In Vivo Imaging Reveals an Essential Role for Neutrophils in Leishmaniasis Transmitted by Sand Flies

Abstract

Infection with the obligate intracellular protozoan Leishmania is thought to be initiated by direct parasitization of macrophages, but the early events following transmission to the skin by vector sand flies have been difficult to examine directly. Using dynamic intravital microscopy and flow cytometry, we observed a rapid and sustained neutrophilic infiltrate at localized sand fly bite sites. Invading neutrophils efficiently captured Leishmania major (L.m.) parasites early after sand fly transmission or needle inoculation, but phagocytosed L.m. remained viable and infected neutrophils efficiently initiated infection. Furthermore, neutrophil depletion reduced, rather than enhanced, the ability of parasites to establish productive infections. Thus, L.m. appears to have evolved to both evade and exploit the innate host response to sand fly bite in order to establish and promote disease.

Infection and Activation of Human Neutrophils with Fluorescent Leishmania infantum

“…Our observation that neutrophil populations exhibit heterogeneous responses to parasites in culture is consistent with observations in vitro and in vivo that PMNs take up, and though they may kill some parasites early in infection [13], they are unable to control Leishmania infection [5,22,23,25]. Our data raise the curious possibility of “silent” parasite uptake by a neutrophil subset.…”

“…survive inside macrophages throughout chronic infection of their mammalian hosts [3]. Reports over the past 5 years have made it clear that neutrophils are among first cells to arrive at the site of a sand fly bite and to take up the parasite [5,6]. An essential step in our understanding of the role of PMNs in leishmaniasis is the bidirectional effect of parasites on neutrophils and vice versa.…”

“…Many studies examine early Leishmania-host cell interactions by document responses of mammalian cell populations in bulk culture [9]. However, methods examining the phenotypic responses of infected immune cells on a percell basis have been invaluable in increasing our understanding of early disease pathogenesis [5,10]. …”

“…It is clear that PMNs play a role in Leishmania infections, but their effect on disease outcome remains unclear [5,6,8,13]. Neutrophils can internalize and kill at least some Leishmania promastigotes in vitro [14–18].…”

“…Neutrophils can internalize and kill at least some Leishmania promastigotes in vitro [14–18]. They are the first cell to migrate to, and take up parasites at the site of in vivo infection with either L. major [5,18–20] or with L. infantum [6] species. Indeed, there appears to be distinct differences in the way various species of Leishmania result in activation and otherwise affect neutrophils [10,21–23].…”