The investigators adhered to the Guide for the Care and Use of Laboratory Animals when conducting research, using animals [28] . The United States Army Medical Research Institute of Infectious Diseases (USAMRIID) animal facilities and animal care and use program are accredited by the Association for Assessment and Accreditation of Laboratory Animal Care International. All infectious material and animals were handled in a maximum-containment biosafety level 4 facility at USAMRIID under standard operating conditions. Antibody purification. IgGl KZ52 was produced and purified as described by Parren et al. [29] and was .98% pure, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and contained ,1 IU of endotoxin/ml, as determined in a quantitative chromagenic Limulus amoebecyte lysate assay (BioWhittaker, Cambrex, http://www.cambrex.com).