion, although some disagree as to the magnitude of TLR4 signaling that Francisella LPS can elicit as well as whether TLR4 plays a minor role or no role in host defense. For example, studies have reported that, in vitro, LPS from F. novicida triggers a low level inflammatory response (96,116) while LVS LPS does not, even at high concentrations (5 g/ml) (6,116,219). However, other studies reported that LVS LPS can induce TLR4 signaling in both transiently transfected HEK293 cells and human monocytes, but only when added at high doses that are likely not biologically relevant (Ն2.5 g/ml) (72).…”

Attenuation and protective efficacy of an O-antigen-deficient mutant of Francisella tularensis LVS

“…The LPS of F. tularensis types A and B is unusual in that the O antigen consists entirely of dideoxyglycoses, the core oligosaccharide contains mannose in place of heptose (Vinogradov et al, 2002(Vinogradov et al, , 1991, and lipid A of the live vaccine strain (LVS) is tetraacylated and lacks phosphate (Vinogradov et al, 2002), while lipid A from a virulent type B isolate also contains a phosphatelinked galactosamine (Phillips et al, 2004). Furthermore, the LPS does not signal through TLR4, is not an agonist for TLR4, and does not induce an inflammatory response Cole et al, 2006;Hajjar et al, 2006), which is probably due to the atypical structure of lipid A. However, apart from failing to incite an inflammatory response by the host, the role of the LPS in virulence and immunoprotection is unclear.…”