Topical Resiquimod Protects against Visceral Infection with Leishmania infantum chagasi in Mice

Abstract

New prevention and treatment strategies are needed for visceral leishmaniasis, particularly ones that can be deployed simply and inexpensively in areas where leishmaniasis is endemic. Synthetic molecules that activate Toll-like receptor 7 and 8 (TLR7/8) pathways have previously been demonstrated to enhance protection against cutaneous leishmaniasis. We initially sought to determine whether the TLR7/8-activating molecule resiquimod might serve as an effective vaccine adjuvant targeting visceral leishmaniasis caused by infection with Leishmania infantum chagasi. Resiquimod was topically applied to the skin of mice either prior to or after systemic infection with L. infantum chagasi, and parasite burdens were assessed. Surprisingly, topical resiquimod application alone, in the absence of vaccination, conferred robust resistance to mice against future intravenous challenge with virulent L. infantum chagasi. This protection against L. infantum chagasi infection persisted as long as 8 weeks after the final topical resiquimod treatment. In addition, in mice with existing infections, therapeutic treatment with topical resiquimod led to significantly lower visceral parasite loads. Resiquimod increased trafficking of leukocytes, including B cells, CD4؉ and CD8 ؉ T cells, dendritic cells, macrophages, and granulocytes, in livers and spleens, which are the key target organs of visceralizing infection. We conclude that topical resiquimod leads to systemic immune modulation and confers durable protection against visceralizing L. infantum chagasi infection, in both prophylactic and therapeutic settings. These studies support continued studies of TLR-modulating agents to determine mechanisms of protection and also provide a rationale for translational development of a critically needed, novel class of topical, preventative, and therapeutic agents for these lethal infections.

Insights on adaptive and innate immunity in canine leishmaniosis

“…In Leishmania, there has already been some progress with the use of ligands in vaccine development using mice models, which is promising for future dog studies. Most recently, Craft et al reported the use of topical resiquimod (a synthetic TLR7/8 activating molecule), inducing protection against L. infantum infection in mice (Craft et al 2014). Topical application to the skin of mice prior to, or following systemic infection resulted in conferred resistance to future intravenous challenge and protection that persisted for as long as 8 weeks after the first topical treatment (Craft et al 2014).…”

“…Most recently, Craft et al reported the use of topical resiquimod (a synthetic TLR7/8 activating molecule), inducing protection against L. infantum infection in mice (Craft et al 2014). Topical application to the skin of mice prior to, or following systemic infection resulted in conferred resistance to future intravenous challenge and protection that persisted for as long as 8 weeks after the first topical treatment (Craft et al 2014). Mice with existing infections were also found to have significantly lower visceral parasite loads following topical resiquimod treatment.…”

“…Mice with existing infections were also found to have significantly lower visceral parasite loads following topical resiquimod treatment. Resiquimod was found to increase the trafficking of leukocytes, B-cells, dendritic cells, macrophages, granulocytes, CD4 + and CD8 + T cells in livers and spleens of the mice (Craft et al 2014). In an earlier study, subcutaneous vaccination with imiquimod, a TLR7/8 agonist, was shown to mediate a Th1 response against L. major antigen, which suppressed Th2 responses following a challenge infection (Zhang and Matlashewski, 2008).…”

“…In an earlier study, subcutaneous vaccination with imiquimod, a TLR7/8 agonist, was shown to mediate a Th1 response against L. major antigen, which suppressed Th2 responses following a challenge infection (Zhang and Matlashewski, 2008). Both studies suggested the use of these compounds as potential vaccine adjuvants for vaccines or topical preventative and therapeutic agents (Zhang and Matlashewski, 2008; Craft et al 2014). …”

Toll-like receptors and cutaneous melanoma

“…Clinical studies have demonstrated the safety and the efficacy of topical application of resiquimod and its analogs in activating the local immune response (64)(65)(66)(67). However, resiquimod injection may induce systemic cytokine release and thus, must only be formulated to cause local immune activation, preventing systemic effects (68).…”