Blocking of CEACAM on myeloma cells increases T-cell reactivity against myeloma cells. (A-C) Mean ± SEM IFN-γ spot numbers of 3 wells per test group from 3 different patients. IFN-γ secretion by T cells cocultured with the CEACAM-1+ and CEACAM-6+ myeloma cell line RPMI-8226 (A, left), the CEACAM-negative myeloma cell line SKMM2 (A, right) or with autologous myeloma cells (B-C). Compared with pretreatment with respective isotype antibodies, a significant increase of IFN-γ secretion was observed when CEACAM-positive myeloma cells were pretreated with mixed anti-CEACAM (–1, –6, and –8; CD166) antibodies (A,C), or with anti-CEACAM-6 mAb, but not anti-CEACAM-8 mAb (C). Further negative control groups contained T cells cocultured with CD138− BM cells or T cells only. (D) Cumulative results from 6 patients showing mean + SEM numbers of IFN-γ secreting cells after coculture with CEACAM-positive RPMI8226 myeloma cells or with CEACAM-negative SKMM2 myeloma cells pretreated with anti-CEACAM (–1, –6, or –8) mAb (black bars) or with respective isotype antibodies (white bars). (E-F) Cumulative results from six patients showing mean + SEM numbers of IFN-γ-secreting cells after coculture with autologous myeloma cells pretreated with mixed anti-CEACAM–1, –6, or –8 mAb (E, black bar), with single antibodies against CEACAM-1, CEACAM-6, or CEACAM-8 (F, black bars) or with respective isotype antibodies (white bars). Significant increase of IFN-γ secretion was observed only upon blocking of CEACAMs on myeloma cells. *Significant differences between test and respective control groups (P < .05, 2-sided Student t test).