Concepts for agonistic targeting of CD40 in immuno-oncology

Abstract

TNF Receptor Superfamily (TNF-R-SF) signaling is a structurally well-defined event that requires proper receptor clustering and trimerization. While the TNF-SF ligands naturally exist as trivalent functional units, the receptors are usually separated on the cell surface. Critically, receptor assembly into functional trimeric signaling complexes occurs through binding of the natural ligand unit. TNF-R-SF members, including CD40, have been key immunotherapeutic targets for over 20 years. CD40, expressed by antigen-presenting cells, endothelial cells, and many tumors, plays a fundamental role in connecting innate and adaptive immunity. The multiple investigated strategies to induce CD40 signaling can be broadly grouped into antibody-based or CD40L-based approaches. Currently, seven different antibodies and one CD40L-based hexavalent fusion protein are in active clinical trials. In this review, we describe the biology and structural properties of CD40, requirements for agonistic signal transduction through CD40 and summarize current attempts to exploit the CD40 signaling pathway for the treatment of cancer.

Keywords: CD40; CD40L; HERA ligands; TNF-R-SF; agonist; cancer; immunotherapy.

Figure 1.

Requirements for TNF-R-SF signaling. TNF-R-SF signaling is a structurally well-defined event that requires proper three-dimensional receptor clustering and trimerization. A. The TNF-SF ligands naturally exist as trivalent functional units and the receptors are usually separated on the cell surface. B. The assembled TNF-SF ligands contain three receptor-binding sites that are located at the three identical clefts between the neighboring monomers. C. Receptor assembly into functional trimeric signaling complexes occurs through binding of the natural ligand unit. D. The interaction of these trimeric TNF-SF ligands with their corresponding receptors, expressed on the surface of other cells, leads to very precise receptor clustering followed by intracellular signal transduction.

Figure 2.

Side effects of CD40 agonistic antibodies versus fusion molecules. The orientation of the target epitope of CD40 agonistic antibodies within the CD40 molecule defines epitope-driven effects. In case the antibody prevents binding of the natural CD40L it inevitably inhibits favorable natural CD40 signaling. Contrarily, cross-linking receptor complexes that have been trimerized by the natural ligand can lead to hyper-clustering and uncontrolled overstimulation with increased risk of toxic side effects and exhaustion of immune cells. Fc-domain driven effects derive from the interaction of the antibody´s Fc domain with Fc receptors on different immune cells leading to unwanted (back-)signaling. Upon Fc receptor engagement these immune cells exert diverse effector functions such as ADCC thereby not only depleting the target cell population but also contributing to unspecific immune system activation. In contrast, agonistic fusion proteins with a hexavalent/Fc-silent structure preclude the aforementioned characteristics, possibly generating a more physiologic activity.