Siglec-1 initiates formation of the virus-containing compartment and enhances macrophage-to-T cell transmission of HIV-1

Help or Hinder: Protein Host Factors That Impact HIV-1 Replication

Interactions between human immunodeficiency virus type 1 (HIV-1) and the host factors or restriction factors of its target cells determine the cell's susceptibility to, and outcome of, infection. Factors intrinsic to the cell are involved at every step of the HIV-1 replication cycle, contributing to productive infection and replication, or severely attenuating the chances of success. Furthermore, factors unique to certain cell types contribute to the differences in infection between these cell types. Understanding the involvement of these factors in HIV-1 infection is a key requirement for the development of anti-HIV-1 therapies. As the list of factors grows, and the dynamic interactions between these factors and the virus are elucidated, comprehensive and up-to-date summaries that recount the knowledge gathered after decades of research are beneficial to the field, displaying what is known so that researchers can build off the groundwork of others to investigate what is unknown. Herein, we aim to provide a review focusing on protein host factors, both well-known and relatively new, that impact HIV-1 replication in a positive or negative manner at each stage of the replication cycle, highlighting factors unique to the various HIV-1 target cell types where appropriate.

Uncloaking the viral glycocalyx: How do viruses exploit glycoimmune checkpoints?

The surfaces of cells and enveloped viruses alike are coated in carbohydrates that play multifarious roles in infection and immunity. Organisms across all kingdoms of life make use of a diverse set of monosaccharide subunits, glycosidic linkages, and branching patterns to encode information within glycans. Accordingly, sugar-patterning enzymes and glycan binding proteins play integral roles in cell and organismal biology, ranging from glycoprotein quality control within the endoplasmic reticulum to lymphocyte migration, coagulation, inflammation, and tissue homeostasis. Unsurprisingly, genes involved in generating and recognizing oligosaccharide patterns are playgrounds for evolutionary conflicts that abound in cross-species interactions, exemplified by the myriad plant lectins that function as toxins. In vertebrates, glycans bearing acidic nine-carbon sugars called sialic acids are key regulators of immune responses. Various bacterial and fungal pathogens adorn their cells in sialic acids that either mimic their hosts' or are stolen from them. Yet, how viruses commandeer host sugar-patterning enzymes to thwart immune responses remains poorly studied. Here, we review examples of viruses that interact with sialic acid-binding immunoglobulin-like lectins (Siglecs), a family of immune cell receptors that regulate toll-like receptor signaling and govern glycoimmune checkpoints, while highlighting knowledge gaps that merit investigation. Efforts to illuminate how viruses leverage glycan-dependent checkpoints may translate into new clinical treatments that uncloak viral antigens and infected cell surfaces by removing or masking immunosuppressive sialoglycans, or by inhibiting viral gene products that induce their biosynthesis. Such approaches may hold the potential to unleash the immune system to clear long intractable chronic viral infections.

Murine alveolar macrophages rapidly accumulate intranasally administered SARS-CoV-2 Spike protein leading to neutrophil recruitment and damage

The trimeric SARS-CoV-2 Spike protein mediates viral attachment facilitating cell entry. Most COVID-19 vaccines direct mammalian cells to express the Spike protein or deliver it directly via inoculation to engender a protective immune response. The trafficking and cellular tropism of the Spike protein in vivo and its impact on immune cells remains incompletely elucidated. In this study, we inoculated mice intranasally, intravenously, and subcutaneously with fluorescently labeled recombinant SARS-CoV-2 Spike protein. Using flow cytometry and imaging techniques, we analyzed its localization, immune cell tropism, and acute functional impact. Intranasal administration led to rapid lung alveolar macrophage uptake, pulmonary vascular leakage, and neutrophil recruitment and damage. When injected near the inguinal lymph node medullary, but not subcapsular macrophages, captured the protein, while scrotal injection recruited and fragmented neutrophils. Widespread endothelial and liver Kupffer cell uptake followed intravenous administration. Human peripheral blood cells B cells, neutrophils, monocytes, and myeloid dendritic cells all efficiently bound Spike protein. Exposure to the Spike protein enhanced neutrophil NETosis and augmented human macrophage TNF-α (tumor necrosis factor-α) and IL-6 production. Human and murine immune cells employed C-type lectin receptors and Siglecs to help capture the Spike protein. This study highlights the potential toxicity of the SARS-CoV-2 Spike protein for mammalian cells and illustrates the central role for alveolar macrophage in pathogenic protein uptake.

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.

Targeted delivery of herpes simplex virus glycoprotein D to CD169+ macrophages using ganglioside liposomes alleviates herpes simplex keratitis in mice

Herpes simplex keratitis (HSK) is a common blinding corneal disease caused by herpes simplex virus type 1 (HSV-1) infection. Antiviral drugs and corticosteroids haven't shown adequate therapeutic efficacy. During the early stage of HSV-1 infection, macrophages serve as the first line of defense. In particular, CD169+ macrophages play an important role in phagocytosis and antigen presentation. Therefore, we constructed GM-gD-lip, a ganglioside GM1 liposome vaccine encapsulating HSV-1 glycoprotein D and targeting CD169+ macrophages. After subconjunctival injection of the vaccine, we evaluated the survival rate and ocular surface lesions of the HSK mice, as well as the virus levels in the tear fluid, corneas, and trigeminal ganglia. We discovered that GM-gD-lip reduced HSV-1 viral load and alleviated the clinical severity of HSK. The GM-gD-lip also increased the number of corneal infiltrating macrophages, especially CD169+ macrophages, and polarized them toward M1. Furthermore, the number of dendritic cells (DCs) and CD8+ T cells in the ocular draining lymph nodes was significantly increased. These findings demonstrated that GM-gD-lip polarized CD169+ macrophages toward M1 to eliminate the virus while cross-presenting antigens to CD8+ T cells via DCs to activate adaptive immunity, ultimately attenuating the severity of HSK. The use of GM-gD-lip as an immunotherapeutic method for the treatment of HSK has significant implications.

Incorporation of the HIV-1 envelope glycoprotein into viral particles is regulated by the tubular recycling endosome in a cell type-specific manner

The HIV-1 envelope glycoprotein (Env) is incorporated into particles during assembly on the plasma membrane (PM). Env initially reaches the PM through the secretory pathway, after which it is rapidly endocytosed via an AP-2- and clathrin-dependent mechanism. Here we show that endocytosed cell surface Env enters the tubular recycling endosome compartment (TRE). Trafficking to the TRE was dependent upon motifs within the CT previously implicated in Env recycling and particle incorporation. Depletion of TRE components MICAL-L1 or EHD1 led to defects in Env incorporation, particle infectivity, and viral replication. Remarkably, defects were limited to cell types defined as nonpermissive for incorporation of CT-deleted Env, including monocyte-derived macrophages, and not observed in 293T, HeLa, or MT-4 cells. This work identifies the TRE as an essential component of Env trafficking and particle incorporation, and provides evidence that the cell type-dependent incorporation of Env is defined by interactions with components of the TRE.

The Implication of Sphingolipids in Viral Infections

Sphingolipids are involved in cell signaling and metabolic pathways, and their metabolites play a critical role in host defense against intracellular pathogens. Here, we review the known mechanisms of sphingolipids in viral infections and discuss the potential implication of the study of sphingolipid metabolism in vaccine and therapeutic development.

Murine Alveolar Macrophages Rapidly Accumulate Intranasally Administered SARS-CoV-2 Spike Protein leading to Neutrophil Recruitment and Damage

The trimeric SARS-CoV-2 Spike protein mediates viral attachment facilitating cell entry. Most COVID-19 vaccines direct mammalian cells to express the Spike protein or deliver it directly via inoculation to engender a protective immune response. The trafficking and cellular tropism of the Spike protein in vivo and its impact on immune cells remains incompletely elucidated. In this study we inoculated mice intranasally, intravenously, and subcutaneously with fluorescently labeled recombinant SARS-CoV-2 Spike protein. Using flow cytometry and imaging techniques we analyzed its localization, immune cell tropism, and acute functional impact. Intranasal administration led to rapid lung alveolar macrophage uptake, pulmonary vascular leakage, and neutrophil recruitment and damage. When injected near the inguinal lymph node medullary, but not subcapsular macrophages, captured the protein, while scrotal injection recruited and fragmented neutrophils. Wide-spread endothelial and liver Kupffer cell uptake followed intravenous administration. Human peripheral blood cells B cells, neutrophils, monocytes, and myeloid dendritic cells all efficiently bound Spike protein. Exposure to the Spike protein enhanced neutrophil NETosis and augmented human macrophage TNF-α and IL-6 production. Human and murine immune cells employed C-type lectin receptors and Siglecs to help capture the Spike protein. This study highlights the potential toxicity of the SARS-CoV-2 Spike protein for mammalian cells and illustrates the central role for alveolar macrophage in pathogenic protein uptake.

Human immunodeficiency virus transmission-Mechanisms underlying the cell-to-cell spread of human immunodeficiency virus

Despite the success of combined antiretroviral therapy in controlling viral load and reducing the risk of human immunodeficiency virus (HIV) transmission, an estimated 1.5 million new infections occurred worldwide in 2021. These new infections are mainly the result of sexual intercourse and thus involve cells present on the genital mucosa, such as dendritic cells (DCs), macrophages (Mø) and CD4+ T lymphocytes. Understanding the mechanisms by which HIV interacts with these cells and how HIV exploits these interactions to establish infection in a new human host is critical to the development of strategies to prevent and control HIV transmission. In this review, we explore how HIV has evolved to manipulate some of the physiological roles of these cells, thereby gaining access to strategic cellular niches that are critical for the spread and pathogenesis of HIV infection. The interaction of HIV with DCs, Mø and CD4+ T lymphocytes, and the role of the intercellular transfer of viral particles through the establishment of the infectious or virological synapses, but also through membrane protrusions such as filopodia and tunnelling nanotubes (TNTs), and cell fusion or cell engulfment processes are presented and discussed.

Antiretrovirals to CCR5 CRISPR/Cas9 gene editing - A paradigm shift chasing an HIV cure

The evolution of drug-resistant viral strains and anatomical and cellular reservoirs of HIV pose significant clinical challenges to antiretroviral therapy. CCR5 is a coreceptor critical for HIV host cell fusion, and a homozygous 32-bp gene deletion (∆32) leads to its loss of function. Interestingly, an allogeneic HSCT from an HIV-negative ∆32 donor to an HIV-1-infected recipient demonstrated a curative approach by rendering the recipient's blood cells resistant to viral entry. Ex vivo gene editing tools, such as CRISPR/Cas9, hold tremendous promise in generating allogeneic HSC grafts that can potentially replace allogeneic ∆32 HSCTs. Here, we review antiretroviral therapeutic challenges, clinical successes, and failures of allogeneic and allogeneic ∆32 HSCTs, and newer exciting developments within CCR5 editing using CRISPR/Cas9 in the search to cure HIV.

KIF16B Mediates Anterograde Transport and Modulates Lysosomal Degradation of the HIV-1 Envelope Glycoprotein

The human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) is incorporated into virions at the site of particle assembly on the plasma membrane (PM). The route taken by Env to reach the site of assembly and particle incorporation remains incompletely understood. Following initial delivery to the PM through the secretory pathway, Env is rapidly endocytosed, suggesting that recycling is required for particle incorporation. Endosomes marked by the small GTPase Rab14 have been previously shown to play a role in Env trafficking. Here, we examined the role of KIF16B, the molecular motor protein that directs outward movement of Rab14-dependent cargo, in Env trafficking. Env colocalized extensively with KIF16B+ endosomes at the cellular periphery, while expression of a motor-deficient mutant of KIF16B redistributed Env to a perinuclear location. The half-life of Env labeled at the cell surface was markedly reduced in the absence of KIF16B, while a normal half-life was restored through inhibition of lysosomal degradation. In the absence of KIF16B, Env expression on the surface of cells was reduced, leading to a reduction in Env incorporation into particles and a corresponding reduction in particle infectivity. HIV-1 replication in KIF16B knockout cells was substantially reduced compared to that in wild-type cells. These results indicated that KIF16B regulates an outward sorting step involved in Env trafficking, thereby limiting lysosomal degradation and enhancing particle incorporation. IMPORTANCE The HIV-1 envelope glycoprotein is an essential component of HIV-1 particles. The cellular pathways that contribute to incorporation of envelope into particles are not fully understood. Here, we have identified KIF16B, a motor protein that directs movement from internal compartments toward the plasma membrane, as a host factor that prevents envelope degradation and enhances particle incorporation. This is the first host motor protein identified that contributes to HIV-1 envelope incorporation and replication.

HIV-1 promotes ubiquitination of the amyloidogenic C-terminal fragment of APP to support viral replication

HIV-1 replication in macrophages and microglia involves intracellular assembly and budding into modified subsets of multivesicular bodies (MVBs), which support both viral persistence and spread. However, the cellular factors that regulate HIV-1's vesicular replication remain poorly understood. Recently, amyloid precursor protein (APP) was identified as an inhibitor of HIV-1 replication in macrophages and microglia via an unknown mechanism. Here, we show that entry of HIV-1 Gag into MVBs is blocked by the amyloidogenic C-terminal fragment of APP, "C99", but not by the non-amyloidogenic product, "C83". To counter this, Gag promotes multi-site ubiquitination of C99 which controls both exocytic sorting of MVBs and further processing of C99 into toxic amyloids. Processing of C99, entry of Gag into MVBs and release of infectious virus could be suppressed by expressing ubiquitination-defective C99 or by γ-secretase inhibitor treatment, suggesting that APP's amyloidogenic pathway functions to sense and suppress HIV-1 replication in macrophages and microglia.

Cyclosporines Antagonize the Antiviral Activity of IFITMProteins by Redistributing Them toward the Golgi Apparatus

Interferon-induced transmembrane proteins (IFITMs) block the fusion of diverse enveloped viruses, likely through increasing the cell membrane's rigidity. Previous studies have reported that the antiviral activity of the IFITM family member, IFITM3, is antagonized by cell pretreatment with rapamycin derivatives and cyclosporines A and H (CsA and CsH) that promote the degradation of IFITM3. Here, we show that CsA and CsH potently enhance virus fusion with IFITM1- and IFITM3-expressing cells by inducing their rapid relocalization from the plasma membrane and endosomes, respectively, towards the Golgi. This relocalization is not associated with a significant degradation of IFITMs. Although prolonged exposure to CsA induces IFITM3 degradation in cells expressing low endogenous levels of this protein, its levels remain largely unchanged in interferon-treated cells or cells ectopically expressing IFITM3. Importantly, the CsA-mediated redistribution of IFITMs to the Golgi occurs on a much shorter time scale than degradation and thus likely represents the primary mechanism of enhancement of virus entry. We further show that rapamycin also induces IFITM relocalization toward the Golgi, albeit less efficiently than cyclosporines. Our findings highlight the importance of regulation of IFITM trafficking for its antiviral activity and reveal a novel mechanism of the cyclosporine-mediated modulation of cell susceptibility to enveloped virus infection.

HIV-Host Cell Interactions

The development of antiretroviral drugs (ARVs) was a great milestone in the management of HIV infection. ARVs suppress viral activity in the host cell, thus minimizing injury to the cells and prolonging life. However, an effective treatment has remained elusive for four decades due to the successful immune evasion mechanisms of the virus. A thorough understanding of the molecular interaction of HIV with the host cell is essential in the development of both preventive and curative therapies for HIV infection. This review highlights several inherent mechanisms of HIV that promote its survival and propagation, such as the targeting of CD4⁺ lymphocytes, the downregulation of MHC class I and II, antigenic variation and an envelope complex that minimizes antibody access, and how they collaboratively render the immune system unable to mount an effective response.

Aberrant protein glycosylation: Implications on diagnosis and Immunotherapy

Glycosylation-mediated post-translational modification is critical for regulating many fundamental processes like cell division, differentiation, immune response, and cell-to-cell interaction. Alterations in the N-linked or O-linked glycosylation pattern of regulatory proteins like transcription factors or cellular receptors lead to many diseases, including cancer. These alterations give rise to micro- and macro-heterogeneity in tumor cells. Here, we review the role of O- and N-linked glycosylation and its regulatory function in autoimmunity and aberrant glycosylation in cancer. The change in cellular glycome could result from a change in the expression of glycosidases or glycosyltransferases like N-acetyl-glucosaminyl transferase V, FUT8, ST6Gal-I, DPAGT1, etc., impact the glycosylation of target proteins leading to transformation. Moreover, the mutations in glycogenes affect glycosylation patterns on immune cells leading to other related manifestations like pro- or anti-inflammatory effects. In recent years, understanding the glycome to cancer indicates that it can be utilized for both diagnosis/prognosis as well as immunotherapy. Studies involving mass spectrometry of proteome, site- and structure-specific glycoproteomics, or transcriptomics/genomics of patient samples and cancer models revealed the importance of glycosylation homeostasis in cancer biology. The development of emerging technologies, such as the lectin microarray, has facilitated research on the structure and function of glycans and glycosylation. Newly developed devices allow for high-throughput, high-speed, and precise research on aberrant glycosylation. This paper also discusses emerging technologies and clinical applications of glycosylation.

Ganglioside-Functionalized Nanoparticles for Chimeric Antigen Receptor T-Cell Activation at the Immunological Synapse

Chimeric Antigen Receptor (CAR) T cell therapy has proven to be an effective strategy against hematological malignancies but persistence and activity against solid tumors must be further improved. One emerging strategy for enhancing efficacy is based on directing CAR T cells to antigen presenting cells (APCs). Activation of CAR T cells at the immunological synapse (IS) formed between APC and T cell is thought to promote strong, persistent antigen-specific T cell-mediated immune responses but requires integration of CAR ligands into the APC/T-cell interface. Here, we demonstrate that CAR ligand functionalized, lipid-coated, biodegradable polymer nanoparticles (NPs) that contain the ganglioside GM3 (GM3-NPs) bind to CD169 (Siglec-1)-expressing APCs and localize to the cell contact site between APCs and CAR T cells upon initiation of cell conjugates. The CD169+ APC/CAR T-cell interface is characterized by a strong optical colocalization of GM3-NPs and CARs, enrichment of F-actin, and recruitment of ZAP-70, indicative of integration of GM3-NPs into a functional IS. Ligands associated with GM3-NPs localized to the APC/T-cell contact site remain accessible to CARs and result in robust T-cell activation. Overall, this work identifies GM3-NPs as a potential antigen delivery platform for active targeting of CD169 expressing APCs and enhancement of CAR T-cell activation at the NP-containing IS.

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.

Viral and Host Factors Regulating HIV-1 Envelope Protein Trafficking and Particle Incorporation

The HIV-1 envelope glycoprotein (Env) is an essential structural component of the virus, serving as the receptor-binding protein and principal neutralizing determinant. Env trimers are incorporated into developing particles at the plasma membrane of infected cells. Incorporation of HIV-1 Env into particles in T cells and macrophages is regulated by the long Env cytoplasmic tail (CT) and the matrix region of Gag. The CT incorporates motifs that interact with cellular factors involved in endosomal trafficking. Env follows an unusual pathway to arrive at the site of particle assembly, first traversing the secretory pathway to the plasma membrane (PM), then undergoing endocytosis, followed by directed sorting to the site of particle assembly on the PM. Many aspects of Env trafficking remain to be defined, including the sequential events that occur following endocytosis, leading to productive recycling and particle incorporation. This review focuses on the host factors and pathways involved in Env trafficking, and discusses leading models of Env incorporation into particles.

NK Cells, Monocytes and Macrophages in HIV-1 Control: Impact of Innate Immune Responses

Rapid and synchronized responses of innate immune cells are an integral part of managing viral spread in acute virus infections. In human immunodeficiency virus type 1 (HIV-1) infection, increased immune control has been associated with the expression of certain natural killer (NK) cell receptors. Further, immune activation of monocytes/macrophages and the presence of specific cytokines was linked to low levels of HIV-1 replication. In addition to the intrinsic antiviral capabilities of NK cells and monocytes/macrophages, interaction between these cell types has been shown to substantially enhance NK cell function in the context of viral infections. This review discusses the involvement of NK cells and monocytes/macrophages in the effective control of HIV-1 and highlights aspects of innate immune crosstalk in viral infections that may be of relevance to HIV-1 infection.

Characterizing Lipid-Coated Mesoporous Silica Nanoparticles as CD169-Binding Delivery System for Rilpivirine and Cabotegravir

Herein, lipid-coated mesoporous silica nanoparticles (LMSN) are investigated as biomimetic delivery vehicle for two antiretroviral compounds (ARVs), rilpivirine (RPV) and cabotegravir (CAB). Monosialodihexosylganglioside (GM3) is incorporated into the membrane to facilitate LMSN binding to CD169 (Siglec-1)-expressing myeloid cells, that are predominantly expressed in secondary lymphoid tissues in vivo. It is demonstrated that in addition to providing CD169-binding functionalities, the lipid membrane around the silica core provides stealth properties that dampen the inflammatory cytokine response to ARVs-loaded LMSN in human monocyte-derived macrophages. Quantification of RPV and CAB releases from nanoparticles, and assessment of antiviral potency to human immunodeficiency virus (HIV-1) infection in vitro reveals that RPV and CAB co-formulated into LMSN retain optimal antiviral potency for 90 days, even upon storage at room temperature, making LMSN an attractive nanoplatform, immune to cold chain requirements. These findings suggest that GM3-LMSN equip the mesoporous silica nanoparticle (MSN) core with lipid-derived properties for surface passivation and lipid-mediated binding that are of high interest for achieving an effective delivery of ARVs to tissue reservoirs of HIV-1 replication.

Advances in HIV-1 Assembly

The assembly of HIV-1 particles is a concerted and dynamic process that takes place on the plasma membrane of infected cells. An abundance of recent discoveries has advanced our understanding of the complex sequence of events leading to HIV-1 particle assembly, budding, and release. Structural studies have illuminated key features of assembly and maturation, including the dramatic structural transition that occurs between the immature Gag lattice and the formation of the mature viral capsid core. The critical role of inositol hexakisphosphate (IP6) in the assembly of both the immature and mature Gag lattice has been elucidated. The structural basis for selective packaging of genomic RNA into virions has been revealed. This review will provide an overview of the HIV-1 assembly process, with a focus on recent advances in the field, and will point out areas where questions remain that can benefit from future investigation.

Virus-Mimicking Polymer Nanoparticles Targeting CD169+ Macrophages as Long-Acting Nanocarriers for Combination Antiretrovirals

Monosialodihexosylganglioside (GM3)-presenting lipid-coated polymer nanoparticles (NPs) that recapitulate the sequestration of human immunodeficiency virus-1 (HIV-1) particles in CD169+ virus-containing compartments (VCCs) of macrophages were developed as carriers for delivery and sustained release of a combination of two antiretrovirals (ARVs), rilpivirine (RPV) and cabotegravir (CAB). RPV and CAB were co-loaded into GM3-presenting lipid-coated polylactic acid (PLA) and poly(lactic-co-glycolic acid) (PLGA) NPs without loss in potency of the drugs. GM3-presenting PLA NPs demonstrated the most favorable release properties and achieved inhibition of HIV-1 infection of primary human macrophages for up to 35 days. Intracellular localization of GM3-presenting PLA NPs in VCCs correlated with retention of intracellular ARV concentrations and sustained inhibition of HIV-1 infection. This work elucidates the design criteria of lipid-coated polymer NPs to utilize CD169+ macrophages as cellular drug depots for eradicating the viral reservoir sites or to achieve long-acting prophylaxis against HIV-1 infection.

LFA1 and ICAM1 are critical for fusion and spread of murine leukemia virus in vivo

Murine leukemia virus (MLV)-presenting cells form stable intercellular contacts with target cells during infection of lymphoid tissue, indicating a role of cell-cell contacts in retrovirus dissemination. Whether host cell adhesion proteins are required for retrovirus spread in vivo remains unknown. Here, we demonstrate that the lymphocyte-function-associated-antigen-1 (LFA1) and its ligand intercellular-adhesion-molecule-1 (ICAM1) are important for cell-contact-dependent transmission of MLV between leukocytes. Infection experiments in LFA1- and ICAM1-deficient mice demonstrate a defect in MLV spread within lymph nodes. Co-culture of primary leukocytes reveals a specific requirement for ICAM1 on donor cells and LFA1 on target cells for cell-contact-dependent spread through trans- and cis-infection. Importantly, adoptive transfer experiments combined with a newly established MLV-fusion assay confirm that the directed LFA1-ICAM1 interaction is important for retrovirus fusion and transmission in vivo. Taken together, our data provide insights on how retroviruses exploit host proteins and the biology of cell-cell interactions for dissemination.

HIV-1 and HTLV-1 Transmission Modes: Mechanisms and Importance for Virus Spread

So far, only two retroviruses, human immunodeficiency virus (HIV) (type 1 and 2) and human T-cell lymphotropic virus type 1 (HTLV-1), have been recognized as pathogenic for humans. Both viruses mainly infect CD4+ T lymphocytes. HIV replication induces the apoptosis of CD4 lymphocytes, leading to the development of acquired immunodeficiency syndrome (AIDS). After a long clinical latency period, HTLV-1 can transform lymphocytes, with subsequent uncontrolled proliferation and the manifestation of a disease called adult T-cell leukemia (ATLL). Certain infected patients develop neurological autoimmune disorder called HTLV-1-associated myelopathy, also known as tropical spastic paraparesis (HAM/TSP). Both viruses are transmitted between individuals via blood transfusion, tissue/organ transplantation, breastfeeding, and sexual intercourse. Within the host, these viruses can spread utilizing either cell-free or cell-to-cell modes of transmission. In this review, we discuss the mechanisms and importance of each mode of transmission for the biology of HIV-1 and HTLV-1.

HIV transmitting mononuclear phagocytes; integrating the old and new

In tissue, mononuclear phagocytes (MNP) are comprised of Langerhans cells, dendritic cells, macrophages and monocyte-derived cells. They are the first immune cells to encounter HIV during transmission and transmit the virus to CD4 T cells as a consequence of their antigen presenting cell function. To understand the role these cells play in transmission, their phenotypic and functional characterisation is important. With advancements in high parameter single cell technologies, new MNPs subsets are continuously being discovered and their definition and classification is in a state of flux. This has important implications for our knowledge of HIV transmission, which requires a deeper understanding to design effective vaccines and better blocking strategies. Here we review the historical research of the role MNPs play in HIV transmission up to the present day and revaluate these studies in the context of our most recent understandings of the MNP system.

Modulation of Cystatin C in Human Macrophages Improves Anti-Mycobacterial Immune Responses to Mycobacterium tuberculosis Infection and Coinfection With HIV

Tuberculosis owes its resurgence as a major global health threat mostly to the emergence of drug resistance and coinfection with HIV. The synergy between HIV and Mycobacterium tuberculosis (Mtb) modifies the host immune environment to enhance both viral and bacterial replication and spread. In the lung immune context, both pathogens infect macrophages, establishing favorable intracellular niches. Both manipulate the endocytic pathway in order to avoid destruction. Relevant players of the endocytic pathway to control pathogens include endolysosomal proteases, cathepsins, and their natural inhibitors, cystatins. Here, a mapping of the human macrophage transcriptome for type I and II cystatins during Mtb, HIV, or Mtb-HIV infection displayed different profiles of gene expression, revealing cystatin C as a potential target to control mycobacterial infection as well as HIV coinfection. We found that cystatin C silencing in macrophages significantly improves the intracellular killing of Mtb, which was concomitant with an increased general proteolytic activity of cathepsins. In addition, downmodulation of cystatin C led to an improved expression of the human leukocyte antigen (HLA) class II in macrophages and an increased CD4⁺ T-lymphocyte proliferation along with enhanced IFN-γ secretion. Overall, our results suggest that the targeting of cystatin C in human macrophages represents a promising approach to improve the control of mycobacterial infections including multidrug-resistant (MDR) TB.

Roles of Virion-Incorporated CD162 (PSGL-1), CD43, and CD44 in HIV-1 Infection of T Cells

Nascent HIV-1 particles incorporate the viral envelope glycoprotein and multiple host transmembrane proteins during assembly at the plasma membrane. At least some of these host transmembrane proteins on the surface of virions are reported as pro-viral factors that enhance virus attachment to target cells or facilitate trans-infection of CD4⁺ T cells via interactions with non-T cells. In addition to the pro-viral factors, anti-viral transmembrane proteins are incorporated into progeny virions. These virion-incorporated transmembrane proteins inhibit HIV-1 entry at the point of attachment and fusion. In infected polarized CD4⁺ T cells, HIV-1 Gag localizes to a rear-end protrusion known as the uropod. Regardless of cell polarization, Gag colocalizes with and promotes the virion incorporation of a subset of uropod-directed host transmembrane proteins, including CD162, CD43, and CD44. Until recently, the functions of these virion-incorporated proteins had not been clear. Here, we review the recent findings about the roles played by virion-incorporated CD162, CD43, and CD44 in HIV-1 spread to CD4⁺ T cells.

Circumcision as an Intervening Strategy against HIV Acquisition in the Male Genital Tract

Unsafe sex with HIV-infected individuals remains a major route for HIV transmission, and protective strategies, such as the distribution of free condoms and pre-or post-prophylaxis medication, have failed to control the spread of HIV, particularly in resource-limited settings and high HIV prevalence areas. An additional key strategy for HIV prevention is voluntary male circumcision (MC). International health organizations (e.g., the World Health Organization, UNAIDS) have recommended this strategy on a larger scale, however, there is a general lack of public understanding about how MC effectively protects against HIV infection. This review aims to discuss the acquisition of HIV through the male genital tract and explain how and why circumcised men are more protected from HIV infection during sexual activity than uncircumcised men who are at higher risk of HIV acquisition.

Human anogenital monocyte-derived dendritic cells and langerin+cDC2 are major HIV target cells

Tissue mononuclear phagocytes (MNP) are specialised in pathogen detection and antigen presentation. As such they deliver HIV to its primary target cells; CD4 T cells. Most MNP HIV transmission studies have focused on epithelial MNPs. However, as mucosal trauma and inflammation are now known to be strongly associated with HIV transmission, here we examine the role of sub-epithelial MNPs which are present in a diverse array of subsets. We show that HIV can penetrate the epithelial surface to interact with sub-epithelial resident MNPs in anogenital explants and define the full array of subsets that are present in the human anogenital and colorectal tissues that HIV may encounter during sexual transmission. In doing so we identify two subsets that preferentially take up HIV, become infected and transmit the virus to CD4 T cells; CD14⁺CD1c⁺ monocyte-derived dendritic cells and langerin-expressing conventional dendritic cells 2 (cDC2).

The Interplay of HIV-1 and Macrophages in Viral Persistence

HIV-1 has evolved mechanisms to evade host cell immune responses and persist for lifelong infection. Latent cellular reservoirs are responsible for this persistence of HIV-1 despite the powerful effects of highly active antiretroviral therapies (HAART) to control circulating viral load. While cellular reservoirs have been extensively studied, much of these studies have focused on peripheral blood and resting memory CD4+ T cells containing latent HIV-1 provirus; however, efforts to eradicate cellular reservoirs have been stunted by reservoirs found in tissues compartments that are not easily accessible. These tissues contain resting memory CD4+ T cells and tissue resident macrophages, another latent cellular reservoir to HIV-1. Tissue resident macrophages have been associated with HIV-1 infection since the 1980s, and evidence has continued to grow regarding their role in HIV-1 persistence. Specific biological characteristics play a vital role as to why macrophages are latent cellular reservoirs for HIV-1, and in vitro and in vivo studies exhibit how macrophages contribute to viral persistence in individuals and animals on antiretroviral therapies. In this review, we characterize the role and evolutionary advantages of macrophage reservoirs to HIV-1 and their contribution to HIV-1 persistence. In acknowledging the interplay of HIV-1 and macrophages in the host, we identify reasons why current strategies are incapable of eliminating HIV-1 reservoirs and why efforts must focus on eradicating reservoirs to find a future functional cure.

Gut Innate Immunity and HIV Pathogenesis

PURPOSE OF REVIEW

In the gastro-intestinal tract, the complex network of multiple innate cell populations play critical roles not only as a first line of defense against invading pathogens and in driving adaptive immune responses but also in maintaining intestinal homeostasis. Here, we describe the roles of various innate immune cell populations in gut immunity and detail studies investigating the impact of acute and chronic HIV infection on these cell populations.

RECENT FINDINGS

Alterations in frequencies, phenotype and/or function of innate lymphoid cells, dendritic cells, macrophages, neutrophils, and innate-like T cells have been reported in people with HIV (PWH), with many of these features persisting despite anti-retroviral therapy and virological suppression. Dysregulated gut innate immunity in PWH is a feature of gut pathogenesis. A greater understanding of the mechanisms driving impairment in the multiple different gut innate immune cell populations and the downstream consequences of an altered innate immune response on host defense and gut homeostasis in PWH is needed to develop more effective HIV treatments and cure strategies.

HIV-infected macrophages resist efficient NK cell-mediated killing while preserving inflammatory cytokine responses

Natural killer (NK) cells are innate cytolytic effectors that target HIV-infected CD4⁺ T cells. In conjunction with antibodies recognizing the HIV envelope, NK cells also eliminate HIV-infected targets through antibody-dependent cellular cytotoxicity (ADCC). However, how these NK cell functions impact infected macrophages is less understood. We show that HIV-infected macrophages resist NK cell-mediated killing. Compared with HIV-infected CD4⁺ T cells, initial innate NK cell interactions with HIV-infected macrophages skew the response toward cytokine production, rather than release of cytolytic contents, causing inefficient elimination of infected macrophages. Studies with chimeric antigen receptor (CAR) T cells demonstrate that the viral envelope is equally accessible on CD4⁺ T cells and macrophages. Nonetheless, ADCC against macrophages is muted compared with ADCC against CD4⁺ T cells. Thus, HIV-infected macrophages employ mechanisms to evade immediate cytolytic NK cell function while preserving inflammatory cytokine responses. These findings emphasize the importance of eliminating infected macrophages for HIV cure efforts.

An OMICs based meta-analysis to support infection state stratification

MOTIVATION

A fundamental problem for disease treatment is that while antibiotics are a powerful counter to bacteria, they are ineffective against viruses. Often, bacterial and viral infections are confused due to their similar symptoms and lack of rapid diagnostics. With many clinicians relying primarily on symptoms for diagnosis, overuse and misuse of modern antibiotics are rife, contributing to the growing pool of antibiotic resistance. To ensure an individual receives optimal treatment given their disease state and to reduce over-prescription of antibiotics, the host response can in theory be measured quickly to distinguish between the two states. To establish a predictive biomarker panel of disease state (viral/bacterial/no-infection) we conducted a meta-analysis of human blood infection studies using Machine Learning (ML).

RESULTS

We focused on publicly available gene expression data from two widely used platforms, Affymetrix and Illumina microarrays as they represented a significant proportion of the available data. We were able to develop multi-class models with high accuracies with our best model predicting 93% of bacterial and 89% viral samples correctly. To compare the selected features in each of the different technologies, we reverse engineered the underlying molecular regulatory network and explored the neighbourhood of the selected features. The networks highlighted that although on the gene-level the models differed, they contained genes from the same areas of the network. Specifically, this convergence was to pathways including the Type I interferon Signalling Pathway, Chemotaxis, Apoptotic Processes, and Inflammatory/Innate Response.

AVAILABILITY

Data and code are available on the Gene Expression Omnibus and github.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Functions of CD169 positive macrophages in human diseases (Review)

CD169⁺ macrophages are a unique type of macrophage subset that differ from M1 and M2 macrophages. CD169⁺ macrophages are present in multiple tissues and organs throughout the body and are primarily expressed in secondary lymphoid organs. These cells are primarily divided across three locations in secondary lymphoid organs: The metallophilic marginal zone of the spleen, the subcapsular sinus and the medulla of the lymph nodes. Due to their unique location distribution in vivo and the presence of the CD169 molecule on their surfaces, CD169⁺ macrophages are reported to serve important roles in several processes, such as phagocytosis, antigen presentation, immune tolerance, viral infection and inflammatory responses. At the same time, it has been reported that CD169⁺ macrophages may also serve an important role in anti-tumour immunity. The present review focuses on the research progress surrounding the function of CD169⁺ macrophages in a variety of diseases, such as viral infection, autoimmune diseases and tumours.

Comparative analysis of human microglial models for studies of HIV replication and pathogenesis

Background

HIV associated neurocognitive disorders cause significant morbidity and mortality despite the advent of highly active antiretroviral therapy. A deeper understanding of fundamental mechanisms underlying HIV infection and pathogenesis in the central nervous system is warranted. Microglia are resident myeloid cells of the brain that are readily infected by HIV and may constitute a CNS reservoir. We evaluated two microglial model cell lines (C20, HMC3) and two sources of primary cell-derived microglia (monocyte-derived microglia [MMG] and induced pluripotent stem cell-derived microglia [iPSC-MG]) as potential model systems for studying HIV-microglia interactions.

Results

All four microglial model cells expressed typical myeloid markers with the exception of low or absent CD45 and CD11b expression by C20 and HMC3, and all four expressed the microglia-specific markers P2RY12 and TMEM119. Marked differences were observed upon gene expression profiling, however, indicating that MMG and iPSC-MG cluster closely together with primary human microglial cells, while C20 and HMC3 were similar to each other but very different from primary microglia. Expression of HIV-relevant genes also revealed important differences, with iPSC-MG and MMG expressing relevant genes at levels more closely resembling primary microglia. iPSC-MG and MMG were readily infected with R5-tropic HIV, while C20 and HMC3 lack CD4 and require pseudotyping for infection. Despite many similarities, HIV replication dynamics and HIV-1 particle capture by Siglec-1 differed markedly between the MMG and iPSC-MG.

Conclusions

MMG and iPSC-MG appear to be viable microglial models that are susceptible to HIV infection and bear more similarities to authentic microglia than two transformed microglia cell lines. The observed differences in HIV replication and particle capture between MMG and iPSC-MG warrant further study.

Electrostatic Surface Properties of Blood and Semen Extracellular Vesicles: Implications of Sialylation and HIV-Induced Changes on EV Internalization

Although extracellular vesicle (EV) surface electrostatic properties (measured as zeta potential, ζ-potential) have been reported by many investigators, the biophysical implications of charge and EV origin remains uncertain. Here, we compared the ζ-potential of human blood EVs (BEVs) and semen EVs (SEVs) from 26 donors that were HIV-infected (HIV+, n = 13) or HIV uninfected (HIV-, n = 13). We found that, compared to BEVs that bear neutral surface charge, SEVs were significantly more negatively charged, even when BEVs and SEVs were from the same individual. Comparison of BEVs and SEVs from HIV- and HIV+ groups revealed subtle HIV-induced alteration in the ζ-potential of EVs, with the effect being more significant in SEVs (∆ζ-potential = -8.82 mV, p-value = 0.0062) than BEVs (∆ζ-potential = -1.4 mV, p-value = 0.0462). These observations were validated by differences in the isoelectric point (IEP) of EVs, which was in the order of HIV + SEV ≤ HIV-SEV ≪ HIV + BEV ≤ HIV-BEV. Functionally, the rate and efficiency of SEV internalization by the human cervical epithelial cell line, primary peripheral blood lymphocytes, and primary blood-derived monocytes were significantly higher than those of BEVs. Mechanistically, removal of sialic acids from the surface of EVs using neuraminidase treatment significantly decreased SEV's surface charge, concomitant with a substantial reduction in SEV's internalization. The neuraminidase effect was independent of HIV infection and insignificant for BEVs. Finally, these results were corroborated by enrichment of glycoproteins in SEVs versus BEVs. Taken together, these findings uncover fundamental tissue-specific differences in surface electrostatic properties of EVs and highlight the critical role of surface charge in EV/target cell interactions.

Stiffness of HIV-1 Mimicking Polymer Nanoparticles Modulates Ganglioside-Mediated Cellular Uptake and Trafficking

The monosialodihexosylganglioside, GM3, and its binding to CD169 (Siglec-1) have been indicated as key factors in the glycoprotein-independent sequestration of the human immunodeficiency virus-1 (HIV-1) in virus-containing compartments (VCCs) in myeloid cells. Here, lipid-wrapped polymer nanoparticles (NPs) are applied as a virus-mimicking model to characterize the effect of core stiffness on NP uptake and intracellular fate triggered by GM3-CD169 binding in macrophages. GM3-functionalized lipid-wrapped NPs are assembled with poly(lactic-co-glycolic) acid (PLGA) as well as with low and high molecular weight polylactic acid (PLAlMW and PLAhMW) cores. The NPs have an average diameter of 146 ± 17 nm and comparable surface properties defined by the self-assembled lipid layer. Due to differences in the glass transition temperature, the Young's modulus (E) differs substantially under physiological conditions between PLGA (E PLGA = 60 ± 32 MPa), PLAlMW (E PLA lMW = 86 ± 25 MPa), and PLAhMW (E PLA hMW = 1.41 ± 0.67 GPa) NPs. Only the stiff GM3-presenting PLAhMW NPs but not the softer PLGA or PLAlMW NPs avoid a lysosomal pathway and localize in tetraspanin (CD9)-positive compartments that resemble VCCs. These observations suggest that GM3-CD169-induced sequestration of NPs in nonlysosomal compartments is not entirely determined by ligand-receptor interactions but also depends on core stiffness.

HIV-Captured DCs Regulate T Cell Migration and Cell-Cell Contact Dynamics to Enhance Viral Spread

Trafficking of cell-associated HIV-1 from the genital mucosa to lymphoid organs represents a critical first step toward systemic infection. Mature DCs capture and transmit HIV-1 to T cells, but insights into DC-to-T cell viral spread dynamics within a 3-dimensional environment is lacking. Using live-cell imaging, we show that mature DCs rapidly compartmentalize HIV-1 within surface-accessible invaginations near the uropod. HIV-1 capture did not interfere with DC migration toward lymph node homing chemo-attractants and their ability to enter lymphatic vessels. However, HIV-captured DCs engaged in prolonged contacts with autologous CD4+ T cells, which led to high T cell infection. Interestingly, we show that surface bound, virion-associated Env induced signal transduction in motile T cells that facilitated prolonged DC:T cell interactions, partially through high-affinity LFA-1 expression. Together, we describe a mechanism by which surface bound HIV-1 particles function as signaling receptors that regulate T cell motility, cell-cell contact dynamics, and productive infection.

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Mature DCs compartmentalize HIV particles near the uropodia via Siglec-1 receptor

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HIV-captured DCs respond to lymph node-homing chemokines and access lymphatics

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Prolonged contacts between HIV-captured DCs and CD4 T cells facilitate virus transfer

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Surface-accessible HIV particles can induce T cell signaling via Env:CD4 engagement

Macrophage Cell-Cell Interactions Promoting HIV-1 Infection

Many pathogens infect macrophages as part of their intracellular life cycle. This is particularly true for viruses, of which HIV-1 is one of the best studied. HIV-1 infection of macrophages has important consequences for viral persistence and pathogenesis, but the mechanisms of macrophage infection remain to be fully elucidated. Despite expressing viral entry receptors, macrophages are inefficiently infected by cell-free HIV-1 virions, whereas direct cell-cell spread is more efficient. Different modes of cell-cell spread have been described, including the uptake by macrophages of infected T cells and the fusion of infected T cells with macrophages, both leading to macrophage infection. Cell-cell spread can also transmit HIV-1 between macrophages and from macrophages to T cells. Here, we describe the current state of the field concerning the cell-cell spread of HIV-1 to and from macrophages, discuss mechanisms, and highlight potential in vivo relevance.

Identification of Host Trafficking Genes Required for HIV-1 Virological Synapse Formation in Dendritic Cells

The lentivirus human immunodeficiency virus (HIV) targets and destroys CD4⁺ T cells, leaving the host vulnerable to life-threatening opportunistic infections associated with AIDS. Dendritic cells (DCs) form a virological synapse (VS) with CD4⁺ T cells, enabling the efficient transfer of virus between the two cells. We have identified cellular factors that are critical in the induction of the VS. We show that ADP-ribosylation factor 1 (ARF1), bridging integrator 1 (BIN1), and Rab GTPases RAB7L1 and RAB8A are important regulators of HIV-1 trafficking to the VS and therefore the infection of CD4⁺ T cells. We found these cellular factors were essential for endosomal protein trafficking and formation of the VS and that depletion of target proteins prevented virus trafficking to the plasma membrane by retaining virus in intracellular vesicles. Identification of key regulators in HIV-1 trans-infection between DC and CD4⁺ T cells has the potential for the development of targeted therapy to reduce trans-infection of HIV-1 in vivo.

Dendritic cells (DCs) are one of the earliest targets of HIV-1 infection acting as a “Trojan horse,” concealing the virus from the innate immune system and delivering it to T cells via virological synapses (VS). To explicate how the virus is trafficked through the cell to the VS and evades degradation, a high-throughput small interfering RNA screen targeting membrane trafficking proteins was performed in monocyte-derived DCs. We identified several proteins including BIN-1 and RAB7L1 that share common roles in transport from endosomal compartments. Depletion of target proteins resulted in an accumulation of virus in intracellular compartments and significantly reduced viral trans-infection via the VS. By targeting endocytic trafficking and retromer recycling to the plasma membrane, we were able to reduce the virus’s ability to accumulate at budding microdomains and the VS. Thus, we identify key genes involved in a pathway within DCs that is exploited by HIV-1 to traffic to the VS.

IMPORTANCE The lentivirus human immunodeficiency virus (HIV) targets and destroys CD4⁺ T cells, leaving the host vulnerable to life-threatening opportunistic infections associated with AIDS. Dendritic cells (DCs) form a virological synapse (VS) with CD4⁺ T cells, enabling the efficient transfer of virus between the two cells. We have identified cellular factors that are critical in the induction of the VS. We show that ADP-ribosylation factor 1 (ARF1), bridging integrator 1 (BIN1), and Rab GTPases RAB7L1 and RAB8A are important regulators of HIV-1 trafficking to the VS and therefore the infection of CD4⁺ T cells. We found these cellular factors were essential for endosomal protein trafficking and formation of the VS and that depletion of target proteins prevented virus trafficking to the plasma membrane by retaining virus in intracellular vesicles. Identification of key regulators in HIV-1 trans-infection between DC and CD4⁺ T cells has the potential for the development of targeted therapy to reduce trans-infection of HIV-1 in vivo.

Cell-to-Cell Spreading of HIV-1 in Myeloid Target Cells Escapes SAMHD1 Restriction

We demonstrate that HIV-1 uses a common two-step cell-to-cell fusion mechanism for massive virus transfer from infected T lymphocytes and dissemination to myeloid target cells, including dendritic cells and macrophages as well as osteoclasts. This cell-to-cell infection process bypasses the restriction imposed by the SAMHD1 host cell restriction factor for HIV-1 replication, leading to the formation of highly virus-productive multinucleated giant cells as observed in vivo in lymphoid and nonlymphoid tissues of HIV-1-infected patients. Since myeloid cells are emerging as important target cells of HIV-1, these results contribute to a better understanding of the role of these myeloid cells in pathogenesis, including cell-associated virus sexual transmission, cell-to-cell virus spreading, and establishment of long-lived viral tissue reservoirs.

Dendritic cells (DCs) and macrophages as well as osteoclasts (OCs) are emerging as target cells of HIV-1 involved in virus transmission, dissemination, and establishment of persistent tissue virus reservoirs. While these myeloid cells are poorly infected by cell-free viruses because of the high expression levels of cellular restriction factors such as SAMHD1, we show here that HIV-1 uses a specific and common cell-to-cell fusion mechanism for virus transfer and dissemination from infected T lymphocytes to the target cells of the myeloid lineage, including immature DCs (iDCs), OCs, and macrophages, but not monocytes and mature DCs. The establishment of contacts with infected T cells leads to heterotypic cell fusion for the fast and massive transfer of viral material into OC and iDC targets, which subsequently triggers homotypic fusion with noninfected neighboring OCs and iDCs for virus dissemination. These two cell-to-cell fusion processes are not restricted by SAMHD1 and allow very efficient spreading of virus in myeloid cells, resulting in the formation of highly virus-productive multinucleated giant cells. These results reveal the cellular mechanism for SAMHD1-independent cell-to-cell spreading of HIV-1 in myeloid cell targets through the formation of the infected multinucleated giant cells observed in vivo in lymphoid and nonlymphoid tissues of HIV-1-infected patients.

Dendritic Cells, the Double Agent in the War Against HIV-1

Human Immunodeficiency Virus (HIV) infects cells from the immune system and has thus developed tools to circumvent the host immunity and use it in its advance. Dendritic cells (DCs) are the first immune cells to encounter the HIV, and being the main antigen (Ag) presenting cells, they link the innate and the adaptive immune responses. While DCs work to promote an efficient immune response and halt the infection, HIV-1 has ways to take advantage of their role and uses DCs to gain faster and more efficient access to CD4⁺ T cells. Due to their ability to activate a specific immune response, DCs are promising candidates to achieve the functional cure of HIV-1 infection, but knowing the molecular partakers that determine the relationship between virus and cell is the key for the rational and successful design of a DC-based therapy. In this review, we summarize the current state of knowledge on how both DC subsets (myeloid and plasmacytoid DCs) act in presence of HIV-1, and focus on different pathways that the virus can take after binding to DC. First, we explore the consequences of HIV-1 recognition by each receptor on DCs, including CD4 and DC-SIGN. Second, we look at cellular mechanisms that prevent productive infection and weapons that turn cellular defense into a Trojan horse that hides the virus all the way to T cell. Finally, we discuss the possible outcomes of DC-T cell contact.

Constitutive Siglec-1 expression confers susceptibility to HIV-1 infection of human dendritic cell precursors

The human dendritic cell (DC) lineage has recently been unraveled by high-dimensional mapping, revealing the existence of a discrete new population of blood circulating DC precursors (pre-DCs). Whether this new DC population possesses specific functional features as compared to the other blood DC subset upon pathogen encounter remained to be evaluated. A unique feature of pre-DCs among blood DCs is their constitutive expression of the viral adhesion receptor Siglec-1. Here, we show that pre-DCs, but not other blood DC subsets, are susceptible to infection by HIV-1 in a Siglec-1-dependent manner. Siglec-1 mediates pre-DC infection of CCR5- and CXCR4-tropic strains. Infection of pre-DCs is further enhanced in the presence of HIV-2/SIVmac Vpx, indicating that Siglec-1 does not counteract restriction factors such as SAMHD1. Instead, Siglec-1 promotes attachment and fusion of viral particles. HIV-1-infected pre-DCs produce new infectious viral particles that accumulate in intracellular compartments reminiscent of the virus-containing compartment of macrophages. Pre-DC activation by toll-like receptor (TLR) ligands induces an antiviral state that inhibits HIV-1 fusion and infection, but Siglec-1 remains functional and mediates replication-independent transfer of HIV-1 to activated primary T lymphocytes. Altogether, Siglec-1-mediated susceptibility to HIV-1 infection of pre-DCs constitutes a unique functional feature that might represent a preferential relationship of this emerging cell type with viruses.

Sphingolipids as Potential Therapeutic Targets against Enveloped Human RNA Viruses

Several notable human diseases are caused by enveloped RNA viruses: influenza, AIDS, hepatitis C, dengue hemorrhagic fever, microcephaly, and Guillain-Barré Syndrome. Being enveloped, the life cycle of this group of viruses is critically dependent on host lipid biosynthesis. Viral binding and entry involve interactions between viral envelope glycoproteins and cellular receptors localized to lipid-rich regions of the plasma membrane. Subsequent infection by these viruses leads to reorganization of cellular membranes and lipid metabolism to support the production of new viral particles. Recent work has focused on defining the involvement of specific lipid classes in the entry, genome replication assembly, and viral particle formation of these viruses in hopes of identifying potential therapeutic targets for the treatment or prevention of disease. In this review, we will highlight the role of host sphingolipids in the lifecycle of several medically important enveloped RNA viruses.

Manipulation of Mononuclear Phagocytes by HIV: Implications for Early Transmission Events

Mononuclear phagocytes are antigen presenting cells that play a key role in linking the innate and adaptive immune systems. In tissue, these consist of Langerhans cells, dendritic cells and macrophages, all of which express the key HIV entry receptors CD4 and CCR5 making them directly infectible with HIV. Mononuclear phagocytes are the first cells of the immune system to interact with invading pathogens such as HIV. Each cell type expresses a specific repertoire of pathogen binding receptors which triggers pathogen uptake and the release of innate immune cytokines. Langerhans cells and dendritic cells migrate to lymph nodes and present antigens to CD4 T cells, whereas macrophages remain tissue resident. Here we review how HIV-1 manipulates these cells by blocking their ability to produce innate immune cytokines and taking advantage of their antigen presenting cell function in order to gain transport to its primary target cells, CD4 T cells.

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.

Glycosylation in health and disease

The glycome describes the complete repertoire of glycoconjugates composed of carbohydrate chains, or glycans, that are covalently linked to lipid or protein molecules. Glycoconjugates are formed through a process called glycosylation and can differ in their glycan sequences, the connections between them and their length. Glycoconjugate synthesis is a dynamic process that depends on the local milieu of enzymes, sugar precursors and organelle structures as well as the cell types involved and cellular signals. Studies of rare genetic disorders that affect glycosylation first highlighted the biological importance of the glycome, and technological advances have improved our understanding of its heterogeneity and complexity. Researchers can now routinely assess how the secreted and cell-surface glycomes reflect overall cellular status in health and disease. In fact, changes in glycosylation can modulate inflammatory responses, enable viral immune escape, promote cancer cell metastasis or regulate apoptosis; the composition of the glycome also affects kidney function in health and disease. New insights into the structure and function of the glycome can now be applied to therapy development and could improve our ability to fine-tune immunological responses and inflammation, optimize the performance of therapeutic antibodies and boost immune responses to cancer. These examples illustrate the potential of the emerging field of 'glycomedicine'.