The role of innate immune signaling in generating tolerance and immunity to orally administered antigens

The gut associated lymphoid tissue (GALT) is exposed to many distinct antigens (Ags). When Ags from food or the intestinal flora are encountered, the GALT promotes a state of non-responsiveness to these innocuous forms of Ag. In contrast, the GALT induces strong mucosal and systemic immune responses to Ags from pathogenic microbes. The mechanisms by which the microenvironment of the GALT influences the development of tolerance and immunity are largely unknown, but recent studies have implicated a role for innate immune signaling via Toll-like receptor (TLR) pathways in modulating immune responses to Ags in the intestinal lumen. We hypothesized that feeding ovalbumin (OVA), a model dietary antigen, to mice induces Ag-specific non-responsiveness because it is presented to T-cells in the absence of innate immune activation. In contrast, we predicted that OVA delivered by an oral Salmonella vaccine generates systemic immunity to OVA because it activates bacteria responsive TLRs. To test our hypothesis, we constructed a Salmonella vaccine expressing OVA. Bacterial TLRs activate the innate immune system by signaling through the TLR adaptor protein MyD88. We predicted that MyD88 -/- mice would not be able to distinguish soluble from bacterially expressed OVA and that both forms of Ag would elicit non-responsiveness. We found, however, that vaccination of MyD88 -/- mice induced both Th1-type and Th2-type serum Ab responses to OVA. Because our laboratory has shown that TLR signaling by the intestinal flora plays a role in generating immunoregulatory cells that prevent allergic responses to food Ags, we predicted that the inability of the flora to signal through most TLRs in MyD88 -/- mice would also prevent the induction of tolerance to OVA in these mice. However, we found that intragastric administration of soluble OVA protein to MyD88 -/- mice induced non-responsiveness to Ag restimulation in vitro. Finally, because many reports have implicated a role for regulatory T-cells (Tregs) in oral tolerance and suggested that their activity is modulated by TLR signaling, we examined the role of MyD88 signaling in the generation and function of Tregs. We report that MyD88-deficiency does not alter the generation and function of CD4 + CD25 + natural Tregs.