Edited by Peter CresswellThe stable effector functionLess (SEFL) antibody was designed as an IgG1 antibody with a constant region that lacks the ability to interact with Fc␥ receptors. The engineering and stability and pharmacokinetic assessments of the SEFL scaffold is described in the accompanying article (Jacobsen, F. W., Stevenson, R., Li, C., Salimi-Moosavi, H., Liu, L., Wen, J., Luo, Q., Daris, K., Buck, L., Miller, S., Ho, S-Y., Wang, W., Chen, Q., Walker, K., Wypych, J., Narhi, L., and Gunasekaran, K. (2017) J. Biol. Chem. 292). The biological properties of these SEFL antibodies were assessed in a variety of human and cynomolgus monkey in vitro assays. Binding of parent molecules and their SEFL variants to human and cynomolgus monkey Fc␥Rs were evaluated using flow cytometry-based binding assays. The SEFL variants tested showed decreased binding affinity to human and cynomolgus Fc␥Rs compared with the wild-type IgG1 antibody. In addition, SEFL variants demonstrated no antibody-dependent cell-mediated cytotoxicity in vitro against Daudi cells with cynomolgus monkey peripheral blood mononuclear cells, and had minimal complement-dependent cytotoxicity activity similar to that of the negative control IgG2 in a CD20؉ human Raji lymphoma cell line. SEFL mutations eliminated off-target antibody-dependent monocyte phagocytosis of cynomolgus monkey platelets, and cynomolgus platelet activation in vitro. These experiments demonstrate that the SEFL modifications successfully eliminated Fc-associated effector binding and functions.Monoclonal antibodies (mAbs) are the largest class of biopharmaceuticals and have diverse clinical applications (1). The choice of therapeutic mAb isotype to develop (IgG1, IgG2, or IgG4) is dependent on the target (cell surface versus soluble), desired biology, safety (risk of immunogenicity and undesired immunological effects), and manufacturability (expression, formulation, and stability). More than 80% of approved therapeutic mAbs are IgG1 isotypes that target cell surface receptors and are effective for oncology indications (2). For these therapeutic approaches, mAb isotypes that can induce cell killing such as complement-dependent cytotoxicity (CDC) 2 and antibody-dependent cell-mediated cytotoxicity (ADCC) are often desirable (3, 4). However, for non-oncologic indications, therapeutic mAbs without cytotoxic effector function may be more appropriate because cell killing may not be a goal of therapy.The Fc portion of IgG has interaction sites for the effector ligands, including Fc␥ receptors (Fc␥RI, Fc␥RII, and Fc␥RIII), C1q complement, and the neonatal Fc receptor (FcRn). IgG isotypes differentially engage Fc␥ receptors and C1q binding to recruit immune effector functions and initiate cytotoxic effector functions, (either ADCC or CDC (5)). Historically, IgG2 or IgG4 isotypes were thought to have minimal cytotoxic effector function, and have been selected for applications where cytotoxic effector function is not required or desirable (5). However, recent evidence suggests that IgG2 is...