Host genetic background determines whether IL-18 deficiency results in increased susceptibility or resistance to murine Leishmania major infection

The effects of Leishmania RNA virus 2 (LRV2) on the virulence factors of L. major and pro-inflammatory biomarkers: an in vitro study on human monocyte cell line (THP-1)

BACKGROUND

Cutaneous Leishmaniasis (CL) is a parasitic disease with diverse outcomes. Clinical diversity is influenced by various factors such as Leishmania species and host genetic background. The role of Leishmania RNA virus (LRV), as an endosymbiont, is suggested to not only affect the pathogenesis of Leishmania, but also impact host immune responses. This study aimed to investigate the influence of LRV2 on the expression of a number of virulence factors (VFs) of Leishmania and pro-inflammatory biomarkers.

MATERIALS AND METHODS

Sample were obtained from CL patients from Golestan province. Leishmania species were identified by PCR (LIN 4, 17), and the presence of LRV2 was checked using the semi-nested PCR (RdRp gene). Human monocyte cell line (THP-1) was treated with three isolates of L. major with LRV2 and one isolate of L. major without LRV2. The treatments with four isolates were administered for the time points: zero, 12, 24, 36, and 48 h after co-infection. The expression levels of Leishmania VFs genes including GP63, HSP83, and MPI, as well as pro-inflammatory biomarkers genes including NLRP3, IL18, and IL1β, were measured using quantitative real-time PCR.

RESULTS

The expression of GP63, HSP83, and MPI revealed up-regulation in LRV2 + isolates compared to LRV2- isolates. The expression of the pro-inflammatory biomarkers including NLRP3, IL1β, and IL18 genes in LRV2- were higher than LRV2 + isolates.

CONCLUSION

This finding suggests that LRV2 + may have a probable effect on the Leishmania VFs and pro-inflammatory biomarkers in the human macrophage model.

Host genetic backgrounds: the key to determining parasite-host adaptation

Parasitic diseases pose a significant threat to global public health, particularly in developing countries. Host genetic factors play a crucial role in determining susceptibility and resistance to infection. Recent advances in molecular and biological technologies have enabled significant breakthroughs in understanding the impact of host genes on parasite adaptation. In this comprehensive review, we analyze the host genetic factors that influence parasite adaptation, including hormones, nitric oxide, immune cells, cytokine gene polymorphisms, parasite-specific receptors, and metabolites. We also establish an interactive network to better illustrate the complex relationship between host genetic factors and parasite-host adaptation. Additionally, we discuss future directions and collaborative research priorities in the parasite-host adaptation field, including investigating the impact of host genes on the microbiome, developing more sophisticated models, identifying and characterizing parasite-specific receptors, utilizing patient-derived sera as diagnostic and therapeutic tools, and developing novel treatments and management strategies targeting specific host genetic factors. This review highlights the need for a comprehensive and systematic approach to investigating the underlying mechanisms of parasite-host adaptation, which requires interdisciplinary collaborations among biologists, geneticists, immunologists, and clinicians. By deepening our understanding of the complex interactions between host genetics and parasite adaptation, we can develop more effective and targeted interventions to prevent and treat parasitic diseases. Overall, this review provides a valuable resource for researchers and clinicians working in the parasitology field and offers insights into the future directions of this critical research area.

Cytokines and Signaling Networks Regulating Disease Outcomes in Leishmaniasis

Cytokines play crucial roles in commencing and coordinating the organized recruitment and activation of immune cells during infection. These molecular regulators play an important part in deciding the fate of disease outcomes in leishmaniasis, a parasitic disease of tropical and subtropical countries. T helper 1 (Th1) cell-mediated inflammatory cytokines usually play a host-protective role, while T helper 2 (Th2) cell activation produces an anti-inflammatory milieu necessary for parasite survival. It is noteworthy that in such a multifaceted disease, the role played by any particular cytokine cannot be generalized as either beneficial or detrimental. For example, a "host-favorable" cytokine in one form of the disease has been found to be "pathogen friendly" in another form of leishmaniasis. On the other hand, the complex signaling network regulating the production of cytokines is further complicated by the nature of the host as well as the presence of other cytokines in the milieu. The present review focuses on the differential roles played by cytokines and the intricate signaling network responsible for the regulation of such cytokines during infection by different species of Leishmania. While many more studies are required in the future to better understand the role of these molecules in both animal models and patient samples, current studies indicate that these molecules are potential candidates to be targeted for therapy against this deadly disease.

Regulation of macrophage subsets and cytokine production in leishmaniasis

Macrophages are host cells for parasites of the genus Leishmania where they multiply inside parasitophorous vacuoles. Paradoxically, macrophages are also the cells responsible for killing or controlling parasite growth, if appropriately activated. In this review, we will cover the patterns of macrophage activation and the mechanisms used by the parasite to circumvent being killed. We will highlight the impacts of the vector bite on macrophage activation. Finally, we will discuss the ontogeny of macrophages that are infected by Leishmania spp.

Role of host genetics and cytokines in Leishmania infection

Cytokines and chemokines are important regulators of innate and specific responses in leishmaniasis, a disease that currently affects 12 million people. We overviewed the current information about influences of genetically engineered mouse models of cytokine and chemokine on leishmaniasis. We found that genetic background of the host, parasite species and sub-strain, as well as experimental design often modify effects of genetically engineered cytokine genes. Next we analyzed genes and QTLs (quantitative trait loci) that control response to Leishmania species in mouse in order to establish relationship between genetic control of cytokine expression and organ pathology. These studies revealed a network-like complexity of the combined effects of the multiple functionally diverse QTLs and their individual specificity. Genetic control of organ pathology and systemic immune response overlap only partially. Some QTLs control both organ pathology and systemic immune response, but the effects of genes and loci with the strongest impact on disease are cytokine-independent, whereas several loci modify cytokines levels in serum without influencing organ pathology. Understanding this genetic control might be important in development of vaccines designed to stimulate certain cytokine spectrum.

A historical review of the role of cytokines involved in leishmaniasis

Leishmaniasis is an infectious disease caused by the Leishmania genus, affecting millions of persons in the world. Despite increased studies, no vaccine has been developed against leishmaniasis, and drug resistance is evolving in some Leishmania species (spp). Innate and acquired immune cells and their associated cytokines interplay together to determine the immune responses related outcomes in leishmaniasis. Interferon (IFN)-γ or macrophage activating factor (MAF) is the first effective lymphokine (LK), with a related function to leishmaniasis, discovered in 1979. This review article discussed the history of cytokines involved in Leishmania infection, and it is the first report demonstrating the involvement in the disease by focusing on cutaneous leishmaniasis. Up to now, the role of many cytokines has been determined and the literature review showed that IL-35 is the latest known cytokine involved in leishmaniasis. This review revealed that the cytokines have pleiotropic effects, depending upon the cytokine environment, generated during the infection and the host genetic background or infecting Leishmania spp. Overall, advances in our knowledge of immune cells and their secreted cytokines, contributing to the protection or pathological process of leishmaniasis may help to reach new approaches for immunotherapy.

Leishmania infantum infection reduces the amyloid β42-stimulated NLRP3 inflammasome activation

Activation of the NLRP3 inflammasome has been shown to play a major role in the neuroinflammation that accompanies Alzheimer's disease (AD); interventions that down regulate the NLRP3 inflammasome could thus be beneficial in AD. Parasite infections were recently shown to be associated with improved cognitive functions in Apolipoprotein E4 (ApoE4)-expressing members of an Amazonian tribe. We verified in an in vitro model whether Leishmania infantum infection could reduce NLRP3. Results obtained in an initial experimental model in which PBMC were LPS primed and nigericin-stimulated showed that L. infantum infection significantly reduced ASC-speck formation (i.e. intracellular inflammasome proteins assembly), as well as the production of activated caspase 5 and IL-1β, but increased that of activated caspase 1 and IL-18. Moreover, L. infantum infection induced the generation of an anti-inflammatory milieu by suppressing the production of TNFα and increasing that of IL-10. These results were replicated when cells that had been LPS-primed were stimulated with Aβ₄₂ and infected with L. infantum. Results herein indicate that Leishmania infection favors an anti-inflammatory milieu, which includes the down-regulation of NLRP3 inflammasome activation, possibly to facilitate its survival inside host cells. A side effect of Leishmaniasis would be the hampering of neuroinflammation; this could play a protective role against AD development.

IL-1 family and Cutaneous Leishmaniasis: A poorly understood relationship

The cytokines of the interleukin (IL) -1 family act in the initiation of an effective immune response in Leishmania infection, represented mainly by the T helper 1 (Th1) profile, in addition to being associated with disease exacerbation and controversial contributions in the Th2 responses. The family also includes members who self-regulate inflammation, such as antagonists and anti-inflammatory cytokines, most of which have not yet been studied in Cutaneous Leishmaniasis (CL) in humans. Here we summarize findings about what is known so far about the role of these cytokines in mice, the main study model, and in humans. We reinforce the importance of studies of these cytokines as new targets in the context of CL.

Cytosolic Recognition of Microbes and Pathogens: Inflammasomes in Action

Infection is a dynamic biological process underpinned by a complex interplay between the pathogen and the host. Microbes from all domains of life, including bacteria, viruses, fungi, and protozoan parasites, have the capacity to cause infection. Infection is sensed by the host, which often leads to activation of the inflammasome, a cytosolic macromolecular signaling platform that mediates the release of the proinflammatory cytokines interleukin-1β (IL-1β) and IL-18 and cleavage of the pore-forming protein gasdermin D, leading to pyroptosis. Host-mediated sensing of the infection occurs when pathogens inject or carry pathogen-associated molecular patterns (PAMPs) into the cytoplasm or induce damage that causes cytosolic liberation of danger-associated molecular patterns (DAMPs) in the host cell. Recognition of PAMPs and DAMPs by inflammasome sensors, including NLRP1, NLRP3, NLRC4, NAIP, AIM2, and Pyrin, initiates a cascade of events that culminate in inflammation and cell death. However, pathogens can deploy virulence factors capable of minimizing or evading host detection. This review presents a comprehensive overview of the mechanisms of microbe-induced activation of the inflammasome and the functional consequences of inflammasome activation in infectious diseases. We also explore the microbial strategies used in the evasion of inflammasome sensing at the host-microbe interaction interface.

Effects of trans-stilbene and terphenyl compounds on different strains of Leishmania and on cytokines production from infected macrophages

Most of the antileishmanial modern therapies are not satisfactory due to high toxicity or emergence of resistance and high cost of treatment. Previously, we observed that two compounds of a small library of trans-stilbene and terphenyl derivatives, ST18 and TR4, presented the best activity and safety profiles against Leishmania infantum promastigotes and amastigotes. In the present study we evaluated the effects of ST18 and the TR4 in 6 different species of Leishmania and the modifications induced by these two compounds in the production of 8 different cytokines from infected macrophages. We observed that TR4 was potently active in all Leishmania species tested in the study showing a leishmanicidal activity higher than that of ST18 and meglumine antimoniate in the most of the species. Moreover, TR4 was able to decrease the levels of IL-10, a cytokine able to render the host macrophage inactive allowing the persistence of parasites inside its phagolysosome, and increase the levels of IL-1β, a cytokine important for host resistance to Leishmania infection by inducible iNOS-mediated production of NO, and IL-18, a cytokine implicated in the development of Th1-type immune response.

How Inflammasomes Inform Adaptive Immunity

An immune response consists of a finely orchestrated interplay between initial recognition of potential microbial threats by the innate immune system and subsequent licensed adaptive immune neutralization. The initial recognition integrates environmental cues derived from pathogen-associated molecular patterns and cell-intrinsic damage-associated molecular patterns to contextualize the insult and inform a tailored adaptive response via T and B lymphocytes. While there are much data to support the role of transcriptional responses downstream of pattern recognition receptors in informing the adaptive immune response, markedly less attention has been paid to the role of post-translational responses to pathogen-associated molecular pattern and damage-associated molecular pattern recognition by the innate immune system, and how this may influence adaptive immunity. A well-characterized post-translational consequence of pattern recognition receptor signaling is the assembly of a multimeric signaling platform, termed the inflammasome, by members of the nucleotide-binding oligomerization domain (Nod), leucine-rich repeat-containing receptors (NLRs), and pyrin and HIN domain (PYHIN) families. Inflammasomes assemble in response to cytosolic perturbations, such as mitochondrial dysfunction and aberrant ion fluxes in the case of the canonical NLRP3 inflammasome or the presence of bacterial lipopolysaccharides in the case of the non-canonical inflammasome. Assembly of the inflammasome allows for the cleavage and activation of inflammatory caspases. These activated inflammatory caspases in turn cleave pro-form inflammatory cytokines into their mature bioactive species and lead to unconventional protein secretion and lytic cell death. In this review, we discuss evidence for inflammasome-mediated instruction and contextualization of infectious and sterile agents to the adaptive immune system.

Immune responses against protozoan parasites: a focus on the emerging role of Nod-like receptors

Nod-like receptors (NLRs) have gained attention in recent years because of the ability of some family members to assemble into a multimeric protein complex known as the inflammasome. The role of NLRs and the inflammasome in regulating innate immunity against bacterial pathogens has been well studied. However, recent studies show that NLRs and inflammasomes also play a role during infections caused by protozoan parasites, which pose a significant global health burden. Herein, we review the diseases caused by the most common protozoan parasites in the world and discuss the roles of NLRs and inflammasomes in host immunity against these parasites.

Innate immunity against Leishmania infections

Leishmaniasis is a major health problem that affects more than 300 million people throughout the world. The morbidity associated with the disease causes serious economic burden in Leishmania endemic regions. Despite the morbidity and economic burden associated with Leishmaniasis, this disease rarely gets noticed and is still categorized under neglected tropical diseases. The lack of research combined with the ability of Leishmania to evade immune recognition has rendered our efforts to design therapeutic treatments or vaccines challenging. Herein, we review the literature on Leishmania from innate immune perspective and discuss potential problems as well as solutions and future directions that could aid in identifying novel therapeutic targets to eliminate this parasite.

IL-18 contributes to susceptibility to Leishmania amazonensis infection by macrophage-independent mechanisms

We evaluated the role of IL-18 during Leishmania amazonensis infection in C57BL/6 mice, using IL-18KO mice. We showed that IL-18 is involved in susceptibility to L. amazonensis, since IL-18KO mice presented reduced lesions and parasite loads. Because macrophages are the host cells of the parasite, we investigated if macrophages were involved in IL-18-mediated susceptibility to L. amazonensis. We showed that macrophages obtained from WT or IL-18KO responded similarly to L. amazonensis infection. Moreover, we showed that C57BL/6 macrophages do not respond to IL-18, since they do not express IL-18R. Therefore, macrophages are not involved in IL-18-mediated susceptibility to L. amazonensis.

An NLRP3 inflammasome-triggered Th2-biased adaptive immune response promotes leishmaniasis

Leishmaniasis is a major tropical disease that can present with cutaneous, mucocutaneous, or visceral manifestation and affects millions of individuals, causing substantial morbidity and mortality in third-world countries. The development of a Th1-adaptive immune response is associated with resistance to developing Leishmania major (L. major) infection. Inflammasomes are key components of the innate immune system that contribute to host defense against bacterial and viral pathogens; however, their role in regulating adaptive immunity during infection with protozoan parasites is less studied. Here, we demonstrated that the NLRP3 inflammasome balances Th1/Th2 responses during leishmaniasis. Mice lacking the inflammasome components NLRP3, ASC, or caspase 1 on a Leishmania-susceptible BALB/c background exhibited defective IL-1β and IL-18 production at the infection site and were resistant to cutaneous L. major infection. Moreover, we determined that production of IL-18 propagates disease in susceptible BALB/c mice by promoting the Th2 cytokine IL-4, and neutralization of IL-18 in these animals reduced L. major titers and footpad swelling. In conclusion, our results indicate that activation of the NLRP3 inflammasome is detrimental during leishmaniasis and suggest that IL-18 neutralization has potential as a therapeutic strategy to treat leishmaniasis patients.

IL-18 gene polymorphism in patients with visceral leishmaniasis in East Azarbaijan, Iran

Visceral leishmaniasis (VL) is a parasitic disease caused by Leishmania species. According to the important role of cellular immunity against VL, this study was directed to determine the frequency of -607A/C and -137G/C genotypes on promoter region of interleukin-18 gene. The study groups included 91 patients with confirmed history of VL, 106 healthy seronegative, and 79 healthy seropositive individuals. All three groups were analyzed by amplification refractory mutation system polymerase chain reaction (ARMS-PCR). The highest rate of -607/A, and -607/C alleles was observed in seronegative individuals (66/67 %) and in the patients (72/83 %). Allele frequency of -607/C is more than -607/A allele in all groups. In position of -137, frequency of -137/G allele in all groups was more than -137/C. Statistical analysis of distribution of genotypes, did not reveal any significant difference among groups. On the basis of the results, there was no significant association between VL and polymorphism of IL-18 promoter. The results of this study showed that IL-18 gene promoter polymorphisms at positions -607 and -137 are not associated with VL in East Azerbaijan, Iran.

How is inflammation initiated? Individual influences of IL-1, IL-18 and HMGB1

Pro-inflammatory cytokines are crucial for fighting infection and establishing immunity. Recently, other proteins, such as danger-associated molecular patterns (DAMPs), have also been appreciated for their role in inflammation and immunity. Following the formation and activation of multiprotein complexes, termed inflammasomes, two cytokines, IL-1β and IL-18, along with the DAMP High Mobility Group Box 1 (HMGB1), are released from cells. Although these proteins all lack classical secretion signals and are released by inflammasome activation, they each lead to different downstream consequences. This review examines how various inflammasomes promote the release of IL-1β, IL-18 and HMGB1 to combat pathogenic situations. Each of these effector molecules plays distinct roles during sterile inflammation, responding to viral, bacterial and parasite infection, and tailoring the innate immune response to specific threats.

How aluminum adjuvants could promote and enhance non-target IgE synthesis in a genetically-vulnerable sub-population

Aluminum-containing adjuvants increase the effectiveness of vaccination, but their ability to augment immune responsiveness also carries the risk of eliciting non-target responses, especially in genetically susceptible individuals. This study reviews the relevant actions of aluminum adjuvants and sources of genetic risk that can combine to adversely affect a vulnerable sub-population. Aluminum adjuvants promote oxidative stress and increase inflammasome activity, leading to the release of IL-1β, IL-18, and IL-33, but not the important regulatory cytokine IL-12. In addition, they stimulate macrophages to produce PGE₂, which also has a role in regulating immune responses. This aluminum-induced cytokine context leads to a T(H)2 immune response, characterized by the further release of IL-3, IL-4, IL-5, IL-9, IL-13, and IgE-potentiating factors such as sCD23. Genetic variants in cytokine genes, such as IL-4, IL-13, IL-33, and IL-18 influence the response to vaccines in children and are also associated with atopy. These genetic factors may therefore define a genetically-vulnerable sub-population, children with a family history of atopy, who may experience an exaggerated T(H)2 immune response to aluminum-containing vaccines. IL-4, sCD23, and IgE are common factors for both atopy and the immune-stimulating properties of aluminum adjuvants. IL-4 is critical in the production of IgE and total IgE up-regulation. IL-4 has also been reported to induce the production of sCD23 and trigger resting sIgM+, sIgD+ B-cells to switch to sIgE+ B-cells, making them targets for IgE-potentiating factors. Further, the actions of IgE-potentiating factors on sIgE+ B-cells are polyclonal and unrestricted, triggering their differentiation into IgE-forming plasma cells. These actions provide a mechanism for aluminum-adjuvant promotion and enhancement of non-target IgE in a genetically vulnerable sub-population. Identification of these individuals may decrease the risk of adverse events associated with the use of aluminum-containing vaccines.

T helper1/t helper2 cells and resistance/susceptibility to leishmania infection: is this paradigm still relevant?

Work in large part on Leishmania major in the 1980s identified two distinct apparently counter-regulatory CD4⁺ T cell populations, T helper (h)1 and Th2, that controlled resistance/susceptibility to infection respectively. However, the generation of IL-4^−/− mice in the 1990s questioned the paramount role of this Th2 archetypal cytokine in the non-healing response to Leishmania infection. The more recent characterization of CD4⁺ T cell regulatory populations and further effector CD4⁺ T helper populations, Th17, Th9, and T follicular (f)h cells as well as the acknowledged plasticity in T helper cell function has further added to the complexity of host pathogen interactions. These interactions are complicated by the multiplicity of cells that respond to CD4⁺ T cell subset signatory cytokines, as well as the diversity of Leishmania species that are often subject to significantly different immune-regulatory controls. In this article we review current knowledge with regard to the role of CD4⁺ T cells and their products during Leishmania infection. In particular we update on our studies using conditional IL-4Rα gene-deficient mice that have allowed dissection of the cell interplay dictating the disease outcomes of the major Leishmania species infecting humans.

Does T Helper Differentiation Correlate with Resistance or Susceptibility to Infection with L. major? Some Insights From the Murine Model

The murine model of Leishmania major infection has been an invaluable tool in understanding T helper differentiation in vivo. The initial evidence for a role of distinct CD4⁺ T helper subsets in the outcome of infection was first obtained with this experimental model. The development of CD4⁺ Th1 cells was associated with resolution of the lesion, control of parasite replication, and resistance to re-infection in most of the mouse strains investigated (i.e., C57BL/6). In contrast, differentiation of CD4⁺ Th2 cells correlated with the development of unhealing lesions, and failure to control parasite load in a few strains (i.e., BALB/c). Since these first reports, an incredible amount of effort has been devoted to understanding the various parameters involved in the differentiation of these, and more recently discovered T helper subsets such as Th17 and T regulatory cells. The discovery of cross-talk between T helper subsets, as well as their plasticity force us to reevaluate the events driving a protective/deleterious T helper immune response following infection with L. major in mice. In this review, we describe the individual contributions of each of these CD4⁺ T helper subsets following L. major inoculation, emphasizing recent advances in the field, such as the impact of different substrains of L. major on the pathogenesis of disease.

Classification and regression tree and spatial analyses reveal geographic heterogeneity in genome wide linkage study of Indian visceral leishmaniasis

Background

Genome wide linkage studies (GWLS) have provided evidence for loci controlling visceral leishmaniasis on Chromosomes 1p22, 6q27, 22q12 in Sudan and 6q27, 9p21, 17q11-q21 in Brazil. Genome wide studies from the major focus of disease in India have not previously been reported.

Methods and Findings

We undertook a GWLS in India in which a primary ∼10 cM (515 microsatellites) scan was carried out in 58 multicase pedigrees (74 nuclear families; 176 affected, 353 total individuals) and replication sought in 79 pedigrees (102 nuclear families; 218 affected, 473 total individuals). The primary scan provided evidence (≥2 adjacent markers allele-sharing LOD≥0.59; nominal P≤0.05) for linkage on Chromosomes 2, 5, 6, 7, 8, 10, 11, 20 and X, with peaks at 6p25.3-p24.3 and 8p23.1-p21.3 contributed to largely by 31 Hindu families and at Xq21.1-q26.1 by 27 Muslim families. Refined mapping confirmed linkage across all primary scan families at 2q12.2-q14.1 and 11q13.2-q23.3, but only 11q13.2-q23.3 replicated (combined LOD = 1.59; P = 0.0034). Linkage at 6p25.3-p24.3 and 8p23.1-p21.3, and at Xq21.1-q26.1, was confirmed by refined mapping for primary Hindu and Muslim families, respectively, but only Xq21.1-q26.1 replicated across all Muslim families (combined LOD 1.49; P = 0.0045). STRUCTURE and SMARTPCA did not identify population genetic substructure related to religious group. Classification and regression tree, and spatial interpolation, analyses confirm geographical heterogeneity for linkages at 6p25.3-p24.3, 8p23.1-p21.3 and Xq21.1-q26.1, with specific clusters of families contributing LOD scores of 2.13 (P = 0.0009), 1.75 (P = 0.002) and 1.84 (P = 0.001), respectively.

Conclusions

GWLS has identified novel loci that show geographical heterogeneity in their influence on susceptibility to VL in India.

Expression, antibody generation, and biological analysis of chicken interleukin-18

The gene encoding mature chicken interleukin-18 (ChIL-18) was cloned into prokaryotic expression vector pET-30a(+), resulting in a recombinant plasmid pET-30a-ChIL-18. After pET-30a-ChIL-18 was transformed into Escherichia coli Rosseta, the expression of ChIL-18 induced by 1 mM IPTG at 37°C was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The expressed fusion protein of 26 kDa was purified with a Ni-NTA affinity column and used to generate a hyperimmune antiserum in a rabbit. The specificity and titer of anti-ChIL-18 serum were analyzed by the enzyme-linked immunosorbent assay. Western blot and immunofluorescence assays indicated that the anti-ChIL-18 antibody specifically reacted with the ChIL-18 expressed from E. coli or ChIL-18-transfected eukaryotic cells. Moreover, the renatured ChIL-18 stimulated the production of nitric oxide (NO) from macrophages via eliciting the secreting of IFN-γ from lymphocytes.

A role for IL-18 in protective immunity against Mycobacterium tuberculosis

Tuberculosis remains the most hazardous bacterial infection worldwide. The causative agent, Mycobacterium tuberculosis, is a facultative intracellular pathogen of resting MPhi. IFN-gamma secreted by natural killer, CD4 Th 1 and CD8 T cells upon instruction by IL-12 and -18 activates MPhi to restrict mycobacterial growth. Production of both cytokines is induced by TLR signalling in DC and MPhi. Mice deficient for the TLR adaptor, MyD88, are highly susceptible to M. tuberculosis infection. Shared usage of MyD88 by signalling cascades for TLR and receptors for IL-1 and IL-18 prompted us to revisit the role of IL-18 during experimental infection with M. tuberculosis. We show that mice deficient for IL-18 and MyD88 but not for IL-18 receptor promptly succumbed to M. tuberculosis infection in contrast to WT or TLR-2/-4 double KO mice indicating that lack of IL-18 contributes to the high susceptibility of MyD88 KO mice to M. tuberculosis. Without IL-18, the protective Th1 response was decreased and hence, mycobacterial propagation was favoured. Neutrophil-driven lung immunopathology concomitant with unrestrained growth of tubercle bacilli are most likely responsible for the premature death of IL-18 KO mice. Thus, IL-18 plays a decisive role in protective immunity against tuberculosis.

Role of interleukin (IL)-18 in experimental paracoccidioidomycosis

Interleukin (IL)-18 has been regarded as a Th1 type cytokine involved in many fungal and parasitic infections. Since there have been no studies, as of yet, evaluating the role of this cytokine in paracoccidioidomycosis (PCM), we assessed the function of IL-18 by using an experimental PCM model. Our results showed that IL-18 knockout (IL-18 -/-) BALB/c were more resistant to Paracoccidioides brasiliensis than their littermate controls (WT). In fact, mortality rate was higher in WT mice and in the first month of infection, the number of colony forming units of the etiologic agent recovered from the lungs was greater in WT mice. In histopathological analyses, well-formed granulomas were seen in both WT and IL-18(-/-) mice. However, substantial differences were observed at the second month of infection when epithelioid cells predominated in the lesions of IL-18(-/-) mice, which could infer that IL-18 postpones pulmonary healing. The levels of IL-10 were significantly higher in IL-18 sufficient mice at early stages of infection and therefore account for the delayed fungal clearance observed in WT mice. TNF-alpha augmented later in the infection of WT mice, seemingly to compensate high levels of IL-10. Our results demonstrated that IL-18 has a critical role in protecting BALB/c mice against disseminated PCM.

Leishmania major: in vitro and in vivo anti-leishmanial activity of paromomycin ointment (Leshcutan) combined with the immunomodulator Imiquimod

Paromomycin at 25, 50 and 100 microg/ml, inhibited the growth of Leishmania major amastigotes by 34.5%, 61.2%, 74.9% and 85.4%, 89.9%, 95.7% on the 2nd and the 4th day of treatment in culture, respectively. Methylbenzethonium chloride at 0.1 and 0.5 microg/ml and Imiquimod at 5 and 10 microg/ml, administered separately, inhibited the parasite development by 39.5% and 65.2% and 31.5% and 47.7%, respectively. Imiquimod (5-10 microg/ml) combined with either paromomycin (25, 50 and 100 microg/ml) or methylbenzethonium chloride (0.1 and 0.5 microg/ml) showed an anti-leishmanial additive effect. A 10 day topical treatment, twice daily, with an ointment containing 15% paromomycin and 12% methylbenzethonium chloride (Leshcutan), either undiluted or diluted 1:5 in soft white paraffin combined with 5% Imiquimod cream (Aldara), was as effective as Leshcutan given alone. The present study suggests that a combination of Aldara and Leshcutan is as effective as Leshcutan given alone in the topical treatment of CL caused by L. major.

Leukotrienes are essential for the control of Leishmania amazonensis infection and contribute to strain variation in susceptibility

Leukotrienes (LTs) are known to be produced by macrophages when challenged with Leishmania, but it is not known whether these lipid mediators play a role in host defense against this important protozoan parasite. In this study, we investigated the involvement of LTs in the in vitro and in vivo response to Leishmania amazonensis infection in susceptible (BALB/c) and resistant (C3H/HePAS) mice. Pharmacologic or genetic deficiency of LTs resulted in impaired leishmanicidal activity of peritoneal macrophages in vitro. In contrast, addition of LTB4 increased leishmanicidal activity and this effect was dependent on the BLT1 receptor. LTB4 augmented NO production in response to L. amazonensis challenge, and studies with a NO synthesis inhibitor revealed that NO was critical for the enhancement of macrophage leishmanicidal activity. Interestingly, macrophages from resistant mice produced higher levels of LTB4 upon L. amazonensis challenge than did those from susceptible mice. In vivo infection severity, as assessed by footpad swelling following s.c. promastigote inoculation, was increased when endogenous LT synthesis was abrogated either pharmacologically or genetically. Taken together, these results for the first time reveal an important role for LTB4 in the protective response to L. amazonensis, identify relevant leishmanicidal mechanisms, and suggest that genetic variation in LTB4 synthesis might influence resistance and susceptibility patterns to infection.

Endogenous interleukin-18 is involved in immunity to Leishmania donovani but its absence does not adversely influence the therapeutic activity of sodium stibogluconate

Immunity to Leishmania donovani is associated with an interleukin (IL)-12 driven T helper 1 (Th1) response. In addition, the ability to respond to chemotherapy with sodium stibogluconate (SSG) requires a fully competent immune response and both Th1 and Th2 responses have been shown to positively influence the outcome of drug treatment. In the present study, the influence of IL-18, which can modulate both interferon (IFN)-gamma and IL-4 production, on the outcome of primary L. donovani infection and SSG therapy following infection was assessed using BALB/c IL-18-deficient and wild type mice. IL-18 deficiency was associated with an increased susceptibility to L. donovani infection, evident by day 40 post infection, resulting in higher parasite burdens in the spleen, liver, and bone marrow compared with wild type control animals. Infected IL-18-deficient mice had significantly lower splenocyte concanavalin A (ConA) induced IFN-gamma production as well as lower serum IL-12 and IFN-gamma levels, indicating a reduced Th1 response. However, drug treatment was equally effective in both mouse strains and restored serum IL-12 and IFN-gamma levels, and IFN-gamma production by ConA stimulated splenocytes of IL-18-deficient mice, to levels equivalent to similarly treated wild type mice.

IL‐18 is redundant in T‐cell responses and in joint inflammation in antigen‐induced arthritis

IL-18 is an important cofactor in Th1 immune responses and it has additional roles in inflammation. Recent reports suggest the contribution of IL-18 to immune responses may vary between mouse strains and immune contexts. We investigated the contribution of IL-18 to T-cell activation and joint inflammation in Ag-induced arthritis (AIA) in C57Bl/6 mice. AIA and cutaneous delayed-type hypersensitivity (DTH) reactions were induced in wild-type (WT) and IL-18-/- C57Bl/6 mice, and Ag-specific T-cell proliferation and IFN-gamma and IL-4 production were measured. The humoral immune response was measured as serum antibody to the disease-initiating Ag, methylated BSA (mBSA). Splenocyte production of IL-6 was measured by ELISA. To confirm the dependence of this model on Th1-cell-mediated immunity, IL-12p40-/- mice were similarly studied. WT mice developed synovitis, joint effusion, cartilage destruction and bone damage associated with induction of DTH, and in vitro Ag-specific T-cell proliferation and IFN-gamma production. Unexpectedly, IL-18-/- mice developed AIA and indices of T-cell activation were similar to those of WT mice. In contrast, IL-12p40-/- mice did not develop AIA, DTH or T-cell activation. WT and IL-18-/- mice, but not IL-12p40-/- mice, developed significantly increased serum antibody to mBSA compared with naive controls. WT and IL-18-/- splenocytes produced high levels of IL-6, whereas IL-12p40-/- cells had significantly lower IL-6 production compared with both. In conclusion, IL-18 is redundant both as a Th1 response cofactor and inflammatory cytokine, whereas IL-12p40-/- is a key cytokine, in AIA in C57Bl/6 mice.

Role of interleukin-18 in allergy and autoimmunity: an explanation for the hygiene hypothesis

The possibility that the hygiene hypothesis is the most reasonable explanation for the increased prevalence of both allergy (in which the immune response is dominated by Th2 cells) and Th1-mediated autoimmunity is supported by the observations that exposure of humans to microbial agents in early life can exert protection against these disorders later. However, there still remains a question about how environmental microbes can decrease both Th1 and Th2 immune-mediated disorders, the two opposite spectrums of the immune responses. Cytokines are considered the main determining factors in the initial differentiation of T cells to Th1 and Th2 subsets. IL-18 as a multifunctional cytokine is capable of polarizing the immune response to both of these distinct subsets depending on the genetic background and cytokine milieu. It is hypothesized that the reduced exposure to microbial agents in early life leads to the aberrant production of IL-18, which in turn influences individuals quite differently depending on their genetic background. In genetically predisposed individuals to allergy, it augments Th2 responses and in genetically predisposed individuals to Th1-autoimmunity it accelerates Th1 responses.

Role of interleukin-18 in experimental infections with Streptococcus pneumoniae

IL-18, a multifunctional cytokine, has been shown to be involved in the immune response to numerous pathogens including several bacterial species. To study its role in infection by the Gram-positive bacterium Streptococcus pneumoniae, wild-type and IL-18 knockout BALB/c mice were compared in murine models of pneumococcal pneumonia, bacteraemia and nasopharyngeal colonization. The influence of IL-18 varied with the infection type, whereby it contributed to increased bacterial loads in pneumonia, reduced levels of colonization and had no effect on levels of bacteraemia following intravenous challenge. Likewise, the influence of IL-18 on pneumonia varied between two infecting pneumococcal strains. Comparison of these results with previous data also suggested that the influence of IL-18 in pneumococcal pneumonia differs with the mouse strain genetic background. Overall, these results demonstrate the complex influence of IL-18 in the response to the pneumococcus.