Insufficient Anthrax Lethal Toxin Neutralization Is Associated with Antibody Subclass and Domain Specificity in the Plasma of Anthrax-Vaccinated Individuals

Abstract

Anthrax vaccine adsorbed (AVA) is a significant line of defense against bioterrorist attack from Bacillus anthracis spores. However, in a subset of individuals, this vaccine may produce a suboptimal quantity of anti-protective antigen (PA), antibodies that are poorly neutralizing, and/or antibody titers that wane over time, necessitating annual boosters. To study individuals with such poor responses, we examine the properties of anti-PA in a subset of vaccinated individuals that make significant quantities of antibody but are still unable to neutralize toxin. In this cohort, characterized by poorly neutralizing antibody, we find that increased IgG4 to IgG1 subclass ratios, low antibody avidity, and insufficient antibody targeting domain 4 associate with improper neutralization. Thus, future vaccines and vaccination schedules should be formulated to improve these deficiencies.

Anthrax Vaccines in the 21st Century

“…In that context, it is reassuring that there is a highly positive correlation between anti-PA IgG titers and TNA activity [ 28 ]. Although this holds overall for combined, aggregate data, there is a differentiation among IgG classes and among the PA domains against which the antibodies are directed [ 29 ]. More specifically, it was found that (a) anti-PA antibodies with a high IgG4/IgG1 ratio are less potent on Lethal Toxin-Neutralizing Activity (LTNA), and (b) IgG1 antibodies directed against epitopes from PA domain 4 are more potent on LTNA [ 29 ].…”

“…Although this holds overall for combined, aggregate data, there is a differentiation among IgG classes and among the PA domains against which the antibodies are directed [ 29 ]. More specifically, it was found that (a) anti-PA antibodies with a high IgG4/IgG1 ratio are less potent on Lethal Toxin-Neutralizing Activity (LTNA), and (b) IgG1 antibodies directed against epitopes from PA domain 4 are more potent on LTNA [ 29 ]. These factors would naturally affect the ultimate protection of the vaccinee against anthrax exposure.…”

“…These factors would naturally affect the ultimate protection of the vaccinee against anthrax exposure. (a) With respect to IgG subclass distribution, it was found in a recent real-world study of 144 AVA vaccinees [ 29 ] that almost half of the cohort (47.9%) neutralized LTNA poorly, and almost a third (30.6%) neutralized LTNA very poorly; poor neutralizers had an IgG4/IgG1 > 1.5 ratio. This finding has an important implication in that IgG4 levels have been found to increase with more AVA doses [ 28 ], implying a potential gradual loss of protection over time under the standard booster schedule.…”

“…There has been substantial research on identifying PA-subunits with high immunogenicity as vaccine candidates [ 29 , 33 , 34 ]. Antibodies targeting PA domain 4 possess high TNA [ 29 , 33 ], thus qualifying well for that purpose.…”

“…There has been substantial research on identifying PA-subunits with high immunogenicity as vaccine candidates [ 29 , 33 , 34 ]. Antibodies targeting PA domain 4 possess high TNA [ 29 , 33 ], thus qualifying well for that purpose. Such “rationally designed vaccines” [ 33 ], composed of PA subunits inducing antibodies with high TNA, would be expected to be optimally effective in anthrax protection and superior to whole-PA-based vaccines (e.g., AVA), which induce antibodies that target, to a large extent, the 20 kDa PA20 region of the PA83 molecule, i.e., the inactive part of PA83, which is separated from it after cleavage by furin-like membrane proteases [ 35 , 36 ].…”