Binding of Leishmania infantum LPG to the midgut is not sufficient to define vector competence in Lutzomyia longipalpis sand flies

Abstract

The major surface lipophosphoglycan (LPG) of Leishmania parasites is critical to vector competence in restrictive sand fly vectors by mediating Leishmania attachment to the midgut epithelium, considered essential to parasite survival and development. However, the relevance of LPG for sand flies that harbor multiple species of Leishmania remains elusive. We tested binding of Leishmania infantum wild type (WT), LPG-defective ({Delta}lpg1 mutants) and add-back lines ({Delta}lpg1 + LPG1) to sand fly midguts in vitro and their survival in Lutzomyia longipalpis sand flies in vivo. Le. infantum WT parasites attached to the Lu. longipalpis midgut in vitro with late-stage parasites binding to midguts in significantly higher numbers compared to early-stage stage promastigotes. {Delta}lpg1 mutants did not bind to Lu. longipalpis midguts, and this was rescued in the {Delta}lpg1 + LPG1 lines, indicating that midgut binding is mediated by LPG. When Lu. longipalpis sand flies were infected with either Le. infantum WT, {Delta}lpg1, or {Delta}lpg1 + LPG1 of the BH46 or BA262 strains, the BH46 {Delta}lpg1 mutant, but not the BA262 {Delta}lpg1 mutant, survived and grew to similar numbers as the WT and {Delta}lpg1 + LPG1 lines. Exposure of BH46 and BA262 {Delta}lpg1 mutants to blood engorged midgut extracts led to the mortality of the BA262 {Delta}lpg1 but not the BH46 {Delta}lpg1 parasites. These findings suggest that Le. infantum LPG protects parasites on a strain-specific basis early in infection, likely against toxic components of blood digestion, however, it is not necessary to prevent Le. infantum evacuation along with the feces in the permissive vector Lu. longipalpis. IMPORTANCEIt is well established that LPG is sufficient to define the vector competence of restrictive sand fly vectors to Leishmania parasites. However, the permissiveness of other sand flies to multiple Leishmania species suggests that other factors might define vector competence for these vectors. In this study, we investigated the underpinnings of Leishmania infantum survival and development in its natural vector Lutzomyia longipalpis. We found out that LPG-mediated midgut binding persists in late-stage parasites. This observation is paradigm-changing and suggests that only a subset of infective metacyclics lose their ability to attach to the midgut with implications for parasite transmission dynamics. However, our data also demonstrate that LPG is not a determining factor in Leishmania infantum retention in the midgut of Lutzomyia longipalpis, a permissive vector. Rather, LPG appears to be more important in protecting some parasite strains from the toxic environment generated during blood meal digestion in the insect gut. Thus, the relevance of LPG in parasite development in permissive vectors appears to be complex and should be investigated on a strain-specific basis.