Cytotoxic activity of anti-FCRL1 immunotoxins. (A) Cytotoxicity of E3(Fv)-PE38 and E9(Fv)-PE38 was conducted on Ramos and OCI-Ly7 cells by cell viability assay. (B) Stability of E9(Fv)-PE38. Immunotoxin (20 ng/mL) was incubated at 4°C (▴) or 37°C (■) for 1, 2, 4, or 8 hours prior to treatment of Ramos cells. The activity at time zero is set as 100%. (C) The correlation between cytotoxicity (x-axis, by IC₅₀s) and FCRL1 expression level (y-axis, by MFIs). The Pearson correlation coefficient (r) is 0.859 (P < .001).