Figure 4.

Immunoglobulins do not bind FREB targeted to the cell surface of 293 cells. FREB was expressed on the cell surface of 293 cells as a FREB–CD4 fusion protein (Left). Control 293 cells were transfected with CD1a cDNA (Right). Cell surface expression of FREB and CD1a was confirmed by flow cytometry by using anti-FREB (a) and anti-CD1a (b) mAbs. Transfected cells were incubated with human IgA (c and d), IgE (e and f), IgM (g and h), and IgG (i and j), which had been previously aggregated by treatment at 65°C for 30 min. Immunoglobulins bound to transfected cells were detected by flow cytometry. Percentages of stained cells are indicated in the upper quadrants. Human IgA partially bound 293 cells, regardless of FREB expression (c and d). Cells stained only with the secondary antibodies fell within the lower quadrants. FSC, forward scatter.