Evidence that calf bronchopneumonia may be accompanied by increased sialylation of circulating immune complexes' IgG

Immune complexes (IC) could have an important role in the pathogenesis of pre-ruminant calves' bronchopneumonia. IC are potent activators of complement and neutrophils and they might be responsible for immune protection, as well as for pulmonary damage. Immunoglobulin G (IgG), as constituents of IC, initiates the effector phase of immune response through binding of Fcγ and complement receptors. The oligosaccharide moieties expressed on IgG can modulate their antigen affinity and effector function. Structural characteristics of IgG molecules from IC in the pre-ruminant calves have not been studied in detail. The aim of our study was to determine if the glycosylation profile of IgG from circulating IC (CIC) in calves with bronchopneumonia differed from those of healthy control calves. A total number of 13 Holstein-Friesian calves, at the age of three months were included in the study. All calves were clinically examined by a veterinarian. Calves were classified by signs of respiratory disease in two groups: healthy (n=6) and diseased (n=7) calves. The CIC from calves' sera were isolated by the polyethylene glycol precipitation (PEG) method. IgG molecules were isolated from PEG precipitates by Protein G affinity method. The level of expression and localization N-acetylglucosamine, galactose, sialic acid, and fucose within the isolated IgG was determined by lectin blot assay. Calves with bronchopneumonia had a statistically significantly increased level of CIC. IgG molecules were isolated from CIC of both healthy and diseased calves. Several other proteins in complex with IgG were detected in both groups of animals. The isolated IgG heavy chains of healthy calves expressed N-acetylglucosamine, galactose, sialic acid, and fucose. The light chains of IgG expressed N-acetylglucosamine, sialic acid, and fucose whereas galactose was not detected in healthy calves. In diseased animals, galactose was detected on light chains, and both heavy and light IgG chains were more sialylated. Proteins in complex with IgG were also lectin reactive, and their glycosylation in diseased animals was different compared to healthy controls. Increased sialylation is a characteristic of anti-inflammatory IgG. The increased sialylation of IgG from CIC in bronchopneumonia might be an attempt of immune system of calves to protect lung tissues against damages provoked by activated cells and secreted pro-inflammatory cytokines. At the same time, increased IgG sialylation could explain the inability of calves' immune system to initiate the process of antigen elimination by activation of Fcγ receptors.