T helper 2 cells control monocyte to tissue-resident macrophage differentiation during nematode infection of the pleural cavity

Th2-biased immune responses to body migrating Ascaris larvae in primary infection are associated with pathology but not protection

Helminth infections lead to an overdispersion of the parasites in humans as well as in animals. We asked whether early immune responses against migrating Ascaris larvae are responsible for the unequal distribution of worms in natural host populations and thus investigated a susceptible versus a resistant mouse strain. In mice, the roundworm larvae develop until the lung stage and thus early anti-Ascaris immune responses against the migrating larvae in the liver and lung can be deciphered. Our data show that susceptible C57BL/6 mice respond to Ascaris larval migration significantly stronger compared to resistant CBA mice and the anti-parasite reactivity is associated with pathology. Increased eosinophil recruitment was detected in the liver and lungs, but also in the spleen and peritoneal cavity of susceptible mice on day 8 post infection compared to resistant mice. In serum, eosinophil peroxidase levels were significantly higher only in the susceptible mice, indicating functional activity of the recruited eosinophils. This effect was associated with an increased IL-5/IL-13 production by innate lymphoid cells and CD4+ T cells and a pronounced type 2 macrophage polarization in the lungs of susceptible mice. Furthermore, a comparison of wildtype BALB/c and eosinophil-deficient dblGATA-1 BALB/c mice showed that eosinophils were not essential for the early control of migrating Ascaris larvae. In conclusion, in primary infection, a strong local and systemic type 2 immune response during hepato-tracheal helminth larval migration is associated with pathology rather than protection.

IL-13 protects from C. difficile colitis

OBJECTIVES

Clostridioides difficile infection (CDI) is the leading hospital-acquired infection in North America. We have previously discovered that antibiotic disruption of the gut microbiota decreases intestinal IL-33 and IL-25 and increases susceptibility to CDI. We further found that IL-33 promotes protection through type 2 Innate Lymphoid Cells (ILC2s), which produce IL-13. However, the contribution of IL-13 to disease has never been explored.

METHODS

We used a validated model of CDI in mice, in which we neutralized via blocking antibodies, or administered recombinant protein, IL-13 to assess the role of this cytokine during infection using weight and clinical scores. Fluorescent activated cell sorting (FACS) was used to characterize myeloid cell population changes in response to IL-13 manipulation.

RESULTS

We found that administration of IL-13 protected, and anti-IL-13 exacerbated CDI. Additionally, we observed alterations to the monocyte/macrophage cells following neutralization of IL-13 as early as day three post infection. We also observed elevated accumulation of myeloid cells by day four post-infection following IL-13 neutralization. Neutralization of the decoy receptor, IL-13Rα2, resulted in protection from disease, likely through increased available endogenous IL-13.

CONCLUSIONS

Our data highlight the protective role of IL-13 in protecting from more severe CDI and the association of poor responses with a dysregulated monocyte-macrophage compartment. These results increase our understanding of type 2 immunity in CDI and may have implications for treating disease in patients.

The Acute Inflammatory Potential of Particles From the Echinococcus granulosus Laminated Layer Is Moderated by Its Calcium Inositol Hexakisphosphate Component

Cystic echinococcosis is caused by the tissue-dwelling larva (hydatid) of Echinococcus granulosus sensu lato. A salient feature is that this larva is protected by the acellular laminated layer (LL). As the parasite grows, the LL sheds abundant particles that can accumulate in the parasite's vicinity. The potential of LL particles to induce inflammation in vivo has not been specifically analysed. It is not known how each of its two major components, namely highly glycosylated mucins and calcium inositol hexakisphosphate (InsP6) deposits, impacts inflammation induced by the LL as a whole. In this work, we show that LL particles injected intraperitoneally cause infiltration of eosinophils, neutrophils and monocytes/macrophages as well as the disappearance of resident (large peritoneal) macrophages. Strikingly, the absence of calcium InsP6 enhanced the recruitment of all the inflammatory cell types analysed. In contrast, oxidation of the mucin carbohydrates caused decreased recruitment of neutrophils. The carbohydrate-oxidised particles caused cell influx nonetheless, which may be explained by possible receptor-independent effects of LL particles on innate immune cells, as suggested by previous works from our group. In summary, LL particles can induce acute inflammatory cell recruitment partly dependent on its mucin glycans, and this recruitment is attenuated by the calcium InsP6 component.

Cooperation of ILC2s and TH2 cells in the expulsion of intestinal helminth parasites

Type 2 immune responses form a critical defence against enteric worm infections. In recent years, mouse models have revealed shared and unique functions for group 2 innate lymphoid cells and T helper 2 cells in type 2 immune response to intestinal helminths. Both cell types use similar innate effector functions at the site of infection, whereas each population has distinct roles during different stages of infection. In this Perspective, we review the underlying mechanisms used by group 2 innate lymphoid cells and T helper 2 cells to cooperate with each other and suggest an overarching model of the interplay between these cell types over the course of a helminth infection.

Krüppel-like Factor (KLF) family members control expression of genes required for serous cavity and alveolar macrophage identities

Tissue-resident macrophages adopt distinct gene expression profiles and exhibit functional specialization based on their tissue of residence. Recent studies have begun to define the signals and transcription factors that induce these identities. Here we describe an unexpected and specific role for the broadly expressed transcription factor Kruppel-like Factor 2 (KLF2) in the development of embryonically derived Large Cavity Macrophages (LCM) in the serous cavities. KLF2 not only directly regulates the transcription of genes previously shown to specify LCM identity, such as retinoic acid receptors and GATA6, but also is required for induction of many other transcripts that define the identity of these cells. We identify a similar role for KLF4 in regulating the identity of alveolar macrophages in the lung. These data demonstrate that broadly expressed transcription factors, such as Group 2 KLFs, can play important roles in the specification of distinct identities of tissue-resident macrophages.

Characterization of the immunosuppressive environment induced by larval Echinococcus granulosus during chronic experimental infection

The larval stage of Echinococcus granulosus causes the chronic infection known as cystic echinococcosis, deploying strong inhibitory mechanisms on host immune responses. Using experimental intraperitoneal infection in C57BL/6 mice, we carried out an in-depth analysis of the local changes in macrophage populations associated with chronic infection. In addition, we analyzed T cells and relevant soluble mediators. Infected animals showed an increase in local cell numbers, mostly accounted for by eosinophils, T cells, and macrophages. Within macrophage populations, the largest increases in cell numbers corresponded to resident large peritoneal macrophages (LPM). Monocyte recruitment appeared to be active, as judged by the increased number of monocytes and cells in the process of differentiation towards LPM, including small (SPM) and converting peritoneal macrophages (CPM). In contrast, we found no evidence of macrophage proliferation. Infection induced the expression of M2 markers in SPM, CPM, and LPM. It also enhanced the expression of the co-inhibitor PD-L1 in LPM, SPM, and CPM and induced the co-inhibitor PD-L2 in SPM and CPM. Therefore, local macrophages acquire M2-like phenotypes with probable suppressive capacities. Regarding T cells, infection induced an increase in the percentage of CD4+ cells that are PD-1+, which represent a potential target of suppression by PD-L1+/PD-L2+ macrophages. In possible agreement, CD4+ T cells from infected animals showed blunted proliferative responses to in vitro stimulation with anti-CD3. Further evidence of immune suppression in the parasite vicinity arose from the observation of an expansion in FoxP3+ CD4+ regulatory T cells and increases in the local concentrations of the anti-inflammatory cytokines TGF-β and IL-1Ra.

Human serous cavity macrophages and dendritic cells possess counterparts in the mouse with a distinct distribution between species

In mouse peritoneal and other serous cavities, the transcription factor GATA6 drives the identity of the major cavity resident population of macrophages, with a smaller subset of cavity-resident macrophages dependent on the transcription factor IRF4. Here we showed that GATA6+ macrophages in the human peritoneum were rare, regardless of age. Instead, more human peritoneal macrophages aligned with mouse CD206+ LYVE1+ cavity macrophages that represent a differentiation stage just preceding expression of GATA6. A low abundance of CD206+ macrophages was retained in C57BL/6J mice fed a high-fat diet and in wild-captured mice, suggesting that differences between serous cavity-resident macrophages in humans and mice were not environmental. IRF4-dependent mouse serous cavity macrophages aligned closely with human CD1c+CD14+CD64+ peritoneal cells, which, in turn, resembled human peritoneal CD1c+CD14-CD64- cDC2. Thus, major populations of serous cavity-resident mononuclear phagocytes in humans and mice shared common features, but the proportions of different macrophage differentiation stages greatly differ between the two species, and dendritic cell (DC2)-like cells were especially prominent in humans.

Pleural macrophages translocate to the lung during infection to promote improved influenza outcomes

Seasonal influenza results in 3 to 5 million cases of severe disease and 250,000 to 500,000 deaths annually. Macrophages have been implicated in both the resolution and progression of the disease, but the drivers of these outcomes are poorly understood. We probed mouse lung transcriptomic datasets using the Digital Cell Quantifier algorithm to predict immune cell subsets that correlated with mild or severe influenza A virus (IAV) infection outcomes. We identified a unique lung macrophage population that transcriptionally resembled small serosal cavity macrophages and whose presence correlated with mild disease. Until now, the study of serosal macrophage translocation in the context of viral infections has been neglected. Here, we show that pleural macrophages (PMs) migrate from the pleural cavity to the lung after infection with IAV. We found that the depletion of PMs increased morbidity and pulmonary inflammation. There were increased proinflammatory cytokines in the pleural cavity and an influx of neutrophils within the lung. Our results show that PMs are recruited to the lung during IAV infection and contribute to recovery from influenza. This study expands our knowledge of PM plasticity and identifies a source of lung macrophages independent of monocyte recruitment and local proliferation.

Macrophages show up in style when Th2 lymphocytes organize their homecoming

Monocytes can differentiate into tissue-resident pleural macrophages, but the mechanisms underlying this process are not yet fully understood. In this issue of Immunity, Finlay et al.1 show that Th2 cytokines promote this differentiation in resistant mice infected with Litomosoides sigmodontis.

The role of monocyte-derived macrophages in the lung: it's all about context

Macrophages are present in every tissue of the body where they play crucial roles in maintaining tissue homeostasis and providing front line defence against pathogens. Arguably, this is most important at mucosal barrier tissues, such as the lung and gut, which are major ports of entry for pathogens. However, a common feature of inflammation, infection or injury is the loss of tissue resident macrophages and accumulation of monocytes from the circulation, which differentiate, to different extents, into macrophages. The exact fate and function of these elicited, monocyte-derived macrophages in infection, injury and inflammation remains contentious. While some studies have documented the indispensable nature of monocytes and their macrophage derivatives in combatting infection and restoration of lung homeostasis following insult, observations from clinical studies and preclinical models of lung infection/injury shows that monocytes and their progeny can become dysregulated in severe pathology, often perpetuating rather than resolving the insult. In this Mini Review, we aim to bring together these somewhat contradictory reports by discussing how the plasticity of monocytes allow them to assume distinct functions in different contexts in the lung, from health to infection, and effective tissue repair to fibrotic disease.