Bacterial flagellin induces IL-6 expression in human basophils

Area of Science:

• Immunology
• Cell Biology

Background:

• Basophils are key players in allergic inflammation, releasing mediators upon allergen-IgE binding.
• Basophils also express pattern-recognition receptors (PRRs) like Toll-like receptors (TLRs) that recognize microbe-associated molecular patterns (MAMPs).
• The role of MAMPs in activating basophils independently of allergen-IgE is not fully understood.

Purpose of the Study:

• To investigate if MAMPs alone can induce Interleukin-6 (IL-6) production in human basophils.
• To identify the specific TLRs involved in MAMP-induced IL-6 production.
• To elucidate the signaling pathways mediating flagellin-induced IL-6 production in basophils.

Main Methods:

• Human basophil cell line (KU812) and primary human basophils were stimulated with various MAMPs (flagellin, lipoteichoic acid, peptidoglycan, poly I:C).
• IL-6 production was measured, and TLR expression (TLR2, TLR3, TLR5) was analyzed using flow cytometry.
• Mitogen-activated protein kinases (MAPKs), signaling molecules (CREB, PKCδ, AKT), and transcription factors (NF-κB, AP-1, CREB) were assessed via phosphorylation and DNA binding activity assays.
• Inhibitors were used to probe signaling pathways, including p38 kinase, ERK, JNK, PKA, PKC, and PI3K.

Main Results:

• Flagellin, but not other tested MAMPs, induced IL-6 production in KU812 cells and primary human basophils.
• KU812 cells expressed TLR5 on the cell surface and cytoplasm, consistent with flagellin recognition.
• Flagellin-induced IL-6 production involved p38 kinase, ERK, CREB, PKCδ, AKT, NF-κB, and AP-1 activation, with PI3K negatively regulating the pathway.

Conclusions:

• Flagellin alone is sufficient to induce IL-6 gene expression in human basophils.
• TLR5 signaling is the primary pathway for flagellin-mediated IL-6 induction in basophils.
• These findings reveal a novel mechanism by which microbial components can influence allergic inflammation independently of allergen-specific IgE.