Competition ELISA, using monoclonal antibodies to the transmissible gastroenteritis antibodies to the transmissible gastroenteritis virus (TGEV) S protein, for serologic differentiation of pigs infected with TGEV or porcine respiratory coronavirus

Monoclonal antibodies (MAB) to subsite A (25C9) and subsite D (44C11) of the S protein of transmissible gastroenteritis virus (TGEV) were used in a blocking ELISA on fixed TGEV-infected swine testis cells to differentiate sera from pigs experimentally inoculated with either TGEV or porcine respiratory coronavirus (PRCV). Serum samples were obtained from pigs at various intervals from postinoculation day (PID) 0 through at least PID 22 to 40. Eleven-day-old pigs, seronegative for TGEV-neutralizing antibodies at the time of inoculation, were inoculated orally and nasally with either the virulent Miller (M5C) strain or the attenuated Purdue (P115) strain of TGEV, or with the ISU-1 strain of PRCV. Gastroenteritis was observed in 100% of the M5C-TGEV-inoculated pigs; but clinical signs of disease were not observed in either the P115-TGEV- or PRCV-inoculated pigs. Virus-neutralization (VN) antibody titer in sera was determined by use of a plaque-reduction assay. Blocking ELISA antibody titer for subsites A and D was determined from the serum dilution that produced 50% reduction in the absorbance values when it competed with biotinylated MAB 25C9 and 44C11, respectively. In sera from the inoculated pigs, the VN antibody titer began to increase by PID 7 and reached maximum by PID 15 to 16. For pigs inoculated with TGEV M5C, subsite A and subsite D blocking antibody titers in the serum paralleled the VN antibody titer, began to increase after PID 7, and reached maximum by PID 15 to 16. The blocking antibody titer to subsites A and D began to increase in the P115-TGEV-inoculated pigs after PID 15 to 16 and reached maximum by PID 22 to 26. Blocking antibody titer to subsite A in PRCV-inoculated pigs behaved similarly to blocking antibody titer to subsite A in the M5C-TGEV-inoculated pigs, reaching maximum by PID 15 to 16; however, blocking antibody titer was not detected for subsite D up to PID 24 (the latest time point examined) in sera from the PRCV-inoculated pigs. Serum antibody responses and clinical signs of disease were monitored in pigs initially inoculated with either M5C-TGEV or -PRCV and challenge-exposed with M5C-TGEV on PID 24. Clinical signs of gastroenteritis were not observed in the M5C-TGEV-inoculated pigs after challenge-exposure with M5C-TGEV. Low increases in VN antibody titer and in subsite A or D blocking antibody titer were detected in the M5C-TGEV-inoculated and challenge-exposed pigs. Of the 12 pigs initially inoculated with PRCV then challenge-exposed with M5C-TGEV, 5 pigs developed diarrhea; the VN and subsite A antibody blocking titers began to increase by postchallenge-exposure day (PCD) 2 and reached maximal titer by PCD 9, increasing approximately 100-fold above the prechallenge-exposure titer. Subsite D antibody-blocking titer began to appear after PCD 9 and, by PCD 12, had reached nearly the same level as that for the primary response to the M5C-TGEV inoculation.