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Antonietta M Lillo,1 Nileena Velappan,1 Julia M Kelliher,1 Austin J Watts,1 Samuel P Merriman,1 Grace Vuyisich,2 Laura M Lilley,2 Kent E Coombs,1 Tara Mastren,2 Munehiro Teshima,1 Benjamin W Stein,2 Gregory L Wagner,2 Srinivas Iyer,1 Andrew RM Bradbury,3 Jennifer Foster Harris,1 Armand E Dichosa,1 Stosh A Kozimor2 1Bioscience Division, Los Alamos National Laboratory, Los Alamos, NM, USA; 2Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM, USA; 3Specifica Inc., Santa Fe, NM, USACorrespondence: Antonietta M LilloUSA Department of Energy, In Care of Los Alamos National Laboratory, Bikini Atoll Road, SM-30, TA-43, Building 0001, Room 220, Los Alamos, NM 87545, USATel +1 505-606-0578Fax +1 505-665-9030Email alillo@lanl.govBackground: Yersinia pestis is a category A infective agent that causes bubonic, septicemic, and pneumonic plague. Notably, the acquisition of antimicrobial or multidrug resistance through natural or purposed means qualifies Y. pestis as a potential biothreat agent. Therefore, high-quality antibodies designed for accurate and sensitive Y. pestis diagnostics, and therapeutics potentiating or replacing traditional antibiotics are of utmost need for national security and public health preparedness.Methods: Here, we describe a set of human monoclonal immunoglobulins (IgG1s) targeting Y. pestis fraction 1 (F1) antigen, previously derived from in vitro evolution of a phage-display library of single-chain antibodies (scFv). We extensively characterized these antibodies and their effect on bacterial and mammalian cells via: ELISA, flow cytometry, mass spectrometry, spectroscopy, and various metabolic assays.Results: Two of our anti-F1 IgG (αF1Ig 2 and αF1Ig 8) stood out for high production yield, specificity, and stability. These two antibodies were additionally attractive in that they displayed picomolar affinity, did not compete when binding Y. pestis, and retained immunoreactivity upon chemical derivatization. Most importantly, these antibodies detected < 1,000 Y. pestis cells in sandwich ELISA, did not harm respiratory epithelial cells, induced Y. pestis agglutination at low concentration (350 nM), and caused apparent reduction in cell growth when radiolabeled at a nonagglutinating concentration (34 nM).Conclusion: These antibodies are amenable to the development of accurate and sensitive diagnostics and immuno/radioimmunotherapeutics.Keywords: immunodiagnostic, radioimmunotherapy, RIT, immunotherapy, radiolabeling, immunoantibiotic, lateral flow assay, LFA