Serological diagnosis of West Nile virus (WNV) infection is complicated by extensive antigenic crossreactivity with other closely related flaviviruses, such as St. Louis encephalitis virus.Here we describe a recombinant, bacterially expressed antigen equivalent to structural domain III of the WNV envelope protein that has allowed clear discrimination of antibody responses to WNV from those against other related flaviviruses in indirect enzyme-linked immunosorbent assays using standardized control antisera and field-collected samples.Since 1999, the United States has experienced annual epidemics of disease in humans and animals caused by West Nile virus (WNV) over an expanding geographical range. To date during 2003, WNV has been isolated in 46 states and the District of Columbia, and more than 8,500 cases of human disease, resulting in 199 deaths, have been reported (1). Outbreaks of WNV disease with neurological manifestations have also been reported in Eastern Europe, North Africa, and Israel since the mid-1990s (reviewed in reference 17). WNV is clearly an emerging and significant public health problem. Research priorities to limit the impact of WNV include the development of more-specific rapid diagnostic assays (5, 18).WNV is a member of the Japanese encephalitis (JE) virus group of the genus Flavivirus, family Flaviviridae; other members include Japanese encephalitis virus (JEV), found throughout Asia, St. Louis encephalitis virus (SLEV), found in the Americas, and Murray Valley encephalitis virus (MVEV), found in Australia and New Guinea. These viruses have a similar ecology and are antigenically related to WNV, and their cocirculation in several regions of the world has complicated the specific diagnosis of infections by these viruses in humans and other vertebrate hosts (10, 15). Cross-reactions in patients ultimately diagnosed with probable dengue virus infections have also been reported in evaluations of WNV testing assays (19).Previously we have reported the identification of WNVspecific neutralizing epitopes within structural domain III of the WNV envelope (E) protein (2). Earlier investigations with other flaviviruses have also reported the presence of virusspecific epitopes within this region of the E protein (e.g., see references 6, 22, and 25), and other authors have suggested the utility of domain III from dengue virus types 1 to 4 or JEV as antigens for specific serological diagnosis of infections with those flaviviruses (10, 24). These observations led us to investigate the utility of a recombinant, bacterially expressed domain III (r-EIII) antigen derived from the envelope protein of a North American WNV strain (385-99) for discrimination of WNV from other JE virus group infections.
MATERIALS AND METHODSExpression and purification of recombinant WNV E protein domain III. Regions corresponding to structural domain III of the WNV strain 385-99 E protein were reverse transcription-PCR amplified for cloning and expression either as a glutathione S-transferase (GST) fusion using the pGEX-2T syste...