Responding to the threat of bioterrorism we have continued to identify and characterize neutralizing monoclonal antibodies (mAbs) to the following biodefense-related microbes which are on the NIAID list of category A or C pathogens: Yersinia pestis (which causes plague, category A), Nipah and Hendra viruses (Henipaviruses) (causing acute infections with high, up to 70%, mortality, category C), Ebola and Marburg viruses (causing hemorrhagic fever with high, up to 90%, mortality, category A), Crimean-Congo virus (CCHFV) (causing hemorrhagic fever with relatively high, up to 50%, mortality, category C), dengue virus (which causes dengue hemorrhagic fever with relatively low mortality, category A), and SARS CoV (causing acute infections with relatively low mortality, category C). Previously, we reported the isolation of henipavirus-neutralizing recombinant hmAbs including one, m102.4, which exhibited exceptionally potent and cross-reactive inhibitory activity against both HeV and NiV. The crystal structure of a variant of this antibody, m102.3, was solved in complex with Hendra virus glycoprotein. This antibody binds to a highly conserved receptor binding site which explains its breadth of neutralization. These results further confirm our proposition that m102.4 has potential as a therapeutic for treatment of diseases caused by henipaviruses, and could save human lives now. It could be also used for prophylaxis, diagnosis and as a research reagent. Animal studies with this antibody continue and more are planned. We have identified several novel potent hmAbs against Yersinia pestis these are the first fully human antibodies against this category A agent. Yersinia pestis is the etiologic agent of plague that has killed more than 200 million people throughout the recorded history of mankind. The occurrence rate is low but persistent with a fatality rate of approximately 10%. Plague has established stable enzootic foci worldwide except in Australia. Present treatment measure only includes antibiotics in the United States and throughout most of the world. Two of the virulent factors, capsid protein F1 and low-calcium response antigen V (LcrV) are proven targets for both active and passive immunization. Current plague vaccines being considered consist of F1 and V subunit proteins and are in human clinical trials. There are mouse monoclonal antibodies against both F1 and V that are passively protective in a mouse model of plague. No human anti-plague monoclonal antibodies are available for prophylactic or therapeutic purposes. In addition, although efficient in reducing the bacterial load, antibiotics may provide little immediate relief to patients who have a high toxin level in their systems. We selected from a human Fab library one antibody against the F1 protein and two antibodies against the V-antigen. These antibodies showed protective effect against Yersinia pestis challenge in a mouse model. The highest protection was achieved when the three antibodies were combined, suggesting a synergistic effect. Multiple dosing also enhanced the protective effect. The two antibodies against V did not compete with each other in binding, providing the mechanistic basis for simultaneous usage. Furthermore, the newly identified human anti-F1 and -V antibodies recognize epitopes that differ from the neutralizing epitopes previously identified using the mouse monoclonal antibodies, suggesting the existence of additional neutralizing epitopes. Kinetic studies indicated that the anti-F1 and V antibodies have a serum half-life of approximately one week, similar to that of other human monoclonal antibodies in mice. These antibodies could have potential as therapeutics and prophylactics against Yersinia pestis. We have also performed preliminary experiment to identify neutralizing hmAbs against dengue virus and Ebola virus. Experiments are ongoing to develop better antigens and identify novel antibodies against these and other microbes of biodefense importance.
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