IL-22 dampens the T cell response in experimental malaria

Immunological mechanisms of inflammatory diseases caused by gut microbiota dysbiosis: A review

The gut microbiota is indispensable for maintaining host health by enhancing the host's digestive capacity, safeguarding the intestinal epithelial barrier, and preventing pathogen invasion. Additionally, the gut microbiota exhibits a bidirectional interaction with the host immune system and promotes the immune system of the host to mature. Dysbiosis of the gut microbiota, primarily caused by factors such as host genetic susceptibility, age, BMI, diet, and drug abuse, is a significant contributor to inflammatory diseases. However, the mechanisms underlying inflammatory diseases resulting from gut microbiota dysbiosis lack systematic categorization. In this study, we summarize the normal physiological functions of symbiotic microbiota in a healthy state and demonstrate that when dysbiosis occurs due to various external factors, the normal physiological functions of the gut microbiota are lost, leading to pathological damage to the intestinal lining, metabolic disorders, and intestinal barrier damage. This, in turn, triggers immune system disorders and eventually causes inflammatory diseases in various systems. These discoveries provide fresh perspectives on how to diagnose and treat inflammatory diseases. However, the unrecognized variables that might affect the link between inflammatory illnesses and gut microbiota, need further studies and extensive basic and clinical research will still be required to investigate this relationship in the future.

Exhausted PD-1+ TOX+ CD8+ T Cells Arise Only in Long-Term Experimental Trypanosoma cruzi Infection

Infection with Trypanosoma cruzi remains the most important neglected zoonosis in Latin America. This infection does not lead to specific symptoms in the acute phase, but chronic infection can result in Chagas disease (CD) with cardiac and/or gastrointestinal manifestations that can lead to death. CD8+ T cells are highly effective and essential to control this infection, but fail to eliminate all parasites. In this study, we show that the CD8+ T cells are modulated by the transient induction of co-inhibitory receptors during acute infection of C57BL/6 mice. Therapeutic intervention strategies with blocking antibodies only had a marginal effect on the elimination of parasite reservoirs. Only long-term chronic infection gave rise to dysfunctional CD8+ T cells, which were characterized by high expression of the inhibitory receptor PD-1 and the co-expression of the transcription factor TOX, which plays a crucial role in the maintenance of the exhausted phenotype. PD-1+ TOX+ CD8+ T cells isolated from the site of infection produced significantly less IFN-γ, TNF-α and Granzyme B than their PD-1- TOX- CD8+ T cell counterparts after T. cruzi-specific stimulation ex vivo. Taken together, we provide evidence that, in the context of experimental infection of mice, the magnitude of the CD8+ T cell response in the acute phase is sufficient for parasite control and cannot be further increased by targeting co-inhibitory receptors. In contrast, persistent long-term chronic infection leads to an increase of exhausted T cells within the tissues of persistence. To our knowledge, this is the first description of infection-induced CD8+ T cells with an exhausted phenotype and reduced cytokine production in muscles of T. cruzi-infected mice.

Structural diversity, functional aspects and future therapeutic applications of human gut microbiome

The research on human gut microbiome, regarded as the black box of the human body, is still at the stage of infancy as the functional properties of the complex gut microbiome have not yet been understood. Ongoing metagenomic studies have deciphered that the predominant microbial communities belong to eubacterial phyla Firmicutes, Bacteroidetes, Proteobacteria, Fusobacteria, Cyanobacteria, Verrucomicrobia and archaebacterial phylum Euryarchaeota. The indigenous commensal microbial flora prevents opportunistic pathogenic infection and play undeniable roles in digestion, metabolite and signaling molecule production and controlling host's cellular health, immunity and neuropsychiatric behavior. Besides maintaining intestinal health via short-chain fatty acid (SCFA) production, gut microbes also aid in neuro-immuno-endocrine modulatory molecule production, immune cell differentiation and glucose and lipid metabolism. Interdependence of diet and intestinal microbial diversity suggests the effectiveness of pre- and pro-biotics in maintenance of gut and systemic health. Several companies worldwide have started potentially exploiting the microbial contribution to human health and have translated their use in disease management and therapeutic applications. The present review discusses the vast diversity of microorganisms playing intricate roles in human metabolism. The contribution of the intestinal microbiota to regulate systemic activities including gut-brain-immunity crosstalk has been focused. To the best of our knowledge, this review is the first of its kind to collate and discuss the companies worldwide translating the multi-therapeutic potential of human intestinal microbiota, based on the multi-omics studies, i.e. metagenomics and metabolomics, as ready solutions for several metabolic and systemic disorders.

The good and the bad about separation anxiety: roles of IL-22 and IL-22BP in liver pathologies

The human liver fulfills several vital tasks daily and possesses an impressive ability to self-regenerate. However, the capacity of this self-healing process can be exhausted by a variety of different liver diseases, such as alcoholic liver damage, viral hepatitis, or hepatocellular carcinoma. Over time, all these diseases generally lead to progressive liver failure that can become fatal if left untreated. Thus, a great effort has been directed towards the development of innovative therapies. The most recently discovered therapies often involve modifying the patient's immune system to enhance a beneficial immune response. Current data suggest that, among others, the cytokine IL-22 might be a promising therapeutical candidate. IL-22 and its endogenous antagonist, IL-22BP, have been under thorough scientific investigation for nearly 20 years. While IL-22 is mainly produced by TH22 cells, ILC3s, NKT cells, or γδ T cells, sources of IL-22BP include dendritic cells, eosinophils, and CD4+ cells. In many settings, IL-22 was shown to promote regenerative potential and, thus, could protect tissues from pathogens and damage. However, the effects of IL-22 during carcinogenesis are more ambiguous and depend on the tumor entity and microenvironment. In line with its capabilities of neutralizing IL-22 in vivo, IL-22BP possesses often, but not always, an inverse expression pattern compared to its ligand. In this comprehensive review, we will summarize past and current findings regarding the roles of IL-22 and IL-22BP in liver diseases with a particular focus on the leading causes of advanced liver failure, namely, liver infections, liver damage, and liver malignancies.

The Intestinal Microbiota: Impacts of Antibiotics Therapy, Colonization Resistance, and Diseases

Trillions of microbes exist in the human body, particularly the gastrointestinal tract, coevolved with the host in a mutually beneficial relationship. The main role of the intestinal microbiome is the fermentation of non-digestible substrates and increased growth of beneficial microbes that produce key antimicrobial metabolites such as short-chain fatty acids, etc., to inhibit the growth of pathogenic microbes besides other functions. Intestinal microbiota can prevent pathogen colonization through the mechanism of colonization resistance. A wide range of resistomes are present in both beneficial and pathogenic microbes. Giving antibiotic exposure to the intestinal microbiome (both beneficial and hostile) can trigger a resistome response, affecting colonization resistance. The following review provides a mechanistic overview of the intestinal microbiome and the impacts of antibiotic therapy on pathogen colonization and diseases. Further, we also discuss the epidemiology of immunocompromised patients who are at high risk for nosocomial infections, colonization and decolonization of multi-drug resistant organisms in the intestine, and the direct and indirect mechanisms that govern colonization resistance to the pathogens.

CD47 blockade reduces the pathologic features of experimental cerebral malaria and promotes survival of hosts with Plasmodium infection

CD47 is an antiphagocytic "don't eat me" signal that inhibits programmed cell removal of self. As red blood cells (RBCs) age they lose CD47 expression and become susceptible to programmed cell removal by macrophages. CD47^-/- mice infected with Plasmodium yoelii, which exhibits an age-based preference for young RBCs, were previously demonstrated to be highly resistant to malaria infection. Our study sought to test the therapeutic benefit of CD47 blockade on ameliorating the clinical syndromes of experimental cerebral malaria (ECM), using the Plasmodium berghei ANKA (Pb-A) murine model. In vitro we tested the effect of anti-CD47 mAb on Plasmodium-infected RBC phagocytosis and found that anti-CD47 treatment significantly increased clearance of Plasmodium-infected RBCs. Infection of C57BL/6 mice with Pb-A is lethal and mice succumb to the clinical syndromes of CM between days 6 and 10 postinfection. Strikingly, treatment with anti-CD47 resulted in increased survival during the cerebral phase of Pb-A infection. Anti-CD47-treated mice had increased lymphocyte counts in the peripheral blood and increased circulating levels of IFN-γ, TNF-α, and IL-22. Despite increased circulating levels of inflammatory cytokines, anti-CD47-treated mice had reduced pathological features in the brain. Survival of ECM in anti-CD47-treated mice was correlated with reduced cellular accumulation in the cerebral vasculature, improved blood-brain barrier integrity, and reduced cytotoxic activity of infiltrating CD8⁺ T cells. These results demonstrate the therapeutic benefit of anti-CD47 to reduce morbidity in a lethal model of ECM, which may have implications for preventing mortality in young African children who are the highest casualties of CM.

Interleukin-22 Influences the Th1/Th17 Axis

Interleukin-22 (IL-22) is secreted by a wide range of immune cells and its downstream effects are mediated by the IL-22 receptor, which is present on non-immune cells in many organs throughout the body. IL-22 is an inflammatory mediator that conditions the tissue compartment by upregulating innate immune responses and is also a homeostatic factor that promotes tissue integrity and regeneration. Interestingly, the IL-22 system has also been linked to many T cell driven inflammatory diseases. Despite this, the downstream effects of IL-22 on the adaptive immune system has received little attention. We have reviewed the literature for experimental data that suggest IL-22 mediated effects on T cells, either transduced directly or via mediators expressed by innate immune cells or non-immune cells in response to IL-22. Collectively, the reviewed data indicate that IL-22 has a hitherto unappreciated influence on T helper cell polarization, or the secretion of signature cytokines, that is context dependent but in many cases results in a reduction of the Th1 type response and to some extent promotion of regulatory T cells. Further studies are needed that specifically address these aspects of IL-22 signaling, which can benefit the understanding and treatment of a wide range of diseases.

Microbiota-Dependent Effects of IL-22

Cytokines are important contributors to immune responses against microbial and environmental threats and are of particular importance at epithelial barriers. These interfaces are continuously exposed to external factors and thus require immune components to both protect the host from pathogen invasion and to regulate overt inflammation. Recently, substantial efforts have been devoted to understanding how cytokines act on certain cells at barrier sites, and why the dysregulation of immune responses may lead to pathogenesis. In particular, the cytokine IL-22 is involved in preserving an intact epithelium, maintaining a balanced microbiota and a functioning defense system against external threats. However, a tight regulation of IL-22 is generally needed, since uncontrolled IL-22 production can lead to the progression of autoimmunity and cancer. Our aim in this review is to summarize novel findings on IL-22 and its interactions with specific microbial stimuli, and subsequently, to understand their contributions to the function of IL-22 and the clinical outcome. We particularly focus on understanding the detrimental effects of dysregulated control of IL-22 in certain disease contexts.

Tim-3 signaling blockade with α-lactose induces compensatory TIGIT expression in Plasmodium berghei ANKA-infected mice

BACKGROUND

Malaria, one of the largest health burdens worldwide, is caused by Plasmodium spp. infection. Upon infection, the host's immune system begins to clear the parasites. However, Plasmodium species have evolved to escape the host's immune clearance. T-cell immunoglobulin and mucin domain 3 (Tim-3), a surface molecule on most immune cells, is often referred to as an exhaustion marker. Galectin (Gal)-9 is a Tim-3 ligand and the T helper (Th) 1 cell response is inhibited when Gal-9 binds to Tim-3. In the present study, dynamic expression of Tim-3 on key populations of lymphocytes during infection periods of Plasmodium berghei and its significance in disease resistance and pathogenesis were explored.

METHODS

Tim-3 expression on critical lymphocyte populations and the proportion of these cells, as well as the levels of cytokines in the sera of infected mice, were detected by flow cytometry. Further, in vitro anti-Tim-3 assay using an anti-Tim-3 antibody and in vivo Tim-3-Gal-9 signaling blockade assays using α-lactose (an antagonist of Gal-9) were conducted. An Annexin V Apoptosis Detection Kit with propidium iodide was used to detect apoptosis. In addition, proteins associated with apoptosis in lung and spleen tissues were confirmed by Western blotting assays.

RESULTS

Increased Tim-3 expression on splenic CD8⁺ and splenic CD4⁺, and circulatory CD4⁺ T cells was associated with a reduction in the proportion of these cells. Furthermore, the levels of interleukin (IL)-2, IL-4, IL-6, IL-22, and interferon (IFN)-γ, but not that of tumor necrosis factor alpha (TNF-α), IL-10, and IL-9, increased to their highest levels at day 4 post-infection and decreased thereafter. Blocking Tim-3 signaling in vitro inhibited lymphocyte apoptosis. Tim-3-Gal-9 signaling blockade in vivo did not protect the mice, but induced the expression of the immunosuppressive molecule, T cell immunoreceptor with Ig and ITIM domains (TIGIT), in Plasmodium berghei ANKA-infected mice.

CONCLUSIONS

Tim-3 on lymphocytes negatively regulates cell-mediated immunity against Plasmodium infection, and blocking Tim-3-galectin 9 signaling using α-lactose did not significantly protect the mice; however, it induced the compensatory expression of TIGIT. Further investigations are required to identify whether combined blockade of Tim-3 and TIGIT signaling could achieve a better protective effect.

Interactions Between the Gut Microbiota and the Host Innate Immune Response Against Pathogens

The mammalian intestine is colonized by over a trillion microbes that comprise the "gut microbiota," a microbial community which has co-evolved with the host to form a mutually beneficial relationship. Accumulating evidence indicates that the gut microbiota participates in immune system maturation and also plays a central role in host defense against pathogens. Here we review some of the mechanisms employed by the gut microbiota to boost the innate immune response against pathogens present on epithelial mucosal surfaces. Antimicrobial peptide secretion, inflammasome activation and induction of host IL-22, IL-17, and IL-10 production are the most commonly observed strategies employed by the gut microbiota for host anti-pathogen defense. Taken together, the body of evidence suggests that the host gut microbiota can elicit innate immunity against pathogens.

Innate Lymphoid Cells in Protection, Pathology, and Adaptive Immunity During Apicomplexan Infection

Apicomplexans are a diverse and complex group of protozoan pathogens including Toxoplasma gondii, Plasmodium spp., Cryptosporidium spp., Eimeria spp., and Babesia spp. They infect a wide variety of hosts and are a major health threat to humans and other animals. Innate immunity provides early control and also regulates the development of adaptive immune responses important for controlling these pathogens. Innate immune responses also contribute to immunopathology associated with these infections. Natural killer (NK) cells have been for a long time known to be potent first line effector cells in helping control protozoan infection. They provide control by producing IL-12 dependent IFNγ and killing infected cells and parasites via their cytotoxic response. Results from more recent studies indicate that NK cells could provide additional effector functions such as IL-10 and IL-17 and might have diverse roles in immunity to these pathogens. These early studies based their conclusions on the identification of NK cells to be CD3–, CD49b+, NK1.1+, and/or NKp46+ and the common accepted paradigm at that time that NK cells were one of the only lymphoid derived innate immune cells present. New discoveries have lead to major advances in understanding that NK cells are only one of several populations of innate immune cells of lymphoid origin. Common lymphoid progenitor derived innate immune cells are now known as innate lymphoid cells (ILC) and comprise three different groups, group 1, group 2, and group 3 ILC. They are a functionally heterogeneous and plastic cell population and are important effector cells in disease and tissue homeostasis. Very little is known about each of these different types of ILCs in parasitic infection. Therefore, we will review what is known about NK cells in innate immune responses during different protozoan infections. We will discuss what immune responses attributed to NK cells might be reconsidered as ILC1, 2, or 3 population responses. We will then discuss how different ILCs may impact immunopathology and adaptive immune responses to these parasites.

HVEM and CD160: Regulators of Immunopathology During Malaria Blood-Stage

CD8⁺ T cells are key players during infection with the malaria parasite Plasmodium berghei ANKA (PbA). While they cannot provide protection against blood-stage parasites, they can cause immunopathology, thus leading to the severe manifestation of cerebral malaria. Hence, the tight control of CD8⁺ T cell function is key in order to prevent fatal outcomes. One major mechanism to control CD8⁺ T cell activation, proliferation and effector function is the integration of co-inhibitory and co-stimulatory signals. In this study, we show that one such pathway, the HVEM-CD160 axis, significantly impacts CD8⁺ T cell regulation and thereby the incidence of cerebral malaria. Here, we show that the co-stimulatory molecule HVEM is indeed required to maintain CD8⁺ T effector populations during infection. Additionally, by generating a CD160^-/- mouse line, we observe that the HVEM ligand CD160 counterbalances stimulatory signals in highly activated and cytotoxic CD8⁺ T effector cells, thereby restricting immunopathology. Importantly, CD160 is also induced on cytotoxic CD8⁺ T cells during acute Plasmodium falciparum malaria in humans. In conclusion, CD160 is specifically expressed on highly activated CD8⁺ T effector cells that are harmful during the blood-stage of malaria.

IL-22 deficiency increases CD4 T cell responses to mucosal immunization

Mucosal vaccines are a promising platform for combatting infectious diseases for which we still lack effective preventative measures. Optimizing these vaccines to generate the best protective immune responses with the least complicated immunization regimen is imperative. Mucosal barriers are the first line of defense against many pathogens and, as such, we looked to their biology for strategies to improve vaccine delivery. Interleukin-22 (IL-22) is a key cytokine in both healthy and inflamed mucosal tissues. IL-22 promotes epithelial cell proliferation and inhibits apoptosis, upregulates mucin and antimicrobial peptides, all of which promote mucosal barrier integrity. In this study, we find that IL-22 impairs the development of a T cell response during mucosal immunization. Compared to wild-type control mice, IL-22 deficient mice had increased antigen-specific CD4 T cell responses to intrarectal immunization using a protein and cholera toxin adjuvant vaccine. When immunized systemically with the same protein antigen adsorbed to alum, no differences in the CD4 T cell response between wild-type and IL-22 deficient mice were detected. This suggests that transiently inhibiting IL-22 during mucosal vaccination could enhance T cell responses. The broad-applicability of this proposed approach would allow for improvement of many existing mucosal vaccine regimens and have positive implications in the development of more efficacious mucosal vaccines.

TH17 cell plasticity: The role of dendritic cells and molecular mechanisms

Upon interaction with dendritic cells (DCs), naïve CD4 T cells differentiate into distinct subsets and orchestrate the development of a physiological immune response. When uncontrolled by cellular and molecular mechanisms, CD4 T cells can also lead to immune mediated inflammatory diseases (IMIDs). Initially, these distinct CD4 T-cell subsets were defined according to the expression of a limited number of cytokines. Later it was revealed that CD4 T cells can acquire much more complex functional phenotypes than previously thought. Experimental data showed that the CD4 T-cell subset T_H17 can secrete IFN-γ and IL-4, which are signature molecules of other T-cell subsets. Furthermore, some T_H17 cells can also explore an anti-inflammatory fate and participate in the resolution of the immune response. A more flexible theory has therefore evolved with the scope to better represent the plastic biology of CD4 T cells. In this context, several aspects still remain unclear. The goal of this review is to discuss the role of extrinsic and intrinsic cellular and molecular mechanisms, which can drive the plasticity of T_H17 cells. In particular, we will outline the role of DCs and the function of transcriptional factors in shaping the fate of T_H17 cells towards either a pathogenic or a regulatory phenotype. Finally, we will discuss whether T_H17 cell plasticity could be a target for new therapies for IMIDs. We indeed envision that when the cellular and molecular mechanisms controlling T_H17 plasticity are known, new therapies, which aim to reset the immune system, will be developed. This will be achieved by either selectively depleting only the pathogenic T_H17 cells or, if possible, re converting these cells from pathogenic to regulatory. This will overcome the challenge posed by the immune suppressive side effects caused by the current therapies, which impair the function of CD4 cells or delete all of them, to the detriment of the patient.

Tissue-specific immunopathology during malaria infection

Systemic inflammation mediated by Plasmodium parasites is central to malaria disease and its complications. Plasmodium parasites reside in erythrocytes and can theoretically reach all host tissues via the circulation. However, actual interactions between parasitized erythrocytes and host tissues, along with the consequent damage and pathological changes, are limited locally to specific tissue sites. Such tissue specificity of the parasite can alter the outcome of malaria disease, determining whether acute or chronic complications occur. Here, we give an overview of the recent progress that has been made in understanding tissue-specific immunopathology during Plasmodium infection. As knowledge on tissue-specific host-parasite interactions accumulates, better treatment modalities and targets may emerge for intervention in malaria disease.

Cutting Edge: Plasmodium falciparum Induces Trained Innate Immunity

Malarial infection in naive individuals induces a robust innate immune response. In the recently described model of innate immune memory, an initial stimulus primes the innate immune system to either hyperrespond (termed training) or hyporespond (tolerance) to subsequent immune challenge. Previous work in both mice and humans demonstrated that infection with malaria can both serve as a priming stimulus and promote tolerance to subsequent infection. In this study, we demonstrate that initial stimulation with Plasmodium falciparum-infected RBCs or the malaria crystal hemozoin induced human adherent PBMCs to hyperrespond to subsequent ligation of TLR2. This hyperresponsiveness correlated with increased H3K4me3 at important immunometabolic promoters, and these epigenetic modifications were also seen in Kenyan children naturally infected with malaria. However, the use of epigenetic and metabolic inhibitors indicated that the induction of trained immunity by malaria and its ligands may occur via a previously unrecognized mechanism(s).

Flt3 ligand expands bona fide innate lymphoid cell precursors in vivo

A common helper-like innate lymphoid precursor (CHILP) restricted to the innate lymphoid cells (ILC) lineage has been recently characterized. While specific requirements of transcription factors for CHILPs development has been partially described, their ability to sense cytokines and react to peripheral inflammation remains unaddressed. Here, we found that systemic increase in Flt3L levels correlated with the expansion of Lineage (Lin)^negα4β7⁺ precursors in the adult murine bone marrow. Expanded Lin^negα4β7⁺ precursors were bona fide CHILPs as seen by their ability to differentiate into all helper ILCs subsets but cNK in vivo. Interestingly, Flt3L-expanded CHILPs transferred into lymphopenic mice preferentially reconstituted the small intestine. While we did not observe changes in serum Flt3L during DSS-induced colitis in mice or plasma from inflammatory bowel disease (IBD) patients, elevated Flt3L levels were detected in acute malaria patients. Interestingly, while CHILP numbers were stable during the course of DSS-induced colitis, they expanded following increased serum Flt3L levels in malaria-infected mice, hence suggesting a role of the Flt3L-ILC axis in malaria. Collectively, our results indicate that Flt3L expands CHILPs in the bone marrow, which might be associated with specific inflammatory conditions.