Human milk effects on neutrophil calcium metabolism: Blockade of calcium influx after agonist stimulation

Neutrophils are the predominant cellular mediators of acute inflammation, a nd human milk suppresses multiple neutrophil functions. We sought to determ ine whether these effects were mediated through disruption of normal intrac ellular Ca2+ homeostasis. Exposure of human neutrophils to human milk, foll owed by washing, resulted in altered Ca2+ transient responses to formyl-pep tide stimulation in which the peak cytosolic free Ca2+ concentration ([free Ca]) was the same as in unexposed cells, but the postpeak decline in [free Ca] was more rapid. This effect was observed after human milk exposures as brief as 10 s, persisted for up to 4 h after human milk removal, and was c oncentration dependent. On the basis of experiments examining Ca2+-free con ditions followed by Ca2+ supplementation, and experiments examining spontan eous and stimulated manganese and barium influx into neutrophils, the human milk effect was due to blockade of Ca2+ influx. Decreased Ca2+ transient r esponses to other physiologic stimuli (IL-8, opsonized Staphylococcus aureu s, and immune complexes) were observed after human milk exposures. Rat inte stinal epithelial cells and HL-60 cells failed to show these effects, sugge sting a selective effect on mature inflammatory cells. Characterization of the Ca2+-blocking activity showed it was heat and acid stable in human milk with a molecular mass between 30-100 kD. Commercial human milk lactoferrin exhibited Ca2+ influx blockade activity, but recombinant human lactoferrin showed none. Separation of the activity by heparin affinity chromatography showed that it was distinct from lactoferrin. Human milk-induced blockade of Ca2+ influx provides a potential mechanism for broad suppression-of neut rophil functions that may contribute to the antiinflammatory properties of human milk.