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

Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Diseases, Bethesda, MD 20892, USA.
Science (Impact Factor: 33.61). 09/2008; 321(5891):970-4. DOI: 10.1126/science.1159194
Source: PubMed


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.

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    • "Intravital microscopy studies conducted in the mouse ear model of L. major infection [19–21] have shown that infiltrating neutrophils engulf the promastigotes before expressing the apoptotic markers required for efferocytosis by dermal DCs. After internalizing the parasitized/apoptotic neutrophils, the dermal DCs are no longer capable of steering protective TH1-responses in the draining lymph node [19–21]. Although efferocytosis has strong impact on DC function and TH development in L. major infection, independent studies showed that macrophage clearance of apoptotic neutrophils may either induce pro- or anti-inflammatory responses in NE-dependent manner, the intracellular fate of the parasite being influenced by the host genetic background [22, 23]. "
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    ABSTRACT: Inhibitors of serine peptidases (ISPs) expressed by Leishmania major enhance intracellular parasitism in macrophages by targeting neutrophil elastase (NE), a serine protease that couples phagocytosis to the prooxidative TLR4/PKR pathway. Here we investigated the functional interplay between ISP-expressing L. major and the kallikrein-kinin system (KKS). Enzymatic assays showed that NE inhibitor or recombinant ISP-2 inhibited KKS activation in human plasma activated by dextran sulfate. Intravital microscopy in the hamster cheek pouch showed that topically applied L. major promastigotes (WT and Δisp2/3 mutants) potently induced plasma leakage through the activation of bradykinin B2 receptors (B2R). Next, using mAbs against kininogen domains, we showed that these BK-precursor proteins are sequestered by L. major promastigotes, being expressed at higher % in the Δisp2/3 mutant population. Strikingly, analysis of the role of kinin pathway in the phagocytic uptake of L. major revealed that antagonists of B2R or B1R reversed the upregulated uptake of Δisp2/3 mutants without inhibiting macrophage internalization of WT L. major. Collectively, our results suggest that L. major ISP-2 fine-tunes macrophage phagocytosis by inhibiting the pericellular release of proinflammatory kinins from surface bound kininogens. Ongoing studies should clarify whether L. major ISP-2 subverts TLR4/PKR-dependent prooxidative responses of macrophages by preventing activation of G-protein coupled B2R/B1R.
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    • "Importantly, animals previously exposed to saliva or vaccinated with a Th1-biased DTH-inducing salivary protein were protected against challenge by infected vector bites (28, 30, 35). This is significant since Peters et al. (36) showed that the innate immune response following sand fly transmission varied significantly from the response induced by needle challenge primarily related to a persistence of a neutrophilic infiltrate at the site of bite enhancing parasite virulence. Additionally, the enhanced virulence of vector-transmission was shown to abrogate protection by Leishmania vaccines tested against needle challenge with parasites largely due to the need for a rapid effector immune response (37). "
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    Frontiers in Public Health 08/2014; 2:99. DOI:10.3389/fpubh.2014.00099
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    • "Several studies highlighted the significance of vector-transmitted models of infection in the assessment of Leishmania vaccines. In contrast to needle-initiated infection, vector-transmission of L. major caused a persistent infiltration of neutrophils to the site of bite that promoted parasite establishment (Peters et al., 2008). Vector-transmission by Lu. longipalpis also enhanced L. mexicana infection, attributed to regurgitation of the promastigote secretory gel alongside parasites (Rogers et al., 2004). "
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