Structural characteristics of circulating immune complexes in calves with bronchopneumonia: Impact on the quiescent leukocytes

Calf bronchopneumonia is accompanied by increased level of circulating immune complexes (CIC), and we analysed size, and protein and lipid constituents of these CIC with an attempt to elucidate the connection between the CIC structural properties and their capacity to modulate leukocyte function. CIC of heathy calves (CICH) and calves with naturally occurring bronchopneumonia (CICD) were isolated by PEG precipitation and analysed by electrophoresis and chromatography. The predominant CIC proteins were IgG, albumin, and transferrin. Affinity isolated serum and CIC IgG coprecipitated several proteins, but only 75 and 80 kDa proteins bound CIC IgG, exclusively. 60 and 65 kDa proteins co-precipitated with CICD IgG, unlike CICH IgG. In both CICH and CICD, oleic acid-containing phospholipids predominated. In CICD, the content of oleic and vaccenic acid was higher than in CICH, while myristic, palmitic, stearic, linoleic and arachidonic acid showed lower content. Dynamic light scattering displayed difference in particle size distribution between CICH and CICD; 1280 nm large particles were present only in CICD. The effect of CICH and CICD on mononuclear cells (MNC) and granulocytes was analysed in vitro. CICH and CICD, with slight difference in intensity, stimulate MNC apoptosis, promote cell cycle arrest of unstimulated MNC, and cell cycle progression of PHA stimulated MNC. Both CIC reduced granulocyte apoptosis after 24 h while after 48 h this effect was detected for CICD only. These results indicate that structural differences of CICH and CICD might interfere with the CIC functional capacity, which we consider important for evaluation of CIC immunoregulatory function.