Differential effects of Francisella tularensis lipopolysaccharide on B lymphocytes

The O-Ag Antibody Response to Francisella Is Distinct in Rodents and Higher Animals and Can Serve as a Correlate of Protection

Identifying correlates of protection (COPs) for vaccines against lethal human (Hu) pathogens, such as Francisella tularensis (Ft), is problematic, as clinical trials are currently untenable and the relevance of various animal models can be controversial. Previously, Hu trials with the live vaccine strain (LVS) demonstrated ~80% vaccine efficacy against low dose (~50 CFU) challenge; however, protection deteriorated with higher challenge doses (~2000 CFU of SchuS4) and no COPs were established. Here, we describe our efforts to develop clinically relevant, humoral COPs applicable to high-dose, aerosol challenge with S4. First, our serosurvey of LVS-vaccinated Hu and animals revealed that rabbits (Rbs), but not rodents, recapitulate the Hu O-Ag dependent Ab response to Ft. Next, we assayed Rbs immunized with distinct S4-based vaccine candidates (S4ΔclpB, S4ΔguaBA, and S4ΔaroD) and found that, across multiple vaccines, the %O-Ag dep Ab trended with vaccine efficacy. Among S4ΔguaBA-vaccinated Rbs, the %O-Ag dep Ab in pre-challenge plasma was significantly higher in survivors than in non-survivors; a cut-off of >70% O-Ag dep Ab predicted survival with high sensitivity and specificity. Finally, we found this COP in 80% of LVS-vaccinated Hu plasma samples as expected for a vaccine with 80% Hu efficacy. Collectively, the %O-Ag dep Ab response is a bona fide COP for S4ΔguaBA-vaccinated Rb and holds significant promise for guiding vaccine trials with higher animals.

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.

Interaction between Lipopolysaccharide and Gut Microbiota in Inflammatory Bowel Diseases

Lipopolysaccharides (LPSs) are bacterial surface glycolipids, produced by Gram-negative bacteria. LPS is known to determine acute inflammatory reactions, particularly in the context of sepsis. However, LPS can also trigger chronic inflammation. In this case, the source of LPS is not an external infection, but rather an increase in endogenous production, which is usually sustained by gut microbiota (GM), and LPS contained in food. The first site in which LPS can exert its inflammatory action is the gut: both GM and gut-associated lymphoid tissue (GALT) are influenced by LPS and shift towards an inflammatory pattern. The changes in GM and GALT induced by LPS are quite similar to the ones seen in IBD: GM loses diversity, while GALT T regulatory (Tregs) lymphocytes are reduced in number, with an increase in Th17 and Th1 lymphocytes. Additionally, the innate immune system is triggered, through the activation of toll-like receptor (TLR)-4, while the epithelium is directly damaged, further triggering inflammation. In this review, we will discuss the importance of the crosstalk between LPS, GM, and GALT, and discuss the possible implications.

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

In the companion article by Yang and colleagues [Yang Y, et al. (2012) Proc Natl Acad Sci USA, 109, 10.1073/pnas.1121631109], we have shown that priming with glycolipid (FtL) from Francisella tularensis live-vaccine strain (i) induces FtL-specific B-1a to produce robust primary responses (IgM >>IgG); (ii) establishes persistent long-term production of serum IgM and IgG anti-FtL at natural antibody levels; and (iii) elicits FtL-specific B-1a memory cells that arise in spleen but rapidly migrate to the peritoneal cavity, where they persist indefinitely but divide only rarely. Here, we show that FtL rechallenge alone induces these PerC B-1a memory cells to divide extensively and to express a unique activation signature. However, FtL rechallenge in the context of a Toll-like receptor 4 agonist-stimulated inflammatory response readily induces these memory cells to migrate to spleen and initiate production of dominant IgM anti-FtL secondary responses. Thus, studies here reveal unique mechanisms that govern B-1a memory development and expression, and introduce B-1a memory as an active participant in immune defenses. In addition, at a practical level, these studies suggest previously unexplored vaccination strategies for pathogen-associated antigens that target the B-1a repertoire.

A molecular mechanism for TNF-α-mediated downregulation of B cell responses

B cell function with age is decreased in class switch recombination (CSR), activation-induced cytidine deaminase (AID), and stability of E47 mRNA. The latter is regulated, at least in part, by tristetraprolin (TTP), which is increased in aged B cells and also negatively regulates TNF-α. In this study, we investigated whether B cells produce TNF-α, whether this changes with age, and how this affects their function upon stimulation. Our hypothesis is that in aging there is a feedback mechanism of autocrine inflammatory cytokines (TNF-α) that lowers the expression of AID and CSR. Our results showed that unstimulated B cells from old BALB/c mice make significantly more TNF-α mRNA and protein than do B cells from young mice, but after stimulation the old make less than the young; thus, they are refractory to stimulation. The increase in TNF-α made by old B cells is primarily due to follicular, but not minor, subsets of B cells. Incubation of B cells with TNF-α before LPS stimulation decreased both young and old B cell responses. Importantly, B cell function was restored by adding anti-TNF-α Ab to cultured B cells. To address a molecular mechanism, we found that incubation of B cells with TNF-α before LPS stimulation induced TTP, a physiological regulator of mRNA stability of the transcription factor E47, which is crucial for CSR. Finally, anti-TNF-α given in vivo increased B cell function in old, but not in young, follicular B cells. These results suggest new molecular mechanisms that contribute to reduced Ab responses in aging.

Novel regulatory functions for Toll-like receptor-activated B cells during intracellular bacterial infection

Infections by intracellular bacterial pathogens remain a major cause of human diseases worldwide. Despite intensive efforts, the development of effective vaccines or immunotherapies against these diseases has largely remained unsuccessful, asking for the exploration of new aspects of the host response to these pathogens. Genetic studies have demonstrated beyond doubt that cell-mediated mechanisms of host defense involving innate immunity and T cells are of crucial importance for the control of these diseases. By contrast, the role of B cells during intracellular bacterial infection has so far received little attention besides their role as antibody-producing cells. However, the general knowledge of B-cell immunology and in particular of their antibody-independent functions has greatly increased during the last years. Recently, it was found in a model of Salmonella typhimurium infection that Toll-like receptor triggering on B cells resulted through interleukin-10 secretion in a marked suppression of innate defense mechanisms ultimately leading to uncontrolled growth of the bacteria and earlier death from the disease during both primary and secondary infections. This article reviews the protective and deleterious roles of B cells during intracellular bacterial infections and discusses how manipulating their antibody-independent functions may be a powerful means to therapeutically improve host resistance against these diseases.

Humoral and cell-mediated immunity to the intracellular pathogen Francisella tularensis

SUMMARY

Francisella tularensis can cause fatal respiratory tularemia in humans and animals and is increasingly being isolated in the United States and several European countries. The correlates of protective immunity against this intracellular bacterium are not known, and currently there are no licensed vaccines available for human use. Cell-mediated immunity has long been believed to be critical for protection, and the importance of humoral immunity is also now recognized. Furthermore, synergy between antibodies, T cell-derived cytokines, and phagocytes appears to be critical to achieve sterilizing immunity against F. tularensis. Thus, novel vaccine approaches should be designed to induce robust antibody and cell-mediated immune responses to this pathogen.

Francisella tularensis infection-derived monoclonal antibodies provide detection, protection, and therapy

Francisella tularensis is the causative agent of tularemia and a potential agent of biowarfare. As an easily transmissible infectious agent, rapid detection and treatment are necessary to provide a positive clinical outcome. As an agent of biowarfare, there is an additional need to prevent infection. We made monoclonal antibodies to the F. tularensis subsp. holarctica live vaccine strain (F. tularensis LVS) by infecting mice with a sublethal dose of bacteria and, following recovery, by boosting the mice with sonicated organisms. The response to the initial and primary infection was restricted to immunoglobulin M antibody directed solely against lipopolysaccharide (LPS). After boosting with sonicated organisms, the specificity repertoire broadened against protein antigens, including DnaK, LpnA, FopA, bacterioferritin, the 50S ribosomal protein L7/L12, and metabolic enzymes. These monoclonal antibodies detect F. tularensis LVS by routine immunoassays, including enzyme-linked immunosorbent assay, Western blot analysis, and immunofluorescence. The ability of the antibodies to protect mice from intradermal infection, both prophylactically and therapeutically, was examined. An antibody to LPS which provides complete protection from infection with F. tularensis LVS and partial protection from infection with F. tularensis subsp. tularensis strain SchuS4 was identified. There was no bacteremia and reduced organ burden within the first 24 h when mice were protected from F. tularensis LVS infection with the anti-LPS antibody. No antibody that provided complete protection when administered therapeutically was identified; however, passive transfer of antibodies against LPS, FopA, and LpnA resulted in 40 to 50% survival of mice infected with F. tularensis LVS.

Francisella tularensis: unravelling the secrets of an intracellular pathogen

Francisella tularensis has been recognized as the causative agent of tularaemia for almost a century. Since its discovery in 1911, it has been shown to infect a wide range of hosts, including humans. As early as the 1920s it was suggested to be an intracellular pathogen, but it has proven to be an enigmatic organism, whose interaction with the host has been difficult to elucidate, and we still have a very limited understanding of the molecular mechanisms of virulence. However, the recent availability of genome sequence data and molecular tools has allowed us to start to understand the molecular basis of F. tularensis pathogenicity, and will facilitate the development of a vaccine to protect against infection.

Native outer membrane proteins protect mice against pulmonary challenge with virulent type A Francisella tularensis

Francisella tularensis is a gram-negative intracellular bacterium and the causative agent of the zoonotic disease tularemia. F. tularensis is a category A select agent and thus a potential agent of bioterrorism. Whereas an F. tularensis live, attenuated vaccine strain (LVS) is the basis of an investigational vaccine, this vaccine is not licensed for human use because of efficacy and safety concerns. In the present study, we immunized mice with isolated native outer membrane proteins (OMPs), ethanol-inactivated LVS (iLVS), or purified LVS lipopolysaccharide (LPS) and assessed the ability of each vaccine preparation to protect mice against pulmonary challenge with the virulent type A F. tularensis strain SchuS4. Antibody isotyping indicated that both Th1 and Th2 antibody responses were generated in mice after immunization with OMPs or iLVS, whereas LPS immunization resulted in only immunoglobulin A production. In survival studies, OMP immunization provided the greatest level of protection (50% survival at 20 days after infection with SchuS4), and there were associated 3-log reductions in the spleen and liver bacterial burdens (compared to nonvaccinated mice). Cytokine quantitation for the sera of SchuS4-challenged mice indicated that OMP and iLVS immunizations induced high levels of tumor necrosis factor alpha and interleukin-2 (IL-2) production, whereas only OMP immunization induced high levels of IL-10 production. By comparison, high levels of proinflammatory cytokines, including RANTES, granulocyte colony-stimulating factor, IL-6, IL-1alpha, IL-12p40, and KC, in nonvaccinated mice indicated that these cytokines may facilitate disease progression. Taken together, the results of this study demonstrate the potential utility of an OMP subunit (acellular) vaccine for protecting mammals against type A F. tularensis.