Moesin activation controls bone resorption and tunneling nanotube-dependent osteoclast fusion

Osteoclasts are multinucleated cells unique in their ability to resorb bone. Osteoclastogenesis involves several steps of actin-driven rearrangements that participate not only in the cell-cell fusion process, but also in the formation of the sealing zone, the adhesive structure determining the resorption area. Despite the importance of these actin cytoskeleton-based processes, their precise mechanisms of regulation are still poorly characterized. Here, we found that moesin, a member of the Ezrin/Radixin/Moesin (ERM) protein family, is activated during osteoclast maturation and plays an instrumental role for both osteoclast fusion and function. In mouse and human osteoclast precursors, moesin is negatively regulated to potentiate their ability to fuse and degrade bone. Accordingly, we demonstrated that moesin depletion decreases membrane-to-cortex attachment and enhances formation of tunneling nanotubes (TNTs), F-actin-containing intercellular bridges that we revealed to trigger osteoclast fusion. In addition, via a β3-integrin/RhoA/SLK pathway and independently of its role in fusion, moesin regulates the number and organization of sealing zones in mature osteoclast, and thus participates in the control of bone resorption. Supporting these findings, we found that moesin-deficient mice are osteopenic with a reduced density of trabecular bones and increased osteoclast abundance and activity. These findings provide a better understanding of the regulation of osteoclast biology, and open new opportunities to specifically target osteoclast activity in bone disease therapy.

Tunneling nanotubes and tumor microtubes-Emerging data on their roles in intercellular communication and pathophysiology: Summary of an International FASEB Catalyst Conference October 2023

In the past decade, there has been a steady rise in interest in studying novel cellular extensions and their potential roles in facilitating human diseases, including neurologic diseases, viral infectious diseases, cancer, and others. One of the exciting new aspects of this field is improved characterization and understanding of the functions and potential mechanisms of tunneling nanotubes (TNTs), which are actin-based filamentous protrusions that are structurally distinct from filopodia. TNTs form and connect cells at long distance and serve as direct conduits for intercellular communication in a wide range of cell types in vitro and in vivo. More researchers are entering this field and investigating the role of TNTs in mediating cancer cell invasion and drug resistance, cellular transfer of proteins, RNA or organelles, and intercellular spread of infectious agents, such as viruses, bacteria, and prions. Even further, the elucidation of highly functional membrane tubes called "tumor microtubes" (TMs) in incurable gliomas has further paved a new path for understanding how and why the tumor type is highly invasive at the cellular level and also resistant to standard therapies. Due to the wide-ranging and rapidly growing applicability of TNTs and TMs in pathophysiology across the spectrum of biology, it has become vital to bring researchers in the field together to discuss advances and the future of research in this important niche of protrusion biology.

Macrophages: Key Cellular Players in HIV Infection and Pathogenesis

Although cells of the myeloid lineages, including tissue macrophages and conventional dendritic cells, were rapidly recognized, in addition to CD4+ T lymphocytes, as target cells of HIV-1, their specific roles in the pathophysiology of infection were initially largely neglected. However, numerous studies performed over the past decade, both in vitro in cell culture systems and in vivo in monkey and humanized mouse animal models, led to growing evidence that macrophages play important direct and indirect roles as HIV-1 target cells and in pathogenesis. It has been recently proposed that macrophages are likely involved in all stages of HIV-1 pathogenesis, including virus transmission and dissemination, but above all, in viral persistence through the establishment, together with latently infected CD4+ T cells, of virus reservoirs in many host tissues, the major obstacle to virus eradication in people living with HIV. Infected macrophages are indeed found, very often as multinucleated giant cells expressing viral antigens, in almost all lymphoid and non-lymphoid tissues of HIV-1-infected patients, where they can probably persist for long period of time. In addition, macrophages also likely participate, directly as HIV-1 targets or indirectly as key regulators of innate immunity and inflammation, in the chronic inflammation and associated clinical disorders observed in people living with HIV, even in patients receiving effective antiretroviral therapy. The main objective of this review is therefore to summarize the recent findings, and also to revisit older data, regarding the critical functions of tissue macrophages in the pathophysiology of HIV-1 infection, both as major HIV-1-infected target cells likely found in almost all tissues, as well as regulators of innate immunity and inflammation during the different stages of HIV-1 pathogenesis.

Giant cells: multiple cells unite to survive

Multinucleated Giant Cells (MGCs) are specialized cells that develop from the fusion of multiple cells, and their presence is commonly observed in human cells during various infections. However, MGC formation is not restricted to infections alone but can also occur through different mechanisms, such as endoreplication and abortive cell cycle. These processes lead to the formation of polyploid cells, eventually resulting in the formation of MGCs. In Entamoeba, a protozoan parasite that causes amoebic dysentery and liver abscesses in humans, the formation of MGCs is a unique phenomenon and not been reported in any other protozoa. This organism is exposed to various hostile environmental conditions, including changes in temperature, pH, and nutrient availability, which can lead to stress and damage to its cells. The formation of MGCs in Entamoeba is thought to be a survival strategy to cope with these adverse conditions. This organism forms MGCs through cell aggregation and fusion in response to osmotic and heat stress. The MGCs in Entamoeba are thought to have increased resistance to various stresses and can survive longer than normal cells under adverse conditions. This increased survival could be due to the presence of multiple nuclei, which could provide redundancy in case of DNA damage or mutations. Additionally, MGCs may play a role in the virulence of Entamoeba as they are found in the inflammatory foci of amoebic liver abscesses and other infections caused by Entamoeba. The presence of MGCs in these infections suggests that they may contribute to the pathogenesis of the disease. Overall, this article offers valuable insights into the intriguing phenomenon of MGC formation in Entamoeba. By unraveling the mechanisms behind this process and examining its implications, researchers can gain a deeper understanding of the complex biology of Entamoeba and potentially identify new targets for therapeutic interventions. The study of MGCs in Entamoeba serves as a gateway to exploring the broader field of cell fusion in various organisms, providing a foundation for future investigations into related cellular processes and their significance in health and disease.

Massively HIV-1-infected macrophages exhibit a severely hampered ability to differentiate into osteoclasts

Introduction

Osteoclasts play a crucial role in bone resorption, and impairment of their differentiation can have significant implications for bone density, especially in individuals with HIV who may be at risk of altered bone health. The present study aimed to investigate the effects of HIV infection on osteoclast differentiation using primary human monocyte-derived macrophages as precursors. The study focused on assessing the impact of HIV infection on cellular adhesion, cathepsin K expression, resorptive activity, cytokine production, expression of co-receptors, and transcriptional regulation of key factors involved in osteoclastogenesis.

Methods

Primary human monocyte-derived macrophages were utilized as precursors for osteoclast differentiation. These precursors were infected with HIV, and the effects of different inoculum sizes and kinetics of viral replication were analyzed. Subsequently, osteoclastogenesis was evaluated by measuring cellular adhesion, cathepsin K expression, and resorptive activity. Furthermore, cytokine production was assessed by monitoring the production of IL-1β, RANK-L, and osteoclasts. The expression levels of co-receptors CCR5, CD9, and CD81 were measured before and after infection with HIV. The transcriptional levels of key factors for osteoclastogenesis (RANK, NFATc1, and DC-STAMP) were examined following HIV infection.

Results

Rapid, massive, and productive HIV infection severely impaired osteoclast differentiation, leading to compromised cellular adhesion, cathepsin K expression, and resorptive activity. HIV infection resulted in an earlier production of IL-1β concurrent with RANK-L, thereby suppressing osteoclast production. Infection with a high inoculum of HIV increased the expression of the co-receptor CCR5, as well as the tetraspanins CD9 and CD81, which correlated with deficient osteoclastogenesis. Massive HIV infection of osteoclast precursors affected the transcriptional levels of key factors involved in osteoclastogenesis, including RANK, NFATc1, and DC-STAMP.

Conclusions

The effects of HIV infection on osteoclast precursors were found to be dependent on the size of the inoculum and the kinetics of viral replication. These findings underscore the importance of understanding the underlying mechanisms to develop novel strategies for the prevention and treatment of bone disorders in individuals with HIV.

Productive HIV-1 infection of tissue macrophages by fusion with infected CD4+ T cells

Macrophages are essential for HIV-1 pathogenesis and represent major viral reservoirs. Therefore, it is critical to understand macrophage infection, especially in tissue macrophages, which are widely infected in vivo, but poorly permissive to cell-free infection. Although cell-to-cell transmission of HIV-1 is a determinant mode of macrophage infection in vivo, how HIV-1 transfers toward macrophages remains elusive. Here, we demonstrate that fusion of infected CD4+ T lymphocytes with human macrophages leads to their efficient and productive infection. Importantly, several tissue macrophage populations undergo this heterotypic cell fusion, including synovial, placental, lung alveolar, and tonsil macrophages. We also find that this mode of infection is modulated by the macrophage polarization state. This fusion process engages a specific short-lived adhesion structure and is controlled by the CD81 tetraspanin, which activates RhoA/ROCK-dependent actomyosin contractility in macrophages. Our study provides important insights into the mechanisms underlying infection of tissue-resident macrophages, and establishment of persistent cellular reservoirs in patients.

Mechanisms of HIV-1 cell-to-cell transfer to myeloid cells

In addition to CD4+ T lymphocytes, cells of the myeloid lineage such as macrophages, dendritic cells (DCs), and osteoclasts (OCs) are emerging as important target cells for HIV-1, as they likely participate in all steps of pathogenesis, including sexual transmission and early virus dissemination in both lymphoid and nonlymphoid tissues where they can constitute persistent virus reservoirs. At least in vitro, these myeloid cells are poorly infected by cell-free viral particles. In contrast, intercellular virus transmission through direct cell-to-cell contacts may be a predominant mode of virus propagation in vivo leading to productive infection of these myeloid target cells. HIV-1 cell-to-cell transfer between CD4+ T cells mainly through the formation of the virologic synapse, or from infected macrophages or dendritic cells to CD4+ T cell targets, have been extensively described in vitro. Recent reports demonstrate that myeloid cells can be also productively infected through virus homotypic or heterotypic cell-to-cell transfer between macrophages or from virus-donor-infected CD4+ T cells, respectively. These modes of infection of myeloid target cells lead to very efficient spreading in these poorly susceptible cell types. Thus, the goal of this review is to give an overview of the different mechanisms reported in the literature for cell-to-cell transfer and spreading of HIV-1 in myeloid cells.

Dysregulation of the IFN-I signaling pathway by Mycobacterium tuberculosis leads to exacerbation of HIV-1 infection of macrophages

While tuberculosis (TB) is a risk factor in HIV-1-infected individuals, the mechanisms by which Mycobacterium tuberculosis (Mtb), the agent of TB in humans, worsens HIV-1 pathogenesis still need to be fully elucidated. Recently, we showed that HIV-1 infection and spread are exacerbated in macrophages exposed to TB-associated microenvironments. Transcriptomic analysis of macrophages conditioned with medium of Mtb-infected human macrophages (cmMTB) revealed an up-regulation of the typeI interferon (IFN-I) pathway, characterized by the overexpression of IFN-inducible genes. Historically, IFN-I are well known for their antiviral functions, but our previous work showed that this is not the case in the context of coinfection with HIV-1. Here, we show that the IFN-I response signature in cmMTB-treated macrophages matches the one observed in the blood of active TB patients, and depends on the timing of incubation with cmMTB. This suggests that the timing of macrophage's exposure to IFN-I can impact their capacity to control HIV-1 infection. Strikingly, we found that cmMTB-treated macrophages are hyporesponsive to extrastimulation with exogenous IFN-I, used to mimic HIV-1 infection. Yet, depleting STAT1 by gene silencing to block the IFN-I signaling pathway reduced TB-induced exacerbation of HIV-1 infection. Altogether, by aiming to understand why TB-derived IFN-I preexposure of macrophages did not induce antiviral immunity against HIV-1, we demonstrated that these cells are hyporesponsive to exogenous IFN-I, a phenomenon that prevents macrophage activation against HIV-1.

Non-Human Primate Models of HIV Brain Infection and Cognitive Disorders

Human Immunodeficiency virus (HIV)-associated neurocognitive disorders are a major burden for people living with HIV whose viremia is stably suppressed with antiretroviral therapy. The pathogenesis of disease is likely multifaceted, with contributions from viral reservoirs including the brain, chronic and systemic inflammation, and traditional risk factors including drug use. Elucidating the effects of each element on disease pathogenesis is near impossible in human clinical or ex vivo studies, facilitating the need for robust and accurate non-human primate models. In this review, we describe the major non-human primate models of neuroHIV infection, their use to study the acute, chronic, and virally suppressed infection of the brain, and novel therapies targeting brain reservoirs and inflammation.

The circle of life: Phases of podosome formation, turnover and reemergence

Podosomes are highly dynamic actin-rich structures in a variety of cell types, especially monocytic cells. They fulfill multiple functions such as adhesion, mechanosensing, or extracellular matrix degradation, thus allowing cells to detect and respond to a changing environment. These abilities are based on an intricate architecture that enables podosomes to sense mechanical properties of their substratum and to transduce them intracellularly in order to generate an appropriate cellular response. These processes are enabled through the tightly orchestrated interplay of more than 300 different components that are dynamically recruited during podosome formation and turnover. In this review, we discuss the different phases of the podosome life cycle and the current knowledge on regulatory factors that impact on the genesis, activity, dissolution and reemergence of podosomes. We also highlight mechanoregulatory processes that become important during these different stages, on the level of individual podosomes, and also at podosome sub- and superstructures.

The balance between p53 isoforms modulates the efficiency of HIV-1 infection in macrophages

Several host factors influence HIV-1 infection and replication. The p53-mediated antiviral role in monocytes-derived macrophages (MDMs) was previously highlighted. Indeed, an increase in p53 level results in a stronger restriction against HIV-1 early replication steps through SAMHD1 activity. In this study, we investigated the potential role of some p53 isoforms in HIV-1 infection. Transfection of isoform-specific siRNA induces distinctive effects on the virus life cycle. For example, in contrast to a siRNA targeting all isoforms, a knockdown of Δ133p53 transcripts reduces virus replication in MDMs that is correlated with a decrease in phosphorylated inactive SAMHD1. Combination of Δ133p53 knockdown and Nutlin-3, a pharmacological inhibitor of MDM2 that stabilizes p53, further reduces susceptibility of MDMs to HIV-1 infection, thus suggesting an inhibitory role of Δ133p53 towards p53 antiviral activity. In contrast, p53β knockdown in MDMs increases the viral production independently of SAMHD1. Moreover, experiments with a Nef-deficient virus show that this viral protein plays a protective role against the antiviral environment mediated by p53. Finally, HIV-1 infection affects the expression pattern of p53 isoforms by increasing p53β and p53γ mRNA levels while stabilizing the protein level of p53α and some isoforms from the p53β subclass. The balance between the various p53 isoforms is therefore an important factor in the overall susceptibility of macrophages to HIV-1 infection, fine-tuning the p53 response against HIV-1. This study brings a new understanding of the complex role of p53 in virus replication processes in myeloid cells. Importance As of today, HIV-1 is still considered as a global pandemic without a functional cure, partly because of the presence of stable viral reservoirs. Macrophages constitute one of these cell reservoirs, contributing to the viral persistence. Studies investigating the host factors involved in cell susceptibility to HIV-1 infection might lead to a better understanding of the reservoir formation and will eventually allow the development of an efficient cure. Our team previously showed the antiviral role of p53 in macrophages, which acts by compromising the early steps of HIV-1 replication. In this study, we demonstrate the involvement of p53 isoforms, which regulates p53 activity and define the cellular environment influencing viral replication. In addition, the results concerning the potential role of p53 in antiviral innate immunity could be transposed to other fields of virology and suggest that knowledge in oncology can be applied to HIV-1 research.

Cellular and molecular actors of myeloid cell fusion: podosomes and tunneling nanotubes call the tune

Different types of multinucleated giant cells (MGCs) of myeloid origin have been described; osteoclasts are the most extensively studied because of their importance in bone homeostasis. MGCs are formed by cell-to-cell fusion, and most types have been observed in pathological conditions, especially in infectious and non-infectious chronic inflammatory contexts. The precise role of the different MGCs and the mechanisms that govern their formation remain poorly understood, likely due to their heterogeneity. First, we will introduce the main populations of MGCs derived from the monocyte/macrophage lineage. We will then discuss the known molecular actors mediating the early stages of fusion, focusing on cell-surface receptors involved in the cell-to-cell adhesion steps that ultimately lead to multinucleation. Given that cell-to-cell fusion is a complex and well-coordinated process, we will also describe what is currently known about the evolution of F-actin-based structures involved in macrophage fusion, i.e., podosomes, zipper-like structures, and tunneling nanotubes (TNT). Finally, the localization and potential role of the key fusion mediators related to the formation of these F-actin structures will be discussed. This review intends to present the current status of knowledge of the molecular and cellular mechanisms supporting multinucleation of myeloid cells, highlighting the gaps still existing, and contributing to the proposition of potential disease-specific MGC markers and/or therapeutic targets.

Virus-Mediated Cell-Cell Fusion

Cell-cell fusion between eukaryotic cells is a general process involved in many physiological and pathological conditions, including infections by bacteria, parasites, and viruses. As obligate intracellular pathogens, viruses use intracellular machineries and pathways for efficient replication in their host target cells. Interestingly, certain viruses, and, more especially, enveloped viruses belonging to different viral families and including human pathogens, can mediate cell-cell fusion between infected cells and neighboring non-infected cells. Depending of the cellular environment and tissue organization, this virus-mediated cell-cell fusion leads to the merge of membrane and cytoplasm contents and formation of multinucleated cells, also called syncytia, that can express high amount of viral antigens in tissues and organs of infected hosts. This ability of some viruses to trigger cell-cell fusion between infected cells as virus-donor cells and surrounding non-infected target cells is mainly related to virus-encoded fusion proteins, known as viral fusogens displaying high fusogenic properties, and expressed at the cell surface of the virus-donor cells. Virus-induced cell-cell fusion is then mediated by interactions of these viral fusion proteins with surface molecules or receptors involved in virus entry and expressed on neighboring non-infected cells. Thus, the goal of this review is to give an overview of the different animal virus families, with a more special focus on human pathogens, that can trigger cell-cell fusion.

HIV-1-Infected Human Macrophages, by Secreting RANK-L, Contribute to Enhanced Osteoclast Recruitment

HIV-1 infection is frequently associated with low bone density, which can progress to osteoporosis leading to a high risk of fractures. Only a few mechanisms have been proposed to explain the enhanced osteolysis in the context of HIV-1 infection. As macrophages are involved in bone homeostasis and are critical host cells for HIV-1, we asked whether HIV-1-infected macrophages could participate in bone degradation. Upon infection, human macrophages acquired some osteoclast features: they became multinucleated, upregulated the osteoclast markers RhoE and β3 integrin, and organized their podosomes as ring superstructures resembling osteoclast sealing zones. However, HIV-1-infected macrophages were not fully differentiated in osteoclasts as they did not upregulate NFATc-1 transcription factor and were unable to degrade bone. Investigating whether infected macrophages participate indirectly to virus-induced osteolysis, we showed that they produce RANK-L, the key osteoclastogenic cytokine. RANK-L secreted by HIV-1-infected macrophages was not sufficient to stimulate multinucleation, but promoted the protease-dependent migration of osteoclast precursors. In conclusion, we propose that, by stimulating RANK-L secretion, HIV-1-infected macrophages contribute to create a microenvironment that favors the recruitment of osteoclasts, participating in bone disorders observed in HIV-1 infected patients.

Tuberculosis-associated IFN-I induces Siglec-1 on tunneling nanotubes and favors HIV-1 spread in macrophages

While tuberculosis (TB) is a risk factor in HIV-1-infected individuals, the mechanisms by which Mycobacterium tuberculosis (Mtb) worsens HIV-1 pathogenesis remain scarce. We showed that HIV-1 infection is exacerbated in macrophages exposed to TB-associated microenvironments due to tunneling nanotube (TNT) formation. To identify molecular factors associated with TNT function, we performed a transcriptomic analysis in these macrophages, and revealed the up-regulation of Siglec-1 receptor. Siglec-1 expression depends on Mtb-induced production of type I interferon (IFN-I). In co-infected non-human primates, Siglec-1 is highly expressed by alveolar macrophages, whose abundance correlates with pathology and activation of IFN-I/STAT1 pathway. Siglec-1 localizes mainly on microtubule-containing TNT that are long and carry HIV-1 cargo. Siglec-1 depletion decreases TNT length, diminishes HIV-1 capture and cell-to-cell transfer, and abrogates the exacerbation of HIV-1 infection induced by Mtb. Altogether, we uncover a deleterious role for Siglec-1 in TB-HIV-1 co-infection and open new avenues to understand TNT biology.

HIV Infection Stabilizes Macrophage-T Cell Interactions To Promote Cell-Cell HIV Spread

Macrophages are susceptible to HIV infection and play an important role in viral dissemination through cell-cell contacts with T cells. However, our current understanding of macrophage-to-T cell HIV transmission is derived from studies that do not consider the robust migration and cell-cell interaction dynamics between these cells. Here, we performed live-cell imaging studies in 3-dimensional (3D) collagen that allowed CD4⁺ T cells to migrate and to locate and engage HIV-infected macrophages, modeling the dynamic aspects of the in situ environment in which these contacts frequently occur. We show that HIV⁺ macrophages form stable contacts with CD4⁺ T cells that are facilitated by both gp120-CD4 and LFA-1-ICAM-1 interactions and that prolonged contacts are a prerequisite for efficient viral spread. LFA-1-ICAM-1 adhesive contacts function to restrain highly motile T cells, since their blockade substantially destabilized macrophage-T cell contacts, resulting in abnormal tethering events that reduced cell-cell viral spread. HIV-infected macrophages displayed strikingly elongated podosomal extensions that were dependent on Nef expression but were dispensable for stable cell-cell contact formation. Finally, we observed persistent T cell infection in dynamic monocyte-derived macrophage (MDM)-T cell cocultures in the presence of single high antiretroviral drug concentrations but achieved complete inhibition with combination therapy. Together, our data implicate macrophages as drivers of T cell infection by altering physiological MDM-T cell contact dynamics to access and restrain large numbers of susceptible, motile T cells within lymphoid tissues.IMPORTANCE Once HIV enters the lymphoid organs, exponential viral replication in T cells ensues. Given the densely packed nature of these tissues, where infected and uninfected cells are in nearly constant contact with one another, efficient HIV spread is thought to occur through cell-cell contacts in vivo However, this has not been formally demonstrated. In this study, we performed live-cell imaging studies within a 3-dimensional space to recapitulate the dynamic aspects of the lymphoid microenvironment and asked whether HIV can alter the morphology, migration capacity, and cell-cell contact behaviors between macrophages and T cells. We show that HIV-infected macrophages can engage T cells in stable contacts through binding of virus- and host-derived adhesive molecules and that stable macrophage-T cell contacts were required for high viral spread. Thus, HIV alters physiological macrophage-T cell interactions in order to access and restrain large numbers of susceptible, motile T cells, thereby playing an important role in HIV progression.

Tumor Necrosis Factor-Alpha Antagonist Interferes With the Formation of Granulomatous Multinucleated Giant Cells: New Insights Into Mycobacterium tuberculosis Infection

More than half of tuberculosis cases in the world are due to resuscitation of dormant Mycobacterium tuberculosis (Mtb) sequestered into cell-derived structures called granulomas. It is fairly admitted that cytokines and more particularly Tumor Necrosis Factor (TNF)-α is critical in the control of Mtb infections and that anti-TNF-α drugs constitute one of the main risk factors for reactivation of latent Mtb infection. The aim of this study was to evaluate the role of etanercept, a dimeric fusion protein consisting of the extracellular ligand-binding portion of the human p75 TNF receptor linked to the Fc portion of human IgG1, in an in vitro model of human tuberculous granuloma. We showed that etanercept slightly delayed the formation of granuloma and reduced the generation of multinuclear giant cells (MGCs). In addition, etanercept exacerbated the expression of M1 polarization genes but also induced interleukin (IL)-10 release. In addition, our results indicated that etanercept inhibited cell fusion in an IL-10-dependent manner. Moreover, adalimumab, a human monoclonal anti-TNF-α IgG1 inhibited MGC formation in granuloma, without altering IL-10 secretion and induced macrophage apoptosis. Taken together, our data provides new insights into the role of TNF-α blockers in MGCs formation and the impact of such immunomodulatory drugs on tuberculous granuloma maturation.

Physiological relevance of ACOT8-Nef interaction in HIV infection

During human immunodeficiency virus (HIV) infection, Nef viral protein plays a crucial role in viral pathogenesis and progression of acquired immunodeficiency syndrome. Nef is expressed in the early stages of infection and alters the cellular environment increasing infectivity, viral replication, and the evasion of host immune response through several mechanisms. Nef has numerous functional domains that allow it to interact with a number of proteins, interfering with intracellular traffic. Among these proteins, human peroxisomal thioesterase 8, ACOT8, has been shown to be an important cellular partner of Nef. It has been suggested that this interaction may be involved in Nef-dependent endocytosis and also in the modulation of lipid composition in membrane rafts. However, the actual role of this interaction, as well as the mechanisms involved, has not yet been fully elucidated. In this review, we focused on the interplay between Nef and ACOT8 proteins, highlighting the possible physiological relevance in HIV infection.

Tuberculosis Exacerbates HIV-1 Infection through IL-10/STAT3-Dependent Tunneling Nanotube Formation in Macrophages

The tuberculosis (TB) bacillus, Mycobacterium tuberculosis (Mtb), and HIV-1 act synergistically; however, the mechanisms by which Mtb exacerbates HIV-1 pathogenesis are not well known. Using in vitro and ex vivo cell culture systems, we show that human M(IL-10) anti-inflammatory macrophages, present in TB-associated microenvironment, produce high levels of HIV-1. In vivo, M(IL-10) macrophages are expanded in lungs of co-infected non-human primates, which correlates with disease severity. Furthermore, HIV-1/Mtb co-infected patients display an accumulation of M(IL-10) macrophage markers (soluble CD163 and MerTK). These M(IL-10) macrophages form direct cell-to-cell bridges, which we identified as tunneling nanotubes (TNTs) involved in viral transfer. TNT formation requires the IL-10/STAT3 signaling pathway, and targeted inhibition of TNTs substantially reduces the enhancement of HIV-1 cell-to-cell transfer and overproduction in M(IL-10) macrophages. Our study reveals that TNTs facilitate viral transfer and amplification, thereby promoting TNT formation as a mechanism to be explored in TB/AIDS potential therapeutics.

Bone degradation machinery of osteoclasts: An HIV-1 target that contributes to bone loss

Bone deficits are frequent in HIV-1-infected patients. We report here that osteoclasts, the cells specialized in bone resorption, are infected by HIV-1 in vivo in humanized mice and ex vivo in human joint biopsies. In vitro, infection of human osteoclasts occurs at different stages of osteoclastogenesis via cell-free viruses and, more efficiently, by transfer from infected T cells. HIV-1 infection markedly enhances adhesion and osteolytic activity of human osteoclasts by modifying the structure and function of the sealing zone, the osteoclast-specific bone degradation machinery. Indeed, the sealing zone is broader due to F-actin enrichment of its basal units (i.e., the podosomes). The viral protein Nef is involved in all HIV-1-induced effects partly through the activation of Src, a regulator of podosomes and of their assembly as a sealing zone. Supporting these results, Nef-transgenic mice exhibit an increased osteoclast density and bone defects, and osteoclasts derived from these animals display high osteolytic activity. Altogether, our study evidences osteoclasts as host cells for HIV-1 and their pathological contribution to bone disorders induced by this virus, in part via Nef.

Mechanisms for Cell-to-Cell Transmission of HIV-1

While HIV-1 infection of target cells with cell-free viral particles has been largely documented, intercellular transmission through direct cell-to-cell contact may be a predominant mode of propagation in host. To spread, HIV-1 infects cells of the immune system and takes advantage of their specific particularities and functions. Subversion of intercellular communication allows to improve HIV-1 replication through a multiplicity of intercellular structures and membrane protrusions, like tunneling nanotubes, filopodia, or lamellipodia-like structures involved in the formation of the virological synapse. Other features of immune cells, like the immunological synapse or the phagocytosis of infected cells are hijacked by HIV-1 and used as gateways to infect target cells. Finally, HIV-1 reuses its fusogenic capacity to provoke fusion between infected donor cells and target cells, and to form infected syncytia with high capacity of viral production and improved capacities of motility or survival. All these modes of cell-to-cell transfer are now considered as viral mechanisms to escape immune system and antiretroviral therapies, and could be involved in the establishment of persistent virus reservoirs in different host tissues.

Tunneling Nanotubes: Intimate Communication between Myeloid Cells

Tunneling nanotubes (TNT) are dynamic connections between cells, which represent a novel route for cell-to-cell communication. A growing body of evidence points TNT towards a role for intercellular exchanges of signals, molecules, organelles, and pathogens, involving them in a diverse array of functions. TNT form among several cell types, including neuronal cells, epithelial cells, and almost all immune cells. In myeloid cells (e.g., macrophages, dendritic cells, and osteoclasts), intercellular communication via TNT contributes to their differentiation and immune functions. Importantly, TNT enable myeloid cells to communicate with a targeted neighboring or distant cell, as well as with other cell types, therefore creating a complex variety of cellular exchanges. TNT also contribute to pathogen spread as they serve as “corridors” from a cell to another. Herein, we addressed the complexity of the definition and in vitro characterization of TNT in innate immune cells, the different processes involved in their formation, and their relevance in vivo. We also assess our current understanding of how TNT participate in immune surveillance and the spread of pathogens, with a particular interest for HIV-1. Overall, despite recent progress in this growing research field, we highlight that further investigation is needed to better unveil the role of TNT in both physiological and pathological conditions.

T Cell-Macrophage Fusion Triggers Multinucleated Giant Cell Formation for HIV-1 Spreading

HIV-1-infected macrophages participate in virus dissemination and establishment of virus reservoirs in host tissues, but the mechanisms for virus cell-to-cell transfer to macrophages remain unknown. Here, we reveal the mechanisms for cell-to-cell transfer from infected T cells to macrophages and virus spreading between macrophages. We show that contacts between infected T lymphocytes and macrophages lead to cell fusion for the fast and massive transfer of CCR5-tropic viruses to macrophages. Through the merge of viral material between T cells and macrophages, these newly formed lymphocyte-macrophage fused cells acquire the ability to fuse with neighboring noninfected macrophages. Together, these two-step envelope-dependent cell fusion processes lead to the formation of highly virus-productive multinucleated giant cells reminiscent of the infected multinucleated giant macrophages detected in HIV-1-infected patients and simian immunodeficiency virus-infected macaques. These mechanisms represent an original mode of virus transmission for viral spreading and a new model for the formation of macrophage virus reservoirs during infection.IMPORTANCE We reveal a very efficient mechanism involved in cell-to-cell transfer from infected T cells to macrophages and subsequent virus spreading between macrophages by a two-step cell fusion process. Infected T cells first establish contacts and fuse with macrophage targets. The newly formed lymphocyte-macrophage fused cells then acquire the ability to fuse with surrounding uninfected macrophages, leading to the formation of infected multinucleated giant cells that can survive for a long time, as evidenced in vivo in lymphoid organs and the central nervous system. This route of infection may be a major determinant for virus dissemination and the formation of macrophage virus reservoirs in host tissues during HIV-1 infection.

HIV-infected macrophages and microglia that survive acute infection become viral reservoirs by a mechanism involving Bim

While HIV kills most of the cells it infects, a small number of infected cells survive and become latent viral reservoirs, posing a significant barrier to HIV eradication. However, the mechanism by which immune cells resist HIV-induced apoptosis is still incompletely understood. Here, we demonstrate that while acute HIV infection of human microglia/macrophages results in massive apoptosis, a small population of HIV-infected cells survive infection, silence viral replication, and can reactivate viral production upon specific treatments. We also found that HIV fusion inhibitors intended for use as antiretroviral therapies extended the survival of HIV-infected macrophages. Analysis of the pro- and anti-apoptotic pathways indicated no significant changes in Bcl-2, Mcl-1, Bak, Bax or caspase activation, suggesting that HIV blocks a very early step of apoptosis. Interestingly, Bim, a highly pro-apoptotic negative regulator of Bcl-2, was upregulated and recruited into the mitochondria in latently HIV-infected macrophages both in vitro and in vivo. Together, these results demonstrate that macrophages/microglia act as HIV reservoirs and utilize a novel mechanism to prevent HIV-induced apoptosis. Furthermore, they also suggest that Bim recruitment to mitochondria could be used as a biomarker of viral reservoirs in vivo.

HIV-1-Mediated BAFF Secretion in Macrophages Does Not Require Endosomal TLRs, Type-I IFN, and Nef, but Depends on the Cellular Phenotype Status

HIV-1 infection is characterized by persistent viral replication, chronic immune activation, and CD4(+) T cell depletion. Moreover, several immune dysfunctions are observed in cells that are not targeted by the virus, such as B cells. Some B cell abnormalities include hypergammaglobulinemia, nonspecific B cell activation, class switching, increased cell turnover, breakage of tolerance, and a loss of the capacity to generate and maintain memory. Several cytokines and growth factors that are increased in the serum of HIV-1-infected individuals have been suggested to directly or indirectly trigger B cell activation, and one of these is BAFF. In this study, we investigate the ability of fully competent (R5-tropic) HIV-1 to induce BAFF production by monocyte-derived macrophages (MDMs). We demonstrate here that HIV-1 drives BAFF production in MDMs in a type-I IFN- and TLR-independent manner. Moreover, we determine that HIV-1 Nef accessory protein is dispensable in BAFF upregulation as a nef-deleted HIV-1 strain is still able to increase BAFF at levels similar to the wild type strain. Finally, we show that the macrophage phenotype status affects HIV-1 replication and BAFF induction, as both were abrogated in MDMs displaying a M1 phenotype. This study provides new useful information about the increased levels of BAFF observed during HIV-1 infection and highlights the importance of macrophages as a source of BAFF, a phenomenon that might contribute to B cell dysfunctions at inflammatory tissue sites in infected individuals.

Reliable and inexpensive expression of large, tagged, exogenous proteins in murine bone marrow-derived macrophages using a second generation lentiviral system

Over the past two decades, researchers have struggled to efficiently express foreign DNA in primary macrophages, impeding research progress. The applications of lipofection, electroporation, microinjection, and viral-mediated transfer typically result in disruptions in macrophage differentiation and function, low expression levels of exogenous proteins, limited efficiency and high cell mortality. In this report, after extensive optimization, we present a method of expressing large tagged proteins at high efficiency, consistency, and low cost using lentiviral infection. This method utilizes laboratory-propagated second generation plasmids to produce efficient virus that can be stored for later use. The expression of proteins up to 150 kDa in size is achieved in 30-70% of cells while maintaining normal macrophage differentiation and morphology as determined by fluorescence microscopy and Western blot analysis. This manuscript delineates the reagents and methods used to produce lentivirus to express exogenous DNA in murine bone marrow-derived macrophages sufficient for single cell microscopy as well as functional assays requiring large numbers of murine bone marrow-derived macrophages.

HIV-1 reprograms the migration of macrophages

Macrophages are motile leukocytes, targeted by HIV-1, thought to play a critical role in host dissemination of the virus. However, whether infection impacts their migration capacity remains unknown. We show that 2-dimensional migration and the 3-dimensional (3D) amoeboid migration mode of HIV-1-infected human monocyte-derived macrophages were inhibited, whereas the 3D mesenchymal migration was enhanced. The viral protein Nef was necessary and sufficient for all HIV-1-mediated effects on migration. In Nef transgenic mice, tissue infiltration of macrophages was increased in a tumor model and in several tissues at steady state, suggesting a dominant role for mesenchymal migration in vivo. The mesenchymal motility involves matrix proteolysis and podosomes, cell structures constitutive of monocyte-derived cells. Focusing on the mechanisms used by HIV-1 Nef to control the mesenchymal migration, we show that the stability, size, and proteolytic function of podosomes are increased via the phagocyte-specific kinase Hck and Wiskott-Aldrich syndrome protein (WASP), 2 major regulators of podosomes. In conclusion, HIV-1 reprograms macrophage migration, which likely explains macrophage accumulation in several patient tissues, which is a key step for virus spreading and pathogenesis. Moreover, Nef points out podosomes and the Hck/WASP signaling pathway as good candidates to control tissue infiltration of macrophages, a detrimental phenomenon in several diseases.

An efficient siRNA-mediated gene silencing in primary human monocytes, dendritic cells and macrophages

Mononuclear phagocytes (MP) comprise monocytes, macrophages (MΦ) and dendritic cells (DC), including their lineage-committed progenitors, which together have an eminent role in health and disease. Lipid-based siRNA-mediated gene inactivation is an established approach to investigate gene function in MP cells. However, although there are few protocols dedicated for siRNA-mediated gene inactivation in primary human DC and MΦ, there are none available for primary human monocytes. Moreover, there is no available method to perform comparative studies of a siRNA-mediated gene silencing in primary monocytes and other MP cells. Here, we describe a protocol optimized for the lipid-based delivery of siRNA to perform gene silencing in primary human blood monocytes, which is applicable to DCs, and differs from the classical route of siRNA delivery into MΦs. Along with this protocol, we provide a comparative analysis of how monocytes, DC and MΦ are efficiently transfected with the target siRNA without affecting cell viability, resulting in strong gene knockdown efficiency, including the simultaneous inactivation of two genes. Moreover, siRNA delivery does not affect classical functions in MP such as differentiation, phagocytosis and migration, demonstrating that this protocol does not induce non-specific major alterations in these cells. As a proof-of-principle, a functional analysis of hematopoietic cell kinase (Hck) shows for the first time that this kinase regulates the protease-dependent migration mode in human monocytes. Collectively, this protocol enables efficient gene inactivation in primary MP, suggesting a wide spectrum of applications such as siRNA-based high-throughput screening, which could ultimately improve our knowledge about MP biology.

Placental macrophages are impaired in chorioamnionitis, an infectious pathology of the placenta

Pregnancy is dependent on maternal-fetal tolerance that may be compromised because of infections or inflammation of the placenta. In this study, we examined whether the context of placental immune tolerance affected the functions of resident macrophages and if their functions were altered during chorioamnionitis, an infectious pathology of the placenta. Macrophages from at-term placentas expressed CD14, exhibited macrophage microbicidal functions, but were less inflammatory than monocyte-derived macrophages. Moreover, placental macrophages spontaneously matured into multinucleated giant cells (MGCs), a property not exhibited by monocyte-derived macrophages, and we detected MGCs of myeloid origin in placental tissue. Compared with placental macrophages, MGCs exhibited a specific phenotype and gene expression signature, consisting of increased cytoskeleton-associated gene expression along with depressed expression of inflammatory response genes. Furthermore, placental macrophages from patients with chorioamnionitis were unable to form MGCs, but this defect was partially corrected by incubating these placental macrophages with control trophoblast supernatants. MGCs formation likely serves to regulate their inflammatory and cytocidal activities in a context that imposes semiallograft acceptance and defense against pathogens.

HIV-1 infection of T cells and macrophages are differentially modulated by virion-associated Hck: a Nef-dependent phenomenon

The proline repeat motif (PxxP) of Nef is required for interaction with the SH3 domains of macrophage-specific Src kinase Hck. However, the implication of this interaction for viral replication and infectivity in macrophages and T lymphocytes remains unclear. Experiments in HIV-1 infected macrophages confirmed the presence of a Nef:Hck complex which was dependent on the Nef proline repeat motif. The proline repeat motif of Nef also enhanced both HIV-1 infection and replication in macrophages, and was required for incorporation of Hck into viral particles. Unexpectedly, wild-type Hck inhibited infection of macrophages, but Hck was shown to enhance infection of primary T lymphocytes. These results indicate that the interaction between Nef and Hck is important for Nef-dependent modulation of viral infectivity. Hck-dependent enhancement of HIV-1 infection of T cells suggests that Nef-Hck interaction may contribute to the spread of HIV-1 infection from macrophages to T cells by modulating events in the producer cell, virion and target cell.

Contributions of HIV-1 Nef to immune dysregulation in HIV-infected patients: a therapeutic target?

INTRODUCTION

HIV accessory protein Nef is a factor responsible for many of the viral pathogenic effects. Progression to AIDS is dramatically delayed and in some well-documented cases completely abolished on infection with naturally occurring HIV strains lacking intact nef sequences in their genomes. The topic of this review is the contribution of Nef to the immune pathology as a possible target in HIV-infected patients.

AREAS COVERED

An overview of known Nef functions accounting for its role in pathogenesis is presented, emphasizing interactions with dendritic cells and macrophages, and Nef-induced exosome secretion, all involved in immune dysregulation during the course of HIV infection. Current approaches to Nef inhibition by different classes of compounds are reviewed.

EXPERT OPINION

Blocking Nef for therapeutic purposes is a challenging endeavor mainly due to intrinsic properties of this HIV accessory protein. Nef has multiple interfaces to interact with host proteins and lacks a catalytic domain. Potential benefits arising from the development of successful inhibitors could however prove beneficial for reducing gradual deterioration of immune system in chronically infected patients in absence of functional cure.

Hck contributes to bone homeostasis by controlling the recruitment of osteoclast precursors

In osteoclasts, Src controls podosome organization and bone degradation, which leads to an osteopetrotic phenotype in src(-/-) mice. Since this phenotype was even more severe in src(-/-)hck(-/-) mice, we examined the individual contribution of Hck in bone homeostasis. Compared to wt mice, hck(-/-) mice exhibited an osteopetrotic phenotype characterized by an increased density of trabecular bone and decreased bone degradation, although osteoclastogenesis was not impaired. Podosome organization and matrix degradation were found to be defective in hck(-/-) osteoclast precursors (preosteoclast) but were normal in mature hck(-/-) osteoclasts, probably through compensation by Src, which was specifically overexpressed in mature osteoclasts. As a consequence of podosome defects, the 3-dimensional migration of hck(-/-) preosteoclasts was strongly affected in vitro. In vivo, this translated by altered bone homing of preosteoclasts in hck(-/-) mice: in metatarsals of 1-wk-old mice, when bone formation strongly depends on the recruitment of these cells, reduced numbers of osteoclasts and abnormal developing trabecular bone were observed. This phenotype was still detectable in adults. In summmary, Hck is one of the very few effectors of preosteoclast recruitment described to date and thereby plays a critical role in bone remodeling.

Macrophage mesenchymal migration requires podosome stabilization by filamin A

Filamin A (FLNa) is a cross-linker of actin filaments and serves as a scaffold protein mostly involved in the regulation of actin polymerization. It is distributed ubiquitously, and null mutations have strong consequences on embryonic development in humans, with organ defects which suggest deficiencies in cell migration. We have reported previously that macrophages, the archetypal migratory cells, use the protease- and podosome-dependent mesenchymal migration mode in dense three-dimensional environments, whereas they use the protease- and podosome-independent amoeboid mode in more porous matrices. Because FLNa has been shown to localize to podosomes, we hypothesized that the defects seen in patients carrying FLNa mutations could be related to the capacity of certain cell types to form podosomes. Using strategies based on FLNa knock-out, knockdown, and rescue, we show that FLNa (i) is involved in podosome stability and their organization as rosettes and three-dimensional podosomes, (ii) regulates the proteolysis of the matrix mediated by podosomes in macrophages, (iii) is required for podosome rosette formation triggered by Hck, and (iv) is necessary for mesenchymal migration but dispensable for amoeboid migration. These new functions assigned to FLNa, particularly its role in mesenchymal migration, could be directly related to the defects in cell migration described during the embryonic development in FLNa-defective patients.

Single-domain antibody-SH3 fusions for efficient neutralization of HIV-1 Nef functions

HIV-1 Nef is essential for AIDS pathogenesis, but this viral protein is not targeted by antiviral strategies. The functions of Nef are largely related to perturbations of intracellular trafficking and signaling pathways through leucine-based and polyproline motifs that are required for interactions with clathrin-associated adaptor protein complexes and SH3 domain-containing proteins, such as the phagocyte-specific kinase Hck. We previously described a single-domain antibody (sdAb) targeting Nef and inhibiting many, but not all, of its biological activities. We now report a further development of this anti-Nef strategy through the demonstration of the remarkable inhibitory activity of artificial Nef ligands, called Neffins, comprised of the anti-Nef sdAb fused to modified SH3 domains. The Neffins inhibited all key activities of Nef, including Nef-mediated CD4 and major histocompatibility complex class I (MHC-I) cell surface downregulation and enhancement of virus infectivity. When expressed in T lymphocytes, Neffins specifically inhibited the Nef-induced mislocalization of the Lck kinase, which contributes to the alteration of the formation of the immunological synapse. In macrophages, Neffins inhibited the Nef-induced formation of multinucleated giant cells and podosome rosettes, and it counteracted the inhibitory activity of Nef on phagocytosis. Since we show here that these effects of Nef on macrophage and T cell functions were both dependent on the leucine-based and polyproline motifs, we confirmed that Neffins disrupted interactions of Nef with both AP complexes and Hck. These results demonstrate that it is possible to inhibit all functions of Nef, both in T lymphocytes and macrophages, with a single ligand that represents an efficient tool to develop new antiviral strategies targeting Nef.

The identification of a small molecule compound that reduces HIV-1 Nef-mediated viral infectivity enhancement

Nef is a multifunctional HIV-1 protein that accelerates progression to AIDS, and enhances the infectivity of progeny viruses through a mechanism that is not yet understood. Here, we show that the small molecule compound 2c reduces Nef-mediated viral infectivity enhancement. When added to viral producer cells, 2c did not affect the efficiency of viral production itself. However, the infectivity of the viruses produced in the presence of 2c was significantly lower than that of control viruses. Importantly, an inhibitory effect was observed with Nef(+) wild-type viruses, but not with viruses produced in the absence of Nef or in the presence of proline-rich PxxP motif-disrupted Nef, both of which displayed significantly reduced intrinsic infectivity. Meanwhile, the overexpression of the SH3 domain of the tyrosine kinase Hck, which binds to a PxxP motif in Nef, also reduced viral infectivity. Importantly, 2c inhibited Hck SH3-Nef binding, which was more marked when Nef was pre-incubated with 2c prior to its incubation with Hck, indicating that both Hck SH3 and 2c directly bind to Nef and that their binding sites overlap. These results imply that both 2c and the Hck SH3 domain inhibit the interaction of Nef with an unidentified host protein and thereby reduce Nef-mediated infectivity enhancement. The first inhibitory compound 2c is therefore a valuable chemical probe for revealing the underlying molecular mechanism by which Nef enhances the infectivity of HIV-1.

Frustrated phagocytosis on micro-patterned immune complexes to characterize lysosome movements in live macrophages

Lysosome mobilization is a key cellular process in phagocytes for bactericidal activities and trans-matrix migration. The molecular mechanisms that regulate lysosome mobilization are still poorly known. Lysosomes are hard to track as they move toward phagosomes throughout the cell volume. In order to anticipate cell regions where lysosomes are recruited to, human and RAW264.7 macrophages were seeded on surfaces that were micro-patterned with immune complexes (ICs) as 4 μm-side squares. Distances between IC patterns were adapted to optimize cell spreading in order to constrain lysosome movements mostly in two dimensions. FcΓ receptors triggered local frustrated phagocytosis, frustrated phagosomes appeared as rings of F-actin dots around the IC patterns as early as 5 min after cells made contact with the substratum. Frustrated phagosomes recruited actin-associated proteins (vinculin, paxillin, and gelsolin). The fusion of lysosomes with frustrated phagosomes was shown by the release of beta-hexosaminidase and the recruitment of Lamp1 to frustrated phagosomes. Lysosomes of RAW264.7 macrophages were labeled with cathepsin-D-mCherry to visualize their movements toward frustrated phagosomes. Lysosomes saltatory movements were markedly slowed down compared to cells layered on non-opsonized patterns. In addition, the linearity of the trajectories and the frequency and duration of contacts of lysosomes with frustrated phagosomes were measured. Our experimental set-up is the first step toward deciphering molecular mechanisms which are involved in lysosome movements in the cytoplasm (speed, directionality, and interaction with phagosomes), and opens the door to approaches such as RNA interference, pharmacological inhibition, or mutant expression.