CXCL17 binds efficaciously to glycosaminoglycans with the potential to modulate chemokine signaling

Identification of orphan GPR25 as a receptor for the chemokine CXCL17

C-X-C motif chemokine ligand 17 (CXCL17) is a small secretory protein primarily expressed in mucosal tissues, which likely functions as a chemoattractant; however, its receptor is controversial. Herein, we identified the rarely studied orphan G protein-coupled receptor 25 (GPR25) as a receptor of CXCL17 via prediction using the newly developed AlphaFold 3 algorithm and experimental validation. In the NanoLuc Binary Technology (NanoBiT)-based β-arrestin recruitment assay, recombinant human CXCL17 could activate human GPR25 in transfected human embryonic kidney (HEK) 293T cells with an EC50 value around 100 nm, but it had no activation effect on the other 17 tested G protein-coupled receptors. Deletion of three conserved C-terminal residues from human CXCL17 almost abolished its activation effect. Alanine replacement of W95 or R178 of human GPR25, two conserved residues in the predicted orthosteric ligand binding pocket, almost abolished its response to CXCL17. Only the pairing of wild-type CXCL17 with wild-type GPR25 could cause shedding of transforming growth factor α and induce chemotactic movement of transfected HEK293T cells. These results were consistent with the AlphaFold 3-predicted binding model, in which the highly conserved C-terminal fragment of CXCL17 inserts into the orthosteric ligand binding pocket of GPR25. According to their expression pattern shown in the Human Protein Atlas, CXCL17 may be an endogenous agonist of GPR25 in humans and other mammals; however, this hypothesis needs to be tested experimentally in future studies. The present deorphanization paves the way for further functional characterization of the orphan receptor GPR25 and the orphan ligand CXCL17.

The N-terminal ELR+ motif of the neutrophil attractant CXCL8 confers susceptibility to degradation by the Group A streptococcal protease, SpyCEP

Streptococcus pyogenes (Group A Streptococcus or GAS) is a major human pathogen for which an effective vaccine is highly desirable. Invasive S. pyogenes strains evade the host immune response in part by producing a cell envelope protease, SpyCEP. This neutralizes chemokines containing an N-terminal Glu-Leu-Arg motif (ELR+ chemokines) by cleavage at a distal C-terminal site within the chemokine. SpyCEP is a component of several S. pyogenes vaccines, yet the molecular determinants underlying substrate selectivity are poorly understood. We hypothesized that chemokine recognition and cleavage is a multistep process involving distinct domains of both substrate and enzyme. We generated a panel of recombinant CXCL8 variants where domains of the chemokine were exchanged or mutated. Chemokine degradation by SpyCEP was assessed by SDS-PAGE, Western blot, and ELISA. Extension of the CXCL8 N-terminus was found to inhibit chemokine cleavage. Reciprocal exchanges of the N-termini of CXCL8 with that of the ELR- chemokine CXCL4 resulted in the generation of loss of function and gain of function substrates. This suggested a key role for the ELR motif in substrate recognition, which was supported directly by alanine substitution of the ELR motif of CXCL8, impairing the parameters, KM, Vmax, and Kcat in kinetic assays with SpyCEP. Collectively, our findings identify the N-terminal ELR motif as a major determinant for recognition by SpyCEP and expose a vulnerability in the mechanism by which the protease recognises its substrates. This likely presents potential avenues for therapeutic intervention via targeted vaccine design and small molecule inhibition.

New insight into a simple high-yielding method for the production of fully folded and functional recombinant human CCL5

Chemokines are proteins important for a range of biological processes from cell-directed migration (chemotaxis) to cell activation and differentiation. Chemokine C-C ligand 5 (CCL5) is an important pro-inflammatory chemokine attracting immune cells towards inflammatory sites through interaction with its receptors CCR1/3/5. Recombinant production of large quantities of CCL5 in Escherichia coli is challenging due to formation of inclusion bodies which necessitates refolding, often leading to low recovery of biologically active protein. To combat this, we have developed a method for CCL5 production that utilises the purification of SUMO tagged CCL5 from E. coli SHuffle cells avoiding the need to reform disulfide bonds through inclusion body purification and yields high quantities of CCL5 (~ 25 mg/L). We demonstrated that the CCL5 produced was fully functional by assessing well-established cellular changes triggered by CCL5 binding to CCR5, including receptor phosphorylation and internalisation, intracellular signalling leading to calcium flux, as well as cell migration. Overall, we demonstrate that the use of solubility tags, SHuffle cells and low pH dialysis constitutes an approach that increases purification yields of active CCL5 with low endotoxin contamination for biological studies.

CXCL17 is a proinflammatory chemokine and promotes neutrophil trafficking

CXCL17, a novel member of the CXC chemokine class, has been implicated in several human pathologies, but its role in mediating immune response is not well understood. Characteristic features of immune response include resident macrophages orchestrating successive and structured recruitment of neutrophils and monocytes to the insult site. Here, we show that Cxcl17 knockout (KO) mice, compared with the littermate wild-type control mice, were significantly impaired in peritoneal neutrophil recruitment post-lipopolysaccharide (LPS) challenge. Further, the KO mice show dysregulated Cxcl1, Cxcr2, and interleukin-6 levels, all of which directly impact neutrophil recruitment. Importantly, the KO mice showed no difference in monocyte recruitment post-LPS challenge or in peritoneal macrophage levels in both unchallenged and LPS-challenged mice. We conclude that Cxcl17 is a proinflammatory chemokine and that it plays an important role in the early proinflammatory response by promoting neutrophil recruitment to the insult site.