RESUMO
BACKGROUND: Immunization with killed Leishmania promastigotes without adjuvant was considered as safe, but gave variable rates of protection. Taking advantage of the immuno-modulatory effect of caffeic acid (CA), a natural polyphenolic antioxidant, we investigated its potentiating effect in autoclaved Leishmania major (ALM)-induced protection of Leishmania major-infected BALB/c. METHODS: First, BALB/c mice were sensitized for 6 weeks either with CA, or ALM alone or combined with caffeic acid (ALM-CA) or Freund's adjuvant (ALM-FA), and subsequently infected with L. major promastigotes. Second, to test the curative effect, CA was given daily for 5 weeks to susceptible mice, starting on week 4 post-infection. Sera, footpads and lymph nodes (LNs) were collected at week 9 post-infection and submitted to biochemical or histological analyses. RESULTS: Compared to the respective controls, our results showed that CA directly healed footpad lesions and reduced the hallmarks of cutaneous leishmaniasis including oxidative inflammation, parasite load, and phagocytes influx and infestation. In sensitized mice, the protection enhanced gradually from ALM-FA, CA, ALM to ALM-CA in parallel to decreased seric IgGt levels. In contrast to ALM-FA, the combined effect of ALM and CA increased specific isotype IgG2, and decreased IL-17 and MCP-1, and phagocyte influx, as attested by the concomitant reduction in myeloperoxidase (MPO) and α-naphthyl acetate esterase (ANAE) activities. ALM-CA shifted IFN-γ/TGF-ß and iNO synthase/arginase1 (iNOS/Arg1) balances in a Th1 immune response that control efficiently cutaneous lesions and LNs hypertrophy, and reactivate the death of infected phagocytes. CONCLUSIONS: Therefore, CA combined with ALM synergizes with L. major antigens for priming innate cells, through early polarization to optimal Th1 response that leads to IFN-γ and iNOS-dependent leishmanicidal activity of neutrophils and macrophages.