The Acquisition of Host-Derived Major Histocompatibility Complex Class II Glycoproteins by Human Immunodeficiency Virus Type 1 Accelerates the Process of Virus Entry and Infection in Human T-Lymphoid Cells

The Incorporation of Host Proteins into the External HIV-1 Envelope

The incorporation of biologically active host proteins into HIV-1 is a well-established phenomenon, particularly due to the budding mechanism of viral egress in which viruses acquire their external lipid membrane directly from the host cell. While this mechanism might seemingly imply that host protein incorporation is a passive uptake of all cellular antigens associated with the plasma membrane at the site of budding, this is not the case. Herein, we review the evidence indicating that host protein incorporation can be a selective and conserved process. We discuss how HIV-1 virions displaying host proteins on their surface can exhibit a myriad of altered phenotypes, with notable impacts on infectivity, homing, neutralization, and pathogenesis. This review describes the canonical and emerging methods to detect host protein incorporation, highlights the well-established host proteins that have been identified on HIV-1 virions, and reflects on the role of these incorporated proteins in viral pathogenesis and therapeutic targeting. Despite many advances in HIV treatment and prevention, there remains a global effort to develop increasingly effective anti-HIV therapies. Given the broad range of biologically active host proteins acquired on the surface of HIV-1, additional studies on the mechanisms and impacts of these incorporated host proteins may inform the development of novel treatments and vaccine designs.

Characterization and Role of Lentivirus-Associated Host Proteins

Enveloped viruses obtain their envelopes during the process of budding from infected cells. During this process, however, these viruses acquire parts of the host cell membranes and host cell-derived proteins as integral parts of their mature envelopes. These host-derived components of viral envelopes may subsequently exhibit various effects on the life cycle of the virus; virus cell interactions, especially host response to virus-incorporated self-proteins; and the pathogenesis of the disease induced by these viruses. Although it was known for some time that various viruses incorporate host cell-derived proteins, the issue of the role of these proteins has received increased attention, specifically in connection with human immunodeficiency virus (HIV) infection and development of acquired immunodeficiency syndrome (AIDS) in humans. The aim of this review is to summarize our current knowledge of the analysis and role of host-derived proteins associated with enveloped viruses, with emphasis on the potential role of these proteins in the pathogenesis of AIDS. Clearly, differences in the clinical outcome of those nonhuman primates infected with simian immunodeficiency virus (SIV) that are disease resistant compared with SIV-infected species that are disease susceptible provide a unique opportunity to determine whether differences in the incorporation of distinct sets of host proteins play a role with distinct clinical outcomes.

Human Immunodeficiency Virus Immune Cell Receptors, Coreceptors, and Cofactors: Implications for Prevention and Treatment

In the last three decades, extensive research on human immunodeficiency virus (HIV) has highlighted its capability to exploit a variety of strategies to enter and infect immune cells. Although CD4(+) T cells are well known as the major HIV target, with infection occurring through the canonical combination of the cluster of differentiation 4 (CD4) receptor and either the C-C chemokine receptor type 5 (CCR5) or C-X-C chemokine receptor type 4 (CXCR4) coreceptors, HIV has also been found to enter other important immune cell types such as macrophages, dendritic cells, Langerhans cells, B cells, and granulocytes. Interestingly, the expression of distinct cellular cofactors partially regulates the rate in which HIV infects each distinct cell type. Furthermore, HIV can benefit from the acquisition of new proteins incorporated into its envelope during budding events. While several publications have investigated details of how HIV manipulates particular cell types or subtypes, an up-to-date comprehensive review on HIV tropism for different immune cells is lacking. Therefore, this review is meant to focus on the different receptors, coreceptors, and cofactors that HIV exploits to enter particular immune cells. Additionally, prophylactic approaches that have targeted particular molecules associated with HIV entry and infection of different immune cells will be discussed. Unveiling the underlying cellular receptors and cofactors that lead to HIV preference for specific immune cell populations is crucial in identifying novel preventative/therapeutic targets for comprehensive strategies to eliminate viral infection.

Molecular determinants of the ratio of inert to infectious virus particles

The ratio of virus particles to infectious units is a classic measurement in virology and ranges widely from several million to below 10 for different viruses. Much evidence suggests a distinction be made between infectious and infecting particles or virions: out of many potentially infectious virions, few infect under regular experimental conditions, largely because of diffusion barriers. Still, some virions are inert from the start; others become defective through decay. And with increasing cell- and molecular-biological knowledge of each step in the replicative cycle for different viruses, it emerges that many processes entail considerable losses of potential viral infectivity. Furthermore, all-or-nothing assumptions about virion infectivity are flawed and should be replaced by descriptions that allow for spectra of infectious propensities. A more realistic understanding of the infectivity of individual virions has both practical and theoretical implications for virus neutralization, vaccine research, antiviral therapy, and the use of viral vectors.

Plasma membrane signaling in HIV-1 infection

Plasma membrane is a multifunctional structure that acts as the initial barrier against infection by intracellular pathogens. The productive HIV-1 infection depends upon the initial interaction of virus and host plasma membrane. Immune cells such as CD4+ T cells and macrophages contain essential cell surface receptors and molecules such as CD4, CXCR4, CCR5 and lipid raft components that facilitate HIV-1 entry. From plasma membrane HIV-1 activates signaling pathways that prepare the grounds for viral replication. Through viral proteins HIV-1 hijacks host plasma membrane receptors such as Fas, TNFRs and DR4/DR5, which results in immune evasion and apoptosis both in infected and uninfected bystander cells. These events are hallmark in HIV-1 pathogenesis that leads towards AIDS. The interplay between HIV-1 and plasma membrane signaling has much to offer in terms of viral fitness and pathogenicity, and a better understanding of this interplay may lead to development of new therapeutic approaches. This article is part of a Special Issue entitled: Viral Membrane Proteins - Channels for Cellular Networking.

Nature, nurture and HIV: The effect of producer cell on viral physiology

Macrophages and CD4-positive T lymphocytes are the major targets and producers of HIV-1. While the molecular details underlying HIV replication in macrophages and T cells become better understood, it remains unclear whether viruses produced by these target cells differ in their biological properties. Recent reports suggest that HIV virions incorporate a large number of producer cell proteins and lipids which have an effect on subsequent viral replication in newly infected cells. The identity and abundance of these incorporated factors varies between different types of producer cells, suggesting that they may influence the replication capacity and pathogenic activity of the virions produced by T cells and macrophages.

Antibody against integrin lymphocyte function-associated antigen 1 inhibits HIV type 1 infection in primary cells through caspase-8-mediated apoptosis

HIV-1 infection induces formation of a virological synapse wherein CD4, chemokine receptors, and cell-adhesion molecules such as lymphocyte function-associated antigen 1 (LFA-1) form localized domains on the cell surface. Studies show that LFA-1 on the surface of HIV-1 particles retains its adhesion function and enhances virus attachment to susceptible cells by binding its counterreceptor intercellular adhesion molecule 1 (ICAM-1). This virus-cell interaction augments virus infectivity by facilitating binding and entry events. In this study, we demonstrate that inhibition of the LFA-1/ICAM-1 interaction by a monoclonal antibody leads to decreased virus production and spread in association with increased apoptosis of HIV-infected primary T cells. The data indicate that the LFA-1/ICAM-1 interaction may limit apoptosis in HIV-1-infected T cells. This phenomenon appears similar to anoikis wherein epithelial cells are protected from apoptosis conferred by ligand-bound integrins. These results have implications for further understanding HIV pathogenesis and replication in peripheral compartments and lymphoid organs.

HLA-C and HIV-1: friends or foes?

The major histocompatibility complex class I protein HLA-C plays a crucial role as a molecule capable of sending inhibitory signals to both natural killer (NK) cells and cytotoxic T lymphocytes (CTL) via binding to killer cell Ig-like receptors (KIR). Recently HLA-C has been recognized as a key molecule in the immune control of HIV-1. Expression of HLA-C is modulated by a microRNA binding site. HLA-C alleles that bear substitutions in the microRNA binding site are more expressed at the cell surface and associated with the control of HIV-1 viral load, suggesting a role of HLA-C in the presentation of antigenic peptides to CTLs. This review highlights the role of HLA-C in association with HIV-1 viral load, but also addresses the contradiction of the association between high cell surface expression of an inhibitory molecule and strong cell-mediated immunity. To explore additional mechanisms of control of HIV-1 replication by HLA-C, we address specific features of the molecule, like its tendency to be expressed as open conformer upon cell activation, which endows it with a unique capacity to associate with other cell surface molecules as well as with HIV-1 proteins.

Role of G protein-coupled vasoactive intestinal peptide receptors in HIV integration

The pathogenesis of HIV infection is closely linked to the replication of the virus in vivo. Even though the progress in anti-HIV-1 chemotherapy in the past several years has been dramatic, the efficient protection against HIV-1 infection still remains one of the most important global challenges. The complete blockage of AIDS progression appears to be difficult with current treatment due to the rapid occurrence of viral drug-resistance, increasing cost and the likelihood of adverse side effects. Furthermore, although originally regarded with high hope, development of a suitable vaccine appears to be years away. The purpose of this article is to describe previous findings regarding a potentially important role of the vasoactive intestinal peptide/pituitary adenylate cyclase-activating polypeptide (VPAC) family of G protein-coupled receptors in HIV-1 infection, to provide evidence for the involvement of these receptors in providing signals that can control the integration of the virus into the host DNA and to report new findings that support a role for VPAC receptors in the facilitation of HIV integration.

CIITA enhances HIV-1 attachment to CD4+ T cells leading to enhanced infection and cell depletion

Activated CD4(+) T cells are more susceptible to HIV infection than resting T cells; the reason for this remains unresolved. Induction of CIITA and subsequent expression of the MHC class II isotype HLA-DR are hallmarks of CD4(+) T cell activation; therefore, we investigated the role of CIITA expression in T cells during HIV infection. CIITA-expressing SupT1 cells display enhanced virion attachment in a gp160/CD4-dependent manner, which results in increased HIV infection, virus release, and T cell depletion. Although increased attachment and infection of T cells correlated with HLA-DR surface expression, Ab blocking, transient expression of HLA-DR without CIITA, and short hairpin RNA knockdown demonstrate that HLA-DR does not directly enhance susceptibility of CIITA-expressing cells to HIV infection. Further analysis of the remaining MHC class II isotypes, HLA-DP and HLA-DQ, MHC class I isotypes, HLA-A, HLA-B, and HLA-C, and the class II Ag presentation genes, invariant chain and HLA-DM, demonstrate that these proteins likely do not contribute to CIITA enhancement of HIV infection. Finally, we demonstrate that in activated primary CD4(+) T cells as HLA-DR/CIITA expression increases there is a corresponding increase in virion attachment. Overall, this work suggests that induction of CIITA expression upon CD4(+) T cell activation contributes to enhanced attachment, infection, virus release, and cell death through an undefined CIITA transcription product that may serve as a new antiviral target.

Class II transactivator (CIITA) enhances cytoplasmic processing of HIV-1 Pr55Gag

Background

The Pr55^gag (Gag) polyprotein of HIV serves as a scaffold for virion assembly and is thus essential for progeny virion budding and maturation. Gag localizes to the plasma membrane (PM) and membranes of late endosomes, allowing for release of infectious virus directly from the cell membrane and/or upon exocytosis. The host factors involved in Gag trafficking to these sites are largely unknown. Upon activation, CD4+ T cells, the primary target of HIV infection, express the class II transcriptional activator (CIITA) and therefore the MHC class II isotype, HLA-DR. Similar to Gag, HLA-DR localizes to the PM and at the membranes of endosomes and specialized vesicular MHC class II compartments (MIICs). In HIV producer cells, transient HLA-DR expression induces intracellular Gag accumulation and impairs virus release.

Methodology/Principal Findings

Here we demonstrate that both stable and transient expression of CIITA in HIV producer cells does not induce HLA-DR-associated intracellular retention of Gag, but does increase the infectivity of virions. However, neither of these phenomena is due to recapitulation of the class II antigen presentation pathway or CIITA-mediated transcriptional activation of virus genes. Interestingly, we demonstrate that CIITA, apart from its transcriptional effects, acts cytoplasmically to enhance Pr160^gag-pol (Gag-Pol) levels and thereby the viral protease and Gag processing, accounting for the increased infectivity of virions from CIITA-expressing cells.

Conclusions/Significance

This study demonstrates that CIITA enhances HIV Gag processing, and provides the first evidence of a novel, post-transcriptional, cytoplasmic function for a well-known transactivator.

CD44 microbeads accelerate HIV-1 infection in T cells

Super-paramagnetic CD44 MicroBeads (Miltenyi) designed for the isolation of infectious HIV-1 from dilute or difficult biological samples dramatically enhance the infectivity of bound HIV virions, even if the original viral suspension is merely incubated with beads. Infection of the CEM T cell line with the NL4-3 virus clone or primary human CD4 T cells with X4- and R5-tropic clones and a clade C primary virus isolate all showed accelerated p24 production and larger fractions of infected target cells. Effects could be detected very early; incubation of virus with the CD44 MicroBeads promoted higher levels of viral integration within the first infection cycle. In summary, CD44 MicroBeads provide the means not only to concentrate dilute viral samples, but also to directly facilitate within days rather than weeks the in vitro expansion of patient isolates independent of coreceptor usage and the performance of HIV replication assays that require a large fraction of infected primary T cells.

Application of proteomics technology for analyzing the interactions between host cells and intracellular infectious agents

Host–pathogen interactions involve protein expression changes within both the host and the pathogen. An understanding of the nature of these interactions provides insight into metabolic processes and critical regulatory events of the host cell as well as into the mechanisms of pathogenesis by infectious microorganisms. Pathogen exposure induces changes in host proteins at many functional levels including cell signaling pathways, protein degradation, cytokines and growth factor production, phagocytosis, apoptosis, and cytoskeletal rearrangement. Since proteins are responsible for the cell biological functions, pathogens have evolved to manipulate the host cell proteome to achieve optimal replication. Intracellular pathogens can also change their proteome to adapt to the host cell and escape from immune surveillance, or can incorporate cellular proteins to invade other cells. Given that the interactions of intracellular infectious agents with host cells are mainly at the protein level, proteomics is the most suitable tool for investigating these interactions. Proteomics is the systematic analysis of proteins, particularly their interactions, modifications, localization and functions, that permits the study of the association between pathogens with their host cells as well as complex interactions such as the host–vector–pathogen interplay. A review on the most relevant proteomic applications used in the study of host–pathogen interactions is presented.

Virus-associated host CD62L increases attachment of human immunodeficiency virus type 1 to endothelial cells and enhances trans infection of CD4+ T lymphocytes

Previous studies have identified several host-derived cell-surface proteins incorporated within emerging human immunodeficiency virus type 1 (HIV-1) particles. Some of these molecules play a role in different steps of the virus life cycle and are often advantageous for the virus. We report here that the leukocyte L-selectin (also called CD62L) remains functional when inserted within the envelope of HIV-1. Indeed, we demonstrate that adsorption of virions to endothelial cells is enhanced upon acquisition of host-derived CD62L. The more important binding of CD62L-bearing HIV-1 particles resulted in a more efficient virus transmission to CD4+ T lymphocytes. Capture and eventual transfer of such CD62L-bearing virions by the endothelium could play a role in the pathogenesis of HIV-1 infection.

The nucleoside triphosphate diphosphohydrolase-1/CD39 is incorporated into human immunodeficiency type 1 particles, where it remains biologically active

Human immunodeficiency virus type 1 (HIV-1) carries a variety of host proteins in addition to virus-encoded structural proteins, both in its envelope and inside the viral particle. Previous studies have reported that the HIV-1 life-cycle is affected by such virus-associated host cell surface proteins. The nucleoside triphosphate diphosphohydrolase-1 (NTPDase1), also known as CD39, is a plasma membrane-bound ectoenzyme that hydrolyzes extracellular ATP and ADP to AMP. It has been shown that CD39 inhibits platelet function, and is thus a critical thromboregulatory molecule. We demonstrate here that host-derived CD39 is acquired by both laboratory-adapted and clinical variants of HIV-1 produced in cellular reservoirs of the virus. Moreover, purified CD39-bearing virions, but not isogenic viruses lacking CD39, display strong ATPase and ADPase activities. It is of particular interest that virions bearing this cellular enzyme can inhibit ADP-induced platelet aggregation, an effect blocked by an NTPDase inhibitor. On the basis of published and the present data on the functionality of human cellular proteins embedded within HIV-1, it can be proposed that these proteins might contribute to some of the immunologic deficiencies seen in infected individuals.

Attachment of human immunodeficiency virus to cells and its inhibition

The entry of enveloped viruses involves virus adsorption followed by close apposition of the viral and plasma membranes. This multistep process is initiated by specific binding interactions between glycoproteins in the viral envelope and appropriate receptors on the cell surface. In the case of HIV-1, attachment of virions to the cell surface is attributed to a high affinity interaction between envelope spike glycoproteins (Env, composed of the surface protein gp120 and the transmembrane protein gp41) and a complex made of the primary CD4 receptor and a seven-transmembrane co-receptor (e.g., CXCR4 or CCR5) (reviewed in [1]). Then a chain of dynamic events take place that enable the viral nucleocapsid to penetrate within the target cell following the destabilization of membrane microenvironment and the formation of a fusion pore.

Attachment factors

As obligate intracellular parasites, viruses must bind to, and enter, permissive host cells in order to gain access to the cellular machinery that is required for their replication. The very large number of mammalian viruses identified to date is reflected in the fact that almost every human and animal cell type is a target for infection by one, or commonly more than one, species of virus. As viruses have adapted to target certain cell types for their propagation, there is exquisite specificity in cellular tropism. This specificity is frequently, but not always, mediated by the first step in the viral replication cycle: attachment of viral surface proteins to receptors expressed on susceptible cells. Viral receptors may be protein, carbohydrate, and/or lipid. Many viruses can use more than one attachment receptor, and indeed may sequentially engage multiple receptors to infect a cell. Thus, it is useful to differentiate between attachment receptors, that simply allow viruses a foothold at the limiting membrane of a cell, and entry receptors that mediate delivery the viral genome into the cytoplasm. For some viruses the attachment factors that promote binding to permissive cells are very well defined, but the sequence of events that triggers viral entry is only now beginning to be understood. For other viruses, despite many efforts, the receptors remain elusive. In this chapter we will confine our review to viruses that infect mammals, with particular focus on human pathogens. We do not intend that this will be an exhaustive overview of viral attachment receptors; instead we will take a number of examples of well-characterized virus-receptor interactions, discuss supporting evidence, and highlight any controversies and uncertainties in the field. We will then conclude with a reflection on general principles of viral attachment, consider some exceptions to these principles, and make some suggestion for future research.

A B-lymphocyte binding peptide from BNRF1 induced antibodies inhibiting EBV-invasion of B-lymphocytes

Epstein-Barr virus (EBV) infects human target cells mainly through gp350/220-CD21 and gp42-MHCII interactions; however, it has been shown that these interactions are dispensable for EBV-invasion of susceptible cells, suggesting that other viral proteins are involved in this process. It is probable that tegument BNRF1/p140 protein is involved in EBV-invasion of target cells, since anti-p140 antibodies inhibit EBV-infection of B-lymphocytes and there is evidence that part of the protein is located on virus surface. Sixty-six peptides, covering the entire BNRF1/p140 sequence, were synthesised and tested in lymphoblastoid cell line binding assays. Peptides 11465 and 11521 bound with high affinity to Raji, Ramos and P3HR-1 cells but not to erythrocytes, showing cell-binding behaviour similar to EBV. These two peptides induced antibodies recognising live EBV-infected cells. Interestingly, peptide-11521 (YVLQNAHQIACHFHSNGTDA) or antibodies induced by this peptide inhibited EBV-binding to B-lymphocytes, suggesting that this p140-region could be involved in EBV and B-lymphocyte interaction.

Envelope glycoproteins are dispensable for insertion of host HLA-DR molecules within nascent human immunodeficiency virus type 1 particles

HLA-DR is a host-derived protein present at the surface of HIV-1. To clarify the mechanism through which this molecule is inserted within viruses, we monitored whether the incorporation process might be influenced by the level of virus-encoded envelope (Env) glycoproteins. Wild-type virions and viruses either lacking or bearing lower levels of Env were produced in different cell types. Results from a virus capture test indicate that HLA-DR is efficiently incorporated and at comparable levels in the tested virus preparations. Therefore, Env does not play an active role in the acquisition of host HLA-DR by emerging HIV-1 particles.

HIV Type 1 Can Act as an APC upon Acquisition from the Host Cell of Peptide-Loaded HLA-DR and CD86 Molecules

It is well documented that a wide range of host-derived cell surface constituents is inserted within HIV type 1 (HIV-1) and located on the exterior of the virion. Although no virus-associated protein of host origin has been shown to be absolutely required for virus replication, studies have revealed that many of these proteins are functional and can affect several steps of the virus life cycle. In this study, we found that HIV-1 acquires peptide-loaded class II MHC (MHC-II) and the costimulatory CD86 molecules from the host cell. Moreover, we present evidence that virions bearing such peptide-loaded MHC-II and CD86 proteins can lead to activation of the transcription factors NF-kappa B and NF-AT in an Ag-specific human T cell line. A linear correlation was found between activation of NF-kappa B and the amount of peptide-loaded MHC-II molecules inserted within HIV-1. Finally, transcription of unintegrated and integrated HIV-1 DNA was promoted upon exposure of peptide-specific human T cells to viruses bearing both peptide-loaded MHC-II and CD86 proteins. These data suggest that HIV-1 can operate as an APC depending on the nature of virus-anchored host cell membrane components. It can be proposed that HIV-1 can manipulate one of its primary targets through the process of incorporation of host-derived proteins.

Galectin-1 Acts as a Soluble Host Factor That Promotes HIV-1 Infectivity through Stabilization of Virus Attachment to Host Cells

The establishment of HIV type 1 (HIV-1) infection is initiated by the stable attachment of the virion to the target cell surface. Although this process relies primarily upon interaction between virus-encoded gp120 and cell surface CD4, a number of distinct interactions influence binding of HIV-1 to host cells. In this study, we report that galectin-1, a dimeric beta-galactoside-binding protein, promotes infection with R5, X4, and R5X4 variants. Galectin-1 acts as a soluble adhesion molecule by facilitating attachment of HIV-1 to the cell surface. This postulate is based on experiments where galectin-1 rendered HIV-1 particles more refractory to various agents that block HIV-1 adsorption and coreceptor binding (i.e., a blocking anti-CD4, soluble CD4, human anti-HIV-1 polyclonal Abs; stromal cell-derived factor-1alpha; RANTES). Experiments performed with the fusion inhibitor T-20 confirmed that galectin-1 is primarily affecting HIV-1 attachment. The relevance of the present findings for the pathogenesis of HIV-1 infection is provided by the fact that galectin-1 is abundantly expressed in the thymus and lymph nodes, organs that represent major reservoirs for HIV-1. Moreover, galectin-1 is secreted by activated CD8(+) T lymphocytes, which are found in high numbers in HIV-1-positive patients. Therefore, it is proposed that galectin-1, which is released in an exocrine fashion at HIV-1 replication sites, can cross-link HIV-1 and target cells and promote a firmer adhesion of the virus to the cell surface, thereby augmenting the efficiency of the infection process. Overall, our findings suggest that galectin-1 might affect the pathogenesis of HIV-1 infection.

Influenza virus activates human immunodeficiency virus type-1 gene expression in human CD4-expressing T cells through an NF-κB-dependent pathway

Influenza virus infection can cause severe complications in human immunodeficiency virus type-1 (HIV-1)-infected individuals leading to an increased risk of complications and death compared to that seen in uninfected individuals. We assessed the capacity of influenza virus (Flu) to modulate transcription of the HIV-1 long terminal repeat (LTR) in human CD4+ T cells. We found that Flu is able to promote expression of both the transiently transfected and stably integrated HIV-1 LTR-driven reporter gene. Experiments performed with Arthrobacter-derived neuraminidase and ammonium chloride revealed that Flu-dependent activation of HIV-1 transcription required an intimate contact between Flu and the target cell and efficient entry of Flu inside human CD4+ T cells. Amplification of a Flu-specific mRNA by RT-PCR indicated that human T cells were indeed productively infected with Flu. Virus preparations rendered noninfectious after UV irradiation could no longer upregulate HIV-1 LTR activity. Furthermore, experiments conducted with wild type and NF-kappaB-mutated HIV-1 LTR-directed reporter vectors suggested that the positive action of Flu on HIV-1 LTR activity was mediated through the induction of NF-kappaB. Our data show that fully competent Flu can lead to NF-kappaB-dependent activation of HIV-1 transcription in CD4+ T cells.

HLA-DR, ICAM-1, CD40, CD40L, and CD86 are incorporated to a similar degree into clinical human immunodeficiency virus type 1 variants expanded in natural reservoirs such as peripheral blood mononuclear cells and human lymphoid tissue cultured ex vivo

To provide additional information on the acquisition of host cell membrane proteins by human immunodeficiency virus type 1 (HIV-1) produced by natural cellular reservoirs, two different field isolates were used to infect ex vivo expanded peripheral blood mononuclear cells (PBMCs) and human lymphoid tissue histocultures. The insertion of host-derived HLA-DR, intercellular adhesion molecule-1 (ICAM-1), CD40, CD40L, and CD86 within HIV-1 particles was evaluated by using specific antibodies linked to a solid matrix to capture ultrafiltrated viral progeny. Overall, our data indicate that neither the HIV-1 co-receptor usage (i.e., T-tropic or macrophage-tropic) nor the cellular source of HIV-1 has an impact on the incorporation process but it was found to be under the influence of the donor source. Given that most viral replication is thought to occur in lymphoid tissues and previous works have shown that HIV-1 life cycle is affected by several virus-anchored host proteins, our results suggest that this phenomenon is likely to contribute to the pathogenesis of this retroviral infection.

Identifying gp85-regions involved in Epstein-Barr virus binding to B-lymphocytes

Epstein-Barr virus lacking glycoprotein gp85 cannot infect B-cells and epithelial cells. The gp85 belongs to the molecular complex required for virus invasion of B-lymphocyte or epithelial cells. Moreover, there is evidence that gp85 is necessary for virus attachment to epithelial cells. Thirty-six peptides from the entire gp85-sequence were tested in epithelial and lymphoblastoid cell line binding assays to identify gp85-regions involved in virus-cell interaction. Five of these peptides presented high binding activity to Raji, Ramos, P3HR-1, and HeLa cells, but not to erythrocytes; Raji-cell affinity constants were between 80 and 140nM. Of these five peptides, 11435 ((181)TYKRVTEKGDEHVLSLVFGK(200)), 11436 ((201)TKDLPDLRGPFSYPSLTSAQ(220)), and 11438 ((241)YFVPNLKDMFSRAVTMTAAS(260)) bound to a 65kDa protein on Raji-cell surface. These peptides and antibodies induced by them (recognising live EBV-infected cells) inhibited Epstein-Barr virus interaction with cord blood lymphocytes. It is thus probable that gp85-regions defined by peptides 11435, 11436, and 11438 are involved in EBV invasion of B-lymphocytes.

The importance of virus-associated host ICAM-1 in human immunodeficiency virus type 1 dissemination depends on the cellular context

The primary objective of this study was to define whether the nature of virion-bound host cell membrane proteins influenced the process of human immunodeficiency virus 1 (HIV-1) capture and transmission. We pulsed cells of monocytoid lineage (established and primary) and CD4-negative epithelial cells transiently expressing DC-SIGN or LFA-1 with isogenic HIV-1 particles either devoid or bearing host-derived ICAM-1 or ICAM-3 before incubation with an indicator cell line. To our surprise, the ICAM-1/LFA-1 association was a more efficient transmission factor than the combined gp120/DC-SIGN and ICAM-3/DC-SIGN interactions. The involvement of the association between virus-bound ICAM-1 and its natural ligand LFA-1 in virus binding and carriage was confirmed when using more physiological cellular targets, i.e., human lymphoid tissues cultured ex vivo. However, the contribution of virus-anchored host ICAM-1 to the process of retention and transmission of HIV-1 could not be confirmed when using primary human cells of macrophage/dendritic lineage as transmitter cells and autologous CD4+ T lymphocytes as targets. Altogether these data underscore the complexity of factors participating in virus-cell contact and efficient dissemination of HIV-1 to target cells.

Assessing human alloimmunization as a strategy for inducing HIV type 1 neutralizing anti-HLA responses

Xenovaccination of rhesus macaques with human HLA Class I and II proteins has been demonstrated to elicit protective immunity against challenge with SIV grown in human cells. To determine if alloimmunization in humans could lead to protective immunity against HIV-1, we prospectively followed a small group of women receiving whole-cell alloimmunization in the form of leukocyte immunotherapy for recurrent spontaneous abortion. Whole-cell vaccine recipients and their respective partners (referred to as donors) provided pre- and postimmune blood samples for analysis. Study participants were HLA typed by sequence-specific PCR and antibodies specific for HLA Class I and II antigens were measured in recipient plasma. To determine if anti-HLA antibody responses detected in recipient plasma samples were capable of neutralizing HIV-1 in vitro, we grew laboratory strain HIV-1(IIIB) and primary isolate HIV-1(301660) in donor-derived CD4(+) T lymphocytes. The ability of purified whole IgG from responding patients to neutralizing infectivity of the respective donor-derived virus was then assayed in vitro. All donor-recipient pairs were determined to be HLA discordant for at least one Class I and one Class II locus. Two of seven female recipients in total made strong anti-HLA antibody responses specific to the HLA haplotype of the male donor in response to the alloimmunization regimen. For one recipient, IgG antibodies specific for donor HLA Class I and II antigens were able to neutralize both HIV-1(IIIB) and a primary isolate HIV-1(301660). In addition polyclonal anti-HLA class II antibodies against a single determinant (DR4) of this donor were also neutralizing. In contrast, the other recipient exhibiting antibodies only against donor HLA Class I antigens did not neutralize HIV-1(IIIB). Using samples from a small number of women undergoing leukocyte immunotherapy, we have demonstrated for the first time that allele-specific anti-HLA antibodies elicited through human alloimmunization are capable of neutralizing HIV-1 in vitro.

Infectious HIV-1 assembles in late endosomes in primary macrophages

Although human immunodeficiency virus type 1 (HIV-1) is generally thought to assemble at the plasma membrane of infected cells, virions have been observed in intracellular compartments in macrophages. Here, we investigated virus assembly in HIV-1-infected primary human monocyte-derived macrophages (MDM). Electron microscopy of cryosections showed virus particles, identified by their morphology and positive labeling with antibodies to the viral p17, p24, and envelope proteins, in intracellular vacuoles. Immunolabeling demonstrated that these compartments contained the late endosomal marker CD63, which was enriched on vesicles within these structures and incorporated into the envelope of budding virions. The virus-containing vacuoles were also labeled with antibodies against LAMP-1, CD81, and CD82, which were also incorporated into the viral envelope. To assess the cellular source of infectious viruses derived from MDM, virus-containing media from infected cells were precipitated with specific antibodies. Only antibodies against antigens found in late endosomes precipitated infectious virus, whereas antibodies against proteins located primarily on the cell surface did not. Our data indicate that most of the infectious HIV produced by primary macrophages is assembled on late endocytic membranes and acquires antigens characteristic of this compartment. This notion has significant implications for understanding the biology of HIV and its cell-cell transmission.

Phosphatidylserine on HIV envelope is a cofactor for infection of monocytic cells

HIV-1 is an enveloped retrovirus that acquires its outer membrane as the virion exits the cell. Because of the association of apoptosis with the progression of AIDS, HIV-1-infected T cells or macrophages might be expected to express elevated levels of surface phosphatidylserine (PS), a hallmark of programmed cell death. Virions produced by these cells would also be predicted to have PS on the surface of their envelopes. In this study, data are presented that support this hypothesis and suggest that PS is required for macrophage infection. The PS-specific protein annexin V was used to enrich for virus particles and to inhibit HIV-1 replication in primary macrophages, but not T cells. HIV-1 replication was also significantly inhibited with vesicles consisting of PS, but not phosphatidylcholine. PS is specifically required for HIV-1 infection because viruses pseudotyped with vesicular stomatitis virus G and amphotropic murine leukemia virus envelopes were not inhibited by PS vesicles or annexin V. These data indicate that PS is an important cofactor for HIV-1 infection of macrophages.

Human macrophages accumulate HIV-1 particles in MHC II compartments

Macrophages are important targets for HIV-1 infection and harbor the virions in an as yet unidentified organelle. To determine the location of HIV-1 in these cells, an extensive analysis of primary human macrophages infected in vitro with HIV-1 was carried out by immuno-electron microscopy. Virus particles were found to accumulate in intracellular multivesicular compartments which were enriched in major histocompatibility complex class II molecules and CD63. These features are characteristics of major histocompatibility complex class II compartments where maturing class II molecules acquire their peptide cargo. The membrane-delimited, electron-dense virus particles of 100-110 nm diameter labeled strongly for HIV-1 p24 antigen, major histocompatibility complex class II molecules, CD63 and, to a lesser extent for HIV-1 gp120 envelope protein and Lamp 1. Our data suggest that virus particles may access the lumen of the major histocompatibility complex class II compartment by budding from the limiting membrane, thus acquiring proteins of this membrane such as class II and CD63. Viral assembly and budding would therefore occur in macrophages by a process similar to the formation of the internal vesicles in multivesicular bodies and at the same location. This could account for the particular content in lipids and proteins previously found in the membrane wrapping HIV particles. Our observations also suggest direct fusion of the virus containing major histocompatibility complex class II compartment with the plasma membrane, leading to massive release of viral particles into the extracellular medium.

New insights into the functionality of a virion-anchored host cell membrane protein: CD28 versus HIV type 1

It is now well established that the HIV type 1 (HIV-1) incorporates a vast array of host-encoded molecules in its envelope during the budding process. Interestingly, it was demonstrated that the attachment process is accentuated by supplementary interactions between virion-anchored host molecules and their cognate ligands. Such an enhancement of the viral attachment process was found to result in an increase of infectivity for both T and macrophage-tropic strains of HIV-1. Given that previous work indicates that HIV-1 is budding at the site of cell-to-cell contact, a location rich in the costimulatory CD28 glycoprotein, we investigated whether CD28 could be efficiently acquired by HIV-1. We have been able to generate progeny viruses bearing or not bearing on their surfaces host-derived CD28 using our previously described transient transfection and expression system. The physical presence of CD28 was found to markedly increase virus infectivity in a CD28/B7-dependent manner following infection of two human lymphoid cell lines expressing high levels of surface B7-1/B7-2, two natural ligands of CD28. The physiological significance of CD28 incorporation was provided by the observation that an anti-CD28 Ab decreased replication in primary human mononuclear cells of clinical isolates of HIV-1 propagated in such cells. A virus precipitation assay revealed that M-, T-, and dual-tropic clinical strains of HIV-1 produced in primary human mononuclear cells do indeed incorporate CD28. These results show for the first time that HIV-1 can incorporate CD28 and the acquisition of this specific host surface glycoprotein modulates the virus life cycle.

Development of a novel allo-independent HIV-1 virus preparation for use in immunoassays

The study of the immunologic response to whole human immunodeficiency virus type 1 (HIV-1) antigen is limited by the presence of highly immunogenic human leukocyte antigen (HLA) alloantigens on the envelope of wild type virus. This paper outlines the production of HIV-1 infectious virions free of HLA for use as whole viral antigens in immunoassays. An infectious molecular clone of HIV-1 was transfected into the K-562 cell line, which does not express HLA on the cell surface. After a 30-day selection period, to ensure stable transfection, cells and culture supernatants were analyzed for productive HIV-1 infection and virion infectivity. An enzyme-linked immunosorbent assay (ELISA) confirmed the presence of p24 in the culture supernatants. Molecular confirmation of HIV-1 transfection was achieved by gene amplification. Flow cytometric analysis was used to identify gp120 on the surface of the infected cells. Viral supernatants were tested for HIV infectivity in peripheral blood mononuclear cells (PBMCs). The usefulness of this viral preparation as whole virus antigens was validated using PBMCs from HIV-infected individuals. These results indicate the successful production of HIV-1 infectious virions, which do not have HLA molecules on their viral envelope, and demonstrate their utility for immunoassays.

VPAC1 is a cellular neuroendocrine receptor expressed on T cells that actively facilitates productive HIV-1 infection

OBJECTIVE

A lack of productive HIV-1 infection of Kit225 compared to Jurkat T cells, despite similar levels of CD4 and HIV-1 chemokine co-receptors, was found to correlate with the expression of vasoactive intestinal peptide/pituitary adenylate cyclase activating polypeptide receptor-1 (VPAC1). We therefore examined a role for this seven-transmembrane G protein-coupled neuroendocrine receptor in modulating HIV-1 infection.

METHODS

Reverse transcription-PCR was used to show the level of VPAC1 expression in different T-cell lines. A signal-blocking antibody to VPAC1 was used to examine its inhibiting effect on HIV-1 infection. Transfection of VPAC1 cDNA in both sense and anti-sense orientation was used to assess the role of VPAC1 in HIV-1 infection. HIV-1 infection was monitored by gag p24 ELISA using HIV-1IIIB or by luciferase activity using pseudo envelope-typed HXB2-NL4-3-luciferase. Analysis of HIV-1 gag DNA and 2-LTR circles was utilized to examine a possible mechanism for the effect of VPAC1.

RESULTS

Using VPAC1 signal blocking antibody, we showed that up to 80% of productive infection with HIV-1IIIB was inhibited. We also demonstrated that HIV-1 gp120 has sequence similarity to the natural ligand for VPAC1 and postulate that it can activate this receptor directly. Transfection of VPAC1 cDNA in the anti-sense orientation resulted in a significant loss, up to 50% of productive infection. In contrast, transfection of cells with VPAC1 in the sense orientation increased the productive infection by more than 15-fold and caused a profound increase in syncytium formation. Furthermore, stimulation of VPAC1 on primary cells facilitated in vitro infection with HIV-1 HXB2-NL4-3. Analysis of HIV-1 gag DNA indicated that VPAC1 does not affect viral entry; however, cells that show negligible expression of VPAC1 may not be productively infected as indicated by a lack of 2-LTR circle formation.

CONCLUSION

We have discovered a cellular receptor, VPAC1, that is a novel and potent facilitator of HIV-1 infection and thus, is a potentially important new target for therapeutic intervention.

Preferential attachment of HIV particles to activated and CD45RO+CD4+ T cells

We have studied the binding of biotinylated HIV particles to various cell lines and peripheral blood mononuclear cells (PBMCs). Viruses were harvested from cultures of cell surface-biotinylated cells productively infected with HIV-IIIB. Labeled HIV particles bound to and infected CD4(+) cell lines and PBMCs. The interaction between gp120 and CD4 contributed in part to HIV binding to CD4(+) cells. However, HIV binding was for the most part independent of CD4 expression and sensitive to polyanion inhibition. Polyanion-sensitive interactions involved heparan sulfate in cell lines but not in primary T cells. Interestingly, HIV binding to primary cells was heterogeneous and targeted discrete subsets of CD4(+) and CD4(-) cells. The CD4(+) T cell subset that displayed high HIV-binding capacity contained mostly CD4(+)CD45RO(+) cells, whereas the subset showing undetectable HIV binding contained higher proportions of CD4(+)CD45RO(-) cells. Consistently, purified CD4(+)CD45RO(-) cells or purified CD4(+) T cells with low virus-binding capacity showed lower HIV entry and delayed HIV replication when compared with purified CD4(+)CD45RO(+) or purified CD4(+) T cells with high virus-binding capacity, respectively. Our data suggest that the binding of HIV to cell surface-expressed CD4 might be inefficient in a subset of CD4(+) T cells and that increased binding of HIV to activated and CD4(+)CD45RO(+) cells may contribute to the higher susceptibility of these cells to HIV infection.

A novel virus capture assay reveals a differential acquisition of host HLA-DR by clinical isolates of human immunodeficiency virus type 1 expanded in primary human cells depending on the nature of producing cells and the donor source

Previous findings indicated that HLA-DR is probably one of the most abundant cellular constituents incorporated within the human immunodeficiency virus type 1 (HIV-1) envelope. Given that the life-cycle of HIV-1 has been reported to be modulated by virion-bound host HLA-DR, an improved version of a virus capture technique was developed to assess the degree of HLA-DR incorporation in several clinical isolates of HIV-1 derived from primary human peripheral blood mononuclear cells (PBMCs) and monocyte-derived macrophages (MDM). Analysis of virus stocks purified from PBMCs and MDM indicated that primary isolates of HIV-1 bearing distinct tropism (i.e. T-, macrophage-, and dual-tropic) all incorporate host cell membrane HLA-DR protein. The amount of incorporated HLA-DR varies among the primary HIV-1 isolates tested. Propagation of some clinical HIV-1 isolates in either autologous PBMCs or MDM resulted in differential incorporation of virion-bound cellular HLA-DR depending on the nature of the virus producer cells. Differences in the degree of HLA-DR incorporation were also noticed when macrophage-tropic isolates of HIV-1 were produced in MDM from different donors. Altogether these data show that the efficiency of HLA-DR incorporation into the envelope of primary isolates of HIV-1 is a multifactorial phenomenon since it is affected by the virus isolate itself, the nature of host cells (i.e. PBMCs or MDM) and the donor source.

Interactions between major histocompatibility complex class II surface expression and HIV: implications for pathogenesis

Although it has been almost 20 years since the first cases of acquired immunodeficiency syndrome (AIDS) were documented, the pathogenesis is still not completely understood. Interactions between major histocompatibility complex (MHC) Class I and human immunodeficiency virus (HIV), resulting in down-regulation of MHC-I surface expression, have been reported to contribute to pathogenesis by suppressing the host's immune response. Interactions between MHC Class II and HIV have also been described, but it is unclear how these contribute to the pathogenesis. MHC-II surface expression on HIV-infected monocytes and monocytic cell lines has been described to be increased as well as decreased when compared to uninfected control monocytes. HIV-specific mechanisms appear to down-regulate MHC-II expression on blood monocytes during HIV-1 infection, whereas host mechanisms up-regulate MHC-II expression in response to infection of blood monocytes as well as brain macrophages. A balance between these two may determine MHC-II expression levels in individual patients. Altogether, HIV seems to be able to benefit from both low and high levels of MHC-II surface expression. The first results in reduced immune surveillance of the host, allowing the virus to replicate faster; the second increases infectivity of the virus as a result of higher MHC-II density on macrophages and virion particles.

Targeting cell-free HIV and virally-infected cells with anti-HLA-DR immunoliposomes containing amphotericin B

OBJECTIVE

To evaluate the ability of liposomes bearing anti-HLA-DR Fab' fragments (immunoliposomes) and containing amphotericin B (AmB) to target and neutralize cell-free HIV-1 particles and virally-infected cells.

METHODS

The effect of AmB on the attachment and fusion of HIV-1(NL4-3) to Jurkat E6.1 cells has been evaluated using a p24 enzymatic assay. The ability of AmB to inhibit HIV-1-based luciferase reporter viruses pseudotyped with HXB2, AML-V and VSV-G envelopes has been evaluated in Jurkat E6.1 cells. The efficacy of free and immunoliposomal AmB to inhibit cell-free HIV, that have incorporated or not HLA-DR molecules, has been evaluated in HLA-DR/negative (NEG) 1G5 T cells and HLA-DR/positive (POS) Mono Mac 1 cells.

RESULTS

AmB inhibited HIV infectivity independently of the nature of viral envelope proteins. Pretreatment of HIV with AmB had no major effect on viral attachment and fusion process to Jurkat E6.1 cells. Immunoliposomal AmB (0.5 microg/ml) led to a 77% inhibition of replication of HLA-DR/POS HIV-1 with no cell toxicity, whereas free AmB had no significant antiviral activity at this concentration. A complete inhibition of viral replication was observed following incubation of viruses with immunoliposomal AmB (2.5 microg/ml). Anti-HLA-DR immunoliposomes containing AmB had no effect on the infectivity of HLA-DR/NEG HIV-1 particles in HLA-DR/NEG T lymphoid cells but completely inhibited replication of viruses in an HLA-DR/POS monocytic cell line.

CONCLUSION

The incorporation of neutralizing agents in anti-HLA-DR immunoliposomes could represent a novel therapeutic strategy to specifically target cell-free HIV particles and virally-infected cells to treat HIV infection more efficiently.

Targeting lymph nodes with liposomes bearing anti-HLA-DR Fab' fragments

The ability of liposomes bearing anti-HLA-DR Fab' fragments to target cells expressing the human HLA-DR determinant of the major histocompatibility complex class II (MHC-II) has been evaluated and compared to that of conventional liposomes. Anti-HLA-DR immunoliposomes did not bind to HLA-DR-negative cells. In contrast, a high level of binding was observed following incubation of immunoliposomes with cells bearing important levels of human HLA-DR. The accumulation of conventional and murine anti-HLA-DR immunoliposomes in different tissues has been investigated following a single subcutaneous injection given in the upper back of C3H mice. Anti-HLA-DR immunoliposomes resulted in a much better accumulation in the cervical and brachial lymph nodes when compared to conventional liposomes. The accumulation in the liver was similar for both liposomal preparations, whereas an approximately twofold decrease in accumulation was observed for immunoliposomes in the spleen. Given that HLA-DR surface marker is expressed on monocyte/macrophages and activated CD4+ T lymphocytes, the primary cellular reservoirs of the human immunodeficiency virus (HIV), the use of liposomes bearing surface-attached anti-HLA-DR could constitute a convenient strategy to more efficiently treat this debilitating retroviral disease. Moreover, the reported incorporation of high amounts of host-encoded HLA-DR proteins by HIV particles renders the use of liposomes bearing anti-HLA-DR antibodies even more attractive.

The effect of viruses on the ability to present antigens via the major histocompatibility complex

We describe viral pathogens that cause significant human disease by their ability to interfere with the expression of major histocompatibility complex class I and II molecules. Herpesviruses and papillomaviruses encode gene products that interfere with the class I pathway of antigen processing and/or peptide translocation. Adenoviruses encode unique gene products that interfere with transport of class I molecules. Influenza virus, measles virus, and HIV interfere with the class II pathway by either suppressing the production of class II molecules or impeding antigen trafficking. Cytomegalovirus interferes with both class I and class II pathways. Better understanding of these mechanisms may lead to further insight into the pathogenesis of viral infections and allow for improved treatments.

Level of ICAM-1 surface expression on virus producer cells influences both the amount of virion-bound host ICAM-1 and human immunodeficiency virus type 1 infectivity

Using virions harvested from 293T cells stably expressing either low or high levels of surface ICAM-1, we determined that the number of virus-embedded host ICAM-1 proteins is positively influenced by the expression level of ICAM-1 on virus producer cells. Moreover, the increase in virion-bound host cell membrane ICAM-1 led to a concomitant enhancement of virus infectivity when a T-cell-tropic strain of human immunodeficiency virus type 1 (HIV-1) was used. The phenomenon was also seen when primary human cells were infected with virions pseudotyped with the envelope protein from a macrophage-tropic HIV-1 isolate, thus ruling out any envelope-specific effect. We also observed that target cells treated with NKI-L16, an anti-LFA-1 antibody known to increase the affinity of LFA-1 for ICAM-1, were markedly more susceptible to infection with HIV-1 particles bearing on their surfaces large numbers of host-derived ICAM-1 proteins. Given that cellular activation of leukocytes is known to modify the conformational state of LFA-1 and induce ICAM-1 surface expression, it is tempting to speculate that activation of virus-infected cells will lead to the production of HIV-1 particles bearing more host ICAM-1 on their surfaces and that such progeny virions will preferentially infect and replicate more efficiently in activated cells which are prevalent in lymphoid organs.