This thesis describes the characterization of antibody-mediated immunity against two pathogens -- Cryptococcus neoformans and Bacillus anthracis. The work is based on the generation and isolation of polyclonal sera that target galactoxylomannan (GalXM), a capsular polysaccharide of C. neoformans, and a novel library of monoclonal antibodies (mAbs) that target protective antigen (PA), one of the toxin components of B. anthracis. In Chapter 2, we determine the strain-related difference in the physical and antigenic properties between the GalXMs isolated from C. neoformans var. neoformans and C. neoformans var. grubii. In Chapter 3, we describe the generation of two GalXM-protein conjugates and evaluate their immunogenicity. In Chapter 4, we review and discuss the mAb studies for 6 toxins, analyzing the prevalence of mAb functions and isotypes. From those studies we determine that only a small fraction of mAbs isolated possess neutralizing activity against toxins. In Chapter 5, we describe a novel phenomenon that combining individually characterized toxin-enhancing mAb with protective mAb significantly augments the protection against anthrax. We provide a mechanism that explains this unexpected finding, and reveal that antibody combinations demonstrate emergent properties that cannot be predicted by and are not reducible to the effects of the individual components. Finally, in Chapter 6, the thesis concludes with a summarization of the results and their significance, as well as their implications for future studies. In summary, this thesis has illustrated the complexity of antibody-mediated immunity against two different pathogens, and the challenge to generate the tools to answer the fundamental questions about antibody-antigen interactions. It is anticipated that the findings in this thesis will provide insights for improving conjugation protocols and generating mAb therapeutics against cryptococcosis and anthrax.
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