Unprecedented selectivity for homologous lectin targets: differential targeting of the viral receptors L-SIGN and DC-SIGN

Abstract

DC-SIGN (CD209) and L-SIGN (CD209L) are two C-type lectin receptors (CLRs) that facilitate SARS-CoV-2 infections as viral co-receptors. SARS-CoV-2 manipulates both DC-SIGN and L-SIGN for enhanced infection, leading to interest in developing receptor antagonists. Despite their structural similarity (82% sequence identity), they function differently. DC-SIGN, found in dendritic cells, shapes the immune response by recognizing pathogen-associated carbohydrate patterns. In contrast, L-SIGN, expressed in airway epithelial endothelial cells, is not directly involved in immunity. COVID-19's primary threat is the hyperactivation of the immune system, potentially reinforced if DC-SIGN engages with exogenous ligands. Therefore, L-SIGN, co-localized with ACE2-expressing cells in the respiratory tract, is a more suitable target for anti-adhesion therapy. However, designing a selective ligand for L-SIGN is challenging due to the high sequence identity of the Carbohydrate Recognition Domains (CRDs) of the two lectins. We here present Man84, a mannose ring modified with a methylene guanidine triazole at position 2. It binds L-SIGN with a K_D of 12.7μM ± 1 μM (ITC) and is the first known L-SIGN selective ligand, showing 50-fold selectivity over DC-SIGN (SPR). The X-ray structure of the L-SIGN CRD/Man84 complex reveals the guanidinium group's role in achieving steric and electrostatic complementarity with L-SIGN. This allows us to trace the source of selectivity to a single amino acid difference between the two CRDs. NMR analysis confirms the binding mode in solution, highlighting Man84's conformational selection upon complex formation. Dimeric versions of Man84 achieve additional selectivity and avidity in the low nanomolar range. These compounds selectively inhibit L-SIGN dependent trans-infection by SARS-CoV-2 and Ebola virus. Man84 and its dimeric constructs display the best affinity and avidity reported to date for low-valency glycomimetics targeting CLRs. They are promising tools for competing with SARS-CoV-2 anchoring in the respiratory tract and have potential for other medical applications.