Immunologic Consequences ofFrancisella tularensisLive Vaccine Strain Infection: Role of the Innate Immune Response in Infection and Immunity

Abstract

Francisella tularensis (Ft), a Gram-negative intracellular bacterium, is the etiologic agent of tularemia. Although attenuated for humans, i.p. infection of mice with <10 Ft live vaccine strain (LVS) organisms causes lethal infection that resembles human tularemia, whereas the LD50 for an intradermal infection is >106 organisms. To examine the immunological consequences of Ft LVS infection on the innate immune response, the inflammatory responses of mice infected i.p. or intradermally were compared. Mice infected i.p. displayed greater bacterial burden and increased expression of proinflammatory genes, particularly in the liver. In contrast to most LPS, highly purified Ft LVS LPS (10 μg/ml) was found to be only minimally stimulatory in primary murine macrophages and in HEK293T cells transiently transfected with TLR4/MD-2/CD14, whereas live Ft LVS bacteria were highly stimulatory for macrophages and TLR2-expressing HEK293T cells. Despite the poor stimulatory activity of Ft LVS LPS in vitro, administration of 100 ng of Ft LVS LPS 2 days before Ft LVS challenge severely limited both bacterial burden and cytokine mRNA and protein expression in the absence of detectable Ab at the time of bacterial challenge, yet these mice developed a robust IgM Ab response within 2 days of infection and survived. These data suggest that prior administration of Ft LVS LPS protects the host by diminishing bacterial burden and blunting an otherwise overwhelming inflammatory response, while priming the adaptive immune response for development of a strong Ab response.

Antigen-specific B-1a antibodies induced byFrancisella tularensisLPS provide long-term protection againstF. tularensisLVS challenge

“…with Ft-LPS protects against lethal i.p. challenge with Ft LVS (7,8). Here, we show that antigen-specific B-1a antibody responses are required for this protection and that protection is induced by Ft-LPS, but not Ft-lipid A.…”

“…We have shown previously that pretreatment of mice with Ft-LPS protects against an otherwise lethal challenge with Ft LVS (7,8). Studies presented here show that this pretreatment induces a dramatic B-1a immune response in which (i) B-1a with BCR that bind Ft-LPS proliferate in spleen and PerC; (ii) most of the Ft-LPS ϩ B-1a cells in spleen differentiate rapidly to plasma cells producing antibodies to Ft-LPS; (iii) serum levels of these antibodies rise shortly after immunization, peak at day 4-5, and then fall slowly to a lower level that is maintained for months; (iv) neither T cells nor TLR4 responsiveness is required for this response; (v) the response is specific for the O-antigen expressed by intact Ft-LPS; and (vi) immunization with Ft-LPS is required for protection against lethal challenge with Ft LVS.…”

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.…”

A conserved and immunodominant lipoprotein of Francisella tularensis is proinflammatory but not essential for virulence

“…The purified LPS of F. tularensis LVS does not stimulate production of tumor necrosis factor-α, interferon-γ, interleukin (IL)-12, IL-10, or nitric oxide by murine macrophages [17,18]. Nonetheless, F. tularensis LVS induces inflammation in mice in vivo [19,20] and can elicit the production of proinflammatory mediators by cultured murine macrophages and dendritic cells [21,22]. Further, human cells of innate immunity respond to culture with the LVS by producing an array of chemokines, cytokines, and adhesion molecules [23,24].…”