Distinct neutrophil C5a receptor inflammatory events in cows initiated by chemoattractant C5a and lipopolysaccharide around parturition and in mid lactation

Abstract

In neutrophils, toll-like receptor and complement component 5a (C5a) signaling are critical pathways regulating innate immunity. In cows, not much is known about the second C5a receptor, complement component 5a receptor 2 (C5AR2). It is an interesting player in sepsis treatment because it is considered to have an anti-inflammatory effect during normal inflammation. Periparturient cows are prone to severe infections, and the objectives of this study were to investigate the expression and functionality of C5AR2 during peripartum. We investigated the effect of 2 major inflammatory stimuli, C5a and lipopolysaccharide (LPS), on the expression of a selected number of genes (C5AR1, C5AR2, TLR4, ITGAM, COX2, and CXCL8) and functions linked to these receptors. Overall, TLR4, ITGAM, and C5AR2, all of which are involved in early inflammation, showed a lower expression in periparturient cows. However, an overall lower expression seems not to be the only explanation for the increased risk of sepsis in periparturient cows. Normally, in response to inflammation and as seen in the mid-lactation group, the expression of these genes increases after stimulation with LPS. However, in periparturient cows, stimulation with LPS led to a decrease in expression of these receptors, indicating a different response of neutrophils in response to LPS during this period. A decrease in ITGAM (coding for CD11b) expression complicates correct neutrophil localization and phagocytosis. Its downregulation upon stimulation might be detrimental for adequate eradication of the pathogen and might increase the risk of an imbalanced inflammation; C5AR2 seems to play a central role in this altered response. In addition, myeloperoxidase (MPO) activity in periparturient cows is lower in response to C5a stimulation. It has been suggested that MPO plays an important role in neutrophil shutdown and, thereby, timely resolution of inflammation. A decreased MPO activity might thus prolong the inflammatory reaction of the neutrophils. This finding was supported by the increased viability of the neutrophils obtained from periparturient cows. Even after stimulation, we found a lower caspase-3 activity in this group, indicating that they might be activated for a longer time compared with the neutrophils from mid-lactation cows. Accordingly, these alterations might contribute to a temporal mismatch in inflammatory responses, as often seen in severe periparturient infections.