Characterization of anti-FCRL1 mAbs. (A) Assessment of cross-reactivity of the representative mAbs E3 and E9 in FACS using 293T cells transfected with each FCRL expression plasmid. Expression of each FCRL on the cells was confirmed by anti-FCRL mAb (data not shown). Cells were stained with E3 or E9 mAb (solid line) or isotype mAb (gray histogram). (B) Topographic epitope mapping of anti-FCRL1 mAbs by mutual competition. The binding of indicator mAbs (listed in rows) to FCRL1-Fc in the presence of more than 100-fold excess amounts of competitor mAbs (listed in columns). Strengths of competition are shown as percentages in each box, which are shaded according to the key at the bottom of the figure.