Increase in apoptotic polymorphonuclear neutrophils in peripheral blood after intramammary infusion of Escherichia coli lipopolysaccharide.

A transient increase in apoptotic polymorphonuclear neutrophils (PMNs) as revealed by the terminal deoxynucleotidyl, transferase-mediated dUTP nick end labeling (TUNEL) technique in bovine jugular and milk vein blood was observed 4 h after intramammary infusion of Escherichia coli lipopolysaccharide (LPS) (jugular vein; before infusion 10.1%, 4h 58.3%: milk vein; before infusion 13.2%, 4 h 76.6%) decrease in PMA-induced oxidative bursts of PMNs was also observed during the same period and continued until 8 h after the infusion. TUNEL-positive cells showed an intention of a Comet tail as detected by a single-cell gel electrophoresis assay (Comet assay) and the morphological apoptotic future, though DNA fragmentation was not clearly detected. A definite decrease in peripheral PMNs and a marked increase in PMNs in the LPS-infused teat cistern were observed during the same period. The migration of milk vein blood-derived PMN and the expression of adhesion receptors (L-selectin and CD18) on PMN were suppressed, accompanied by an increase in apoptotic cells. TUNEL-positive PMN observed in normal animals showed a reduced migration capacity. The increase in apoptotic PMNs observed in the LPS-infused cattle was thought to be due to the remaining intravenous spontaneous apoptotic cells existing under the normal condition (the aging cell), and this increase appeared to lower the expression of adhesion receptors and the migration capacity. Decreased PMA-induced oxidative burst activity in PMN was thought to be derived from these aging cells and immature band cells appearing in the circulation as a subsequent event of leukopenia and/or severe stress associated with mastitis. The results from the present study indicate the possibility that the function of PMN in the circulation at early stages of bovine mastitis is regulated by the kinetics of PMN aging.