In Vitro Production of Neutrophils Extracellular Traps Is Affected by the Lactational Stage of Dairy Cows

Simple Summary Polymorphonuclear leukocytes (PMN) are the first line and most abundant immune cells against infection. They kill bacteria via phagocytosis, degranulation, and the formation of neutrophil extracellular traps (NETs). The formation of NETs is always accompanied by the production of reactive oxygen species (ROS). We aimed to research the NETs and ROS formation capacity of PMN in different lactational stages of Holstein cows. We also validated a model to mimic infection and inflammation to study the NETs and ROS formation capacity in vitro. Results show that the basal NETs and ROS values of PMN harvested from peripartum and lactating cows were higher than those in nulliparous heifers. The in vitro stimulation, using inflammatory products, of PMN derived from nulliparous heifers increased the production of ROS and NETs. PMN isolated from peripartum and lactating cows are primed to produce NETs and ROS, and this potentially contributes to the cows’ unfavorable inflammatory and immune status at these critical periods in their lactation cycle. The basal NETs and ROS production is lower in nulliparous heifers. Thus, they are an excellent model to mimic and study fundamental aspects of the production of NETs and ROS in vitro. Abstract We aimed to research the neutrophil extracellular traps (NETs) and reactive oxygen species (ROS) formation capacity of polymorphonuclear cells (PMN) during different lactational stages of Holstein cows. We also aimed to validate a model which could mimic infection and inflammation in vitro by adding increasing concentrations of lipopolysaccharide (LPS) and phorbol 12-myristate 13-acetate (PMA) to PMN suspensions isolated from nulliparous heifers and evaluate their capacity to produce NETs and ROS. In 3 replicates, we collected blood from nulliparous heifers (n = 3), cows at the end of gestation (n = 3), early postpartum (n = 3) and in mid-lactation (n = 3) in which PMN were isolated. The production of ROS in PMN were assessed using the 2’,7’-Dichlorofluorescein diacetate method, while the SYTOX Orange and Quant-iT™ PicoGreen dsDNA ultra-sensitive nucleic fluorescent acid staining methods were applied in order to quantitatively analyze the formation of NETs. Statistical analyses were performed via linear regression models using the replicate as a random. ROS values of PMN harvested from peripartum cows were 1.3 times increased compared with those in nulliparous heifers (p < 0.01). Compared with nulliparous heifers, the production of NETs by PMN isolated from mid-lactation and postpartum cows was 2.1 and 2.5 times higher (p < 0.01), respectively. In 3 replicates, in vitro stimulation of PMN isolated from nulliparous heifers (n = 3) with LPS linearly increased the production of ROS and NETs (R2 = 0.96 and 0.86, respectively). Similarly, when PMN isolated from nulliparous heifers were stimulated with PMA, a linear increase in the production of ROS (R2 = 0.99) and NETs (R2 = 0.78) was observed. The basal NETs and ROS production is lower in nulliparous heifers. Thus, they are an excellent model to mimic inflammation and study fundamental aspects of the production of NETs and ROS in vitro.