The Milk Mononuclear Phagocyte

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Abstract
A large number of mononuclear phagocytes are present in human colostrum and milk and are transferred from mother to baby. Although one can only speculate on their role in vivo, there is a modest amount of information about their characteristics in vitro which will be reviewed in this paper. About 80% of the one to two million leukocytes in colostrum and early human milk are mononuclear phagocytes1,2 as determined by esterase staining,3 latex ingestion, and glass adherence. While See Table in the PDF Document their concentration diminishes with longer lactation, the number of macrophages secreted daily is large (Table 1). The milk mononuclear phagocyte spreads avidly onto glass surfaces extending long filamentous processes into the environment (Fig 1). Trypan blue is rapidly taken up into the lysozomes of living cells (Fig 2) as is acridine orange. These cells are lipid laden (Fig 3) and on ultrastructural analysis one sees that the lipid inclusions are not always membrane bound (Fig 4). Mitochondria, an indication of oxidative metabolism, and rough and smooth endoplasmic reticulum, suggesting active protein synthesis, are plentiful. Comparisons between milk and blood mononuclear phagocytes are listed in Table 2. Lysozyme is one of the proteins synthesized and secreted by the milk mononuclear phagocyte. By competitive binding, radioimmunoassay milk macrophages can be shown to release 100 ± 8 ng lysozyme/ 5 x 105 cells into the supernatant; the level is reduced to 20 ± 4 ng when these cells are cultured in the presence of 10 µg/ml of cyclohexamide, an inhibitor of protein synthesis. Moreover, it appears that these cells make C3 and C4.
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