Highlights d A complex HER2 + breast cancer ecosystem is reconstituted and quantitatively described d The effects of the drug trastuzumab (Herceptin) are directly visualized ex vivo d Trastuzumab promotes long cancer-immune interactions and an ADCC immune response d Trastuzumab and CAFs have antagonist immunomodulation effects

Our results suggest that topoiomerase II-reactive agents may prove to be clinically useful drugs for patients with malignant gliomas.

A major challenge in cancer research is the complexity of the tumor microenvironment, which includes the host immunological setting. Inspired by the emerging technology of organ-on-chip, we achieved 3D co-cultures in microfluidic devices (integrating four cell populations: cancer, immune, endothelial, and fibroblasts) to reconstitute ex vivo a human tumor ecosystem (HER2 + breast cancer). We visualized and quantified the complex dynamics of this tumor-onchip, in the absence or in the presence of the drug trastuzumab (Herceptin), a targeted antibody therapy directed against the HER2 receptor. We uncovered the capacity of the drug trastuzumab to specifically promote long cancer-immune interactions (>50 min), recapitulating an anti-tumoral ADCC (antibody-dependent cell-mediated cytotoxicity) immune response. Cancer-associated fibroblasts (CAFs) antagonized the effects of trastuzumab. These observations constitute a proof of concept that tumors-on-chip are powerful platforms to study ex vivo immunocompetent tumor microenvironments, to characterize ecosystem-level drug responses, and to dissect the roles of stromal components.