Increased susceptibility of IgA-deficient mice to pulmonary Francisella tularensis live vaccine strain infection

Protective potential of outer membrane vesicles derived from a virulent strain of Francisella tularensis

Francisella tularensis secretes tubular outer membrane vesicles (OMVs) that contain a number of immunoreactive proteins as well as virulence factors. We have reported previously that isolated Francisella OMVs enter macrophages, cumulate inside, and induce a strong pro-inflammatory response. In the current article, we present that OMVs treatment of macrophages also enhances phagocytosis of the bacteria and suppresses their intracellular replication. On the other hand, the subsequent infection with Francisella is able to revert to some extent the strong pro-inflammatory effect induced by OMVs in macrophages. Being derived from the bacterial surface, isolated OMVs may be considered a "non-viable mixture of Francisella antigens" and as such, they present a promising protective material. Immunization of mice with OMVs isolated from a virulent F. tularensis subsp. holarctica strain FSC200 prolonged the survival time but did not fully protect against the infection with a lethal dose of the parent strain. However, the sera of the immunized animals revealed unambiguous cytokine and antibody responses and proved to recognize a set of well-known Francisella immunoreactive proteins. For these reasons, Francisella OMVs present an interesting material for future protective studies.

Francisella and Antibodies

Immune responses to intracellular pathogens depend largely upon the activation of T helper type 1-dependent mechanisms. The contribution of B cells to establishing protective immunity has long been underestimated. Francisella tularensis, including a number of subspecies, provides a suitable model for the study of immune responses against intracellular bacterial pathogens. We previously demonstrated that Francisella infects B cells and activates B-cell subtypes to produce a number of cytokines and express the activation markers. Recently, we documented the early production of natural antibodies as a consequence of Francisella infection in mice. Here, we summarize current knowledge on the innate and acquired humoral immune responses initiated by Francisella infection and their relationships with the immune defense systems.

Immunoglobulin A deficiency in children, an undervalued clinical issue

Immunoglobulin A (IgA) is the principal antibody in secretions that bathe the gastrointestinal and respiratory mucosal surfaces and acts as an important first line of defense against invasion of pathogenic micro-organisms. The reported prevalence rate of complete IgA deficiency in healthy children ranges from 1:170 to 1:400, and as a solitary condition, it is often considered of limited clinical importance. However, patients with IgA deficiency can develop recurrent respiratory and gastrointestinal infections, as well as allergic and autoimmune diseases. In children referred for recurrent respiratory tract infections, the observed prevalence rate increases more than tenfold. This review discusses several aspects of IgA deficiency in children, including immunologic and microbiome changes in early childhood and the potential consequences of this condition in later life. It illustrates the importance of early identification of children with impaired IgA production who deserve appropriate clinical care and follow-up.

Development of experimental pneumococcal vaccine for mucosal immunization

Streptococcus pneumonia is an important human pathogen that causes various severe diseases such as pneumonia, otitis and meningitis. Vaccination against S. pneumoniae is implemented in many developed countries. The presently used vaccines are safe, well tolerated but relatively expensive and require modification due to the immunological changes of the epidemic strains. This paper describes the development of a new pneumococcal vaccine candidate for immunization on mucosal surfaces. For this purpose the antigens of chimeric protein PSPF, previously suggested for an injectable S. pneumoniae vaccine, were expressed on the surface of the live probiotic strain Enterococcus faecium L3. Experiments on laboratory mice vaccinated with live bacteria demonstrated the appearance of the specific IgA and IgG which provide protection against the lethal S. pneumoniae infection.

Maternal Immunization Confers Protection to the Offspring against an Attaching and Effacing Pathogen through Delivery of IgG in Breast Milk

Owing to immature immune systems and impaired colonization resistance mediated by the microbiota, infants are more susceptible to enteric infections. Maternal antibodies can provide immunity, with maternal vaccination offering a protective strategy. We find that oral infection of adult females with the enteric pathogen Citrobacter rodentium protects dams and offspring against oral challenge. Parenteral immunization of dams with heat-inactivated C. rodentium reduces pathogen loads and mortality in offspring but not mothers. IgG, but not IgA or IgM, transferred through breast milk to the intestinal lumen of suckling offspring, coats the pathogen and reduces intestinal colonization. Protective IgG largely recognizes virulence factors encoded within the locus of enterocyte effacement (LEE) pathogenicity island, including the adhesin Intimin and T3SS filament EspA, which are major antigens conferring protection. Thus, pathogen-specific IgG in breast milk induced during maternal infection or immunization protects neonates against infection with an attaching and effacing pathogen.

Intranasal administration of a two-dose adjuvanted multi-antigen TMV-subunit conjugate vaccine fully protects mice against Francisella tularensis LVS challenge

Tularemia is a fatal human disease caused by Francisella tularensis, a Gram-negative encapsulated coccobacillus bacterium. Due to its low infectious dose, ease of aerosolized transmission, and lethal effects, the CDC lists F. tularensis as a Category A pathogen, the highest level for a potential biothreat agent. Previous vaccine studies have been conducted with live attenuated, inactivated, and subunit vaccines, which have achieved partial or full protection from F. tularensis live vaccine strain (LVS) challenge, but no vaccine has been approved for human use. We demonstrate the improved efficacy of a multi-antigen subunit vaccine by using Tobacco Mosaic virus (TMV) as an antigen carrier for the F. tularensis SchuS4 proteins DnaK, OmpA, SucB and Tul4 (DOST). The magnitude and quality of immune responses were compared after mice were immunized by subcutaneous or intranasal routes of administration with a TMV-DOST mixture, with or without four different adjuvants. Immune responses varied in magnitude and isotype profile, by antigen, by route of administration, and by protection in an F. tularensis LVS challenge model of disease. Interestingly, our analysis demonstrates an overwhelming IgG2 response to SucB after intranasal dosing, as well as a robust cellular response, which may account for the improved two-dose survival imparted by the tetravalent vaccine, compared to a previous study that tested efficacy of TMV-DOT. Our study provides evidence that potent humoral, cellular and mucosal immunity can be achieved by optimal antigen combination, delivery, adjuvant and appropriate route of administration, to improve vaccine potency and provide protection from pathogen challenge.

Defective anti-polysaccharide IgG vaccine responses in IgA deficient mice

We report that IgA^-/- mice exhibit specific defects in IgG antibody responses to various polysaccharide vaccines (Francisella tularensis LPS and Pneumovax), but not protein vaccines such as Fluzone. This defect further included responses to polysaccharide-protein conjugate vaccines (Prevnar and Haemophilus influenzae type b-tetanus toxoid vaccine). In agreement with these findings, IgA^-/- mice were protected from pathogen challenge with protein- but not polysaccharide-based vaccines. Interestingly, after immunization with live bacteria, IgA^+/+ and IgA^-/- mice were both resistant to lethal challenge and their IgG anti-polysaccharide antibody responses were comparable. Immunization with live bacteria, but not purified polysaccharide, induced production of serum B cell-activating factor (BAFF), a cytokine important for IgG class switching; supplementing IgA^-/- cell cultures with BAFF enhanced in vitro polyclonal IgG production. Taken together, these findings show that IgA deficiency impairs IgG class switching following vaccination with polysaccharide antigens and that live bacterial immunization can overcome this defect. Since IgA deficient patients also often show defects in antibody responses following immunization with polysaccharide vaccines, our findings could have relevance to the clinical management of this population.

Supplemental feeding of a gut microbial metabolite of linoleic acid, 10-hydroxy-cis-12-octadecenoic acid, alleviates spontaneous atopic dermatitis and modulates intestinal microbiota in NC/nga mice

The present study investigated the antiallergic and anti-inflammatory effects of 10-hydroxy-cis-12-octadecenoic acid (HYA), a novel gut microbial metabolite of linoleic acid, in NC/Nga mice, a model of atopic dermatitis (AD). Feeding HYA decreased the plasma immunoglobulin E level and skin infiltration of mast cells with a concomitant decrease in dermatitis score. HYA feeding decreased TNF-α and increased claudin-1, a tight junction protein, levels in the mouse skin. Cytokine expression levels in the skin and intestinal Peyer's patches cells suggested that HYA improved the Th1/Th2 balance in mice. Immunoglobulin A concentration in the feces of the HYA-fed mice was approximately four times higher than that in the control mice. Finally, denaturing gradient gel electrophoresis of the PCR-amplified 16 S rRNA gene of fecal microbes indicated the modification of microbiota by HYA. Taken together, the alterations in the intestinal microbiota might be, at least in part, associated with the antiallergic effect of HYA.

Antibodies and tuberculosis

Tuberculosis (TB) remains a major public health problem internationally, causing 9.6 million new cases and 1.5 million deaths worldwide in 2014. The Bacillus Calmette-Guérin vaccine is the only licensed vaccine against TB, but its protective effect does not extend to controlling the development of infectious pulmonary disease in adults. The development of a more effective vaccine against TB is therefore a pressing need for global health. Although it is established that cell-mediated immunity is necessary for the control of latent infection, the presupposition that such immunity is sufficient for vaccine-induced protection has recently been challenged. A greater understanding of protective immunity against TB is required to guide future vaccine strategies against TB.

In contrast to cell-mediated immunity, the human antibody response against M.tb is conventionally thought to exert little immune control over the course of infection. Humoral responses are prominent during active TB disease, and have even been postulated to contribute to immunopathology. However, there is evidence to suggest that specific antibodies may limit the dissemination of M.tb, and potentially also play a role in prevention of infection via mucosal immunity. Further, antibodies are now understood to confer protection against a range of intracellular pathogens by modulating immunity via Fc-receptor mediated phagocytosis. In this review, we will explore the evidence that antibody-mediated immunity could be reconsidered in the search for new vaccine strategies against TB.

IgA modulates respiratory dysfunction as a sequela to pulmonary chlamydial infection as neonates

Neonatal Chlamydia lung infections are associated with serious sequelae such as asthma and airway hyper-reactivity in children and adults. Our previous studies demonstrated the importance of Th-1 type cytokines, IL-12 and IFN-γ in protection against neonatal pulmonary chlamydial challenge; however, the role of the humoral arm of defense has not been elucidated. We hypothesized that B-cells and IgA, the major mucosal antibody, play a protective role in newborns against development of later life respiratory sequelae to Chlamydia infection. Our studies using neonatal mice revealed that all WT and IgA-deficient (IgA(-/-)) animals survived a sublethal pulmonary Chlamydia muridarum challenge at one day after birth with similar reduction in bacterial burdens over time. In contrast, all B-cell-deficient (μMT) mice succumbed to infection at the same challenge dose correlating to failure to control bacterial burdens in the lungs. Although IgA may not be important for bacterial clearance, we observed IgA(-/-) mice displayed greater respiratory dysfunction 5 weeks post challenge. Specifically, comparative respiratory functional analyses revealed a significant shift upward in P-V loops, and higher dynamic resistance in IgA(-/-) animals. This study provides insight(s) into the protective role of IgA in neonates against pulmonary chlamydial infection induced respiratory pathological sequelae observed later in life.

Francisella tularensis type B ΔdsbA mutant protects against type A strain and induces strong inflammatory cytokine and Th1-like antibody response in vivo

Francisella tularensis subspecies tularensis is a highly virulent intracellular bacterial pathogen, causing the disease tularemia. However, a safe and effective vaccine for routine application against F. tularensis has not yet been developed. We have recently constructed the deletion mutants for the DsbA homolog protein (ΔdsbA/FSC200) and a hypothetical protein IglH (ΔiglH/FSC200) in the type B F. tularensis subsp. holarctica FSC200 strain, which exerted different protection capacity against parental virulent strain. In this study, we further investigated the immunological correlates for these different levels of protection provided by ΔdsbA/FSC200 and ΔiglH/FSC200 mutants. Our results show that ΔdsbA/FSC200 mutant, but not ΔiglH/FSC200 mutant, induces an early innate inflammatory response leading to strong Th1-like antibody response. Furthermore, vaccination with ΔdsbA/FSC200 mutant, but not with ΔiglH/FSC200, elicited protection against the subsequent challenge with type A SCHU S4 strain in mice. An immunoproteomic approach was used to map a spectrum of antigens targeted by Th1-like specific antibodies, and more than 80 bacterial antigens, including novel ones, were identified. Comparison of tularemic antigens recognized by the ΔdsbA/FSC200 post-vaccination and the SCHU S4 post-challenge sera then revealed the existence of 22 novel SCHU S4 specific antibody clones.

Role of Interleukin-12 in Protection against Pulmonary Infection with Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a common pathogen associated with nosocomial pneumonia and is an increasing threat for severe community-acquired pneumonia. We have now investigated the role of interleukin-12 (IL-12) in protective immunity against lung infection with MRSA. The importance of IL-12 in protection from pulmonary MRSA infection was demonstrated by the finding that IL-12p35-deficient mice had a lower survival rate, higher bacterial burdens in lung and spleen, and decreased expression of interferon gamma (IFN-γ) in the lung compared to wild-type mice. These effects were completely reversed by replacement intranasal therapy with recombinant IL-12. Furthermore, exogenous IL-12 treatment of wild-type mice 24 h before pulmonary challenge with a lethal dose of MRSA significantly improved bacterial clearance and resulted in protection from death. The IL-12-treated mice had increased numbers of lung natural killer (NK) cells and neutrophils and higher levels of IFN-γ in the lung and serum compared to untreated mice. The major source of IL-12-driven IFN-γ expression in the lung was the NK cell, and the direct target of pulmonary IFN-γ was the lung macrophage, as shown using mice with a macrophage-specific defect in interferon gamma (IFN-γ) signaling (MIIG mice). Importantly, combination therapy with linezolid and IL-12 following intranasal MRSA infection significantly increased survival compared to that of mice receiving linezolid or IL-12 alone. These results indicate that IL-12-based immunotherapy may hold promise for treatment of MRSA pneumonia.

Enhanced differentiation of intraepithelial lymphocytes in the intestine of polymeric immunoglobulin receptor-deficient mice

To clarify the effect of secretory IgA (sIgA) deficiency on gut homeostasis, we examined intraepithelial lymphocytes (IELs) in the small intestine (SI) of polymeric immunoglobulin receptor-deficient (pIgR(-/-) ) mice. The pIgR(-/-) mice exhibited the accumulation of CD8αβ(+) T-cell receptor (TCR)-αβ(+) IELs (CD8αβ(+) αβ-IELs) after weaning, but no increase of CD8αβ(+) γδ-IELs was detected in pIgR(-/-) TCR-β(-/-) mice compared with pIgR(+/+) TCR-β(-/-) mice. When 5-bromo-2'-deoxyuridine (BrdU) was given for 14 days, the proportion of BrdU-labelled cells in SI-IELs was not different between pIgR(+/+) mice and pIgR(-/-) mice. However, the proportion of BrdU-labelled CD8αβ(+) -IELs became higher in pIgR(-/-) mice than pIgR(+/+) mice 10 days after discontinuing BrdU-labelling. Intravenously transferred splenic T cells migrated into the intraepithelial compartments of pIgR(+/+) TCR-β(-/-) mice and pIgR(-/-) TCR-β(-/-) mice to a similar extent. In contrast, in the case of injection of immature bone marrow cells, CD8αβ(+) αβ-IELs increased much more in the SI of pIgR(-/-) TCR-β(-/-) mice than pIgR(+/+) TCR-β(-/-) mice 8 weeks after the transfer. αβ-IELs from pIgR(-/-) mice could produce more interferon-γ and interleukin-17 than those of pIgR(+/+) mice, and intestinal permeability tended to increase in the SI of pIgR(-/-) mice with aging. Taken together, these results indicate that activated CD8αβ(+) αβ-IELs preferentially accumulate in pIgR(-/-) mice through the enhanced differentiation of immature haematopoietic precursor cells, which may subsequently result in the disruption of epithelial integrity.

Border Patrol Gone Awry: Lung NKT Cell Activation by Francisella tularensis Exacerbates Tularemia-Like Disease

The respiratory mucosa is a major site for pathogen invasion and, hence, a site requiring constant immune surveillance. The type I, semi-invariant natural killer T (NKT) cells are enriched within the lung vasculature. Despite optimal positioning, the role of NKT cells in respiratory infectious diseases remains poorly understood. Hence, we assessed their function in a murine model of pulmonary tularemia—because tularemia is a sepsis-like proinflammatory disease and NKT cells are known to control the cellular and humoral responses underlying sepsis. Here we show for the first time that respiratory infection with Francisella tularensis live vaccine strain resulted in rapid accumulation of NKT cells within the lung interstitium. Activated NKT cells produced interferon-γ and promoted both local and systemic proinflammatory responses. Consistent with these results, NKT cell-deficient mice showed reduced inflammatory cytokine and chemokine response yet they survived the infection better than their wild type counterparts. Strikingly, NKT cell-deficient mice had increased lymphocytic infiltration in the lungs that organized into tertiary lymphoid structures resembling induced bronchus-associated lymphoid tissue (iBALT) at the peak of infection. Thus, NKT cell activation by F. tularensis infection hampers iBALT formation and promotes a systemic proinflammatory response, which exacerbates severe pulmonary tularemia-like disease in mice.

NKT cells are innate-like lymphocytes with a demonstrated role in a wide range of diseases. Often cited for their ability to rapidly produce a variety of cytokines upon activation, they have long been appreciated for their ability to “jump-start” the immune system and to shape the quality of both the innate and adaptive response. This understanding of their function has been deduced from in vitro experiments or through the in vivo administration of highly potent, chemically synthesized lipid ligands, which may not necessarily reflect a physiologically relevant response as observed in a natural infection. Using a mouse model of pulmonary tularemia, we report that intranasal infection with the live vaccine strain of F. tularensis rapidly activates NKT cells and promotes systemic inflammation, increased tissue damage, and a dysregulated immune response resulting in increased morbidity and mortality in infected mice. Our data highlight the detrimental effects of NKT cell activation and identify a potential new target for therapies against pulmonary tularemia.

Conserved surface accessible nucleoside ABC transporter component SP0845 is essential for pneumococcal virulence and confers protection in vivo

Streptococcus pneumoniae is a leading cause of bacterial pneumonia, sepsis and meningitis. Surface accessible proteins of S. pneumoniae are being explored for the development of a protein-based vaccine in order to overcome the limitations of existing polysaccharide-based pneumococcal vaccines. To identify a potential vaccine candidate, we resolved surface-associated proteins of S. pneumoniae TIGR4 strain using two-dimensional gel electrophoresis followed by immunoblotting with antisera generated against whole heat-killed TIGR4. Ten immunoreactive spots were identified by mass spectrometric analysis that included a putative lipoprotein SP0845. Analysis of the inferred amino acid sequence of sp0845 homologues from 36 pneumococcal strains indicated that SP0845 was highly conserved (>98% identity) and showed less than 11% identity with any human protein. Our bioinformatic and functional analyses demonstrated that SP0845 is the substrate-binding protein of an ATP-binding cassette (ABC) transporter that is involved in nucleoside uptake with cytidine, uridine, guanosine and inosine as the preferred substrates. Deletion of the gene encoding SP0845 renders pneumococci avirulent suggesting that it is essential for virulence. Immunoblot analysis suggested that SP0845 is expressed in in vitro grown pneumococci and during mice infection. Immunofluorescence microscopy and flow cytometry data indicated that SP0845 is surface exposed in encapsulated strains and accessible to antibodies. Subcutaneous immunization with recombinant SP0845 induced high titer antibodies in mice. Hyperimmune sera raised against SP0845 promoted killing of encapsulated pneumococcal strains in a blood bactericidal assay. Immunization with SP0845 protected mice from intraperitoneal challenge with heterologous pneumococcal serotypes. Based on its surface accessibility, role in virulence and ability to elicit protective immunity, we propose that SP0845 may be a potential candidate for a protein-based pneumococcal vaccine.

Host-pathogen interactions and immune evasion strategies in Francisella tularensis pathogenicity

Francisella tularensis is an intracellular Gram-negative bacterium that causes life-threatening tularemia. Although the prevalence of natural infection is low, F. tularensis remains a tier I priority pathogen due to its extreme virulence and ease of aerosol dissemination. F. tularensis can infect a host through multiple routes, including the intradermal and respiratory routes. Respiratory infection can result from a very small inoculum (ten organisms or fewer) and is the most lethal form of infection. Following infection, F. tularensis employs strategies for immune evasion that delay the immune response, permitting systemic distribution and induction of sepsis. In this review we summarize the current knowledge of F. tularensis in an immunological context, with emphasis on the host response and bacterial evasion of that response.

B cell subsets are activated and produce cytokines during early phases of Francisella tularensis LVS infection

Francisella tularensis, a facultative intracellular Gram-negative bacterium, causes the illness tularemia. The infection of mice with live vaccine strain is considered to be a model of human tularemia. F. tularensis infects predominantly such phagocytic cells as macrophages or neutrophils, but it also infects non-phagocytic hepatocytes, epithelial cells, and murine and human B cell lines. Based on work with the murine tularemia model, we report here that F. tularensis LVS infects peritoneal CD19(+) cells - exclusively B-1a cells - early after intraperitoneal infection in vivo. The peritoneal and consequently spleen CD19(+) cells are activated by the F. tularensis LVS infection to express the activation markers from MHC class II, CD25, CD54, CD69, and the co-stimulatory molecules CD80 and CD86. As early as 12 h post-infection, the peritoneal CD19(+) cells produce IFN-γ, IL-1β, IL-4, IL-6, IL-12, IL-17, IL-23, and TNF-α. The spleen CD19(+) cells respond to infection with some delay. Moreover, the F. tularensis infected A20 B cell line activates CD3(+) spleen cells isolated from naïve mice. Thus, the data presented here suggest that B cells have all the attributes to actively participate in the induction and regulation of the adaptive immune response during early stages of F. tularensis infection.

LcrV delivered via type III secretion system of live attenuated Yersinia pseudotuberculosis enhances immunogenicity against pneumonic plague

Here, we constructed a Yersinia pseudotuberculosis mutant strain with arabinose-dependent regulated and delayed shutoff of crp expression (araC P(BAD) crp) and replacement of the msbB gene with the Escherichia coli msbB gene to attenuate it. Then, we inserted the asd mutation into this construction to form χ10057 [Δasd-206 ΔmsbB868::P(msbB) msbB(EC) ΔP(crp21)::TT araC P(BAD) crp] for use with a balanced-lethal Asd-positive (Asd(+)) plasmid to facilitate antigen synthesis. A hybrid protein composed of YopE (amino acids [aa]1 to 138) fused with full-length LcrV (YopE(Nt138)-LcrV) was synthesized in χ10057 harboring an Asd(+) plasmid (pYA5199, yopE(Nt138)-lcrV) and could be secreted through a type III secretion system (T3SS) in vitro and in vivo. Animal studies indicated that mice orally immunized with χ10057(pYA5199) developed titers of IgG response to whole-cell lysates of Y. pestis (YpL) and subunit LcrV similar to those seen with χ10057(pYA3332) (χ10057 plus an empty plasmid). However, only immunization of mice with χ10057(pYA5199) resulted in a significant secretory IgA response to LcrV. χ10057(pYA5199) induced a higher level of protection (80% survival) against intranasal (i.n.) challenge with ~240 median lethal doses (LD50) (2.4 × 10(4) CFU) of Y. pestis KIM6+(pCD1Ap) than χ10057(pYA3332) (40% survival). Splenocytes from mice vaccinated with χ10057(pYA5199) produced significant levels of gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), and interleukin-17 (IL-17) after restimulation with LcrV and YpL antigens. Our results suggest that it is possible to use an attenuated Y. pseudotuberculosis strain delivering the LcrV antigen via the T3SS as a potential vaccine candidate against pneumonic plague.

IFN-γ, but not IL-17A, is required for survival during secondary pulmonary Francisella tularensis Live Vaccine Stain infection

IL-17 and IFN-γ production by Th17 and Th1 cells, respectively, is critical for survival during primary respiratory infection with the pathogenic bacterium, Francisella tularensis Live Vaccine Strain (LVS). The importance, however, of these T cell subsets and their soluble mediators is not well understood during a secondary or memory response. We measured the number of CD4(+) T cells producing IFN-γ or IL-17 in the spleen and lungs of vaccinated mice on day four of the secondary response using intracellular cytokine staining in order to identify protective T cell subsets participating in the memory response. Few bacteria were present in spleens of vaccinated mice on day four and a T cell response was not observed. In the lung, where more bacteria were present, there was a robust Th1 response in vaccinated mice but Th17 cells were not present at higher numbers in vaccinated mice compared to unvaccinated mice. These data show that the lung is the dominant site of the secondary immune response and suggest that Th17 cells are not required for survival during secondary challenge. To further investigate the importance of IFN-γ and IL-17 during the secondary response to F. tularensis, we neutralized either IFN-γ or IL-17 in vivo using monoclonal antibody treatment. Vaccinated mice treated with anti-IFN-γ lost more weight and had higher bacterial burdens compared to vaccinated mice treated with isotype control antibody. In contrast, treatment with anti-IL-17A antibody did not alter weight loss profiles or bacterial burdens compared to mice treated with isotype control antibody. Together, these results suggested that IFN-γ is required during both primary and secondary respiratory F. tularensis infection. IL-17, on the other hand, is only critical during the primary response to respiratory F. tularensis but dispensable during the secondary response.