IL 17F His Human

What is IL-17F and how does it relate to other IL-17 family members?

IL-17F is a prominent member of the IL-17 family of cytokines that regulates both innate and adaptive immunity. It shares approximately 50% amino acid sequence identity with IL-17A, making them the most closely related members of the IL-17 protein family. Both cytokines are highly conserved at the C terminus and contain five spatially conserved cysteine residues that mediate dimerization . These proteins can exist as homodimers (IL-17A/A or IL-17F/F) or as the IL-17A/F heterodimer. All three forms signal through the assembly of a ternary complex with the IL-17RA and IL-17RC receptors .

The IL-17 family consists of six members (IL-17A through IL-17F), with IL-17A and IL-17F being the most extensively studied. These cytokines are highly pro-inflammatory and serve as "signature cytokines" of effector T helper 17 (Th17) cells. They play critical roles in host defense against extracellular bacteria and fungi, particularly at mucosal barriers, and have been implicated in various inflammatory and autoimmune diseases .

What cellular sources produce IL-17F in humans?

IL-17F is produced by multiple cell types involved in both innate and adaptive immunity. The primary cellular sources include:

These cells are often strategically positioned at barrier tissues such as the skin, gut, and lung, where they play key roles in antimicrobial defense . The Th17 subset of CD4+ T cells represents the predominant adaptive immune cell type producing IL-17F. Differentiation of this lineage from naive CD4+ T cells requires signals from IL-6 and transforming growth factor-β (TGF-β), while maintenance depends on IL-23 and IL-21 . Interestingly, pathogenic Th17 cells can also be generated in a TGF-β-independent manner, and commensal bacteria, particularly segmented filamentous bacteria, strongly induce Th17 cells in the lamina propria of the small intestine .

What is the structural basis for IL-17F and IL-17A/F heterodimer receptor binding?

The crystal structures of IL-17A, IL-17F, and the IL-17A/F heterodimer provide crucial insights into their receptor interactions. X-ray analysis of the human IL-17A/F heterodimer reveals a "two-faced cytokine" that closely mimics both IL-17A and IL-17F on its respective faces . This unique structural characteristic explains the intermediate binding properties of the heterodimer.

IL-17A and IL-17F signal through a heterotrimeric receptor complex comprising IL-17RA and IL-17RC, both of which are single-pass type I membrane receptors containing an intracellular signaling motif called the SEFIR domain . Structural information from human IL-17A and IL-17F homodimers and their respective binary complexes with IL-17RA has revealed that receptor binding induces allosteric changes in the cytokine dimer. These changes lead to the formation of asymmetric binary and ternary receptor complexes, which contrasts sharply with TGF-β and BMPs that form symmetrical signaling complexes in a 2:2:2 stoichiometry .

The binding affinities of these cytokines to their receptors vary significantly. Human IL-17RA shows widely different affinities for human IL-17A, IL-17F, and IL-17A/F: compared to IL-17A, IL-17F binds with at least 100-fold lower affinity to IL-17RA but binds with equally high affinity to IL-17RC. The IL-17A/F heterodimer shows intermediate binding affinity to IL-17RA .

How does the IL-17A/F heterodimer interact with its receptors, and what are the functional implications?

The IL-17A/F heterodimer exhibits unique receptor binding properties that differentiate it from the homodimers. Unexpectedly, despite the much higher affinity of IL-17RA toward IL-17A, crystallographic studies have shown that IL-17RA can bind to the "F-face" of the heterodimer. Site-directed mutagenesis experiments have demonstrated that IL-17RA can also bind to the "A-face" of IL-17A/F with similar affinity .

Furthermore, IL-17RC does not discriminate between the two faces of the cytokine heterodimer either. This flexibility in receptor binding enables the formation of two topologically-distinct heterotrimeric complexes with potentially different signaling properties . These findings suggest a more complex signaling mechanism than previously thought, where the heterodimer might elicit distinct biological responses depending on which face interacts with which receptor.

The biological potencies of IL-17A, IL-17F, and IL-17A/F in inducing IL-6 and CXCL1 expression in cell-based assays correlate with their binding affinities toward the IL-17RA receptor . This explains why IL-17A has stronger pro-inflammatory effects than IL-17F in most experimental systems, while the IL-17A/F heterodimer exhibits intermediate activity.

What are optimal approaches for studying IL-17F signaling in experimental systems?

When investigating IL-17F signaling, researchers should consider several methodological approaches:

• Receptor binding assays: To characterize the interaction between His-tagged IL-17F and IL-17RA/RC receptors, surface plasmon resonance or bioluminescence resonance energy transfer can be employed.

• Cell-based functional assays: IL-17F acts on fibroblasts, epithelial cells, and endothelial cells to induce pro-inflammatory responses. Researchers can measure:

  • Pro-inflammatory cytokine production (IL-6, IL-1, TNF)

  • Chemokine expression (IL-8, CXCL1, CXCL5, CCL2, CCL7)

  • Matrix metalloproteinase production (MMP1, MMP3, MMP13)

  • Antimicrobial factor expression (defensins, S100 proteins)

• Signaling pathway analysis: IL-17F signals primarily through TRAF6-dependent activation of the NF-κB and MAP kinase pathways. Phosphorylation assays and reporter gene systems can be used to monitor these pathways .

• In vivo models: For studying neutrophil recruitment and inflammation, the air pouch model has been validated for IL-17F research .

• Comparative analysis: When using His-tagged IL-17F, comparing its activity with non-tagged versions is essential to ensure the tag doesn't interfere with biological function.

A recently identified 20-amino acid IL-17A/F-derived peptide (nIL-17) mimics the pro-inflammatory actions of the full-length proteins and could serve as a useful tool for signaling studies . This peptide activates IL-17RA/C-dependent intracellular signaling in NIH-3T3 mouse embryonic fibroblast cells and human dermal blood endothelial cells (HDBECs), inducing cytokine, chemokine, and adhesion molecule expression .

How can researchers distinguish between effects of IL-17F homodimers versus IL-17A/F heterodimers?

Distinguishing between the biological effects of IL-17F homodimers and IL-17A/F heterodimers presents a significant experimental challenge. Several strategies can be employed:

• Recombinant protein approaches: Use purified recombinant IL-17F homodimers and IL-17A/F heterodimers in parallel experiments. His-tagged versions can facilitate purification and detection.

• Selective neutralization: Employ antibodies or inhibitors with differential specificity for IL-17A versus IL-17F. The novel antibody Ab-IPL-IL-17, which targets a bioactive 20 amino acid IL-17A/F-derived peptide (nIL-17), can neutralize both the peptide and IL-17A/F with minimal off-target effects compared to secukinumab .

• Receptor mutants: Generate cells expressing IL-17RA or IL-17RC variants with altered binding preference for either the A-face or F-face of the heterodimer.

• Dose-response analysis: Since IL-17A binds IL-17RA with higher affinity than IL-17F, while both bind IL-17RC with similar affinity, dose-response curves may help distinguish their effects.

• Genetic approaches: In experimental systems where possible, selective knockout of IL-17A or IL-17F can help delineate their individual contributions.

When interpreting results, researchers should consider that the IL-17A/F heterodimer can interact with receptors through either its A-face or F-face, potentially forming topologically distinct signaling complexes with different biological outcomes . This complexity necessitates careful experimental design and interpretation.

Role in Disease and Therapeutic Applications

Several therapeutic approaches targeting the IL-17 pathway are in development or clinical use:

The therapeutic efficacy of these approaches may vary depending on the relative contributions of IL-17A, IL-17F, and IL-17A/F to specific diseases, highlighting the importance of understanding their distinct roles in pathogenesis .

How do IL-17F and IL-17A/F coordinate with other cytokine networks in inflammatory responses?

IL-17F functions within a complex network of cytokine interactions that shape inflammatory responses. These interactions include:

• Upstream regulation: IL-17F production by Th17 cells is regulated by several cytokines, including IL-6, TGF-β, IL-23, and IL-21. Different cytokine combinations can generate functionally distinct Th17 cells, including pathogenic and non-pathogenic subsets .

• Downstream effects: IL-17F induces the secretion of:

  • Pro-inflammatory cytokines (IL-6, IL-1, TNF)

  • Granulopoietic factors (G-CSF, GM-CSF)

  • Antimicrobial factors (defensins, S100 proteins)

  • Chemokines (IL-8, CXCL1, CXCL5, CCL2, CCL7)

  • Matrix metalloproteinases (MMP1, MMP3, MMP13)

• Synergistic interactions: IL-17F often functions synergistically with other cytokines like TNF-α, amplifying inflammatory responses beyond what either cytokine alone would produce.

• Microbial influence: Commensal bacteria, particularly segmented filamentous bacteria, strongly induce Th17 cells in the lamina propria, influencing IL-17F production. Antibiotic-mediated depletion of these bacteria inhibits Th17 differentiation .

Understanding these complex interactions requires consideration of the tissue microenvironment, temporal sequence of cytokine exposure, and cell-type specific responses. Systems biology approaches that integrate this complexity may help develop more precise and effective therapeutic interventions.

What are emerging techniques for studying the structural dynamics of IL-17F interactions with its receptors?

Advanced techniques to study IL-17F-receptor interactions include:

• Cryo-electron microscopy (cryo-EM): This technique can capture IL-17F and IL-17A/F in complex with their receptors in near-native conditions, potentially revealing conformational changes that occur during receptor engagement.

• Single-molecule FRET: This approach can monitor real-time conformational changes in IL-17F or its receptors during complex formation, providing insights into the dynamics of these interactions.

• Hydrogen-deuterium exchange mass spectrometry (HDX-MS): This technique identifies regions of proteins that undergo conformational changes upon binding, helping map interaction interfaces and allosteric effects.

• Molecular dynamics simulations: Computational approaches can model the dynamic behavior of IL-17F-receptor complexes, particularly the conformational changes induced by receptor binding.

• Site-directed mutagenesis combined with structural studies: This approach has already revealed that IL-17RA can bind to both the "A-face" and "F-face" of the IL-17A/F heterodimer with similar affinity, and that IL-17RC does not discriminate between the two faces . Further mutations could elucidate the specific residues critical for these interactions.

These techniques can help address key questions about how the "two-faced" nature of the IL-17A/F heterodimer influences receptor binding and subsequent signaling events. Understanding these structural dynamics may guide the development of more specific therapeutic agents that modulate IL-17 signaling in disease contexts while preserving beneficial host defense functions.