Noninfectious recombinant antigen for detection of St. Louis encephalitis virus-specific antibodies in serum by enzyme-linked immunosorbent assay

Proper surveillance of virus activity and a timely response to viral outbreaks depend upon the rapid diagnosis of viral infections. The immunoglobulin M (IgM) antibody-capture enzyme-linked immunosorbent assay (MAC-ELISA) is a fast, sensitive test routinely used for the diagnosis of the medically important West Nile and St. Louis encephalitis flaviviruses. However, the suckling mouse brain-derived (SMB) antigen used in this assay is tedious to prepare and has a risk of exposing personnel to live virus and hazardous chemicals. We report the development of a St. Louis encephalitis virus (SLEV) noninfectious recombinant antigen that is a safe and easily produced alternative antigen for use in diagnostic assays. The expression plasmid pCB8SJ2, containing the premembrane and envelope structural protein-encoding regions of SLEV, was constructed to express secreted extracellular virus-like particles (VLPs) from CHO cells. Blind-coded human serum panels were assembled from patients having recent SLEV, West Nile virus (WNV), Powassan virus, or La Crosse encephalitis virus infections to assess the sensitivity and specificity of assays with SLEV VLP or SMB antigen. MAC-ELISAs with either antigen had comparable sensitivity for the detection of IgM antibodies against SLEV. Importantly, when these two antigens were tested against a human serum panel from patients having recent WNV or Powassan virus infections, the SLEV VLPs were less likely than SMB antigen to detect flavivirus cross-reactive IgM antibodies. An optimized IgG antibody capture ELISA (GAC-ELISA) with both WNV and SLEV VLPs was developed to circumvent the frequently observed higher background in the antigen-capture IgG-ELISA (ACG-ELISA). For the detection of IgG antibodies against WNV, the GAC-ELISA resulted in a statistically significant higher performance accuracy (P = 0.003) than the ACG-ELISA when the WNV VLP antigen was used in both assays. However, no statistical difference was observed in the assay performance of the GAC-ELISA with SLEV VLP or the ACG-ELISA with SLEV SMB antigen.