Bitter-sweet symphony: defining the role of dendritic cell gp120 receptors in HIV infection

Myeloid C-Type Lectin Receptors in Tuberculosis and HIV Immunity: Insights Into Co-infection?

C-type lectin receptors (CLRs) are carbohydrate binding pattern recognition receptors (PRRs) which play a central role in host recognition of pathogenic microorganisms. Signaling through CLRs displayed on antigen presenting cells dictates important innate and adaptive immune responses. Several pathogens have evolved mechanisms to exploit the receptors or signaling pathways of the CLR system to gain entry or propagate in host cells. CLR responses to high priority pathogens such as Mycobacterium tuberculosis (Mtb), HIV, Ebola, and others are described and considered potential avenues for therapeutic intervention. Mtb and HIV are the leading causes of death due to infectious disease and have a synergistic relationship that further promotes aggressive disease in co-infected persons. Immune recognition through CLRs and other PRRs are important determinants of disease outcomes for both TB and HIV. Investigations of CLR responses to Mtb and HIV, to date, have primarily focused on single infection outcomes and do not account for the potential effects of co-infection. This review will focus on CLRs recognition of Mtb and HIV motifs. We will describe their respective roles in protective immunity and immune evasion or exploitation, as well as their potential as genetic determinants of disease susceptibility, and as avenues for development of therapeutic interventions. The potential convergence of CLR-driven responses of the innate and adaptive immune systems in the setting of Mtb and HIV co-infection will further be discussed relevant to disease pathogenesis and development of clinical interventions.

Human Blood-Circulating Basophils Capture HIV-1 and Mediate Viral trans-Infection of CD4+ T Cells

Cell-associated HIV-1 infection has been proposed to play a pivotal role in the spread of HIV-1 infection. Granulocytes are a category of white blood cells, comprising mainly basophils, neutrophils, and eosinophils, and participate in various inflammatory reactions and defense against pathogens. Here, we investigated the role of human blood granulocytes in the dissemination of HIV-1. These cells were found to express a variety of HIV-1 attachment factors (HAFs). Basophils expressed HAFs dendritic cell (DC)-specific intercellular adhesion molecule 3 (ICAM3)-grabbing nonintegrin (DC-SIGN), DC immunoreceptor (DCIR), heparan sulfate proteoglycan (HSPG), and α4β7 integrin and mediated the most efficient capture of HIV-1 on the cell surface. Neutrophils were found to express DCIR and demonstrated limited efficiency of viral capture. Eosinophils expressed α4β7 integrin but exhibited little or no virus-binding capacity. Intriguingly, following direct contact with CD4+ T cells, viruses harbored on the surface of basophils were transferred to T cells. The contact between basophils and CD4+ T cells and formation of infectious synapses appeared necessary for efficient HIV-1 spread. In HIV-1-infected individuals, the frequency of basophils remained fairly stable over the course of disease, regardless of CD4+ T depletion or the emergence of AIDS-associated opportunistic infections. Collectively, our results provide novel insights into the roles of granulocytes, particularly basophils, in HIV-1 dissemination. Thus, strategies designed to prevent basophil-mediated viral capture and transfer may be developed into a new form of therapy.

IMPORTANCE

Cell-associated HIV-1 infection has been proposed to play a pivotal role in the spread of HIV-1 infection. Here, we demonstrated that human blood-circulating granulocytes, particularly basophils, can capture HIV-1 and mediate viral trans-infection of CD4+ T cells. The expression of a variety of HIV-1 attachment factors, such as the C-type lectins, etc., facilitates viral capture and transfer. Intriguingly, the frequency of basophils in patients with different levels of CD4+ T counts remains fairly stable during the course of disease. Our results provide novel insights into the roles of granulocytes, particularly basophils, in HIV-1 dissemination. We suggest that strategies designed to prevent basophil-mediated viral capture and transfer may be a new direction for the development of anti-HIV therapy.

HIV-1 gp120 protein downregulates Nef induced IL-6 release in immature dentritic cells through interplay of DC-SIGN

HIV-1 replication is a tightly controlled mechanism which demands the interplay of host as well as viral factors. Both gp120 (envelope glycoprotein) and Nef (regulatory protein) have been correlated with the development of AIDS disease in independent studies. In this context, the ability of HIV-1 to utilize immature dentritic cells for transfer of virus is pivotal for early pathogenesis. The presence of C-type lectins on dendritic cells (DCs) like DC-SIGN, are crucial in inducing antiviral immunity to HIV-1. Both gp120 and Nef induce the release of cytokines leading to multiple effects of viral pathogenesis. Our study elucidated for the first time the cross-talk of the signaling mechanism of these two viral proteins in immature monocyte derived dentritic cells (immDCs). Further, gp120 was found to downregulate the IL-6 release by Nef, depending on the interaction with DC-SIGN. A cascade of signaling followed thereafter, including the activation of SOCS-3, to mediate the diminishing effect of gp120. Our results also revealed that the anti-apoptotic signals emanated from Nef was put to halt by gp120 through inhibition of Nef induced STAT3. Thus our results implicate that the signaling generated by gp120 and Nef, undergoes a switch-over mechanism that significantly contributes to the pathogenesis of HIV-1 and widens our view towards the approach on battling the viral infection.

Simian immunodeficiency virus interactions with macaque dendritic cells

This chapter summarizes advances in the following areas: (1) dendritic cell (DC)-mediated simian immunodeficiency virus (SIV) transmission, (2) role of DCs in innate and adaptive immunity against SIV, and (3) approaches to harness DC function to induce anti-SIV responses. The nonhuman primate (NHP) model of human immunodeficiency virus (HIV) infection in rhesus macaques and other Asian NHP species is highly relevant to advance the understanding of virus-host interactions critical for transmission and disease pathogenesis. HIV infection is associated with changes in frequency, phenotype, and function of the two principal subsets of DCs, myeloid DCs and plasmacytoid DCs. DC biology during pathogenic SIV infection is strikingly similar to that observed in HIV-infected patients. The NHP models provide an opportunity to dissect the requirements for DC-driven SIV infection and to understand how SIV distorts the DC system to its advantage. Furthermore, the SIV model of mucosal transmission enables the study of the earliest events of infection at the portal of entry that cannot be studied in humans, and, importantly, the involvement of DCs. Nonpathogenic infection in African NHP hosts allows investigations into the role of DCs in disease control. Understanding how DCs are altered during SIV infection is critical to the design of therapeutic and preventative strategies against HIV.

HIV infection of dendritic cells subverts the IFN induction pathway via IRF-1 and inhibits type 1 IFN production

Many viruses have developed mechanisms to evade the IFN response. Here, HIV-1 was shown to induce a distinct subset of IFN-stimulated genes (ISGs) in monocyte-derived dendritic cells (DCs), without detectable type I or II IFN. These ISGs all contained an IFN regulatory factor 1 (IRF-1) binding site in their promoters, and their expression was shown to be driven by IRF-1, indicating this subset was induced directly by viral infection by IRF-1. IRF-1 and -7 protein expression was enriched in HIV p24 antigen-positive DCs. A HIV deletion mutant with the IRF-1 binding site deleted from the long terminal repeat showed reduced growth kinetics. Early and persistent induction of IRF-1 was coupled with sequential transient up-regulation of its 2 inhibitors, IRF-8, followed by IRF-2, suggesting a mechanism for IFN inhibition. HIV-1 mutants with Vpr deleted induced IFN, showing that Vpr is inhibitory. However, HIV IFN inhibition was mediated by failure of IRF-3 activation rather than by its degradation, as in T cells. In contrast, herpes simplex virus type 2 markedly induced IFNβ and a broader range of ISGs to higher levels, supporting the hypothesis that HIV-1 specifically manipulates the induction of IFN and ISGs to enhance its noncytopathic replication in DCs.

HIV-1-infected dendritic cells show 2 phases of gene expression changes, with lysosomal enzyme activity decreased during the second phase

Dendritic cells (DCs) play a key role in the pathogenesis of HIV infection. HIV interacts with these cells through 2 pathways in 2 temporal phases, initially via endocytosis and then via de novo replication. Here the transcriptional response of human DCs to HIV-1 was studied in these phases and at different stages of the virus replication cycle using purified HIV-1 envelope proteins, and inactivated and viable HIV-1. No differential gene expression was detected in response to envelope. However, more than 100 genes were differentially expressed in response to entry of viable and inactivated HIV-1 in the first phase. A completely different set of genes was differentially expressed in the second phase, predominantly in response to viable HIV-1, including up-regulation of immune regulation genes, whereas genes encoding lysosomal enzymes were down-regulated. Cathepsins B, C, S, and Z RNA and protein decreased, whereas cathepsin L was increased, probably reflecting a concomitant decrease in cystatin C. The net effect was markedly diminished cathepsin activity likely to result in enhanced HIV-1 survival and transfer to contacting T lymphocytes but decreased HIV-1 antigen processing and presentation to these T cells.

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.

Maturation of blood-derived dendritic cells enhances human immunodeficiency virus type 1 capture and transmission

Dendritic cells (DCs) are specialized antigen-presenting cells. However, DCs exposed to human immunodeficiency virus type 1 (HIV-1) are also able to transmit a vigorous cytopathic infection to CD4(+) T cells, a process that has been frequently related to the ability of DC-SIGN to bind HIV-1 envelope glycoproteins. The maturation of DCs can increase the efficiency of HIV-1 transmission through trans infection. We aimed to comparatively study the effect of maturation in monocyte-derived DCs (MDDCs) and blood-derived myeloid DCs during the HIV-1 capture process. In vitro capture and transmission of envelope-pseudotyped HIV-1 and its homologous replication-competent virus to susceptible target cells were assessed by p24(gag) detection, luciferase activity, and both confocal and electron microscopy. Maturation of MDDCs or myeloid DCs enhanced the active capture of HIV-1 in a DC-SIGN- and viral envelope glycoprotein-independent manner, increasing the life span of trapped virus. Moreover, higher viral transmission of mature DCs to CD4(+) T cells was highly dependent on active viral capture, a process mediated through cholesterol-enriched domains. Mature DCs concentrated captured virus in a single large vesicle staining for CD81 and CD63 tetraspanins, while immature DCs lacked these structures, suggesting different intracellular trafficking processes. These observations help to explain the greater ability of mature DCs to transfer HIV-1 to T lymphocytes, a process that can potentially contribute to the viral dissemination at lymph nodes in vivo, where viral replication takes place and there is a continuous interaction between susceptible T cells and mature DCs.

Galactosyl ceramide expressed on dendritic cells can mediate HIV-1 transfer from monocyte derived dendritic cells to autologous T cells

Mucosa, comprising epithelial and dendritic cells, are the major sites for Human Immunodeficiency Virus type 1 (HIV-1) transmission. There, DCs can capture incoming HIV-1 and in turn transfer virus to CD4(+) T lymphocytes in a two-phase process, thereby initiating HIV-1 dissemination. We show that the glycosphingolipid Galactosyl Ceramide (GalCer), acting as mucosal epithelial receptor for HIV-1, was expressed by human monocyte derived immature DCs (iDCs), human primary DCs isolated from blood and mucosal tissue and in situ on mucosal tissue and acts as HIV-1-gp41 receptor. Blocking both GalCer and CD4 with specific mAbs results in a >95% transfer inhibition of HIV-1 from human monocyte-derived iDCs to autologous resting T cells. GalCer interaction with HIV-1 controls the early infection-independent phase of HIV-1 transfer to T cells. Thus, GalCer appears as an initial receptor for HIV-1, common to both mucosal epithelial cells and iDCs.

Impaired accessory cell function in a human dendritic cell line after human immunodeficiency virus infection

We generated human dendritic cell (DC) hybridoma cell lines by fusing HGPRT-deficient promonocytic U937 cells with immature DCs obtained by culturing peripheral blood monocytes with interleukin-4 (IL-4; 1,000 U/ml) and granulocyte-macrophage colony-stimulating factor (100 U/ml) for 7 days and mature DCs by treatment with tumor necrosis factor alpha (12.5 microg/ml) for 3 days. Only one fusion with immature DCs was successful and yielded four cell lines--HB-1, HB-2, HB-3, and HB-9--with an overall fusion efficiency of 0.0015%. The cell lines were stable in long-term culture, displayed morphological features typical of DCs, and expressed distinct class I and class II molecules not present on U937 (A*031012, B*51011, Cw*0701, DRB3*01011 52, and DR5*01011). A representative cell line, HB-2, that expressed DC markers including CD83, CD80 and CD86 could be induced to produce IL-12 through CD40 stimulation. After human immunodeficiency virus (HIV) infection, there was impairment of antigen-presenting cell (APC) function, which was manifested by an inability to stimulate allogeneic T-cell responses. There was no change in expression of major histocompatibility complex class I and class II antigens, CD83, CD40, CD4, CD11c, CD80, CD86, CD54, and CD58, or IL-12 production in the HIV-infected HB-2 cells. The HIV-infected HB-2 cells induced T-cell apoptosis in the cocultures. T-cell proliferation could be partially restored by using ddI, indinivir, and blocking anti-gp120 and anti-IL-10 antibodies. Our data suggest that there are multiple mechanisms that DCs use to inhibit T-cell responses in HIV-infected patients. The HB-2 cell line could be a useful model system to study APC function in HIV-infected DCs.

Lactoferrin prevents dendritic cell-mediated human immunodeficiency virus type 1 transmission by blocking the DC-SIGN--gp120 interaction

One of the cell types first encountered by human immunodeficiency virus type 1 (HIV-1) following sexual transmission are dendritic cells (DC). DC capture HIV-1 through C-type lectin receptors, of which the best studied example is DC-SIGN, which mediates HIV-1 internalization. DC can keep the virus infectious for several days and are able to transmit HIV-1 to CD4(+) T cells. We tested proteins from milk and serum for their ability to block DC-mediated HIV-1 transmission, of which bovine lactoferrin (bLF) is the most potent inhibitor. bLF binds strongly to DC-SIGN, thus preventing virus capture and subsequent transmission. Interestingly, bLF is a much more efficient inhibitor of transmission than human lactoferrin. Since bLF is nontoxic and easy to purify in large quantities, it is an interesting candidate microbicide against HIV-1. Another advantage of bLF is its ability to block HIV-1 replication in T cells. DC-mediated capture of a bLF-resistant HIV-1 variant that was selected during long-term culturing in T cells could still be blocked by bLF. This underscores the usefulness of bLF as a microbicide drug to prevent HIV-1 transmission.

Dichotomy between CD1a+ and CD83+ dendritic cells in lymph nodes during SIV infection of macaques

The prevalence and differentiation of dendritic cells (DC) in lymphoid tissue of simian immunodeficiency virus (SIV)-infected cynomolgus monkeys was studied during disease progression. Lymph node biopsies were consecutively obtained from clinical rapid and slow progressors until the development of disease consistent with simian acquired immunodeficiency syndrome (sAIDS) occurred. Quantitative evaluation of CD1a+ DC and the expression of DC antigens related to maturation (CD83, DC-LAMP and S100b) were performed at the single cell level by in situ image analysis. Despite a persistent prevalence of CD1a+ DC in lymphoid tissue during disease progression, there was a subsequent drop of mature CD83+, DC-LAMP+ and S100b+ DC, correlating with the decline of CD4+ T cells in blood. Thus, disease progression to sAIDS was associated with impaired maturation of DC, and lack of CD83, DC-LAMP and S100b expression.

The role of dendritic cell C-type lectin receptors in HIV pathogenesis

Dendritic cells play a major role in HIV pathogenesis. Epithelial dendritic cells appear to be one of the first cells infected after sexual transmission and transfer of the virus to CD4 lymphocytes, simultaneously activating these cells to produce high levels of HIV replication. Such transfer may occur locally in inflamed mucosa or after dendritic cells have matured and migrated to local lymph nodes. Therefore, the mechanism of binding, internalization, infection and transfer of HIV to CD4 lymphocytes is of great interest. Recently, the role of the C-type lectin DC-SIGN as a dendritic cell receptor for HIV has been intensively studied with in vitro monocyte-derived dendritic cells. However, it is clear that other C-type lectin receptors such as Langerin on Langerhan cells and mannose receptor on dermal dendritic cells are at least equally important for gp120 binding on epithelial dendritic cells. C-type lectin receptors play a role in virus transfer to T cells, either via de novo infection ("cis transfer") or without infection ("in trans" or transinfection). Both these processes are important in vitro, and both may have a role in vivo, although the low-level infection of immature dendritic cells may be more important as it leads to R5 HIV strain selection and persistence of virus within dendritic cells for at least 24 h, sufficient for these cells to transit to lymph nodes. The exact details of these processes are currently the subject of intense study.

Decreased stimulation of CD4+ T cell proliferation and IL-2 production by highly enriched populations of HIV-infected dendritic cells

APC infection and dysfunction may contribute to the immunopathogenesis of HIV disease. In this study, we examined immunologic function of highly enriched populations of HIV-infected monocyte-derived dendritic cells (DC). Compared with uninfected DC, HIV-infected DC markedly down-regulated surface expression of CD4. HIV p24(+) DC were then enriched by negative selection of CD4(+)HIV p24(-) DC and assessed for cytokine secretion and immunologic function. Although enriched populations of HIV-infected DC secreted increased IL-12p70 and decreased IL-10, these cells were poor stimulators of allogeneic CD4(+) T cell proliferation and IL-2 production. Interestingly, HIV-infected DC secreted HIV gp120 and the addition of soluble (s) CD4 (a known ligand for HIV gp120) to DC-CD4(+) T cell cocultures restored T cell proliferation in a dose-dependent manner. By contrast, addition of antiretroviral drugs did not affect CD4(+) T cell proliferation. Furthermore, recombinant HIV gp120 inhibited proliferation in uninfected cocultures of allogeneic DC and CD4(+) T cells, an effect that was also reversed by addition of sCD4. In summary, we show that HIV gp120 produced by DC infected by HIV in vitro impairs normal CD4(+) T cell function and that sCD4 completely reverses HIV gp120-mediated immunosuppression. We hypothesize that HIV-infected DC may contribute to impaired CD4(+) T cell-mediated immune responses in vivo and that agents that block this particular immunosuppression may be potential immune adjuvants in HIV-infected individuals.

The influence of cytokines, chemokines and their receptors on HIV-1 replication in monocytes and macrophages

Monocytes, macrophages and dendritic cells play an important role in the initial infection and contribute to its pathogenesis throughout the course of infection. Myeloid cells express CD4 and chemokine receptors known for HIV-1 fusion and entry. The beta-chemokine receptor, CCR5, is the major co-receptor in conjunction with CD4 for macrophage (M)-tropic or (R5) isolates of HIV-1, whereas the alpha-chemokine receptor, CXCR4, facilitates entry of T-tropic or (X4) HIV-1 strains. Cells of myeloid lineage may be infected predominantly with R5- strains, although infection with dual-tropic isolates of HIV-1 (exhibiting the capacity to use CCR-5 and/or CXCR-4 for entry) or some strains of X4- isolates has also been reported. The expression of chemokine receptors, HIV-1 infection and replication is under continuous regulation by a complex cytokine network produced by a variety of cells. The effects of cytokines/chemokines on HIV-1 replication in cells of myeloid lineage can be inhibitory (IFN-alpha, IFN-beta, IFN-gamma, GM-CSF, IL-10, IL-13 and IL-16 and beta-chemokines), stimulatory (M-CSF, TNF-alpha, TNF-beta, IL-1, IL-6) or bifunction al, that is both inhibitory and stimulatory (IL-4). This review focuses on the overall expression of chemokine receptors on cells of myeloid lineage and considers the mechanisms of entry of R5-, X4- and dual-tropic strains of HIV-1 into these cells. The effects of cytokines/chemokines on viral entry and productive HIV-1 infection are also reviewed.

Natural type 1 interferon producing cells in HIV infection

Natural type 1 interferon producing cells (IPCs) are in the first line of defense against infectious pathogens. Besides the known properties of type 1 interferons in inhibiting human immunodeficiency virus (HIV) replication, the recent characterization of human IPCs and the possibility to purify them for in vitro studies has greatly accelerated the study of their role in HIV infection. The blood IPC numbers and function are decreased in HIV primary infection and in advanced stages of HIV infection. Loss of circulating IPCs correlates with a high HIV viral load and the occurrence of opportunistic infections. Moreover, HIV can directly infect IPCs in vitro, providing a potential explanation for their in vivo depletion. Thus, the balance between IPCs and HIV replication might be critical in determining the control or progression of HIV infection.

Diversity of receptors binding HIV on dendritic cell subsets

The ability of HIV-1 to use dendritic cells (DCs) for transport and to transfer virus to activated T cells in the lymph node may be crucial in early HIV-1 pathogenesis. We have characterized primary DCs for the receptors involved in viral envelope attachment and observed that C-type lectin receptor (CLR) binding was predominant in skin DCs, whereas binding to emigrating and tonsil DCs was CD4-dependent. No one CLR was solely responsible for envelope binding on all skin DC subsets. DC-SIGN (DC-specific ICAM-3-grabbing nonintegrin) was only expressed by CD14(+)CDla(lo) dermal DCs. The mannose receptor was expressed by CD1a(hi) and CD14(+)CDla(lo) dermal DCs, and langerin was expressed by Langerhans cells. The diversity of CLRs able to bind HIV-1 in skin DCs may reflect their ability to bind a range of microbial glycoproteins.

Infectious and whole inactivated simian immunodeficiency viruses interact similarly with primate dendritic cells (DCs): differential intracellular fate of virions in mature and immature DCs

As potential targets for human immunodeficiency virus type 1 and simian immunodeficiency virus (HIV-1 and SIV), dendritic cells (DCs) likely play a significant role in the onset and spread of infection as well as in the induction of antiviral immunity. Using the SIV-macaque system to study the very early events in DC-virus interactions, we compared chemically inactivated SIV having conformationally and functionally intact envelope glycoproteins (2,2'-dithiodipyridine [AT-2] SIV) to infectious and heat-treated SIV. Both human and macaque DCs interact similarly with SIV without detectable effects on DC viability, phenotype, or endocytic function. As assessed by measuring cell-associated viral RNA, considerable amounts of virus are captured by the DCs and this is reduced when the virus is heat treated or derived from a strain that expresses low levels of envelope glycoprotein. Immunostaining for SIV proteins and electron microscopy indicated that few intact virus particles are retained at the periphery of the endocytically active, immature DCs. This contrasts with a perinuclear localization of numerous virions in large vesicular compartments deeper within mature DCs (in which macropinocytosis is down-regulated). Both immature and mature DCs are capable of clathrin-coated pit-mediated uptake of SIV, supporting the notion that the receptor-mediated uptake of virus can occur readily in mature DCs. While large numbers of whole viruses were preferentially found in mature DCs, both immature and mature DCs contained similar amounts of viral RNA, suggesting that different uptake/virus entry mechanisms are active in immature and mature DCs. These findings have significant implications for cell-to-cell transmission of HIV-1 and SIV and support the use of AT-2 SIV, an authentic but noninfectious form of virus, as a useful tool for studies of processing and presentation of AT-2 SIV antigens by DCs.