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Therapeutic monoclonal antibodies contribute to elimination of tumor cells by different mechanisms including blocking of receptor signaling, induction of antibody dependent cellular cytotoxicity (ADCC), antibody dependent cellular phagocytosis (ADCP), complement dependent cytotoxicity (CDC), and induction of adaptive immunity. FcγR-expressing monocytes and macrophages have recently been identified as important immune effectors contributing to the activity of monoclonal antibodies in vivo. Therefore, we analyzed the FcγR-dependent effector functions mediated by both cell subsets upon treatment with glycoengineered (GE) antibodies (having enhanced FcγRIIIa (CD16a) binding affinity), wild-type (WT), and Fc-mutated antibodies that lack binding to FcγRs (P329G LALA mutation). Trastuzumab, cetuximab, rituximab and ofatumumab were used as WT antibodies, whereas GE trastuzumab, GA201 (targeting EGFR) and GA101 (targeting CD20) were used as GE antibodies. To better reflect the plasticity of myeloid lineage cells we evaluated the activities on classical and non-classical/intermediate monocytes, M1 and M2c macrophages. We found that GE and WT antibodies displayed comparable binding and induced similar monocyte/macrophage-mediated ADCC and ADCP in absence of unspecific endogenous IgGs. Interestingly, in their presence, i.e. in a situation that more closely mimics physiological conditions, GE antibodies displayed significantly superior binding and induced higher levels of ADCC/ADCP compared to WT antibodies. Data collected by imaging further indicated that the degree of phagocytized cells is higher and faster in presence of GE antibodies and that M2c macrophages perform serial phagocytosis by engulfing 2-3 tumor cells. Fc-mutated P329G LALA variant showed no binding and no functional activity. A direct comparison of the three anti-CD20 antibodies (GA101, rituximab and ofatumumab), pointed out that GA101 mediated significantly stronger elimination of tumor cells further underscoring the relevance of combination of both glycoengineering and cell death induction for monocyte/macrophage-mediated killing of tumor cells. These data show, for the first time that, in addition to enhancing NK cell-mediated cytotoxicity (ADCC), glycoengineering also enhances monocyte and macrophage cytotoxic and phagocytic activities through enhanced binding to FcγRIIIa under conditions that more closely resemble the physiological setting. Citation Format: Herter Sylvia, Christian Klein, Martina Birk, Thomas Weber, Pablo Umana, Marina Bacac. Modulation of monocyte- and macrophage-mediated antibody-dependent cell phagocytosis and cytotoxicity (ADCP/ADCC) by Fc engineering of therapeutic antibodies. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3631. doi:10.1158/1538-7445.AM2014-3631