Casein effects on the myeloperoxidase-mediated oxygen-dependent bactericidal activity of bovine neutrophils

Effects of casein on the respiratory burst and bactericidal activity of bovine neutrophils were studied in vitro using chemiluminescence measurements of neutrophils and the myeloperoxidase (MPO)/peroxide/chloride bactericidal system in neutrophil-free cultures. At physiological concentrations, casein inhibited the respiratory burst responses of unstimulated bovine neutrophils and those stimulated with opsonized zymosan or phorbol-12-myristate-13-acetate (PMA). The inhibition did not result from impaired activation of neutrophils by the phagocytic stimuli, reduced light transmission or cellular toxicity. Neutrophil respiratory burst activity and the bactericidal activity associated with this phenomenon, i.e. the MPO/peroxide/chloride bactericidal system, were inhibited by casein through the modification of the oxygen radical generating pathway or oxygen radical scavenging. It has long been known that milk inhibits the bactericidal activity of neutrophils. However, the previously undocumented mechanism of action of the major milk protein requires further examination in order to determine whether the neutrophil oxygen-dependent bactericidal pathway plays an important role in defending the lactating bovine udder against bacteria causing mastitis.