Abstract • 16

Human milk macrophages are supposed to play an important role in the protection of the newborn against infection. We investigated the generation of superoxide anion (O2-) by monocytes and human milk macrophages after stimulation with opsonized and unopsonized zymosan.

Generally 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 stimulus monocytes generated less O2- than human milk macrophages (150,8 ±34,5 nmol / mg protein vs. 176,1 ±18 nmol O2- / mg protein, p<0,05). This shows 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. After addition of cytochalasin B equal inhibition of O2- generation was observed regardless of the cell type or stimulus used.

These results indicate that human milk macrophages are stimulated better by serum-independent mechanisms than monocytes, what could make sense since e.g. complement and immunoglobulin G are present in the gut of the neonate only in small amounts.