CASEIN EFFECTS ON THE MYELOPEROXIDASE-MEDIATED OXYGEN-DEPENDENT BACTERICIDAL ACTIVITY OF BOVINE NEUTROPHILS

Effects of casein on the respiratory burst and bactericidal activity o f bovine neutrophils were studied in vitro using chemiluminescence mea surements of neutrophils and the myeloperoxidase (MPO)/peroxide/chlori de bactericidal system in neutrophil-free cultures. At physiological c oncentrations, casein inhibited the respiratory burst responses of uns timulated bovine neutrophils and those stimulated with opsonized zymos an or phorbol-12-myristate-13-acetate (PMA). The inhibition did not re sult from impaired activation of neutrophils by the phagocytic stimuli , reduced light transmission or cellular toxicity. Neutrophil respirat ory burst activity and the bactericidal activity associated with this phenomenon, i.e. the MPO/peroxide/chloride bactericidal system, were i nhibited by casein through the modification of the oxygen radical gene rating pathway or oxygen radical scavenging. It has long been known th at milk inhibits the bactericidal activity of neutrophils. However, th e previously undocumented mechanism of action of the major milk protei n requires further examination in order to determine whether the neutr ophil oxygen-dependent bactericidal pathway plays an important role in defending the lactating bovine udder against bacteria causing mastiti s.