ILC2 Activation by Protozoan Commensal Microbes

A gut commensal protozoan determines respiratory disease outcomes by shaping pulmonary immunity

The underlying mechanisms used by the intestinal microbiota to shape disease outcomes of the host are poorly understood. Here, we show that the gut commensal protozoan, Tritrichomonas musculis (T.mu), remotely shapes the lung immune landscape to facilitate perivascular shielding of the airways by eosinophils. Lung-specific eosinophilia requires a tripartite immune network between gut-derived inflammatory group 2 innate lymphoid cells and lung-resident T cells and B cells. This network exacerbates the severity of allergic airway inflammation while hindering the systemic dissemination of pulmonary Mycobacterium tuberculosis. The identification of protozoan DNA sequences in the sputum of patients with severe allergic asthma further emphasizes the relevance of commensal protozoa in human disease. Collectively, these findings demonstrate that a commensal protozoan tunes pulmonary immunity via a gut-operated lung immune network, promoting both beneficial and detrimental disease outcomes in response to environmental airway allergens and pulmonary infections.

Gut microbiota as a key regulator of intestinal mucosal immunity

Gut microbiota is a complex microbial community with the ability of maintaining intestinal health. Intestinal homeostasis largely depends on the mucosal immune system to defense external pathogens and promote tissue repair. In recent years, growing evidence revealed the importance of gut microbiota in shaping intestinal mucosal immunity. Therefore, according to the existing findings, this review first provided an overview of intestinal mucosal immune system before summarizing the regulatory roles of gut microbiota in intestinal innate and adaptive immunity. Specifically, this review delved into the gut microbial interactions with the cells such as intestinal epithelial cells (IECs), macrophages, dendritic cells (DCs), neutrophils, and innate lymphoid cells (ILCs) in innate immunity, and T and B lymphocytes in adaptive immunity. Furthermore, this review discussed the main effects of gut microbiota dysbiosis in intestinal diseases and offered future research prospects. The review highlighted the key regulatory roles of gut microbiota in intestinal mucosal immunity via various host-microbe interactions, providing valuable references for the development of microbial therapy in intestinal diseases.

NETosis in Parasitic Infections: A Puzzle That Remains Unsolved

Neutrophils are the key players in the innate immune system, being weaponized with numerous strategies to eliminate pathogens. The production of extracellular traps is one of the effector mechanisms operated by neutrophils in a process called NETosis. Neutrophil extracellular traps (NETs) are complex webs of extracellular DNA studded with histones and cytoplasmic granular proteins. Since their first description in 2004, NETs have been widely investigated in different infectious processes. Bacteria, viruses, and fungi have been shown to induce the generation of NETs. Knowledge is only beginning to emerge about the participation of DNA webs in the host's battle against parasitic infections. Referring to helminthic infections, we ought to look beyond the scope of confining the roles of NETs solely to parasitic ensnarement or immobilization. Hence, this review provides detailed insights into the less-explored activities of NETs against invading helminths. In addition, most of the studies that have addressed the implications of NETs in protozoan infections have chiefly focused on their protective side, either through trapping or killing. Challenging this belief, we propose several limitations regarding protozoan-NETs interaction. One of many is the duality in the functional responses of NETs, in which both the positive and pathological aspects seem to be closely intertwined.

The notable global heterogeneity in the distribution of COVID-19 cases and the association with pre-existing parasitic diseases

BACKGROUND

The coronavirus Disease 2019 (COVID-19) is a respiratory disease that has caused extensive ravages worldwide since being declared a pandemic by the World Health Organization (WHO). Unlike initially predicted by WHO, the incidence and severity of COVID-19 appeared milder in many Low-to-Middle-Income Countries (LMIC). To explain this noticeable disparity between countries, many hypotheses, including socio-demographic and geographic factors, have been put forward. This study aimed to estimate the possible association of parasitic diseases with COVID-19 as either protective agents or potential risk factors.

METHODS/PRINCIPAL FINDINGS

A country-level ecological study using publicly available data of countries was conducted. We conceptualized the true number of COVID-19 infections based on a function of test positivity rate (TPR) and employed linear regression analysis to assess the association between the outcome and parasitic diseases. We considered demographic, socioeconomic, and geographic confounders previously suggested. A notable heterogeneity was observed across WHO regions. The countries in Africa (AFRO) showed the lowest rates of COVID-19 incidence, and the countries in the Americas (AMRO) presented the highest. The multivariable model results were computed using 165 countries, excluding missing values. In the models analyzed, lower COVID-19 incidence rates were consistently observed in malaria-endemic countries, even accounting for potential confounding variables, Gross Domestic Product (GDP) per capita, the population aged 65 and above, and differences in the duration of COVID-19. However, the other parasitic diseases were not significantly associated with the spread of the pandemic.

CONCLUSIONS/SIGNIFICANCE

This study suggests that malaria prevalence is an essential factor that explains variability in the observed incidence of COVID-19 cases at the national level. Potential associations of COVID-19 with schistosomiasis and soil-transmitted helminthiases (STHs) are worthy of further investigation but appeared unlikely, based on this analysis, to be critical factors of the variability in COVID-19 epidemic trends. The quality of publicly accessible data and its ecological design constrained our research, with fundamental disparities in monitoring and testing capabilities between countries. Research at the subnational or individual level should be conducted to explore hypotheses further.

Remote regulation of type 2 immunity by intestinal parasites

The intestinal tract is the target organ of most parasitic infections, including those by helminths and protozoa. These parasites elicit prototypical type 2 immune activation in the host's immune system with striking impact on the local tissue microenvironment. Despite local containment of these parasites within the intestinal tract, parasitic infections also mediate immune adaptation in peripheral organs. In this review, we summarize the current knowledge on how such gut-tissue axes influence important immune-mediated resistance and disease tolerance in the context of coinfections, and elaborate on the implications of parasite-regulated gut-lung and gut-brain axes on the development and severity of airway inflammation and central nervous system diseases.

Innate Lymphoid Cells and Natural Killer Cells in Bacterial Infections: Function, Dysregulation, and Therapeutic Targets

Infectious diseases represent one of the largest medical challenges worldwide. Bacterial infections, in particular, remain a pertinent health challenge and burden. Moreover, such infections increase over time due to the continuous use of various antibiotics without medical need, thus leading to several side effects and bacterial resistance. Our innate immune system represents our first line of defense against any foreign pathogens. This system comprises the innate lymphoid cells (ILCs), including natural killer (NK) cells that are critical players in establishing homeostasis and immunity against infections. ILCs are a group of functionally heterogenous but potent innate immune effector cells that constitute tissue-resident sentinels against intracellular and extracellular bacterial infections. Being a nascent subset of innate lymphocytes, their role in bacterial infections is not clearly understood. Furthermore, these pathogens have developed methods to evade the host immune system, and hence permit infection spread and tissue damage. In this review, we highlight the role of the different ILC populations in various bacterial infections and the possible ways of immune evasion. Additionally, potential immunotherapies to manipulate ILC responses will be briefly discussed.

Tissue-Specific Molecular Markers and Heterogeneity in Type 2 Innate Lymphoid Cells

Innate lymphoid cells (ILCs) are the most recently described group of lymphoid subpopulations. These tissue-resident cells display a heterogeneity resembling that observed on different groups of T cells, hence their categorization as cytotoxic NK cells and helper ILCs type 1, 2 and 3. Each one of these groups is highly diverse and expresses different markers in a context-dependent manner. Type 2 innate lymphoid cells (ILC2s) are activated in response to helminth parasites and regulate the immune response. They are involved in the etiology of diseases associated with allergic responses as well as in the maintenance of tissue homeostasis. Markers associated with their identification differ depending on the tissue and model used, making the study and understanding of these cells a cumbersome task. This review compiles evidence for the heterogeneity of ILC2s as well as discussion and analyses of molecular markers associated with their identity, function, tissue-dependent expression, and how these markers contribute to the interaction of ILC2s with specific microenvironments to maintain homeostasis or respond to pathogenic challenges.

Regulation of Gastrointestinal Immunity by Metabolites

The gastrointestinal tract contains multiple types of immune cells that maintain the balance between tolerance and activation at the first line of host defense facing non-self antigens, including dietary antigens, commensal bacteria, and sometimes unexpected pathogens. The maintenance of homeostasis at the gastrointestinal tract requires stringent regulation of immune responses against various environmental conditions. Dietary components can be converted into gut metabolites with unique functional activities through host as well as microbial enzymatic activities. Accumulating evidence demonstrates that gastrointestinal metabolites have significant impacts on the regulation of intestinal immunity and are further integrated into the immune response of distal mucosal tissue. Metabolites, especially those derived from the microbiota, regulate immune cell functions in various ways, including the recognition and activation of cell surface receptors, the control of gene expression by epigenetic regulation, and the integration of cellular metabolism. These mucosal immune regulations are key to understanding the mechanisms underlying the development of gastrointestinal disorders. Here, we review recent advancements in our understanding of the role of gut metabolites in the regulation of gastrointestinal immunity, highlighting the cellular and molecular regulatory mechanisms by macronutrient-derived metabolites.

Interleukin-25-mediated resistance against intestinal trematodes does not depend on the generation of Th2 responses

Background

The cytokine interleukin-25 (IL-25) is recognized as the most relevant initiator of protective T helper 2 (Th2) responses in intestinal helminth infections. This cytokine induces resistance against several species of intestinal helminths, including the trematode Echinostoma caproni. E. caproni has been extensively used as an experimental model to study the factors determining resistance to intestinal infections. In the study reported here, we assessed the role of IL-25 in the generation of resistance in mice infected with E. caproni.

Methods

The factors that determine the production of IL-25 in mice experimentally infected with E. caproni were determined, as were the consequences of IL-25 production in terms of polarization of the immune response and resistance to infection.

Results

Our results show that the role of IL-25 in the polarization of the immune response differs between the primary and secondary immune responses. IL-25 is required for the development of a Th2 phenotype in primary E. caproni infections, but it can also promote the differentiation to Th2 memory cell subsets that enhance type-2 immunity in memory responses. However, the development of Th2 responses does not induce resistance to infection. The Th2 phenotype does not elicit resistance, and IL-25 is responsible for the resistance regardless of its type-2 cytokine activity and activation of signal transducer and activator of transcription (STAT6). Alternative activation of macrophages induced by IL-25 can be implicated in the resistance to infection.

Conclusions

In contrast to primary infection, secondary infection elicits a type-2 immune response even in the absence of IL-25 expression. Despite the development of a type-2 response, mice are susceptible to secondary infection associated with the lack of IL-25. Resistance to infection is due to the production of IL-25, which acts autonomously from Th2 response in terms of parasite clearance.