Immunization with a Toll-like receptor 7 and/or 8 agonist vaccine adjuvant increases protective immunity against Leishmania major in BALB/c mice.

Activation of Toll-like receptors (TLRs) on antigen-presenting cells of the innate immune system initiates, amplifies, and directs the antigen-specific acquired immune response. Ligands that stimulate TLRs therefore represent potential vaccine adjuvants. In the present study, we determined whether imiquimod and its related compound R848, which are TLR7 and/or TLR8 agonists, represent potential vaccine adjuvants when delivered topically, subcutaneously, or intramuscularly. Using the Leishmania major infection model in BALB/c mice, vaccination with crude Leishmania antigen was not protective against subsequent challenge infection unless it was administered with R848 or a topical application of imiquimod containing cream on the skin. Subcutaneous vaccination with these adjuvants mediated a TH1 response against L. major antigen, which appeared to suppress the TH2 response following a challenge infection. Protective immunity was generated following subcutaneous vaccination but not intramuscular vaccination. These observations suggest that topically administered imiquimod or subcutaneously injected R848 represent potential vaccine adjuvants to enhance the TH1 response, which can be used with existing or new vaccine formulations.