Trastuzumab Triggers Phagocytic Killing of High HER2 Cancer Cells In Vitro and In Vivo by Interaction with Fcγ Receptors on Macrophages

Abstract

Trastuzumab has been used for the treatment of HER2-overexpressing breast cancer for more than a decade, but the mechanisms of action for the therapy are still being actively investigated. Ab-dependent cell-mediated cytotoxicity mediated by NK cells is well recognized as one of the key mechanisms of action for trastuzumab, but trastuzumab-mediated Ab-dependent cellular phagocytosis (ADCP) has not been established. In this study, we demonstrate that macrophages, by way of phagocytic engulfment, can mediate ADCP and cancer cell killing in the presence of trastuzumab. Increased infiltration of macrophages in the tumor tissue was associated with enhanced efficacy of trastuzumab whereas depletion of macrophages resulted in reduced antitumor efficacy in mouse xenograft tumor models. Among the four mouse FcγRs, FcγRIV exhibits the strongest binding affinity to trastuzumab. Knockdown of FcγRIV in mouse macrophages reduced cancer cell killing and ADCP activity triggered by trastuzumab. Consistently, an upregulation of FcγRIV expression by IFN-γ triggered an increased ADCP activity by trastuzumab. In an analogous fashion, IFN-γ priming of human macrophages increased the expression of FcγRIII, the ortholog of murine FcγRIV, and increased trastuzumab-mediated cancer cell killing. Thus, in two independent systems, the results indicated that activation of macrophages in combination with trastuzumab can serve as a therapeutic strategy for treating high HER2 breast cancer by boosting ADCP killing of cancer cells.

Macrophage-Mediated Trogocytosis Leads to Death of Antibody-Opsonized Tumor Cells

“…In this context, actin and CD45 accumulation at the macrophage:cancer cell interface did not indicate differences between RAW264.7 and J774A.1 macrophages when analyzed by fluorescence microscopy. However, by contrast with our observations and those of others (6), it was recently reported that RAW264.7 cells phagocytose trastuzumab-opsonized BT-474 cells (44). Further studies are required to determine the reasons for this apparent discrepancy.…”

HER2 signaling regulates the tumor immune microenvironment and trastuzumab efficacy

“…Results obtained in an animal model using an anti-CCL2 monoclonal antibody, which indicated improved monocyte/macrophage (CD11b+ F4/80+) infiltration in the TME associated with higher trastuzumab anti-tumor inhibitory activity, supported the role of these immune cells in trastuzumab efficacy. However, we could not exclude that other CD68+ cells, such as dendritic cells, that are recruited by CCL2 20 might also infiltrate these tumors. As dendritic cells express the Fcγ receptor, they may also contribute to the responsiveness of TRAR-low tumors to trastuzumab by taking up tumor cell fragments opsonized by the antibody and priming CD4+ or CD8+ T lymphocytes.…”

IL-15 enhances the anti-tumor activity of trastuzumab against breast cancer cells but causes fatal side effects in humanized tumor mice (HTM)

“…A trastuzumab-induced tumor infiltration by macrophages with phagocytotic activity has been recently demonstrated by Shi et al [41]. The importance of anti-tumor activity in myeloid cells has been explicitly shown by a depletion of macrophages, which resulted in reduced anti-tumor-efficacy in mouse xenograft tumor models.…”