Interactions of antibody with the capsular polypeptide of Bacillus anthracis

Abstract

Bacillus anthracis is the etiologic agent of anthrax. The bacterium is surrounded by a capsule entirely assembled from gamma (g)-linked D-glutamic acid (gDPGA). The polymer is produced immediately following spore germination and is a considerable obstacle to the immune system. Previous studies have indicated that monoclonal antibodies (mAbs) reactive with the bacterial capsule are effective reagents for antimicrobial therapy and immunodiagnosis; both applications have tremendous potential in the field of bio-defense. gDPGA is a unique antigen; some of its characteristics include high molecular weight, resistance to degradation, negative charge, and repeating subunits. The work presented in this dissertation was carried out to more fully understand how mAbs interact with gDPGA. We determined that binding may occur in a stereo-selective manner, despite the repetitious character of the polymer. Transfer of a murine IgG3 anti-gDPGA variable region to human IgG1-4 constant domains resulted in severe binding alterations. Affinity maturation of a gDPGA-reactive mAb F26G3 facilitated the assessment of subclass and binding attributes as independent variables. These studies highlighted the importance of capsular reactivity to protection. Finally, the function of murine IgG3-specific post-translational modifications were examined. Deglycosylation altered the fundamental capsular reactivity of murine IgG3. Together, the results indicate that mAb:gDPGA interactions are surprisingly complex. Murine IgG3 anti-gDPGA mAbs retain characteristics not observed in any other murine or human antibody; suggesting a purposeful mechanism to manage encapsulated pathogens.