Spontaneous aggregation as a mechanism for human monocyte purification

A previously unreported property of human mononuclear phagocytes is the ability of these cells to spontaneously aggregate. Fresh mononuclear cells obtained after plateletpheresis were noted to spontaneously form large cellular aggregates. Dual parameter immunofluorescence analysis demonstrated that the aggregating cells were positive for the monocyte marker CD11 (complement receptor, type 3) but were negative for the lymphocyte marker CD3 (T3 antigen). In addition, less than 5% of the nonaggregating cells were CD11⁺, suggesting that almost all CD11⁺ cells aggregated. Cellular aggregates were independent of cell concentration and formed more efficiently at 4 °C than at either 22 or 37 °C. Based on these observations, a purification procedure utilizing Ficoll-Hypaque separation, spontaneous aggregation at 4 °C, and transient plastic adherence resulted in a sevenfold enrichment of the CD11⁺ peripheral blood monocytes. Purified monocytes were contaminated with less than 2% CD3 cells. The size, growth, and adherence characteristics as well as cytologic stains indicated that the monocytes were not significantly altered by the purification procedure. Thus, spontaneous aggregation is an efficient and convenient method for the isolation of large numbers of purified monocytes.