Progenitor cell sensitivity to SAg-directed T-cell killing. Highly purified CD34⁺ cells were used as targets. (A) Anti-CD19-Fab-SEAm fusion protein or C215-Fab-SEAm control fusion protein. Results are the mean ± SD % cytotoxicity. (B) An indirect system including MoAbs against CD34 or CD19 followed by RAM and PA-SEAm. Each value is the mean of triplicate samples. The E:T ratio was 40:1, and specific cytotoxicity was measured in a ⁵¹Cr-release assay.