Opsonophagocytosis versus lectinophagocytosis in human milk macrophages.

Some important immunoprotective effects of human breast milk have been attributed to the presence of macrophages. We investigated the generation of superoxide anion (O2-) by monocytes and human milk macrophages after stimulation with opsonized and unopsonized zymosan in the absence and presence of mannose as an inhibitor to investigate lectinophagocytic and opsonophagocytic properties. Peripheral blood monocytes generated more O2- than human milk macrophages (417,4 + 79,1 nmol O2-/mg protein vs. 216,1 +/-15,1 nmol O2-/mg protein, p<0,05) after stimulation with opsonized zymosan. When unopsonized zymosan was used as a serum-independent stimulus monocytes generated slightly less O2- in comparison to human milk macrophages (150,8 +/- 34,5 nmol/mg protein vs. 176,1 +/- 18 nmol O2-/mg protein, p<0,05). These findings demonstrate that the proportion of opsonin-independent phagocytosis in human milk macrophages is higher than in monocytes (82% vs. 36%). When mannose was used as an inhibitor a significantly higher reduction of O2- generation occurred in human milk macrophages compared to monocytes stimulated with opsonized zymosan, whereas no difference was found when unopsonized zymosan was used. These results indicate that human milk macrophages are stimulated to a greater extent by opsonin-independent mechanisms than blood borne monocytes. As the colostrum and the intestinal environment of the neonate offers only a little amount of opsonins like complement and immunoglobulin G, such a differentiation to lectinophagocytic properties could bear a great advantage for protective functions of human milk macrophages.

Superoxide anion generation in human milk macrophages: opsonin-dependent versus opsonin-independent stimulation compared with blood monocytes.

Macrophages are believed to play an important role within the immunoprotective effects of human breast milk. It was the purpose of this study to evaluate the capability of human milk macrophages (MMPhi) to generate superoxide anions (O2(-)) in comparison with peripheral blood monocytes (BMo) after stimulation with opsonized and unopsonized zymosan. Potential inhibitors of attachment and phagocytosis such as mannose and cytochalasin B were used. Expression of the mannose receptor on MMPhi was demonstrated by staining with MAb. BMo generated more O2(-) than MMPhi (417 +/- 79 versus 216 +/- 15 nmol O2(-)/mg protein, p < 0.05) after stimulation with opsonized zymosan. When unopsonized zymosan was used as a serum-independent stimulus, BMo generated slightly less O2(-) in comparison with MMPhi (150 +/- 34 versus 176 +/- 18 nmol O2(-)/mg protein, p < 0.05). These findings imply a higher proportion of opsonin-independent phagocytosis in MMPhi than in BMo (82 versus 36 %). Preincubation with mannose resulted in a significantly higher reduction of O2(-) generation in MMPhi compared with BMo stimulated with opsonized zymosan, whereas no difference was found when unopsonized zymosan was used. After addition of cytochalasin B, equal inhibition of O2(-) generation was observed regardless of the cell type or stimulus used. Thus, MMPhi are stimulated to a greater extent by serum-independent mechanisms than BMo. As opsonins like complement or IgG are rare in the colostrum and the neonatal intestinal environment, such a differentiation toward serum-independent phagocytic abilities could play an important role for protective functions of human MMPhi. Possible involvement of the mannose receptor and the beta-glucan receptor in this specialization are discussed.