Isotype-specific antibody responses to bovine coronavirus structural proteins in serum, feces, and mucosal secretions from experimentally challenge-exposed colostrum-deprived calves

Abstract

Five newborn isolation-reared colostrum-deprived calves were inoculated orally and intranasally when they were 20 to 30 hours old and challenge exposed when they were 21 days old with a suspension of virulent bovine coronavirus (BCV). Blood, feces, nasal swabs, tears, saliva, and bronchoalveolar lavage (BAL) fluids were collected from each calf prior to inoculation and then weekly for 5 post-inoculation weeks. An ELISA was used to quantitate the immunoglobulin isotype titers of BCV antibodies in all samples. An immunoblot assay was used to determine the antibody isotype responses to BCV structural proteins in all the samples, except saliva. At postinoculation days 2 to 3, all calves had severe watery diarrhea, shed BCV in their feces, and had evidence of BCV replication in their upper respiratory tract. After challenge exposure, no calves became ill and no evidence of BCV replication in the respiratory or intestinal tracts was detected. At postinoculation week 1, IgM responses to the N protein were seen in mucosal secretions (except nasal fluid) and feces. At postinoculation weeks 2 and 3, IgA was predominant in mucosal secretions and feces directed toward all the BCV proteins (except the E2 protein in BAL fluid). After challenge exposure, an increase (or failure to decrease) in most IgA and some IgG1 titers to BCV proteins was seen. The increases in IgA titers were to all viral proteins in all mucosal secretions and feces, except to the N and E1 viral proteins in feces. The IgG1 titer increases were to the E2 proteins in tears and BAL fluid and to the E3 protein in BAL fluid.(ABSTRACT TRUNCATED AT 250 WORDS)