Comprehensive Analysis and Selection of Anthrax Vaccine Adsorbed Immune Correlates of Protection in Rhesus Macaques

Abstract

T o date, there has not been a systematic evaluation of the relationship between anthrax vaccine-stimulated humoral and cell-mediated immune responses, their relative contributions to protection, or their comparative importance when used singly or in combination to predict the probability of survival in animal models or in humans.Anthrax toxin protective antigen (PA) is the primary immunogen in licensed anthrax vaccines in the United States and the European Union, as well as in many of the second-generation anthrax vaccines in current development (1). Consequently, the quantitative analysis of anti-PA IgG antibody levels and lethal toxin neutralization activity (TNA) in serum are generally accepted as immunological correlates of protection (COP) for vaccine efficacy in animal models (2). Anti-PA IgG levels and TNA are also considered pivotal for cross-species predictions of anthrax vaccine efficacy in humans, for whom clinical efficacy studies are either impractical or ethically infeasible (3, 4) (http://www.fda .gov/AdvisoryCommittees/CommitteesMeetingMaterials/Blood VaccinesandOtherBiologics/VaccinesandRelatedBiologicalProducts AdvisoryCommittee/ucm239733.htm). Anti-PA IgG and TNA levels, however, are but one part of the spectrum of humoral and cell-mediated immune responses that may contribute to protection. The COP for anthrax may differ depending on vaccine formulations, schedules, and routes of administration (5-10).The U.S.-licensed anthrax vaccine adsorbed (AVA) (BioThrax) was approved in 1970 for the prevention of anthrax in humans (11)(12)(13)(14). The original regimen for AVA was a subcutaneous (s.c.) six-dose primary schedule at 0, 0.5, 1, 6, 12, and 18 months, with subsequent annual boosters. In May 2012, the U.S. Food and Drug Administration (FDA) approved the AVA regimen as an intramuscular (i.m.) three-dose priming schedule at 0, 1, and 6 months, with boosters at 12 and 18 months and annually thereafter (http://www .fda.gov/BiologicsBloodVaccines/Vaccines/ApprovedProducts /ucm304758.htm). These recent changes in the schedule and administration route were based on data from the Centers for Disease Control and Prevention Anthrax Vaccine Research Program (AVRP) (12, 13). The goals of the AVRP were to improve the AVA safety profile and ensure efficacy while minimizing the number of doses required. The study objectives included determining immunological correlates of protection, documenting vaccine efficacy, and optimizing the vaccination schedule and route of administration (14). Due to the low prevalence of inhalation anthrax and the ethical concerns of conducting an efficacy trial in humans, vaccine efficacy and duration of protection were evaluated in rhesus macaques (Macaca mulatta) (15).The AVRP nonhuman primate (NHP) study used the 0-, 1-, and 6-month intramuscular priming series (3-i.m.) with a full human dose or saline dilutions of AVA to modulate the immune response. The NHP were challenged with high-dose (median, 504ϫ the 50% lethal dose [LD 50 ]) aerosolized Bacillus anthracis spores at m...

Humoral and Cell-Mediated Immune Responses to Alternate Booster Schedules of Anthrax Vaccine Adsorbed in Humans

“…A previously reported cross-species COP bridging between NHPs and humans estimated that at the lowest levels of serum antibody at month 42, the 4-IM group had a predicted minimum probability of survival from 86.8% to 90.2%, compared to 95.8 to 98.0% in the 5-IM group and 98.1 to 99.5% in the 7-IM annual booster group (1,3). Of additional importance, therefore, is whether the anamnestic response kinetics in the reduced booster schedules are sufficient to meet or exceed those of the annual boosters and to provide protection against acute infection.…”

“…The immunological characteristics of these long-term protective responses in NHPs have been reported previously, and anti-PA IgG was identified as the most accurate immune correlate of protection (COP) (1). Serum antibody levels decline in humans and NHPs in the absence of boosters.…”

“…The primary immunogen in AVA is anthrax toxin protective antigen (PA). Serum anti-PA antibody levels are accurate immune correlates of protection in nonhuman primate (NHP) models of inhalation anthrax and for predicted probability of survival in humans (1)(2)(3). There is a significant lack of data in humans regarding the onset, duration, quantitative analysis, and functional activity of humoral antibody and cell-mediated immunity (CMI) responses following priming and boosting with AVA.…”

Unique Inflammatory Mediators and Specific IgE Levels Distinguish Local from Systemic Reactions after Anthrax Vaccine Adsorbed Vaccination

“…The primary immunogen of AVA is protective antigen (PA), a component of anthrax toxin that is nontoxic alone. In many animal models, the magnitude and toxin neutralization ability of the humoral response to PA is correlated with protection against B. anthracis spore challenge (8)(9)(10)(11)(12). Since AVA became mandatory for select populations in 1998, concerns have arisen regarding the relatively high rate of adverse events (AEs), especially in women (13,14).…”

Mechanistic Analysis of the Effect of Deamidation on the Immunogenicity of Anthrax Protective Antigen

“…Internalization of LT and ET leads to introduction of the enzymatically active effector proteins LF and EF into the cell cytosol where they exert their cytotoxicity (2). Vaccines based on PA or a recombinant form of PA (rPA) elicit antibodies, in particular functional toxinneutralizing antibodies (TNAs), that have been correlated with protection against the disease (4)(5)(6).…”