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.