Evaluation of the baculovirus-expressed S glycoprotein of transmissible gastroenteritis virus (TGEV) as antigen in a competition ELISA to differentiate porcine respiratory coronavirus from TGEV antibodies in pigs

The spike (S) glycoprotein of the Miller strain of transmissible gastroente ritis virus (TGEV) was recently cloned and expressed in baculovirus. The re combinant S protein was used as the coating antigen in a competition (block ing) enzyme-linked immunosorbent assay (ELISA) in combination with monoclon al antibodies to the S protein epitope A (conserved on TGEV and porcine res piratory coronavirus [PRCV]) or epitope D (present on TGEV only) to differe ntiate PRCV- from TGEV-induced antibodies. One set (set A) of 125 serum sam ples were collected at different times after inoculation of caesarean-deriv ed, colostrum-deprived (n = 52) and conventional young pigs (n = 73) with 1 of the 2 porcine coronaviruses or uninoculated negative controls (TGEV/PRC V/negative = 75/30/20). A second set (set B) of 63 serum samples originated from adult sows inoculated with PRCV and the recombinant TGEV S protein or with mock-protein control and then exposed to virulent TGEV after challeng e of their litters. Sera from set A were used to assess the accuracy indica tors (sensitivity, specificity, accuracy) of the fixed-cell blocking ELISA, which uses swine testicular cells infected with the M6 strain of TGEV as t he antigen source (ELISA 1) and the newly developed ELISA based on the reco mbinant S protein as antigen (ELISA 2). The sera from set B (adults) were t ested for comparison. The plaque reduction virus neutralization test was us ed as a confirmatory test for the presence of antibodies to TGEV/PRCV in th e test sera. The accuracy indicators for both ELISAs suggest that different ial diagnosis can be of practical use at least 3 weeks after inoculation by testing the dual (acute/convalescent) samples from each individual in conj unction with another confirmatory (virus neutralization) antibody assay to provide valid and complete differentiation information. Moreover, whereas E LISA 1 had 10-20% false positive results to epitope D for PRCV-infected pig s (set A samples), no false-positive results to epitope D occurred using EL ISA 2, indicating its greater specificity. The progression of seroresponses to the TGEV S protein epitopes A or D, as measured by the 2 ELISAs, was si milar for both sets (A and B) of samples. Differentiation between TGEV and PRCV antibodies (based on seroresponses to epitope D) was consistently meas ured after the third week of inoculation.