Phagocytosis of EA mediated by the chimeric receptor I-IIA-IIA in COS-1 cells expressing I-IIA-IIA alone or I-IIA-IIA plus FcγRIIB2. (A) The percent of cells that phagocytose EA and the phagocytic index (PI, number of ingested EA/100 transfected COS-1 cells) for three representative experiments are shown. (B) Expression of the chimeric receptor as determined by flow cytometry. Cells were stained with MoAb 32.2 to measure I-IIA-IIA expression in the presence (–––––) or absence (.........) of FcγRIIB2. (. . . .), Indicates transfectants stained with isotype control antibody. The expression of FcγRIIB was determined by flow cytometry after staining with MoAb IV.3. Mean fluorescence intensity (MFI) for FcγRIIB2 in experiments 1, 2, and 3 was 104, 91, and 80, respectively. The fluorescence histogram shows that the expression of I-IIA-IIA (MFI) in the presence or absence of FcγRIIB2 was virtually the same. Thus, FcγRIIB2 decreased phagocytosis by I-IIA-IIA without changing the cell-surface expression of I-IIA-IIA.