NKT cell activation by Leishmania mexicana LPG: Description of a novel pathway

Leishmania major-derived lipophosphoglycan influences the host's early immune response by inducing platelet activation and DKK1 production via TLR1/2

Background

Platelets are rapidly deployed to infection sites and respond to pathogenic molecules via pattern recognition receptors (TLR, NLRP). Dickkopf1 (DKK1) is a quintessential Wnt antagonist produced by a variety of cell types including platelets, endothelial cells, and is known to modulate pro-inflammatory responses in infectious diseases and cancer. Moreover, DKK1 is critical for forming leukocyte-platelet aggregates and induction of type 2 cell-mediated immune responses. Our previous publication showed activated platelets release DKK1 following Leishmania major recognition.

Results

Here we probed the role of the key surface virulence glycoconjugate lipophosphoglycan (LPG), on DKK1 production using null mutants deficient in LPG synthesis (Δlpg1- and Δlpg2-). Leishmania-induced DKK1 production was reduced to control levels in the absence of LPG in both mutants and was restored upon re-expression of the cognate LPG1 or LPG2 genes. Furthermore, the formation of leukocyte-platelet aggregates was dependent on LPG. LPG mediated platelet activation and DKK1 production occurs through TLR1/2.

Conclusion

Thus, LPG is a key virulence factor that induces DKK1 production from activated platelets, and the circulating DKK1 promotes Th2 cell polarization. This suggests that LPG-activated platelets can drive innate and adaptive immune responses to Leishmania infection.

Leishmaniasis: Immune Cells Crosstalk in Macrophage Polarization

Leishmaniasis is a complex infectious parasitic disease caused by protozoa of the genus Leishmania, belonging to a group of neglected tropical diseases. It establishes significant global health challenges, particularly in socio-economically disadvantaged regions. Macrophages, as innate immune cells, play a crucial role in initiating the inflammatory response against the pathogens responsible for this disease. Macrophage polarization, the process of differentiating macrophages into pro-inflammatory (M1) or anti-inflammatory (M2) phenotypes, is essential for the immune response in leishmaniasis. The M1 phenotype is associated with resistance to Leishmania infection, while the M2 phenotype is predominant in susceptible environments. Notably, various immune cells, including T cells, play a significant role in modulating macrophage polarization by releasing cytokines that influence macrophage maturation and function. Furthermore, other immune cells can also impact macrophage polarization in a T-cell-independent manner. Therefore, this review comprehensively examines macrophage polarization's role in leishmaniasis and other immune cells' potential involvement in this intricate process.

Leishmania mexicana Lipophosphoglycan Activates Dermal γδ T Cells with Participation of TLR2

PURPOSE

Leishmania transmission by sand flies is detected by dermal cells that recognize ligands, such as lipophosphoglycan (LPG) on the promastigote glycocalyx. Resident dermal cells include γδ T cells, that recognize antigens by TCR or innate receptors, such as TLRs. We analyzed the response of dermal γδ T cells to Leishmania mexicana infections or inoculation of LPG, and whether parasite LPG activates γδ T cells through TLR2.

METHODS

We stimulated γδ T cells with LPG and analyzed colocalization of LPG and TLR2 by confocal microscopy. Activation of TLR2 was evaluated by IκBα phosphorylation. BALB/c mice were inoculated with L. mexicana or LPG in the dermis of earlobes, and LPG⁺ TLR2⁺ γδ T cells were analyzed by flow cytometry. TNF⁺ γδ T cells were examined in earlobe dermis by confocal microscopy.

RESULTS

Stimulation with purified LPG showed activation of TLR2 with IκBα phosphorylation in γδ T cells. Inoculation of L. mexicana parasites or LPG into earlobe dermis showed co-expression of LPG⁺ and TLR2⁺ in γδ T cells, demonstrating their interaction during infections. A subset of γδ T cells (LPG⁺ and TLR2^-) provided evidence that additional receptors recognize LPG. Inoculation of LPG enhanced overall γδ T cell numbers, including those expressing TNF, whereas infection with the parasite mostly enhanced γδ T cells expressing TNF.

CONCLUSION

L. mexicana LPG is a ligand recognized by TLR2 on γδ-T cells leading to their activation, although contribution of other receptors cannot be ruled out and need to be analyzed to elucidate their contribution during Leishmania infections.

Virulence factors of Leishmania parasite: Their paramount importance in unraveling novel vaccine candidates and therapeutic targets

Leishmaniasis is a neglected tropical disease and remains a global concern for healthcare. It is caused by an opportunistic protozoan parasite belonging to the genus Leishmania and affects millions worldwide. This disease is mainly prevalent in tropical and subtropical regions and is associated with a high risk of public morbidity and mortality if left untreated. Transmission of this deadly disease is aggravated by the bite of female sand-fly vectors (Phlebotomus and Lutzomyia). With time, significant advancement in leishmaniasis-related research has been carried out to cope with the disease burden. Still, the Leishmania parasite has also co-evolved with its host and adapted successfully within the host's lethal milieu/environment. Thus, understanding and knowledge of various leishmanial virulence factors responsible for the parasitic infection are essential for exploring drug targets and vaccine candidates. The present review elucidates the importance of virulence factors in pathogenesis and summarizes the major leishmanial virulence molecules contributing to the parasitic infection during host-pathogen interaction. Furthermore, we have also elaborated on the potential contribution of leishmanial virulence proteins in developing vaccine candidates and exploring novel therapeutics against this parasitic disease. We aim to represent a clearer picture of parasite pathogenesis within the human host that can further aid in unraveling new strategies to fight against the deadly infection of leishmaniasis.

Paradoxical immune response in leishmaniasis: the role of toll-like receptors in disease progression

Toll-like receptors (TLRs), members of pattern recognition receptors, are expressed on many cells of the innate immune system and their engagements with antigens regulates specific immune responses. TLRs signalling influences species-specific immune responses during Leishmania infection, thus, TLRs play a decisive role towards elimination or exacerbation of Leishmania infection. To date, there is no single therapeutic or prophylactic approach that fully effective against Leishmaniasis. An in-depth understanding of the mechanisms by which Leishmania species evade, or exploit host immune machinery could lead to the development of novel therapeutic approaches for the prevention and management of leishmaniasis. In this review, the role of TLRs in the induction of a paradoxical immune response in leishmaniasis was discussed. This review focuses on highlighting the novel interplay of TLR2/TLR9 driven resistance or susceptibility to 5 clinically important Leishmania species in human. The activation of TLR2/TLR9 can induce a diverse anti-Leishmania activities depending on the species of infecting Leishmania parasite. Infection with L. infantum and L. mexicana initiate TLR2/9 activation leading to host protective immune response while infection with L. major, L. donovani, and L. amazonensis trigger either a TLR2/9 related protective or non-protective immune responses. These findings suggest that TLR2 and TLR9 are targets worth pursuing either for modulation or blockage to trigger host protective immune response towards leishmaniasis.

Miltefosine enhances infectivity of a miltefosine-resistant Leishmania infantum strain by attenuating its innate immune recognition

Background

Miltefosine (MIL) is currently the only oral drug available to treat visceral leishmaniasis but its use as first-line monotherapy has been compromised by an increasing treatment failure. Despite the scarce number of resistant clinical isolates, MIL-resistance by mutations in a single aminophospholipid transporter gene can easily be selected in a laboratory environment. These mutations result in a reduced survival in the mammalian host, which can partially be restored by exposure to MIL, suggesting a kind of drug-dependency.

Methodology/Principal findings

To enable a combined study of the infection dynamics and underlying immunological events for differential in vivo survival, firefly luciferase (PpyRE9) / red fluorescent protein (DsRed) double-reporter strains were generated of MIL-resistant (MIL-R) and syngeneic MIL-sensitive (MIL-S) Leishmania infantum. Results in C57Bl/6 and BALB/c mice show that MIL-R parasites induce an increased innate immune response that is characterized by enhanced influx and infection of neutrophils, monocytes and dendritic cells in the liver and elevated serum IFN-γ levels, finally resulting in a less efficient establishment in liver macrophages. The elevated IFN-γ levels were shown to originate from an increased response of hepatic NK and NKT cells to the MIL-R parasites. In addition, we demonstrated that MIL could increase the in vivo fitness of MIL-R parasites by lowering NK and NKT cell activation, leading to a reduced IFN-γ production.

Conclusions/Significance

Differential induction of innate immune responses in the liver was found to underlie the attenuated phenotype of a MIL-R parasite and its peculiar feature of drug-dependency. The impact of MIL on hepatic NK and NKT activation and IFN-γ production following recognition of a MIL-R strain indicates that this mechanism may sustain infections with resistant parasites and contribute to treatment failure.

Visceral leishmaniasis is a neglected tropical disease that is fatal if left untreated. Miltefosine is currently the only oral drug available but is increasingly failing to cure patients, resulting in its discontinuation as first-line drug in some endemic areas. To understand these treatment failures, we investigated the complex interplay of the parasite with the host immune system in the presence and absence of miltefosine. Our data indicate that miltefosine-resistant Leishmania parasites become severely hampered in their in vivo infectivity, which could be attributed to the induction of a pronounced innate immune response. Interestingly, the infection deficit was partially restored in the presence of miltefosine. Our results further indicate that miltefosine can exacerbate infections with resistant parasites by reducing innate immune recognition. This study provides new insights into the complex interplay between parasite, drug and host and discloses an immune-related mechanism of treatment failure.

Immune response to Leishmania mexicana: the host-parasite relationship

Leishmaniosis is currently considered a serious public health problem and it is listed as a neglected tropical disease by World Health Organization (WHO). Despite the efforts of the scientific community, it has not been possible to develop an effective vaccine. Current treatment consists of antimonials that is expensive and can cause adverse effects. It is essential to fully understand the immunopathogenesis of the disease to develop new strategies to prevent, treat and eradicate the disease. Studies on animal models have shown a new paradigm in the resolution or establishment of infection by Leishmania mexicana where a wide range of cytokines, antibodies and cells are involved. In recent years, the possibility of a new therapy with monoclonal antibodies has been considered, where isotype, specificity and concentration are critical for effective therapy. Would be better to create/generate a vaccine to induce host protection or produce passive immunization with engineering monoclonal antibodies to a defined antigen? This review provides an overview that includes the current known information on the immune response that are involved in the complex host-parasite relationship infection caused by L. mexicana.

Leishmania lipophosphoglycan components: A potent target for synthetic neoglycoproteins as a vaccine candidate for leishmaniasis

Leishmania is an obligate intracellular pathogen that invades phagocytic host cells. Due to its high morbidity and mortality rates, leishmaniasis attracts significant attention. The disease, which is caused by Leishmania parasites, is distributed worldwide, particularly among developing communities, and causes fatal complications if not treated expediently. Unfortunately, the existing treatments are not preventive and do not impede Leishmania infection. Many drugs available for leishmaniasis are becoming less effective due to emerging resistance in some Leishmania species. Other drugs have drawbacks such as low cost-effectiveness, toxicity, and side effects. The World Health Organization (WHO) considers leishmaniasis to be a major public health problem and suggests that the best prevention is to develop a vaccine for this dangerous disease. In this review, we focus on the unique components of lipophosphoglycan (LPG), a component of the Leishmania cell wall, particularly [Galp(1 → 4)-β-[Manp-(1 → 2)-α-Manp-(1 → 2)-α]-Manp] in the cryptic tetrasaccharide cap, and on synthetic approaches as a potent candidate for a leishmaniasis vaccine.

Leishmania species-dependent functional duality of toll-like receptor 2

Toll-like receptors (TLRs) are a subset of pattern recognition receptors (PRR) in innate immunity and act as a connecting link between innate and adaptive immune systems. During Leishmania infection, the activation of TLRs influences the pathogen-specific immune responses, which may play a decisive role in determining the outcome of infection, toward elimination or survival of the pathogen. Antigen-presenting cells (APCs) of the innate immune system such as macrophages, dendritic cells (DCs), neutrophils, natural killer (NK) cells, and NKT cells express TLR2, which plays a crucial role in the parasite recognition and elicitation of immune responses in Leishmania infection. Depending on the infecting Leishmania species, the TLR2 pathways may result in a host-protective or a disease-exacerbating response. While Leishmania major and Leishmania donovani infections trigger TLR2-related host-protective and non-protective immune responses, Leishmania mexicana and Leishmania infantum infections are reported to elicit TLR2-mediated host-protective responses and Leishmania amazonensis and Leishmania braziliensis infections are reported to evoke a disease-exacerbating response. These findings illustrate that TLR2-related effector functions are diverse and may be exerted in a species- or strain-dependent manner. TLR2 agonists or antagonists may have therapeutic potentials to trigger the desired immune response during leishmaniasis. In this review, we discuss the TLR2-related immune responses during leishmaniasis and highlight the novel insights into the possible role of TLR2-driven resistance or susceptibility to Leishmania.

Next Generation Sequencing of the Pig αβ TCR Repertoire Identifies the Porcine Invariant NKT Cell Receptor

Swine represent the only livestock with an established invariant NKT (iNKT) cell-CD1d system. In this study, we exploited the fact that pig iNKT cells can be purified using a mouse CD1d tetramer reagent to establish their TCR repertoire by next generation sequencing. CD1d tetramer-positive pig cells predominantly expressed an invariant Vα-Jα rearrangement, without nontemplate nucleotide diversity, homologous to the Vα24-Jα18 and Vα14-Jα18 rearrangements of human and murine iNKT cells. The coexpressed β-chain used a Vβ segment homologous to the semivariant Vβ11 and Vβ8.2 segments of human and murine iNKT cell receptors. Molecular modeling found that contacts within CD1d and CDR1α that underlie fine specificity differences between mouse and human iNKT cells are conserved between pigs and humans, indicating that the response of porcine and human iNKT cells to CD1d-restricted Ags may be similar. Accordingly, pigs, which are an important species for diverse fields of biomedical research, may be useful for developing human-based iNKT cell therapies for cancer, infectious diseases, and other disorders. Our study also sequenced the expressed TCR repertoire of conventional porcine αβ T cells, which identified 48 Vα, 50 Jα, 18 Vβ, and 18 Jβ sequences, most of which correspond to human gene segments. These findings provide information on the αβ TCR usage of pigs, which is understudied and deserves further attention.

Elucidating in vitro and in vivo phenotypic behaviour of L. infantum/L. major natural hybrids

The clinical manifestation and course of Leishmania infections depend on factors such as species, virulence and host-immunity. Although trypanosomatids are considered to have clonal propagation, genetic hybridization has produced successful natural hybrid lineages. Hybrids displaying strong selective advantages may have an impact on pathogenesis and the eco-epidemiology of leishmaniasis. Thus, characterization of phenotypic properties of Leishmania hybrids could bring significant insight into the biology, infectivity, pathogenicity and transmission dynamics of these atypical strains. The present study focuses on phenotypic features and survival capacity of Leishmania infantum/Leishmania major hybrid isolates as compared with representative putative parental species, L. infantum and L. major. In vitro assays (growth kinetics, susceptibility to different conditions) and in vivo infection (parasite detection and histopathological alterations) showed that hybrids present higher growth capacity and decreased susceptibility to reactive oxygen species. Furthermore, evaluation of infected spleen tissue suggests that hybrids induce a stronger immune reaction than their putative parents, leading to the development of white pulp hyperplasia in B-lymphocyte compartments. Overall, these hybrids have shown high plasticity in terms of their general behaviour within the different phenotypic parameters, suggesting that they might have acquired genetic features conferring different mechanisms to evade host cells.

The Janus Face of NKT Cell Function in Autoimmunity and Infectious Diseases

Natural killer T cells (NKT) are a subset of T lymphocytes bridging innate and adaptive immunity. These cells recognize self and microbial glycolipids bound to non-polymorphic and highly conserved CD1d molecules. Three NKT cell subsets, type I, II, and NKT-like expressing different antigen receptors (TCR) were described and TCR activation promotes intracellular events leading to specific functional activities. NKT can exhibit different functions depending on the secretion of soluble molecules and the interaction with other cell types. NKT cells act as regulatory cells in the defense against infections but, on the other hand, their effector functions can be involved in the pathogenesis of several inflammatory disorders due to their exposure to different microbial or self-antigens, respectively. A deep understanding of the biology and functions of type I, II, and NKT-like cells as well as their interplay with cell types acting in innate (neuthrophils, innate lymphoid cells, machrophages, and dendritic cells) and adaptive immunity (CD4⁺,CD8⁺, and double negative T cells) should be important to design potential immunotherapies for infectious and autoimmune diseases.