Development and partial validation of a recombinant E2-based indirect ELISA for detection of specific IgM antibody responses against classical swine fever virus

• Recombinant E2 protein of CSFV was successfully expressed in Escherichia coli.
• This is the first report of development of ELISA for detection of IgM antibodies against CSFV.
• IgM and IgG antibody responses were detected in HCLV vaccinated pigs.

Detecting classical swine fever virus specific antibody responses is critical for prevention and control of CSF. In this study, a ΔE2-based indirect ELISA was developed to detect specific IgM antibodies against CSFV. The optimized conditions that were determined experimentally are: a ΔE2 antigen concentration of 0.5 μg/ml, a serum sample dilution of 1/100 incubated at 37 °C for 1.5 h, and a HRP conjugated rabbit anti-pig IgM dilution of 1/50,000 incubated at 37 °C for 1 h. Three hundred clinical sera were tested with ΔE2-IgM-ELISA and IDEXX ELISA and the positive rates were 77.3% (232/300) and 71.7% (215/300), respectively. Concordance rate between them was 80.3% (241/300). The 59 inconsistent sera were tested further: among the 21 IDEXX ELISA +/ΔE2-IgM-ELISA − and 38 IDEXX ELISA +/ΔE2-IgM-ELISA − samples, 17 and 24 were determined positive by virus neutralization test; 15 and 25 were tested positive by ΔE2-IgG-ELISA, respectively. In addition, the E2-specific IgM antibody response in 15 vaccinated piglets could be detected 2 weeks post-vaccination and earlier than specific IgG antibody. It increased regularly and reached high levels by 6 weeks post-vaccination. The ΔE2-IgM-ELISA could be used for clinical detection and for exploring the kinetics of IgM antibody response.