CXCL17 Human, His

What is CXCL17 and what are its alternative names?

CXCL17 is the 17th member of the CXC chemokine family, discovered as a mucosal chemokine involved in immune cell recruitment and antimicrobial activity. It is also known as Dendritic Cell and Monocyte Chemokine-like Protein (DMC), VEGF Coregulated Chemokine 1 (VCC1), and Small Inducible Cytokine Subfamily B Member 17 (SCYB17). The protein plays functional roles in maintaining homeostasis at mucosal barriers, regulating myeloid cell recruitment, promoting angiogenesis, and controlling microorganisms . CXCL17 is strategically expressed in mucosal tissues, particularly in the respiratory system, where it is produced both under steady state and inflammatory conditions, suggesting its involvement in local immune responses .

What is the molecular structure of human CXCL17?

Human CXCL17 is initially synthesized as a 119 amino acid pro-peptide with a molecular weight of approximately 13,819 Da. This precursor protein contains six cysteine residues, with four participating in the formation of the CXC motif. Post-translational processing removes a 22 amino acid sequence from the amino-terminal domain (including two cysteine residues), resulting in a mature protein of approximately 11.5 kDa (98 amino acids) . This maturation process is functionally significant, as the four-cysteine mature peptide demonstrates approximately two-fold greater chemoattractant activity compared to the six-cysteine pro-peptide . The structural classification of CXCL17 has been subject to debate, with some researchers questioning whether it truly follows the typical CXC-type chemokine structural pattern .

Where is CXCL17 predominantly expressed in humans?

CXCL17 expression is largely restricted to mucosal tissues. In healthy human tissues, CXCL17 shows basal expression in fetal lung samples, adult trachea, bronchial and bronchiolar epithelium, and alveolar cells. It is also expressed throughout the gastrointestinal tract, including the stomach, small intestine, and colon, specifically within the epithelial lining at the luminal surface . Immunohistochemistry studies have confirmed stronger CXCL17 expression near epithelial layers exposed to the lumen of either intestine or bronchus, suggesting active secretion into these spaces. Additionally, distinct cells within mucosal tissues, morphologically resembling macrophages and plasma cells, exhibit cytoplasmic staining for CXCL17 . This strategic expression pattern at mucosal interfaces supports CXCL17's proposed roles in innate immunity and mucosal barrier function.

How does CXCL17 contribute to antimicrobial defense at mucosal surfaces?

CXCL17 demonstrates significant antimicrobial activities, contributing to the sterility and innate immunity of mucosal surfaces. Studies testing CXCL17 against a panel of pathogenic and opportunistic bacteria have revealed potent antimicrobial effects . The mechanism of antimicrobial action appears to involve peptide-mediated bacterial membrane disruption, similar to other antimicrobial chemokines like CCL28 . This antimicrobial function aligns with CXCL17's strategic expression in mucosal tissues, particularly in bronchi, where it may directly contribute to pathogen clearance. Additionally, CXCL17 recruits myeloid cells including macrophages, monocytes, and dendritic cells to mucosal sites, thereby enhancing local immune surveillance and pathogen elimination . The dual function of CXCL17 as both a chemotactic agent for immune cells and a direct antimicrobial peptide represents an efficient defense strategy at vulnerable mucosal interfaces, where rapid pathogen control is essential for preventing systemic infections.

What are the dual roles of CXCL17 in health and disease, particularly in respiratory disorders?

What are the optimal conditions for handling recombinant human CXCL17 in laboratory experiments?

Recombinant human CXCL17 for research applications is typically produced in Escherichia coli expression systems, purified to >96% as confirmed by SDS-PAGE and HPLC analyses, and supplied as a sterile filtered white lyophilized powder . For optimal stability and activity retention, CXCL17 should be stored desiccated at -20°C . When reconstituting lyophilized CXCL17, it is recommended to use sterile, neutral pH buffer (PBS or similar) and prepare small aliquots to minimize freeze-thaw cycles.

The biological activity of recombinant CXCL17 can be assessed by its ability to induce VEGF expression in murine endothelial cells, with active preparations typically demonstrating an ED50 of less than 5 μg/ml, corresponding to a specific activity greater than 200 IU/mg . For chemotaxis assays, researchers should note that the mature four-cysteine form of CXCL17 (post-translational cleavage) exhibits approximately two-fold greater chemoattractant activity compared to the six-cysteine pro-peptide , which may influence experimental design and interpretation.

What cell models are appropriate for studying CXCL17 chemotactic function?

Several cell models have been validated for investigating CXCL17 chemotactic activity. The human monocytic cell line THP-1 has been extensively used and shows responsiveness to CXCL17, particularly after overnight incubation with prostaglandin E2 (PGE2) . For real-time chemotaxis assays, the TAXIScan system has been successfully employed to investigate migratory responses of both human monocytes and THP-1 cells to CXCL17 gradients .

When designing chemotaxis experiments with CXCL17, researchers should consider that:

• Resting THP-1 cells show a trend toward directional migration along a CXCL17 gradient, but this response is significantly enhanced by overnight incubation with PGE2 .

• Freshly isolated human monocytes may not display obvious migration to CXCL17 without appropriate priming or activation .

• The GPR35 antagonist ML145 does not significantly impair THP-1 cell migratory responses to CXCL17, suggesting involvement of alternative receptors .

• CXCL17 is susceptible to cleavage by chymase, although this appears to have limited effect on its ability to recruit THP-1 cells .

Other cell models reported to respond to CXCL17 include murine J774 macrophages and myeloid-derived immature cells, which may be appropriate depending on specific research questions .

How can researchers accurately measure CXCL17 expression in tissue samples?

Accurate measurement of CXCL17 expression in tissue samples can be achieved through multiple complementary approaches. Quantitative real-time PCR (Q-PCR) has been validated for measuring CXCL17 mRNA expression in both human and murine tissues, with results showing good correlation with microarray data . For protein-level detection, immunohistochemistry (IHC) has successfully confirmed CXCL17 expression in normal human bronchus, tongue, and gastrointestinal tract tissues .

When performing IHC for CXCL17, researchers should note that expression is typically stronger near epithelial layers exposed to the lumen (of intestine or bronchus), with additional distinct cytoplasmic staining in cells morphologically resembling macrophages and plasma cells within the tissue . These macrophages typically show indented nuclei and clear cytoplasm, while plasma cells exhibit peripheral nuclei with distinct chromatin staining and abundant cytoplasm .

For quantitative measurement of CXCL17 protein in biological fluids such as bronchoalveolar lavage, ELISA methods have been employed successfully, revealing significant upregulation in conditions like idiopathic pulmonary fibrosis . When designing expression studies, researchers should consider that CXCL17 expression may vary considerably between healthy and disease states, particularly in respiratory disorders.

How should researchers interpret contradictory findings regarding CXCL17 receptor identity?

The controversy surrounding CXCL17's receptor requires careful consideration when designing receptor-targeting experiments. When interpreting contradictory findings, researchers should note that:

• CXCL17 induces calcium flux in GPR35-transfected cells but fails to signal in other GPR35-dependent assays including β-arrestin recruitment, inositol phosphate production, and receptor endocytosis .

• Silencing or antagonism of GPR35 does not inhibit migration of THP-1 cells or primary human monocytes in response to CXCL17 gradients, suggesting involvement of an unidentified receptor .

• CXCL17-induced migration is inhibited by Pertussis toxin, indicating involvement of Gαi/o-coupled signaling regardless of the exact receptor identity .

To address these contradictions, researchers should employ multiple complementary approaches when studying CXCL17 receptor interactions, including:

• Using both GPR35-transfected and untransfected cell systems in parallel

• Employing multiple readouts of receptor activation (calcium flux, arrestin recruitment, chemotaxis)

• Including appropriate positive controls for GPR35 activation with known ligands

• Testing the effects of receptor antagonists and Pertussis toxin on CXCL17 responses

These strategies can help distinguish GPR35-dependent and -independent effects of CXCL17, contributing to a more comprehensive understanding of its signaling mechanisms.

What factors influence CXCL17 function in different experimental contexts?

Several factors can significantly impact CXCL17 function in experimental settings, potentially explaining variable results across studies:

• Post-translational processing: The mature four-cysteine form of CXCL17 (post cleavage) exhibits approximately two-fold greater chemoattractant activity compared to the six-cysteine pro-peptide . Researchers should verify which form they are working with.

• Cell activation state: THP-1 cells show enhanced migration to CXCL17 after overnight incubation with PGE2, highlighting the importance of cellular priming .

• Enzymatic modification: CXCL17 is susceptible to cleavage by chymase, though this appears to have limited effect on its chemotactic function for THP-1 cells . Other enzymatic modifications might occur in different contexts.

• Receptor expression heterogeneity: The controversies surrounding CXCL17's receptor suggest that different cell types might express varying levels of GPR35 and/or alternative CXCL17 receptors, leading to context-dependent responses.

• Concentration-dependent effects: Like many chemokines, CXCL17 likely exhibits bell-shaped dose-response curves where both too low and too high concentrations may result in suboptimal responses.

Researchers should systematically control for these variables and clearly report experimental conditions to facilitate interpretation and reproducibility of CXCL17 studies.

How does the structural classification debate impact functional studies of CXCL17?

The debate regarding CXCL17's structural classification as a CXC-type chemokine has important implications for functional studies. Secondary structure predictions for CXCL17 using the DSC server predict predominantly α-helical regions, in contrast to the canonical chemokine secondary structure (three β-strands and one α-helix) observed for CXCL8 . Additionally, sequence alignment demonstrates that the four-cysteine mature form of CXCL17 does not follow the typical CXC-type cysteine pattern and may be too short to adopt a conventional chemokine fold .

These structural considerations impact functional studies in several ways:

• Structure-function relationships: If CXCL17 adopts an unconventional structure compared to typical chemokines, structure-based predictions of its functional domains may be misleading.

• Receptor interactions: The unique structural features of CXCL17 may explain its atypical receptor interactions and the ongoing controversy regarding GPR35.

• Oligomerization potential: Many chemokines form functional dimers or higher-order oligomers. If CXCL17's structure differs significantly from typical chemokines, its oligomerization properties may also differ.

• Experimental design: When designing experiments involving structure-based mutations or truncations of CXCL17, researchers should consider the structural uncertainty and validate predictions with multiple approaches.

To address these challenges, researchers are advised to combine computational modeling with experimental structural biology techniques (circular dichroism, NMR, crystallography) when investigating CXCL17's structure-function relationships, rather than relying solely on homology-based predictions from other chemokines.

What is the potential of CXCL17 as a biomarker or therapeutic target in respiratory diseases?

CXCL17 shows significant promise as both a biomarker and therapeutic target in respiratory diseases, particularly in idiopathic pulmonary fibrosis (IPF) where it is strongly upregulated in bronchoalveolar lavage fluids . As a biomarker, CXCL17 may offer valuable diagnostic or prognostic information, especially in interstitial lung diseases where early detection and disease monitoring remain challenging.

The dual role of CXCL17 in both protective and pathogenic processes presents interesting therapeutic possibilities. In conditions where CXCL17's antimicrobial activities might be beneficial, such as respiratory infections, enhancing its expression or activity could potentially augment host defense. Conversely, in fibrotic or inflammatory conditions where elevated CXCL17 contributes to pathology, targeted inhibition might be therapeutic.

Future research should focus on:

• Validating CXCL17 as a diagnostic/prognostic biomarker in larger patient cohorts

• Determining whether CXCL17 levels correlate with disease severity or treatment response

• Developing targeted approaches to modulate CXCL17 activity in specific disease contexts

• Investigating the relationship between CXCL17 and other inflammatory mediators in respiratory pathologies

These investigations will help clarify whether CXCL17-targeted therapies could benefit patients with respiratory disorders while minimizing potential adverse effects on protective mucosal immunity.

What are the methodological challenges in studying CXCL17's antimicrobial activities?

Investigating CXCL17's antimicrobial properties presents several methodological challenges that researchers should address:

• Defining optimal antimicrobial assay conditions: The activity of antimicrobial peptides often depends on pH, ionic strength, and the presence of serum proteins. Researchers should test CXCL17 under conditions that mimic the mucosal environment.

• Distinguishing direct killing from immune cell recruitment effects: CXCL17's dual functions as both a direct antimicrobial peptide and a recruiter of immune cells complicate in vivo studies. Careful experimental design using purified systems and appropriate controls is necessary to differentiate these mechanisms.

• Determining structure-activity relationships: The specific domains or structural features of CXCL17 responsible for its antimicrobial activity remain to be fully characterized. Structure-function studies with truncated or mutated peptides can help identify the antimicrobial domain(s).

• Assessing activity against relevant pathogens: CXCL17's expression in mucosal tissues suggests it may have evolved to target specific mucosal pathogens. Testing against a comprehensive panel of mucosal pathogens, including bacteria, fungi, and viruses, would provide valuable insights.

• Evaluating potential synergies: CXCL17 likely acts in concert with other antimicrobial peptides and immune factors at mucosal surfaces. Investigating potential synergistic or antagonistic interactions would better reflect the in vivo situation.