Granulomatous Inflammation in Tuberculosis and Sarcoidosis: Does the Lymphatic System Contribute to Disease?

Distinguishing multiple roles of T cell and macrophage involvement in determining lymph node fates during Mycobacterium tuberculosis infection

Tuberculosis (TB) is a disease of major public health concern with an estimated one-fourth of the world currently infected with M. tuberculosis (Mtb) bacilli. Mtb infection occurs after inhalation of Mtb, following which, highly structured immune structures called granulomas form within lungs to immunologically restrain and physically constrain spread of infection. Most lung granulomas are successful at controlling or even eliminating their bacterial loads, but others fail to control infection and promote disease. Granulomas also form within lung-draining lymph nodes (LNs), variably affecting immune function. Both lung and LN granulomas vary widely in ability to control infection, even within a single host, with outcomes ranging from bacterial clearance to uncontrolled bacterial growth. While lung granulomas are well-studied, data on LN granulomas are scarce; it is unknown what mechanisms drive LN Mtb infection progression and variability in severity. Recent data suggest that LN granulomas are niches for bacterial replication and can reduce control over lung infection. To identify mechanisms driving LN Mtb infection, we developed a multi-scale compartmental model that includes multiple lung-draining LNs, blood. We calibrated to data from a nonhuman primate TB model (one of the only models that parallels human TB infection). Our model predicts temporal trajectories for LN macrophage, T-cell, and Mtb populations during simulated Mtb infection. We also predict a clinically measurable infection feature from PET/CT imaging, FDG avidity. Using uncertainty and sensitivity analysis methods, we identify key mechanisms driving LN granuloma fate, T-cell efflux rates from LNs, and a role for LNs in pulmonary infection control.

Mistakes to avoid in the management of abdominal tuberculosis

INTRODUCTION

The diagnosis and management of abdominal tuberculosis, i.e Gastrointestinal Tuberculosis (GITB) and tuberculous peritonitis (TBP) is challenging. Abdominal tuberculosis, presenting usually with abdominal pain, intestinal obstruction, and constitutional symptoms, is typically a paucibacillary condition. The diagnosis hinges on a correct interpretation of clinical, radiological, histological, biochemical, and microbiological findings as also appropriately assessing response to therapy.

AREAS COVERED

The authors review potential missteps that could occur in managing GITB and TBP sourced from published literature and clinical experience. These include avoiding excess use of tests with limited accuracy, understanding limitations of ascitic adenosine deaminase (ADA) and granulomas, avoiding empirical antitubercular therapy (ATT) where possible but also understanding that microbiological tests may not always be positive, and finally not to bank solely on subjective clinical responses but to use objective markers in assessing response to therapy. In addition, diagnosis of predisposing immunosuppressed states, attention to nutrition, appropriate management of sequelae with endoscopic dilatation/surgery, and early surgery when indicated are some of the additional issues discussed.

EXPERT OPINION

In future, a more secure diagnosis banking on the use of better microbiological tools, multiparameter-based models, artificial intelligence-based approaches, and use of advances in -omics-based approaches can improve diagnosis and avoid some missteps.

Serum chitotriosidase activity in South African patients with sarcoidosis and tuberculosis

Background

Chitotriosidase is a chitinase enzyme that is expressed selectively through activated macrophages in humans. Increased activity of chitotriosidase in both bronchoalveolar lavage samples and serum of patients with sarcoidosis has been reported. It has been proposed that chitotriosidase could be used as a potential biomarker for diagnosis, monitoring and prognosis in sarcoidosis patients. However, no studies in a South African (SA) cohort have evaluated this potential role.

Objectives

To analyse serum chitotriosidase activity in treated and untreated sarcoidosis patients, healthy controls and patients with tuberculosis (TB). Sarcoidosis and TB are two diseases of differing aetiology that may be clinically difficult to distinguish between in the SA setting, which is a high-burden area for TB. We hoped to determine whether chitotriosidase activity levels could help differentiate the one disease from the other.

Methods

Serum chitotriosidase activity was measured in an SA cohort of treated and untreated sarcoidosis patients and compared with controls. In addition, activity in sarcoidosis patients was compared with that in TB patients. Overall, chitotriosidase activity was assayed in the serum of 12 biopsy-proven sarcoidosis patients before treatment, 9 sarcoidosis patients after at least a month's treatment, 10 patients with confirmed pulmonary and/or disseminated TB before treatment, and 12 healthy controls. Plasma chitotriosidase activity was assayed as previously described using 4-methylumbelliferyl-β-D-N,N',N″-triacetylchitotriose as a substrate.

Results

Significantly higher serum chitotriosidase activity was observed in sarcoidosis patients, both untreated and treated, compared with controls (p<0.05). Sarcoidosis patients had higher chitotriosidase levels than TB patients, but this difference was not significant. While chitotriosidase activity was lower in patients with TB than in those with sarcoidosis, levels were elevated compared with controls.

Conclusion

Chitotriosidase activity in patients with sarcoidosis was greater than in those with TB, and also greater compared with controls. The increased chitotriosidase activity in sarcoidosis suggests that this enzyme may be involved in the disease pathogenesis. Further investigation is required to validate these findings.

Study synopsis

What the study adds. Serum chitotriosidase activity in South African sarcoidosis and tuberculosis (TB) patients was evaluated. The study adds to the research assessing the significance of serum chitotriosidase in patients with sarcoidosis and TB.Implications of the findings. Chitotriosidase enzyme activity could potentially serve as a biomarker of possible diagnostic and/or prognostic value in patients with sarcoidosis.

The lymphatic vasculature in lung function and respiratory disease

The lymphatic vasculature maintains tissue homeostasis via fluid drainage in the form of lymph and immune surveillance due to migration of leukocytes through the lymphatics to the draining lymph nodes. Lymphatic endothelial cells (LECs) form the lymphatic vessels and lymph node sinuses and are key players in shaping immune responses and tolerance. In the healthy lung, the vast majority of lymphatic vessels are found along the bronchovascular structures, in the interlobular septa, and in the subpleural space. Previous studies in both mice and humans have shown that the lymphatics are necessary for lung function from the neonatal period through adulthood. Furthermore, changes in the lymphatic vasculature are observed in nearly all respiratory diseases in which they have been analyzed. Recent work has pointed to a causative role for lymphatic dysfunction in the initiation and progression of lung disease, indicating that these vessels may be active players in pathologic processes in the lung. However, the mechanisms by which defects in lung lymphatic function are pathogenic are understudied, leaving many unanswered questions. A more comprehensive understanding of the mechanistic role of morphological, functional, and molecular changes in the lung lymphatic endothelium in respiratory diseases is a promising area of research that is likely to lead to novel therapeutic targets. In this review, we will discuss our current knowledge of the structure and function of the lung lymphatics and the role of these vessels in lung homeostasis and respiratory disease.

Pyroptosis in inflammation-related respiratory disease

Pyroptosis is commonly induced by the gasdermin (GSDM) family and is accompanied by the release of inflammatory cytokines such as IL-1β and IL-18. Recently, increasing evidence suggests that pyroptosis plays a role in respiratory diseases. This review aimed to summarize the roles and mechanisms of pyroptosis in inflammation-related respiratory diseases. There are several pathways involved in pyroptosis, such as the canonical inflammasome-induced pathway, non-canonical inflammasome-induced pathway, caspase-1/3/6/7/GSDMB pathway, caspase-8/GSDMC pathway, caspase-8/GSDMD pathway, and caspase-3/GSEME pathway. Pyroptosis may be involved in asthma, chronic obstructive pulmonary disease (COPD), lung cancer, acute lung injury (ALI), silicosis, pulmonary hypertension (PH), and tuberculosis (TB), in which the NLRP3 inflammasome-induced pathway is mostly highlighted. Pyroptosis contributes to the deterioration of asthma, COPD, ALI, silicosis, and PH. In addition, pyroptosis has dual effects on lung cancer and TB. Additionally, whether pyroptosis participates in cystic fibrosis (CF) and sarcoidosis or not is largely unknown, though the activation of NLRP3 inflammasome is found in CF and sarcoidosis. In conclusion, pyroptosis may play a role in inflammation-related respiratory diseases, providing new therapeutic targets.

Targeting Molecular Inflammatory Pathways in Granuloma as Host-Directed Therapies for Tuberculosis

Globally, more than 10 million people developed active tuberculosis (TB), with 1.4 million deaths in 2020. In addition, the emergence of drug-resistant strains in many regions of the world threatens national TB control programs. This requires an understanding of host-pathogen interactions and finding novel treatments including host-directed therapies (HDTs) is of utter importance to tackle the TB epidemic. Mycobacterium tuberculosis (Mtb), the causative agent for TB, mainly infects the lungs causing inflammatory processes leading to immune activation and the development and formation of granulomas. During TB disease progression, the mononuclear inflammatory cell infiltrates which form the central structure of granulomas undergo cellular changes to form epithelioid cells, multinucleated giant cells and foamy macrophages. Granulomas further contain neutrophils, NK cells, dendritic cells and an outer layer composed of T and B lymphocytes and fibroblasts. This complex granulomatous host response can be modulated by Mtb to induce pathological changes damaging host lung tissues ultimately benefiting the persistence and survival of Mtb within host macrophages. The development of cavities is likely to enhance inter-host transmission and caseum could facilitate the dissemination of Mtb to other organs inducing disease progression. This review explores host targets and molecular pathways in the inflammatory granuloma host immune response that may be beneficial as target candidates for HDTs against TB.

Macrophage-specific responses to human- and animal-adapted tubercle bacilli reveal pathogen and host factors driving multinucleated cell formation

The Mycobacterium tuberculosis complex (MTBC) is a group of related pathogens that cause tuberculosis (TB) in mammals. MTBC species are distinguished by their ability to sustain in distinct host populations. While Mycobacterium bovis (Mbv) sustains transmission cycles in cattle and wild animals and causes zoonotic TB, M. tuberculosis (Mtb) affects human populations and seldom causes disease in cattle. The host and pathogen determinants underlying host tropism between MTBC species are still unknown. Macrophages are the main host cell that encounters mycobacteria upon initial infection, and we hypothesised that early interactions between the macrophage and mycobacteria influence species-specific disease outcome. To identify factors that contribute to host tropism, we analysed blood-derived primary human and bovine macrophages (hMϕ or bMϕ, respectively) infected with Mbv and Mtb. We show that Mbv and Mtb reside in different cellular compartments and differentially replicate in hMϕ whereas both Mbv and Mtb efficiently replicate in bMϕ. Specifically, we show that out of the four infection combinations, only the infection of bMϕ with Mbv promoted the formation of multinucleated giant cells (MNGCs), a hallmark of tuberculous granulomas. Mechanistically, we demonstrate that both MPB70 from Mbv and extracellular vesicles released by Mbv-infected bMϕ promote macrophage multinucleation. Importantly, we extended our in vitro studies to show that granulomas from Mbv-infected but not Mtb-infected cattle contained higher numbers of MNGCs. Our findings implicate MNGC formation in the contrasting pathology between Mtb and Mbv for the bovine host and identify MPB70 from Mbv and extracellular vesicles from bMϕ as mediators of this process.

The identification of host and pathogen factors contributing to host-pathogen interaction is crucial to understand the pathogenesis and dissemination of tuberculosis. This is particularly the case in deciphering the mechanistic basis for host-tropism across the MTBC. Here, we show that in vitro, M. bovis but not M. tuberculosis induces multinucleated cell formation in bovine macrophages. We identified host and pathogen mechanistic drivers of multinucleated cell formation: MPB70 as the M. bovis factor and bovine macrophage extracellular vesicles. Using a cattle experimental infection model, we confirmed differential multinucleated cell formation in vivo. Thus, we have identified host and pathogen factors that contribute to host tropism in human/bovine tuberculosis. Additionally, this work provides an explanation for the long-standing association of multinucleated cells with tuberculosis pathogenesis.

Mycobacterium tuberculosis cords within lymphatic endothelial cells to evade host immunity

The ability of Mycobacterium tuberculosis to form serpentine cords is intrinsically related to its virulence, but specifically how M. tuberculosis cording contributes to pathogenesis remains obscure. Here, we show that several M. tuberculosis clinical isolates form intracellular cords in primary human lymphatic endothelial cells (hLECs) in vitro and in the lymph nodes of patients with tuberculosis. We identified via RNA-Seq a transcriptional program that activated, in infected-hLECs, cell survival and cytosolic surveillance of pathogens pathways. Consistent with this, cytosolic access was required for intracellular M. tuberculosis cording. Mycobacteria lacking ESX-1 type VII secretion system or phthiocerol dimycocerosates expression, which failed to access the cytosol, were indeed unable to form cords within hLECs. Finally, we show that M. tuberculosis cording is a size-dependent mechanism used by the pathogen to avoid its recognition by cytosolic sensors and evade either resting or IFN-γ-induced hLEC immunity. These results explain the long-standing association between M. tuberculosis cording and virulence and how virulent mycobacteria use intracellular cording as strategy to successfully adapt and persist in the lymphatic tracts.