HIV-1 gp120 mannoses induce immunosuppressive responses from dendritic cells

Lack of complex N-glycans on HIV-1 envelope glycoproteins preserves protein conformation and entry function

The HIV-1 envelope glycoprotein complex (Env) is the focus of vaccine development aimed at eliciting humoral immunity. Env's extensive and heterogeneous N-linked glycosylation affects folding, binding to lectin receptors, antigenicity and immunogenicity. We characterized recombinant Env proteins and virus particles produced in mammalian cells that lack N-acetylglucosaminyltransferase I (GnTI), an enzyme necessary for the conversion of oligomannose N-glycans to complex N-glycans. Carbohydrate analyses revealed that trimeric Env produced in GnTI(-/-) cells contained exclusively oligomannose N-glycans, with incompletely trimmed oligomannose glycans predominating. The folding and conformation of Env proteins was little affected by the manipulation of the glycosylation. Viruses produced in GnTI(-/-) cells were infectious, indicating that the conversion to complex glycans is not necessary for Env entry function, although virus binding to the C-type lectin DC-SIGN was enhanced. Manipulating Env's N-glycosylation may be useful for structural and functional studies and for vaccine design.

HIV-1-specific antibody responses during acute and chronic HIV-1 infection

PURPOSE OF REVIEW

The humoral immune response to HIV-1 throughout infection is comprised of complex mixtures of antibody isotypes with numerous HIV-1 specificities. However, unlike antibody responses to most infections, protective antibody responses are delayed and do not arise until long after HIV-1 latency is established. We review recent data on HIV-1-specific antibody isotypes induced following HIV-1 transmission: to understand the effects of HIV-1 on B cell and T cell effector responses, to understand the timing of the rise and fall of different anti-HIV-1 antibodies and to understand how antibodies could contribute to protective immunity if they were either pre-existing or elicited immediately after HIV-1 transmission.

RECENT FINDINGS

Studies of the earliest events following infection by the transmitted/founder virus have recently revealed that early destruction of B cell generative microenvironments may be responsible for delay of potentially protective anti-HIV-1 antibody responses. Unlike the initial CD8 T cell response to HIV-1, the initial induced antibody response is usually ineffective in controlling virus replication during acute HIV-1 infection.

SUMMARY

The antibody isotypes and specificities elicited during HIV-1 infection can provide a window into deciphering the detrimental effects of HIV-1 on B cell and T cell responses. Additionally, further characterization of the virus inhibitory capabilities of anti-HIV-1 antibody isotypes can define the spectrum of potential protective HIV-1 antibodies that could be readily elicited by experimental vaccines and adjuvants.

Solution and crystal molecular dynamics simulation study of m4-cyanovirin-N mutants complexed with di-mannose

Cyanovirin-N (CVN) is a highly potent anti-HIV carbohydrate-binding agent that establishes its microbicide activity through interaction with mannose-rich glycoprotein gp120 on the virion surface. The m4-CVN and P51G-m4-CVN mutants represent simple models for studying the high-affinity binding site, B(M). A recently determined 1.35 A high-resolution structure of P51G-m4-CVN provided details on the di-mannose binding mechanism, and suggested that the Arg-76 and Glu-41 residues are critical components of high mannose specificity and affinity. We performed molecular-dynamics simulations in solution and a crystal environment to study the role of Arg-76. Network analysis and clustering were used to characterize the dynamics of Arg-76. The results of our explicit solvent solution and crystal simulations showed a significant correlation with conformations of Arg-76 proposed from x-ray crystallographic studies. However, the crystal simulation showed that the crystal environment strongly biases conformational sampling of the Arg-76 residue. The solution simulations demonstrated no conformational preferences for Arg-76, which would support its critical role as the residue that locks the ligand in the bound state. Instead, a comparative analysis of trajectories from >50 ns of simulation for two mutants revealed the existence of a very stable eight-hydrogen-bond network between the di-mannose ligand and predominantly main-chain atoms. This network may play a key role in the specific recognition and strong binding of mannose oligomers in CVN and its homologs.

The role of IL-10 in regulating immunity to persistent viral infections

The immune system has evolved multipronged responses that are critical to effectively defend the body from invading pathogens and to clear infection. However, the same weapons employed to eradicate infection can have caustic effects on normal bystander cells. Therefore, tight regulation is vital and the host must balance engendering correct and sufficient immune responses to pathogens while limiting errant and excessive immunopathology. To accomplish this task, a complex network of positive and negative immune signals are delivered, which in most instances successfully eliminate the pathogen. However, in response to some viral infections, immune function is rapidly suppressed leading to viral persistence. Immune suppression is a critical obstacle to the control of many persistent viral infections such as HIV, hepatitis C, and hepatitis B virus, which together affect more than 500 million individuals worldwide. Thus, the ability to therapeutically enhance immunity is a potentially powerful approach to resolve persistent infections. The host-derived cytokine IL-10 is a key player in the establishment and perpetuation of viral persistence. This chapter discusses the role of IL-10 in viral persistence and explores the exciting prospect of therapeutically blocking IL-10 to increase antiviral immunity and vaccine efficacy.

Biology of HIV mucosal transmission

PURPOSE OF REVIEW

HIV-1 mucosal transmission plays a critical role in HIV-1 infection and AIDS pathogenesis. This review summarizes the latest advances in biological studies of HIV-1 mucosal transmission, highlighting the implications of these studies in the development of microbicides to prevent HIV-1 transmission.

RECENT FINDINGS

New studies of initial HIV-1 infection using improved culture models updated the current view of mucosal transmission. Mechanistic studies enhanced our understanding of cell-cell transmission of HIV-1 mediated by the major target cells, including dendritic cells, CD4(+) T cells, and macrophages. Increasing evidence indicated the significance of host factors and immune responses in HIV-1 mucosal infection and transmission.

SUMMARY

Recent progress in HIV-1 mucosal infection and transmission enriches our knowledge of virus-host interactions and viral pathogenesis. Functional studies of HIV-1 interactions with host cells can provide new insights into the design of more effective approaches to combat HIV-1 infection and AIDS.

Dendritic cells transmit HIV-1 through human small intestinal mucosa

To dissect the early events in the transmission of HIV-1 from mother to child, we investigated whether DCs participate in HIV-1 entry into human small intestinal mucosa. We isolated human MNLs from jejunal lamina propria and identified a subpopulation of CD11c(+)HLA-DR(+) MNLs that expressed DC-SIGN, CD83, CD86, CD206, and CCR7, indicating a DC phenotype. Jejunal DCs also expressed the HIV-1 receptor CD4 and coreceptors CCR5 and CXCR4 and in suspension rapidly took up cell-free HIV-1. HIV-1 inoculated onto the apical surface of explanted jejunum was transported by lamina propria DCs through the mucosa and transmitted in trans to blood and intestinal lymphocytes. These findings indicate that in addition to intestinal epithelial cells, which we showed previously transcytose infectious HIV-1 to indicator cells, intestinal DCs play an important role in transporting HIV-1 through the intestinal mucosa and the subsequent transmission to T cells.

Pathogen recognition by DC-SIGN shapes adaptive immunity

Dendritic cells (DCs) tailor adaptive immune responses to specific pathogens. This diversity is mediated by cooperation between different pattern recognition receptors that are triggered by specific pathogens. DC-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) is a pattern recognition receptor with a broad pathogen recognition specificity as a result of its affinity for mannose and fucose carbohydrates. DC-SIGN induces very diverse immune responses to different pathogens, such as bacteria, fungi, helminths and viruses. Recent data show that DC-SIGN triggering by pathogens modulates Toll-like receptor signaling at the level of nuclear factor-kappaB. In this article, we will discuss the signaling pathways induced by DC-SIGN and its central role in the regulation of adaptive immunity to bacterial, fungal and viral pathogens.

Targeting of DC-SIGN on human dendritic cells by minor fimbriated Porphyromonas gingivalis strains elicits a distinct effector T cell response

The oral mucosal pathogen Porphyromonas gingivalis expresses at least two adhesins: the 67-kDa mfa-1 (minor) fimbriae and the 41-kDa fimA (major) fimbriae. In periodontal disease, P. gingivalis associates in situ with dermal dendritic cells (DCs), many of which express DC-SIGN (DC-specific ICAM-3 grabbing nonintegrin; CD209). The cellular receptors present on DCs that are involved in the uptake of minor/major fimbriated P. gingivalis, along with the effector immune response induced, are presently unclear. In this study, stably transfected human DC-SIGN(+/-) Raji cell lines and monocyte-derived DCs (MoDCs) were pulsed with whole, live, wild-type Pg381 or isogenic major (DPG-3)-, minor (MFI)-, or double fimbriae (MFB)-deficient mutant P. gingivalis strains. The influence of blocking Abs, carbohydrates, full-length glycosylated HIV-1 gp120 envelope protein, and cytochalasin D on the uptake of strains and on the immune responses was determined in vitro. We show that the binding of minor fimbriated P. gingivalis strains to Raji cells and MoDCs is dependent on DC-SIGN, whereas the double fimbriae mutant strain does not bind. Binding to DC-SIGN on MoDCs is followed by the internalization of P. gingivalis into DC-SIGN-rich intracellular compartments, and MoDCs secrete low levels of inflammatory cytokines and remain relatively immature. Blocking DC-SIGN with HIV-1 gp120 prevents the uptake of minor fimbriated strains and deregulates the expression of inflammatory cytokines. Moreover, MoDCs promote a Th2 or Th1 effector response, depending on whether they are pulsed with minor or major fimbriated P. gingivalis strains, respectively, suggesting distinct immunomodulatory roles for the two adhesins of P. gingivalis.

Utilizing the FIV model to understand dendritic cell dysfunction and the potential role of dendritic cell immunization in HIV infection

Dendritic cells (DC) are potent antigen presenting cells which initiate and coordinate the immune response making them central targets of and attractive candidates for manipulation in chronic lentiviral infections. Emerging evidence suggests that DC immune function is disrupted during both acute and chronic infection with human immunodeficiency virus (HIV), simian immunodeficiency virus (SIV), and feline immunodeficiency virus (FIV). Despite some early promising data, the use of DC for lentiviral immunotherapy has not fulfilled its expected potential and has been complicated by the large number of variables involved in DC harvesting, purifying, and antigen loading. Pre-clinical studies aimed at identifying successful strategies for DC augmentation of current HIV treatment protocols are needed. Over the past two decades, the FIV model for HIV infection has increased the understanding of retroviral pathogenesis, and studies have begun using the FIV model to study DC dysfunction and DC-mediated immunotherapy. Careful consideration of the many variables involved in DC function and therapy should help develop protocols to explore the potential of DC vaccine-based therapies for lentiviral infection.

Signalling through C-type lectin receptors: shaping immune responses

Crosstalk between pattern recognition receptors (PRRs) expressed by dendritic cells orchestrates T helper (T_H) cell differentiation through the induction of specific cytokine expression profiles, tailored to invading pathogens. C-type lectin receptors (CLRs) have an important role in orchestrating the induction of signalling pathways that regulate adaptive immune responses.

CLRs can control adaptive immunity at various levels by inducing signalling on their own, through crosstalk with other PRRs or by inducing carbohydrate-specific signalling pathways.

DC-specific ICAM3-grabbing non-integrin (DC-SIGN) interacts with mannose-carrying pathogens including Mycobacterium tuberculosis, HIV-1, measles virus and Candida albicans to activate the serine/threonine protein kinase RAF1. RAF1 signalling leads to the acetylation of Toll-like receptor (TLR)-activated nuclear factor-κB (NF-κB) subunit p65 and affects cytokine expression, such as inducing the upregulation of interleukin-10 (IL-10).

DC-associated C-type lectin 1 (dectin 1) triggering by a broad range of fungal pathogens, such as C. albicans, Aspergillus fumigatus and Pneumocystis carinii, results in protective antifungal immunity through the crosstalk of two independent signalling pathways — one through spleen tyrosine kinase (SYK) and one through RAF1 — that are essential for the expression of T_H1 and T_H17 cell polarizing cytokines.

Crosstalk between the SYK and RAF1 pathways is both synergistic and antagonizing to fine-tune NF-κB activity: although Ser276 phosphorylation of p65 leads to enhanced transcriptional activity of p65 itself through acetylation, it also inhibits the transcriptional activity of the NF-κB subunit RELB by sequestering it in p65–RELB dimers, which are transcriptionally inactive.

The diversity in CLR-mediated signalling provides some major challenges for the researches to elucidate and manipulate the signalling properties of this exciting family of receptors. However, the recent advances strongly support the use of CLR targeting vaccination strategies using dendritic cells to induce or redirect adaptive immune responses as well as improve antigen delivery.

Here, Teunis Geijtenbeek and Sonja Gringhuis discuss the role of the signalling pathways induced by C-type lectin receptors in determining T helper cell lineage commitment and describe how these pathways can be exploited for the development of new vaccination strategies.

C-type lectin receptors (CLRs) expressed by dendritic cells are crucial for tailoring immune responses to pathogens. Following pathogen binding, CLRs trigger distinct signalling pathways that induce the expression of specific cytokines which determine T cell polarization fates. Some CLRs can induce signalling pathways that directly activate nuclear factor-κB, whereas other CLRs affect signalling by Toll-like receptors. Dissecting these signalling pathways and their effects on host immune cells is essential to understand the molecular mechanisms involved in the induction of adaptive immune responses. In this Review we describe the role of CLR signalling in regulating adaptive immunity and immunopathogenesis and discuss how this knowledge can be harnessed for the development of innovative vaccination approaches.

HIV-1 envelope induces memory B cell responses that correlate with plasma antibody levels after envelope gp120 protein vaccination or HIV-1 infection

Successful vaccines (i.e., tetanus and diphtheria) can induce long-lived Ab levels that are maintained by bone marrow plasma cells and plasma Ab levels do not correlate with numbers of blood memory B cells. Destruction of CD4(+) T cells early in HIV-1 acute infection may result in insufficient induction of neutralizing Ab responses; thus, an HIV-1 vaccine should elicit high levels of durable Abs by long-lived plasma cells to be protective. We asked if HIV-1 envelope-specific memory responses were sustained by memory B cells in the settings of HIV-1 gp120 envelope vaccination and chronic HIV-1 infection. Levels of anti-HIV-1 envelope plasma Abs and memory B cells were found to correlate in both settings. Moreover, whereas the expected half-life of plasma Ab levels to protein vaccines was >10 years when maintained by long-lived plasma cells, anti-envelope Ab level half-lives were approximately 33-81 wk in plasma from antiretroviral drug-treated HIV-1(+) subjects. In contrast, anti-p55 Gag Ab level half-life was 648 wk, and Ab titers against influenza did not decay in-between yearly or biennial influenza vaccine boosts in the same patients. These data demonstrated that HIV-1 envelope induces predominantly short-lived memory B cell-dependent plasma Abs in the settings of envelope vaccination and HIV-1 infection. The inability to generate high titers of long-lived anti-envelope Abs is a major hurdle to overcome for the development of a successful HIV-1 vaccine.

Innate signaling by the C-type lectin DC-SIGN dictates immune responses

Effective immune responses depend on the recognition of pathogens by dendritic cells (DCs) through pattern recognition receptors (PRRs). These receptors induce specific signaling pathways that lead to the induction of immune responses against the pathogens. It is becoming evident that C-type lectins are also important PRRs. In particular, the C-type lectin DC-SIGN has emerged as a key player in the induction of immune responses against numerous pathogens by modulating TLR-induced activation. Recent reports have begun to elucidate the molecular mechanisms underlying these immune responses. Upon pathogen binding, DC-SIGN induces an intracellular signaling pathway with a central role for the serine/threonine kinase Raf-1. For several pathogens that interact with DC-SIGN, including Mycobacterium tuberculosis and HIV-1, Raf-1 activation leads to acetylation of NF-kappaB subunit p65, which induces specific gene transcription profiles. In addition, other DC-SIGN-ligands induce different signaling pathways downstream of Raf-1, indicating that DC-SIGN-signaling is tailored to the pathogen. In this review we will discuss in detail the current knowledge about DC-SIGN signaling and its implications on immunity.

Modulation of cellular tropism and innate antiviral response by viral glycans

Arthropod-borne viruses (arboviruses) are a significant cause of human and animal disease worldwide. Multiple interactions between virus and the host innate immune system ultimately determine the pathogenesis and clinical outcome of the infection. Evidence is rapidly emerging that suggests viral glycans play a key role in viral pathogenesis by regulating host cell tropism and interactions with the host innate immune response. Glycan-mediated interactions are especially important for arboviruses which must adapt to variable glycosylation systems and cellular receptors within both vertebrate and invertebrate hosts. This review focuses on emerging evidence which supports a crucial role for viral glycans in mediating host cell tropism and regulating the innate antiviral response.

Enzymatic removal of mannose moieties can increase the immune response to HIV-1 gp120 in vivo

The Env glycoproteins gp120 and gp41 are used in humoral immunity-based vaccines against human immunodeficiency virus (HIV-1) infection. One among many obstacles to such a vaccine is the structural defenses of Env glycoproteins that limit their immunogenicity. For example, gp120 mannose residues can induce immunosuppressive responses in vitro, including IL-10 expression, via mannose C-type lectin receptors on antigen-presenting cells. Here, we have investigated whether mannose removal alters gp120 immunogenicity in mice. Administering demannosylated gp120 (D-gp120) in the T(H)2-skewing adjuvant Alum induced approximately 50-fold higher titers of anti-gp120 IgG, compared to unmodified gp120. While the IgG subclass profile was predominantly T(H)2-associated IgG1, Abs of the T(H)1-associated IgG2a and IgG3 subclasses were also detectable in D-gp120 recipients. Immunizing with D-gp120 also improved T-cell responses. Giving an IL-10 receptor blocking MAb together with unmodified gp120 in Alum increased the anti-gp120 IgG titer, implicating IL-10 as a possible mediator of auto-suppressive responses to gp120.

IL-10 is up-regulated in multiple cell types during viremic HIV infection and reversibly inhibits virus-specific T cells

Murine models indicate that interleukin-10 (IL-10) can suppress viral clearance, and interventional blockade of IL-10 activity has been proposed to enhance immunity in chronic viral infections. Increased IL-10 levels have been observed during HIV infection and IL-10 blockade has been shown to enhance T-cell function in some HIV-infected subjects. However, the categories of individuals in whom the IL-10 pathway is up-regulated are poorly defined, and the cellular sources of IL-10 in these subjects remain to be determined. Here we report that blockade of the IL-10 pathway augmented in vitro proliferative capacity of HIV-specific CD4 and CD8 T cells in individuals with ongoing viral replication. IL-10 blockade also increased cytokine secretion by HIV-specific CD4 T cells. Spontaneous IL-10 expression, measured as either plasma IL-10 protein or IL-10 mRNA in peripheral blood mononuclear cells (PBMCs), correlated positively with viral load and diminished after successful antiretroviral therapy. IL-10 mRNA levels were up-regulated in multiple PBMC subsets in HIV-infected subjects compared with HIV-negative controls, particularly in T, B, and natural killer (NK) cells, whereas monocytes were a major source of IL-10 mRNA in HIV-infected and -uninfected individuals. These data indicate that multiple cell types contribute to IL-10-mediated immune suppression in the presence of uncontrolled HIV viremia.

A yeast glycoprotein shows high-affinity binding to the broadly neutralizing human immunodeficiency virus antibody 2G12 and inhibits gp120 interactions with 2G12 and DC-SIGN

The human immunodeficiency virus type 1 (HIV-1) envelope (Env) protein contains numerous N-linked carbohydrates that shield conserved peptide epitopes and promote trans infection by dendritic cells via binding to cell surface lectins. The potent and broadly neutralizing monoclonal antibody 2G12 binds a cluster of high-mannose-type oligosaccharides on the gp120 subunit of Env, revealing a conserved and highly exposed epitope on the glycan shield. To find an effective antigen for eliciting 2G12-like antibodies, we searched for endogenous yeast proteins that could bind to 2G12 in a panel of Saccharomyces cerevisiae glycosylation knockouts and discovered one protein that bound weakly in a Delta pmr1 strain deficient in hyperglycosylation. 2G12 binding to this protein, identified as Pst1, was enhanced by adding the Delta mnn1 deletion to the Delta pmr1 background, ensuring the exposure of terminal alpha1,2-linked mannose residues on the D1 and D3 arms of high-mannose glycans. However, optimum 2G12 antigenicity was found when Pst1, a heavily N-glycosylated protein, was expressed with homogenous Man(8)GlcNAc(2) structures in Delta och1 Delta mnn1 Delta mnn4 yeast. Surface plasmon resonance analysis of this form of Pst1 showed high affinity for 2G12, which translated into Pst1 efficiently inhibiting gp120 interactions with 2G12 and DC-SIGN and blocking 2G12-mediated neutralization of HIV-1 pseudoviruses. The high affinity of the yeast glycoprotein Pst1 for 2G12 highlights its potential as a novel antigen to induce 2G12-like antibodies.

Cytokine production by human herpesvirus 8-infected dendritic cells

We have shown previously that human herpesvirus 8 (HHV-8)-infected dendritic cells (DCs) undergo incomplete maturation and have a defective antigen-presenting function. Here, we examined the effects of HHV-8 infection on cytokine production, which is critical to the function of DCs. We detected expression of interleukin (IL)-6, tumour necrosis factor (TNF)-alpha, macrophage inflammatory protein (MIP)-1alpha, MIP-1beta, RANTES and IL-12p40 from 2 to 6 h post-infection, and these peaked by 15-24 h. Expression of these factors decreased 24-48 h post-infection, with the exception of TNF-alpha which remained high throughout the entire 72 h. Interestingly, while IL-12p40 expression increased post-infection, bioactive IL-12p70 was not detected in the supernatants. These results suggest an intentional skewing of cytokine production in HHV-8-infected DCs towards induction of a Th2 response.

Immune dysregulation in human immunodeficiency virus infection: know it, fix it, prevent it?

Infection of humans by the human immunodeficiency virus (HIV) causes a progressive, multifactorial impairment of the immune system eventually leading to the acquired immunodeficiency syndrome (AIDS). No cure or vaccine exists yet against HIV infection. More worrisome is the fact that despite having identified HIV as the cause of the AIDS, we still do not understand what pathogenic mechanisms lead to the debacle of the immune system. In this review we consider the extent and the limits of our knowledge of HIV pathogenesis, and how this knowledge may be used to design preventive and therapeutic approaches.

Protein/peptide and DNA vaccine delivery by targeting C-type lectin receptors

C-type lectin receptors (CLRs) are a class of pathogen-recognition receptors that are actively investigated in the field of vaccine delivery. Many of their properties have functions linked to the immune system. These receptors are expressed abundantly on antigen-presenting cells and are considered to be the sentinels of immune surveillance owing to their endocytic nature and the ability to recognize a diverse range of pathogens through recognition of pathogen-associated molecular patterns. CLRs are also involved in the processes of antigen presentation mediated through the induction of dendritic cell maturation and cytokine production. These properties engender CLRs to be ideal for vaccine targeting. Conversely, CLRs also function to recognize glycosylated self-antigens to induce homeostatic control and tolerance. In this review, we will describe the various preclinical/clinical vaccination strategies to target antigens and plasmid DNA to this diverse class of receptors.

Antigenic mimicry of the HIV envelope by AIDS-associated pathogens

Only one broadly neutralizing anti-HIV antibody, 2G12, recognises the envelope sugars of HIV. In the present study, we show that 2G12 also recognises Candida albicans and Candida tropicalis with high affinity (11 nmol/l) through a carbohydrate-dependent interaction (50% inhibitory concentration for D-fructose, 12 mmol/l). This is the first report of a neutralizing HIV antibody displaying cross-reactivity with another pathogen, revealing that the carbohydrate neutralization determinant of HIV, defined by 2G12, is more widespread amongst immunogenic, microbial surfaces than previously recognized.

Ross River virus envelope glycans contribute to type I interferon production in myeloid dendritic cells

Alphaviruses are mosquito-transmitted viruses that cause significant human disease, and understanding how these pathogens successfully transition from the mosquito vector to the vertebrate host is an important area of research. Previous studies demonstrated that mosquito and mammalian-cell-derived alphaviruses differentially induce type I interferons (alpha/beta interferon [IFN-alpha/beta]) in myeloid dendritic cells (mDCs), where the mosquito cell-derived virus is a poor inducer of IFN-alpha/beta compared to the mammalian-cell-derived virus. Furthermore, the reduced IFN-alpha/beta induction by the mosquito cell-derived virus is attributed to differential N-linked glycosylation. To further evaluate the role of viral envelope glycans in regulating the IFN-alpha/beta response, studies were performed to assess whether the mosquito cell-derived virus actively inhibits IFN-alpha/beta induction or is simply a poor inducer of IFN-alpha/beta. Coinfection studies using mammalian- and mosquito cell-derived Ross River virus (mam-RRV and mos-RRV, respectively) indicated that mos-RRV was unable to suppress IFN-alpha/beta induction by mam-RRV in mDC cultures. Additionally, a panel of mutant viruses lacking either individual or multiple N-linked glycosylation sites was used to demonstrate that N-linked glycans were essential for high-level IFN-alpha/beta induction by the mammalian-cell-derived virus. These results suggest that the failure of the mosquito cell-derived virus to induce IFN-alpha/beta is due to a lack of complex carbohydrates on the virion rather than the active suppression of the DC antiviral response.

RNA viruses and the mitogenic Raf/MEK/ERK signal transduction cascade

The Raf/MEK/ERK signal transduction cascade belongs to the mitogen-activated protein kinase (MAPK) cascades. Raf/MEK/ERK signaling leads to stimulus-specific changes in gene expression, alterations in cell metabolism or induction of programmed cell death (apoptosis), and thus controls cell differentiation and proliferation. It is induced by extracellular agents, including pathogens such as RNA viruses. Many DNA viruses are known to induce cellular signaling via this pathway. As these pathogens partly use the DNA synthesis machinery for their replication, they aim to drive cells into a proliferative state. In contrast, the consequences of RNA virus-induced Raf/MEK/ERK signaling were less clear for a long time, but since the turn of the century the number of publications on this topic has rapidly increased. Research on this virus/host-interaction will broaden our understanding of its relevance in viral replication. This important control center of cellular responses is differently employed to support the replication of several important human pathogenic RNA viruses including influenza, Ebola, hepatitis C and SARS corona viruses.

The C-type lectin surface receptor DCIR acts as a new attachment factor for HIV-1 in dendritic cells and contributes to trans- and cis-infection pathways

The dynamic interplay between dendritic cells (DCs) and human immunodeficiency virus type-1 (HIV-1) is thought to result in viral dissemination and evasion of antiviral immunity. Although initial observations suggested that the C-type lectin receptor (CLR) DC-SIGN was responsible for the trans-infection function of the virus, subsequent studies demonstrated that trans-infection of CD4(+) T cells with HIV-1 can also occur through DC-SIGN-independent mechanisms. We demonstrate that a cell surface molecule designated DCIR (for DC immunoreceptor), a member of a recently described family of DC-expressing CLRs, can participate in the capture of HIV-1 and promote infection in trans and in cis of autologous CD4(+) T cells from human immature monocyte-derived DCs. The contribution of DCIR to these processes was revealed using DCIR-specific siRNAs and a polyclonal antibody specific for the carbohydrate recognition domain of DCIR. Data from transfection experiments indicated that DCIR acts as a ligand for HIV-1 and is involved in events leading to productive virus infection. Finally, we show that the neck domain of DCIR is important for the DCIR-mediated effect on virus binding and infection. These results point to a possible role for DCIR in HIV-1 pathogenesis by supporting the productive infection of DCs and promoting virus propagation.

Evolution of DC-SIGN use revealed by fitness studies of R5 HIV-1 variants emerging during AIDS progression

Background

At early stages of infection CCR5 is the predominant HIV-1 coreceptor, but in approximately 50% of those infected CXCR4-using viruses emerge with disease progression. This coreceptor switch is correlated with an accelerated progression. However, those that maintain virus exclusively restricted to CCR5 (R5) also develop AIDS. We have previously reported that R5 variants in these "non-switch virus" patients evolve during disease progression towards a more replicative phenotype exhibiting altered CCR5 coreceptor interactions. DC-SIGN is a C-type lectin expressed by dendritic cells that HIV-1 may bind and utilize for enhanced infection of T cells in trans. To further explore the evolution of the R5 phenotype we analyzed sequential R5 isolates obtained before and after AIDS onset, i.e. at the chronic stage and during end-stage disease, with regard to efficiency of DC-SIGN use in trans-infections.

Results

Results from binding and trans-infection assays showed that R5 viruses emerging during end-stage AIDS disease displayed reduced ability to use DC-SIGN. To better understand viral determinants underlying altered DC-SIGN usage by R5 viruses, we cloned and sequenced the HIV-1 env gene. We found that end-stage R5 viruses lacked potential N-linked glycosylation sites (PNGS) in the gp120 V2 and V4 regions, which were present in the majority of the chronic stage R5 variants. One of these sites, amino acid position 160 (aa160) in the V2 region, also correlated with efficient use of DC-SIGN for binding and trans-infections. In fitness assays, where head-to-head competitions between chronic stage and AIDS R5 viruses were setup in parallel direct and DC-SIGN-mediated infections, results were further supported. Competitions revealed that R5 viruses obtained before AIDS onset, containing the V2 PNGS at aa160, were selected for in the trans-infection. Whereas, in agreement with our previous studies, the opposite was seen in direct target cell infections where end-stage viruses out-competed the chronic stage viruses.

Conclusion

Results of our study suggest R5 virus variants with diverse fitness for direct and DC-SIGN-mediated trans-infections evolve within infected individuals at end-stage disease. In addition, our results point to the importance of a glycosylation site within the gp120 V2 region for efficient DC-SIGN use of HIV-1 R5 viruses.

The carbohydrate at asparagine 386 on HIV-1 gp120 is not essential for protein folding and function but is involved in immune evasion

BACKGROUND

The HIV-1 envelope glycoprotein gp120, which mediates viral attachment to target cells, consists for approximately 50% of sugar, but the role of the individual sugar chains in various aspects of gp120 folding and function is poorly understood. Here we studied the role of the carbohydrate at position 386. We identified a virus variant that had lost the 386 glycan in an evolution study of a mutant virus lacking the disulfide bond at the base of the V4 domain.

RESULTS

The 386 carbohydrate was not essential for folding of wt gp120. However, its removal improved folding of a gp120 variant lacking the 385-418 disulfide bond, suggesting that it plays an auxiliary role in protein folding in the presence of this disulfide bond. The 386 carbohydrate was not critical for gp120 binding to dendritic cells (DC) and DC-mediated HIV-1 transmission to T cells. In accordance with previous reports, we found that N386 was involved in binding of the mannose-dependent neutralizing antibody 2G12. Interestingly, in the presence of specific substitutions elsewhere in gp120, removal of N386 did not result in abrogation of 2G12 binding, implying that the contribution of N386 is context dependent. Neutralization by soluble CD4 and the neutralizing CD4 binding site (CD4BS) antibody b12 was significantly enhanced in the absence of the 386 sugar, indicating that this glycan protects the CD4BS against antibodies.

CONCLUSION

The carbohydrate at position 386 is not essential for protein folding and function, but is involved in the protection of the CD4BS from antibodies. Removal of this sugar in the context of trimeric Env immunogens may therefore improve the elicitation of neutralizing CD4BS antibodies.