Complement biosynthesis in human breast-milk macrophages and blood monocytes

The availability of techniques for establishment of primary, long term monolayers of breast milk macrophages and blood monocytes permitted a direct comparison of biosynthetic functions of human tissue macrophage and its progenitor. In addition to previously described morphological differences, four specific characteristics of the breast milk macrophage were identified: (i) a reduced rate of total protein secretion relative to the monocyte; (ii) abrogation of the 3 day lag in complement secretion regularly observed in blood monocyte cultures; (iii) an increase in the secretion of complement proteins C2 and factor B; and (iv) an increase in the ratio of C2 : factor B secretion. These differences did not result from cell-cell interactions between monocytes and macrophages, stable factors elucidated by monocyte or macrophage monolayers, or heat-stable factors in breast milk. In addition, both monocytes and macrophages synthesized and secreted C3 in an apparently native but haemolytically inactive form. The differences observed suggest that macrophages may modulate local availability of complement proteins in tissues or in the early phase of an inflammatory response.