Abstract
Purpose: Cutaneous leishmaniasis (CL), a parasitic skin disease transmitted by sand flies, is characterized by complex interactions between Leishmania parasites and sand fly saliva, influencing both transmission and disease progression. This study investigated the immunogenicity of partial Phlebotomus papatasi salivary protein PpSP42 (or PP42 in short) and a novel recombinant fusion protein composed of Leishmania major stress-inducible protein 1 (LmSTI1) and PP42, produced in Escherichia coli. Methods: BALB/c mice were immunized subcutaneously with the fusion protein or with PP42 subunits separately, using resiquimod as an adjuvant, and subsequently challenged with L. major promastigotes and sand fly salivary gland homogenate. Lesion development and parasite burden as well immune responses were evaluated by measuring key cytokines (IFN-γ, IL-4, IL-10, TNF-α) and antibodies (IgG, IgG1, IgG2a) up to eight weeks post-challenge. Results: Immunization with either PP42 or the fusion protein significantly attenuated lesion development (30-40% reduction in lesion size after 8 weeks; P < 0.001) and parasite burden (∼100-fold lower after 8 weeks; P < 0.05) compared to the PBS control. This protection was accompanied by significantly higher levels of IFN-γ and TNF-α (P < 0.05 for both). Also, an elevated IgG2a/IgG1 ratio (P < 0.01), indicative of a Th1-biased immune response was observed when the fusion protein was used for immunization. Conclusion: These findings suggest while immunization with PP42 protein alone resulted in lesion control and reduced parasite load, compared to the control group, these effects were less pronounced than those observed in the fusion protein group. Hence, LmSTI1-PP42 fusion protein warrants further investigation as a potential vaccine candidate against L. major infection.