Abstract
Our previous studies demonstrated the potential of the sublingual (s.l.) route for delivering vaccines capable of inducing mucosal as well as systemic immune responses. Those findings prompted us to attempt to identify possible inductive mechanism of s.l. vaccination for immune responses. Within 2 h after s.l. administration with cholera toxin (CT), significantly higher numbers of MHC class II(+) cells accumulated in the s.l. mucosa. Of note, there were brisk expression levels of both CCL19 and CCL21 in cervical lymph nodes (CLN) 24 h after s.l. vaccination with CT. In reconstitution experiments using OVA-specific CD4(+) or CD8(+) T cells, s.l. vaccination elicited strong Ag-specific T cell proliferation mainly in CLN. Interestingly, Ag-specific T cell proliferation completely disappeared in CD11c-depleted and CCR7(-/-) mice but not in Langerin-depleted, macrophage-depleted, and CCR6(-/-) mice. Similar to CD4(+) T cell responses, induction of Ag-specific IgG (systemic) and IgA (mucosal) Ab responses were significantly reduced in CD11c-depleted and CCR7(-/-) mice after s.l. vaccination with OVA plus CT. Although CD8alpha(-) dendritic cells ferried Ag from the s.l. mucosa, both migratory CD8alpha(-) and resident CD8alpha(+) dendritic cells were essential to prime CD4(+) T cells in the CLN. On the basis of these findings, we believe that CCR7 expressed CD8alpha(-)CD11c(+) cells ferry Ag in the s.l. mucosa, migrate into the CLN, and share the Ag with resident CD8alpha(+)CD11c(+) cells for the initiation of Ag-specific T and B cell responses following s.l. challenge. We propose that the s.l. mucosa is one of the effective mucosal inductive sites regulated by the CCR7-CCL19/CCL21 pathway.