Antibodies against anthrax: mechanisms of action and clinical applications

Abstract

B. anthracis is a bioweapon of primary importance and its pathogenicity depends on its lethal and edema toxins, which belong to the A-B model of bacterial toxins, and on its capsule. These toxins are secreted early in the course of the anthrax disease and for this reason antibiotics must be administered early, in addition to other limitations. Antibodies (Abs) may however neutralize those toxins and target this capsule to improve anthrax treatment, and many Abs have been developed in that perspective. These Abs act at various steps of the cell intoxication and their mechanisms of action are detailed in the present review, presented in correlation with structural and functional data. The potential for clinical application is discussed for Abs targeting each step of entry, with four of these molecules already advancing to clinical trials. Paradoxically, certain Abs may also enhance the lethal toxin activity and this aspect will also be presented. The unique paradigm of Abs neutralizing anthrax toxins thus exemplifies how they may act to neutralize A-B toxins and, more generally, be active against infectious diseases.

Keywords: anthrax toxins; antibodies; edema factor; lethal factor; protective antigen.

Figure 1

Structures of protective antigen (PA), lethal factor (LF) and edema factor (EF) subunits. PA structure has been obtained using file 1acc from the Protein Data Bank [6]. LF and EF structures are derived from the files 1j7n [7] and 1xfv [8], respectively. For each subunit, the different domains are identified on ribbon models and their respective function is indicated. The color code utilized on ribbon models was re-utilized for schematic rendering.

Figure 2

The different steps of anthrax toxins entry, and their inhibition by antibodies. (A) Various steps of anthrax toxins entry. PA₈₃ binds to its cell receptors and is processed by furin on the cell surface. PA₂₀ is released and PA₆₃ remains attached to the receptor. Heptamerization of PA₆₃ induces the formation of LF/EF binding site. The toxin complex is then endocytosed. (B) Inhibition of the various steps of anthrax toxins entry by Abs. Neutralizing Abs act at each entry step: binding of PA₈₃ to its receptors (1), PA₈₃ cleavage by furin (2), PA₂₀ release (3), PA₆₃ heptamerization (4), LF/EF binding to the heptamer by targeting PA (5) or LF/EF (6), and endocytosis of the toxin (7).

Figure 3

Neutralizing epitopes inhibiting PA-ATRs interactions compared with anthrax toxin receptors (ATRs) binding site. (A) Three-dimensional localization of the neutralizing epitopes on PA. PA structure has been obtained using file 1acc from the Protein Data Bank [6] and was colored in grey. Residues of the four PA loops constituting the ATRs binding site, residues 340-348 (part of domain II), residues 654-662, 681-688, and 714-716 (part of domain IV) were colored in red. The core of the four well-defined PA neutralizing epitopes were delimited in blue for 14B7, in green for 35PA₈₃, in purple for 1-F1 and in yellow for 2-B12. After Ab binding to the core epitope, the interface between Ab and PA should cover a larger surface. (B) Localization of the neutralizing epitopes on PA sequence. Localization of the four epitopes was given as sequence alignments; residues constituting ATRs binding site were colored in red on PA sequence.