Gamma interferon and interleukin-1 receptor 1 regulate neutrophil recruitment to the corneal stroma in a murine model of Onchocerca volvulus keratitis

Highlighting the Relevance of CD8+ T Cells in Filarial Infections

The T cell immune responses in filarial infections are primarily mediated by CD4+ T cells and type 2-associated cytokines. Emerging evidence indicates that CD8+ T cell responses are important for anti-filarial immunity, however, could be suppressed in co-infections. This review summarizes what we know so far about the activities of CD8+ T cell responses in filarial infections, co-infections, and the associations with the development of filarial pathologies.

Wolbachia: endosymbiont of onchocercid nematodes and their vectors

Background

Wolbachia is an obligate intracellular maternally transmitted, gram-negative bacterium which forms a spectrum of endosymbiotic relationships from parasitism to obligatory mutualism in a wide range of arthropods and onchocercid nematodes, respectively. In arthropods Wolbachia produces reproductive manipulations such as male killing, feminization, parthenogenesis and cytoplasmic incompatibility for its propagation and provides an additional fitness benefit for the host to protect against pathogens, whilst in onchocercid nematodes, apart from the mutual metabolic dependence, this bacterium is involved in moulting, embryogenesis, growth and survival of the host.

Methods

This review details the molecular data of Wolbachia and its effect on host biology, immunity, ecology and evolution, reproduction, endosymbiont-based treatment and control strategies exploited for filariasis. Relevant peer-reviewed scientic papers available in various authenticated scientific data bases were considered while writing the review.

Conclusions

The information presented provides an overview on Wolbachia biology and its use in the control and/or treatment of vectors, onchocercid nematodes and viral diseases of medical and veterinary importance. This offers the development of new approaches for the control of a variety of vector-borne diseases.

Graphic Abstract

In silico drug repurposing for filarial infection predicts nilotinib and paritaprevir as potential inhibitors of the Wolbachia 5'-aminolevulinic acid synthase

Filarial infections affect millions of individuals and are responsible for some notorious disabilities. Current treatment options involve repeated mass drug administrations, which have been met with several challenges despite some successes. Administration of doxycycline, an anti-Wolbachia agent, has shown clinical effectiveness but has several limitations, including long treatment durations and contraindications. We describe the use of an in silico drug repurposing approach to screening a library of over 3200 FDA-approved medications against the filarial endosymbiont, Wolbachia. We target the enzyme which catalyzes the first step of heme biosynthesis in the Wolbachia. This presents an opportunity to inhibit heme synthesis, which leads to depriving the filarial worm of heme, resulting in a subsequent macrofilaricidal effect. High throughput virtual screening, molecular docking and molecular simulations with binding energy calculations led to the identification of paritaprevir and nilotinib as potential anti-Wolbachia agents. Having higher binding affinities to the catalytic pocket than the natural substrate, these drugs have the structural potential to bind and engage active site residues of the wolbachia 5'-Aminolevulinic Acid Synthase. We hereby propose paritaprevir and nilotinib for experimental validations as anti-Wolbachia agents.

Anti-Wolbachia therapy for onchocerciasis & lymphatic filariasis: Current perspectives

Onchocerciasis and lymphatic filariasis (LF) are human filarial diseases belonging to the group of neglected tropical diseases, leading to permanent and long-term disability in infected individuals in the endemic countries such as Africa and India. Microfilaricidal drugs such as ivermectin and albendazole have been used as the standard therapy in filariasis, although their efficacy in eliminating the diseases is not fully established. Anti-Wolbachia therapy employs antibiotics and is a promising approach showing potent macrofilaricidal activity and also prevents embryogenesis. This has translated to clinical benefits resulting in successful eradication of microfilarial burden, thus averting the risk of adverse events from target species as well as those due to co-infection with loiasis. Doxycycline shows potential as an anti-Wolbachia treatment, leading to the death of adult parasitic worms. It is readily available, cheap and safe to use in adult non-pregnant patients. Besides doxycycline, several other potential antibiotics are also being investigated for the treatment of LF and onchocerciasis. This review aims to discuss and summarise recent developments in the use of anti-Wolbachia drugs to treat onchocerciasis and LF.

IL-17 plays a central role in initiating experimental Candida albicans infection in mouse corneas

The pathogenesis of fungal infection in the cornea remains largely unclear. To understand how the immune system influences the progression of fungal infection in corneas, we inoculated immunocompetent BALB/c mice, neutrophil- or CD4⁺ T-cell-depleted BALB/c mice, and nude mice with Candida albicans. We found that only immunocompetent BALB/c mice developed typical Candida keratitis (CaK), while the other mouse strains lacked obvious clinical manifestations. Furthermore, CaK development was blocked in immunocompetent mice treated with anti-IL-17A or anti-IL-23p19 to neutralize IL-17 activity. However, no significant effects were observed when Treg cells, γδ T cells, or IFN-γ were immunodepleted. Upon infection, the corneas of BALB/c mice were infiltrated with IL-17-producing leukocytes, including neutrophils and, to a lesser degree, CD4⁺ T cells. In contrast, leukocyte recruitment to corneas was significantly diminished in nude mice. Indeed, nude mice produced much less chemokines (e.g. CXCL1, CXCL2, CXCL10, CXCL12, CCL2, and IL-6) in response to inoculation. Remarkably, addition of CXCL2 during inoculation restored CaK induction in nude mice. In contrast to its therapeutic effect on CaK, neutralization of IL-17 exacerbated Candida-induced dermatitis in skin. We conclude that IL-17, mainly produced by neutrophils and CD4⁺ T cells in the corneas, is essential in the pathogenesis of CaK.

The symbiotic role of Wolbachia in Onchocercidae and its impact on filariasis

Symbiotic associations between eukaryotes and microorganisms are frequently observed in nature, and range along the continuum between parasitism and mutualism. The genus Wolbachia contains well-known intracellular bacteria of arthropods that induce several reproductive phenotypes that benefit the transmission of the bacteria. Interestingly, Wolbachia bacteria have been found in the Onchocercidae, a family of filarial nematodes, including species that cause human filarial diseases, e.g. lymphatic filariasis and onchocerciasis. The endosymbiont is thought to be mutualistic in the Onchocercidae, and to provide essential metabolites to the filariae. Currently, Wolbachia bacteria are targets of antibiotic therapy with tetracyclines, which have profound effects on the development, viability and fertility of filarial parasites. This overview article presents the Onchocercidae and Wolbachia, and then discusses the origin and the nature of the symbiosis. It highlights the contribution of Wolbachia to the survival of the filariae and to the development of pathology. Finally, the infection control implications for filariases are debated. Potential directions for future research are also discussed.

Differential induction of Th2- and Th1-associated responses by filarial antigens and endosymbiotic Wolbachia in a murine model of river blindness

Immune responses to filarial parasites like the river blindness inducing Onchocerca volvulus are obscured by combined reactions to the filarial nematodes themselves and their endosymbiont bacteria Wolbachia. Overall, infection with filarial nematodes induces a strong Th2 response characterized by IL-5 production and to a lesser degree a Th1 response and IFNγ production. Neutrophil and eosinophil infiltration into the corneal stroma are hallmark features of Onchocerca volvulus stimulation in a mouse model of river blindness. To determine the splenic and corneal response to filarial antigens in the absence of Wolbachia, C57BL/6 mice were immunized subcutaneously with either endosymbiotic Wolbachia alone, a soluble extract from the filaria Acanthocheilonema viteae that does not contain Wolbachia, or both, and injected into the corneal stroma. Neutrophil and eosinophil infiltration into the cornea was assessed by immunohistochemistry. In addition, Th1- and Th2-associated responses to filaria or Wolbachia were investigated by determining IL-5 and IFN-γ production by splenocytes. We found that A. viteae in the absence of Wolbachia induced IL-5 production and eosinophil infiltration, but not IFN-γ. Conversely, Wolbachia induced IFN-γ production and no migration of eosinophils. There was no difference in neutrophil infiltration. Together, these findings demonstrate a distinct Th-associated phenotype induced by filaria and Wolbachia.

Tolerance of infections

A host has two methods to defend against pathogens: It can clear the pathogens or reduce their impact on health in other ways. The first, resistance, is well studied. Study of the second, which ecologists call tolerance, is in its infancy. Tolerance measures the dose response curve of a host's health in reaction to a pathogen and can be studied in a simple quantitative manner. Such studies hold promise because they point to methods of treating infections that put evolutionary pressures on microbes different from antibiotics and vaccines. Studies of tolerance will provide an improved foundation to describe our interactions with all microbes: pathogenic, commensal, and mutualistic. One obvious mechanism affecting tolerance is the intensity of an immune response; an overly exuberant immune response can cause collateral damage through immune effectors and because of the energy allocated away from other physiological functions. There are potentially many other tolerance mechanisms, and here we systematically describe tolerance using a variety of animal systems.

Onchocerciasis: the role of Wolbachia bacterial endosymbionts in parasite biology, disease pathogenesis, and treatment

The discovery of Wolbachia intracellular bacteria within filarial nematodes, including Onchocerca volvulus, the causative agent of onchocerciasis or "river blindness," has delivered a paradigm shift in our understanding of the parasite's biology, to where we now know that the bacterial endosymbionts are essential for normal development of larvae and embryos and may support the long-term survival of adult worms. The apparent mutualistic dependency has also offered a novel approach to the treatment of onchocerciasis through the use of antibiotics to eliminate Wolbachia, delivering for the first time a treatment which has significant macrofilaricidal efficacy. Studies with other filarial nematode species have also highlighted a role for Wolbachia in transmission and infection of the mammalian host through a fascinating manipulation of mast cell-mediated vasodilation to enhance infectivity of vector-borne larvae. Wolbachia has also been identified as the principal driver of innate and adaptive Th1 inflammatory immunity, which can either contribute to disease pathogenesis or, with the Wolbachia-mediated recruitment of mast cells, enhance infectivity. The Wolbachia activation of innate inflammation also drives inflammatory adverse events in response to chemotherapy with either diethylcarbamazine (DEC) or ivermectin. In this review we summarize the experimental and field trial data which have uncovered the importance of Wolbachia symbiosis in onchocerciasis.

Lipoteichoic acid from Lactobacillus plantarum induces nitric oxide production in the presence of interferon-γ in murine macrophages

Lipoteichoic acid (LTA) is a major immuno-stimulating component of Gram-positive bacteria. LTA from the beneficial bacterium Lactobacillus plantarum induces weak nitric oxide (NO) production in murine macrophages. Currently, it is not clear if LTA from L. plantarum is able to stimulate the innate immune response, even in the presence of inflammation. In the present study, we prepared highly pure and structurally intact LTA from L. plantarum and investigated its ability to induce NO in the presence of interferon (IFN)-γ in the RAW 264.7 murine macrophage cell line and bone marrow-derived macrophages (BMMs) from mice. L. plantarum LTA alone was unable to induce NO production, even at 30μg/ml. However, LTA in the presence of IFN-γ significantly induced NO production in RAW 264.7 cells. The observed NO production was inhibited by a NO synthase (NOS) inhibitor l-NAME and an inducible NOS (iNOS) inhibitor l-NIL, suggesting that iNOS is specifically required for this action. Western blot analysis and reverse transcription and polymerase chain reaction further confirmed that L. plantarum LTA increased protein and mRNA levels of iNOS, respectively. However, such induction was substantially inhibited in BMMs from Toll-like receptor 2 (TLR2)-deficient mice and the macrophages treated with an inhibitor blocking platelet-activating factor receptor. In addition, L. plantarum LTA plus IFN-γ induced IFN-β expression and STAT1 phosphorylation, which are key pathways for inducing iNOS expression. Electrophoretic mobility shift assay demonstrated that L. plantarum LTA in the presence of IFN-γ remarkably increased the DNA-binding activity of NF-κB transcription factor, which is known to be involved in the iNOS gene expression. Collectively, these results suggest that LTA from L. plantarum alone has no inflammatory potential but does induce NO production under conditions of inflammation, such as the presence of IFN-γ.

Decreased leukocyte accumulation and delayed Bordetella pertussis clearance in IL-6-/- mice

IL-6, a pleiotropic cytokine primarily produced by the innate immune system, has been implicated in the development of acquired immune responses, though its roles are largely undefined and may vary in the context of different diseases. Using a murine model of infection, we established that IL-6 influences the adaptive immune responses against the endemic human respiratory pathogen Bordetella pertussis. IL-6 was induced in the lungs of C57BL/6 mice by B. pertussis. IL-6(-/-) mice showed a protracted infectious course and were less efficiently protected by B. pertussis vaccination than wild-type mice. Abs from IL-6(-/-) mice, though lower in titer, efficiently reduced B. pertussis numbers in IL-6-sufficient mice. Pulmonary leukocyte recruitment and splenic or pulmonary T cell cytokine responses to B. pertussis, including Th1 and Th17 cytokine production, were lower in IL-6(-/-) mice than in wild-type mice. Adoptive transfer of immune wild-type CD4(+) cells ameliorated the defect of IL-6(-/-) mice in the control of B. pertussis numbers. Together, these results reveal the dysregulation of multiple aspects of adaptive immune responses in B. pertussis-infected IL-6(-/-) mice and suggest that IL-6 is involved in regulating Ab generation, pulmonary leukocyte accumulation, and T cell cytokine production in response to B. pertussis as well as the generation of effective vaccine-induced immunity against this pathogen.

Lymphatic filariasis and onchocerciasis

Lymphatic filariasis and onchocerciasis are parasitic helminth diseases that constitute a serious public health issue in tropical regions. The filarial nematodes that cause these diseases are transmitted by blood-feeding insects and produce chronic and long-term infection through suppression of host immunity. Disease pathogenesis is linked to host inflammation invoked by the death of the parasite, causing hydrocoele, lymphoedema, and elephantiasis in lymphatic filariasis, and skin disease and blindness in onchocerciasis. Most filarial species that infect people co-exist in mutualistic symbiosis with Wolbachia bacteria, which are essential for growth, development, and survival of their nematode hosts. These endosymbionts contribute to inflammatory disease pathogenesis and are a target for doxycycline therapy, which delivers macrofilaricidal activity, improves pathological outcomes, and is effective as monotherapy. Drugs to treat filariasis include diethylcarbamazine, ivermectin, and albendazole, which are used mostly in combination to reduce microfilariae in blood (lymphatic filariasis) and skin (onchocerciasis). Global programmes for control and elimination have been developed to provide sustained delivery of drugs to affected communities to interrupt transmission of disease and ultimately eliminate this burden on public health.

Human conjunctival transcriptome analysis reveals the prominence of innate defense in Chlamydia trachomatis infection

Trachoma is the leading infectious cause of blindness and is endemic in 52 countries. There is a critical need to further our understanding of the host response during disease and infection, as millions of individuals are still at risk of developing blinding sequelae. Infection of the conjunctival epithelial cells by the causative bacterium, Chlamydia trachomatis, stimulates an acute host response. The main clinical feature is a follicular conjunctivitis that is incompletely defined at the tissue-specific gene expression and molecular levels. To explore the features of disease and the response to infection, we measured host gene expression in conjunctival samples from Gambian children with active trachoma and healthy controls. Genome-wide expression and transcription network analysis identified signatures characteristic of the expected infiltrating immune cell populations, such as neutrophils and T/B lymphocytes. The expression signatures were also significantly enriched for genes in pathways which regulate NK cell activation and cytotoxicity, antigen processing and presentation, chemokines, cytokines, and cytokine receptors. The data suggest that in addition to polymorph and adaptive cellular responses, NK cells may contribute to a significant component of the conjunctival inflammatory response to chlamydial infection.

History and physiology of immune privilege

Immune privilege is the condition in which selected immune responses are suppressed or excluded in certain organs, such as the eye. Immune privilege in the eye was described over 130 years ago, but its significance was not appreciated until the early 1950s. Investigations beginning in the 1970s ushered in a new era and revealed that ocular immune privilege is due to anatomical, physiological, and immunoregulatory processes that prevent the induction and expression of immune-mediated inflammation. It is widely believed that immune privilege is an adaptation for reducing immune-mediated injury to ocular cells that have limited or no capacity for regeneration.

Pretreatment with lipopolysaccharide modulates innate immunity in corneal fibroblasts challenged with Aspergillus fumigatus

Aspergillus fumigatus triggers inflammatory responses through Toll-like receptors (TLRs), particularly TLR2 and TLR4. In this study, we tested the hypothesis that lipopolysaccharide (LPS), a ligand of TLR4, can induce tolerance of A. fumigatus hyphae in telomerase-immortalized human stroma fibroblasts (THSFs). The THSFs were pretreated with low-dose LPS for various times and then challenged with A. fumigatus hyphae. We observed that pretreatment of THSFs with low-dose LPS resulted in diminished production of cytokines IL-8 and IL-6 and elevated expression of antimicrobial peptides, such as CC chemokine-ligand 20 (CCL20) and thymosin β4 (Tβ4), upon subsequent A. fumigatus challenge. Furthermore, LPS pretreatment also resulted in suppression of polymorphonuclear leukocyte (PMN) migration. Our results suggested that THSFs pretreated with LPS develop a state of A. fumigatus hyphae tolerance. This may induce protective mechanisms during fungal keratitis to prevent an excessive inflammatory response and to control infection of the cornea.