IL-33/ST2 axis is involved in disease progression in the spleen during Leishmania donovani infection

Leishmania exploits host cAMP/EPAC/calcineurin signaling to induce an IL-33-mediated anti-inflammatory environment for the establishment of infection

Host anti-inflammatory responses are critical for the progression of visceral leishmaniasis, and the pleiotropic cytokine interleukin (IL)-33 was found to be upregulated in infection. Here, we documented that IL-33 induction is a consequence of elevated cAMP-mediated exchange protein activated by cAMP (EPAC)/calcineurin-dependent signaling and essential for the sustenance of infection. Leishmania donovani-infected macrophages showed upregulation of IL-33 and its neutralization resulted in decreased parasite survival and increased inflammatory responses. Infection-induced cAMP was involved in IL-33 production and of its downstream effectors PKA and EPAC, only the latter was responsible for elevated IL-33 level. EPAC initiated Rap-dependent phospholipase C activation, which triggered the release of intracellular calcium followed by calcium/calmodulin complex formation. Screening of calmodulin-dependent enzymes affirmed involvement of the phosphatase calcineurin in cAMP/EPAC/calcium/calmodulin signaling-induced IL-33 production and parasite survival. Activated calcineurin ensured nuclear localization of the transcription factors, nuclear factor of activated T cell 1 and hypoxia-inducible factor 1 alpha required for IL-33 transcription, and we further confirmed this by chromatin immunoprecipitation assay. Administering specific inhibitors of nuclear factor of activated T cell 1 and hypoxia-inducible factor 1 alpha in BALB/c mouse model of visceral leishmaniasis decreased liver and spleen parasite burden along with reduction in IL-33 level. Splenocyte supernatants of inhibitor-treated infected mice further documented an increase in tumor necrosis factor alpha and IL-12 level with simultaneous decrease of IL-10, thereby indicating an overall disease-escalating effect of IL-33. Thus, this study demonstrates that cAMP/EPAC/calcineurin signaling is crucial for the activation of IL-33 and in effect creates anti-inflammatory responses, essential for infection.

Regulatory T cells in parasite infections: susceptibility, specificity and specialisation

Regulatory T cells (Tregs) are essential to control immune system responses to innocuous self-specificities, intestinal and environmental antigens. However, they may also interfere with immunity to parasites, particularly in chronic infection. Susceptibility to many parasite infections is, to a greater or lesser extent, controlled by Tregs, but often they play a more prominent role in moderating the immunopathological consequences of parasitism, and dampening bystander reactions in an antigen-nonspecific manner. More recently, Treg subtypes have been defined which may preferentially act in different contexts; we also discuss the degree to which this specialisation is now being mapped onto how Tregs maintain the delicate balance between tolerance, immunity, and pathology in infection.

Regulatory Functions of Hypoxia in Host-Parasite Interactions: A Focus on Enteric, Tissue, and Blood Protozoa

Body tissues are subjected to various oxygenic gradients and fluctuations and hence can become transiently hypoxic. Hypoxia-inducible factor (HIF) is the master transcriptional regulator of the cellular hypoxic response and is capable of modulating cellular metabolism, immune responses, epithelial barrier integrity, and local microbiota. Recent reports have characterized the hypoxic response to various infections. However, little is known about the role of HIF activation in the context of protozoan parasitic infections. Growing evidence suggests that tissue and blood protozoa can activate HIF and subsequent HIF target genes in the host, helping or hindering their pathogenicity. In the gut, enteric protozoa are adapted to steep longitudinal and radial oxygen gradients to complete their life cycle, yet the role of HIF during these protozoan infections remains unclear. This review focuses on the hypoxic response to protozoa and its role in the pathophysiology of parasitic infections. We also discuss how hypoxia modulates host immune responses in the context of protozoan infections.

Endogenous IL-33 Accelerates Metacestode Growth during Late-Stage Alveolar Echinococcosis

During the course of the infectious disease alveolar echinococcosis (AE), the larval stage of Echinococcus multilocularis develops in the liver, where an initial Th1/Th17 immune response may allow its elimination in resistant individuals. In patients susceptible to infection and disease, the Th2 response initiates later, inducing tolerance to the parasite. The role of interleukin 33 (IL-33), an alarmin released during necrosis and known to drive a Th2 immune response, has not yet been described during AE. Wild-type (WT) and IL-33-/- C57BL/6J mice were infected by peritoneal inoculation with E. multilocularis metacestodes and euthanized 4 months later, and their immune response were analyzed. Immunofluorescence staining and IL-33 enzyme-linked immunosorbent assay (ELISA) were also performed on liver samples from human patients with AE. Overall, metacestode lesions were smaller in IL-33-/- mice than in WT mice. IL-33 was detected in periparasitic tissues, but not in mouse or human serum. In infected mice, endogenous IL-33 modified peritoneal macrophage polarization and cytokine profiles. Th2 cytokine concentrations were positively correlated with parasite mass in WT mice, but not in IL-33-/- mice. In human AE patients, IL-33 concentrations were higher in parasitic tissues than in distant liver parenchyma. The main sources of IL-33 were CD31+ endothelial cells of the neovasculature, present within lymphoid periparasitic infiltrates together with FOXP3+ Tregs. In the murine model, periparasitic IL-33 correlated with accelerated parasite growth putatively through the polarization of M2-like macrophages and release of immunosuppressive cytokines IL-10 and transforming growth factor β1 (TGF-β1). We concluded that IL-33 is a key alarmin in AE that contributes to the tolerogenic effect of systemic Th2 cytokines. IMPORTANCE Infection with the metacestode stage of Echinococcus multilocularis, known as alveolar echinococcosis, is the most severe cestodosis worldwide. However, less than 1% of exposed individuals, in which the immune system is unable to control the parasite, develop the disease. The factors responsible for this interindividual variability are not fully understood. In this in vivo study comparing wild-type and IL-33-/- infected mice, together with data from human clinical samples, we determined that IL-33, an alarmin released following tissue injury and involved in the pathogenesis of cancer and asthma, accelerates the progression of the disease by modulating the periparasitic microenvironment. This suggests that targeting IL-33 could be of interest for the management of patients with AE, and that IL-33 polymorphisms could be responsible for increased susceptibility to AE.

A DNA Nanoraft-Based Cytokine Delivery Platform for Alleviation of Acute Kidney Injury

Cytokine immunotherapy represents an attractive strategy to stimulate robust immune responses for renal injury repair in ischemic acute kidney injury (AKI). However, its clinical application is hindered by its nonspecificity to kidney, short circulation half-life, and severe side effects. An ideal cytokine immunotherapy for AKI requires preferential delivery of cytokines with accurate dosage to the kidney and sustained-release of cytokines to stimulate the immune responses. Herein, we developed a DNA nanoraft cytokine by precisely arranging interleukin-33 (IL-33) nanoarray on rectangle DNA origami, through which IL-33 can be preferentially delivered to the kidney for alleviation of AKI. A nanoraft carrying precisely quantified IL-33 predominantly accumulated in the kidney for up to 48 h. Long-term sustained-release of IL-33 from nanoraft induced rapid expansion of type 2 innate lymphoid cells (ILC 2s) and regulatory T cells (Tregs) and achieved better treatment efficiency compared to free IL-33 treatment. Thus, our study demonstrates that a nanoraft can serve as a structurally well-defined delivery platform for cytokine immunotherapy in ischemic AKI and other renal diseases.

Role of Cytokines in Experimental and Human Visceral Leishmaniasis

Visceral Leishmaniasis (VL) is the most fatal form of disease leishmaniasis. To date, there are no effective prophylactic measures and therapeutics available against VL. Recently, new immunotherapy-based approaches have been established for the management of VL. Cytokines, which are predominantly produced by helper T cells (Th) and macrophages, have received great attention that could be an effective immunotherapeutic approach for the treatment of human VL. Cytokines play a key role in forming the host immune response and in managing the formation of protective and non-protective immunities during infection. Furthermore, immune response mediated through different cytokines varies from different host or animal models. Various cytokines viz. IFN-γ, IL-2, IL-12, and TNF-α play an important role during protection, while some other cytokines viz. IL-10, IL-6, IL-17, TGF-β, and others are associated with disease progression. Therefore, comprehensive knowledge of cytokine response and their interaction with various immune cells is very crucial to determine appropriate immunotherapies for VL. Here, we have discussed the role of cytokines involved in VL disease progression or host protection in different animal models and humans that will determine the clinical outcome of VL and open the path for the development of rapid and accurate diagnostic tools as well as therapeutic interventions against VL.