Complement-Opsonized HIV-1 Overcomes Restriction in Dendritic Cells

Control of complement-induced inflammatory responses to SARS-CoV-2 infection by anti-SARS-CoV-2 antibodies

Dysregulated immune responses contribute to the excessive and uncontrolled inflammation observed in severe COVID-19. However, how immunity to SARS-CoV-2 is induced and regulated remains unclear. Here, we uncover the role of the complement system in the induction of innate and adaptive immunity to SARS-CoV-2. Complement rapidly opsonizes SARS-CoV-2 particles via the lectin pathway. Complement-opsonized SARS-CoV-2 efficiently induces type-I interferon and pro-inflammatory cytokine responses via activation of dendritic cells, which are inhibited by antibodies against the complement receptors (CR) 3 and 4. Serum from COVID-19 patients, or monoclonal antibodies against SARS-CoV-2, attenuate innate and adaptive immunity induced by complement-opsonized SARS-CoV-2. Blocking of CD32, the FcγRII antibody receptor of dendritic cells, restores complement-induced immunity. These results suggest that opsonization of SARS-CoV-2 by complement is involved in the induction of innate and adaptive immunity to SARS-CoV-2 in the acute phase of infection. Subsequent antibody responses limit inflammation and restore immune homeostasis. These findings suggest that dysregulation of the complement system and FcγRII signaling may contribute to severe COVID-19.

A role for tunneling nanotubes in virus spread

Tunneling nanotubes (TNTs) are actin-rich intercellular conduits that mediate distant cell-to-cell communication and enable the transfer of various cargos, including proteins, organelles, and virions. They play vital roles in both physiological and pathological processes. In this review, we focus on TNTs in different types of viruses, including retroviruses such as HIV, HTLV, influenza A, herpesvirus, paramyxovirus, alphavirus and SARS-CoV-2. We summarize the viral proteins responsible for inducing TNT formation and explore how these virus-induced TNTs facilitate intercellular communication, thereby promoting viral spread. Furthermore, we highlight other virus infections that can induce TNT-like structures, facilitating the dissemination of viruses. Moreover, TNTs promote intercellular spread of certain viruses even in the presence of neutralizing antibodies and antiviral drugs, posing significant challenges in combating viral infections. Understanding the mechanisms underlying viral spread via TNTs provides valuable insights into potential drug targets and contributes to the development of effective therapies for viral infections.

HIV-1 Trans Infection via TNTs Is Impeded by Targeting C5aR

Nonadjacent immune cells communicate through a complex network of tunneling nanotubes (TNTs). TNTs can be hijacked by HIV-1, allowing it to spread between connected cells. Dendritic cells (DCs) are among the first cells to encounter HIV-1 at mucosal sites, but they are usually efficiently infected only at low levels. However, HIV-1 was demonstrated to productively infect DCs when the virus was complement-opsonized (HIV-C). Such HIV-C-exposed DCs mediated an improved antiviral and T-cell stimulatory capacity. The role of TNTs in combination with complement in enhancing DC infection with HIV-C remains to be addressed. To this aim, we evaluated TNT formation on the surface of DCs or DC/CD4+ T-cell co-cultures incubated with non- or complement-opsonized HIV-1 (HIV, HIV-C) and the role of TNTs or locally produced complement in the infection process using either two different TNT or anaphylatoxin receptor antagonists. We found that HIV-C significantly increased the formation of TNTs between DCs or DC/CD4+ T-cell co-cultures compared to HIV-exposed DCs or co-cultures. While augmented TNT formation in DCs promoted productive infection, as was previously observed, a significant reduction in productive infection was observed in DC/CD4+ T-cell co-cultures, indicating antiviral activity in this setting. As expected, TNT inhibitors significantly decreased infection of HIV-C-loaded-DCs as well as HIV- and HIV-C-infected-DC/CD4+ T-cell co-cultures. Moreover, antagonizing C5aR significantly inhibited TNT formation in DCs as well as DC/CD4+ T-cell co-cultures and lowered the already decreased productive infection in co-cultures. Thus, local complement mobilization via DC stimulation of complement receptors plays a pivotal role in TNT formation, and our findings herein might offer an exciting opportunity for novel therapeutic approaches to inhibit trans infection via C5aR targeting.

Revisiting an IgG Fc Loss-of-Function Experiment: the Role of Complement in HIV Broadly Neutralizing Antibody b12 Activity

The role of the complement system in HIV-1 immunity and pathogenesis is multifaceted, and an improved understanding of complement activities mediated by HIV-1-specific antibodies has the potential to inform and advance clinical development efforts. A seminal nonhuman primate challenge experiment suggested that complement was dispensable for the protective effect of the early broadly neutralizing antibody (bnAb) b12, but recent experiments have raised questions about the breadth of circumstances under which this conclusion may hold. Here, we reassess the original observation using Fc variants of IgG1 b12 that enhance complement activity and report that complement fixation on recombinant antigen, virions, and cells and complement-dependent viral and cellular lysis in vitro vary among bnAbs. Specifically, while the clinically significant V3 glycan-specific bnAb 10-1074 demonstrates activity, we found that b12 does not meaningfully activate the classical complement cascade. Consistent with avid engagement by C1q and its complex system of regulatory factors, these results suggest that complement-mediated antibody activities demonstrate a high degree of context dependence and motivate revisiting the role of complement in antibody-mediated prevention of HIV-1 infection by next-generation bnAbs in new translational studies in animal models.

Complement Potentiates Immune Sensing of HIV-1 and Early Type I Interferon Responses

Complement-opsonized HIV-1 triggers efficient antiviral type I interferon (IFN) responses in dendritic cells (DCs), which play an important role in protective responses at the earliest stages in retroviral infection. In contrast, HIV-1 suppresses or escapes sensing by STING- and MAVS-associated sensors. Here, we identified a complement receptor-mediated sensing pathway, where DCs are activated in CCR5/RLR (RIG-I/MDA5)/MAVS/TBK1-dependent fashion. Increased fusion of complement-opsonized HIV-1 via complement receptor 4 and CCR5 leads to increased incoming HIV-1 RNA in the cytoplasm, sensed by a nonredundant cooperative effect of RIG-I and MDA5. Moreover, complement-opsonized HIV-1 down-modulated the MAVS-suppressive Raf-1/PLK1 pathway, thereby opening the antiviral recognition pathway via MAVS. This in turn was followed by MAVS aggregation and subsequent TBK1/IRF3/NF-κB activation in DCs exposed to complement- but not non-opsonized HIV-1. Our data strongly suggest that complement is important in the induction of efficient antiviral immune responses by preventing HIV-1 suppressive mechanisms as well as inducing specific cytosolic sensors.

P80 Natural Essence Exerts Efficient Anti-HIV-1- as Well as Adjuvant Effects in DCs

Dendritic cells (DCs), as well as complement, play a major role during human immunodeficiency virus 1 (HIV-1) entry and infection at mucosal sites. Together, DCs and complement are key points for understanding host defence against HIV-1 infection and for studying the impact of new drugs on the regulation of innate host-pathogen interactions and adaptive immunity. For this, we evaluated the antiviral effect of the P80 natural essence (Longan extract) on interactions of non- and complement-opsonized HIV-1 with DCs. In viability assays, we first illustrated the effects of P80 natural essence on DC function. We found that P80 concentrations above 1.5% caused increased cell death, while at concentrations between 0.5% and 1% the compound exerted efficient antiviral effects in DCs and illustrated an adjuvant effect regarding DC activation. DC maturation, as well as co-stimulatory capacity, were significantly improved by P80 natural essence via p38 MAPK phosphorylation in presence of the viral challenge independent of the opsonization pattern. These findings might be exploited for future therapeutic options to target DC subsets directly at mucosal sites by P80 natural essence and to block entry of both, non- and complement-opsonized HIV-1.

Mucosal Dendritic Cell Subsets Control HIV-1's Viral Fitness

Dendritic cell (DC) subsets are abundantly present in genital and intestinal mucosal tissue and are among the first innate immune cells that encounter human immunodeficiency virus type 1 (HIV-1) after sexual contact. Although DCs have specific characteristics that greatly enhance HIV-1 transmission, it is becoming evident that most DC subsets also have virus restriction mechanisms that exert selective pressure on the viruses during sexual transmission. In this review we discuss the current concepts of the immediate events following viral exposure at genital mucosal sites that lead to selection of specific HIV-1 variants called transmitted founder (TF) viruses. We highlight the importance of the TF HIV-1 phenotype and the role of different DC subsets in establishing infection. Understanding the biology of HIV-1 transmission will contribute to the design of novel treatment strategies preventing HIV-1 dissemination.

Innate Immune Response Against HIV-1

The innate immune system is comprised of both cellular and humoral players that recognise and eradicate invading pathogens. Therefore, the interplay between retroviruses and innate immunity has emerged as an important component of viral pathogenesis. HIV-1 infection in humans that results in hematologic abnormalities and immune suppression is well represented by changes in the CD4/CD8 T cell ratio and consequent cell death causing CD4 lymphopenia. The innate immune responses by mucosal barriers such as complement, DCs, macrophages, and NK cells as well as cytokine/chemokine profiles attain great importance in acute HIV-1 infection, and thus, prevent mucosal capture and transmission of HIV-1. Conversely, HIV-1 has evolved to overcome innate immune responses through RNA-mediated rapid mutations, pathogen-associated molecular patterns (PAMPs) modification, down-regulation of NK cell activity and complement receptors, resulting in increased secretion of inflammatory factors. Consequently, epithelial tissues lining up female reproductive tract express innate immune sensors including anti-microbial peptides responsible for forming primary barriers and have displayed an effective potent anti-HIV activity during phase I/II clinical trials.

HIV-1 subverts the complement system in semen to enhance viral transmission

Semen is important in determining HIV-1 susceptibility but it is unclear how it affects virus transmission during sexual contact. Mucosal Langerhans cells (LCs) are the first immune cells to encounter HIV-1 during sexual contact and have a barrier function as LCs are restrictive to HIV-1. As semen from people living with HIV-1 contains complement-opsonized HIV-1, we investigated the effect of complement on HIV-1 dissemination by human LCs in vitro and ex vivo. Notably, pre-treatment of HIV-1 with semen enhanced LC infection compared to untreated HIV-1 in the ex vivo explant model. Infection of LCs and transmission to target cells by opsonized HIV-1 was efficiently inhibited by blocking complement receptors CR3 and CR4. Complement opsonization of HIV-1 enhanced uptake, fusion, and integration by LCs leading to an increased transmission of HIV-1 to target cells. However, in the absence of both CR3 and CR4, C-type lectin receptor langerin was able to restrict infection of complement-opsonized HIV-1. These data suggest that complement enhances HIV-1 infection of LCs by binding CR3 and CR4, thereby bypassing langerin and changing the restrictive nature of LCs into virus-disseminating cells. Targeting complement factors might be effective in preventing HIV-1 transmission.

Role of Complement Receptors (CRs) on DCs in Anti-HIV-1 Immunity

Upon entry of human immunodeficiency virus 1 (HIV-1) into the host, innate immune mechanisms are acting as a first line of defense, that considerably also modify adaptive immunity by the provision of specific signals. Innate and adaptive immune responses are intimately linked and dendritic cells (DCs) together with complement (C) play an important role in regulation of adaptive immunity. Initially, the role of complement was considered to primarily support – or COMPLEMENT - cytolytic actions of antibodies or antibody-complexed antigens (immune complexes, ICs) or directly kill the pathogens by complement-mediated lysis. Recently, the role of complement was revised and found to significantly augmenting and modulating adaptive immunity, in particular against viruses. Complement and DCs are therefore predestined to open novel avenues for antiviral research and potential therapeutic interventions. Recent studies on interactions of complement-opsonized HIV-1 with DCs demonstrated a high potential of such primed DCs to initiate efficient antiviral and cytotoxic anti-HIV-1 immunity and complement-coated viral particles shift DCs functions via CR3 and CR4 in an antithetic manner. This review will focus on our current knowledge of CR3 and CR4 actions on DCs during HIV-1 binding and the outcome of infection influenced by entry and signaling pathways.

In the Crosshairs: RNA Viruses OR Complement?

Complement, a part of the innate arm of the immune system, is integral to the frontline defense of the host against innumerable pathogens, which includes RNA viruses. Among the major groups of viruses, RNA viruses contribute significantly to the global mortality and morbidity index associated with viral infection. Despite multiple routes of entry adopted by these viruses, facing complement is inevitable. The initial interaction with complement and the nature of this interaction play an important role in determining host resistance versus susceptibility to the viral infection. Many RNA viruses are potent activators of complement, often resulting in virus neutralization. Yet, another facet of virus-induced activation is the exacerbation in pathogenesis contributing to the overall morbidity. The severity in disease and death associated with RNA virus infections shows a tip in the scale favoring viruses. Growing evidence suggest that like their DNA counterparts, RNA viruses have co-evolved to master ingenious strategies to remarkably restrict complement. Modulation of host genes involved in antiviral responses contributed prominently to the adoption of unique strategies to keep complement at bay, which included either down regulation of activation components (C3, C4) or up regulation of complement regulatory proteins. All this hints at a possible “hijacking” of the cross-talk mechanism of the host immune system. Enveloped RNA viruses have a selective advantage of not only modulating the host responses but also recruiting membrane-associated regulators of complement activation (RCAs). This review aims to highlight the significant progress in the understanding of RNA virus–complement interactions.

Complement opsonization of HIV affects primary infection of human colorectal mucosa and subsequent activation of T cells

HIV transmission via genital and colorectal mucosa are the most common routes of dissemination. Here, we explored the effects of free and complement-opsonized HIV on colorectal tissue. Initially, there was higher antiviral responses in the free HIV compared to complement-opsonized virus. The mucosal transcriptional response at 24 hr revealed the involvement of activated T cells, which was mirrored in cellular responses observed at 96 hr in isolated mucosal T cells. Further, HIV exposure led to skewing of T cell phenotypes predominantly to inflammatory CD4+ T cells, that is Th17 and Th1Th17 subsets. Of note, HIV exposure created an environment that altered the CD8+ T cell phenotype, for example expression of regulatory factors, especially when the virions were opsonized with complement factors. Our findings suggest that HIV-opsonization alters the activation and signaling pathways in the colorectal mucosa, which promotes viral establishment by creating an environment that stimulates mucosal T cell activation and inflammatory Th cells.

CR4 Signaling Contributes to a DC-Driven Enhanced Immune Response Against Complement-Opsonized HIV-1

Dendritic cells (DCs) possess intrinsic cellular defense mechanisms to specifically inhibit HIV-1 replication. In turn, HIV-1 has evolved strategies to evade innate immune sensing by DCs resulting in suboptimal maturation and poor antiviral immune responses. We previously showed that complement-opsonized HIV-1 (HIV-C) was able to efficiently infect various DC subsets significantly higher than non-opsonized HIV-1 (HIV) and therefore also mediate a higher antiviral immunity. Thus, complement coating of HIV-1 might play a role with respect to viral control occurring early during infection via modulation of DCs. To determine in detail which complement receptors (CRs) expressed on DCs was responsible for infection and superior pro-inflammatory and antiviral effects, we generated stable deletion mutants for the α-chains of CR3, CD11b, and CR4, CD11c using CRISPR/Cas9 in THP1-derived DCs. We found that CD11c deletion resulted in impaired DC infection as well as antiviral and pro-inflammatory immunity upon exposure to complement-coated HIV-1. In contrast, sole expression of CD11b on DCs shifted the cells to an anti-inflammatory, regulatory DC type. We here illustrated that CR4 comprised of CD11c and CD18 is the major player with respect to DC infection associated with a potent early pro-inflammatory immune response. A more detailed characterization of CR3 and CR4 functions using our powerful tool might open novel avenues for early therapeutic intervention during HIV-1 infection.

A role for antibodies in natural HIV control

PURPOSE OF REVIEW

Rare patients naturally control HIV replication without antiretroviral therapy. Understanding the mechanisms implicated in natural HIV control will inform the development of immunotherapies against HIV. Elite controllers are known for developing efficient antiviral T-cell responses, but recent findings suggest that antibody responses also play a significant role in HIV control. We review the key studies that uncovered a potent memory B-cell response and highly functional anti-HIV antibodies in elite controllers, and explore the mechanisms that may account for the distinct properties of their humoral response.

RECENT FINDINGS

Elite controllers maintain a large HIV-specific memory B-cell pool that is sustained by efficient T follicular helper function. Neutralizing antibody rarely show high titers in controllers, but seem capable, at least in certain cases, of neutralizing contemporaneous viral strains. In addition, elite controllers display a unique HIV-specific antibody profile in terms of isotype, antigen specificity, and glycosylation pattern, resulting in polyfunctional antibody effector functions that may promote infected cell lysis and prime effectors of the antiviral immune response.

SUMMARY

Lessons from elite controller studies argue for the importance of integrating the many parameters defining a polyfunctional antibody response when evaluating candidate vaccines and immunotherapeutic approaches directed at HIV.

Immunotherapy as an Antifungal Strategy in Immune Compromised Hosts

Purpose of Review

IFIs cause high morbidity and mortality in the immunocompromised host worldwide. Although highly effective, conventional antifungal chemotherapy faces new challenges due to late diagnosis and increasing numbers of drug-resistant fungal strains. Thus, antifungal immunotherapy represents a viable treatment option, and recent advances in the field are summarized in this review.

Recent Findings

Antifungal immunotherapies include application of immune cells as well as the administration of cytokines, growth factors, and antibodies. Novel strategies to treat IFIs in the immunocompromised host target intracellular signaling pathways using SMTs such as checkpoint inhibitors.

Summary

Studies using cytokines or chemokines exerted a potential adjuvant role to conventional antifungal therapy, but issues on toxicity for some agents have to be resolved. Cell-based immunotherapies are very labor-intense and costly, but NK cell transfer and CAR T cell therapy provide exciting strategies to combat IFIs. Antibody-mediated protection and checkpoint inhibition are additional novel immunotherapeutic approaches.

Molecular Model for the Surface-Catalyzed Protein Self-Assembly

The importance of cell surfaces in the self-assembly of proteins is widely accepted. One biologically significant event is the assembly of amyloidogenic proteins into aggregates, which leads to neurodegenerative disorders like Alzheimer's and Parkinson's diseases. The interaction of amyloidogenic proteins with cellular membranes appears to dramatically facilitate the aggregation process. Recent findings indicate that, in the presence of surfaces, aggregation occurs at physiologically low concentrations, suggesting that interaction with surfaces plays a critical role in the disease-prone aggregation process. However, the molecular mechanisms behind the on-surface aggregation process remain unclear. Here, we provide a theoretical model that offers a molecular explanation. According to this model, monomers transiently immobilized to surfaces increase the local monomer protein concentration and thus work as nuclei to dramatically accelerate the entire aggregation process. This physical-chemical theory was verified by experimental studies, using mica surfaces, to examine the aggregation kinetics of amyloidogenic α-synuclein protein and non-amyloidogenic cytosine deaminase APOBEC3G.

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.

Friend retrovirus studies reveal complex interactions between intrinsic, innate and adaptive immunity

Approximately 4.4% of the human genome is comprised of endogenous retroviral sequences, a record of an evolutionary battle between man and retroviruses. Much of what we know about viral immunity comes from studies using mouse models. Experiments using the Friend virus (FV) model have been particularly informative in defining highly complex anti-retroviral mechanisms of the intrinsic, innate and adaptive arms of immunity. FV studies have unraveled fundamental principles about how the immune system controls both acute and chronic viral infections. They led to a more complete understanding of retroviral immunity that begins with cellular sensing, production of type I interferons, and the induction of intrinsic restriction factors. Novel mechanisms have been revealed, which demonstrate that these earliest responses affect not only virus replication, but also subsequent innate and adaptive immunity. This review on FV immunity not only surveys the complex host responses to a retroviral infection from acute infection to chronicity, but also highlights the many feedback mechanisms that regulate and counter-regulate the various arms of the immune system. In addition, the discovery of molecular mechanisms of immunity in this model have led to therapeutic interventions with implications for HIV cure and vaccine development.

Negative and Positive Selection Pressure During Sexual Transmission of Transmitted Founder HIV-1

Sexual transmission of HIV-1 consists of processes that exert either positive or negative selection pressure on the virus. The sum of these selection pressures lead to the transmission of only one specific HIV-1 strain, termed the transmitted founder virus. Different dendritic cell subsets are abundantly present at mucosal sites and, interestingly, these DC subsets exert opposite pressure on viral selection during sexual transmission. In this review we describe receptors and cellular compartments in DCs that are involved in HIV-1 communication leading to either viral restriction by the host or further dissemination to establish a long-lived reservoir. We discuss the current understanding of host antiretroviral restriction factors against HIV-1 and specifically against the HIV-1 transmitted founder virus. We will also discuss potential clinical implications for exploiting these intrinsic restriction factors in developing novel therapeutic targets. A better understanding of these processes might help in developing strategies against HIV-1 infections by targeting dendritic cells.

Co- but not Sequential Infection of DCs Boosts Their HIV-Specific CTL-Stimulatory Capacity

Pathogenic bacteria and their microbial products activate dendritic cells (DCs) at mucosal surfaces during sexually transmitted infections (STIs) and therefore might also differently shape DC functions during co-infection with HIV-1. We recently illustrated that complement (C) coating of HIV-1 (HIV-C), as primarily found during the acute phase of infection before appearance of HIV-specific antibodies, by-passed SAMHD1-mediated restriction in DCs and therefore mediated an increased DC activation and antiviral capacity. To determine whether the superior antiviral effects of HIV-C-exposed DCs also apply during STIs, we developed a co-infection model in which DCs were infected with Chlamydia spp. simultaneously (HIV-C/Chlam-DCs or HIV/Chlam-DCs) or a sequential infection model, where DCs were exposed to Chlamydia for 3 or 24 h (Chlam-DCs) followed by HIV-1 infection. Co-infection of DCs with HIV-1 and Chlamydia significantly boosted the CTL-stimulatory capacity compared to HIV-1-loaded iDCs and this boost was independent on the opsonization pattern. This effect was lost in the sequential infection model, when opsonized HIV-1 was added delayed to Chlamydia-loaded DCs. The reduction in the CTL-stimulatory capacity of Chlam-DCs was not due to lower HIV-1 binding or infection compared to iDCs or HIV-C/Chlam-DCs, but due to altered fusion and internalization mechanisms within DCs. The CTL-stimulatory capacity of HIV-C in Chlam-DCs correlated with significantly reduced viral fusion compared to iDCs and HIV-C/Chlam-DCs and illustrated considerably increased numbers of HIV-C-containing vacuoles than iDCs. The data indicate that Chlamydia co-infection of DCs mediates a transient boost of their HIV-specific CTL-stimulatory and antiviral capacity, while in the sequential infection model this is reversed and associated with hazard to the host.

Intertwined: SAMHD1 cellular functions, restriction, and viral evasion strategies

SAMHD1 was initially described for its ability to efficiently restrict HIV-1 replication in myeloid cells and resting CD4⁺ T cells. However, a growing body of evidence suggests that SAMHD1-mediated restriction is by far not limited to lentiviruses, but seems to be a general concept that applies to most retroviruses and at least a number of DNA viruses. SAMHD1 anti-viral activity was long believed to be solely due to its ability to deplete cellular dNTPs by enzymatic degradation. However, since its discovery, several new functions have been attributed to SAMHD1. It has been demonstrated to bind nucleic acids, to modulate innate immunity, as well as to participate in the DNA damage response and resolution of stalled replication forks. Consequently, it is likely that SAMHD1-mediated anti-viral activity is not or not exclusively mediated through its dNTPase activity. Therefore, in this review, we summarize current knowledge on SAMHD1 cellular functions and systematically discuss how these functions could contribute to the restriction of a broad range of viruses besides retroviruses: herpesviruses, poxviruses and hepatitis B virus. Furthermore, we aim to highlight different ways how viruses counteract SAMHD1-mediated restriction to bypass the SAMHD1-mediated block to viral infection.

Fcγ Receptor Type I (CD64)-Mediated Impairment of the Capacity of Dendritic Cells to Activate Specific CD8 T Cells by IgG-opsonized Friend Virus

Dendritic cells (DCs) express Fcγ receptors (FcγRs) for the binding immune complexes (ICs) consisting of IgG and antigens (Ags). IC–FcγR interactions have been demonstrated to enhance activation and antigen-presenting functions of DCs. Utilizing Friend virus (FV), an oncogenic mouse retrovirus, we investigated the effect of IgG-opsonization of retroviral particles on the infection of DCs and the subsequent presentation of viral antigens by DCs to virus-specific CD8 T cells. We found that opsonization by virus-specific non-neutralizing IgG abrogated DC infection and as a consequence significantly reduced the capacity of DCs to activate virus-specific CD8 T cells. Effects of IgG-opsonization were mediated by the high-affinity FcγR type I, CD64, expressed on DCs. Our results suggest that different opsonization patterns on the retroviral surface modulate infection and antigen-presenting functions of DCs, whereby, in contrast to complement, IgG reduces the capacity of DCs to activate cytotoxic T cell (CTL) responses.

Expression of complement receptor 3 (CR3) and regulatory protein CD46 on dendritic cells of antiretroviral naïve and treated HIV-1 infected individuals: Correlation with immune activation status

During infection and budding, human immunodeficiency virus-1 (HIV-1) acquires regulators of Complement Activation (RCAs) along with the host cell membrane on the viral envelope. Activation of host complement system results in opsonization of virus by complement fragments, however the virus evades complement mediated lysis (CoML) by virtue of the RCAs on the viral envelope. The RCAs on HIV-1 envelope process complement protein C3 into various fragments that promote viral entry and infection of cells through different complement receptors. Complement opsonized HIV-1 has been shown in vitro to infect dendritic cells (DCs) in a CR3 dependent manner, although the role of CR3 and CD46 in natural HIV-1 infection is not clear. Surface expression of CR3 and CD46 on DC subsets of 30 antiretroviral naïve, 31 treated (cART) HIV-1 infected individuals and 30 seronegative controls was measured by flow cytometry and plasma levels of cytokines and complement activity (C3c levels) were quantitated by sandwich ELISA. Significantly lower surface expression of CR3 and CD46 was observed on DC subsets in naïve and treated HIV-1 infected individuals compared to controls. Significantly higher complement activation and plasma levels of IL-4, IL-8, IL-10 and IFN-γ were observed in treatment naïve HIV-1 infected individuals than controls. Significantly lower plasma levels of IL-4, IL-6, IL-8 and IL-10 were observed in treated vs. naïve HIV-1 infected individuals. Our findings suggest that alterations in expression of CR3 and CD46 on DCs along with complement activity could be factors that influence viral persistence and HIV-1 disease progression and need to be further evaluated.

Role of the complement system in the tumor microenvironment

The complement system has traditionally been considered a component of innate immunity against invading pathogens and "nonself" cells. Recent studies have demonstrated the immunoregulatory functions of complement activation in the tumor microenvironment (TME). The TME plays crucial roles in tumorigenesis, progression, metastasis and recurrence. Imbalanced complement activation and the deposition of complement proteins have been demonstrated in many types of tumors. Plasma proteins, receptors, and regulators of complement activation regulate several biological functions of stromal cells in the TME and promote the malignant biological properties of tumors. Interactions between the complement system and cancer cells contribute to the proliferation, epithelial-mesenchymal transition, migration and invasion of tumor cells. In this review, we summarize recent advances related to the function of the complement system in the TME and discuss the therapeutic potential of targeting complement-mediated immunoregulation in cancer immunotherapy.

Analysis of Complement-Mediated Lysis of Simian Immunodeficiency Virus (SIV) and SIV-Infected Cells Reveals Sex Differences in Vaccine-Induced Immune Responses in Rhesus Macaques

An effective human immunodeficiency virus (HIV) vaccine has yet to be developed, and defining immune correlates of protection against HIV infection is of paramount importance to inform future vaccine design. The complement system is a component of innate immunity that can directly lyse pathogens and shape adaptive immunity. To determine if complement lysis of simian immunodeficiency virus (SIV) and/or SIV-infected cells represents a protective immune correlate against SIV infection, sera from previously vaccinated and challenged rhesus macaques were analyzed for the induction of antibody-dependent complement-mediated lysis (ADCML). Importantly, the vaccine regimen, consisting of a replication-competent adenovirus type 5 host-range mutant SIV recombinant prime followed by a monomeric gp120 or oligomeric gp140 boost, resulted in overall delayed SIV acquisition only in females. Here, sera from all vaccinated animals induced ADCML of SIV and SIV-infected cells efficiently, regardless of sex. A modest correlation of SIV lysis with a reduced infection rate in males but not females, together with a reduced peak viremia in all animals boosted with gp140, suggested a potential for influencing protective efficacy. Gag-specific IgG and gp120-specific IgG and IgM correlated with SIV lysis in females, while Env-specific IgM correlated with SIV-infected cell lysis in males, indicating sex differences in vaccine-induced antibody characteristics and function. In fact, gp120/gp140-specific antibody functional correlates between antibody-dependent cellular cytotoxicity, antibody-dependent phagocytosis, and ADCML as well as the gp120-specific IgG glycan profiles and the corresponding ADCML correlations varied depending on the sex of the vaccinees. Overall, these data suggest that sex influences vaccine-induced antibody function, which should be considered in the design of globally effective HIV vaccines in the future.IMPORTANCE An HIV vaccine would thwart the spread of HIV infection and save millions of lives. Unfortunately, the immune responses conferring universal protection from HIV infection are poorly defined. The innate immune system, including the complement system, is an evolutionarily conserved, basic means of protection from infection. Complement can prevent infection by directly lysing incoming pathogens. We found that vaccination against SIV in rhesus macaques induces antibodies that are capable of directing complement lysis of SIV and SIV-infected cells in both sexes. We also found sex differences in vaccine-induced antibody species and their functions. Overall, our data suggest that sex affects vaccine-induced antibody characteristics and function and that males and females might require different immune responses to protect against HIV infection. This information could be used to generate highly effective HIV vaccines for both sexes in the future.

Immunomodulatory Role of Complement Proteins in the Neuropathology Associated with Opiate Abuse and HIV-1 Co-Morbidity

The complement system which is a critical mediator of innate immunity plays diverse roles in the neuropathogenesis of HIV-1 infection such as clearing HIV-1 and promoting productive HIV-1 replication. In the development of HIV-1 associated neurological disorders (HAND), there may be an imbalance between complement activation and regulation, which may contribute to the neuronal damage as a consequence of HIV-1 infection. It is well recognized that opiate abuse exacerbates HIV-1 neuropathology, however, little is known about the role of complement proteins in opiate induced neuromodulation, specifically in the presence of co-morbidity such as HIV-1 infection. Complement levels are significantly increased in the HIV-1-infected brain, thus HIV-induced complement synthesis may represent an important mechanism for the pathogenesis of AIDS in the brain, but remains underexplored. Anti-HIV-1 antibodies are able to initiate complement activation in HIV-1 infected CNS cells such as microglia and astrocytes during the course of disease progression; however, this complement activation fails to clear and eradicate HIV-1 from infected cells. In addition, the antiretroviral agents used for HIV therapy cause dysregulation of lipid metabolism, endothelial, and adipocyte cell function, and activation of pro-inflammatory cytokines. We speculate that both HIV-1 and opiates trigger a cytokine-mediated pro-inflammatory stimulus that modulates the complement cascade to exacerbate the virus-induced neurological damage. We examined the expression levels of C1q, SC5b-9, C5L2, C5aR, C3aR, and C9 key members of the complement cascade both in vivo in post mortem brain frontal cortex tissue from patients with HAND who used/did not use heroin, and in vitro using human microglial cultures treated with HIV tat and/or heroin. We observed significant expression of C1q and SC5b-9 by immunofluorescence staining in both the brain cortical and hippocampal region in HAND patients who abused heroin. Additionally, we observed increased gene expression of C5aR, C3aR, and C9 in the brain tissue of both HIV-1 infected patients with HAND who abused and did not abuse heroin, as compared to HIV negative controls. Our results show a significant increase in the expression of complement proteins C9, C5L2, C5aR, and C3aR in HIV transfected microglia and an additional increase in the levels of these complement proteins in heroin-treated HIV transfected microglia. This study highlights the a) potential roles of complement proteins in the pathogenesis of HIV-1-related neurodegenerative disorders; b) the combined effect of an opiate, like heroin, and HIV viral protein like HIV tat on complement proteins in normal human microglial cells and HIV transfected microglial cells. In the context of HAND, targeting selective steps in the complement cascade could help ameliorating the HIV burden in the CNS, thus investigations of complement-related therapeutic approaches for the treatment of HAND are warranted.

C-Type Lectin Receptors in Antiviral Immunity and Viral Escape

C-type lectin receptors (CLRs) are important pattern recognition receptors involved in recognition and induction of adaptive immunity to pathogens. Certain CLRs play an important role in viral infections as they efficiently interact with viruses. However, it has become clear that deadly viruses subvert the function of CLRs to escape antiviral immunity and promote infection. In particular, viruses target CLRs to suppress or modulate type I interferons that play a central role in the innate and adaptive defense against viruses. In this review, we discuss the function of CLRs in binding to enveloped viruses like HIV-1 and Dengue virus, and how uptake and signaling cascades have decisive effects on the outcome of infection.

Complement Evasion Strategies of Viruses: An Overview

Being a major first line of immune defense, the complement system keeps a constant vigil against viruses. Its ability to recognize large panoply of viruses and virus-infected cells, and trigger the effector pathways, results in neutralization of viruses and killing of the infected cells. This selection pressure exerted by complement on viruses has made them evolve a multitude of countermeasures. These include targeting the recognition molecules for the avoidance of detection, targeting key enzymes and complexes of the complement pathways like C3 convertases and C5b-9 formation - either by encoding complement regulators or by recruiting membrane-bound and soluble host complement regulators, cleaving complement proteins by encoding protease, and inhibiting the synthesis of complement proteins. Additionally, viruses also exploit the complement system for their own benefit. For example, they use complement receptors as well as membrane regulators for cellular entry as well as their spread. Here, we provide an overview on the complement subversion mechanisms adopted by the members of various viral families including Poxviridae, Herpesviridae, Adenoviridae, Flaviviridae, Retroviridae, Picornaviridae, Astroviridae, Togaviridae, Orthomyxoviridae and Paramyxoviridae.

Generation of Human Monocyte-derived Dendritic Cells from Whole Blood

Dendritic cells (DCs) recognize foreign structures of different pathogens, such as viruses, bacteria, and fungi, via a variety of pattern recognition receptors (PRRs) expressed on their cell surface and thereby activate and regulate immunity. The major function of DCs is the induction of adaptive immunity in the lymph nodes by presenting antigens via MHC I and MHC II molecules to naïve T lymphocytes. Therefore, DCs have to migrate from the periphery to the lymph nodes after the recognition of pathogens at the sites of infection. For in vitro experiments or DC vaccination strategies, monocyte-derived DCs are routinely used. These cells show similarities in physiology, morphology, and function to conventional myeloid dendritic cells. They are generated by interleukin 4 (IL-4) and granulocyte-macrophage colony-stimulating factor (GM-CSF) stimulation of monocytes isolated from healthy donors. Here, we demonstrate how monocytes are isolated and stimulated from anti-coagulated human blood after peripheral blood mononuclear cell (PBMC) enrichment by density gradient centrifugation. Human monocytes are differentiated into immature DCs and are ready for experimental procedures in a non-clinical setting after 5 days of incubation.

Tetherin/BST-2: Restriction Factor or Immunomodulator?

BACKGROUND

Cell-mediated immune (CMI) responses are critical for the control of HIV-1 infection and their importance was highlighted by the existence of viral proteins, particularly Vpu and Nef, that antagonize these responses. Pandemic HIV-1 Vpu counteracts Tetherin/BST-2, a host factor that could prevent the release of HIV-1 virions by tethering virions on the cell surface, but a link between Tetherin and HIV-1 CMI responses has not yet been demonstrated in vivo. In vitro, the virological and immunological impact of Tetherin-mediated accumulation of virions ranged from enhanced or diminished cell-to-cell spread to enhanced recognition by virus-specific antibodies for natural killer cellmediated lysis. However, Tetherin-restricted virions could be internalized through an endocytosis motif in the Tetherin cytoplasmic tail.

METHODS

Given the uncertainties on which in vitro results manifest in vivo and the dearth of knowledge on how Tetherin influences retroviral immunity, in vivo retrovirus infections in mice encoding wild-type, null and endocytosis-defective Tetherin were performed. Here, we review and highlight the results from these in vivo studies.

RESULTS

Current data suggests that endocytosis-defective Tetherin functions as a potent innate restriction factor. By contrast, endocytosis-competent Tetherin, the form found in most mammals including humans and the form counteracted by HIV-1 Vpu, was linked to stronger CMI responses in mice.

CONCLUSION

We propose that the main role of endocytosis-competent Tetherin is not to directly restrict retroviral replication, but to promote a more effective CMI response against retroviruses.

Cell-Free versus Cell-to-Cell Infection by Human Immunodeficiency Virus Type 1 and Human T-Lymphotropic Virus Type 1: Exploring the Link among Viral Source, Viral Trafficking, and Viral Replication

Human immunodeficiency virus type 1 (HIV-1) and human T-lymphotropic virus type 1 (HTLV-1) are complex retroviruses mainly infecting CD4(+) T lymphocytes. In addition, antigen-presenting cells such as dendritic cells (DCs) are targeted in vivo by both viruses, although to a lesser extent. Interaction of HIV-1 with DCs plays a key role in viral dissemination from the mucosa to CD4(+) T lymphocytes present in lymphoid organs. While similar mechanisms may occur for HTLV-1 as well, most HTLV-1 data were obtained from T-cell studies, and little is known regarding the trafficking of this virus in DCs. We first compared the efficiency of cell-free versus cell-associated viral sources of both retroviruses at infecting DCs. We showed that both HIV-1 and HTLV-1 cell-free particles are poorly efficient at productively infecting DCs, except when DC-SIGN has been engaged. Furthermore, while SAMHD-1 accounts for restriction of cell-free HIV-1 infection, it is not involved in HTLV-1 restriction. In addition, cell-free viruses lead mainly to a nonproductive DC infection, leading to trans-infection of T-cells, a process important for HIV-1 spread but not for that of HTLV-1. Finally, we show that T-DC cell-to-cell transfer implies viral trafficking in vesicles that may both increase productive infection of DCs ("cis-infection") and allow viral escape from immune surveillance. Altogether, these observations allowed us to draw a model of HTLV-1 and HIV-1 trafficking in DCs.

Virus-specific antibodies allow viral replication in the marginal zone, thereby promoting CD8(+) T-cell priming and viral control

Clinically used human vaccination aims to induce specific antibodies that can guarantee long-term protection against a pathogen. The reasons that other immune components often fail to induce protective immunity are still debated. Recently we found that enforced viral replication in secondary lymphoid organs is essential for immune activation. In this study we used the lymphocytic choriomeningitis virus (LCMV) to determine whether enforced virus replication occurs in the presence of virus-specific antibodies or virus-specific CD8(+) T cells. We found that after systemic recall infection with LCMV-WE the presence of virus-specific antibodies allowed intracellular replication of virus in the marginal zone of spleen. In contrast, specific antibodies limited viral replication in liver, lung, and kidney. Upon recall infection with the persistent virus strain LCMV-Docile, viral replication in spleen was essential for the priming of CD8(+) T cells and for viral control. In contrast to specific antibodies, memory CD8(+) T cells inhibited viral replication in marginal zone but failed to protect mice from persistent viral infection. We conclude that virus-specific antibodies limit viral infection in peripheral organs but still allow replication of LCMV in the marginal zone, a mechanism that allows immune boosting during recall infection and thereby guarantees control of persistent virus.

Mouse Siglec-1 Mediates trans-Infection of Surface-bound Murine Leukemia Virus in a Sialic Acid N-Acyl Side Chain-dependent Manner

Siglec-1 (sialoadhesin, CD169) is a surface receptor on human cells that mediates trans-enhancement of HIV-1 infection through recognition of sialic acid moieties in virus membrane gangliosides. Here, we demonstrate that mouse Siglec-1, expressed on the surface of primary macrophages in an interferon-α-responsive manner, captures murine leukemia virus (MLV) particles and mediates their transfer to proliferating lymphocytes. The MLV infection of primary B-cells was markedly more efficient than that of primary T-cells. The major structural protein of MLV particles, Gag, frequently co-localized with Siglec-1, and trans-infection, primarily of surface-bound MLV particles, efficiently occurred. To explore the role of sialic acid for MLV trans-infection at a submolecular level, we analyzed the potential of six sialic acid precursor analogs to modulate the sialylated ganglioside-dependent interaction of MLV particles with Siglec-1. Biosynthetically engineered sialic acids were detected in both the glycolipid and glycoprotein fractions of MLV producer cells. MLV released from cells carrying N-acyl-modified sialic acids displayed strikingly different capacities for Siglec-1-mediated capture and trans-infection; N-butanoyl, N-isobutanoyl, N-glycolyl, or N-pentanoyl side chain modifications resulted in up to 92 and 80% reduction of virus particle capture and trans-infection, respectively, whereas N-propanoyl or N-cyclopropylcarbamyl side chains had no effect. In agreement with these functional analyses, molecular modeling indicated reduced binding affinities for non-functional N-acyl modifications. Thus, Siglec-1 is a key receptor for macrophage/lymphocyte trans-infection of surface-bound virions, and the N-acyl side chain of sialic acid is a critical determinant for the Siglec-1/MLV interaction.