Role of Toll-like receptors in host responses to a virulence antigen of Streptococcus mutans expressed by a recombinant, attenuated Salmonella vector vaccine

In the present study, we investigated the role of Toll-like receptors (TLRs) in host responses to the saliva-binding region (SBR) of Streptococcus mutans expressed by a recombinant, attenuated Salmonella vaccine. C57BL/6 wild type (wt), TLR2−/−, TLR4−/− and MyD88−/− mice were immunized by the intranasal route on days 0, 18 and boosted on day 98 with Salmonella typhimurium BRD 509 containing a plasmid encoding SBR. Serum and saliva samples were collected throughout the experiment and assessed for antibody activity by ELISA. Evidence is provided that the induction of a serum IgG2a (Th1-type) anti-SBR antibody response involved TLR2 signaling, whereas the anti-Salmonella response involved signaling through TLR4. The adaptor molecule MyD88 was not essential for the induction of a primary Th1-type response to SBR or Salmonella, but was necessary for a secondary response to SBR. Furthermore, the absence of TLR2, TLR4 or MyD88 resulted in enhanced Th2-type serum IgG1 anti-SBR and anti-Salmonella responses. Mucosal IgA responses to SBR were TLR2-, TLR4- and MyD88-dependent, while IgA responses to Salmonella were TLR4- and MyD88-dependent.