Empowering GITR-targeted agonistic antibody-based immunotherapy by Fc engineering

Alongside the clinical success, immune checkpoint blockade therapy has notable limitations: they elicit differential responses between individuals and tumor types. Additional therapeutic approaches to boost anti-tumor immune responses are being developed to improve outcomes. One promising approach is targeting the immuno-stimulatory receptor Glucocorticoid-induced tumor necrosis factor receptor [TNFR] family-related protein (GITR) using agonistic antibodies. This study elucidates the role of human Fc-gamma receptors (FcγRs) in the activity of GITR monoclonal antibodies (mAbs). We applied Fc-engineering and glycoengineering approaches to modify the FcγR interactions of GITR mAb and characterized the anti-tumor immune response in-vivo of these Fc-variants. By utilizing a mouse model that recapitulates human FcγRs expression, we showed the engagement of different human FcγRs by GITR mAb significantly dictates their anti-tumor activity. While the engagement of the inhibitory hFcγRIIB compromises the efficacy of GITR mAb, engagement of the activating hFcγRIIA and/or hFcγRIIIA enhances the anti-tumor activity of this mAb. Our study identified an Fc-optimized human IgG scaffold of GITR agonistic mAb that results in enhanced therapeutic effects by increasing the CD8/T regulatory (Treg) ratio within the tumor microenvironment (TME). This activity is mediated by multiple mechanisms, including GITR agonism, Fc-dependent Treg depletion, and engagement of beneficial FcγR-signaling pathway on myeloid cells, all contributing to the enhanced effect by the Fc-optimized mAbs compared to the wild-type IgG1. Altogether, the data suggest that the anti-tumor activity of GITR mAbs can be increased when engaging the activating FcγR pathways and provides a rationale for Fc-engineering GITR human mAbs to increase their therapeutic activity.