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

Abstract

SUMMARY 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 postinoculation 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 IgGl titer increases were to the E2 proteins in tears and bal fluid and to the E3 protein in bal fluid. In serum, IgM to the N and E3 proteins appeared first, followed by IgGl to mainly the N and E2 proteins, and then more moderate and slower IgG2 and IgA responses. Challenge exposure resulted in an increase (or failure to decrease) in IgGl reactions to all bcv proteins; in IgG2 reactions to E2 and E3 proteins; in IgA reactions to E1, E2, and E3 proteins; and IgM reactions to the N protein only. The N protein elicited the greatest antibody responses, followed by the E2 and E3 proteins, and least by the E1 protein in serum, feces, or mucosal secretions. The E1 and E2 proteins elicited no detectable IgM responses in serum or mucosal secretions. Our findings indicate that there may be local antibody production at mucosal sites of viral replication, as well as antibody production at associated systemic sites resulting in serum antibodies. Immunoglobulin A antibodies on mucosal surfaces to some or all of the bcv proteins may be important in protection from bcv reinfection.