DNA plasmid coding for <i>Phlebotomus sergenti</i> salivary protein PsSP9, a member of the SP15 family of proteins, protects against <i>Leishmania tropica</i>

<div><p>Background</p><p>The vector-borne disease leishmaniasis is transmitted to humans by infected female sand flies, which transmits <i>Leishmania</i> parasites together with saliva during blood feeding. In Iran, cutaneous leishmaniasis (CL) is caused by <i>Leishmania (L</i>.<i>) major</i> and <i>L</i>. <i>tropica</i>, and their main vectors are <i>Phlebotomus (Ph</i>.<i>) papatasi</i> and <i>Ph</i>. <i>sergenti</i>, respectively. Previous studies have demonstrated that mice immunized with the salivary gland homogenate (SGH) of <i>Ph</i>. <i>papatasi</i> or subjected to bites from uninfected sand flies are protected against <i>L</i>. <i>major</i> infection.</p><p>Methods and results</p><p>In this work we tested the immune response in BALB/c mice to 14 different plasmids coding for the most abundant salivary proteins of <i>Ph</i>. <i>sergenti</i>. The plasmid coding for the salivary protein <i>PsSP9</i> induced a DTH response in the presence of a significant increase of IFN-γ expression in draining lymph nodes (dLN) as compared to control plasmid and no detectable PsSP9 antibody response. Animals immunized with whole <i>Ph</i>. <i>sergenti</i> SGH developed only a saliva-specific antibody response and no DTH response. Mice immunized with whole <i>Ph</i>. <i>sergenti</i> saliva and challenged intradermally with <i>L</i>. <i>tropica</i> plus <i>Ph</i>. <i>sergenti</i> SGH in their ears, exhibited no protective effect. In contrast, <i>PsSP9</i>-immunized mice showed protection against <i>L</i>. <i>tropica</i> infection resulting in a reduction in nodule size, disease burden and parasite burden compared to controls. Two months post infection, protection was associated with a significant increase in the ratio of IFN-γ to IL-5 expression in the dLN compared to controls.</p><p>Conclusion</p><p>This study demonstrates that while immunity to the whole <i>Ph</i>. <i>sergenti</i> saliva does not induce a protective response against cutaneous leishmaniasis in BALB/c mice, <i>PsSP9</i>, a member of the <i>PpSP15</i> family of <i>Ph</i>. <i>sergenti</i> salivary proteins, provides protection against <i>L</i>. <i>tropica</i> infection. These results suggest that this family of proteins in <i>Ph</i>. <i>sergenti</i>, <i>Ph</i>. <i>duboscqi</i> and <i>Ph</i>. <i>papatasi</i> may have similar immunogenic and protective properties against different <i>Leishmania</i> species. Indeed, this anti-saliva immunity may act as an adjuvant to accelerate the cell-mediated immune response to co-administered <i>Leishmania</i> antigens, or even cause the activation of infected macrophages to remove parasites more efficiently. These findings highlight the idea of applying arthropod saliva components in vaccination approaches for diseases caused by vector-borne pathogens.</p></div>