The acquisition of host-encoded proteins by nascent HIV-1

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.

Cellular proteins detected in HIV-1

It has been known for some time that HIV-1 virions contain cellular proteins in addition to proteins encoded by the viral genome. Recent studies have vastly increased the number of host proteins detected in HIV-1. This review summarises the current findings on several cellular proteins present in these virions, including some functional studies on their potential roles in the viral replication cycle and pathogenesis. Because retroviruses require extensive assistance from host proteins and pathways, the data from biochemical characterisations of HIV-1 serve as an important starting point for understanding the role of cellular proteins that act in or influence the biology of HIV-1. Additionally, a better understanding of the interactions between cellular proteins and viral components might provide more targets for anti-HIV therapeutic intervention and provide for a better understanding of how HIV-1 alters the immune system. The extensive study of HIV-1 has already brought new insights to the fields of immunology and vaccine science. In the same way, knowledge of viral--cellular protein interactions might assist our understanding of important cellular pathways.

Combining human antisera to human leukocyte antigens, HIVgp120 and 70 kDa heat shock protein results in broadly neutralizing activity to HIV-1

OBJECTIVE

To elicit broadly neutralizing antibody activity by combining polyclonal human serum IgG antibodies with HIVgp120, human leukocyte antigen (HLA) class I or class II and 70 kDa heat shock protein.

DESIGN

: In addition to HIV antigens, HIV-1 virions express HLA class I, HLA class II and 70 kDa heat shock protein molecules, which have quantitative and functional significance. The complementary effect of combining human polyclonal IgG antibodies with these antigens may result in effective broad spectrum neutralizing activity.

METHODS

Polyclonal human sera with IgG antibodies and monoclonal antibody to HLA class I or class II, HIVgp120 and 70 kDa heat shock protein were selected and used in single, double or triple combinations. Dose-dependent inhibition studies of HIV-1 clades A, B, C and D were carried out using human CD4 T cells treated with the combinations of human sera and with monoclonal antibodies for clade B. The results are presented as half maximal (IC50) inhibitory concentration and maximum inhibition by these sera.

RESULTS

The half maximal (IC50) inhibitory concentration of clade B HIV-1 infection with single or a combination of two antisera was higher than those with three antisera, which also showed maximum inhibition of HIV-1. Further investigations of human sera with HIV-1 clades C and D also showed lower half maximal (IC50) inhibitory concentrations and higher maximum inhibition with combinations of the three antisera, but this was not seen with clade A.

CONCLUSION

A novel vaccination strategy eliciting broadly neutralizing antibody activity to the CCR5-using HIV-1 clades B, C and D has been demonstrated by the trimolecular complex of human antisera with HLA class II or class I, HIVgp120 and 70 kDa heat shock protein.

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.

Gp120 V3-dependent impairment of R5 HIV-1 infectivity due to virion-incorporated CCR5

Entry of R5 human immunodeficiency virus type 1 (HIV-1) into target cells requires sequential interactions of the envelope glycoprotein gp120 with the receptor CD4 and the coreceptor CCR5. We investigated replication of 45 R5 viral clones derived from the HIV-1JR-FLan library carrying 0-10 random amino acid substitutions in the gp120 V3 loop. It was found that 6.7% (3/45) of the viruses revealed >or=10-fold replication suppression in PM1/CCR5 cells expressing high levels of CCR5 compared with PM1 cells expressing low levels of CCR5. In HIV-1V3L#08, suppression of replication was not associated with entry events and viral production but with a marked decrease in infectivity of nascent progeny virus. HIV-1V3L#08, generated from infected PM1/CCR5 cells, was 98% immunoprecipitated by anti-CCR5 monoclonal antibody T21/8, whereas the other infectious viruses were only partially precipitated, suggesting that incorporation of larger amounts of CCR5 into the virions caused impairment of viral infectivity in HIV-1V3L#08. The results demonstrate the implications of an alternative influence of CCR5 on HIV-1 replication.

Galectin-1 promotes HIV-1 infectivity in macrophages through stabilization of viral adsorption

Following primary infection with human immunodeficiency virus type-1 (HIV-1), macrophages are thought to play an important role, as they are one of the first target cells the virus encounters and can also sustain a significant production of viruses over extended periods of time. While the interaction between the primary cellular receptor CD4 and the virus-encoded external envelope glycoprotein gp120 initiates the infection process, it has been suggested that various host factors are exploited by HIV-1 to facilitate adsorption onto the cell surface. Macrophages and other cells found at the infection site can secrete a soluble mammalian lectin, galectin-1, which binds to beta-galactoside residues through its carbohydrate recognition domain. Being a dimer, galectin-1 can cross-link ligands expressed on different constituents to mediate adhesion between cells or between cells and pathogens. We report here that galectin-1, but not galectin-3, increased HIV-1 infectivity in monocyte-derived macrophages (MDMs). This phenomenon was likely due to an enhancement of virus adsorption kinetics, which facilitates HIV-1 entry. The fusion inhibitors T-20 and TAK779 remained effective at reducing infection even in the presence of galectin-1, indicating that the galectin-1-mediated effect is occurring at a step prior to fusion. Together, our data suggest that galectin-1 can facilitate HIV-1 infection in MDMs by promoting early events of the virus replicative cycle (i.e. adsorption).

Host factors influencing susceptibility to HIV infection and AIDS progression

Transmission of HIV first results in an acute infection, followed by an apparently asymptomatic period that averages ten years. In the absence of antiretroviral treatment, most patients progress into a generalized immune dysfunction that culminates in death. The length of the asymptomatic period varies, and in rare cases infected individuals never progress to AIDS. Other individuals whose behavioral traits put them at high-risk of HIV transmission, surprisingly appear resistant and never succumb to infection. These unique cases highlight the fact that susceptibility to HIV infection and progression to disease are complex traits modulated by environmental and genetic factors. Recent evidence has indicated that natural variations in host genes can influence the outcome of HIV infection and its transmission. In this review we summarize the available literature on the roles of cellular factors and their genetic variation in modulating HIV infection and disease progression.

Inhibition of HIV-1 entry by antibodies: potential viral and cellular targets

Vaccine-induced antibodies that interfere with viral entry are the protective correlate of most existing prophylactic vaccines. However, for highly variable viruses such as HIV-1, the ability to elicit broadly neutralizing antibody responses through vaccination has proven to be extremely difficult. The major targets for HIV-1 neutralizing antibodies are the viral envelope glycoprotein trimers on the surface of the virus that mediate receptor binding and entry. HIV-1 has evolved many mechanisms on the surface of envelope glycoproteins to evade antibody-mediated neutralization, including the masking of conserved regions by glycan, quaternary protein interactions and the presence of immunodominant variable elements. The primary challenge in the development of an HIV-1 vaccine that elicits broadly neutralizing antibodies therefore lies in the design of suitable envelope glycoprotein immunogens that circumvent these barriers. Here, we describe neutralizing determinants on the viral envelope glycoproteins that are defined by their function in receptor binding or by rare neutralizing antibodies isolated from HIV-infected individuals. We also describe the nonvariable cellular receptors involved in the HIV-1 entry process, or other cellular proteins, and ongoing studies to determine if antibodies against these proteins have efficacy as therapeutic reagents or, in some cases, as vaccine targets to interfere with HIV-1 entry.

Antigen-presenting particle technology using inactivated surface-engineered viruses: induction of immune responses against infectious agents

Background

Developments in cell-based and gene-based therapies are emerging as highly promising areas to complement pharmaceuticals, but present day approaches are too cumbersome and thereby limit their clinical usefulness. These shortcomings result in procedures that are too complex and too costly for large-scale applications. To overcome these shortcomings, we described a protein delivery system that incorporates over-expressed proteins into viral particles that are non-infectious and stable at room temperature. The system relies on the biological process of viral egress to incorporate cellular surface proteins while exiting their host cells during lytic and non-lytic infections.

Results

We report here the use of non-infectious surface-engineered virion particles to modulate immunity against three infectious disease agents – human immunodeficiency virus type 1 (HIV-1), herpes simplex virus (HSV), and Influenza. Surface-engineering of particles are accomplished by genetic modification of the host cell surface that produces the egress budding viral particle. Human peripheral blood lymphocytes from healthy donors exposed to CD80/B7.1, CD86/B7.2, and/or antiCD3 single-chain antibody surface-engineered non-infectious HIV-1 and HSV-2 particles stimulate T cell proliferation, whereas particles released from non-modified host cells have no T cell stimulatory activity. In addition to T cell proliferation, HIV-based particles specifically suppress HIV-1 replication (both monocytotropic and lymphocytotropic strains) 55 to 96% and HSV-based particles specifically induce cross-reactive HSV-1/HSV-2 anti-herpes virus antibody production. Similar surface engineering of influenza-based particles did not modify the intrinsic ability of influenza particles to stimulate T cell proliferation, but did bestow on the engineered particles the ability to induce cross-strain anti-influenza antibody production.

Conclusion

We propose that non-infectious viral particles can be surface-engineered to produce antigen-presenting particles that mimic antigen-presenting cells to induce immune responses in human peripheral blood lymphocytes. The viral particles behave as "biological carriers" for recombinant proteins, thereby establishing a new therapeutic paradigm for molecular medicine.

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.

Human immunodeficiency virus type 1 replication in dendritic cell-T-cell cocultures is increased upon incorporation of host LFA-1 due to higher levels of virus production in immature dendritic cells

Dendritic cells (DCs) act as a portal for invasion by human immunodeficiency virus type-1 (HIV-1). Here, we investigated whether virion-incorporated host cell membrane proteins can affect virus replication in DC-T-cell cocultures. Using isogenic viruses either devoid of or bearing host-derived leukocyte function-associated antigen 1 (LFA-1), we showed that HIV-1 production is augmented when LFA-1-bearing virions are used compared to that for viral entities lacking this adhesion molecule. This phenomenon was observed in immature monocyte-derived DCs (IM-MDDCs) only and not in DCs displaying a mature phenotype. The increase is not due to higher virus production in responder CD4(+) T cells but rather is linked with a more important productive infection of IM-MDDCs. We provided evidence that virus-associated host LFA-1 molecules do not affect a late event in the HIV-1 life cycle but rather exert an effect on an early step in virus replication. We demonstrated that the enhancement of productive infection of IM-MDDCs that is conferred by virus-anchored host LFA-1 involves the protein kinase A (PKA) and PKC signal transduction pathways. The biological significance of this phenomenon was established by performing experiments with virus stocks produced in primary human cells and anti-LFA-1 antibodies. Together, our results indicate that the association between some virus-bound host proteins and their natural cognate ligands can modulate de novo HIV-1 production by IM-MDDCs. Therefore, the additional interactions between virus-bound host cell membrane constituents and counter receptors on the surfaces of DCs can influence HIV-1 replication in IM-MDDC-T-cell cocultures.

Dynamic variation in the cellular origin of HIV type 1 during treatment of tuberculosis in dually infected subjects

HIV-1 replication remains elevated in dually infected HIV-1/TB subjects at completion of antituberculosis therapy. A viral immunocapture assay was used to examine the cellular origin of HIV-1 within plasma from HIV-1/TB subjects at time of diagnosis of pulmonary TB, at end of TB treatment, and 6 months after completion of treatment. Asymptomatic HIV-1-infected subjects without TB (HIV-1/C) served as controls. Both activated immature macrophage (CD36(+)) and CD4 T cell (CD26(+)) compartments contributed to viral load. Changes in the activation status of either cellular compartment paralleled their contribution to viral load. Levels of HIV-1 originating from activated (HLA-DR(+)) cells and from CD36(+) and CD26(+) mononuclear cells resolved to levels observed in HIV-1/C by the end of treatment. HIV-1 isolated by anti-CD3 immunocapture from HIV-1/TB patients remained significantly higher than from HIV-1/C patients at the end of TB treatment and at 12 months follow-up. Therefore, viral production by lymphocytes extends well beyond the completion of TB treatment.

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.

Primary attachment of murine leukaemia virus vector mediated by particle-associated heparan sulfate proteoglycan

Target cell entry of murine leukaemia virus vectors proceeds via primary attachment, independent of the viral envelope protein and subsequent envelope-receptor interaction. Although much attention has been paid to modifying the latter for target cell specificity, the initial binding interaction has been overlooked, despite its opposing involvement both in providing the virus available for receptor binding and in depleting free virus. As a first step towards modifying primary attachment, both to provide specificity and to enhance vector availability, we sought to determine the nature of this interaction. Following an initial screen of GAGs (glycosaminoglycans) for their ability to inhibit virus binding and transduction, we have shown that production of virus from cells in which GAG sulfation is inhibited, or treatment of virus with heparinase III, reduces both particle attachment and infection. Detection in purified virus preparations of a neo-epitope generated by heparinase III confirmed the presence of virus-associated HSPG [HS (heparan sulfate) proteoglycan], acquired from the producer cell. We propose that host-acquired cell-surface HSPG (potentially including syndecan-2) provides a means of virus attachment to target cells that precedes specific receptor interaction and membrane fusion. Inhibition of HS biosynthesis may provide a sufficiently reduced background of primary binding such that novel mechanisms of attachment, ideally with appropriate target cell specificity, can be introduced.

Proteomic and biochemical analysis of purified human immunodeficiency virus type 1 produced from infected monocyte-derived macrophages

Human immunodeficiency virus type 1 (HIV-1) infects CD4(+) T lymphocytes and monocytes/macrophages, incorporating host proteins in the process of assembly and budding. Analysis of the host cell proteins incorporated into virions can provide insights into viral biology. We characterized proteins in highly purified HIV-1 virions produced from human monocyte-derived macrophages (MDM), within which virus buds predominantly into intracytoplasmic vesicles, in contrast to the plasmalemmal budding of HIV-1 typically seen with infected T cells. Liquid chromatography-linked tandem mass spectrometry of highly purified virions identified many cellular proteins, including 33 previously described proteins in HIV-1 preparations from other cell types. Proteins involved in many different cellular structures and functions were present, including those from the cytoskeleton, adhesion, signaling, intracellular trafficking, chaperone, metabolic, ubiquitin/proteasomal, and immune response systems. We also identified annexins, annexin-binding proteins, Rab proteins, and other proteins involved in membrane organization, vesicular trafficking, and late endosomal function, as well as apolipoprotein E, which participates in cholesterol transport, immunoregulation, and modulation of cell growth and differentiation. Several tetraspanins, markers of the late endosomal compartment, were also identified. MDM-derived HIV contained 26 of 37 proteins previously found in exosomes, consistent with the idea that HIV uses the late endosome/multivesicular body pathway during virion budding from macrophages.

The tyrosine kinase inhibitor genistein blocks HIV-1 infection in primary human macrophages

Binding of HIV-1 envelope glycoprotein (Env) to its cellular receptors elicits a variety of signaling events, including the activation of select tyrosine kinases. To evaluate the potential role of such signaling, we examined the effects of the tyrosine kinase inhibitor, genistein, on HIV-1 entry and infection of human macrophages using a variety of assays. Without altering cell viability, cell surface expression of CD4 and CCR5 or their abilities to interact with Env, genistein inhibited infection of macrophages by reporter gene-encoding, beta-lactamase containing, or wild type virions, as well as Env-mediated cell-fusion. The observation that genistein blocked virus infection if applied before, during or immediately after the infection period, but not 24h later; coupled with a more pronounced inhibition of infection in the reporter gene assays as compared to both beta-lactamase and p24 particle entry assays, imply that genistein exerts its inhibitory effects on both entry and early post-entry steps. These findings suggest that other exploitable targets, or steps, of the HIV-1 infection process may exist and could serve as additional opportunities for the development of new therapeutics.

HIV accomplices and adversaries in macrophage infection

Cell surface and intracellular proteins in macrophages influence various steps in the life cycle of lentiviruses. Characterization of these restriction and/or cofactors is essential to understanding how macrophages become unwitting HIV hosts and in fact, can coexist with a heavy viral burden. Although many of the cellular pathways co-opted by HIV in macrophages mimic those seen in CD4+ T cells, emerging evidence reveals cellular constituents of the macrophage, which may be uniquely usurped by HIV. For example, in addition to CD4 and CCR5, membrane annexin II facilitates early steps in infection of macrophages, but not in T cells. Blockade of this pathway effectively diminishes macrophage infection. Viral binding engages a macrophage-centric signaling pathway and a transcriptional profile, including genes such as p21, which benefit the virus. Once inside the cell, multiple host cell molecules are engaged to facilitate virus replication and assembly. Although the macrophage is an enabler, it also possesses innate antiviral mechanisms, including apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like 3G (APOBEC3) family DNA-editing enzymes to inhibit replication of HIV. Differential expression of these enzymes, which are largely neutralized by HIV to protect its rebirth, is associated with resistance or susceptibility to the virus. Higher levels of the cytidine deaminases endow potential HIV targets with a viral shield, and IFN-alpha, a natural inducer of macrophage APOBEC expression, renders macrophages tougher combatants to HIV infection. These and other manipulatable pathways may give the macrophage a fighting chance in its battle against the virus.

A study of the Topoisomerase II activity in HIV-1 replication using the ferrocene derivatives as probes

Human Topoisomerase II is present in two isoforms, 170KDa alpha and 180KDa beta. Both the isoforms play a crucial role in maintenance of topological changes during DNA replication and recombination. It has been shown that Topoisomerase II activity is required for HIV-1 replication and the enzyme is phosphorylated during early time points of HIV-1 replication. In the present study, we have studied the molecular action of Topoisomerase II inhibitors, azalactone ferrocene (AzaFecp), Thiomorpholide amido methyl ferrocene (ThioFecp), and Ruthenium benzene amino pyridine (Ru(ben)Apy) on cell proliferation and also on various events of HIV-1 replication cycle. The Topoisomerase II beta over-expressing neuroblastoma cell line shows a higher sensitivity to these compounds compared to the Sup-T1 cell line. All the three Topoisomerase II inhibitors show significant anti-HIV activity at nanomolar concentrations against an Indian isolate of HIV-1(93IN101) in Sup-T1 cell line. An analysis of action of these compounds on proviral DNA synthesis at 5h of post-infection shows that they inhibit proviral DNA synthesis as well as the formation of pre-integration complexes completely. Further analysis, using polymerase chain reaction and western blot, showed that both the Topoisomerase II alpha and beta isoforms are present in the pre-integration complexes, suggesting their significant role in HIV-1 replication.

Metabolic Biotinylation of Lentiviral Pseudotypes for Scalable Paramagnetic Microparticle-Dependent Manipulation

Nonviral, host-derived proteins on lentiviral vector surfaces can have a profound effect on the vector's biology as they can both promote infection and provide resistance to complement inactivation. We have exploited this to engineer a specific posttranslational modification of a "nonenvelope," virally associated protein. The bacterial biotin ligase (BirA) and a modified human DeltaLNGFR have been introduced into HEK293T cells and their protein products directed to the lumen of the endoplasmic reticulum. The BirA then couples biotin to an acceptor peptide that has been fused to the DeltaLNGFR. This results in the covalent linkage of biotin to the extracellular domain of the DeltaLNGFR expressed on the cell surface. Lentiviral vectors from these cells are metabolically labeled with biotin in the presence of free biotin. These biotinylated lentiviral vectors have a high affinity for streptavidin paramagnetic particles and, once captured, are easily manipulated in vitro. This is illustrated by the concentration of lentiviral vectors pseudotyped with either the VSV-G or an amphotropic envelope in excess of 4500-fold. This new cell line has the potential for widespread application to envelope pseudotypes compatible with lentiviral vector production.

Broad neutralization and complement-mediated lysis of HIV-1 by PEHRG214, a novel caprine anti-HIV-1 polyclonal antibody

OBJECTIVES

To assess the potency, breadth of action, and mechanism of action of the polyclonal goat anti-HIV antibody, PEHRG214.

DESIGN

Typical human antibody responses to HIV-1 infection are unable to neutralize virus efficiently, clear the infection, or prevent disease progression. However, more potent neutralizing antibodies may be capable of playing a pivotal role in controlling HIV replication in vivo. PEHRG214 is a polyclonal caprine antibody raised against purified HIV-associated proteins, such that epitopes that are immunologically silent in humans may potentially be recognized in another species. It has been administered safely to HIV-infected individuals in Phase I clinical trials.

METHODS

The anti-HIV activity of PEHRG214 was assessed using neutralization and virion lysis assays. The target proteins for PEHRG214 activity were investigated using flow cytometry and by adsorption of anti-cell antibodies from the antibody cocktail.

RESULTS

PEHRG214 strongly neutralized a diverse range of primary HIV-1 isolates, encompassing subtypes A to E and both CCR5 and CXCR4 phenotypes. Neutralization was enhanced by the presence of complement. PEHRG214 also induced complement-mediated lysis of all HIV-1 isolates tested, and recognized or cross-reacted with a number of host cell proteins. Lysis was abrogated by adsorption with T and/or B cells expressing GPI-linked proteins, but not by GPI-deficient B cells or red blood cells.

CONCLUSIONS

PEHRG214 was found to potently neutralize and lyse HIV-1 particles. By targeting host cell proteins present in the viral envelope, which are conserved among all strains tested, PEHRG214 potentially opens up a highly novel means of eliminating circulating virus in infected individuals.

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.

LFA-1 is a key determinant for preferential infection of memory CD4+ T cells by human immunodeficiency virus type 1

Memory CD4+ T cells are considered a stable latent reservoir for human immunodeficiency virus type 1 (HIV-1) and a barrier to eradication of this retroviral infection in patients under therapy. It has been shown that memory CD4+ T cells are preferentially infected with HIV-1, but the exact mechanism(s) responsible for this higher susceptibility remains obscure. Previous findings indicate that incorporation of host-derived intercellular adhesion molecule 1 (ICAM-1) in HIV-1 increases virus infectivity. To measure the putative involvement of virus-anchored ICAM-1 in the preferential infection of memory cells by HIV-1, quiescent and activated naive and memory T-cell subsets were exposed to isogenic virions either lacking or bearing ICAM-1. Memory CD4+ T cells were found to be more susceptible than naive CD4+ T cells to infection with ICAM-1-bearing virions, as exemplified by a more important virus replication, an increase in integrated viral DNA copies, and a more efficient entry process. Interactions between virus-associated host ICAM-1 and cell surface LFA-1 under a cluster formation seem to be responsible for the preferential HIV-1 infection of the memory cell subset. Altogether, these data shed light on a potential mechanism by which HIV-1 preferentially targets long-lived memory CD4+ T cells.

Rab5 and Rab7, but Not ARF6, Govern the Early Events of HIV-1 Infection in Polarized Human Placental Cells

Trophoblasts, the structural cells of the placenta, are thought to play a determinant role in in utero HIV type 1 (HIV-1) transmission. We have accumulated evidence suggesting that HIV-1 infection of these cells is associated with uptake by an unusual clathrin/caveolae-independent endocytic pathway and that endocytosis is followed by trafficking through multiple organelles. Furthermore, part of this trafficking involves the transit of HIV-1 from transferrin-negative to EEA1 and transferrin-positive endosomes, suggesting a merger from nonclassical to classical endocytic pathways in these cells. In the present article, the relationship between the presence of HIV-1 within specific endosomes and infection was studied. We demonstrate that viral infection is virtually lost when endosome inhibitors are added shortly after exposure to HIV-1. Thus, contrary to what is seen in CD4+ T lymphocytes, the initial presence of HIV-1 within the endosomes is mandatory for infection to take place. Importantly, this process is independent of the viral envelope proteins gp120 and gp41. The Rab family of small GTPases coordinates the vesicular transport between the different endocytic organelles. Experiments performed with various expression vectors indicated that HIV-1 infection in polarized trophoblasts relies on Rab5 and Rab7 without the contribution of Arf6 or Rab11. Furthermore, we conclude that Rab5 drives movements from raft-rich region to early endosomes, and this transit is required for subsequently reaching late endosomes via Rab7. This complex trafficking is mandatory for HIV-1 infection to proceed in human polarized trophoblasts.

Exploiting common targets in human fertilization and HIV infection: development of novel contraceptive microbicides

The continued high rates of unintended pregnancies and the unrelentless expansion of the acquired immune deficiency syndrome (AIDS) epidemic, especially in less developed countries, warrant the development of novel strategies to help individuals avoid these risks. Dually active compounds displaying contraceptive and microbicidal anti-human immunodeficiency virus (anti-HIV) properties constitute one such strategy. Sharing the same anatomical and functional context, sperm fertilization and genital infection by HIV offer an opportunity for simultaneous intervention. Some of the molecules and mechanisms used by sperm to fertilize the oocyte are similar, if not identical, to those used by HIV while infecting host cells. An example of common structures is the lipid membrane surrounding the spermatozoon and the HIV core. Disruption of its architecture by surface-active compounds exerts both spermicidal and virucidal activity. A more specific alteration of lipid rafts [membrane microdomains enriched in cholesterol and glycosylphosphatidylinositol (GPI)-anchored proteins] by beta-cyclodextrins also results in similar effects. During fertilization and infection, both sperm and HIV interact with their target cell receptors through chemical charges, hydrophobic forces and carbohydrate recognition. Anionic polymers such as cellulose sulphate and polystyrene sulphonate (PSS) inhibit sperm and HIV cell binding. Because some of the molecules involved in this interaction, e.g. heparin sulphate proteoglycan, are also used by other pathogens to infect their target tissues, polyanions exert broad antimicrobial activity as well. During fertilization and infection, sperm and HIV, as well as other microbes, use signal transduction molecules and mechanisms such as adenyl cyclase/cyclic adenosine monophosphate (cAMP)-dependent kinase, calcium and tyrosine phosphorylation, whose inhibition has been shown to impair sperm function and HIV replication. These commonalities at the level of sperm and HIV structure, cell binding and fusion processes, and signalling pathways therefore provide the biological framework to develop bifunctional inhibitors with both antimicrobial and contraceptive properties.

Statins Could Be Used to Control Replication of Some Viruses, Including HIV-1

Statins are mainly known for their plasma cholesterol-lowering properties and are widely used for the prevention of cardiovascular diseases. They however also exert pleiotropic effects through a variety of mechanisms, among which several immunosuppressive effects that are unrelated to their cholesterol-lowering activity. Interestingly, there has been recent evidence of antiviral effects, including preliminary studies on the efficacy of statins against HIV-1. This paper more particularly focuses on the specific inhibition of the binding of leukocyte function-associated antigen-1 (LFA-1) to intercellular adhesion molecule (ICAM-1) by statins, independently of the inhibition of HMGCoA reductase. Targeting the statin-binding site within LFA-1 or regulating LFA-1 affinity by inhibiting prenylation of the small GTPases could prove useful to treat inflammatory, autoimmune diseases and possibly viral infections, including HIV-1.

Lymphoid tissue targeting of anti-HIV drugs using liposomes

Considering that HIV-1 accumulates and replicates actively within lymphoid tissues, any strategy that will decrease viral stores in these tissues might be beneficial to the infected host. Follicular dendritic cells (FDC), B lymphocytes, antigen-presenting cells like macrophages, and activated CD4(+) T cells are abundant in lymphoid tissues, and all express substantial levels of the HLA-DR determinant of the major histocompatibility complex class II (MHC-II). Monocyte-derived macrophages, which are also CD4(+) and express HLA-DR, are considered to be the most frequent hosts of HIV-1 in tissues of infected individuals. This chapter describes a method for the generation of sterically stabilized immunoliposomes grafted with anti-HLA-DR antibodies that allows efficient delivery of drugs to lymphoid tissues. The method first involves the production of murine HLA-DR (clone Y-17, IgG(2b)) and human HLA-DR (clone 2.06, IgG(1)) antibodies from hybridomas in mice and their purification from ascites fluids. This step is followed by the production of Fab' fragments of antibodies 2.06 and Y-17 that are grafted at the surface of sterically stabilized immunoliposomes instead of the complete IgG to reduce their immunogenicity. The preparation of sterically stabilized liposomes, the composition of which allows an efficient entrapment and retention of several drugs, by the method of thin lipid film hydratation followed by extrusion through polycarbonate membranes is then described. This step is followed by the removal of unencapsulated drug, when present, by low-speed centrifugation of the liposomal preparation through a Sephadex G-50 column. These liposomes contain a fixed amount of poly(ethylene glycol) chain terminated by a maleimide reactive group for the coupling of Fab' fragments. The procedure for the coupling of Fab' fragments at the surface of sterically stabilized liposomes and the removal of uncoupled fragments of antibodies is described. In vitro binding studies of sterically stabilized immunoliposomes to cell lines expressing different surface levels of the mouse or human HLA-DR determinant of MHC-II demonstrate that these liposomes are very specific. When compared with conventional liposomes, the subcutaneous administration in the upper back, below the neck, of mice of anti-HLA-DR immunoliposomes resulted in a 2.9 and 1.6 times greater accumulation in the cervical and brachial lymph nodes, respectively. The use of sterically stabilized immunoliposomes increases 2 to 4.6 times the concentration of liposomes in all tissues, with a peak accumulation at 240 h in brachial, inguinal, and popliteal lymph nodes and at 360 h or greater in cervical lymph nodes. A single bolus injection of indinavir given subcutaneously to mice results in no significant drug levels in lymphoid organs. Most of the injected drug accumulates in the liver and is totally cleared within 24 h postadministration. In contrast, sterically stabilized immunoliposomes are very efficient in delivering high concentrations of indinavir to lymphoid tissues for at least 15 days postinjection. The drug accumulation in all tissues leads to a 21- to 126-fold increased accumulation when compared with the free agent. Anti-HLA-DR immunoliposomes containing indinavir are as efficient as the free agent in inhibiting HIV-1 replication in PM1 cells that express high levels of cell surface HLA-DR. Sterically stabilized anti-HLA-DR immunoliposomes mostly accumulate in the cortex in which follicles (B cells and FDCs) are located, and in parafollicular areas in which T cells, interdigitating dendritic cells, and other accessory cells are abundant. The delivery of drugs in this area of the lymph nodes could represent a convenient strategy to inhibit more efficiently HIV-1 replication. Although the method described in this chapter is specific to the coupling of anti-HLA-DR antibodies, any antibody fragment or peptide specific for an antigen present in relatively large quantities at the surface of lymphoid cells, that is anchored to the surface of sterically stabilized liposomes with an appropriate coupling method, can be used to concentrate drugs within target tissues and improve the therapeutic effect of drugs.

Human immunodeficiency virus type 1 env clones from acute and early subtype B infections for standardized assessments of vaccine-elicited neutralizing antibodies

Induction of broadly cross-reactive neutralizing antibodies is a high priority for AIDS vaccine development but one that has proven difficult to be achieved. While most immunogens generate antibodies that neutralize a subset of T-cell-line-adapted strains of human immunodeficiency virus type 1 (HIV-1), none so far have generated a potent, broadly cross-reactive response against primary isolates of the virus. Even small increments in immunogen improvement leading to increases in neutralizing antibody titers and cross-neutralizing activity would accelerate vaccine development; however, a lack of uniformity in target strains used by different investigators to assess cross-neutralization has made the comparison of vaccine-induced antibody responses difficult. Thus, there is an urgent need to establish standard panels of HIV-1 reference strains for wide distribution. To facilitate this, full-length gp160 genes were cloned from acute and early subtype B infections and characterized for use as reference reagents to assess neutralizing antibodies against clade B HIV-1. Individual gp160 clones were screened for infectivity as Env-pseudotyped viruses in a luciferase reporter gene assay in JC53-BL (TZM-bl) cells. Functional env clones were sequenced and their neutralization phenotypes characterized by using soluble CD4, monoclonal antibodies, and serum samples from infected individuals and noninfected recipients of a recombinant gp120 vaccine. Env clones from 12 R5 primary HIV-1 isolates were selected that were not unusually sensitive or resistant to neutralization and comprised a wide spectrum of genetic, antigenic, and geographic diversity. These reference reagents will facilitate proficiency testing and other validation efforts aimed at improving assay performance across laboratories and can be used for standardized assessments of vaccine-elicited neutralizing antibodies.

Induction of humoral immune responses following vaccination with envelope-containing, formaldehyde-treated, thermally inactivated human immunodeficiency virus type 1

The lack of success of subunit human immunodeficiency virus type 1 (HIV-1) vaccines to date suggests that multiple components or a complex virion structure may be required. We previously demonstrated retention of the major conformational epitopes of HIV-1 envelope following thermal treatment of virions. Moreover, antibody binding to some of these epitopes was significantly enhanced following thermal treatment. These included the neutralizing epitopes identified by monoclonal antibodies 1b12, 2G12, and 17b, some of which have been postulated to be partially occluded or cryptic in native virions. Based upon this finding, we hypothesized that a killed HIV vaccine could be derived to elicit protective humoral immune responses. Shedding of HIV-1 envelope has been described for some strains of HIV-1 and has been cited as one of the major impediments to developing an inactivated HIV-1 vaccine. In the present study, we demonstrate that treatment of virions with low-dose formaldehyde prior to thermal inactivation retains the association of viral envelope with virions. Moreover, mice and nonhuman primates vaccinated with formaldehyde-treated, thermally inactivated virions produce antibodies capable of neutralizing heterologous strains of HIV in peripheral blood mononuclear cell-, MAGI cell-, and U87-based infectivity assays. These data indicate that it is possible to create an immunogen by using formaldehyde-treated, thermally inactivated HIV-1 virions to induce neutralizing antibodies. These findings have broad implications for vaccine development.

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.

Human immunodeficiency virus type 1-induced macrophage gene expression includes the p21 gene, a target for viral regulation

In contrast to CD4+ T cells, human immunodeficiency virus type 1 (HIV-1)-infected macrophages typically resist cell death, support viral replication, and consequently, may facilitate HIV-1 transmission. To elucidate how the virus commandeers the macrophage's intracellular machinery for its benefit, we analyzed HIV-1-infected human macrophages for virus-induced gene transcription by using multiple parameters, including cDNA expression arrays. HIV-1 infection induced the transcriptional regulation of genes associated with host defense, signal transduction, apoptosis, and the cell cycle, among which the cyclin-dependent kinase inhibitor 1A (CDKN1A/p21) gene was the most prominent. p21 mRNA and protein expression followed a bimodal pattern which was initially evident during the early stages of infection, and maximum levels occurred concomitant with active HIV-1 replication. Mechanistically, viral protein R (Vpr) independently regulates p21 expression, consistent with the reduced viral replication and lack of p21 upregulation by a Vpr-negative virus. Moreover, the treatment of macrophages with p21 antisense oligonucleotides or small interfering RNAs reduced HIV-1 infection. In addition, the synthetic triterpenoid and peroxisome proliferator-activated receptor gamma ligand, 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid (CDDO), which is known to influence p21 expression, suppressed viral replication. These data implicate p21 as a pivotal macrophage facilitator of the viral life cycle. Moreover, regulators of p21, such as CDDO, may provide an interventional approach to modulate HIV-1 replication.

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.

Contribution of virus-like particles to the immunogenicity of human immunodeficiency virus type 1 Gag-derived vaccines in mice

The human immunodeficiency virus type 1 (HIV-1) Gag protein is a major target antigen for cytotoxic-T-lymphocyte-based vaccine strategies because of its high level of conservation. The murine model has been used extensively to evaluate potential HIV-1 vaccines. However, the biology of HIV-1 Gag is somewhat different in human and murine tissues. The ability of HIV-1 Gag to form virus-like particles (VLPs) in human cells is severely curtailed in murine cells. Hence, it is not known whether immunizing mice with expression vectors encoding HIV-1 Gag provides an accurate assessment of the immunogenicity of these candidate vaccines in primates. In this report, we made use of a chimeric Moloney murine leukemia virus (MMLV)-HIV-1 Gag in which the p17 matrix domain of HIV-1 was replaced with the p15 matrix and p12 domains from MMLV. Murine cells expressing this construct released significant amounts of VLPs. The construct preserved H-2d-restricted antigenic determinants in the remaining portion of HIV-1 Gag, allowing immunogenicity studies to be performed with mice. We demonstrated that immunizing mice with plasmid DNA or adenoviral vectors encoding this chimeric Gag did not significantly increase the HIV-1 Gag-specific cellular or humoral immune response when compared to immunization with a myristoylation-incompetent version of the construct. Thus, the release of VLPs formed in vivo may not play a major role in the immunogenicity of vectors expressing HIV-1 Gag constructs.

Evaluating neutralizing antibodies against HIV, SIV, and SHIV in luciferase reporter gene assays

Neutralizing antibody assays are powerful tools for assessing humoral immunity in AIDS virus infection and vaccine development. Although many different assays have been described, all are based on the same principle, measuring reductions in virus infectivity. This unit describes two assays utilizing a genetically engineered cell lines that are susceptible to infection by most strains of HIV-1, SIV, and SHIV. One assay is designed for optimal performance with uncloned viruses produced in either PBMC or CD4+ T cell lines. A second assay is designed for single-cycle infection with molecularly cloned pseudoviruses produced by transfection in 293T cells. Both assays are performed in a 96-well format and use tat-responsive luciferase reporter gene expression as readout.

Interaction between the cytoplasmic domain of ICAM-1 and Pr55Gag leads to acquisition of host ICAM-1 by human immunodeficiency virus type 1

We have examined the molecular basis for the selective incorporation of the adhesion molecule ICAM-1 within human immunodeficiency virus type 1 (HIV-1). The process of ICAM-1 incorporation was investigated by using different ICAM-1 constructs in combination with virus capture and immunoprecipitation studies, Western blot and confocal microscopy analyses, and infectivity assays. Experiments conducted with viruses bearing a truncated version of ICAM-1 revealed that the cytoplasmic domain of ICAM-1 governs insertion of this adhesion molecule into HIV-1. Further experiments suggested that there is an association between ICAM-1 and the virus-encoded Pr55(Gag) polyprotein. This study represents the first demonstration that structural Gag polyproteins play a key role in the uptake of a host-derived cell surface by the virus entity. Taken together, our results indicate that interactions between viral and cellular proteins are responsible for the selective uptake of host ICAM-1 by HIV-1. This observation describes a new strategy by which HIV-1 can modulate its replicative cycle, considering that insertion of ICAM-1 within nascent virions has been shown to increase virus infectivity.

HIV-1 cell to cell transfer across an Env-induced, actin-dependent synapse

Direct cell–cell transfer is an efficient mechanism of viral dissemination within an infected host, and human immunodeficiency virus 1 (HIV-1) can exploit this mode of spread. Receptor recognition by HIV-1 occurs via interactions between the viral surface envelope glycoprotein (Env), gp120, and CD4 and a chemokine receptor, CCR5 or CXCR4. Here, we demonstrate that the binding of CXCR4-using HIV-1–infected effector T cells to primary CD4⁺/CXCR4⁺ target T cells results in rapid recruitment to the interface of CD4, CXCR4, talin, and lymphocyte function–associated antigen 1 on the target cell, and of Env and Gag on the effector cell. Recruitment of these membrane molecules into polarized clusters was dependent on Env engagement of CD4 and CXCR4 and required remodelling of the actin cytoskeleton. Transfer of Gag from effector to target cell was observed by 1 h after conjugate formation, was independent of cell–cell fusion, and was probably mediated by directed virion fusion with the target cell. We propose that receptor engagement by Env directs the rapid, actin-dependent recruitment of HIV receptors and adhesion molecules to the interface, resulting in a stable adhesive junction across which HIV infects the target cell.

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.

Insertion of host-derived costimulatory molecules CD80 (B7.1) and CD86 (B7.2) into human immunodeficiency virus type 1 affects the virus life cycle

Human immunodeficiency virus type 1 (HIV-1) carries virus-encoded and host-derived proteins. Recent advances in the functional characterization of host molecules inserted into mature virus particles have revealed that HIV-1 biology is influenced by the acquisition of host cell membrane components. The CD28/B7 receptor/ligand system is considered one of the fundamental elements of the normal immune response. Two major cell types that harbor HIV-1 in vivo, i.e., monocytes/macrophages and CD4+ T cells, express the costimulatory molecules CD80 (B7.1) and CD86 (B7.2). We investigated whether CD80 and CD86 are efficiently acquired by HIV-1, and if so, whether these host-encoded molecules can contribute to the virus life cycle. Here we provide the first evidence that the insertion of CD80 and CD86 into HIV-1 increases virus infectivity by facilitating the attachment and entry process due to interactions with their two natural ligands, CD28 and CTLA-4. Moreover, we demonstrate that NF-kappaB is induced by CD80- and CD86-bearing virions when they are combined with the engagement of the T-cell receptor/CD3 complex, an event that is inhibited upon surface expression of CTLA-4. Finally, both CD80 and CD86 were found to be efficiently incorporated into R5- and X4-tropic field strains of HIV-1 expanded in cytokine-treated macrophages. Thus, besides direct interactions between the virus envelope glycoproteins and cell surface constituents, such as CD4 and some specific chemokine coreceptors, HIV-1 may attach to target cells via interactions between cell-derived molecules incorporated into virions and their natural ligands. These findings support the theory that HIV-1-associated host proteins alter virus-host dynamics.

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.

Envelope glycoproteins are not required for insertion of host ICAM-1 into human immunodeficiency virus type 1 and ICAM-1-bearing viruses are still infectious despite a suboptimal level of trimeric envelope proteins

Previous works have indicated that incorporation of surface glycoprotein into retroviruses such as the human immunodeficiency virus type 1 (HIV-1) is not a highly specific process because several cellular glycoproteins can be inserted within the mature viral particle. The mechanism(s) that govern the acquisition of such host constituents have remained so far elusive. In this study, we have investigated the role played by the viral envelope (Env) of HIV-1 in the acquisition of host intercellular adhesion molecule type I (ICAM-1). ICAM-1 proteins were still present on viruses carrying much lower levels of gp120/gp41 due to a mutation in the matrix (MA) domain or on Env-deficient viruses when produced in immortalized and primary human cell lines. Interestingly, infectivity of an HIV-1 MA mutant that carry a suboptimal amount of Env proteins was restored to a certain degree by the presence of ICAM-1 when infection was performed in cells expressing an activated form of its natural counter-ligand, LFA-1.

The host cell MAP kinase ERK-2 regulates viral assembly and release by phosphorylating the p6gag protein of HIV-1

The host cell MAP kinase ERK-2 incorporated within human immunodeficiency virus type 1 particles plays a critical role in virus infectivity by phosphorylating viral proteins. Recently, a fraction of the virus incorporated late (L) domain-containing p6(gag) protein, which has an essential function in the release of viral particles from the cell surface, was reported to be phosphorylated by an unknown virus-associated cellular protein kinase (Muller, B., Patschinsky, T., and Krausslich, H. G. (2002) J. Virol. 76, 1015-1024). The present study demonstrates the contribution of the MAP kinase ERK-2 in p6(gag) phosphorylation. According to mutational analysis, a single ERK-2-phosphorylated threonine residue, belonging to a highly conserved phosphorylation MAP kinase consensus site, was identified at position 23 within p6(gag). Substitution by an alanine of the Thr(23) phosphorylable residue within the pNL4.3 molecular clone was found to decrease viral release from various cell types. As observed from electron microscopy experiments, most virions produced from this molecular clone remained incompletely separated from the host cell membrane with an immature morphology and displayed a reduced infectivity in single round infection experiments. Analysis of protein processing by Western blotting experiments revealed an incomplete Pr55(gag) maturation and a reduction in the virion-associated reverse transcriptase proteins was observed that was not related to differences in intracellular viral protein expression. Altogether, these data suggest that phosphorylation of p6(gag) protein by virus-associated ERK-2 is involved in the budding stage of HIV-1 life cycle.

Genetic and phenotypic analyses of human immunodeficiency virus type 1 escape from a small-molecule CCR5 inhibitor

We have described previously the generation of an escape variant of human immunodeficiency virus type 1 (HIV-1), under the selection pressure of AD101, a small molecule inhibitor that binds the CCR5 coreceptor (A. Trkola, S. E. Kuhmann, J. M. Strizki, E. Maxwell, T. Ketas, T. Morgan, P. Pugach, S. X. L. Wojcik, J. Tagat, A. Palani, S. Shapiro, J. W. Clader, S. McCombie, G. R. Reyes, B. M. Baroudy, and J. P. Moore, Proc. Natl. Acad. Sci. USA 99:395-400, 2002). The escape mutant, CC101.19, continued to use CCR5 for entry, but it was at least 20,000-fold more resistant to AD101 than the parental virus, CC1/85. We have now cloned the env genes from the the parental and escape mutant isolates and made chimeric infectious molecular clones that fully recapitulate the phenotypes of the corresponding isolates. Sequence analysis of the evolution of the escape mutants suggested that the most relevant changes were likely to be in the V3 loop of the gp120 glycoprotein. We therefore made a series of mutant viruses and found that full AD101 resistance was conferred by four amino acid changes in V3. Each change individually caused partial resistance when they were introduced into the V3 loop of a CC1/85 clone, but their impact was dependent on the gp120 context in which they were made. We assume that these amino acid changes alter how the HIV-1 Env complex interacts with CCR5. Perhaps unexpectedly, given the complete dependence of the escape mutant on CCR5 for entry, monomeric gp120 proteins expressed from clones of the fully resistant isolate failed to bind to CCR5 on the surface of L1.2-CCR5 cells under conditions where gp120 proteins from the parental virus and a partially AD101-resistant virus bound strongly. Hence, the full impact of the V3 substitutions may only be apparent at the level of the native Env complex.

Quantitation of HLA class II protein incorporated into human immunodeficiency type 1 virions purified by anti-CD45 immunoaffinity depletion of microvesicles

Among the many host cell-derived proteins found in human immunodeficiency virus type 1 (HIV-1), HLA class II (HLA-II) appears to be selectively incorporated onto virions and may contribute to mechanisms of indirect imunopathogenesis in HIV infection and AIDS. However, the amount of HLA-II on the surface of HIV-1 particles has not been reliably determined due to contamination of virus preparations by microvesicles containing host cell proteins, including HLA-II. Even rigorous sucrose density centrifugation is unable to completely separate HIV-1 from microvesicles. CD45, a leukocyte integral membrane protein, is found on microvesicles, yet appears to be excluded from HIV-1 particles. Exploiting this observation, we have developed a CD45-based immunoaffinity depletion method for removing CD45-containing microvesicles that yields highly purified preparations of virions. Examination of CD45-depleted HIV-1(MN) by high-pressure liquid chromatography, protein sequencing, and amino acid analyses determined a molar ratio of HLA-II to Gag of 0.04 to 0.05 in the purified virions, corresponding to an estimated average of 50 to 63 native HLA-II complexes (i.e., a dimer of alpha and beta heterodimers) per virion. These values are approximately 5- to 10-fold lower than those previously determined for other virion preparations that contained microvesicles. Our observations demonstrate the utility of CD45 immunoaffinity-based approaches for producing highly purified retrovirus preparations for applications that would benefit from the use of virus that is essentially free of microvesicles.

AIDS in Africa: the impact of coinfections on the pathogenesis of HIV-1 infection

The intimate relationship between the HIV-1 life-cycle and the activation state of cells supporting viral replication results in a dynamic interaction between coinfections and HIV-1 replication in dually infected people. The immunologic impact of recurrent coinfections has the potential to increase viral replication, viral genotypic heterogeneity and CD4 T lymphocyte loss, leading to accelerated decline in immune function, reduced survival and increased HIV-1 transmission risk. These effects may play a particularly significant role in the HIV-1 epidemic in sub-Saharan Africa. The mechanisms underlying these effects on virus-host dynamics are reviewed and data describing the impact of tuberculosis, malaria, schistosomiasis and genital ulceration on HIV-1 infection are presented.

HIV-1 entry and its inhibition

Entry of HIV-1 virions into cells is a complex and dynamic process carried out by envelope (Env) glycoproteins on the surface of the virion that promote the thermodynamically unfavorable fusion of highly stable viral and target cell membranes. Insight gained from studies of the mechanism of viral entry allowed insight into the design of novel inhibitors of HIV-1 entry, several of which are now in clinical trials. This review highlights the mechanism by which viral and cellular proteins mediate entry of HIV-1 into permissive cells, with an emphasis on targeting this process in the design of novel therapies that target distinct steps of the entry process, including antagonizing receptor binding events and blocking conformational changes intimately involved in membrane fusion.