Gastrointestinal antibody responses in axenic mice to topically administered Escherichia coli.

Immunoglobulin levels were determined in gastrointestinal secretions of both conventional and axenic mice, as was the stability of the immunoglobulin classes in these secretions. Axenic animals were then administered nonviable Escherichia coli O111:B4 by topical application into the oral cavity. Immunoglobulin (Ig) A, IgG1, AND IgG2 were detected in the gastrointestinal secretions, with an apparent gradation in stability (IgA greater than IgG1 greater than IgG2) under the conditions investigated. Specific antibodies (IgA and IgG) to both the somatic and the flagellar antigens of E. coli were demonstrated. The time course of the responses to the polysaccharide and protein antigens suggested that a secondary response to lipolysaccharide was being observed in the secretions. Examination of the autoclaved diet fed to the germfree animals indicated the presence of antigenically intact polysaccharide materials (i.e., lipopolysaccharides), whereas antigenic bacterial proteins were uniformly undetectable. Therefore, endotoxin in the diet was apparently causing a primary type of sensitization in the mice, whereas the active oral immunization regimen induced a secondary antibacterial response in the secretions.