Antibody targeting of phosphatidylserine for the detection and immunotherapy of cancer

Abstract

Phosphatidylserine (PS) is a negatively charged phospholipid in all eukaryotic cells that is actively sequestered to the inner leaflet of the cell membrane. Exposure of PS on apoptotic cells is a normal physiological process that triggers their rapid removal by phagocytic engulfment under noninflammatory conditions via receptors primarily expressed on immune cells. PS is aberrantly exposed in the tumor microenvironment and contributes to the overall immunosuppressive signals that antagonize the development of local and systemic antitumor immune responses. PS-mediated immunosuppression in the tumor microenvironment is further exacerbated by chemotherapy and radiation treatments that result in increased levels of PS on dying cells and necrotic tissue. Antibodies targeting PS localize to tumors and block PS-mediated immunosuppression. Targeting exposed PS in the tumor microenvironment may be a novel approach to enhance immune responses to cancer.

Keywords: bavituximab; imaging; immunosuppression; immunotherapy; phos-phatidylserine; tumor microenvironment.

Figure 1

PS-targeting antibodies interact with exposed PS in the tumor microenvironment to activate immune cells.(p)(p)Notes: PS exposure in the tumor microenvironment activates PS receptors in immune cells and causes these cells to adopt an immunosuppressive phenotype. PS-targeting antibodies 2aG4, 3G4, bavituximab, 1N11, and mch1N11 bind exposed PS via β2GP1. Binding of the PS-targeting complex blocks the interaction between PS and the PS receptors on immune cells and activates these cells via their FcγR.(p)(p)Abbreviations: ADCC, antibody-dependent cell-mediated cytotoxicity; β2GP1, beta 2 glycoprotein-1; FcγR, Fc gamma receptor; IL, interleukin; MDSC, myeloid-derived suppressor cell; PS, phosphatidylserine; TAMs, Tyro3, Axl, Mer receptor tyrosine kinases; TIMs, transmembranes, immunoglobulins, and mucins; TGF, transforming growth factor; TNF, tumor necrosis factor.

Figure 2

PS-targeting antibodies interact with β2GP1 and PS to form a high avidity complex.(p)(p)Notes: PS-targeting antibodies 2aG4, 3G4, bavituximab, 1N11, and mch1N11 bind circulating monomeric β2GP1 with low affinity. When sufficient surface exposure is present on a target cell, PS-targeting antibody interacts with two molecules of β2GP1 to form a high avidity (1 nM KD) complex. Domain V of β2GP1 interacts with PS exposed on the target cell. KD, equilibrium dissociation constant.(p)(p)Abbreviations: Ab, antibody; β2GP1, beta 2 glycoprotein-1; PS, phosphatidylserine.

Figure 3

PET imaging of PS exposed in tumors.(p)(p)Notes: Mice implanted with tumors (PC3-luc) are injected with ¹²⁴I-labeled 1N11 F(ab′)₂ (PGN650) or control F(ab′)₂ probes and imaged by bioluminescence in left panels and PET in middle and right panels. The yellow circle marks the tumor uptake of ¹²⁴I-PGN650. Reproduced from Stafford JH, Hao G, Best AM, Sun X, Thorpe PE. Highly specific PET imaging of prostate tumors in mice with an iodine-124-labeled antibody fragment that targets phosphatidylserine. PLoS One. 2013;8(12):e84864.¹¹²(p)(p)Abbreviations: Ctrl, control; F(ab′)₂, bivalent antigen-binding fragment of an antibody; I-124, iodine 124; luc, luciferase; PET, positron emission tomography; PS, phosphatidylserine.