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Background  Phlebotomine insects harbor bacterial, viral and parasitic pathogens that can cause diseases of public health importance.  Lutzomyia longipalpis  is the main vector of visceral leishmaniasis in the New World. Insects can mount a powerful innate immune response to pathogens. Defensin peptides take part in this response and are known to be active against Gram-positive and Gram-negative bacteria, and some parasites. We studied the expression of a defensin gene from  Lutzomyia longipalpis  to understand its role in sand fly immune response.    Methods  We identified, sequenced and evaluated the expression of a  L .  longipalpis  defensin gene by semi-quantitative RT-PCR. The gene sequence was compared to other vectors defensins and expression was determined along developmental stages and after exposure of adult female  L .  longipalpis  to bacteria and  Leishmania .    Results  Phylogenetic analysis showed that the  L .  longipalpis  defensin is closely related to a defensin from the Old World sand fly  Phlebotomus duboscqi . Expression was high in late L4 larvae and pupae in comparison to early larval stages and newly emerged flies. Defensin expression was modulated by oral infection with bacteria. The Gram-positive  Micrococcus luteus  induced early high defensin expression, whilst the Gram-negative entomopathogenic  Serratia marcescens  induced a later response. Bacterial injection also induced defensin expression in adult insects. Female sand flies infected orally with  Leishmania mexicana  showed no significant difference in defensin expression compared to blood fed insects apart from a lower defensin expression 5 days post  Leishmania  infection. When  Leishmania  was introduced into the hemolymph by injection there was no induction of defensin expression until 72 h later.    Conclusions  Our results suggest that  L .  longipalpis  modulates defensin expression upon bacterial and  Leishmania  infection, with patterns of expression that are distinct among bacterial species and routes of infection.

Bacterial feeding, Leishmania infection and distinct infection routes induce differential defensin expression in Lutzomyia longipalpis