Subset 4 IGs bind to viable human B lymphocytes. Subset 4 mAbs (nos. 183, 240, 342) were incubated with Ramos or Jurkat cell line cells under nonpermeabilizing conditions. mAb141, which expresses IGHV4-34 but does not belong to subset 4, and mAb360, which is a member of subset 1 and does not express IGHV4-34, were used as comparators. Apoptosis was measured by annexin V staining. (A) Representative contour plots of CLL mAb binding to Ramos cells. Binding detected by FITC-labeled F(ab′)2 fragments of goat polyclonal antibodies (pAbs) specific for human IgG. (B) Average binding of live (annexin V−) and apoptotic/dead (annexin V+) Ramos and Jurkat cells by the following biotinylated CLL mAbs: subset 4 mAbs183, 240 and 342; non-subset 4 IGHV4-34-expressing CLL mAbs 141 and DO8; and subset 2 mAb282 (IGHV3-21/IGLV3-21). Binding was detected by streptavidin PE-Cy7. Data obtained from four independent experiments. Error bars represent SEM.