Administration of a Synthetic TLR4 Agonist Protects Mice from Pneumonic Tularemia

Abstract

Francisella tularensis is a Gram-negative intracellular pathogen that causes the zoonosis tularemia. Because F. tularensis LPS causes weak TLR4 activation, we hypothesized that administration of a synthetic TLR4 agonist, aminoalkyl glucosaminide phosphate (AGP), would boost the innate immune system and compensate for reduced TLR4 stimulation. Intranasal administration of AGPs induced intrapulmonary production of proinflammatory cytokines and chemokines. Mice treated with AGPs before and after inhalation of Francisella novicida exhibited augmented cytokine and inflammatory responses to infection; reduced bacterial replication in lung, liver, and spleen; and increased survival, whereas all PBS-treated control mice died within 4 days of infection, all AGP-treated mice showed prolonged time-to-death, and 30–60% of AGP-treated mice survived. The protective effect of AGP was lost in mice lacking IFN-γ. Long-term survivors developed specific Th1 splenocyte responses and specific Abs dominated by IgG2 isotypes. Survivors were fully protected from rechallenge with aerosolized F. novicida. Thus, preventive administration of AGP successfully modulated innate immune responses to aerosolized F. novicida, leading to protective immunity to pneumonic tularemia. This is the first report of the protective effect of a TLR ligand on resistance to F. novicida-induced pneumonic tularemia.

Toll-Like Receptor 3 Agonist Protection against ExperimentalFrancisella tularensisRespiratory Tract Infection

“…Poly(I:C) afforded protection against a mouse model of genital herpes simplex virus type 2 (17). Other investigators have demonstrated that TLR4 (24) or TLR9 (12, 13) agonist application increased cytokine production and prolonged survival in F. tularensis inhalation (24) or i.p. challenge mouse models (12,13).…”

“…Until such vaccines are available and widely distributed, alternate methods of broad range protection must be investigated. One interesting strategy is to engender innate immune resistance against mucosal pathogens (13,17,24). We have investigated the potential of Toll-like receptor (TLR) agonists recognized by TLRs highly expressed by respiratory epithelial cells.…”

Mucosal Administration of Flagellin Protects Mice from Streptococcus pneumoniae Lung Infection

“…The rationale is to promote innate responses that greatly exceed in magnitude, quality, and dynamics the innate response triggered by the pathogen itself. The effectiveness of TLR agonists for therapeutic treatment of infectious diseases has been demonstrated in several animal models, including models of respiratory infections (6,21,35). TLR5 senses bacterial flagellins, which are the main constituents of flagella.…”

Inhalation of Francisella novicida ΔmglA causes replicative infection that elicits innate and adaptive responses but is not protective against invasive pneumonic tularemia

“…However, in contrast to intranasal infection with an iglC-deficient mutant of F. novicida [8], inhalation of F. novicida ΔmglA does not confer protection against wild type pulmonary infection. We have demonstrated that protection against inhaled U112 infection in this system is feasible by the administration of intranasal TLR4 agonists [31]. We speculate that the lack of protective immunity induced by ΔmglA infection may be explained by the variable magnitude of IFN-γ production by sensitized splenocytes.…”