Coordination Between T Helper Cells, Inkt Cells, And Their Follicular Helper Subsets In The Humoral Immune Response Against Clostridium Difficile Toxin B

Activation of iNKT cells with the CD1d-binding glycolipid adjuvant alpha-galactosylceramide (alpha-GC) enhances humoral immunity specific for coadministered T-dependent Ag. However, the relationship between the iNKT cell and the classic T helper (Th) or T follicular helper (Tfh) function following this immunization modality remains unclear. We show that immunization with the C-terminal domain (CTD) of Clostridium difficile toxin B (TcdB), accompanied by activation of iNKT cells with alpha-GC, led to enhanced production of CTD-specific IgG, which was CD1d- and iNKT cell-dependent and associated with increased neutralization of active TcdB. Immunization with CTD plus alpha-GC followed by NP hapten-linked CTD increased NP-specific IgG1 titers in an NKT-dependent manner, suggesting that iNKT activation could enhance Th or Tfh function or that iNKT and iNKTfh cells could provide supplemental, yet independent, B cell help. Th, Tfh, iNKT, and iNKTfh cells were, therefore, examined quantitatively, phenotypically, and functionally following immunization with CTD or with CTD plus alpha-GC. Our results demonstrated that alpha-GC-activated iNKT cells had no direct effect on the numbers, phenotype, or function of Th or Tfh cells. However, CD4(+) T cell-specific ablation of the Bc16 transcription factor demonstrated that Tfh and iNKTfh cells both contributed to B cell help. This work extends our understanding of the immune response to vaccination and demonstrates an important contribution by NKTfh cells to humoral immunity.