Bacillus anthracis Overcomes an Amino Acid Auxotrophy by Cleaving Host Serum Proteins.

UNLABELLED: Bacteria sustain an infection by acquiring nutrients from the host to support replication. The host sequesters these nutrients as a growth-restricting strategy, a concept termed "nutritional immunity." Historically, the study of nutritional immunity has centered on iron uptake because many bacteria target hemoglobin, an abundant circulating protein, as an iron source. Left unresolved are the mechanisms that bacteria use to attain other nutrients from host sources, including amino acids. We employed a novel medium designed to mimic the chemical composition of human serum, and we show here that Bacillus anthracis, the causative agent of anthrax disease, proteolyzes human hemoglobin to liberate essential amino acids which enhance its growth. This property can be traced to the actions of InhA1, a secreted metalloprotease, and extends to at least three other serum proteins, including serum albumin. The results suggest that we must also consider proteolysis of key host proteins to be a way for bacterial pathogens to attain essential nutrients, and we provide an experimental framework to determine the host and bacterial factors involved in this process. IMPORTANCE: The mechanisms by which bacterial pathogens acquire nutrients during infection are poorly understood. Here we used a novel defined medium that approximates the chemical composition of human blood serum, blood serum mimic (BSM), to better model the nutritional environment that pathogens encounter during bacteremia. Removing essential amino acids from BSM revealed that two of the most abundant proteins in blood-hemoglobin and serum albumin-can satiate the amino acid requirement for Bacillus anthracis, the causative agent of anthrax. We further demonstrate that hemoglobin is proteolyzed by the secreted protease InhA1. These studies highlight that common blood proteins can be a nutrient source for bacteria. They also challenge the historical view that hemoglobin is solely an iron source for bacterial pathogens.

Live attenuated Francisella novicida vaccine protects against Francisella tularensis pulmonary challenge in rats and non-human primates.

Francisella tularensis causes the disease tularemia. Human pulmonary exposure to the most virulent form, F. tularensis subsp. tularensis (Ftt), leads to high morbidity and mortality, resulting in this bacterium being classified as a potential biothreat agent. However, a closely-related species, F. novicida, is avirulent in healthy humans. No tularemia vaccine is currently approved for human use. We demonstrate that a single dose vaccine of a live attenuated F. novicida strain (Fn iglD) protects against subsequent pulmonary challenge with Ftt using two different animal models, Fischer 344 rats and cynomolgus macaques (NHP). The Fn iglD vaccine showed protective efficacy in rats, as did a Ftt iglD vaccine, suggesting no disadvantage to utilizing the low human virulent Francisella species to induce protective immunity. Comparison of specific antibody profiles in vaccinated rat and NHP sera by proteome array identified a core set of immunodominant antigens in vaccinated animals. This is the first report of a defined live attenuated vaccine that demonstrates efficacy against pulmonary tularemia in a NHP, and indicates that the low human virulence F. novicida functions as an effective tularemia vaccine platform.

Lethal factor, but not edema factor, is required to cause fatal anthrax in cynomolgus macaques after pulmonary spore challenge.

Inhalational anthrax is caused by inhalation of Bacillus anthracis spores. The ability of B. anthracis to cause anthrax is attributed to the plasmid-encoded A/B-type toxins, edema toxin (edema factor and protective antigen) and lethal toxin (lethal factor and protective antigen), and a poly-d-glutamic acid capsule. To better understand the contribution of these toxins to the disease pathophysiology in vivo, we used B. anthracis Ames strain and isogenic toxin deletion mutants derived from the Ames strain to examine the role of lethal toxin and edema toxin after pulmonary spore challenge of cynomolgus macaques. Lethal toxin, but not edema toxin, was required to induce sustained bacteremia and death after pulmonary challenge with spores delivered via bronchoscopy. After intravenous challenge with bacilli to model the systemic phase of infection, lethal toxin contributed to bacterial proliferation and subsequent host death to a greater extent than edema toxin. Deletion of protective antigen resulted in greater loss of virulence after intravenous challenge with bacilli than deletion of lethal toxin or edema toxin alone. These findings are consistent with the ability of anti-protective antigen antibodies to prevent anthrax and suggest that lethal factor is the dominant toxin that contributes to the escape of significant numbers of bacilli from the thoracic cavity to cause anthrax after inhalation challenge with spores.

IgG subclass and heavy chain domains contribute to binding and protection by mAbs to the poly γ-D-glutamic acid capsular antigen of Bacillus anthracis.

Bacterial capsules are common targets for antibody-mediated immunity. The capsule of Bacillus anthracis is unusual among capsules because it is composed of a polymer of poly-γ-d-glutamic acid (γdPGA). We previously generated murine IgG3 monoclonal antibodies (mAbs) to γdPGA that were protective in a murine model of pulmonary anthrax. IgG3 antibodies are characteristic of the murine response to polysaccharide antigens. The goal of the present study was to produce subclass switch variants of the γdPGA mAbs (IgG3 → IgG1 → IgG2b → IgG2a) and assess the contribution of subclass to antibody affinity and protection. Subclass switch antibodies had identical variable regions but differed in their heavy chains. The results showed that a switch from the protective IgG3 to IgG1, IgG2b or IgG2a was accompanied by i) a loss of protective activity ii) a change in mAb binding to the capsular matrix, and iii) a loss of affinity. These results identify a role for the heavy chain constant region in mAb binding. Hybrid mAbs were constructed in which the CH1, CH2 or CH3 heavy chain constant domains from a non-protective, low binding IgG2b mAb were swapped into the protective IgG3 mAb. The IgG3 mAb that contained the CH1 domain from IgG2b showed no loss of affinity or protection. In contrast, swapping the CH2 or CH3 domains from IgG2b into IgG3 produced a reduction in affinity and a loss of protection. These studies identify a role for the constant region of IgG heavy chains in affinity and protection against an encapsulated bacterial pathogen.

Autophagy protects against active tuberculosis by suppressing bacterial burden and inflammation.

Autophagy is a cell biological pathway affecting immune responses. In vitro, autophagy acts as a cell-autonomous defense against Mycobacterium tuberculosis, but its role in vivo is unknown. Here we show that autophagy plays a dual role against tuberculosis: antibacterial and anti-inflammatory. M. tuberculosis infection of Atg5(fl/fl) LysM-Cre(+) mice relative to autophagy-proficient littermates resulted in increased bacillary burden and excessive pulmonary inflammation characterized by neutrophil infiltration and IL-17 response with increased IL-1α levels. Macrophages from uninfected Atg5(fl/fl) LysM-Cre(+) mice displayed a cell-autonomous IL-1α hypersecretion phenotype, whereas T cells showed propensity toward IL-17 polarization during nonspecific activation or upon restimulation with mycobacterial antigens. Thus, autophagy acts in vivo by suppressing both M. tuberculosis growth and damaging inflammation.

Expression of either lethal toxin or edema toxin by Bacillus anthracis is sufficient for virulence in a rabbit model of inhalational anthrax.

The development of therapeutics against biothreats requires that we understand the pathogenesis of the disease in relevant animal models. The rabbit model of inhalational anthrax is an important tool in the assessment of potential therapeutics against Bacillus anthracis. We investigated the roles of B. anthracis capsule and toxins in the pathogenesis of inhalational anthrax in rabbits by comparing infection with the Ames strain versus isogenic mutants with deletions of the genes for the capsule operon (capBCADE), lethal factor (lef), edema factor (cya), or protective antigen (pagA). The absence of capsule or protective antigen (PA) resulted in complete avirulence, while the presence of either edema toxin or lethal toxin plus capsule resulted in lethality. The absence of toxin did not influence the ability of B. anthracis to traffic to draining lymph nodes, but systemic dissemination required the presence of at least one of the toxins. Histopathology studies demonstrated minimal differences among lethal wild-type and single toxin mutant strains. When rabbits were coinfected with the Ames strain and the PA- mutant strain, the toxin produced by the Ames strain was not able to promote dissemination of the PA- mutant, suggesting that toxigenic action occurs in close proximity to secreting bacteria. Taken together, these findings suggest that a major role for toxins in the pathogenesis of anthrax is to enable the organism to overcome innate host effector mechanisms locally and that much of the damage during the later stages of infection is due to the interactions of the host with the massive bacterial burden.

Identification of circulating bacterial antigens by in vivo microbial antigen discovery.

UNLABELLED: Detection of microbial antigens in clinical samples can lead to rapid diagnosis of an infection and administration of appropriate therapeutics. A major barrier in diagnostics development is determining which of the potentially hundreds or thousands of antigens produced by a microbe are actually present in patient samples in detectable amounts against a background of innumerable host proteins. In this report, we describe a strategy, termed in vivo microbial antigen discovery (InMAD), that we used to identify circulating bacterial antigens. This technique starts with "InMAD serum," which is filtered serum that has been harvested from BALB/c mice infected with a bacterial pathogen. The InMAD serum, which is free of whole bacterial cells, is used to immunize syngeneic BALB/c mice. The resulting "InMAD immune serum" contains antibodies specific for the soluble microbial antigens present in sera from the infected mice. The InMAD immune serum is then used to probe blots of bacterial lysates or bacterial proteome arrays. Bacterial antigens that are reactive with the InMAD immune serum are precisely the antigens to target in an antigen immunoassay. By employing InMAD, we identified multiple circulating antigens that are secreted or shed during infection using Burkholderia pseudomallei and Francisella tularensis as model organisms. Potential diagnostic targets identified by the InMAD approach included bacterial proteins, capsular polysaccharide, and lipopolysaccharide. The InMAD technique makes no assumptions other than immunogenicity and has the potential to be a broad discovery platform to identify diagnostic targets from microbial pathogens. IMPORTANCE: Effective treatment of microbial infection is critically dependent on early diagnosis and identification of the etiological agent. One means for rapid diagnosis is immunoassay for antigens that are shed into body fluids during infection. Immunoassays can be inexpensive, rapid, and adaptable to a point-of-care format. A major impediment to immunoassay for diagnosis of infectious disease is identification of appropriate antigen targets. This report describes a strategy that can be used for identification of microbial antigens that are shed into serum during infection by the biothreats Burkholderia pseudomallei and Francisella tularensis. Termed InMAD (in vivo microbial antigen discovery), the strategy has the potential for application to a broad spectrum of microbial pathogens.

Antibodies contribute to effective vaccination against respiratory infection by type A Francisella tularensis strains.

Pneumonic tularemia is a life-threatening disease caused by inhalation of the highly infectious intracellular bacterium Francisella tularensis. The most serious form of the disease associated with the type A strains can be prevented in experimental animals through vaccination with the attenuated live vaccine strain (LVS). The protection is largely cell mediated, but the contribution of antibodies remains controversial. We addressed this issue in a series of passive immunization studies in Fischer 344 (F344) rats. Subcutaneous LVS vaccination induced a robust serum antibody response dominated by IgM, IgG2a, and IgG2b antibodies. Prophylactic administration of LVS immune serum or purified immune IgG reduced the severity and duration of disease in naïve rats challenged intratracheally with a lethal dose of the virulent type A strain SCHU S4. The level of resistance increased with the volume of immune serum given, but the maximum survivable SCHU S4 challenge dose was at least 100-fold lower than that shown for LVS-vaccinated rats. Protection correlated with reduced systemic bacterial growth, less severe histopathology in the liver and spleen during the early phase of infection, and bacterial clearance by a T cell-dependent mechanism. Our results suggest that treatment with immune serum limited the sequelae associated with infection, thereby enabling a sterilizing T cell response to develop and resolve the infection. Thus, antibodies induced by LVS vaccination may contribute to the defense of F344 rats against respiratory infection by type A strains of F. tularensis.

Cowpox virus inhibits human dendritic cell immune function by nonlethal, nonproductive infection.

Orthopoxviruses encode multiple proteins that modulate host immune responses. We determined whether cowpox virus (CPXV), a representative orthopoxvirus, modulated innate and acquired immune functions of human primary myeloid DCs and plasmacytoid DCs and monocyte-derived DCs (MDDCs). A CPXV infection of DCs at a multiplicity of infection of 10 was nonproductive, altered cellular morphology, and failed to reduce cell viability. A CPXV infection of DCs did not stimulate cytokine or chemokine secretion directly, but suppressed toll-like receptor (TLR) agonist-induced cytokine secretion and a DC-stimulated mixed leukocyte reaction (MLR). LPS-stimulated NF-κB nuclear translocation and host cytokine gene transcription were suppressed in CPXV-infected MDDCs. Early viral immunomodulatory genes were upregulated in MDDCs, consistent with early DC immunosuppression via synthesis of intracellular viral proteins. We conclude that a nonproductive CPXV infection suppressed DC immune function by synthesizing early intracellular viral proteins that suppressed DC signaling pathways.

Pre- and postexposure protection against virulent anthrax infection in mice by humanized monoclonal antibodies to Bacillus anthracis capsule.

One of the two essential virulence factors of Bacillus anthracis is the poly-γ-D-glutamic acid (γDPGA) capsule. Five γDPGA-specific antibody antigen-binding fragments (Fabs) were generated from immunized chimpanzees. The two selected for further study, Fabs 11D and 4C, were both converted into full-length IgG1 and IgG3 mAbs having human IgG1 or IgG3 constant regions. These two mAbs had similar binding affinities, in vitro opsonophagocytic activities, and in vivo efficacies, with the IgG1 and IgG3 subclasses reacting similarly. The mAbs bound to γDPGA specifically with estimated binding affinities (K(d)) of 35-70 nM and effective affinities (effective K(d)) of 0.1-0.3 nM. The LD(50) in an opsonophagocytic bactericidal assay was ≈10 ng/mL of 11D or 4C. A single 30-µg dose of either mAb given to BALB/c mice 18 h before challenge conferred about 50% protection against a lethal intratracheal spore challenge by the virulent B. anthracis Ames strain. More importantly, either mAb given 8 h or 20 h after challenge provided significant protection against lethal infection. Thus, these anti-γDPGA mAbs should be useful, alone or in combination with antitoxin mAbs, for achieving a safe and efficacious postexposure therapy for anthrax.

The Fischer 344 rat reflects human susceptibility to francisella pulmonary challenge and provides a new platform for virulence and protection studies.

BACKGROUND: The pathogenesis of Francisella tularensis, the causative agent of tularemia, has been primarily characterized in mice. However, the high degree of sensitivity of mice to bacterial challenge, especially with the human virulent strains of F. tularensis, limits this animal model for screening of defined attenuated vaccine candidates for protection studies. METHODS AND FINDINGS: We analyzed the susceptibility of the Fischer 344 rat to pulmonary (intratracheal) challenge with three different subspecies (subsp) of F. tularensis that reflect different levels of virulence in humans, and characterized the bacterial replication profile in rat bone marrow-derived macrophages (BMDM). In contrast to the mouse, Fischer 344 rats exhibit a broader range of sensitivity to pulmonary challenge with the human virulent subsp. tularensis and holarctica. Unlike mice, Fischer rats exhibited a high degree of resistance to pulmonary challenge with LVS (an attenuated derivative of subsp. holarctica) and subsp. novicida. Within BMDM, subsp. tularensis and LVS showed minimal replication, subsp. novicida showed marginal replication, and subsp. holartica replicated robustly. The limited intramacrophage replication of subsp. tularensis and novicida strains was correlated with the induction of nitric oxide production. Importantly, Fischer 344 rats that survived pulmonary infection with subsp. novicida were markedly protected against subsequent pulmonary challenge with subsp. tularensis, suggesting that subsp. novicida may be a useful platform for the development of vaccines against subsp. tularensis. CONCLUSIONS: The Fischer 344 rat exhibits similar sensitivity to F. tularensis strains as that reported for humans, and thus the Fischer 344 ray may serve as a better animal model for tularemia vaccine development.

The pathogenesis of acute pulmonary viral and bacterial infections: investigations in animal models.

Acute viral and bacterial infections in the lower respiratory tract are major causes of morbidity and mortality worldwide. The proper study of pulmonary infections requires interdisciplinary collaboration among physicians and biomedical scientists to develop rational hypotheses based on clinical studies and to test these hypotheses in relevant animal models. Animal models for common lung infections are essential to understand pathogenic mechanisms and to clarify general mechanisms for host protection in pulmonary infections, as well as to develop vaccines and therapeutics. Animal models for uncommon pulmonary infections, such as those that can be caused by category A biothreat agents, are also very important because the infrequency of these infections in humans limits in-depth clinical studies. This review summarizes our understanding of innate and adaptive immune mechanisms in the lower respiratory tract and discusses how animal models for selected pulmonary pathogens can contribute to our understanding of the pathogenesis of lung infections and to the search for new vaccines and therapies.

Regulatory interactions of a virulence-associated serine/threonine phosphatase-kinase pair in Bacillus anthracis.

In the current study, we examined the regulatory interactions of a serine/threonine phosphatase (BA-Stp1), serine/threonine kinase (BA-Stk1) pair in Bacillus anthracis. B. anthracis STPK101, a null mutant lacking BA-Stp1 and BA-Stk1, was impaired in its ability to survive within macrophages, and this correlated with an observed reduction in virulence in a mouse model of pulmonary anthrax. Biochemical analyses confirmed that BA-Stp1 is a PP2C phosphatase and dephosphorylates phosphoserine and phosphothreonine residues. Treatment of BA-Stk1 with BA-Stp1 altered BA-Stk1 kinase activity, indicating that the enzymatic function of BA-Stk1 can be influenced by BA-Stp1 dephosphorylation. Using a combination of mass spectrometry and mutagenesis approaches, three phosphorylated residues, T165, S173, and S214, in BA-Stk1 were identified as putative regulatory targets of BA-Stp1. Further analysis found that T165 and S173 were necessary for optimal substrate phosphorylation, while S214 was necessary for complete ATP hydrolysis, autophosphorylation, and substrate phosphorylation. These findings provide insight into a previously undescribed Stp/Stk pair in B. anthracis.

New classes of orthopoxvirus vaccine candidates by functionally screening a synthetic library for protective antigens.

The licensed smallpox vaccine, comprised of infectious vaccinia, is no longer popular as it is associated with a variety of adverse events. Safer vaccines have been explored such as further attenuated viruses and component designs. However, these alternatives typically provide compromised breadth and strength of protection. We conducted a genome-level screening of cowpox, the ancestral poxvirus, in the broadly immune-presenting C57BL/6 mouse as an approach to discovering novel components with protective capacities. Cowpox coding sequences were synthetically built and directly assayed by genetic immunization for open-reading frames that protect against lethal pulmonary infection. Membrane and non-membrane antigens were identified that partially protect C57BL/6 mice against cowpox and vaccinia challenges without adjuvant or regimen optimization, whereas the 4-pox vaccine did not. New vaccines might be developed from productive combinations of these new and existing antigens to confer potent, broadly efficacious protection and be contraindicated for none.

Vaccination of Fischer 344 rats against pulmonary infections by Francisella tularensis type A strains.

Pneumonic tularemia caused by inhalation of the type A strains of Francisella tularensis is associated with high morbidity and mortality in humans. The only vaccine known to protect humans against this disease is the attenuated live vaccine strain (LVS), but it is not currently registered for human use. To develop a new generation of vaccines, multiple animal models are needed that reproduce the human response to F. tularensis infection and vaccination. We examined the potential use of Fischer 344 rat as such a model. Fischer 344 rats were very sensitive to intratracheal infection with the virulent type A strain SCHU S4 and generally succumbed less than 2 weeks after infection. Similar to humans and non-human primates, Fischer 344 rats vaccinated with LVS by subcutaneous or intradermal routes were protected against a greater range of respiratory SCHU S4 challenge doses than has been reported for LVS vaccinated mice. Intratracheal LVS vaccination also induced effective immunity, but it was less protective when the challenge dose exceeded 10(5) SCHU S4. LVS vaccination did not prevent SCHU S4 infection but rather controlled bacterial growth and pathology, leading to the eventual clearance of the bacteria. Our results suggest that the Fischer 344 rat may be a good model for studying pneumonic tularemia and evaluating potential vaccine candidates.

Rapid detection of the poly-gamma-D-glutamic acid capsular antigen of Bacillus anthracis by latex agglutination.

Latex agglutination has been used to detect capsular polysaccharides from a variety of bacteria in body fluids. A latex agglutination assay was constructed for detection of the poly-gamma-D-glutamic acid (gammaDPGA) capsular polypeptide of Bacillus anthracis in serum from animal models of pulmonary anthrax. The assay was able to detect gammaDPGA in serum from infected animals at concentrations of 100 to 200 ng/mL.

T cells from lungs and livers of Francisella tularensis-immune mice control the growth of intracellular bacteria.

Parenteral and respiratory vaccinations with the intracellular bacterium Francisella tularensis have been studied using the live vaccine strain (LVS) in a mouse model, and spleen cells from immune mice are often used for immunological studies. However, mechanisms of host immunological responses may be different in nonlymphoid organs that are important sites of infection, such as lung and liver. Using parenteral (intradermal) or respiratory (cloud aerosol) vaccination, here we examine the functions of resulting LVS-immune liver or lung cells, respectively. Surprisingly, LVS was considerably more virulent when administered by cloud aerosol than by intranasal instillation, suggesting method-dependent differences in initial localization and/or dissemination patterns. Only low doses were sublethal, and resolution of sublethal cloud aerosol infection was dependent on gamma interferon (IFN-gamma), tumor necrosis factor alpha, and inducible nitric oxide synthase. Nonetheless, survival of cloud aerosol or parenteral infection resulted in the development of a protective immune response against lethal LVS intraperitoneal or aerosol challenge, reflecting development of systemic secondary immunity in both cases. Such immunity was further detected by directly examining the functions of LVS-immune lung or liver lymphocytes in vitro. Lung lymphocytes primed by respiratory infection, as well as liver lymphocytes primed by parenteral infection, clearly controlled in vitro intracellular bacterial growth primarily via mechanisms that were not dependent on IFN-gamma activity. Thus, our results indicate functional similarities between immune T cells residing in spleens, livers, and lungs of LVS-immune mice.

Killed but metabolically active Bacillus anthracis vaccines induce broad and protective immunity against anthrax.

Bacillus anthracis is the causative agent of anthrax. We have developed a novel whole-bacterial-cell anthrax vaccine utilizing B. anthracis that is killed but metabolically active (KBMA). Vaccine strains that are asporogenic and nucleotide excision repair deficient were engineered by deleting the spoIIE and uvrAB genes, rendering B. anthracis extremely sensitive to photochemical inactivation with S-59 psoralen and UV light. We also introduced point mutations into the lef and cya genes, which allowed inactive but immunogenic toxins to be produced. Photochemically inactivated vaccine strains maintained a high degree of metabolic activity and secreted protective antigen (PA), lethal factor, and edema factor. KBMA B. anthracis vaccines were avirulent in mice and induced less injection site inflammation than recombinant PA adsorbed to aluminum hydroxide gel. KBMA B. anthracis-vaccinated animals produced antibodies against numerous anthrax antigens, including high levels of anti-PA and toxin-neutralizing antibodies. Vaccination with KBMA B. anthracis fully protected mice against challenge with lethal doses of toxinogenic unencapsulated Sterne 7702 spores and rabbits against challenge with lethal pneumonic doses of fully virulent Ames strain spores. Guinea pigs vaccinated with KBMA B. anthracis were partially protected against lethal Ames spore challenge, which was comparable to vaccination with the licensed vaccine anthrax vaccine adsorbed. These data demonstrate that KBMA anthrax vaccines are well tolerated and elicit potent protective immune responses. The use of KBMA vaccines may be broadly applicable to bacterial pathogens, especially those for which the correlates of protective immunity are unknown.

Discriminating virulence mechanisms among Bacillus anthracis strains by using a murine subcutaneous infection model.

Bacillus anthracis strains harboring virulence plasmid pXO1 that encodes the toxin protein protective antigen (PA), lethal factor, and edema factor and virulence plasmid pXO2 that encodes capsule biosynthetic enzymes exhibit different levels of virulence in certain animal models. In the murine model of pulmonary infection, B. anthracis virulence was capsule dependent but toxin independent. We examined the role of toxins in subcutaneous (s.c.) infections using two different genetically complete (pXO1(+) pXO2(+)) strains of B. anthracis, strains Ames and UT500. Similar to findings for the pulmonary model, toxin was not required for infection by the Ames strain, because the 50% lethal dose (LD(50)) of a PA-deficient (PA(-)) Ames mutant was identical to that of the parent Ames strain. However, PA was required for efficient s.c. infection by the UT500 strain, because the s.c. LD(50) of a UT500 PA(-) mutant was 10,000-fold higher than the LD(50) of the parent UT500 strain. This difference between the Ames strain and the UT500 strain could not be attributed to differences in spore coat properties or the rate of germination, because s.c. inoculation with the capsulated bacillus forms also required toxin synthesis by the UT500 strain to cause lethal infection. The toxin-dependent phenotype of the UT500 strain was host phagocyte dependent, because eliminating Gr-1(+) phagocytes restored virulence to the UT500 PA(-) mutant. These experiments demonstrate that the dominant virulence factors used to establish infection by B. anthracis depend on the route of inoculation and the bacterial strain.

Effect of Bacillus anthracis virulence factors on human dendritic cell activation.

Bacillus anthracis possesses three primary virulence factors: capsule, lethal toxin (LT), and edema toxin (ET). Dendritic cells (DCs) are critical to innate and acquired immunity and represent potential targets for these factors. We examined the ability of B. anthracis spores and bacilli to stimulate human monocyte-derived DC (MDDC), primary myeloid DC (mDC), and plasmacytoid DC (pDC) cytokine secretion. Exposure of MDDCs and mDCs to spores or vegetative bacilli of the genetically complete strain UT500 induced significantly increased cytokine secretion. Spores lacking genes required for capsule biosynthesis stimulated significantly higher cytokine secretion than UT500 spores from mDCs, but not MDDCs. In contrast, bacilli lacking capsule stimulated significantly higher cytokine secretion than UT500 bacilli in both MDDCs and mDCs. Spores or bacilli lacking both LT and ET stimulated significantly higher cytokine secretion than UT500 spores or bacilli, respectively, in both mDCs and MDDCs. pDCs exposed to spores or bacilli did not produce significant amounts of cytokines even when virulence factors were absent. In conclusion, B. anthracis employs toxins as well as capsule to inhibit human MDDC and mDC cytokine secretion, whereas human pDCs respond poorly even when capsule or both toxins are absent.