Antibody-mediated cellular cytotoxicity (ADCC), a key immune effector mechanism, relies on the binding of antigen-antibody complexes to Fcγ receptors expressed on immune cells. Antibodies lacking core fucosylation show a large increase in affinity for FcγRIIIa leading to an improved receptor-mediated effector function. Although afucosylated IgGs exist naturally, a next generation of recombinant therapeutic, glycoenginereed antibodies is currently being developed to exploit this finding. In this study, the crystal structures of a glycosylated Fcγ receptor complexed with either afucosylated or fucosylated Fc were determined allowing a detailed, molecular understanding of the regulatory role of Fc-oligosaccharide core fucosylation in improving ADCC. The structures reveal a unique type of interface consisting of carbohydratecarbohydrate interactions between glycans of the receptor and the afucosylated Fc. In contrast, in the complex structure with fucosylated Fc, these contacts are weakened or nonexistent, explaining the decreased affinity for the receptor. These findings allow us to understand the higher efficacy of therapeutic antibodies lacking the core fucose and also suggest a unique mechanism by which the immune system can regulate antibody-mediated effector functions.immunoglobulin | afucosylation | antibody effector function | X-ray crystallography

In higher organisms the formation of the steroid scaffold is catalysed exclusively by the membrane-bound oxidosqualene cyclase (OSC; lanosterol synthase). In a highly selective cyclization reaction OSC forms lanosterol with seven chiral centres starting from the linear substrate 2,3-oxidosqualene. Valuable data on the mechanism of the complex cyclization cascade have been collected during the past 50 years using suicide inhibitors, mutagenesis studies and homology modelling. Nevertheless it is still not fully understood how the enzyme catalyses the reaction. Because of the decisive role of OSC in cholesterol biosynthesis it represents a target for the discovery of novel anticholesteraemic drugs that could complement the widely used statins. Here we present two crystal structures of the human membrane protein OSC: the target protein with an inhibitor that showed cholesterol lowering in vivo opens the way for the structure-based design of new OSC inhibitors. The complex with the reaction product lanosterol gives a clear picture of the way in which the enzyme achieves product specificity in this highly exothermic cyclization reaction.

Halogen bonding triggers activity: Increasing binding affinity was observed for a series of covalent human Cathepsin L inhibitors by exchanging an aryl ring H atom with Cl, Br, and I, which undergo halogen bonding with the CO group of Gly61 in the S3 pocket of the enzyme. Fluorine, in contrast, strongly avoids halogen bonding (see scheme). The strong distance and angle dependence of halogen bonding was confirmed for biological systems.

Inhibition of dipeptidyl peptidase IV (DPP-IV), the main glucagon-like peptide 1 (GLP1)-degrading enzyme, has been proposed for the treatment of type II diabetes. We expressed and purified the ectodomain of human DPP-IV in Pichia pastoris and determined the X-ray structure at 2.1 A resolution. The enzyme consists of two domains, the catalytic domain, with an alpha/beta hydrolase fold, and a beta propeller domain with an 8-fold repeat of a four-strand beta sheet motif. The beta propeller domain contributes two important functions to the molecule that have not been reported for such structures, an extra beta sheet motif that forms part of the dimerization interface and an additional short helix with a double Glu sequence motif. The Glu motif provides recognition and a binding site for the N terminus of the substrates, as revealed by the complex structure with diprotin A, a substrate with low turnover that is trapped in the tetrahedral intermediate of the reaction in the crystal.

We developed cergutuzumab amunaleukin (CEA-IL2v, RG7813), a novel monomeric CEA-targeted immunocytokine, that comprises a single IL-2 variant (IL2v) moiety with abolished CD25 binding, fused to the C-terminus of a high affinity, bivalent carcinoembryonic antigen (CEA)-specific antibody devoid of Fc-mediated effector functions. Its molecular design aims to (i) avoid preferential activation of regulatory T-cells vs. immune effector cells by removing CD25 binding; (ii) increase the therapeutic index of IL-2 therapy by (a) preferential retention at the tumor by having a lower dissociation rate from CEA-expressing cancer cells vs. IL-2R-expressing cells, (b) avoiding any FcγR-binding and Fc effector functions and (c) reduced binding to endothelial cells expressing CD25; and (iii) improve the pharmacokinetics, and thus convenience of administration, of IL-2. The crystal structure of the IL2v-IL-2Rβγ complex was determined and CEA-IL2v activity was assessed using human immune effector cells. Tumor targeting was investigated in tumor-bearing mice using 89Zr-labeled CEA-IL2v. Efficacy studies were performed in (a) syngeneic mouse models as monotherapy and combined with anti-PD-L1, and in (b) xenograft mouse models in combination with ADCC-mediating antibodies. CEA-IL2v binds to CEA with pM avidity but not to CD25, and consequently did not preferentially activate Tregs. In vivo, CEA-IL2v demonstrated superior pharmacokinetics and tumor targeting compared with a wild-type IL-2-based CEA immunocytokine (CEA-IL2wt). CEA-IL2v strongly expanded NK and CD8+ T cells, skewing the CD8+:CD4+ ratio toward CD8+ T cells both in the periphery and in the tumor, and mediated single agent efficacy in syngeneic MC38-CEA and PancO2-CEA models. Combination with trastuzumab, cetuximab and imgatuzumab, all of human IgG1 isotype, resulted in superior efficacy compared with the monotherapies alone. Combined with anti-PD-L1, CEA-IL2v mediated superior efficacy over the respective monotherapies, and over the combination with an untargeted control immunocytokine. These preclinical data support the ongoing clinical investigation of the cergutuzumab amunaleukin immunocytokine with abolished CD25 binding for the treatment of CEA-positive solid tumors in combination with PD-L1 checkpoint blockade and ADCC competent antibodies.

In two series of small-molecule ligands, one inhibiting human cathepsin L (hcatL) and the other MEK1 kinase, biological affinities were found to strongly increase when an aryl ring of the inhibitors is substituted with the larger halogens Cl, Br, and I, but to decrease upon F substitution. X-ray co-crystal structure analyses revealed that the higher halides engage in halogen bonding (XB) with a backbone C=O in the S3 pocket of hcatL and in a back pocket of MEK1. While the S3 pocket is located at the surface of the enzyme, which provides a polar environment, the back pocket in MEK1 is deeply buried in the protein and is of pronounced apolar character. This study analyzes environmental effects on XB in protein-ligand complexes. It is hypothesized that energetic gains by XB are predominantly not due to water replacements but originate from direct interactions between the XB donor (Caryl-X) and the XB acceptor (C=O) in the correct geometry. New X-ray co-crystal structures in the same crystal form (space group P2(1)2(1)2(1)) were obtained for aryl chloride, bromide, and iodide ligands bound to hcatL. These high-resolution structures reveal that the backbone C=O group of Gly61 in most hcatL co-crystal structures maintains water solvation while engaging in XB. An aryl-CF3-substituted ligand of hcatL with an unexpectedly high affinity was found to adopt the same binding geometry as the aryl halides, with the CF3 group pointing to the C=O group of Gly61 in the S3 pocket. In this case, a repulsive F2C-F⋅⋅⋅O=C contact apparently is energetically overcompensated by other favorable protein-ligand contacts established by the CF3 group.

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