Role of TLR signaling inFrancisella tularensis-LPS-induced, antibody-mediated protection againstFrancisella tularensischallenge

Abstract

Immunization with Ft-LPS provokes an antigen-specific, B-1a cell-derived antibody response that protects WT mice against an otherwise lethal challenge with Ft LVS. However, this same regimen offers limited protection to TLR2(-/-) mice, despite production of WT levels of anti-Ft-LPS antibodies. As Ft-LPS exhibits no TLR2 agonist activity, and macrophage-induced cytokine production in response to Ft LVS is overwhelmingly TLR2-dependent, we hypothesized that treatment of TLR2(-/-) mice with an alternative, MyD88-dependent TLR agonist would compensate for reduced recognition of Ft LVS in TLR2(-/-) mice and thereby, restore Ft-LPS-mediated protection. Administration of the nontoxic TLR4 agonist, synthetic Escherichia coli MPL, at the time of Ft-LPS immunization or Ft LVS challenge, fully protected TLR2(-/-) mice, whereas treatment of WT or TLR2(-/-) mice with MPL alone conferred partial protection. The TLR5 agonist, flagellin, also synergized with Ft-LPS to protect TLR2(-/-) mice from lethal Ft LVS challenge. In contrast to Ft LVS, Ft-LPS pretreatment failed to protect mice against i.n. challenge with Ft Schu S4, whereas MPL, administered in the absence or presence of Ft-LPS, conferred significant, albeit partial, protection. MPL treatment of macrophages increased the uptake of Ft LVS and decreased intracellular bacterial survival while shifting the macrophage-differentiation phenotype from "alternatively activated" to "classically activated". Collectively, our data suggest that optimal, Ft-LPS-mediated protection against Ft LVS infection requires two discrete events, i.e., production of Ft-LPS-specific antibody, as well as TLR-mediated macrophage activation, to fully control Francisella infection.

Monophosphoryl Lipid A Enhances Efficacy of a Francisella tularensis LVS-Catanionic Nanoparticle Subunit Vaccine against F. tularensis Schu S4 Challenge by Augmenting both Humoral and Cellular Immunity

“…We and others have shown previously that antibodies to this endogenous LPS are produced, making up the majority of the anti-F. tularensis IgM response and a small fraction of the IgG response in LVS-V-immunized mice (25,(41)(42)(43). However, F. tularensis LPS-mediated protection is effective only in a narrow 2-day window prior to challenge and does not yield lasting protection against systemic or aerosol challenge, although partial protection against intradermal challenge and low-virulence strains was observed in previous studies (13,(41)(42)(43)(44)(45). Inclusion of synthetic MPL in the LVS-V vaccine augmented IgG titers significantly (Fig.…”

“…F. tularensis expresses a lipopolysaccharide (LPS) that has an atypical, tetraacylated structure that fails to stimulate Toll-like receptor 4 (TLR4) (6)(7)(8). However, components of live F. tularensis are recognized via TLR2, the absent in melanoma 2 (AIM2) inflammasome, and stimulator of interferon genes (STING) that leads to the induction of type I interferon (9)(10)(11)(12)(13)(14)(15)(16)(17)(18). F. tularensis replicates intracellularly, preferentially in macrophages (19,20).…”

Antigen-specific memory in B-1a and its relationship to natural immunity

“…FtL itself does not supply a substitute inflammatory stimulus because it has minimal TLR2 and TLR4 agonist activity and is shown not to induce inflammation in mice (6)(7)(8). Furthermore, primary anti-FtL responses in TLR4 −/− (2) and TLR2 −/− mice have been shown to be indistinguishable from WT (9).…”

Interleukin-17 Protects against the Francisella tularensis Live Vaccine Strain but Not against a Virulent F. tularensis Type A Strain

“…However, emerging evidence suggests that in vitro and in vivo immune responses differ between type A Francisella and the LVS (7)(8)(9)(10). In addition, immunotherapeutic strategies that confer potent protection against pulmonary LVS infection only confer partial or negligible protection against pulmonary infection with type A Francisella (6,11,12). Due to the differences in induced and protective immune responses between attenuated and virulent F. tularensis subspecies, it is important to study the immune response to virulent strains of F. tularensis that cause disease in humans in order to determine truly protective correlates of immunity with relevance to human disease.…”