Effect of phagocytosed fat and casein on the intraphagosomal pH in bovine polymorphonuclear leukocytes.

Abstract Polymorphonuclear leukocytes (PMN) were isolated from bovine blood or milk and incubated with heat-killed yeast stained with pH indicator dyes. Observation of changes in the color of the phagocytosed yeast allowed a determination of the temporal and maximal depression in the pH of the newly formed phagosomes. Results indicated that the maximal depression of pH was to approximately 5.0 for PMN isolated from both sources, although cells from milk produced fewer (P < 0.01) phagosomes reaching pH 5.0 than cells from blood. The effect of casein or milk fat globules on the temporal depression in pH of phagosomes was determined by preincubating PMN from blood with isolated preparations of casein and milk fat globules. Results indicated that the prior ingestion of fat globules by PMN inhibited the subsequent depression of pH within phagosomes compared with untreated control cells (P < 0.05). Ingestion of casein micelles, however, had no effect (P > 0.05). The results are discussed with reference to previous reports that PMN in milk have reduced phagocytic ability and bactericidal properties compared with PMN in the systemic circulation.