Fig. 2.

PolyI:C enhances IgA production in the NALT via the TLR3-TICAM1 pathway. After the intranasal administration of an HA vaccine and polyI:C, CD103⁺ DCs incorporate polyI:C and are activated through the TLR3-TICAM1 pathway. CD103⁺ DCs express BAFF and APRIL to activate B cells. PolyI:C also induces Th2-type cytokines, including IL-4, IL-6, and IL-10, in a TLR3-dependent manner. Furthermore, polyI:C enhances the formation of germinal centers and the expression of the Aicda gene, coding AID protein. Th2-type cytokines and TGF-β signaling promote IgA class switching in the NALT. The secretory IgA produced is secreted to the nasal mucosa through the polymeric immunoglobulin receptor expressed on epithelial cells in the NALT. AID, activation-induced cytidine deaminase; BAFF, B-cell-activating factor; DC, dendritic cell; HA, hemagglutinin; TGF-β, transforming growth factor β; TLR, toll-like receptor.