Anthrax vaccination induced anti-lethal factor IgG: Fine specificity and neutralizing capacity
Introduction
Bacillus anthracis, a gram-positive, spore-forming bacterium, is the causative agent of anthrax infection. Infection can be initiated by cutaneous, gastrointestinal, or inhalational routes, with the inhalational route resulting in 45–90% mortality [1]. Systemic infection is characterized by an extremely high blood concentration of bacilli, resulting in high concentrations of the secreted tripartite toxin. This toxin is composed of three polypeptides: protective antigen (PA), lethal factor (LF), and edema factor (EF). PA binds to its cellular receptor(s), Tumor Endothelial Marker 8 (TEM8) or Capillary Morphogenesis Protein 2 (CMG2) [1], [2], [3], and is cleaved by a furin-like membrane endoprotease. The resulting 63 kDa fragment oligomerizes and, when endocytosed, carries EF and/or LF into the cell [1], [4], [5]. Edema toxin (ET), composed of PA and EF, is an adenylate cyclase that results in edema and can be lethal when injected into animals [6]. This toxin has also been shown to impair macrophage phagocytosis and increase cAMP levels [1], [7]. Lethal toxin (LT), formed by the combination of PA with LF, is a zinc-dependent protease that causes lysis of intoxicated macrophages and is lethal in animal models [1], [8]. Following PA-mediated translocation of LF into the cytosol, target cells such as macrophages release pro-inflammatory cytokines inducing endothelial cell death by apoptosis and leading to vascular collapse [1], [8], [9], [10], [11], [12], [13], [14].
The design of the current United States anthrax vaccine (Anthrax Vaccine Absorbed, AVA) is predicated on the fact that PA serves as a crucial component of both LT and ET, and antibodies against PA are known to provide protection from disease in animals [15], [16]. This vaccine is produced from a cell-free filtrate of an attenuated bovine isolate (V770-NP1-R) that produces a higher fraction of PA [17]. However, all three toxin components (PA, LF, and EF) are present in the product [17], [18]. While it is clear that antibodies to PA are the primary method of protection generated following AVA immunization, studies with mouse models have demonstrated the protective significance of antibodies to LF alone [19]. Antibodies directed against LF have been shown to provide protection against challenge with toxin or bacteria in several experimental animal models [14], [20], [21], [22], [23], [24], [25]. Additionally, the protective capacity of neutralizing antibodies directed against PA can be greatly enhanced by the addition of LF neutralizing antibodies [21]. Limited human data exists characterizing the fine-specificity and potential for protection of antibodies to LF following AVA immunization. This study evaluated plasma from a large cohort (n = 1000) of AVA immunized individuals for the quantitative levels of LF specific antibodies as well as for the presence of binding to sequential B cell epitopes that contribute to functional protection. Antibodies directed against two antigenic regions of LF, one in the PA binding domain and one in the ADP-ribotransferase-like domain, are able to provide protection in an in vivo mouse model of lethal toxin challenge. These data suggest that development of new active and passive vaccination strategies that incorporate these LF antigenic regions will lead to improved protection against anthrax.
Section snippets
Human subjects and sample collection
Individuals who were vaccinated with the currently licensed AVA were enrolled in this study (n = 1000). Participants provided informed consent and information about vaccination history, sex, age, and ethnicity. One hundred non-vaccinated individuals were recruited to provide control samples. Institutional Review Board approval was obtained from the Oklahoma Medical Research Foundation, Oklahoma University Health Sciences Center, Walter Reed Army Medical Center, Washington, DC and Womack Army
AVA vaccination results in the development of antibodies directed against LF in a small subset of individuals
Individuals who had received at least the first three doses of the AVA anthrax vaccine series were recruited and provided plasma. Participants provided vaccination history and self-reported demographics, including sex, age, and ethnicity. European Americans comprised the majority of the cohort (63.7%), but other racial/ethnic groups were well represented, with 134 African Americans, 75 Hispanics, 23 Asians, and 131 individuals of American Indian, Pacific Island or mixed ethnicity (Table 1). Of
Discussion
The predominant antibody response generated following at least three doses of US anthrax vaccine was, as expected, targeted against PA, although a small subset (6.9%) also generated LF-specific antibodies. In contrast to the US vaccine, the UK vaccine contains both a significant amount of PA and LF, but individuals vaccinated with the UK vaccine typically develop an anti-LF response which is at least one log lower than their anti-PA response [34]. In a number of studies, monoclonal antibodies
Acknowledgements
This work was supported by funds from the National Institute of Allergy and Infectious Diseases (NIAID) through grant U19AI062629 and NCRR grant P20RR15577, OMRF J. Donald Capra Fellowship Support, and the OMRF Lou C. Kerr Chair in Biomedical Research. Local protocol development and management was supported by Walter Reed Army Medical Center Vaccine Healthcare Centers Network/Allergy-Immunology Department and Womack Army Medical Center, Fort Bragg Regional VHC.
The opinions and assertions
References (45)
- et al.
Interactions between anthrax toxin receptors and protective antigen
Curr Opin Microbiol
(2005) - et al.
Bacillus anthracis edema toxin causes extensive tissue lesions and rapid lethality in mice
Am J Pathol
(2005) - et al.
Effect of bacillus anthracis lethal toxin on human peripheral blood mononuclear cells
FEBS Lett
(2002) - et al.
Involvement of domain ii in toxicity of anthrax lethal factor
J Biol Chem
(2004) - et al.
Microbial metalloproteases and pathogenesis
Microbes Infect
(2000) - et al.
Characterization of the human immune response to the uk anthrax vaccine
FEMS Immunol Med Microbiol
(2004) - et al.
Immune response to two different dosing schedules of the anthrax vaccine precipitated (avp) vaccine
Vaccine
(2007) - et al.
Age-related changes in the immune response to influenza vaccination in a racially diverse, healthy elderly population
Vaccine
(2006) - et al.
Associations between snps in toll-like receptors and related intracellular signaling molecules and immune responses to measles vaccine: preliminary results
Vaccine
(2008) - et al.
Variations in measles vaccine-specific humoral immunity by polymorphisms in slam and cd46 measles virus receptors
J Allergy Clin Immunol
(2007)