Antibody-dependent cell-mediated cytotoxicity (ADCC) mediated by CD56 + natural killer (NK) cells may contribute to the activity of trastuzumab in HER-2-amplified tumours. In this study, we investigated the possibility that trastuzumab might induce ADCC against HER-2-non-amplified breast cancer cells.

Induction of NK cell-mediated ADCC was examined in trastuzumab-treated HER-2-non-amplified breast cancer cell lines. HER-2 protein levels were also determined in tumour and autologous normal tissue samples from patients with HER-2 negative breast cancer.

Trastuzumab induced a significant ADCC response in the HER-2-amplified HCC1954 and SKBR3 cell lines, and in all five of the non-amplified cell lines, which had low levels of detectable HER-2 by western blot (CAL-51, CAMA-1, MCF-7, T47D, and EFM19). Trastuzumab did not induce ADCC in the K562 control cell line or MDA-MB-468, which has very low levels of HER-2 detectable by enzyme-linked immunosorbent assay (ELISA) only. HER-2 protein was detected by ELISA in 14/15 patient tumour samples classified as HER-2-non-amplified. Significantly lower levels of HER-2 were detected in normal autologous tissue compared with tumour samples from the same patients.

Our results suggest that HER-2-non-amplified breast cancer cells, with low but detectable levels of HER-2 protein, can bind trastuzumab and initiate ADCC.