In vitro correlate of immunity in a rabbit model of inhalational anthrax

A serological correlate of vaccine-induced immunity was identified in the rabbit model of inhalational anthrax. Animals were inoculated intramuscularly at 0 and 4 weeks with varying doses of Anthrax Vaccine Adsorbed (AVA) ranging from a human dose to a 1:256 dilution in phosphate-buffered saline (PBS). At 6 and 10 weeks, both the quantitative anti-protective antigen (PA) IgG ELISA and the toxin-neutralizing antibody (TNA) assays were used to measure antibody levels to PA. Rabbits were aerosol-challenged at 10 weeks with a lethal dose (84-133 LD(50)) of Bacillus anthracis spores. All the rabbits that received the undiluted and 1:4 dilution of vaccine survived, whereas those receiving the higher dilutions of vaccine (1:16, 1:64 and 1:256) had deaths in their groups. Results showed that antibody levels to PA at both 6 and 10 weeks were significant (P<0.0001) predictors of survival.

Protection against anthrax lethal toxin challenge by genetic immunization with a plasmid encoding the lethal factor protein

The ability of genetic vaccination to protect against a lethal challenge of anthrax toxin was evaluated. BALB/c mice were immunized via gene gun inoculation with eucaryotic expression vector plasmids encoding either a fragment of the protective antigen (PA) or a fragment of lethal factor (LF). Plasmid pCLF4 contains the N-terminal region (amino acids [aa] 10 to 254) of Bacillus anthracis LF cloned into the pCI expression plasmid. Plasmid pCPA contains a biologically active portion (aa 175 to 764) of B. anthracis PA cloned into the pCI expression vector. One-micrometer-diameter gold particles were coated with plasmid pCLF4 or pCPA or a 1:1 mixture of both and injected into mice via gene gun (1 microg of plasmid DNA/injection) three times at 2-week intervals. Sera were collected and analyzed for antibody titer as well as antibody isotype. Significantly, titers of antibody to both PA and LF from mice immunized with the combination of pCPA and pCLF4 were four to five times greater than titers from mice immunized with either gene alone. Two weeks following the third and final plasmid DNA boost, all mice were challenged with 5 50% lethal doses of lethal toxin (PA plus LF) injected intravenously into the tail vein. All mice immunized with pCLF4, pCPA, or the combination of both survived the challenge, whereas all unimmunized mice did not survive. These results demonstrate that DNA-based immunization alone can provide protection against a lethal toxin challenge and that DNA immunization against the LF antigen alone provides complete protection.

The role of antibodies to Bacillus anthracis and anthrax toxin components in inhibiting the early stages of infection by anthrax spores

Vaccines which are efficacious against anthrax, such as the human vaccine, Anthrax Vaccine Absorbed (AVA), contain the protective antigen (PA) component of the anthrax toxins as the major protective immunogen. Although AVA protects against inhalational anthrax, the immune responses to and role in protection of PA and possibly other antigens have yet to be fully elucidated. Sera from animals immunized with a toxin-producing, unencapsulated live vaccine strain of Bacillus anthracis have been reported to have anti-spore activities associated with the antitoxin humoral response. The authors performed studies to determine whether anti-PA antibody (Ab)-containing preparations stimulated spore uptake by phagocytes and suppressed the germination of spores in vitro. AVA- and PA-immune sera from several species enhanced the phagocytosis by murine peritoneal macrophages of spores of the virulent Ames and the Sterne vaccine strains. Antitoxin Abs appeared to contribute significantly, although not solely, to the enhanced uptake. Rabbit antisera to PA purified from either Sterne or a PA-producing pX01-cured recombinant, affinity-purified anti-PA IgG, and monkey antisera to AVA were used to assess the role of anti-PA ABS: Rabbit anti-PA Abs promoted the uptake of spores of the PA-producing strains Sterne, Ames and RP42, a mutant of Sterne producing only PA, but not of the pX01-Sterne-1 strain, Ames strain, or RP4, a mutant of Sterne with deletions in the loci encoding PA and the oedema factor (EF) toxin component and producing only the lethal factor toxin component. Rabbit anti-PA and monkey anti-AVA Abs also significantly inhibited spore germination in vitro compared to preimmune serum or medium. Spore-associated proteins recognized by anti-PA Abs were detected by electron microscopy and confirmed by immunoblotting of spore coat extracts. Thus, the anti-PA Ab-specific immunity induced by AVA has anti-spore activity and might have a role in impeding the early stages of infection with B. anthracis spores.

Application of recovery tests in the validation of immunoassays for assessing the immunogenicity of B. anthracis PA vaccine

In the quantitative assessment of polyclonal serum antibodies, the complex composition and characteristics of the analyte population (serum antibodies) restricts the capability of constructing appropriately defined calibration standards. This fact limits the application of the conservative recovery tests to the validation of immunoassays aimed at determining serum antibody levels. The present report describes a modification of recovery tests that overcomes this impediment. The modified approach is based on a dilution analysis system, where a given immune serum is serially diluted in normal serum and the antibody titers in each of the derived diluted samples are then determined. Expected sample titers (calculated on the basis of the relevant dilution factors) are plotted against the respective observed results, and the resulting recovery curve is then examined by means of a regression analysis, according to the standard rules of the conservative recovery analysis. This approach was tried with two immunoassay systems, Enzyme Linked Immunosorbent Assay (ELISA) and Neutralizing Antibodies (NtAb) immunoassays, aimed at assessing the immunogenicity in guinea pigs of B. anthracis protective antigen (PA) vaccine. In a series of feasibility studies using a recovery simulation model (dilutions made in the immunoassay diluent, rather than in normal serum) the average recovery levels in ELISA and NtAb immunoassays were 0.99 +/- 0.011 and 1.02 +/- 0.04 respectively, and the 99% confidence intervals contained the target 100% value. Regression lines were proved to be linear demonstrating R > 0.97 in all cases. The 99% confidence intervals around the observed slopes and intercepts always contained the corresponding target values 1 and 0. The relative standard deviation (RSD) in the ELISA and NtAb immunoassays was found to be 0.01 and 0.025 respectively. All of the above experimental results were not affected by the serum antibody titer, or by day-to-day variations embodied in these immunoassay systems. When true recovery tests were applied to the above immunoassays, essentially identical results were obtained. In both assays the correlation coefficients were in the range of 0.96-1, recoveries were found to be in the range of 0.90-1.06, and RSD values were in the range of 0.02-0.025. All the recovery deviations from the target value of 1 were not statistically significant. The hitherto observed experimental findings illustrate the capability of the dilution analysis system to allow the application of recovery tests to the validation of quantitative immunoassays, which are based on the procedure of serum titrations.

Adverse reactions to anthrax immunisation in a military field hospital

OBJECTIVE

To determine the outcome of anthrax immunisation.

METHODS

Adverse reactions (occurrence, nature, severity and incapacity) and immune responses to a voluntary programme of anthrax immunisation at 0, 3, 6, and 24 weeks were monitored by questionnaire and voluntary blood sampling in 129 members, including 24 immunised 7 years previously (immunes), of a military field hospital alerted for possible deployment.

RESULTS

Follow-up was complete in 85%. Ninety-eight (76%) received the first anthrax immunisation. Uptake was greater (p = 0.015) in immunes. Initial prevalence of adverse reaction was 63%. Subsequent uptake and adverse reaction dwindled significantly (p < 0.001). Only 28 (22%) were immunised at 24 weeks. Proportions reporting adverse reactions following the initial immunisation were greater in immunes (p = 0.046) and officers (p = 0.02). There was no significant (p = 0.36) correlation between uptake of immunisation and prevalence of adverse reaction. Antecedent adverse reaction did not reduce the proportion of participants accepting immunisation subsequently. The nature of adverse reactions (47% local, 24% systemic and 27% both) and severity were the same throughout. Forty-five percent of adverse reactions caused incapacity. Seventy-four percent of these had pain in the injected arm (+/- systemic symptoms) which prevented lifting or driving for 48 hours in 63%. Immune responses were greater in immunes.

CONCLUSIONS

It was concluded that anthrax immunisation results in a higher than expected prevalence of adverse reaction with initial incapacity of military significance affecting 18%. Greater immune responses may increase adverse reaction but this does not affect acceptance of anthrax immunisation. Poor completion rates necessitate development of a new anthrax immunisation strategy.