Gut Microbes Egested during Bites of Infected Sand Flies Augment Severity of Leishmaniasis via Inflammasome-Derived IL-1β

Abstract

Leishmania donovani parasites are the cause of visceral leishmaniasis and are transmitted by bites from phlebotomine sand flies. A prominent feature of vector-transmitted Leishmania is the persistence of neutrophils at bite sites, where they protect captured parasites, leading to enhanced disease. Here, we demonstrate that gut microbes from the sand fly are egested into host skin alongside Leishmania parasites. The egested microbes trigger the inflammasome, leading to a rapid production of interleukin-1β (IL-1β), which sustains neutrophil infiltration. Reducing midgut microbiota by pretreatment of Leishmania-infected sand flies with antibiotics or neutralizing the effect of IL-1β in bitten mice abrogates neutrophil recruitment. These early events are associated with impairment of parasite visceralization, indicating that both gut microbiota and IL-1β are important for the establishment of Leishmania infections. Considering that arthropods harbor a rich microbiota, its potential egestion after bites may be a shared mechanism that contributes to severity of vector-borne disease.

TLR2 Signaling in Skin Nonhematopoietic Cells Induces Early Neutrophil Recruitment in Response to Leishmania major Infection

“…Neutrophils play important roles not only in the cutaneous forms but also in the experimental visceral form of the disease (Dey et al, 2018, McFarlane et al, 2008, Sacramento et al, 2015, Smelt et al, 2000. Decreased neutrophil recruitment in the liver and spleen of L. infantuminfected Tlr2 -/mice was recently described (Sacramento et al, 2017) further implying the importance of TLR2 signalling in the inflammatory response to infection with Leishmania spp inducing visceral diseases.…”

“…Upregulation of KC mRNA and CXCL6 were previously shown to be associated with neutrophil recruitment following L. major infection (Muller et al, 2001, Uyttenhove et al, 2011. In addition to chemokines, other factors produced by the parasites (van Zandbergen et al, 2002) or contributed by the sand fly during natural infection such as egested bacteria, salivary gland products and a proteophosphoglycan gel, are likely to also participate in neutrophil recruitment (de Moura et al, 2010, Dey et al, 2018, Giraud et al, 2018. Thus, multiple factors may contribute and synergize to promote neutrophil recruitment at the site of infection.…”

Macrophage priming is dispensable for NLRP3 inflammasome activation and restriction of Leishmania amazonensis replication

“…24 Importantly, many independent groups have shown that several species of Leishmania, such as Leishmania amazonensis (L. amazonensis), Leishmania major (L. major), and Leishmania braziliensis (L. braziliensis), are able to induce NLRP3 inflammasome activation in bone marrow-derived macrophages (BMDMs), via both the canonic and noncanonic pathways. [25][26][27][28][29][30][31] However, robust IL-1 and CASP1 secretion can only be achieved with previous priming by TLRs agonists, such as LPS, 25,27,29,30 and the reasons for these particular requirements have never been addressed in the context of Leishmania infection.…”

Protective or Detrimental? Understanding the Role of Host Immunity in Leishmaniasis

“…An additional explanation for the large and sustained neutrophil infiltration at the bite sites was recently proposed and involves the immunomodulatory properties of gut microbes from infected sandflies that are co-egested with Leishmania parasites into the skin (Figure 1). Using a VL BALB/c mouse model intradermally infected in the ears by L. donovani, Dey et al demonstrated that the microbiota of the sandfly midgut enhances the early recruitment of neutrophils and the activation of the inflammasome in these cells, with the production of interleukin (IL)-1β, a potent proinflammatory cytokine [30]. These effects were abrogated with the pre-treatment of sandflies with antibiotic cocktails to decrease the microbial population prior to infection, or when mice were treated with an IL-1 receptor (IL1R) antagonist.…”

“…These effects were abrogated with the pre-treatment of sandflies with antibiotic cocktails to decrease the microbial population prior to infection, or when mice were treated with an IL-1 receptor (IL1R) antagonist. The authors concluded that the microbe-mediated IL-1β production serves as an autocrine signal that amplifies neutrophil infiltration at the infection sites and may also aid in parasite dissemination to the spleen [30]. Figure 1.…”