Oral vaccination induces protective immunity both systemically and at the site of infection. Live attenuated bacterial vaccine vectors with potent Toll-like receptor (TLR) ligands are attractive vessels for reliably delivering vaccine antigen to the gut. Whether TLR signaling through the adaptor molecule MyD88 is required to initiate an antibody response remains unclear. Using a novel ovalbumin (OVA)-expressing recombinant Salmonella vaccine, we demonstrated that the dependence of serum antibody responses on MyD88-mediated signaling varied with immunization route. Oral and intraperitoneal vaccination, but not subcutaneous footpad injection induced predominantly Th2 and TGF-β dependent antibody isotypes in MyD88^-/- mice. Adoptively transferred, MyD88-sufficient, activated, antigen-specific CD4⁺ T cells accumulated in draining lymph nodes of orally vaccinated MyD88^-/- mice but failed to produce IFN-γ. CD1d tetramer staining revealed that oral vaccination with Salmonella -OVA activated invariant (i)NKT cells in wild type (WT), but not MyD88^-/- mice. Anti-CD1d neutralizing antibody treatment reduced the OVA-specific serum IgG2c response in WT mice only. Our data suggest that the type of adaptive immune response generated to this live attenuated vaccine depends on MyD88-mediated signals and vaccination route.

The regions where gastrointestinal helminth infection is endemic overlap with regions that house populations targeted by global health organizations for improved vaccine coverage. Moreover, some diseases remain refractory to effective vaccination, particularly within these helminth-endemic regions. Yet, the impact of chronic parasitic infection on vaccine efficacy has not been extensively explored. We observed that chronic infection with the intestinal nematode Heligmosomoides polygyrus reduced antigen-specific humoral responses to both intramuscular injection with OVA adsorbed to alum and oral vaccination with Salmonella-OVA. Activated, vaccine antigen-specific CD4⁺ T cells accumulated in the draining MLNs of both uninfected and helminth-infected mice orally vaccinated with Salmonella-OVA, suggesting that reduced antibody responses in helminth-infected mice were not due to impaired antigen-specific T effector responses. Helminth infection increased frequencies of adoptively transferred vaccine antigen-specific IFN-γ, IL-4, and IL-10 producing CD4⁺ T cells in the draining MLN. Our findings suggest that helminth infection-induced IL-10-secreting CD4⁺ T regulatory cells may suppress vaccine-induced humoral responses and underscore the potential need to treat parasitic infection before mass vaccination campaigns in helminth-endemic areas.