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.