Structural Basis of Interleukin-5 Inhibition by the Small Cyclic Peptide AF17121

Proinflammatory Cytokines in Chronic Respiratory Diseases and Their Management

Pulmonary homeostasis can be agitated either by external environmental insults or endogenous factors produced during respiratory/pulmonary diseases. The lungs counter these insults by initiating mechanisms of inflammation as a localized, non-specific first-line defense response. Cytokines are small signaling glycoprotein molecules that control the immune response. They are formed by numerous categories of cell types and induce the movement, growth, differentiation, and death of cells. During respiratory diseases, multiple proinflammatory cytokines play a crucial role in orchestrating chronic inflammation and structural changes in the respiratory tract by recruiting inflammatory cells and maintaining the release of growth factors to maintain inflammation. The issue aggravates when the inflammatory response is exaggerated and/or cytokine production becomes dysregulated. In such instances, unresolving and chronic inflammatory reactions and cytokine production accelerate airway remodeling and maladaptive outcomes. Pro-inflammatory cytokines generate these deleterious consequences through interactions with receptors, which in turn initiate a signal in the cell, triggering a response. The cytokine profile and inflammatory cascade seen in different pulmonary diseases vary and have become fundamental targets for advancement in new therapeutic strategies for lung diseases. There are considerable therapeutic approaches that target cytokine-mediated inflammation in pulmonary diseases; however, blocking specific cytokines may not contribute to clinical benefit. Alternatively, broad-spectrum anti-inflammatory approaches are more likely to be clinically effective. Herein, this comprehensive review of the literature identifies various cytokines (e.g., interleukins, chemokines, and growth factors) involved in pulmonary inflammation and the pathogenesis of respiratory diseases (e.g., asthma, chronic obstructive pulmonary, lung cancer, pneumonia, and pulmonary fibrosis) and investigates targeted therapeutic treatment approaches.

The β Common Cytokine Receptor Family Reveals New Functional Paradigms From Structural Complexities

Cytokines are small proteins that are critical for controlling the growth and activity of hematopoietic cells by binding to cell surface receptors and transmitting signals across membranes. The β common (βc) cytokine receptor family, consisting of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin (IL)-3, and IL-5 cytokine receptors, is an architype of the heterodimeric cytokine receptor systems. We now know that signaling by cytokine receptors is not always an "all or none" phenomenon. Subtle alterations of the cytokine:receptor complex can result in differential or selective signaling and underpin a variety of diseases including chronic inflammatory conditions and cancers. Structural biology techniques, such as X-ray crystallography and cryo-electron microscopy alongside cell biology studies, are providing detailed insights into cytokine receptor signaling. Recently, we found that the IL-3 receptor ternary complex forms higher-order assemblies, like those found earlier for the GM-CSF receptor, and demonstrated that functionally distinct biological signals arise from different IL-3 receptor oligomeric assemblies. As we enhance our understanding of the structural nuances of cytokine-receptor interactions, we foresee a new era of theranostics whereby structurally guided mechanism-based manipulation of cytokine signaling through rational/targeted protein engineering will harness the full potential of cytokine biology for precision medicine.

Structure-based improvement of the binding affinity and recognition specificity of peptide competitors to target pediatric IL-5R/IL-5 interaction by gluing halogen bonds at their complex interface

Human interleukin-5 (IL-5) cytokine mediates the development of eosinophils and is involved in a variety of immune inflammatory responses that play a major role in the pathogenesis of childhood asthma, leukemia, and other pediatric allergic diseases. The immunomodulatory cytokine functions by binding to its cognate cell surface receptor IL-5R in a sheet-by-sheet manner, which can be conformationally mimicked and competitively disrupted by a double-stranded cyclic AF18748 peptide. In this study, we systematically examined the co-crystallized complex structure of human IL-5R with AF18748 peptide and rationally designed a halogen bond to glue at the protein-peptide complex interface by substituting the indole moiety of AF18748 Trp13 residue with a halogen atom (X = F, Cl, Br, or I). High-level theoretical calculations imparted presence of the halogen bond between the oxygen atom (O) of IL-5R Glu58 backbone and the halogen atom (X) of AF18748 Trp13 side chain. Experimental assays confirmed that the halogen bond can promote peptide binding moderately or considerably. More importantly, the halogen bond not only enhances peptide affinity to IL-5R, but also improves peptide selectivity for its cognate IL-5R over other noncognate IL-R proteins. As might be expected, the affinity and selectivity conferred by halogen bond increase consistently in the order: H < F < Cl < Br < I. Structural modeling revealed that the halogen bond plus its vicinal π-cation-π stacking co-define a ringed noncovalent system at the complex interface, which involves a synergistic effect to effectively improve the peptide binding potency and recognition specificity.

Molecular design and systematic optimization of a halogen-bonding system between the asthma interleukin-5 receptor and its cyclic peptide ligand

Human interleukin-5 (IL-5) functions as an important pro-inflammatory factor by binding to its specific receptor, IL-5Rα, which has been implicated in the pathogenesis of asthma. Previously, a disulfide-bonded cyclic peptide AF17121 obtained from random library screening and sequence variation was found to competitively disrupt the cognate IL-5Rα/IL-5 interaction with moderate potency. In this study, the crystal complex of IL-5Rα with AF17121 was investigated at structural and energetic levels. It is revealed that the side-chain indole moiety of the AF17121 Trp5 residue is a potential site for a stem putative halogen bond (X-bond) with IL-5Rα, which is just located within the key 3 EXXR6 motif region recognized specifically by IL-5Rα. We systematically examined four halogen substitution types at five positions of the indole moiety; QM/MM calculations theoretically unraveled that only halogenations at 5 and 6 positions can form effective X-bonds with the side-chain hydroxyl oxygen of the IL-5Rα Thr21 residue and the backbone carbonyl oxygen of Ala66 residue, respectively. Binding assays observed that I-substitution at the 5 position and Br-substitution at the 6 position can result in two potent halogenated peptides, [5I]AF17121 and [6Br]AF17121, which are improved by 1.6-fold and 3.5-fold relative to the native AF17121, respectively. 5I/6Br-double substitution, resulting in [5I/6Br]AF17121, can further enhance the peptide affinity by 7.5-fold. Structural analysis revealed that the X-bond stemming from 6Br-substitution is also involved in an orthogonal interaction system with a H-bond; they share a common backbone carbonyl oxygen acceptor of IL-5Rα Ala66 residue and exhibit a significant synergistic effect between them.

Characterisation of IL-23 receptor antagonists and disease relevant mutants using fluorescent probes

Association of single nucleotide polymorphisms in the IL-23 receptor with several auto-inflammatory diseases, led to the heterodimeric receptor and its cytokine-ligand IL-23, becoming important drug targets. Successful antibody-based therapies directed against the cytokine have been licenced and a class of small peptide antagonists of the receptor have entered clinical trials. These peptide antagonists may offer therapeutic advantages over existing anti-IL-23 therapies, but little is known about their molecular pharmacology. In this study, we use a fluorescent version of IL-23 to characterise antagonists of the full-length receptor expressed by living cells using a NanoBRET competition assay. We then develop a cyclic peptide fluorescent probe, specific to the IL23p19:IL23R interface and use this molecule to characterise further receptor antagonists. Finally, we use the assays to study the immunocompromising C115Y IL23R mutation, demonstrating that the mechanism of action is a disruption of the binding epitope for IL23p19.