Regulatory T cells dampen pulmonary inflammation and lung injury in an animal model of pneumocystis pneumonia

Dynamic Changes of Regulatory T Cells/CD4⁺ T Cells in Peripheral Blood of Adult Kidney Transplant Recipients: A Comparison of Pediatric and Adult Kidney Donors

BACKGROUND Inducing transplantation tolerance and monitoring the recipient's immune status to improve allograft survival remains the main goal for kidney transplantation (KTx). MATERIAL AND METHODS A total of 53 renal transplantation patients and 20 healthy individuals were assigned to the post-transplantation and healthy groups, respectively; 10 recipients with stable renal function for 2 years after kidney transplantation were assigned to Group C. Eleven kidney transplantation recipients were hospitalized due to lung infection. Flow cytometry was used to measure levels of Tregs/CD4⁺ T cells. RESULTS The Tregs/CD4⁺ T cells ratio reached homeostasis 6 months after KTx, with no significant difference between Group D (healthy control group) and pre-surgery or Group C (2 years after KTx group). The pediatric donor group and the adult donor group reached immune homeostasis 3 months after the operation. Immune homeostasis is maintaining a balance between immune tolerance and immunogenicity. There was no significant difference in graft function between the pediatric and adult donor groups before surgery, 1 day after surgery, 1 week after surgery, 2 weeks after surgery, and 1 month after surgery; however, graft function was significantly better in the pediatric donor group compared with the adult donor group at 3 mouths (eGFR: 51.7 (40.4-66.2) vs 73.0 (55.7-90.2), P=0.008<0.05) and 6 months (eGFR: 52.2 (37.5-62.8) vs 80.5 (64.1-90.4), P<0.001) after surgery. Pediatric donor kidneys reached immune homeostasis 3 months after surgery, with better graft function at this time compared with adult donor kidneys. The proportion of Tregs/CD4⁺ T cells in recipients with a pulmonary infection after KTx was lower than in those with infection recovery. CONCLUSIONS Expanding the use of pediatric kidneys should be further explored by the transplantation community. The proportion of Tregs/CD4⁺ T cells in recipients with a pulmonary infection after KTx was lower than in those with infection recovery.

Differential roles of regulatory T cells in acute respiratory infections

Acute respiratory infections trigger an inflammatory immune response with the goal of pathogen clearance; however, overexuberant inflammation causes tissue damage and impairs pulmonary function. CD4+FOXP3+ regulatory T cells (Tregs) interact with cells of both the innate and the adaptive immune system to limit acute pulmonary inflammation and promote its resolution. Tregs also provide tissue protection and coordinate lung tissue repair, facilitating a return to homeostatic pulmonary function. Here, we review Treg-mediated modulation of the host response to respiratory pathogens, focusing on mechanisms underlying how Tregs promote resolution of inflammation and repair of acute lung injury. We also discuss potential strategies to harness and optimize Tregs as a cellular therapy for patients with severe acute respiratory infection and discuss open questions in the field.

Th17/Treg Imbalance: Implications in Lung Inflammatory Diseases

Regulatory T cells (Tregs) and T helper 17 cells (Th17) are two CD4⁺ T cell subsets with antagonist effects. Th17 cells promote inflammation, whereas Tregs are crucial in maintaining immune homeostasis. Recent studies suggest that Th17 cells and Treg cells are the foremost players in several inflammatory diseases. In this review, we explore the present knowledge on the role of Th17 cells and Treg cells, focusing on lung inflammatory diseases, such as chronic obstructive pulmonary disease (COPD), acute respiratory distress syndrome (ARDS), sarcoidosis, asthma, and pulmonary infectious diseases.

Genetic Mouse Models of Pneumocystis Pneumonia

Pneumocystis jirovecii causes pneumonia in immunocompromised patients. A major challenge in drug susceptibility testing and in understanding host/pathogen interactions is that Pneumocystis spp. are not viable in vitro. Continuous culture of the organism is not currently available, and therefore, developing new drug targets is very limited. Due to this limitation, mouse models of Pneumocystis pneumonia have proven to be an invaluable resource to researchers. In this chapter, we provide an overview of selected methods used in mouse models of infection including, in vivo Pneumocystis murina propagation, routes of transmission, genetic mouse models available, a P. murina life form-specific model, a mouse model of PCP immune reconstitution inflammatory syndrome (IRIS), and the experimental parameters associated with these models.

Platelet Dysregulation in the Pathobiology of COVID-19

Coronavirus disease 2019 (COVID-19) encompasses a broad spectrum of clinical manifestations caused by infection with severe acute respiratory syndrome coronavirus 2.Patients with severe disease present with hyperinflammation which can affect multiple organs which often include observations of microvascular and macrovascular thrombi. COVID-19 is increasingly recognized as a thromboinflammatory disease where alterations of both coagulation and platelets are closely linked to mortality and clinical outcomes. Although platelets are most well known as central mediators of hemostasis, they possess chemotactic molecules, cytokines, and adhesion molecules that are now appreciated as playing an important role in the regulation of immune response. This review summarizes the current knowledge of platelet alterations observed in the context of COVID-19 and their impact upon disease pathobiology.

Multi-Omics Integration Reveals Only Minor Long-Term Molecular and Functional Sequelae in Immune Cells of Individuals Recovered From COVID-19

The majority of COVID-19 patients experience mild to moderate disease course and recover within a few weeks. An increasing number of studies characterized the long-term changes in the specific anti-SARS-CoV-2 immune responses, but how COVID-19 shapes the innate and heterologous adaptive immune system after recovery is less well known. To comprehensively investigate the post-SARS-CoV-2 infection sequelae on the immune system, we performed a multi-omics study by integrating single-cell RNA-sequencing, single-cell ATAC-sequencing, genome-wide DNA methylation profiling, and functional validation experiments in 14 convalescent COVID-19 and 15 healthy individuals. We showed that immune responses generally recover without major sequelae after COVID-19. However, subtle differences persist at the transcriptomic level in monocytes, with downregulation of the interferon pathway, while DNA methylation also displays minor changes in convalescent COVID-19 individuals. However, these differences did not affect the cytokine production capacity of PBMCs upon different bacterial, viral, and fungal stimuli, although baseline release of IL-1Ra and IFN-γ was higher in convalescent individuals. In conclusion, we propose that despite minor differences in epigenetic and transcriptional programs, the immune system of convalescent COVID-19 patients largely recovers to the homeostatic level of healthy individuals.

The Association between Regulatory T Cell Subpopulations and Severe Pneumonia Post Renal Transplantation

Severe pneumonia accounts for the majority of morbidity and mortality in renal allograft recipients due to immunosuppressant maintenance. Regulatory T cells (Tregs), which are involved in tackling infections under immunosuppressive conditions, are rarely uncovered. We aimed to investigate the relationship between various Treg subpopulations and severe pneumonia after kidney transplantation (KTx). KTx recipients with pneumonia were divided into severe pneumonia and mild pneumonia groups. The frequencies and absolute numbers (Ab No.) of total Tregs (CD4+CD25+FoxP3+), six subsets of Tregs (Helios+/-, CD39+/-, and CD45RA+/-), and T cells, B cells, and NK cells were assessed from peripheral blood via flow cytometry using the $t$ or Mann-Whitney test and receiver operating curve analysis. We also determined the median fluorescence intensity (MFI) of human leukocyte antigen- (HLA-) DR on monocytes and CD64 on neutrophils. Logistic regression was used to identify the risk factors of disease progression, and Pearson’s correlation analysis was performed to identify relationships between the measured immune indices and patients’ clinical information. Our research indicated that Treg subpopulations were strongly associated with severe pneumonia progression post KTx. Based on the monitoring of Treg subpopulations, better-individualized prevention and therapy might be achieved for patients with severe pneumonia post KTx.

Epigenetic Control of Regulatory T Cell Stability and Function: Implications for Translation

FoxP3+ regulatory T (Treg) cells maintain immune homeostasis, promote self-tolerance, and have an emerging role in resolving acute inflammation, providing tissue protection, and repairing tissue damage. Some data suggest that FoxP3+ T cells are plastic, exhibiting susceptibility to losing their function in inflammatory cytokine-rich microenvironments and paradoxically contributing to inflammatory pathology. As a result, plasticity may represent a barrier to Treg cell immunotherapy. Here, we discuss controversies surrounding Treg cell plasticity and explore determinants of Treg cell stability in inflammatory microenvironments, focusing on epigenetic mechanisms that clinical protocols could leverage to enhance efficacy and limit toxicity of Treg cell-based therapeutics.

Crosstalk of Microorganisms and Immune Responses in Autoimmune Neuroinflammation: A Focus on Regulatory T Cells

Regulatory T cells (Tregs) are the major determinant of peripheral immune tolerance. Many Treg subsets have been described, however thymus-derived and peripherally induced Tregs remain the most important subpopulations. In multiple sclerosis, a prototypical autoimmune disorder of the central nervous system, Treg dysfunction is a pathogenic hallmark. In contrast, induction of Treg proliferation and enhancement of their function are central immune evasion mechanisms of infectious pathogens. In accordance, Treg expansion is compartmentalized to tissues with high viral replication and prolonged in chronic infections. In friend retrovirus infection, Treg expansion is mainly based on excessive interleukin-2 production by infected effector T cells. Moreover, pathogens seem also to enhance Treg functions as shown in human immunodeficiency virus infection, where Tregs express higher levels of effector molecules such as cytotoxic T-lymphocyte-associated protein 4, CD39 and cAMP and show increased suppressive capacity. Thus, insights into the molecular mechanisms by which intracellular pathogens alter Treg functions might aid to find new therapeutic approaches to target central nervous system autoimmunity. In this review, we summarize the current knowledge of the role of pathogens for Treg function in the context of autoimmune neuroinflammation. We discuss the mechanistic implications for future therapies and provide an outlook for new research directions.

Bias of the Immune Response to Pneumocystis murina Does Not Alter the Ability of Neonatal Mice to Clear the Infection

Newborn mice are unable to clear Pneumocystis (PC) infection with the same efficiency as adults due, in part, to their inability to develop a robust immune response to infection until three weeks of age. It is known that infants tend develop a Th2 skewed response to antigen so we sought to determine whether a biased cytokine response altered the clearance of PC infection in neonatal mice. P. murina infection in neonatal mice resulted in increased IL-4 expression by CD4 T cells and myeloid cells, augmented IL-13 secretion within the airways and increased arginase activity in the airways, indicative of Th2-type responses. P. murina-infected IL-4Rα^-/- neonates had a shift towards Th1 cytokine production and increased numbers of CD4 and CD8 T cells within the lung as well as elevated levels of P. murina-specific IgG. IFNγ^-/- and IL-23 p19^-/- mice had altered CD4-T cell-dependent cytokine and cell responses. Though we could alter the T helper cell environment in neonatal knockout mice, there was no loss in the ability of these pups to clear infection. It is possible that the Th2 phenotype normally seen in neonatal mice protects the developing lung from pro-inflammatory immune responses without compromising host defense against P. murina.

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.

Lymphocyte subset analysis to evaluate the prognosis of HIV-negative patients with pneumocystis pneumonia

Objectives

We analysed the peripheral blood lymphocyte subsets of human immunodeficiency virus (HIV)-negative patients infected with pneumocystis pneumonia (PCP) to determine the relationships between the levels of different types of lymphocytes and the prognosis of patients.

Methods

We retrospectively reviewed HIV-negative patients with PCP diagnosed in our department. All the eligible patients underwent lymphocyte subset analysis on admission.

Results

A total of 88 HIV-negative PCP patients were enrolled in the study. In univariate analyses, low CD4+ T cell count, low CD8+ T cell count, and low natural killer cell (NK cell) count were associated with higher in-hospital mortality. CD8+ T cell count ≤300/μL was found to be an independent risk factor for poor prognosis in multivariate logistical regression analysis (p = 0.015, OR = 11.526, 95% CI = 1.597–83.158). Although low CD4+ T cell and NK cell counts were not independent risk factors, the mortality rates of PCP patients decreased as the CD4+ T cell and NK cell counts increased.

Conclusion

The immune process of Pneumocystis jirovecii infection is complex but important. We propose that lymphocyte subsets could give clinicians a better understanding of patient immune status, helping with the early identification of potentially lethal infections and treatment decision making, such as adjusting the immunosuppressive regimen and choosing an appropriate patient monitoring level.

Single-Cell TCR Sequencing Reveals the Dynamics of T Cell Repertoire Profiling During Pneumocystis Infection

T cell responses play critical roles in host adaptive immunity against Pneumocystis. However, the dynamics and diversity of the T cell immune repertoire in human immunodeficiency virus (HIV)-negative Pneumocystis pneumonia (PCP) remains unclear. In this study, single-cell RNA and single-cell T cell receptor (TCR) sequencing were applied to cells sorted from lung tissues of mice infected with Pneumocystis. Our findings demonstrated the clonal cells were mainly composed of CD4⁺ T cells in response to Pneumocystis, which were marked by highly expressed genes associated with T cell activation. Mice infected with Pneumocystis showed reduced TCR diversity in CD4⁺ T cells and increased diversity in CD8⁺ T cells compared with uninfected controls. Furthermore, Th17 cells were mostly clonal CD4⁺ T cells, which exhibited the phenotype of tissue-resident memory-like Th17 cells. In addition, Pneumocystis-infected mice showed biased usage of TCRβ VDJ genes. Taken together, we characterized the transcriptome and TCR immune repertoires profiles of expanded T cell clones, which demonstrate a skewed TCR repertoire after Pneumocystis infection.

Toward an understanding of regulatory T cells in COVID-19: A systematic review

A more detailed understanding of Treg cells in COVID-19 infection will broaden our knowledge of the COVID-19 immunopathology and give us more insight into the curative immune-based strategies. We systematically searched electronic databases (PubMed, Google Scholar, EMBASE) and identified 18 eligible studies. Despite the inconsistencies between the results, we observed a trend toward decreasing Treg levels in severe COVID-19 patients. This finding underlines the hypothesis that Tregs play a role in the pathogenesis of COVID-19. Further studies on Tregs' functional aspects are necessary to illustrate Tregs' potential role in COVID-19 disease.

Estradiol resolves pneumonia via ERβ in regulatory T cells

Current treatments for pneumonia (PNA) are focused on the pathogens. Mortality from PNA-induced acute lung injury (PNA-ALI) remains high, underscoring the need for additional therapeutic targets. Clinical and experimental evidence exists for potential sex differences in PNA survival, with males having higher mortality. In a model of severe pneumococcal PNA, when compared with male mice, age-matched female mice exhibited enhanced resolution characterized by decreased alveolar and lung inflammation and increased numbers of Tregs. Recognizing the critical role of Tregs in lung injury resolution, we evaluated whether improved outcomes in female mice were due to estradiol (E2) effects on Treg biology. E2 promoted a Treg-suppressive phenotype in vitro and resolution of PNA in vivo. Systemic rescue administration of E2 promoted resolution of PNA in male mice independent of lung bacterial clearance. E2 augmented Treg expression of Foxp3, CD25, and GATA3, an effect that required ERβ, and not ERα, signaling. Importantly, the in vivo therapeutic effects of E2 were lost in Treg-depleted mice (Foxp3^DTR mice). Adoptive transfer of ex vivo E2-treated Tregs rescued Streptococcuspneumoniae–induce PNA-ALI, a salutary effect that required Treg ERβ expression. E2/ERβ was required for Tregs to control macrophage proinflammatory responses. Our findings support the therapeutic role for E2 in promoting resolution of lung inflammation after PNA via ERβ Tregs.

Inflammation in Pulmonary Arterial Hypertension

Pulmonary artery hypertension (PAH) is a devastating cardiopulmonary disease characterized by vascular remodeling and obliteration of the precapillary pulmonary arterioles. Alterations in the structure and function of pulmonary vessels result in the resistance of blood flow and can progress to right-sided heart failure, causing significant morbidity and mortality. There are several types of PAH, and the disease can be familial or secondary to an underlying medical condition such as a connective tissue disorder or infection. Regardless of the cause, the exact pathophysiology and cellular interactions responsible for disease development and progression are largely unknown.There is significant evidence to suggest altered immune and vascular cells directly participate in disease progression. Inflammation has long been hypothesized to play a vital role in the development of PAH, as an altered or skewed immune response favoring a proinflammatory environment that can lead to the infiltration of cells such as lymphocytes, macrophages, and neutrophils. Current treatment strategies focus on the dilation of partially occluded vessels; however, such techniques have not resulted in an effective strategy to reverse or prevent vascular remodeling. Therefore, current studies in human and animal models have attempted to understand the underlying pathophysiology of pulmonary hypertension (PH), specifically focusing on the inflammatory cascade predisposing patients to disease so that better therapeutic targets can be developed to potentially reverse or prevent disease progression.The purpose of this chapter is to provide a comprehensive review of the expanding literature on the inflammatory process that participates in PH development while highlighting important and current studies in both animal and human models. While our primary focus will be on cells found in the adaptive and innate immune system, we will review all potential causes of PAH, including cells of the endothelium, pulmonary lymphatics, and genetic mutations predisposing patients. In addition, we will discuss current therapeutic options while highlighting potential future treatments and the questions that still remain unanswered.

Correlation between clinical outcome and tissue inflammatory response in kidney transplant recipients with cryptococcosis

Cryptococcosis is the second most common invasive fungal infection reported in renal transplant recipients. Tissue granulomatous inflammation is necessary to contain Cryptococcus infection. This study aims to analyze the granuloma patterns and in situ expression of regulatory T (Treg) immune response in tissue samples from 12 renal transplant recipients with cryptococcosis. Fungal isolates were molecularly identified as Cryptococcus neoformans species complex. A detailed characterization of granulomas in tissue samples from 12 kidney transplant recipients with cryptococcosis was described by checking six lung and six skin biopsies by conventional histology and for immunohistochemical detection of CD4 and Treg markers: forkhead box P3 (FoxP3), interleukin (IL)-10 and transforming-growth factor (TGF)-β. Granulomas were classified as compact, loose or mixed. Patients with mixed (n = 4) and compact (n = 3) granulomatous inflammation patterns were associated with a better prognosis and presented a higher number of CD4+FoxP3+T cells compared to the group of patients with loose granulomas. In counterpart, three out of five patients with loose granulomas died with cryptococcosis. We suggest that Treg may have a protective role in the tissue response to Cryptococcus infection given its association with compact and mixed granulomas in patients with better clinical outcomes.

Immunologic Effects of Sirolimus in Patients With Vascular Anomalies

Emerging data have suggested that sirolimus may be a treatment option for complicated vascular anomalies (VAs). The present study aimed to investigate the immunologic effects of sirolimus treatment for 6 months in patients with VAs. Blood samples obtained from the patients enrolled in 2 multicenter studies to investigate the efficacy of sirolimus for VAs before and after sirolimus treatment for 6 months were used. Data for total white blood cell count, absolute lymphocyte count, serum immunoglobulins (Igs) levels (IgG, IgA, IgM), lymphocyte proliferation assays with mitogens including phytohemagglutinin and concanavalin A, and flow cytometric analysis of lymphocyte subsets were evaluated. A total of 18 patients with VAs receiving sirolimus treatment were included in the study. Comparisons of white blood cell, absolute lymphocyte count, IgG, IgA, IgM, and reaction rates of phytohemagglutinin and concanavalin A revealed no significant differences before and after treatment. No significant differences were observed in the absolute counts of lymphocyte subtypes before and after treatment, except for regulatory T-cell counts, which were significantly decreased after treatment. Severe infections were not observed during sirolimus treatment. The immunologic parameters assessed in the present study were hardly affected by sirolimus treatment for 6 months in patients with VAs.

The Binary Classification Of Chronic Diseases

Acute diseases start with an insult and end when insult disappears. If the trauma induces an immune reaction (which happens in most cases), this reaction must be terminated with some type of resolution mechanism, when the cause of the trauma ceases. Chronicity develops if insult is permanent or if the resolution mechanism is defective. Another way to reach disease chronicity is a positive feedback loop, whereby the immune reaction activates an internal, insult-like reaction. A distinction between chronic states characterized by a persistent, low suppressive effect and those characterized by a persistent, high suppressive effect of regulatory T cells (Treg), is proposed. This two-class division represents two ways to reach chronicity: (a) by maintaining inflammatory reaction long after insult disappears (“low Treg”), or (b) by suppressing inflammatory reaction prior to the disappearance of insult (“high Treg”). This two-class division may explain the strong association between certain pathogens and cancer, on one hand, and between several other pathogens and autoimmunity, on the other hand. The weak association between autoimmune diseases and HIV infection and the relatively weak association between autoimmune diseases and cancer may be elucidated as well. In addition, the model rationalizes why immune-modulating drugs, which are effective in cancer, are also effective in “high Treg” viral infections, while corticosteroids, which are generally effective in autoimmune diseases, are also effective in other “low Treg” diseases (such as asthma, atopic dermatitis, and “low Treg” infections) but are not effective in solid malignancies and “high Treg” infections. Moreover, the model expounds why certain bacteria inhibit tumor growth and why these very bacteria induce autoimmune diseases.

Circulating and Pulmonary T-cell Populations Driving the Immune Response in Non-HIV Immunocompromised Patients with Pneumocystis jirovecii Pneumonia

Background: Previous studies in human subjects have mostly been confined to peripheral blood lymphocytes for Pneumocystis infection. We here aimed to compare circulating and pulmonary T-cell populations derived from human immunodeficiency virus (HIV)-uninfected immunocompromised patients with Pneumocystis jirovecii pneumonia (PCP) in order to direct new therapies. Methods: Peripheral blood and bronchoalveolar lavage samples were collected from patients with and without PCP. Populations of Th1/Tc1, Th2/Tc2, Th9/Tc9, and Th17/Tc17 CD4⁺ and CD8⁺ T cells were quantified using multiparameter flow cytometry. Results: No significant differences were found between PCP and non-PCP groups in circulating T cells. However, significantly higher proportions of pulmonary Th1 and Tc9 were observed in the PCP than in the non-PCP group. Interestingly, our data indicated that pulmonary Th1 was negatively correlated with disease severity, whereas pulmonary Tc9 displayed a positive correlation in PCP patients. Conclusions: Our findings suggest that pulmonary expansion of Th1 and Tc9 subsets may play protective and detrimental roles in PCP patients, respectively. Thus, these specific T-cell subsets in the lungs may serve as targeted immunotherapies for patients with PCP.

Pulmonary influenza A virus infection leads to suppression of the innate immune response to dermal injury

The innate immune system is responsible for many important functions in the body including responding to infection, clearing cancerous cells, healing wounds, and removing foreign substances. Although many of these functions happen simultaneously in life, most laboratory studies of the innate immune response focus on one activity. How the innate immune system responds to concurrent insults in different parts of the body is not well understood. This study explores the impact of a lung infection on the cutaneous wound healing process. We used two complimentary models of injury: the excisional tail wound and subcutaneous implantation of polyvinyl alcohol (PVA) sponges. These models allow for assessment of the rate of closure and measurement of cellular and cytokine responses during acute wound healing, respectively. When mice with these healing wounds were infected with influenza A virus (IAV) in the lung there was a delay in wound healing. The viral lung infection suppressed the innate immune response in a healing wound, including cellular infiltrate, chemokines, growth factors, and cytokines. However, there was not a global immune suppression as there was an increase in inflammation systemically in mice with both infection and healing wounds compared to mice with only wounds or IAV infection. In addition, the lung immune response was largely unaffected indicating that responding to a lung infection is prioritized over a healing wound. This study introduces the concept of immune triage, in that when faced with multiple insults the immune system prioritizes responses. This paradigm likely applies to many situations that involve the innate immune system, and understanding how the innate immune system handles multiple insults is essential to understanding how it can efficiently clear pathogens while responding to other inflammatory events.

In a natural setting, the innate immune system is frequently faced with multiple insults, against which it must mount overlapping inflammatory responses. We are interested in how the innate immune system deals with multiple, simultaneously occurring inflammatory insults, and if the response to one will take priority. For example, the innate immune system is essential in mediating both the early control of pathogen replication in infected tissue and in the early stages of wound healing. Pulmonary infections occur frequently in injured patient populations; therefore, we set out to determine the impact of a respiratory infection on a healing wound. To examine this, mice with healing dermal wounds were infected with influenza A virus (IAV), a common cause of viral pneumonia. We found that the innate immune response to the lung infection took priority at the expense of the healing wound, in that the initial control of viral replication in the lung was not impacted, while the wound healing response was suppressed. Very little work has been done examining how the immune response can respond to overlapping inflammatory insults. Our work shows that not all immune responses are created equal, and that the cells of the innate immune system are preferentially routed towards fighting a lung infection rather than the healing dermal wound. This apparent prioritization of the innate immune response opens up a new direction of study. It is relevant to many fields where competing insults may alter the disease state.

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.

A novel role for coinhibitory receptors/checkpoint proteins in the immunopathology of sepsis

Coinhibitory molecules, such as PD-1, CTLA-4, 2B4, and BTLA, are an important new family of mediators in the pathophysiology of severe bacterial and/or fungal infection, as well as the combined insults of shock and sepsis. Further, the expression of these molecules may serve as indicators of the immune status of the septic individual. Using PD-1:PD-L as an example, we discuss in this review how such checkpoint molecules may affect the host response to infection by mediating the balance between effective immune defense and immune-mediated tissue injury. Additionally, we explore how the up-regulation of PD-1 and/or PD-L1 expression on not only adaptive immune cells (e.g., T cells), but also on innate immune cells (e.g., macrophages, monocytes, and neutrophils), as well as nonimmune cells during sepsis and/or shock contributes to functional alterations often with detrimental sequelae.

Adaptive Immunity to Cryptococcus neoformans Infections

The Cryptococcus neoformans/Cryptococcus gattii species complex is a group of fungal pathogens with different phenotypic and genotypic diversity that cause disease in immunocompromised patients as well as in healthy individuals. The immune response resulting from the interaction between Cryptococcus and the host immune system is a key determinant of the disease outcome. The species C. neoformans causes the majority of human infections, and therefore almost all immunological studies focused on C. neoformans infections. Thus, this review presents current understanding on the role of adaptive immunity during C. neoformans infections both in humans and in animal models of disease.

New advances in understanding the host immune response to Pneumocystis

Pneumocystis jirovecii causes clinical pneumonia in immunocompromised hosts. Despite this, the inability to cultivate this organism in vitro has likely hindered the field in ascertaining the true impact of Pneumocystis in human infection. However the recent release of the genome as well as in advances in understanding host genetics, and other risk factors for infection and robust experimental models of disease have shed new light on the impact of this fungal pathogen as to better define populations at risk. This review will highlight these recent advances as well as highlight future needed areas of research.

B Cell Requirement for Robust Regulatory T Cell Responses to Friend Retrovirus Infection

Regulatory T cells (Tregs) are immunosuppressive cells of the immune system that control autoimmune reactivity. Tregs also respond during immune reactions to infectious agents in order to limit immunopathological damage from potent effectors such as CD8⁺ cytolytic T lymphocytes. We have used the Friend virus (FV) model of retroviral infection in mice to investigate how viral infections induce Tregs. During acute FV infection, there is significant activation and expansion of thymus-derived (natural) Tregs that suppress virus-specific CD8⁺ T cell responses. Unlike conventional T cells, the responding Tregs are not virus specific, so the mechanisms that induce their expansion are of great interest. We now show that B cells provide essential signals for Treg expansion during FV infection. Treg responses are greatly diminished in B cell-deficient mice but can be restored by adoptive transfers of B cells at the time of infection. The feeble Treg responses in B cell-deficient mice are associated with enhanced virus-specific CD8⁺ T cell responses and accelerated virus control during the first 2 weeks of infection. In vitro experiments demonstrated that B cells promote Treg activation and proliferation through a glucocorticoid-induced receptor superfamily member 18 (GITR) ligand-dependent mechanism. Thus, B cells play paradoxically opposing roles during FV infection. They provide proliferative signals to immunsosuppressive Tregs, which slows early virus control, and they also produce virus-specific antibodies, which are essential for long-term virus control.

When infectious agents invade a host, numerous immunological mechanisms are deployed to limit their replication, neutralize their spread, and destroy the host cells harboring the infection. Since immune responses also have a strong capacity to damage host cells and tissues, their magnitude, potency, and duration are under regulatory control. Regulatory T cells are an important component of this control, and the mechanisms that induce them to respond and exert immunosuppressive regulation are of great interest. In the current report, we show that B cells, the cells responsible for making pathogen-specific antibodies, are also involved in promoting the expansion of regulatory T cells during a retroviral infection. In vitro studies demonstrated that they do so via stimulation of the Tregs through interactions between cell surface molecules: GITR interactions with its ligand (GITRL) on B cells and GITR on regulatory T cells. These findings point the way toward therapeutics to better treat infections and autoimmune diseases.

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

Pneumocystis remains an important pathogen of immunosuppressed patients, causing a potentially life-threatening pneumonia. Despite its medical importance, the immune responses required to control infection, including the role of interleukin-17 (IL-17), which is important in controlling other fungal infections, have not been clearly defined. Using flow cytometry and intracellular cytokine staining after stimulation with phorbol myristate acetate and ionomycin, we examined gamma interferon (IFN-γ), IL-4, IL-5, and IL-17 production by lung lymphocytes in immunocompetent C57BL/6 mice over time following infection with Pneumocystismurina We also examined the clearance of Pneumocystis infection in IL-17A-deficient mice. The production of both IFN-γ and IL-17 by pulmonary lymphocytes increased during infection, with maximum production at approximately days 35 to 40, coinciding with peak Pneumocystis levels in the lungs, while minimal changes were seen in IL-4- and IL-5-positive cells. The proportion of cells producing IFN-γ was consistently higher than for cells producing IL-17, with peak levels of ∼25 to 30% of CD3⁺ T cells for the former compared to ∼15% for the latter. Both CD4⁺ T cells and γδ T cells produced IL-17. Administration of anti-IFN-γ antibody led to a decrease in IFN-γ-positive cells, and an increase in IL-5-positive cells, but did not impact clearance of Pneumocystis infection. Despite the increases in IL-17 production during infection, IL-17A-deficient mice cleared Pneumocystis infection with kinetics similar to C57BL/6 mice. Thus, while IL-17 production in the lungs is increased during Pneumocystis infection in immunocompetent mice, IL-17A is not required for control of Pneumocystis infection.

Fasciola hepatica reinfection potentiates a mixed Th1/Th2/Th17/Treg response and correlates with the clinical phenotypes of anemia

Background

Fascioliasis is a severe zoonotic disease of worldwide extension caused by liver flukes. In human fascioliasis hyperendemic areas, reinfection and chronicity are the norm and anemia is the main sign. Herein, the profile of the Th1/Th2/Th17/Treg expression levels is analyzed after reinfection, correlating them with their corresponding hematological biomarkers of morbidity.

Methodology/Principal findings

The experimental design reproduces the usual reinfection/chronicity conditions in human fascioliasis endemic areas and included Fasciola hepatica primo-infected Wistar rats (PI) and rats reinfected at 8 weeks (R8), and at 12 weeks (R12), and negative control rats. In a cross-sectional study, the expression of the genes associated with Th1 (Ifng, Il12a, Il12b, Nos2), Th2 (Il4, Arg1), Treg (Foxp3, Il10, Tgfb, Ebi3), and Th17 (Il17) in the spleen and thymus was analyzed. After 20 weeks of primary infection, PI did not present significant changes in the expression of those genes when compared to non-infected rats (NI), but an increase of Il4, Arg1 and Ifng mRNA in the spleen was observed in R12, suggesting the existence of an active mixed Th1/Th2 systemic immune response in reinfection. Foxp3, Il10, Tgfb and Ebi3 levels increased in the spleen in R12 when compared to NI and PI, indicating that the Treg gene expression levels are potentiated in chronic phase reinfection. Il17 gene expression levels in R12 in the spleen increased when compared to NI, PI and R8. Gene expression levels of Il10 in the thymus increased when compared to NI and PI in R12. Ifng expression levels in the thymus increased in all reinfected rats, but not in PI. The clinical phenotype was determined by the fluke burden, the rat body weight and the hemogram. Multivariate mathematical models were built to describe the Th1/Th2/Th17/Treg expression levels and the clinical phenotype. In reinfection, two phenotypic patterns were detected: i) one which includes only increased splenic Ifng expression levels but no Treg expression, correlating with severe anemia; ii) another which includes increased splenic Ifng and Treg expression levels, correlating with a less severe anemia.

Conclusions/Significance

In animals with established F. hepatica infection a huge increase in the immune response occurs, being a mixed Th2/Treg associated gene expression together with an expression of Ifng. Interestingly, a Th17 associated gene expression is also observed. Reinfection in the chronic phase is able to activate a mixed immune response (Th1/Th2/Th17/Treg) against F. hepatica but T and B proliferation to mitogens is strongly suppressed in all infected rats vs control in the advanced chronic phase independently of reinfection The systemic immune response is different in each group, suggesting that suppression is mediated by different mechanisms in each case. Immune suppression could be due to the parasite in PI and R8 rats and the induction of suppressive cells such as Treg in R12. This is the first study to provide fundamental insight into the immune profile in fascioliasis reinfection and its relation with the clinical phenotypes of anemia.

Immune reconstitution inflammatory syndrome associated with pulmonary pathogens

Immune reconstitution inflammatory syndrome (IRIS) is an exaggerated immune response to a variety of pathogens in response to antiretroviral therapy-mediated recovery of the immune system in HIV-infected patients. Although IRIS can occur in many organs, pulmonary IRIS, associated with opportunistic infections such as Mycobacterium tuberculosis and Pneumocystis jirovecii, is particularly associated with high morbidity and mortality. The pathology of IRIS is associated with a variety of innate and adaptive immune factors, including CD4⁺ T-cells, CD8⁺ T-cells, γδ T-cells, natural killer cells, macrophages, the complement system and surfactant proteins, Toll-like receptors and pro-inflammatory cytokines and chemokines. Although there are numerous reports about the immune factors involved in IRIS, the mechanisms involved in the development of pulmonary IRIS are poorly understood. Here, we propose that studies using gene-deficient murine and nonhuman primate models will help to identify the specific molecular targets associated with the development of IRIS. An improved understanding of the mechanisms involved in the pathology of pulmonary IRIS will help to identify potential biomarkers and therapeutic targets in this syndrome.

Mechanisms of pulmonary IRIS in HIV-infected individuals recently initiated on ART are poorly definedhttp://ow.ly/AAOR301Bh36

Regulatory T Cell Induction and Retention in the Lungs Drives Suppression of Detrimental Type 2 Th Cells During Pulmonary Cryptococcal Infection

Lethal disease caused by the fungus Cryptococcus neoformans is a consequence of the combined failure to control pulmonary fungal replication and immunopathology caused by induced type 2 Th2 cell responses in animal models. In order to gain insights into immune regulatory networks, we examined the role of regulatory T (Treg) cells in suppression of Th2 cells using a mouse model of experimental cryptococcosis. Upon pulmonary infection with Cryptococcus, Treg cells accumulated in the lung parenchyma independently of priming in the draining lymph node. Using peptide-MHC class II molecules to identify Cryptococcus-specific Treg cells combined with genetic fate-mapping, we noted that a majority of the Treg cells found in the lungs were induced during the infection. Additionally, we found that Treg cells used the transcription factor, IFN regulatory factor 4, to dampen harmful Th2 cell responses, as well as mediate chemokine retention of Treg cells in the lungs. Taken together, induction and IFN regulatory factor 4-dependent localization of Treg cells in the lungs allow Treg cells to suppress the deleterious effects of Th2 cells during cryptococcal infection.

Loss- and Gain-of-Function Approaches Indicate a Dual Role Exerted by Regulatory T Cells in Pulmonary Paracoccidioidomycosis

Paracoccidioidomycosis (PCM), is a pulmonary fungal disease whose severity depends on the adequate development of T cell immunity. Although regulatory T (Treg) cells were shown to control immunity against PCM, deleterious or protective effects were described in different experimental settings. To clarify the function of Treg cells in pulmonary PCM, loss-and gain-of-function approaches were performed with Foxp3^GFP knock-in mice and immunodeficient Rag1^-/- mice, respectively, which were intratracheally infected with 10⁶ yeast cells. The activity of Foxp3-expressing Treg cells in pulmonary PCM was determined in Foxp3^GFP transgenic mice. First, it was verified that natural Treg cells migrate to the lungs of infected mice, where they become activated. Depletion of Treg cells led to reduced fungal load, diminished pathogen dissemination and increased Th1/Th2/Th17 immunity. Further, adoptive transfer of diverse T cell subsets to Rag1^-/- mice subsequently infected by the pulmonary route demonstrated that isolated CD4⁺Foxp3⁺ Treg cells were able to confer some degree of immunoprotection and that CD4⁺Foxp3^- T cells alone reduced fungal growth and enhanced T cell immunity, but induced vigorous inflammatory reactions in the lungs. Nevertheless, transfer of Treg cells combined with CD4⁺Foxp3^- T cells generated more efficient and balanced immune Th1/Th2/Th17 responses able to limit pathogen growth and excessive tissue inflammation, leading to regressive disease and increased survival rates. Altogether, these loss- and gain-of-function approaches allow us to clearly demonstrate the dual role of Treg cells in pulmonary PCM, their deleterious effects by impairing T cell immunity and pathogen eradication, and their protective role by suppressing exacerbated tissue inflammation.

Paracoccidioidomycosis (PCM), the most relevant deep mycosis in Latin America, is caused by the fungus Paracoccidioides brasiliensis. The involvement of regulatory T (Treg) cells in the immunity against PCM has been previously demonstrated, however its underlying mechanisms are still to be elucidated. Using Foxp3^GFP transgenic mice, we found that natural Tregs migrate to the lungs of infected mice, where they become activated. Depletion of Treg cells led to reduced fungal load, diminished pathogen dissemination and increased Th1/Th2/Th17 immunity. Further, after performing adoptive transfer experiments using immunodeficient Rag1^-/- mice, we demonstrated that isolated CD4⁺Foxp3⁺ Treg cells were able to confer some degree of immunoprotection and that CD4⁺Foxp3^- T cells alone contributed to fungal elimination and enhanced T cell immunity, but induced vigorous inflammatory reactions in the lungs. Nevertheless, transfer of Tregs combined with CD4⁺Foxp3^- T cells generated more efficient and balanced immune responses able to limit pathogen growth and excessive tissue inflammation, leading to regressive disease and higher survival rates. Altogether, these loss- and gain-of-function approaches allow us to propose a dual role for Treg cells in pulmonary PCM, their deleterious effects by impairing T cell immunity and pathogen eradication, and their protective role by suppressing exacerbated tissue inflammation.

Treatment with Interleukin-7 Restores Host Defense against Pneumocystis in CD4+ T-Lymphocyte-Depleted Mice

Pneumocystis pneumonia (PCP) is a major cause of morbidity and mortality in patients with HIV infection. CD4(+) T lymphocytes are critical for host defense against this infection, but in the absence of CD4(+) T lymphocytes, CD8(+) T lymphocytes may provide limited host defense. The cytokine interleukin-7 (IL-7) functions to enhance lymphocyte proliferation, survival, and recruitment of immune cells to sites of infection. However, there is little known about the role of IL-7 in PCP or its potential use as an immunotherapeutic agent. We hypothesized that treatment with recombinant human IL-7 (rhIL-7) would augment host defense against Pneumocystis and accelerate pathogen clearance in CD4-depleted mice. Control and CD4-depleted mice were infected with Pneumocystis, and rhIL-7 was administered via intraperitoneal injection. Our studies indicate that endogenous murine IL-7 is part of the normal host response to Pneumocystis murina and that administration of rhIL-7 markedly enhanced clearance of Pneumocystis in CD4-depleted mice. Additionally, we observed increased recruitment of CD8(+) T lymphocytes to the lungs and decreased apoptosis of pulmonary CD8(+) T lymphocytes in rhIL-7-treated animals compared to those in untreated mice. The antiapoptotic effect of rhIL-7 was associated with increased levels of Bcl-2 protein in T lymphocytes. rhIL-7 immunotherapy in CD4-depleted mice also increased the number of gamma interferon (IFN-γ)-positive CD8(+) central memory T lymphocytes in the lungs. We conclude that rhIL-7 has a potent therapeutic effect in the treatment of murine Pneumocystis pneumonia in CD4-depleted mice. This therapeutic effect is mediated through enhanced recruitment of CD8(+) T cells and decreased apoptosis of lung T lymphocytes, with a preferential action on central memory CD8(+) T lymphocytes.

The Immune Fulcrum: Regulatory T Cells Tip the Balance Between Pro- and Anti-inflammatory Outcomes upon Infection

Regulatory T cells (Tregs) are indispensable for immune homeostasis and the prevention of autoimmunity. In the context of infectious diseases, Tregs are multidimensional. Here, we describe how they may potentiate effector responses by assisting in recruitment of T cells into the infection site to resolve infection, facilitate accelerated antigen-specific memory responses, limit pathology, and contribute to disease resolution and healing, to the great benefit of the host. We also explore the villainous functions of Tregs during infection by reviewing several diseases in which the depletion or reduction in Treg frequency allows for better generation of effector memory, and results in acute resolution of infection, as opposed to chronicity or severe long-term outcomes. We describe findings generated using mouse models of infection as well as experiments performed using human cells and tissues. We propose that Tregs represent an immunologic fulcrum, promoting both pathogen clearance and damage control by preventing excessive destruction of infected tissues though unchecked immune responses.

Expanding regulatory T cells alleviates chikungunya virus-induced pathology in mice

Chikungunya virus (CHIKV) infection is a reemerging pandemic human arboviral disease. CD4⁺ T cells were previously shown to contribute to joint inflammation in the course of CHIKV infection in mice. The JES6-1 anti-IL-2 antibody selectively expands mouse regulatory T cells (Tregs) by forming a complex with IL-2. In this study, we show that the IL-2 JES6-1-mediated expansion of Tregs ameliorates CHIKV-induced joint pathology. It does so by inhibiting the infiltration of CD4⁺ T cells due to the induction of anergy in CHIKV-specific CD4⁺ effector T cells. These findings suggest that activation of Tregs could also become an alternative approach to control CHIKV-mediated disease.

IMPORTANCE Chikungunya virus (CHIKV) has reemerged as a pathogen of global significance. Patients infected with CHIKV suffer from incapacitating joint pain that severely affects their daily functioning. Despite the best efforts, treatment is still inadequate. While T cell-mediated immunopathology in CHIKV infections has been reported, the role of regulatory T cells (Tregs) has not been explored. The JES6-1 anti-interleukin 2 (IL-2) antibody has been demonstrated to selectively expand mouse Tregs by forming a complex with IL-2. We reveal here that IL-2 JES6-1-mediated expansion of Tregs ameliorates CHIKV-induced joint pathology in mice by neutralizing virus-specific CD4⁺ effector T (Teff) cells. We show that this treatment abrogates the infiltration of pathogenic CD4⁺ T cells through induction of anergy in CHIKV-specific CD4⁺ Teff cells. This is the first evidence where the role of Tregs is demonstrated in CHIKV pathogenesis, and its expansion could control virus-mediated immunopathology.

Anti-CD20 antibody therapy and susceptibility to Pneumocystis pneumonia

Anti-CD20 antibody therapy has been a useful medication for managing non-Hodgkin's lymphoma as well as autoimmune diseases characterized by autoantibody generation. CD20 is expressed during most developmental stages of B lymphocytes; thus, CD20 depletion leads to B-lymphocyte deficiency. As the drug has become more widely used, there has been an increase in the number of case reports of patients developing Pneumocystis pneumonia. The role of anti-CD20 in Pneumocystis jirovecii infection is under debate due to the fact that most patients receiving it are on a regimen of multiple immunosuppressive medications. To address the specific role of CD20 depletion in host immunity against Pneumocystis, we examined a murine anti-CD20 depleting antibody. We demonstrated that anti-CD20 alone is permissive for Pneumocystis infection and that anti-CD20 impairs components of type II immunity, such as production of interleukin-4 (IL-4), IL-5, and IL-13 by whole-lung cells, in response to Pneumocystis murina. We also demonstrated that CD4(+) T cells from mice treated with anti-CD20 during Pneumocystis infection are incapable of mounting a protective immune response when transferred into Rag1(-/-) mice. Thus, CD20(+) cells are critical for generating protective CD4(+) T-cell immune responses against this organism.

T cell subsets in human airways prior to and following endobronchial administration of endotoxin

BACKGROUND AND OBJECTIVES

Bronchial instillation of lipopolysaccharide (LPS) provides a reversible model of lung inflammation that may resemble early stages of acute respiratory distress syndrome (ARDS). We investigated the distributions of T-cell subsets in the human airways and sought to determine whether pro- and anti-inflammatory T cells are involved in the local immune response to lung inflammation.

METHODS

Bronchoalveolar lavage (BAL) was performed in 15 healthy volunteers, after which Escherichia coli LPS (4 ng/kg) was administered. BAL was repeated at 2, 4, 6, 8 or 24 h after instillation of LPS.

RESULTS

BALF CD4+ and CD8+ T cells were characterized by expression of activation markers (HLA-DR+CD38+), the proportion of cells expressing naïve markers (CD45RA+CD27+CCR7+) was lower, and that of cells expressing effector memory markers (CD45RA-CD27+CCR7-) was higher, compared with peripheral blood. Bronchial LPS induced a local inflammatory response with recruitment of CD4+ (P=0.014), CD8+ T cells (P=0.034), an increase in the proportion of CD4+CD25+CD127lowFoxp3+ regulatory T cells (Tregs) (P=0.045) and a tendency towards an increase in CD4+CD161+ cells (P=0.071) were observed.

CONCLUSIONS

A unique distribution of T cells with little day-to-day variation was found in human airways. An increase in Tregs after endobronchial LPS suggests a role for Tregs during early stages of pulmonary inflammation.

Pathological and protective immunity to Pneumocystis infection

Pneumocystis jirovecii is a common opportunistic infection in the HIV-positive population and is re-emerging as a growing clinical concern in the HIV-negative immunosuppressed population. Newer targeted immunosuppressive therapies and the discovery of rare genetic mutations have furthered our understanding of the immunity required to clear Pneumocystis infection. The immune system can also mount a pathologic response against Pneumocystis following removal of immunosuppression and result in severe damage to the host lung. The current review will examine the most recent epidemiologic studies about the incidence of Pneumocystis in the HIV-positive and HIV-negative populations in the developing and developed world and will detail methods of diagnosis for Pneumocystis pneumonia. Finally, this review aims to summarize the known mediators of immunity to Pneumocystis and detail the pathologic immune response leading to Pneumocystis-related immune reconstitution inflammatory syndrome.

Active players in resolution of shock/sepsis induced indirect lung injury: immunomodulatory effects of Tregs and PD-1

The immunomodulatory effects of PD-1 and CD4(+)CD25(+) Tregs in the resolution of ALI are still poorly understood. Accordingly, 1 million Tregs were isolated from spleens of WT C57BL/6 or PD-1(-/-) mice (magnetical bead purification and subsequent labeling with/without Vybrant dye) and then AT into mice subjected to Hem shock during their resuscitation period, which were subsequently subjected to CLP/septic challenge (24 h post-Hem) to induce iALI. Initially, we demonstrated that Vybrant-labeled AT Tregs appear in the lungs of iALI mice. Subsequently, we found that AT of WT Tregs induced a significant repression of the indices of lung injury: a reduction of neutrophil influx to the lung tissue and a decrease of lung apoptosis compared with vehicle-treated iALI mice. In addition, these mice had substantially higher concentrations of BALF and lung-tissue IL-10 but significantly decreased levels of lung KC. However, these beneficial effects of the AT of Tregs were lost with the administration of PD-1(-/-) mouse Tregs to the recipient WT mice. ALI was exacerbated in these recipient mice receiving AT PD-1(-/-) Tregs to the same extent as iALI mice that did not receive Tregs. These data imply that Tregs can act directly to modify the innate immune response induced by experimental iALI, and this is mediated, in part, by PD-1. Hence, the manipulation of Tregs may represent a plausible target for treating iALI.

Clonotype-specific avidity influences the dynamics and hierarchy of virus-specific regulatory and effector CD4(+) T-cell responses

A key component of immunity against viruses, CD4(+) T cells expand and differentiate into functional subsets upon primary infection, where effector (Teff) cells facilitate infection control and regulatory (Treg) cells mitigate immunopathology. After secondary infection, Teff cells mount a robust response from the memory pool. Here, we show that Treg-cell responses are diminished upon secondary infection, and Treg-cell response dynamics are associated more with T-cell receptors (TCRs) repertoire and avidity than with epitope specificity. In the murine model, the IA(b) M209 epitope of respiratory syncytial virus is recognized by both CD4(+) Treg and Teff cells, while the IA(b) M226 epitope is recognized almost exclusively by CD4(+) Teff cells expressing high avidity TCR Vβ8.1/8.2 and dominating the CD4(+) T-cell response during primary and secondary infections. IA(b) M209 -Teff cells express relatively low avidity TCRs during early primary infection, but high avidity TCR Vβ7-expressing IA(b) M209 -Teff cells emerge during the late phase, and become dominant after secondary infection. The emerging high avidity IA(b) M209 -Teff cells outcompete IA(b) M209 -Treg cells that share the same epitope, but have low avidity and are restricted to TCR Vβ2 and Vβ6 subpopulations. These data indicate that MHC-peptide-TCR interactions can produce different kinetic and functional profiles in CD4(+) T-cell populations even when responding to the same epitope.

Modulation of Treg function improves adenovirus vector-mediated gene expression in the airway

Virus vector-mediated gene transfer has been developed as a treatment for cystic fibrosis (CF) airway disease, a lethal inherited disorder caused by somatic mutations in the cystic fibrosis transmembrane conductance regulator gene. The pathological proinflammatory environment of CF as well as the naïve and adaptive immunity induced by the virus vector itself limits the effectiveness of gene therapy for CF airway. Here, we report the use of an HDAC inhibitor, valproic acid (VPA), to enhance the activity of the regulatory T cells (T(reg)) and to improve the expression of virus vector-mediated gene transfer to the respiratory epithelium. Our study demonstrates the potential utility of VPA, a drug used for over 50 years in humans as an anticonvulsant and mood-stabilizer, in controlling inflammation and improving the efficacy of gene transfer in CF airway.

An increased alveolar CD4 + CD25 + Foxp3 + T-regulatory cell ratio in acute respiratory distress syndrome is associated with increased 30-day mortality

Purpose

Cell therapy may become an option for lung injury treatment. However, no data are available on the alveolar presence and time course of CD4+ CD25 + Foxp3 + T-regulatory lymphocyte cells (Tregs) in acute respiratory distress syndrome (ARDS). Accordingly, we (1) measured the ratio of CD4 + CD25 + Foxp3 + Tregs to all (CD4+) lymphocytes in the bronchoalveolar lavage (BAL) of ARDS patients and of control subjects without lung disease and (2) assessed their impact on 30-day mortality.

Methods

In a prospective study, the ratios of CD4 + CD25 + Foxp3 + T-regulatory cells to all CD4+ cells were measured (FACS) within 24 h of the patients’ ICU referral in the BAL and in the blood of 47 patients with ARDS (32 males, 15 females; mean age 44 years ±13) as well as in 8 controls undergoing elective abdominal surgery (5 men, 3 women; mean age 49 years ±4). BAL concentrations of several cytokines were also measured in ARDS patients.

Results

Tregs were detected in the BAL of control subjects and ARDS patients. However, the mean ratio of Tregs to all CD4+ lymphocytes was threefold greater in ARDS non-survivors (16.5 %; p = 0.025) and almost twofold greater in ARDS survivors (9.0 %; p = 0.015) compared to controls (5.9 %). Multivariate Cox regression analysis revealed the ratio of CD4 + CD25 + Foxp3 + T-regulatory lymphocytes to all CD4+ lymphocytes in the BAL to be an important and independent prognostic factor for 30-day survival (HR 6.5; 95 % CI, 1.7–25; p = 0.006).

Conclusion

An increased T-regulatory cell ratio in the admission BAL of patients with ARDS is an important and independent risk factor for 30-day mortality.

Involvement of high mobility group box 1 and the therapeutic effect of recombinant thrombomodulin in a mouse model of severe acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is accompanied by severe lung inflammation induced by various diseases. Despite the severity of the symptoms, therapeutic strategies have been ineffective. High mobility group box 1 (HMGB1), which was identified originally as a DNA binding protein, has been proposed as a mediator of acute lung injury. In addition to its anti-coagulant activity, recombinant thrombomodulin (rTM) possesses an ability to suppress the inflammatory response through neutralizing HMGB1. T regulatory (T(reg)) cells in the lungs are reported to modify innate immune responses during resolution of acute lung injury. In the present study, we investigated the therapeutic effect of rTM, and the contribution of T(reg) cells to this effect, in a mouse model of severe ARDS. C57BL/6 mice received sequential intratracheal administration of α-galactosylceramide (α-GalCer) and lipopolysaccharide (LPS), which resulted in the development of severe ARDS. HMGB1 levels in the lungs increased to a higher level in ARDS mice compared to those in mice treated with LPS alone. HMGB1 was expressed in the infiltrating neutrophils and macrophages in lungs. T(reg) cells were reduced significantly in the lungs of ARDS mice compared to those in mice treated with LPS alone. rTM administration prolonged the survival time and ameliorated the development of ARDS, which was associated with increased T(reg) cells and synthesis of interleukin (IL)-10 and transforming growth factor (TGF)-β in the lungs. These results suggest that HMGB1 is involved in the development of severe ARDS and rTM shows therapeutic effects through promoting the accumulation of T(reg) cells at the inflammatory sites.

Dectin immunoadhesins and pneumocystis pneumonia

The opportunistic pathogen Pneumocystis jirovecii is a significant cause of disease in HIV-infected patients and others with immunosuppressive conditions. Pneumocystis can also cause complications in treatment following antiretroviral therapy or reversal of immunosuppressive therapy, as the newly reconstituted immune system can develop a pathological inflammatory response to remaining antigens or a previously undetected infection. To target β-(1,3)-glucan, a structural component of the Pneumocystis cell wall with immune-stimulating properties, we have developed immunoadhesins consisting of the carbohydrate binding domain of Dectin-1 fused to the Fc regions of the 4 subtypes of murine IgG (mIgG). These immunoadhesins bind β-glucan with high affinity, and precoating the surface of zymosan with Dectin-1:Fc can reduce cytokine production by macrophages in an in vitro stimulation assay. All Dectin-1:Fc variants showed specificity of binding to the asci of Pneumocystis murina, but effector activity of the fusion molecules varied depending on Fc subtype. Dectin-1:mIgG2a Fc was able to reduce the viability of P. murina in culture through a complement-dependent mechanism, whereas previous studies have shown the mIgG1 Fc fusion to increase macrophage-dependent killing. In an in vivo challenge model, systemic expression of Dectin-1:mIgG1 Fc significantly reduced ascus burden in the lung. When administered postinfection in a model of immune reconstitution inflammatory syndrome (IRIS), both Dectin-1:mIgG1 and Dectin-1:mIgG2a Fc reduced hypoxemia despite minimal effects on fungal burden in the lung. Taken together, these data indicate that molecules targeting β-glucan may provide a mechanism for treatment of fungal infection and for modulation of the inflammatory response to Pneumocystis and other pathogens.

T cell-mediated host immune defenses in the lung

Evidence has increasingly shown that the lungs are a major site of immune regulation. A robust and highly regulated immune response in the lung protects the host from pathogen infection, whereas an inefficient or deleterious response can lead to various pulmonary diseases. Many cell types, such as epithelial cells, dendritic cells, macrophages, neutrophils, eosinophils, and B and T lymphocytes, contribute to lung immunity. This review focuses on the recent advances in understanding how T lymphocytes mediate pulmonary host defenses against bacterial, viral, and fungal pathogens.

STAT4-dependent and -independent Th2 responses correlate with protective immunity against lung infection with Pneumocystis murina

Although it is clear that the loss of CD4(+) T cells is a predisposing factor for the development of Pneumocystis pneumonia, specific Th mechanisms mediating protection are not well understood. Th1, Th2, and Th17 responses have each been implicated in protective responses during infection. As STAT4 may promote Th1 and Th17 development, yet antagonize Th2 development, we investigated its role in Pneumocystis murina host defense. STAT4 was required for Th1 and, unexpectedly, Th2 responses in the lungs of C57BL/6 (BL/6) and BALB/c mice 14 d postchallenge, but only BALB/c Stat4(-/-) mice demonstrated susceptibility to P. murina lung infection. BL/6 Stat4(-/-), but not BALB/c Stat4(-/-), mice maintained an enhanced alternatively activated (M2) macrophage signature in the lungs, which we have previously reported to be associated with enhanced P. murina clearance. In addition, anti-P. murina class-switched Abs were increased in BL/6 Stat4(-/-) mice, but not BALB/c Stat4(-/-) mice. Supporting our experimental observations, plasma from HIV-infected individuals colonized with Pneumocystis jirovecii contained significantly lower levels of the Th2 cytokines IL-4, IL-5, and IL-13 compared with HIV-infected individuals who were not colonized. Collectively, our data suggest that robust local and systemic Th2-mediated responses are critical for immunity to Pneumocystis.

Characterization of a distinct host response profile to Pneumocystis murina asci during clearance of pneumocystis pneumonia

Pneumocystis spp. are yeast-like fungi that cause pneumocystis pneumonia (PcP) in immunocompromised individuals and exacerbate chronic lung diseases in immunocompetent individuals. The Pneumocystis life cycle includes trophic forms and asci (cyst forms). The cell walls of Pneumocystis asci contain β-1,3-D-glucan, and treatment of PcP with β-1,3-D-glucan synthase inhibitors, such as anidulafungin, results in depletion of asci, but not trophic forms. The pulmonary host response during immune reconstitution (IR)-mediated clearance of PcP in anidulafungin-treated and untreated mice was characterized to identify ascus-specific responses. During IR, similar numbers of trophic forms were present in the anidulafungin-treated and untreated mice; however, asci were only present in the untreated mice. IR resulted in a significant reduction of trophic forms from the lungs in both groups and asci in the untreated group. The presence of asci in untreated mice correlated with increased β-glucan content in the lungs. The untreated mice mounted immune responses associated with a deleterious host inflammatory response, including increased CD8(+) T cell influx and expression of macrophage inflammatory response markers. A more robust cellular response was also observed in the untreated mice, with increased numbers of macrophages and neutrophils that were associated with greater lung damage. Markers of a Th17 response were also elevated in the untreated mice. These results suggest that the host mounts unique responses to asci and trophic forms. That these 2 life cycle stages provoked distinct host response profiles has significant implications for clearance and interpretation of the host immune responses to PcP.

Anti-CD25 treatment depletes Treg cells and decreases disease severity in susceptible and resistant mice infected with Paracoccidioides brasiliensis

Regulatory T (Treg) cells are fundamental in the control of immunity and excessive tissue pathology. In paracoccidioidomycosis, an endemic mycosis of Latin America, the immunoregulatory mechanisms that control the progressive and regressive forms of this infection are poorly known. Due to its modulatory activity on Treg cells, we investigated the effects of anti-CD25 treatment over the course of pulmonary infection in resistant (A/J) and susceptible (B10.A) mice infected with Paracoccidioides brasiliensis. We verified that the resistant A/J mice developed higher numbers and more potent Treg cells than susceptible B10.A mice. Compared to B10.A cells, the CD4⁺CD25⁺Foxp3⁺ Treg cells of A/J mice expressed higher levels of CD25, CTLA4, GITR, Foxp3, LAP and intracellular IL-10 and TGF-β. In both resistant and susceptible mice, anti-CD25 treatment decreased the CD4⁺CD25⁺Foxp3⁺ Treg cell number, impaired indoleamine 2,3-dioxygenase expression and resulted in decreased fungal loads in the lungs, liver and spleen. In A/J mice, anti-CD25 treatment led to an early increase in T cell immunity, demonstrated by the augmented influx of activated CD4⁺ and CD8⁺ T cells, macrophages and dendritic cells to the lungs. At a later phase, the mild infection was associated with decreased inflammatory reactions and increased Th1/Th2/Th17 cytokine production. In B10.A mice, anti-CD25 treatment did not alter the inflammatory reactions but increased the fungicidal mechanisms and late secretion of Th1/Th2/Th17 cytokines. Importantly, in both mouse strains, the early depletion of CD25⁺ cells resulted in less severe tissue pathology and abolished the enhanced mortality observed in susceptible mice. In conclusion, this study is the first to demonstrate that anti-CD25 treatment is beneficial to the progressive and regressive forms of paracoccidioidomycosis, potentially due to the anti-CD25-mediated reduction of Treg cells, as these cells have suppressive effects on the early T cell response in resistant mice and the clearance mechanisms of fungal cells in susceptible mice.