728 THE MILK MONONUCLEAR PHAGOCYTE IS A MACROPHAGE

Abstract

Mononuclear phagocytic cells (MM) account for 86±2 (±1 SD) of the 1.8±.2×106/cc milk leukocytes in the first week of lactation. MM were isolated by differential centrifugation and glass adherence and their state of differentiation studied with histochemical immunologic, functional and metabolic assays. 31±9% of glass adherent latex ingesting cells stained for peroxidase activity compared to 100% of similarly selected blood mononuclear phagocytes (BM). 83±9% MM and 97±2 BM formed rosettes with SRBC coated with IgM anti-SRBC and the first four complement components (C3bSRBC). 96±3% MM and 97±2% BM formed rosettes with IgGSRBC. Titration of the IgGSRBC to BM or MM ratio required for cosetting revealed that MM had >4 fold avidity for IgGSRBC than BM. MM spread and elongate on glass more rapidly than BM. This phenomenon, the low peroxidase activity, and high avidity for IgGSRBC of MM all suggest that the MM is a macrophage (MØ). IgGSRBC phagocytosis occurred in 85±4% MM and 90±5% BM under similar conditions but C3bSRBC phagocytosis occurred in neither MMnor BM suggesting that the MM is not an activated MØ. MM and BM IgGSRBC phagocytosis were completely inhibited by NaF. BM pretreated with cell free human milk (MS) resembled MM in the degree of spreading on glass, number of lysosomes, low level of peroxidase and presence of lipid droplets. Intracellular IgA, previously shown to be released from MM (Pittard et al., Ped. Res., vol. 11, No. 4, 721, 1977) was detected by immunofluorescence both in MM and in BM cultured for 24 hours in MS.