Injectional anthrax infection due to heroin use induces strong immunological memory

Impact of HLA Polymorphism on the Immune Response to Bacillus Anthracis Protective Antigen in Vaccination versus Natural Infection

The causative agent of anthrax, Bacillus anthracis, evades the host immune response and establishes infection through the production of binary exotoxins composed of Protective Antigen (PA) and one of two subunits, lethal factor (LF) or edema factor (EF). The majority of vaccination strategies have focused upon the antibody response to the PA subunit. We have used a panel of humanised HLA class II transgenic mouse strains to define HLA-DR-restricted and HLA-DQ-restricted CD4+ T cell responses to the immunodominant epitopes of PA. This was correlated with the binding affinities of epitopes to HLA class II molecules, as well as the responses of two human cohorts: individuals vaccinated with the Anthrax Vaccine Precipitated (AVP) vaccine (which contains PA and trace amounts of LF), and patients recovering from cutaneous anthrax infections. The infected and vaccinated cohorts expressing different HLA types were found to make CD4+ T cell responses to multiple and diverse epitopes of PA. The effects of HLA polymorphism were explored using transgenic mouse lines, which demonstrated differential susceptibility, indicating that HLA-DR1 and HLA-DQ8 alleles conferred protective immunity relative to HLA-DR15, HLA-DR4 and HLA-DQ6. The HLA transgenics enabled a reductionist approach, allowing us to better define CD4+ T cell epitopes. Appreciating the effects of HLA polymorphism on the variability of responses to natural infection and vaccination is vital in planning protective strategies against anthrax.

A Comparison of the Adaptive Immune Response between Recovered Anthrax Patients and Individuals Receiving Three Different Anthrax Vaccines

Several different human vaccines are available to protect against anthrax. We compared the human adaptive immune responses generated by three different anthrax vaccines or by previous exposure to cutaneous anthrax. Adaptive immunity was measured by ELISPOT to count cells that produce interferon (IFN)-γ in response to restimulation ex vivo with the anthrax toxin components PA, LF and EF and by measuring circulating IgG specific to these antigens. Neutralising activity of antisera against anthrax toxin was also assayed. We found that the different exposures to anthrax antigens promoted varying immune responses. Cutaneous anthrax promoted strong IFN-γ responses to all three antigens and antibody responses to PA and LF. The American AVA and Russian LAAV vaccines induced antibody responses to PA only. The British AVP vaccine produced IFN-γ responses to EF and antibody responses to all three antigens. Anti-PA (in AVA and LAAV vaccinees) or anti-LF (in AVP vaccinees) antibody titres correlated with toxin neutralisation activities. Our study is the first to compare all three vaccines in humans and show the diversity of responses against anthrax antigens.

CD4+ T Cells Targeting Dominant and Cryptic Epitopes from Bacillus anthracis Lethal Factor

Anthrax is an endemic infection in many countries, particularly in the developing world. The causative agent, Bacillus anthracis, mediates disease through the secretion of binary exotoxins. Until recently, research into adaptive immunity targeting this bacterial pathogen has largely focused on the humoral response to these toxins. There is, however, growing recognition that cellular immune responses involving IFNγ producing CD4+ T cells also contribute significantly to a protective memory response. An established concept in adaptive immunity to infection is that during infection of host cells, new microbial epitopes may be revealed, leading to immune recognition of so called ‘cryptic’ or ‘subdominant’ epitopes. We analyzed the response to both cryptic and immunodominant T cell epitopes derived from the toxin component lethal factor and presented by a range of HLA-DR alleles. Using IFNγ-ELISpot assays we characterized epitopes that elicited a response following immunization with synthetic peptide and the whole protein and tested their capacities to bind purified HLA-DR molecules in vitro. We found that DR1 transgenics demonstrated T cell responses to a greater number of domain III cryptic epitopes than other HLA-DR transgenics, and that this pattern was repeated with the immunodominant epitopes, as a greater proportion of these epitopes induced a T cell response when presented within the context of the whole protein. Immunodominant epitopes LF_457-476 and LF_467-487 were found to induce a T cell response to the peptide, as well as to the whole native LF protein in DR1 and DR15, but not in DR4 transgenics. The analysis of Domain I revealed the presence of several unique cryptic epitopes all of which showed a strong to moderate relative binding affinity to HLA-DR4 molecules. However, none of the cryptic epitopes from either domain III or I displayed notably high binding affinities across all HLA-DR alleles assayed. These responses were influenced by the specific HLA alleles presenting the peptide, and imply that construction of future epitope string vaccines which are immunogenic across a wide range of HLA alleles could benefit from a combination of both cryptic and immunodominant anthrax epitopes.

Antimicrobial Treatment for Systemic Anthrax: Analysis of Cases from 1945 to 2014 Identified Through a Systematic Literature Review

Systemic anthrax is associated with high mortality. Current national guidelines, developed for the individualized treatment of systemic anthrax, outline the use of combination intravenous antimicrobials for a minimum of 2 weeks, bactericidal and protein synthesis inhibitor antimicrobials for all cases of systemic anthrax, and at least 3 antimicrobials with good blood-brain barrier penetration for anthrax meningitis. However, in an anthrax mass casualty incident, large numbers of anthrax cases may create challenges in meeting antimicrobial needs. To further inform our understanding of the role of antimicrobials in treating systemic anthrax, a systematic review of the English-language literature was conducted to identify cases of systemic anthrax treated with antimicrobials for which a clinical outcome was recorded. A total of 149 cases of systemic anthrax were identified. Among the identified 59 cases of cutaneous anthrax, 33 were complicated by meningitis (76% mortality), while 26 simply had evidence of the systemic inflammatory response syndrome (4% mortality); 21 of 26 (81%) of this latter group received monotherapy. Subsequent analysis regarding combination antimicrobial therapy was restricted to the remaining 123 cases of more severe anthrax (overall 67% mortality). Recipients of combination bactericidal and protein synthesis inhibitor therapy had a 45% survival versus 28% in the absence of combination therapy (p = 0.07). For meningitis cases (n = 77), survival was greater for those receiving 3 or more antimicrobials over the course of treatment (3 of 4; 75%), compared to receipt of 1 or 2 antimicrobials (12 of 73; 16%) (p = 0.02). Median parenteral antimicrobial duration was 14 days. Combination bactericidal and protein synthesis inhibitor therapy may be appropriate in severe anthrax disease, particularly anthrax meningitis, in a mass casualty incident.

Whole genome protein microarrays for serum profiling of immunodominant antigens of Bacillus anthracis

A commercial Bacillus anthracis (Anthrax) whole genome protein microarray has been used to identify immunogenic Anthrax proteins (IAP) using sera from groups of donors with (a) confirmed B. anthracis naturally acquired cutaneous infection, (b) confirmed B. anthracis intravenous drug use-acquired infection, (c) occupational exposure in a wool-sorters factory, (d) humans and rabbits vaccinated with the UK Anthrax protein vaccine and compared to naïve unexposed controls. Anti-IAP responses were observed for both IgG and IgA in the challenged groups; however the anti-IAP IgG response was more evident in the vaccinated group and the anti-IAP IgA response more evident in the B. anthracis-infected groups. Infected individuals appeared somewhat suppressed for their general IgG response, compared with other challenged groups. Immunogenic protein antigens were identified in all groups, some of which were shared between groups whilst others were specific for individual groups. The toxin proteins were immunodominant in all vaccinated, infected or other challenged groups. However, a number of other chromosomally-located and plasmid encoded open reading frame proteins were also recognized by infected or exposed groups in comparison to controls. Some of these antigens e.g., BA4182 are not recognized by vaccinated individuals, suggesting that there are proteins more specifically expressed by live Anthrax spores in vivo that are not currently found in the UK licensed Anthrax Vaccine (AVP). These may perhaps be preferentially expressed during infection and represent expression of alternative pathways in the B. anthracis “infectome.” These may make highly attractive candidates for diagnostic and vaccine biomarker development as they may be more specifically associated with the infectious phase of the pathogen. A number of B. anthracis small hypothetical protein targets have been synthesized, tested in mouse immunogenicity studies and validated in parallel using human sera from the same study.

The serodominant secreted effector protein of Salmonella, SseB, is a strong CD4 antigen containing an immunodominant epitope presented by diverse HLA class II alleles

Detailed characterization of the protective T-cell response in salmonellosis is a pressing unmet need in light of the global burden of human Salmonella infections and the likely contribution of CD4 T cells to immunity against this intracellular infection. In previous studies screening patient sera against antigen arrays, SseB was noteworthy as a serodominant target of adaptive immunity, inducing significantly raised antibody responses in HIV-seronegative compared with seropositive patients. SseB is a secreted protein, part of the Espa superfamily, localized to the bacterial surface and forming part of the translocon of the type III secretion system (T3SS) encoded by Salmonella pathogenicity island 2. We demonstrate here that SseB is also a target of CD4 T-cell immunity, generating a substantial response after experimental infection in human volunteers, with around 0·1% of the peripheral repertoire responding to it. HLA-DR/peptide binding studies indicate that this protein encompasses a number of peptides with ability to bind to several different HLA-DR alleles. Of these, peptide 11 (p11) was shown in priming of both HLA-DR1 and HLA-DR4 transgenic mice to contain an immunodominant CD4 epitope. Analysis of responses in human donors showed immunity focused on p11 and another epitope in peptide 2. The high frequency of SseB-reactive CD4 T cells and the broad applicability to diverse HLA genotypes coupled with previous observations of serodominance and protective vaccination in mouse challenge experiments, make SseB a plausible candidate for next-generation Salmonella vaccines.

Host immunity to Bacillus anthracis lethal factor and other immunogens: implications for vaccine design

Infections of humans with Bacillus anthracis are an issue with respect to the biothreat both to civilians and military personnel, infections of individuals by infected livestock in endemic regions and, recently, infections of intravenous drug users injecting anthrax-contaminated heroin. Existing vaccination regimens are reliant on protective antigen neutralization induced by repeated boosts with the AVA or AVP vaccines. However, there is ongoing interest in updated approaches in light of the intensive booster regime and extent of reactogenicity inherent in the current protocols. Several other immunogens from the B. anthracis proteome have been characterized in recent years, including lethal factor. Lethal factor induces strong CD4 T-cell immunity and encompasses immunodominant epitopes of relevance across diverse HLA polymorphisms. Taken together, recent studies emphasize the potential benefits of vaccines able to confer synergistic immunity to protective antigen and to other immunogens, targeting both B-cell and T-cell repertoires.

Anthrax lethal factor as an immune target in humans and transgenic mice and the impact of HLA polymorphism on CD4+ T cell immunity

Bacillus anthracis produces a binary toxin composed of protective antigen (PA) and one of two subunits, lethal factor (LF) or edema factor (EF). Most studies have concentrated on induction of toxin-specific antibodies as the correlate of protective immunity, in contrast to which understanding of cellular immunity to these toxins and its impact on infection is limited. We characterized CD4⁺ T cell immunity to LF in a panel of humanized HLA-DR and DQ transgenic mice and in naturally exposed patients. As the variation in antigen presentation governed by HLA polymorphism has a major impact on protective immunity to specific epitopes, we examined relative binding affinities of LF peptides to purified HLA class II molecules, identifying those regions likely to be of broad applicability to human immune studies through their ability to bind multiple alleles. Transgenics differing only in their expression of human HLA class II alleles showed a marked hierarchy of immunity to LF. Immunogenicity in HLA transgenics was primarily restricted to epitopes from domains II and IV of LF and promiscuous, dominant epitopes, common to all HLA types, were identified in domain II. The relevance of this model was further demonstrated by the fact that a number of the immunodominant epitopes identified in mice were recognized by T cells from humans previously infected with cutaneous anthrax and from vaccinated individuals. The ability of the identified epitopes to confer protective immunity was demonstrated by lethal anthrax challenge of HLA transgenic mice immunized with a peptide subunit vaccine comprising the immunodominant epitopes that we identified.

Anthrax is of concern with respect to human exposure in endemic regions, concerns about bioterrorism and the considerable global burden of livestock infections. The immunology of this disease remains poorly understood. Vaccination has been based on B. anthracis filtrates or attenuated spore-based vaccines, with more recent trials of next-generation recombinant vaccines. Approaches generally require extensive vaccination regimens and there have been concerns about immunogenicity and adverse reactions. An ongoing need remains for rationally designed, effective and safe anthrax vaccines. The importance of T cell stimulating vaccines is inceasingly recognized. An essential step is an understanding of immunodominant epitopes and their relevance across the diverse HLA immune response genes of human populations. We characterized CD4 T cell immunity to anthrax Lethal Factor (LF), using HLA transgenic mice, as well as testing candidate peptide epitopes for binding to a wide range of HLA alleles. We identified anthrax epitopes, noteworthy in that they elicit exceptionally strong immunity with promiscuous binding across multiple HLA alleles and isotypes. T cell responses in humans exposed to LF through either natural anthrax infection or vaccination were also examined. Epitopes identified as candidates were used to protect HLA transgenic mice from anthrax challenge.