Phospholipid composition of human monocytes and alterations occurring due to culture and stimulation by C3b.

The phospholipid composition of human peripheral blood monocytes has not been previously reported, due to difficulty in isolating these cells in a purified state. In this study, monocytes were purified by counterflow centrifugation without selective adherence, and were characterized with the use of fluorescent monoclonal antibodies to T and B lymphocytes and monocytes by flow cytometry. These platelet-free cell preparations contained less than 5% T cells and less than 3% B cells. Isolated monocytes, which were rapidly frozen after isolation, contained phospholipids (in order of decreasing concentrations) as follows: phosphatidylcholine greater than phosphatidylethanolamine greater than sphingomyelin greater than phosphatidylserine greater than phosphatidylinositol greater than cardiolipin. A small amount of lyso-PC, but no lyso-PE, phosphatidic acid or lyso-PI, was found. The effect of culturing these cells in the presence or absence of a known stimulant of monocyte prostaglandin E and thromboxane release, the C3b fragment of the third component of human complement (C3), was studied with regard to phospholipid composition. Monocytes cultured without stimulant for 24 h contained 3-4% more sphingomyelin than did uncultured cells, and lyso-PC concentrations were consistently elevated. The addition of the stimulant C3b to cultured cells resulted in enhancement of release of immunoreactive prostaglandin E into culture supernatants, without affecting the release of lysosomal enzymes. Analysis of the phospholipid content of cells cultured in the presence of C3b revealed that there was a significant decrease in total PI compared to cells cultured in the absence of C3b, in addition to an increased concentration of sphingomyelin and lyso-PC when compared to freshly isolated cells. These changes occurred in the absence of elevated concentrations of phosphatidic acid.