Immune responses of the bovine fetus and neonate to Escherichia coli: quantitation and qualitation of the humoral immune response.

The humoral immune responses of fetuses and neonates of Escherichia coli O:26:K60:NM were studied in 26 Angus-Hereford crossbred calves. Bacterin (5.0 X 10(10) organism) was injected in utero directly into the amniotic fluid of seventeen 7- to 8.5-month-old fetuses (principals). Saline solution was injected in the same manner into 9 control fetuses. Colostrum-deprived neonates were allotted to 10 groups and either were euthanatized at birth or were subjected to oral revaccination, challenge inoculation with the homologous organism, or both. The resistance to challenge exposure was a function of previous in utero injection of bacterin, age when challenged, and dose of challenge organisms used. Control calves were susceptible to only a large challenge dose, whereas almost all of the prinicipal calves were resistant. Revaccination of principal calves with bacterin at birth, exposure to the large challenge dose, or both, caused a marked increase in anti-O26 passive hemagglutination titers. Results of quantitative and qualitative radioimunossay indicated that the immune response to the O26 antigen was mainly of the immunoglobulin M (IgM) class, although there were also demonstrable changes in immunoglobulins (Ig) G1 and G2. The actively acquired immune responses were serotype specific, and there was no cross reactivity with 4 other E coli serotypes. An unidentified immunoprecipitate band was observed in immunoelectrophoretograms of whole bovine serum which may represent another class of Ig or which may be a subclass of IgG1 or IgG2.