Toll-like receptor 2 activators modulate oral tolerance in mice

Background: Toll-like receptor 2 (TLR2) is a widely expressed pattern recognition receptor critical for innate immunity. TLR2 is also a key regulator of mucosal immunity implicated in the development of allergic disease. TLR2 activators are found in many common foods, but the role of TLR2 in oral tolerance and allergic sensitization to foods is not well understood. Objective: The purpose of this study was to evaluate the impacts of TLR2 expression and TLR2 activation on oral tolerance to food antigens in a murine model. Methods: Mice were fed ovalbumin (OVA) or peanut butter with or without the addition of low doses of TLR2 activators Pam₃CSK₄ or FSL-1. Oral tolerance was assessed by analysing antibody responses after a systemic antigen challenge. OVA-specific Tregs were assessed in the Peyer's patches, mesenteric lymph nodes, and spleen in wild-type and TLR2^-/- mice. Low-dose Pam₃CSK₄ was also tested as an oral adjuvant. Results: Oral tolerance was successfully induced in both wild-type and TLR2^-/- recipient mice, with an associated regulatory T-cell response. Oral TLR2 activation, with low-dose Pam₃CSK₄ or FSL-1, during oral antigen exposure was found to alter oral tolerance and was associated with the development of substantial IgE and IgA responses to foods upon systemic challenge. Low-dose oral Pam₃CSK₄ treatment also selectively enhanced antigen-specific IgA responses to oral antigen exposure. Conclusions and Clinical Relevance: TLR2 is not necessary for oral tolerance induction, but oral TLR2 activation modulates humoral IgE and IgA responses during tolerance development. Low-dose Pam₃CSK₄ is also an effective oral adjuvant that selectively enhances IgA production. These observations are pertinent to the optimization of oral allergen immunotherapy and oral vaccine development.