ADCC with enriched NK cells from healthy donors. (A) NK cells efficiently lyse cryopreserved primary B-lineage ALL blasts in the presence of CD19-4G7chim. Mean specific lysis induced by CD19-4G7chim, averaged over 10 experiments with enriched NK cells (> 90% CD16⁺CD56⁺) from 7 healthy donors and cryopreserved primary CD19⁺CD20^– preB/cALL blasts from 3 different patients at several effector-target (E/T) ratios. Open symbols indicate the absence of antibody; closed symbols, with CD19-4G7chim; triangles, NK cells not stimulated with IL-2; squares, NK cells stimulated with IL-2 at 40 IU/mL. Although the donor-target pairs each produced different levels of spontaneous lysis, NK cells from each donor showed significantly enhanced lysis after addition of CD19-4G7chim (P = .02, paired Wilcoxon rank sum test). (B) ADCC with CD19-4G7chim or blocking of HLA class I result in similar lysis of B-lineage ALL blasts. In 3 additional experiments, blocking of HLA class I by incubation with the murine IgG2a antibody W6/32 (α HLA-I, which binds to conformation-dependent epitopes, relevant for NK cell inhibitory receptors) resulted in strongly increased lysis. Blocking with W6/32 Fab fragments, used to exclude induction of ADCC by W6/32 itself, resulted in comparable lysis. These results suggest that interactions between HLA class I on cALL blasts and NK cells inhibit lysis. However, enhanced lysis of target cells is possible by ADCC with CD19-4G7chim. Underlying mechanisms remain to be investigated. A chimeric CD20 antibody of the same isotype as CD19-4G7chim (IgG1) did not increase specific lysis. Thus, ADCC mediated by CD19-4G7chim was specific. Furthermore, the parental murine 4G7 antibody did not induce enhanced lysis, demonstrating that a human Fc portion was needed for ADCC. Error bars represent SDs of all experiments.