Leishmania infantum Exoproducts Inhibit Human Invariant NKT Cell Expansion and Activation

Abstract

Leishmania infantum is one of the major parasite species associated with visceral leishmaniasis, a severe form of the disease that can become lethal if untreated. This obligate intracellular parasite has developed diverse strategies to escape the host immune response, such as exoproducts (Exo) carrying a wide range of molecules, including parasite virulence factors, which are potentially implicated in early stages of infection. Herein, we report that L. infantum Exo and its two fractions composed of extracellular vesicles (EVs) and vesicle-depleted-exoproducts (VDEs) inhibit human peripheral blood invariant natural killer T (iNKT) cell expansion in response to their specific ligand, the glycolipid α-GalactosylCeramide (α-GalCer), as well as their capacity to promptly produce IL-4 and IFNγ. Using plate-bound CD1d and α-GalCer, we found that Exo, EV, and VDE fractions reduced iNKT cell activation in a dose-dependent manner, suggesting that they prevented α-GalCer presentation by CD1d molecules. This direct effect on CD1d was confirmed by the observation that CD1d:α-GalCer complex formation was impaired in the presence of Exo, EV, and VDE fractions. Furthermore, lipid extracts from the three compounds mimicked the inhibition of iNKT cell activation. These lipid components of L. infantum exoproducts, including EV and VDE fractions, might compete for CD1-binding sites, thus blocking iNKT cell activation. Overall, our results provide evidence for a novel strategy through which L. infantum can evade immune responses of mammalian host cells by preventing iNKT lymphocytes from recognizing glycolipids in a TCR-dependent manner.

Guidelines for the purification and characterization of extracellular vesicles of parasites

“…Some in vitro studies have shown activation of toll-like receptors (TLRs) by EVs of some Leishmania species. Other works have also demonstrated the activation of macrophages and other immune cells by Leishmania EVs (Belo et al, 2017;Silverman & Reiner, 2011). However, it remains to be investigated whether this stimulation is sufficient for these vesicles to have an effect as a potential adjuvant.…”

“…Leishmania comprises several species of a unicellular protozoan parasite, which is responsible for different clinical forms of leishmaniasis, an NTD that causes major health problems in tropical and subtropical areas (Scott & Novais, 2016). These parasites have a digenetic life cycle: they live as highly motile promastigote forms in the sand fly vector and differentiate into a less motile amastigote forms in the mammalian host macrophages (Belo et al., 2017). EVs released by Leishmania spp.…”

Extracellular Vesicles Released by Leishmania: Impact on Disease Development and Immune System Cells

“…Besides macrophages and DCs, Leishmania-EVs can modulate other immune cells. EVs released by L. infantum inhibited the expansion of peripheral iNKT (Invariant Natural Killer T) cells and the production of IL-4 and IFN-γ by this cell type [44]. Experiments using CD1d specific ligands (glycolipid α-GalactosylCeramide (α-GalCer) suggest that lipids present in L. infantum-EVs and other exocomponents released by the parasites may compete for the CD1 binding site, inhibiting iNKT activation [44].…”

“…EVs released by L. infantum inhibited the expansion of peripheral iNKT (Invariant Natural Killer T) cells and the production of IL-4 and IFN-γ by this cell type [44]. Experiments using CD1d specific ligands (glycolipid α-GalactosylCeramide (α-GalCer) suggest that lipids present in L. infantum-EVs and other exocomponents released by the parasites may compete for the CD1 binding site, inhibiting iNKT activation [44]. In addition, our group showed that murine B-1 cells (a subtype of B lymphocytes) stimulated with EVs released by L. amazonensis produced higher levels of NO, compared to non-stimulated B-1 cells [45].…”

Invariant Natural Killer T and Mucosal-Associated Invariant T Cells in Asthmatic Patients

“…For instance, these cells have been shown to perform an intravascular immune surveillance function in the liver, spleen, and lung ( 40 – 43 ). The primary function of iNKT cells is to protect the host from infections ( 24 , 44 , 45 ). However, in some conditions, iNKT cell activation favors tissue injury, including lung (Figure 1 ), as discussed below.…”