Activation of canine neutrophils by platelet-activating factor

Neutrophils are essential for innate immunity as the first line of defence. Neutrophils act as phagocytic white blood cells to kill bacteria and other microorganisms. A strong respiratory burst of neutrophils, dependent on reactive oxygen species, is produced during phagocytosis. Platelet-activating factor (PAF) is a signalling molecule with several prominent roles in tissue injury, inflammation, and platelet aggregation. However, the detailed mechanisms and intracellular signalling pathways involved in PAF-mediated neutrophil activation remain unclear. Here, we investigated the effect of PAF on changes in calcium concentration ([Ca²⁺]_i) and oxygen radical (O₂^-) generation in activating canine neutrophils. We further evaluated these effects of PAF with inhibition of G protein-coupled receptors using the specific inhibitor suramin. Blood samples were collected from a total of five dogs and neutrophils were isolated. PAF stimulation of canine neutrophils caused an increase in [Ca²⁺]_i as well as the generation of O₂^-, and the PAF receptor was sensitive to suramin. The results suggested that PAF stimulation of canine neutrophils may cause Ca²⁺ influx from the endoplasmic reticulum into the cytoplasm (as the first wave) and then trigger store-operated Ca²⁺ entry (as the second wave), which is an important intracellular signal transduction pathway for neutrophil activation. Furthermore, O₂^- generation by PAF stimulation may depend on the intracellular signalling pathway, with increasing inositol trisphosphate levels and [Ca²⁺]_i via G protein-coupled receptors. The finding that PAF-activating platelet aggregation is involved in canine neutrophil activation suggests a close relationship between haemostasis and neutrophil activation in dogs, offering new insight into the response to infection.