TRIF signaling is essential for TLR4-driven IgE class switching

Area of Science:

• Immunology
• Cell Biology
• Molecular Biology

Background:

• Toll-like receptor 4 (TLR4) ligands, such as lipopolysaccharide (LPS), induce B cell immunoglobulin (Ig) isotype switching.
• Interleukin-4 (IL-4) is a key cytokine promoting IgE and IgG1 class switching.

Purpose of the Study:

• To elucidate the distinct roles of TLR4 adaptor molecules MyD88 and TRAM/TRIF in LPS-induced IgE and IgG1 class switching in mouse B cells.
• To investigate the signaling pathways, including NF-κB activation, involved in IgE class switching.

Main Methods:

• Utilized knockout mouse models for adaptor molecules (TRAM, TRIF, MyD88) and wild-type B cells.
• Stimulated B cells with LPS plus IL-4 or anti-CD40 plus IL-4.
• Assessed germline transcript (GLT) expression (Cε, Cγ1), Ig secretion (IgE, IgG1), Aicda expression, and NF-κB p65 nuclear translocation.
• Employed an NF-κB inhibitor (JSH-23) to assess its impact on class switching.

Main Results:

• TRAM/TRIF-dependent signaling was essential for Cε germline transcript (GLT) expression and IgE secretion following LPS plus IL-4 stimulation.
• MyD88-deficient B cells showed reduced but not abolished IgE secretion, with normal Cε GLT expression.
• Cγ1 GLT expression and IgG1 secretion were modestly reduced in TRAM/TRIF-deficient cells and significantly reduced in all knockout strains (TRAM/TRIF/MyD88).
• TRIF-deficient B cells exhibited impaired sustained NF-κB p65 nuclear translocation and binding to the Iε promoter.
• NF-κB inhibition selectively blocked Cε GLT expression and IgE secretion, with minimal impact on Cγ1 GLT and IgG1.

Conclusions:

• Sustained NF-κB activation, driven by TRIF signaling, is indispensable for LPS plus IL-4-induced Cε locus activation and IgE class switching.
• Distinct signaling pathways mediated by MyD88 and TRAM/TRIF differentially regulate IgE and IgG1 class switching.