Retinoic acid drives intestine-specific adaptation of effector ILC2s originating from distant sites.

Adaptation of immune cells to tissue-specific microenvironments is a crucial process in homeostasis and inflammation. Here, we show that murine effector type 2 innate lymphoid cells (ILC2s) from various organs are equally effective in repopulating ILC2 niches in other anatomical locations where they adapt tissue-specific phenotypes of target organs. Single-cell transcriptomics of ILC2 populations revealed upregulation of retinoic acid (RA) signaling in ILC2s during adaptation to the small intestinal microenvironment, and RA signaling mediated reprogramming of kidney effector ILC2s toward the small intestinal phenotype in vitro and in vivo. Inhibition of intestinal ILC2 adaptation by blocking RA signaling impaired worm expulsion during Strongyloides ratti infection, indicating functional importance of ILC2 tissue imprinting. In conclusion, this study highlights that effector ILC2s retain the ability to adapt to changing tissue-specific microenvironments, enabling them to exert tissue-specific functions, such as promoting control of intestinal helminth infections.

Tuberculostearic Acid-Containing Phosphatidylinositols as Markers of Bacterial Burden in Tuberculosis.

One-fourth of the global human population is estimated to be infected with strains of the Mycobacterium tuberculosis complex (MTBC), the causative agent of tuberculosis (TB). Using lipidomic approaches, we show that tuberculostearic acid (TSA)-containing phosphatidylinositols (PIs) are molecular markers for infection with clinically relevant MTBC strains and signify bacterial burden. For the most abundant lipid marker, detection limits of ∼102 colony forming units (CFUs) and ∼103 CFUs for bacterial and cell culture systems were determined, respectively. We developed a targeted lipid assay, which can be performed within a day including sample preparation─roughly 30-fold faster than in conventional methods based on bacterial culture. This indirect and culture-free detection approach allowed us to determine pathogen loads in infected murine macrophages, human neutrophils, and murine lung tissue. These marker lipids inferred from mycobacterial PIs were found in higher levels in peripheral blood mononuclear cells of TB patients compared to healthy individuals. Moreover, in a small cohort of drug-susceptible TB patients, elevated levels of these molecular markers were detected at the start of therapy and declined upon successful anti-TB treatment. Thus, the concentration of TSA-containing PIs can be used as a correlate for the mycobacterial burden in experimental models and in vitro systems and may prospectively also provide a clinically relevant tool to monitor TB severity.

Corrigendum: Eosinophils and Neutrophils Eliminate Migrating Strongyloides ratti Larvae at the Site of Infection in the Context of Extracellular DNA Trap Formation.

[This corrects the article DOI: 10.3389/fimmu.2021.715766.].

Evaluation of Myeloperoxidase as Target for Host-Directed Therapy in Tuberculosis In Vivo.

Due to the rise of tuberculosis cases infected with multi and extensively drug-resistant Mycobacterium tuberculosis strains and the emergence of isolates resistant to antibiotics newly in clinical use, host-directed therapies targeting pathogenesis-associated immune pathways adjunct to antibiotics may ameliorate disease and bacterial clearance. Active tuberculosis is characterized by neutrophil-mediated lung pathology and tissue destruction. Previously, we showed that preventing M. tuberculosis induced necrosis in human neutrophils by inhibition of myeloperoxidase (MPO) promoted default apoptosis and subsequent control of mycobacteria by macrophages taking up the mycobacteria-infected neutrophils. To translate our findings in an in vivo model, we tested the MPO inhibitor 4-aminobenzoic acid hydrazide (ABAH) in C3HeB/FeJ mice, which are highly susceptible to M. tuberculosis infection manifesting in neutrophil-associated necrotic granulomas. MPO inhibition alone or as co-treatment with isoniazid, a first-line antibiotic in tuberculosis treatment, did not result in reduced bacterial burden, improved pathology, or altered infiltrating immune cell compositions. MPO inhibition failed to prevent M. tuberculosis induced neutrophil necrosis in C3Heb/FeJ mice in vivo as well as in murine neutrophils in vitro. In contrast to human neutrophils, murine neutrophils do not respond to M. tuberculosis infection in an MPO-dependent manner. Thus, the murine C3HeB/FeJ model does not fully resemble the pathomechanisms in active human tuberculosis. Consequently, murine infection models of tuberculosis are not necessarily adequate to evaluate host-directed therapies targeting neutrophils in vivo.

Eosinophils and Neutrophils Eliminate Migrating Strongyloides ratti Larvae at the Site of Infection in the Context of Extracellular DNA Trap Formation.

Parasitic nematodes such as hookworms actively penetrate the skin of their hosts, encountering skin-resident innate immune cells that represent the host´s first line of defense. Here we use Strongyloides ratti as a model for an intestinal helminth parasite with tissue migrating stages. We show that interception and killing of migrating larvae in mice during a 1st infection occurred predominantly in skin and muscle tissue before larvae migrated via lung and head tissue to the intestine. Inhibition of larval migration was even more efficient in immune mice during a 2nd infection where larvae barely left the site of entry i.e. the foot. Using cell-deficient mice we show that interception in the tissue was predominantly mediated by neutrophils and eosinophils while basophils and mast cells were dispensable in vivo. Likewise, neutrophils and eosinophils inhibited S. ratti L3 motility in vitro in the context of ETosis. Thereby eosinophils were strictly dependent on the presence of anti-S. ratti antibodies while neutrophils inhibited L3 motility as such. Also, MPO and MMP-9 were released by neutrophils in response to L3 alone, but immune plasma further stimulated MPO release in an antibody-dependent manner. In summary, our findings highlight the central role of the skin as first line of defense against helminth parasites in both, innate and adaptive immunity.

WNT6/ACC2-induced storage of triacylglycerols in macrophages is exploited by Mycobacterium tuberculosis.

In view of emerging drug-resistant tuberculosis (TB), host-directed adjunct therapies are urgently needed to improve treatment outcomes with currently available anti-TB therapies. One approach is to interfere with the formation of lipid-laden "foamy" macrophages in the host, as they provide a nutrient-rich host cell environment for Mycobacterium tuberculosis (Mtb). Here, we provide evidence that Wnt family member 6 (WNT6), a ligand of the evolutionarily conserved Wingless/Integrase 1 (WNT) signaling pathway, promotes foam cell formation by regulating key lipid metabolic genes including acetyl-CoA carboxylase 2 (ACC2) during pulmonary TB. Using genetic and pharmacological approaches, we demonstrated that lack of functional WNT6 or ACC2 significantly reduced intracellular triacylglycerol (TAG) levels and Mtb survival in macrophages. Moreover, treatment of Mtb-infected mice with a combination of a pharmacological ACC2 inhibitor and the anti-TB drug isoniazid (INH) reduced lung TAG and cytokine levels, as well as lung weights, compared with treatment with INH alone. This combination also reduced Mtb bacterial numbers and the size of mononuclear cell infiltrates in livers of infected mice. In summary, our findings demonstrate that Mtb exploits WNT6/ACC2-induced storage of TAGs in macrophages to facilitate its intracellular survival, a finding that opens new perspectives for host-directed adjunctive treatment of pulmonary TB.

Limited role of mast cells during infection with the parasitic nematode Litomosoides sigmodontis.

Mast cells are innate effector cells that due to their localization in the tissue form the first line of defense against parasites. We have previously shown that specifically mucosal mast cells were essential for the termination of the intestinal Strongyloides ratti infection. Here, we analyze the impact of mast cells on the immune response and defense against the tissue-dwelling filarial nematode Litomosoides sigmodontis using mast cell-deficient Cpa3cre mice. Despite an increase and an activation of mast cells at the site of infection in wildtype BALB/c mice the outcome of L. sigmodontis infection was not changed in mast cell-deficient BALB/c Cpa3cre mice. In Cpa3cre mice neither vascular permeability induced by blood-sucking mites nor the migration of L3 was altered compared to Cpa3 wildtype littermates. Worm burden in the thoracic cavity was alike in the presence and absence of mast cells during the entire course of infection. Although microfilaremiae in the peripheral blood increased in mast cell-deficient mice at some time points, the infection was cleared with comparable kinetics in the presence and absence of mast cells. Moreover, mast cell deficiency had no impact on the cytokine and antibody response to L. sigmodontis. In summary, our findings suggest that mast cells are not mandatory for the initiation of an appropriate immune response and host defense during L. sigmodontis infection in mice.

Basophils Are Dispensable for the Control of a Filarial Infection.

Basophils are innate effector cells that contribute to allergic reactions and provide protection against parasites. Using basophil-deficient Mcpt8-cre mice, we have previously shown that these granulocytes contributed to the immune mediated early control of the gastrointestinal helminth Strongyloides ratti in mice. In this study, we analyze the impact of basophils on the immune response and defense against the tissue-dwelling filarial helminth parasite Litomosoides sigmodontis Although basophils and IgE increased at the site of infection, the absence of basophils did not change the outcome of L. sigmodontis infection. Worm burden in the thoracic cavity and microfilaremiae in the peripheral blood were alike in L. sigmodontis-infected Mcpt8-cre mice compared with Mcpt8 wild type littermates during the entire course of infection. Analysis of the cytokine and Ab response to L. sigmodontis revealed no consistent alterations in the absence of basophils. Furthermore, basophil-deficient and -competent mice were protected to the same extent during a secondary infection with L. sigmodontis In summary, our findings suggest that basophils are dispensable for the initiation of the appropriate immune response and host defense against L sigmodontis infection in mice.

Targeting neutrophils for host-directed therapy to treat tuberculosis.

M. tuberculosis is one of the prime killers from infectious diseases worldwide. Infections with multidrug-resistant variants counting for almost half a million new cases per year are steadily on the rise. Tuberculosis caused by extensively drug-resistant variants that are even resistant against newly developed or last resort antibiotics have to be considered untreaTable Susceptible tuberculosis already requires a six-months combinational therapy which requires further prolongation to treat drug-resistant infections. Such long treatment schedules are often accompanied by serious adverse effects causing patients to stop therapy. To tackle the global tuberculosis emergency, novel approaches for treatment need to be urgently explored. Host-directed therapies that target components of the defense system represent such a novel approach. In this review, we put a spotlight on neutrophils and neutrophil-associated effectors as promising targets for adjunct host-directed therapies to improve antibiotic efficacy and reduce both, treatment time and long-term pathological sequelae.

Strategies to Improve Vaccine Efficacy against Tuberculosis by Targeting Innate Immunity.

The global tuberculosis epidemic is the most common cause of death after infectious disease worldwide. Increasing numbers of infections with multi- and extensively drug-resistant variants of the Mycobacterium tuberculosis complex, resistant even to newly discovered and last resort antibiotics, highlight the urgent need for an efficient vaccine. The protective efficacy to pulmonary tuberculosis in adults of the only currently available vaccine, M. bovis BCG, is unsatisfactory and geographically diverse. More importantly, recent clinical studies on new vaccine candidates did not prove to be better than BCG, yet. Here, we propose and discuss novel strategies to improve efficacy of existing anti-tuberculosis vaccines. Modulation of innate immune responses upon vaccination already provided promising results in animal models of tuberculosis. For instance, neutrophils have been shown to influence vaccine efficacy, both, positively and negatively, and stimulate specific antibody secretion. Modulating immune regulatory properties after vaccination such as induction of different types of innate immune cell death, myeloid-derived suppressor or regulatory T cells, production of anti-inflammatory cytokines such as IL-10 may have beneficial effects on protection efficacy. Incorporation of lipid antigens presented via CD1 molecules to T cells have been discussed as a way to enhance vaccine efficacy. Finally, concepts of dendritic cell-based immunotherapies or training the innate immune memory may be exploitable for future vaccination strategies against tuberculosis. In this review, we put a spotlight on host immune networks as potential targets to boost protection by old and new tuberculosis vaccines.