Nonlytic exocytosis of Cryptococcus neoformans from neutrophils in the brain vasculature

Elements of dissemination in cryptococcosis

As healthcare improves and our ability to support patients with compromised immune systems increases, such patients become more vulnerable to microbes in the environment. These include fungal pathogens such as Cryptococcus neoformans, the primary cause of fungal meningitis and a top priority pathogen on the World Health Organization fungal pathogen list. Like many other environmental pathogens, C. neoformans must adapt to and thrive in diverse environments in order to cause disease: (i) the environmental niche, (ii) the lungs following inhalation of infectious particles, (iii) the bloodstream and/or lymphatic system during dissemination, and (iv) the central nervous system (CNS), where it causes a deadly cryptococcal meningoencephalitis. Because CNS infection is the driver of mortality and the presenting illness, understanding the dissemination process from both host and fungal perspectives is important for treating these infections. In this review, we discuss the different stages of dissemination, how fungal cells interact with host cells during disease, and the ability to adapt to different environments within hosts.

Toxoplasma infection induces an aged neutrophil population in the CNS that is associated with neuronal protection

BACKGROUND

Infection with the protozoan parasite Toxoplasma gondii leads to the formation of lifelong cysts in neurons that can have devastating consequences in the immunocompromised. In the immunocompetent individual, anti-parasitic effector mechanisms and a balanced immune response characterized by pro- and anti-inflammatory cytokine production establishes an asymptomatic infection that rarely leads to neurological symptoms. Several mechanisms are known to play a role in this successful immune response in the brain including T cell production of IFNγ and IL-10 and the involvement of CNS resident cells. This limitation of clinical neuropathology during chronic infection suggests a balance between immune response and neuroprotective mechanisms that collectively prevent clinical manifestations of disease. However, how these two vital mechanisms of protection interact during chronic Toxoplasma infection remains poorly understood.

MAIN TEXT

This study demonstrates a previously undescribed connection between innate neutrophils found chronically in the brain, termed "chronic brain neutrophils" (CBNeuts), and neuroprotective mechanisms during Toxoplasma infection. Lack of CBNeuts during chronic infection, accomplished via systemic neutrophil depletion, led to enhanced infection and deleterious effects on neuronal regeneration and repair mechanisms in the brain. Phenotypic and transcriptomic analysis of CBNeuts identified them as distinct from peripheral neutrophils and revealed two main subsets of CBNeuts that display heterogeneity towards both classical effector and neuroprotective functions in an age-dependent manner. Further phenotypic profiling defined expression of the neuroprotective molecules NRG-1 andErbB4 by these cells, and the importance of this signaling pathway during chronic infection was demonstrated via NRG-1 treatment studies.

CONCLUSIONS

In conclusion, this work identifies CBNeuts as a heterogenous population geared towards both classical immune responses and neuroprotection during chronic Toxoplasma infection and provides the foundation for future mechanistic studies of these cells.

An Image Processing Algorithm for Facile and Reproducible Quantification of Vomocytosis

Vomocytosis is a process that occurs when internalized fungal pathogens escape from phagocytes without compromising the viability of the pathogen and the host cell. Manual quantification of time-lapse microscopy videos is currently used as the standard to study pathogen behavior and vomocytosis incidence. However, human-driven quantification of vomocytosis (and the closely related phenomenon, exocytosis) is incredibly burdensome, especially when a large volume of cells and interactions needs to be analyzed. In this study, we designed a MATLAB algorithm that measures the extent of colocalization between the phagocyte and fungal cell (Cryptococcus neoformans; CN) and rapidly reports the occurrence of vomocytosis in a high throughput manner. Our code processes multichannel, time-lapse microscopy videos of cocultured CN and immune cells that have each been fluorescently stained with unique dyes and provides quantitative readouts of the spatiotemporally dynamic process that is vomocytosis. This study also explored metrics, such as the rate of change of pathogen colocalization with the host cell, that could potentially be used to predict vomocytosis occurrence based on the quantitative data collected. Ultimately, the algorithm quantifies vomocytosis events and reduces the time for video analysis from over 1 h to just 10 min, a reduction in labor of 83%, while simultaneously minimizing human error. This tool significantly minimizes the vomocytosis analysis pipeline, accelerates our ability to elucidate unstudied aspects of this phenomenon, and expedites our ability to characterize CN strains for the study of their epidemiology and virulence.

A Multi-Fluorophore Staining Scheme for Identification and Quantification of Vomocytosis

Vomocytosis is a process by which fungal pathogens, for instance, Cryptococcus neoformans (CN), escape from the digestive phagolysosome of phagocytic cells after ingestion. Interestingly, this expulsion leaves both the pathogen and phagocyte unharmed, and is believed to be an important mechanism by which CNs disseminate throughout infected hosts. This phenomenon was discovered in 2006, and research to date has relied almost entirely on quantification via manual counting of vomocytosis events in time-lapse microscopy videos. This archaic method has the significant disadvantages of requiring excessive labor in manual analysis, limited throughput capabilities, and low accuracy due to subjectivity. Here, we present an alternative method to measure vomocytosis rates using a multi-fluorophore reporter system comprised of two in situ staining steps during infection and a flow cytometry readout. This approach overcomes the limitations of conventional time lapse microscopy methods, with key advantages of high throughput capability, simple procedural steps, and accurate objective readouts. This study rigorously characterizes this vomocytosis reporter system in CN-infected MΦ and DC cultures via fluorescence microscopy, confocal microscopy, and flow cytometry. Here, this fluorescent tool is used to observe differences in expulsion rates after phagosome-modifying drug treatments and additionally utilized to distinguish differences in biochemical compositions among fluorescence-activated cell sorted fungal populations via Raman spectroscopy. Furthermore, this reporter scheme is demonstrated to be adaptable for use in measuring potential biomaterial particle expulsion events. Ultimately, the fluorescent reporter system presented here provides a universal tool for vomocytosis rate measurement of phagocytosed material. This facile approach opens the door to previously unfeasible types of vomocytosis-related studies such as high throughput treatment mechanistic screening and downstream characterization of expelled material.

Innate Pulmonary Phagocytes and Their Interactions with Pathogenic Cryptococcus Species

Cryptococcus neoformans is an opportunistic fungal pathogen that causes over 180,000 annual deaths in HIV/AIDS patients. Innate phagocytes in the lungs, such as dendritic cells (DCs) and macrophages, are the first cells to interact with the pathogen. Neutrophils, another innate phagocyte, are recruited to the lungs during cryptococcal infection. These innate cells are involved in early detection of C. neoformans, as well as the removal and clearance of cryptococcal infections. However, C. neoformans has developed ways to interfere with these processes, allowing for the evasion of the host's innate immune system. Additionally, the innate immune cells have the ability to aid in cryptococcal pathogenesis. This review discusses recent literature on the interactions of innate pulmonary phagocytes with C. neoformans.

Vomocytosis of Cryptococcus neoformans cells from murine, bone marrow-derived dendritic cells

Cryptococcus neoformans (CN) cells survive within the acidic phagolysosome of macrophages (MΦ) for extended times, then escape without impacting the viability of the host cell via a phenomenon that has been coined 'vomocytosis'. Through this mechanism, CN disseminate throughout the body, sometimes resulting in a potentially fatal condition-Cryptococcal Meningitis (CM). Justifiably, vomocytosis studies have focused primarily on MΦ, as alveolar MΦ within the lung act as first responders that ultimately expel this fungal pathogen. Herein, we hypothesize that dendritic cells (DCs), an innate immune cell with attributes that include phagocytosis and antigen presentation, can also act as 'vomocytes'. Presciently, this report shows that vomocytosis of CN indeed occurs from murine, bone marrow-derived DCs. Primarily through time-lapse microscopy imaging, we show that rates of vomocytosis events from DCs are comparable to those seen from MΦ and further, are independent of the presence of the CN capsule and infection ratios. Moreover, the phagosome-altering drug bafilomycin A inhibits this phenomenon from DCs. Although DC immunophenotype does not affect the total number of vomocytic events, we observed differences in the numbers of CN per phagosome and expulsion times. Interestingly, these observations were similar in murine, bone marrow-derived MΦ. This work not only demonstrates the vomocytic ability of DCs, but also investigates the complexity of vomocytosis regulation in this cell type and MΦ under multiple modulatory conditions. Understanding the vomocytic behavior of different phagocytes and their phenotypic subtypes is needed to help elucidate the full picture of the dynamic interplay between CN and the immune system. Critically, deeper insight into vomocytosis could reveal novel approaches to treat CM, as well as other immune-related conditions.

Host immune responses in the central nervous system during fungal infections

Fungal infections in the central nervous system (CNS) cause high morbidity and mortality. The frequency of CNS mycosis has increased over the last two decades as more individuals go through immunocompromised conditions for various reasons. Nevertheless, options for clinical interventions for CNS mycoses are still limited. Thus, there is an urgent need to understand the host-pathogen interaction mechanisms in CNS mycoses for developing novel treatments. Although the CNS has been regarded as an immune-privileged site, recent studies demonstrate the critical involvement of immune responses elicited by CNS-resident and CNS-infiltrated cells during fungal infections. In this review, we discuss mechanisms of fungal invasion in the CNS, fungal pathogen detection by CNS-resident cells (microglia, astrocytes, oligodendrocytes, neurons), roles of CNS-infiltrated leukocytes, and host immune responses. We consider that understanding host immune responses in the CNS is crucial for endeavors to develop treatments for CNS mycosis.

Fungal CNS Infections in Africa: The Neuroimmunology of Cryptococcal Meningitis

Cryptococcal meningitis (CM) is the leading cause of central nervous system (CNS) fungal infections in humans, with the majority of cases reported from the African continent. This is partly due to the high burden of HIV infection in the region and reduced access to standard-of-care including optimal sterilising antifungal drug treatments. As such, CM is responsible for 10-15% of all HIV-related mortality, with a large proportion being preventable. Immunity to the causative agent of CM, Cryptococcus neoformans, is only partially understood. IFNγ producing CD4⁺ T-cells are required for the activation of myeloid cells, especially macrophages, to enable fungal killing and clearance. However, macrophages may also act as a reservoir of the fungal yeast cells, shielding them from host immune detection thus promoting latent infection or persistent chronic inflammation. In this chapter, we review the epidemiology and pathogenesis of CNS fungal infections in Africa, with a major focus on CM, and the antifungal immune pathways operating to protect against C. neoformans infection. We also highlight the areas of research and policy that require prioritisation to help reduce the burden of CNS fungal diseases in Africa.

Fine Particulate Matter (PM2.5) Promotes CD146 Expression in Alveolar Epithelial Cells and Cryptococcus neoformans Pulmonary Infection

Air pollution is a leading cause of increasing infectious lung diseases. Pulmonary cryptococcosis is a fatal fungal pneumonia in acquired immunodeficiency syndrome patients. In some cases, the pathogen Cryptococcus neoformans also develops dormant nodules in immunocompetent individuals. In the present study, we demonstrated that fine particulate matter (PM₂.₅) increased CD146 expression in alveolar epithelial cells and promoted C. neoformans pulmonary infection. Aryl hydrocarbon receptor (AhR) signaling was required for increased expression of CD146 in epithelial cells treated with PM₂.₅. In a murine model of pulmonary infection, PM₂.₅ promoted fungal infection, and CD146 deficiency decreased the fugal burden of C. neoformans. Our study may highlight the importance of air pollution to lung mycosis and CD146 as a target for preventing infectious lung diseases.

Animal Models of Cryptococcus neoformans in Identifying Immune Parameters Associated With Primary Infection and Reactivation of Latent Infection

Cryptococcus species are environmental fungal pathogens and the causative agents of cryptococcosis. Infection occurs upon inhalation of infectious particles, which proliferate in the lung causing a primary infection. From this primary lung infection, fungal cells can eventually disseminate to other organs, particularly the brain, causing lethal meningoencephalitis. However, in most cases, the primary infection resolves with the formation of a lung granuloma. Upon severe immunodeficiency, dormant cryptococcal cells will start proliferating in the lung granuloma and eventually will disseminate to the brain. Many investigators have sought to study the protective host immune response to this pathogen in search of host parameters that keep the proliferation of cryptococcal cells under control. The majority of the work assimilates research carried out using the primary infection animal model, mainly because a reactivation model has been available only very recently. This review will focus on anti-cryptococcal immunity in both the primary and reactivation models. An understanding of the differences in host immunity between the primary and reactivation models will help to define the key host parameters that control the infections and are important for the research and development of new therapeutic and vaccine strategies against cryptococcosis.

The Diverse Roles of Phagocytes During Bacterial and Fungal Infections and Sterile Inflammation: Lessons From Zebrafish

The immediate and natural reaction to both infectious challenges and sterile insults (wounds, tissue trauma or crystal deposition) is an acute inflammatory response. This inflammatory response is mediated by activation of the innate immune system largely comprising professional phagocytes (neutrophils and macrophages). Zebrafish (danio rerio) larvae possess many advantages as a model organism, including their genetic tractability and highly conserved innate immune system. Exploiting these attributes and the live imaging potential of optically transparent zebrafish larvae has greatly contributed to our understanding of how neutrophils and macrophages orchestrate the initiation and resolution phases of inflammatory responses. Numerous bacterial and fungal infection models have been successfully established using zebrafish as an animal model and studies investigating neutrophil and macrophage behavior to sterile insults have also provided unique insights. In this review we highlight how examining the larval zebrafish response to specific bacterial and fungal pathogens has uncovered cellular and molecular mechanisms behind a variety of phagocyte responses, from those that protect the host to those that are detrimental. We also describe how modeling sterile inflammation in larval zebrafish has provided an opportunity to dissect signaling pathways that control the recruitment, and fate, of phagocytes at inflammatory sites. Finally, we briefly discuss some current limitations, and opportunities to improve, the zebrafish model system for studying phagocyte biology.