Pulmonary Interleukin-17-Positive Lymphocytes Increase during Pneumocystis murina Infection but Are Not Required for Clearance of Pneumocystis

Description of a Murine Model of Pneumocystis Pneumonia

Pneumocystis pneumonia is a serious lung infection caused by an original ubiquitous fungus with opportunistic behavior, referred to as Pneumocystis jirovecii. P. jirovecii is the second most common fungal agent among invasive fungal infections after Candida spp. Unfortunately, there is still an inability to culture P. jirovecii in vitro, and so a great impairment to improve knowledge on the pathogenesis of Pneumocystis pneumonia. In this context, animal models have a high value to address complex interplay between Pneumocystis and the components of the host immune system. Here, we propose a protocol for a murine model of Pneumocystis pneumonia. Animals become susceptible to Pneumocystis by acquiring an immunocompromised status induced by iterative administration of steroids within drinking water. Thereafter, the experimental infection is completed by an intranasal challenge with homogenates of mouse lungs containing Pneumocystis murina. The onset of clinical signs occurs within 5 weeks following the infectious challenge and immunosuppression can then be withdrawn. At termination, lungs and bronchoalveolar lavage (BAL) fluids from infected mice are analyzed for fungal load (qPCR) and immune response (flow cytometry and biochemical assays). The model is a useful tool in studies focusing on immune responses initiated after the establishment of Pneumocystis pneumonia.

IFN-γ Limits Immunopathogenesis but Delays Fungal Clearance during Pneumocystis Pneumonia

High levels of IFN-γ are produced in the lung during an adaptive immune response to Pneumocystis, but the effects of this prototypical Th1 cytokine on fungal clearance and immunopathogenesis have not been fully defined. Therefore, Pneumocystis-infected immunodeficient mice were immune reconstituted and administered control or anti-IFN-γ neutralizing Ab to determine how IFN-γ regulates the balance between host defense and immune-mediated lung injury. Mice treated with anti-IFN-γ demonstrated an initial worsening of Pneumocystis pneumonia-related immunopathogenesis, with greater weight loss, heightened lung inflammation, and more severe pulmonary function deficits than control mice. However, IFN-γ neutralization also enhanced macrophage phagocytosis of Pneumocystis and accelerated fungal clearance. When anti-IFN-γ-treated mice were also given IL-4 and IL-13 to promote a Th2-biased lung environment, the accelerated fungal clearance was preserved, but the severity of immunopathogenesis was reduced, and a more rapid recovery was observed. A direct suppressive effect of IFN-γ on macrophages was required but was not solely responsible for delayed fungal clearance, suggesting that IFN-γ acts through multiple mechanisms that likely include modulation of both macrophage and Th polarization. Enhanced Pneumocystis clearance in anti-IFN-γ-treated and IFN-γR-deficient mice was associated with significantly elevated IL-17+ CD4+ T cells and IL-17 protein in the lungs. Furthermore, neutralization of IL-17, but not IL-4, signaling blocked the accelerated fungal clearance observed in anti-IFN-γ-treated mice. Together, these data demonstrate that although IFN-γ delays fungal clearance by suppressing the lung Th17 response, it also serves an important regulatory role that limits immunopathogenesis and preserves pulmonary function.

Integrated multi-omics analyses reveal the altered transcriptomic characteristics of pulmonary macrophages in immunocompromised hosts with Pneumocystis pneumonia

Introduction

With the extensive use of immunosuppressants, immunosuppression-associated pneumonitis including Pneumocystis jirovecii pneumonia (PCP) has received increasing attention. Though aberrant adaptive immunity has been considered as a key reason for opportunistic infections, the characteristics of innate immunity in these immunocompromised hosts remain unclear.

Methods

In this study, wild type C57BL/6 mice or dexamethasone-treated mice were injected with or without Pneumocystis. Bronchoalveolar lavage fluids (BALFs) were harvested for the multiplex cytokine and metabolomics analysis. The single-cell RNA sequencing (scRNA-seq) of indicated lung tissues or BALFs was performed to decipher the macrophages heterogeneity. Mice lung tissues were further analyzed via quantitative polymerase chain reaction (qPCR) or immunohistochemical staining.

Results

We found that the secretion of both pro-inflammatory cytokines and metabolites in the Pneumocystis-infected mice are impaired by glucocorticoids. By scRNA-seq, we identified seven subpopulations of macrophages in mice lung tissues. Among them, a group of Mmp12⁺ macrophages is enriched in the immunocompetent mice with Pneumocystis infection. Pseudotime trajectory showed that these Mmp12⁺ macrophages are differentiated from Ly6c⁺ classical monocytes, and highly express pro-inflammatory cytokines elevated in BALFs of Pneumocystis-infected mice. In vitro, we confirmed that dexamethasone impairs the expression of Lif, Il1b, Il6 and Tnf, as well as the fungal killing capacity of alveolar macrophage (AM)-like cells. Moreover, in patients with PCP, we found a group of macrophages resembled the aforementioned Mmp12⁺ macrophages, and these macrophages are inhibited in the patient receiving glucocorticoid treatment. Additionally, dexamethasone simultaneously impaired the functional integrity of resident AMs and downregulated the level of lysophosphatidylcholine, leading to the suppressed antifungal capacities.

Conclusion

We reported a group of Mmp12⁺ macrophages conferring protection during Pneumocystis infection, which can be dampened by glucocorticoids. This study provides multiple resources for understanding the heterogeneity and metabolic changes of innate immunity in immunocompromised hosts, and also suggests that the loss of Mmp12⁺ macrophages population contributes to the pathogenesis of immunosuppression-associated pneumonitis.

Mucosal-Associated Invariant T Cells Accumulate in the Lungs during Murine Pneumocystis Infection but Are Not Required for Clearance

Pneumocystis is a fungal pathogen that can cause pneumonia in immunosuppressed hosts and subclinical infection in immunocompetent hosts. Mucosal-associated invariant T (MAIT) cells are unconventional lymphocytes with a semi-invariant T-cell receptor that are activated by riboflavin metabolites that are presented by the MHC-1b molecule MR1. Although Pneumocystis can presumably synthesize riboflavin metabolites based on whole-genome studies, the role of MAIT cells in controlling Pneumocystis infection is unknown. We used a co-housing mouse model of Pneumocystis infection, combined with flow cytometry and qPCR, to characterize the response of MAIT cells to infection in C57BL/6 mice, and, using MR1^−/− mice, which lack MAIT cells, to examine their role in clearing the infection. MAIT cells accumulated in the lungs of C57BL/6 mice during Pneumocystis infection and remained at increased levels for many weeks after clearance of infection. In MR1^−/− mice, Pneumocystis infection was cleared with kinetics similar to C57BL/6 mice. Thus, MAIT cells are not necessary for control of Pneumocystis infection, but the prolonged retention of these cells in the lungs following clearance of infection may allow a more rapid future response to other pathogens.

Immune Response in Pneumocystis Infections According to the Host Immune System Status

The host immune response is critical in Pneumocystis pneumonia (PCP). Immunocompetent hosts can eliminate the fungus without symptoms, while immunodeficient hosts develop PCP with an unsuitable excessive inflammatory response leading to lung damage. From studies based on rodent models or clinical studies, this review aimed to better understand the pathophysiology of Pneumocystis infection by analysing the role of immune cells, mostly lymphocytes, according to the immune status of the infected host. Hence, this review first describes the immune physiological response in infected immunocompetent hosts that are able to eliminate the fungus. The objective of the second part is to identify the immune elements required for the control of the fungus, focusing on specific immune deficiencies. Finally, the third part concentrates on the effect of the different immune elements in immunocompromised subjects during PCP, to better understand which cells are detrimental, and which, on the contrary, are beneficial once the disease has started. This work highlights that the immune response associated with a favourable outcome of the infection may differ according to the immune status of the host. In the case of immunocompetency, a close communication between B cells and TCD4 within tertiary lymphocyte structures appears critical to activate M2 macrophages without much inflammation. Conversely, in the case of immunodeficiency, a pro-inflammatory response including Th1 CD4, cytotoxic CD8, NK cells, and IFNγ release seems beneficial for M1 macrophage activation, despite the impact of inflammation on lung tissue.

Evolved to protect, designed to destroy: IL-17-producing γδ T cells in infection, inflammation, and cancer

T cells of the gamma delta (γδ) lineage are evolutionary conserved from jawless to cartilaginous and bony fish to mammals and represent the "swiss army knife" of the immune system capable of antigen-dependent or independent responses, memory, antigen presentation, regulation of other lymphocytes, tissue homeostasis, and mucosal barrier maintenance, to list a few. Over the last 10 years, γδ T cells that produce the cytokine IL-17 (γδT17) have taken a leading position in our understanding of how our immune system battles infection, inflicts tissue damage during inflammation, and gets rewired by the tumor microenvironment. A lot of what we know about γδT17 cells stems from mouse models, however, increasing evidence implicates these cells in numerous human diseases. Herein, we aim to give an overview of the most common mouse models that have been used to study the role of γδT17 cells in infection, inflammation, and cancer, while at the same time we will evaluate evidence for their importance in humans. We hope and believe that in the next 10 years, means to take advantage of the protective and destructive properties of γδ T and in particular γδT17 cells will be part of our standard immunotherapy toolkit.

Characterization of p57, a Stage-Specific Antigen of Pneumocystis murina

Pneumocystis has a large multicopy gene family encoding proteins related to the major surface glycoprotein (Msg), whose functions are largely unknown. We expressed one such protein of Pneumocystis murina, p57, which is encoded by 3 highly conserved genes, and demonstrated by immunoblot that immunocompetent mice that were immunized with crude Pneumocystis antigens or that had cleared Pneumocystis infection developed antibodies to p57. Using hyperimmune anti-p57 serum combined with immunolabeling, we found that p57 was expressed by small trophic forms and intracystic bodies, whereas it was not expressed on larger trophic forms or externally by cysts. Expression of p57 and Msg by trophic forms was largely mutually exclusive. Treatment of infected animals with caspofungin inhibited cyst formation and markedly decreased p57 expression. While p57 expression was seen in immunocompetent mice infected with Pneumocystis, immunization with recombinant p57 did not result in altered cytokine expression by lymphocytes or in diminished infection in such mice. Thus, p57 appears to be a stage-specific antigen of Pneumocystis that is expressed on intracystic bodies and young trophic forms and may represent a mechanism to conserve resources in organisms during periods of limited exposure to host immune responses.

IL-17 Inversely Correlated with IL-10 via the STAT3 Gene in Pneumocystis-Infected Mice

Background

Pneumocystis pneumonia (PCP) remains a common opportunistic infection in immunosuppressed individuals. Current studies showed that multiple immune cells and cytokines took part in the host defense against Pneumocystis (PC). However, the roles of IL-17 and IL-10 in the development of PCP have not been elucidated.

Methods

IL-10 and IL-17 levels in serum from PCP mice were detected via ELISA. The percentages of B10 cells, IL-10⁺ macrophages, and IL-10⁺ T cells in the lung from IL-17^–/– PCP mice and Th17 cells and IL-17⁺γδT cells in IL-10^–/– PCP mice were examined via flow cytometry. Also, antibody neutralization examination was also performed to elucidate the relationship of IL-17 and IL-10 in the PCP model.

Results

We noted the increase of IL-17 and IL-10 levels in serum from mice infected with Pneumocystis. Furthermore, deficiency of IL-17 or IL-10 could lead to the delayed clearance of Pneumocystis and more severed lung damage. Our data also demonstrated that IL-17 deficiency enhanced the serum IL-10 level and the percentages of B10 cells, IL-10⁺ macrophages, and IL-10⁺ T cells in the lung from PCP mice. Interestingly, we also noted an increase of the IL-17 level in serum and Th17 cell and IL-17⁺γδT cell percentages in the lung from IL-10^–/– PCP mice. Using antibody neutralization experiments, we found that the STAT3 gene might play a critical role in the interplay of IL-17 and IL-10 in PCP.

Conclusion

Taken together, our results demonstrated that IL-17 and IL-10 could play the protective roles in the progression of PCP and the inverse correlation of them might be mediated by STAT3.

The Contribution of Host Cells to Pneumocystis Immunity: An Update

Pneumocystis is a ubiquitous atypical fungus that is distributed globally. The genus comprises morphologically similar but genetically heterogeneous species that have co-evolved with specific mammalian hosts as obligate intra-pulmonary pathogens. In humans, Pneumocystis jirovecii is the causative organism of Pneumocystis pneumonia (PCP) in immunocompromised individuals, a serious illness frequently leading to life-threatening respiratory failure. Initially observed in acquired immunodeficiency syndrome (AIDS) patients, PCP is increasingly observed in immunocompromised non-AIDS patients. The evolving epidemiology and persistently poor outcomes of this common infection will require new strategies for diagnosis and treatment. A deeper understanding of host immune responses and of the cells that mediate them will improve the chance of developing new treatment strategies. This brief review provides an update on recent studies on the role of host immunity against Pneumocystis.

Helper T-cell responses and pulmonary fungal infections

The mucosal surface of the respiratory tract encounters microbes, such as fungal particles, with every inhaled breath. When pathogenic fungi breach the physical barrier and innate immune system within the lung to establish an infection, adaptive immunity is engaged, often in the form of helper CD4 T-cell responses. Type 1 responses, characterized by interferon-γ production from CD4 cells, promote clearance of Histoplasma capsulatum and Cryptococcus neoformans infection. Likewise, interleukin-17A (IL-17A) production from Th17 cells promotes immunity to Blastomyces dermatitidis and Coccidioides species infection by recruiting neutrophils. In contrast the development of T helper type 2 responses, characterized by IL-5 production from T cells and eosinophil influx into the lungs, drives allergic bronchopulmonary aspergillosis and poor outcomes during C. neoformans infection. Experimental vaccines against several endemic mycoses, including Histoplasma capsulatum, Coccidioides, Cryptococcus and Blastomyces dermatitidis, induce protective T-cell responses and foreshadow the development of vaccines against pulmonary fungal infections for use in humans. Additionally, recent work using antifungal T cells as immunotherapy to protect immune-compromised patients from opportunist fungal infections also shows great promise. This review covers the role of T-cell responses in driving protection and pathology in response to pulmonary fungal infections, and highlights promising therapeutic applications of antifungal T cells.

IL-9 Deficiency Promotes Pulmonary Th17 Response in Murine Model of Pneumocystis Infection

Introduction

Pneumocystis pneumonia (PCP) remains a severe complication with high mortality in immunocompromised patients. It has been well accepted that CD4⁺ T cells play a major role in controlling Pneumocystis infection. Th9 cells were the main source of IL-9 with multifaced roles depending on specific diseases. It is unclear whether IL-9/Th9 contributes to the immune response against PCP. The current study aims to explore the role of IL-9 and the effect of IL-9 on Th17 cells in murine model of PCP.

Materials and methods

Mice were intratracheally injected with 1 × 10⁶Pneumocystis organisms to establish the murine model of Pneumocystis infection. Pneumocystis burden was detected by TaqMan real-time PCR. Using IL-9-deficient (IL-9^−/−) mice, flow cytometry, real-time PCR and enzyme-linked immunosorbent assay (ELISA) were conducted to investigate the immune function related to Th17 response in defense against Pneumocystis infection.

Results

Reduced Pneumocystis burden was observed in lungs in IL-9^−/− mice compared with WT mice at 3-week postinfection. IL-9^−/−mice exhibited stronger Th17 immune responses than WT PCP mice through flow cytometer and real-time PCR. ELISA revealed higher levels of IL-17 and IL-23 in bronchoalveolar lavage fluid from IL-9^−/− mice than WT mice. And IL-9 deficiency promoted Th17 differentiation from CD4⁺ naive T cells. IL-17A neutralization increased Pneumocystis burden in IL-9^−/− mice.

Conclusion

Although similar basic clearance of Pneumocystis organisms was achieved in both WT and IL-9^−/− PCP mice, IL-9 deficiency could lower Pneumocystis organism burden and promote pulmonary Th17 cells response in the early stage of infection.