The measurement of specific cell: Cell interactions by dual‐parameter flow cytometry

The Fc receptor‐mediated aggregation of antibody‐coated spleen cells from the P388D1 mouse macrophage line was followed using a novel flow cytometric technique. P388D1 and spleen cells were directly labeled with green‐emitting (fluorescein isothiocyanate) and red‐emitting (substituted rhodamine isothiocyanate) flurophores, respectively. They were mixed, incubated in suspension at 4°C, and analyzed for aggregation with a dual laser flow cytometer. Unconjugated cells appeared as particles which were either red or green, while conjugates were detected as particles which were both red and green. Using this assay procedure, 5 × 104 cells were analyzed in 2‐3 min for the percentages of conjugates, free spleen cells, and free P388D1 cells. Intercellular aggregation required both antibody on the spleen cells and free Fc receptors on the P388D1 cells; nonspecific aggregates accounted for 1% or less of the total particles analyzed. Measurements of the fluorescence distributions within conjugates indicated that the majority of conjugates contained a single P388D1 cell bound to 1–3 spleen cells, and that only heterophilic aggregation occurred. The flow cytometric technique described here should be applicable for the measurement of the initial events of intercellular aggregation in other systems as well.