The HIV-1 envelope glycoproteins: fusogens, antigens, and immunogens

Targeting human T cells by retroviral vectors displaying antibody domains selected from a phage display library

To generate T cell-specific retroviral vectors an scFv phage display library derived from immunized mice was selected for binding to the human T cell line Molt-4/8. The scFv cDNAs recovered from the selected phages were transiently expressed as an N-terminal fusion of the spleen necrosis virus (SNV) transmembrane protein (TM) subunit of the viral envelope protein (Env) in the cell line DSH-cxl, which packages the beta-galactosidase gene into SNV particles. Screening of supernatants from about 150 transfections resulted in the identification of 5 scFvs that mediated efficient transduction of Molt-4/8 cells. Using stable packaging cell lines vector preparations with titers greater than 10(4) EFU/ml on human T cells were obtained. The scFv 7A5 in particular was able to mediate selective transduction of human T cells with high efficiency. Titers of up to 106 EFU/ml were reached on Molt-4/8, Jurkat, and A301 cells, while titers on HeLa cells, TE671 cells, 293T cells, and HT1080 cells were below 102 EFU/ml. Transduction of stimulated primary human peripheral blood cells, which consisted mainly of T cells, was about fivefold more efficient than transduction of B cells. Western blot analysis of supernatant from the 7A5 packaging cells demonstrated incorporation of 7A5-TM into vector particles and indicated proteolytic processing of the coexpressed unmodified TM during particle formation. Binding of bacterially expressed 7A5-scFv to a panel of cell lines correlated well with the transduction results. These data provide the first proof of concept that a general approach can be taken to obtain scFvs able to mediate selective gene transfer into target cells.

Sequential CD4-coreceptor interactions in human immunodeficiency virus type 1 Env function: soluble CD4 activates Env for coreceptor-dependent fusion and reveals blocking activities of antibodies against cryptic conserved epitopes on gp120

We devised an experimental system to examine sequential events by which the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) interacts with CD4 and coreceptor to induce membrane fusion. Recombinant soluble CD4 (sCD4) activated fusion between effector cells expressing Env and target cells expressing coreceptor (CCR5 or CXCR4) but lacking CD4. sCD4-activated fusion was dose dependent, occurred comparably with two- and four-domain proteins, and demonstrated Env-coreceptor specificities parallel to those reported in conventional fusion and infectivity systems. Fusion activation occurred upon sCD4 preincubation and washing of the Env-expressing effector cells but not the coreceptor-bearing target cells, thereby demonstrating that sCD4 exerts its effects by acting on Env. These findings provide direct functional evidence for a sequential two-step model of Env-receptor interactions, whereby gp120 binds first to CD4 and becomes activated for subsequent functional interaction with coreceptor, leading to membrane fusion. We used the sCD4-activated system to explore neutralization by the anti-gp120 human monoclonal antibodies 17b and 48d. These antibodies reportedly bind conserved CD4-induced epitopes involved in coreceptor interactions but neutralize HIV-1 infection only weakly. We found that 17b and 48d had minimal effects in the standard cell fusion system using target cells expressing both CD4 and coreceptor but potently blocked sCD4-activated fusion with target cells expressing coreceptor alone. Both antibodies strongly inhibited sCD4-activated fusion by Envs from genetically diverse HIV-1 isolates. Thus, the sCD4-activated system reveals conserved Env-blocking epitopes that are masked in native Env and hence not readily detected by conventional systems.

Differential signalling of the chemokine receptor CXCR4 by stromal cell-derived factor 1 and the HIV glycoprotein in rat neurons and astrocytes

CXCR4 is the Gi protein-linked seven-transmembrane receptor for the alpha chemokine stromal cell-derived factor 1 (SDF-1), a chemoattractant for lymphocytes. This receptor is highly conserved between human and rodent. CXCR4 is also a coreceptor for entry of human immunodeficiency virus (HIV) in T cells and is expressed in the CNS. To investigate how these CXCR4 ligands influence CNS development and/or function, we have examined the expression and signalling of this chemokine receptor in rat neurons and astrocytes in vitro. CXCR4 transcripts and protein are synthesized by both cell types and in E15 brain neuronal progenitors. In these progenitors, SDF-1, but not gp120 (the HIV glycoprotein), induced activation of extracellular signal regulated kinases (ERKs) 1/2 and a dose-dependent chemotactic response. This chemotaxis was inhibited by Pertussis toxin, which uncouples Gi proteins and the bicyclam AMD3100, a highly selective CXCR4 antagonist, as well as by an inhibitor of the MAP kinase pathway. In differentiated neurons, both SDF-1 and the glycoprotein of HIV, gp120, triggered activation of ERKs with similar kinetics. These effects were significantly inhibited by Pertussis toxin and the CXCR4 antagonist. Rat astrocytes also responded to SDF-1 signalling by phosphorylation of ERKs but, in contrast to cortical neurons, no kinase activation was induced by gp120. Thus neurons and astrocytes can respond differently to signalling by SDF-1 and/or gp120. As SDF-1 triggers directed migration of neuronal progenitors, this alpha chemokine may play a role in cortex development. In differentiated neurons, both natural and viral ligands of CXCR4 activate ERKs and may therefore influence neuronal function.

Dissociation of HIV-1 from follicular dendritic cells during HAART: mathematical analysis

Follicular dendritic cells (FDC) provide a reservoir for HIV type 1 (HIV-1) that may reignite infection if highly active antiretroviral therapy (HAART) is withdrawn before virus on FDC is cleared. To estimate the treatment time required to eliminate HIV-1 on FDC, we develop deterministic and stochastic models for the reversible binding of HIV-1 to FDC via ligand-receptor interactions and examine the consequences of reducing the virus available for binding to FDC. Analysis of these models shows that the rate at which HIV-1 dissociates from FDC during HAART is biphasic, with an initial period of rapid decay followed by a period of slower exponential decay. The speed of the slower second stage of dissociation and the treatment time required to eradicate the FDC reservoir of HIV-1 are insensitive to the number of virions bound and their degree of attachment to FDC before treatment. In contrast, the expected time required for dissociation of an individual virion from FDC varies sensitively with the number of ligands attached to the virion that are available to interact with receptors on FDC. Although most virions may dissociate from FDC on the time scale of days to weeks, virions coupled to a higher-than-average number of ligands may persist on FDC for years. This result suggests that HAART may not be able to clear all HIV-1 trapped on FDC and that, even if clearance is possible, years of treatment will be required.

Targeting of HIV gp120 by oligonucleotide-photosensitizer conjugates. Light-induced damages

Some guanine-rich oligonucleotides inhibit HIV infectivity through interaction with the gp120 glycoprotein. Besides, photoinactivation of viruses attracts attention for blood decontamination. The feasibility of targeting a red light-absorbing chlorin-type photosensitizer to gp120 through covalent coupling with 8-mer phosphodiester oligodeoxynucleotides is investigated. Some conjugates inhibit binding of antibodies directed to gp120. Inhibition is significantly increased upon red light activation. The activity of the conjugates correlates with their ability to self-associate, a process strongly favored by the propensity of the hydrophobic chlorin moiety to dimerize. Thus, the photosensitizer moiety both promotes structures with a higher affinity for gp120 and, upon light activation, can induce site-directed damages to the protein.

Characterization of primary isolate-like variants of simian-human immunodeficiency virus

Several different strains of simian-human immunodeficiency virus (SHIV) that contain the envelope glycoproteins of either T-cell-line-adapted (TCLA) strains or primary isolates of human immunodeficiency virus type 1 (HIV-1) are now available. One of the advantages of these chimeric viruses is their application to studies of HIV-1-specific neutralizing antibodies in preclinical AIDS vaccine studies in nonhuman primates. In this regard, an important consideration is the spectrum of antigenic properties exhibited by the different envelope glycoproteins used for SHIV construction. The antigenic properties of six SHIV variants were characterized here in neutralization assays with recombinant soluble CD4 (rsCD4), monoclonal antibodies, and serum samples from SHIV-infected macaques and HIV-1-infected individuals. Neutralization of SHIV variants HXBc2, KU2, 89.6, and 89.6P by autologous and heterologous sera from SHIV-infected macaques was restricted to an extent that these viruses may be considered heterologous to one another in their major neutralization determinants. Little or no variation was seen in the neutralization determinants on SHIV variants 89.6P, 89.6PD, and SHIV-KB9. Neutralization of SHIV HXBc2 by sera from HXBc2-infected macaques could be blocked with autologous V3-loop peptide; this was less true in the case of SHIV 89.6 and sera from SHIV 89.6-infected macaques. The poorly immunogenic but highly conserved epitope for monoclonal antibody IgG1b12 was a target for neutralization on SHIV variants HXBc2, KU2, and 89.6 but not on 89.6P and KB9. The 2G12 epitope was a target for neutralization on all five SHIV variants. SHIV variants KU2, 89.6, 89.6P, 89.6PD, and KB9 exhibited antigenic properties characteristic of primary isolates by being relatively insensitive to neutralization in peripheral blood mononuclear cells with serum samples from HIV-1-infected individuals and 12-fold to 38-fold less sensitive to inhibition with recombinant soluble CD4 than TCLA strains of HIV-1. The utility of nonhuman primate models in AIDS vaccine development is strengthened by the availability of SHIV variants that are heterologous in their neutralization determinants and exhibit antigenic properties shared with primary isolates.

Effects of the human CD38 glycoprotein on the early stages of the HIV-1 replication cycle

CD38 displays lateral association with the HIV-1 receptor CD4. This association is potentiated by the HIV-1 envelope glycoprotein gp120. The aim of this work was to evaluate the CD38 role in T cell susceptibility to HIV-1 infection. Using laboratory X4 HIV-1 strains and X4 and X4/R5 primary isolates, we found that CD38 expression was negatively correlated to cell susceptibility to infection, evaluated as percentage of infected cells, release of HIV p24 in the supernatants, and cytopathogenicity. This correlation was at first suggested by results obtained in a panel of human CD4(+) T cell lines expressing different CD38 levels (MT-4, MT-2, C8166, CEMx174, Supt-1, and H9) and then demonstrated using CD38 transfectants of MT-4 cells (the line with the lowest CD38 expression). To address whether CD38 affected viral binding, we used mouse T cells that are non-permissive for productive infection. Gene transfection in mouse SR.D10.CD4(-).F1 T cells produced four lines expressing human CD4 and/or CD38. Ability of CD4(+)CD38(+)cells to bind HIV-1 or purified recombinant gp120 was significantly lower than that of CD4(+)CD38(-) cells. These data suggest that CD38 expression inhibits lymphocyte susceptibility to HIV infection, probably by inhibiting gp120/CD4-dependent viral binding to target cells.-Savarino, A., Bottarel, F., Calosso, L., Feito, M. J., Bensi, T., Bragardo, M., Rojo, J. M., Pugliese, A., Abbate, I., Capobianchi, M. R., Dianzani, F., Malavasi, F., and Dianzani, U. Effects of the human CD38 glycoprotein on the early stages of theHIV-1 replication cycle.

Species-specific, postentry barriers to primate immunodeficiency virus infection

By using replication-defective vectors derived from human immunodeficiency virus type 1 (HIV-1), simian immunodeficiency virus (SIV(mac)), and murine leukemia virus (MuLV), all of which were pseudotyped with the vesicular stomatitis virus (VSV) G glycoprotein, the efficiency of postentry, early infection events was examined in target cells of several mammalian species. Titers of HIV-1 vectors were significantly lower than those of SIV(mac) and MuLV vectors in most cell lines and primary cells from Old World monkeys. By contrast, most New World monkey cells exhibited much lower titers for the SIV(mac) vector compared with those of the HIV-1 vector. Prosimian cells were resistant to both HIV-1 and SIV(mac) vectors, although the MuLV vector was able to infect these cells. Cells from other mammalian species were roughly equivalent in susceptibility to the three vectors, with the exception of rabbit cells, which were specifically resistant to the HIV-1 vector. The level of HIV-1 vector expression was very low in transduced cells of rodent, rabbit, cow, and pig origin. Early postentry restriction of primate immunodeficiency virus infection exhibits patterns largely coincident with species borders and applies to diverse cell types within an individual host, suggesting the involvement of species-specific, widely expressed cellular factors.

Determinants of CD4 independence for a human immunodeficiency virus type 1 variant map outside regions required for coreceptor specificity

Although infection by human immunodeficiency virus (HIV) typically requires an interaction between the viral envelope glycoprotein (Env), CD4, and a chemokine receptor, CD4-independent isolates of HIV and simian immunodeficiency virus have been described. The structural basis and underlying mechanisms for this phenotype are unknown. We have derived a variant of HIV-1/IIIB, termed IIIBx, that acquired the ability to utilize CXCR4 without CD4. This virus infected CD4-negative T and B cells and fused with murine 3T3 cells that expressed human CXCR4 alone. A functional IIIBx env clone exhibited several mutations compared to the CD4-dependent HXBc2 env, including the striking loss of five glycosylation sites. By constructing env chimeras with HXBc2, the determinants for CD4 independence were shown to map outside the V1/V2 and V3 hypervariable loops, which determine chemokine receptor specificity, and at least partly within an area on the gp120 core that has been implicated in forming a conserved chemokine receptor binding site. We also identified a point mutation in the C4 domain that could render the IIIBx env clone completely CD4 dependent. Mutations in the transmembrane protein (TM) were also required for CD4 independence. Remarkably, when the V3 loop of a CCR5-tropic Env was substituted for the IIIBx Env, the resulting chimera was found to utilize CCR5 but remained CD4 independent. These findings show that Env determinants for chemokine receptor specificity are distinct from those that mediate CD4-independent use of that receptor for cell fusion and provide functional evidence for multiple steps in the interaction of Env with chemokine receptors. Combined with our observation that the conserved chemokine receptor binding site on gp120 is more exposed on the IIIBx gp120 (T. L. Hoffman, C. C. LaBranche, W. Zhang, G. Canziani, J. Robinson, I. Chaiken, J. A. Hoxie, and R. W. Doms, Proc. Natl. Acad. Sci. USA 96:6359-6364, 1999), the findings from this study suggest novel approaches to derive and design Envs with exposed chemokine receptor binding sites for vaccine purposes.

Use of a gp120 binding assay to dissect the requirements and kinetics of human immunodeficiency virus fusion events

Binding of the extracellular subunit of human immunodeficiency type 1 (HIV-1) envelope (Env) glycoprotein (gp120) to CD4 triggers the induction or exposure of a highly conserved coreceptor binding site in gp120 that helps mediate membrane fusion. Characterizing the structural features involved in gp120-coreceptor binding and the conditions under which binding occurs is important for understanding the fusion process, the evolution of pathogenic strains in vivo, the identification of novel anti-HIV compounds, and the development of HIV vaccines that utilize triggered structures of Env. Here we use the kinetics of interaction between CCR5 and gp120 to understand temporal and structural changes that occur during viral fusion. Using saturation binding and homologous competition analysis, we estimated the K(d) of interaction between CCR5 and gp120 from the macrophage tropic HIV-1 strain JRFL to be 4 nM. Unlike Env-mediated fusion, gp120 binding to CCR5 did not require divalent cations or elevated temperatures. Binding was not significantly affected by the pH of binding, G-protein coupling of CCR5, or partial gp120 deglycosylation. Oligomeric, uncleaved JRFL gp140 failed to bind CCR5 despite its ability to bind CD4 and monoclonal antibody 17b, suggesting that the uncleaved ectodomain of gp41 interferes with full exposure of the chemokine receptor binding site. Exposure of the chemokine receptor binding site on gp120 could be induced rapidly by CD4, but exposure of this site was lost upon CD4 dissociation from gp120, indicating that the conformational changes in gp120 induced by CD4 binding are fully reversible. The functional gp120-soluble CD4 complex was remarkably stable over time and temperature ranges, offering the possibility that complexes in which the highly conserved coreceptor binding site in gp120 is exposed can be used for vaccine development.

Antibodies from HIV-positive and AIDS patients bind to an HIV envelope multivalent vaccine

A major problem impeding development of an effective HIV vaccine is the rapid antigenic variability that is characteristic of several envelope glycoprotein epitopes. Frequent mutations alter the composition of the most immunogenic regions of the envelope glycoprotein. We have prepared a synthetic immunogen representing the evolution of the major hypervariable epitopes on the envelope glycoprotein (gp120) of HIV-1. Five synthetic constructs, representing each of the HIV-1 gp120 hypervariable epitopes were tested for recognition by antibodies from patients infected with HIV-1 from different geographic regions worldwide. An HIV-1 human plasma panel provided a representation of the antibodies recognizing subtype-specific epitope sequences prevalent at different parts of the world. The vaccine construct was recognized by antibodies from HIV-1-positive individuals infected with subtypes A, B, C, D, E, and F. Antibodies in pooled HIV-1 patient sera from San Francisco also recognized all five constructs. This complex immunogen was recognized by antibodies in sera from individual HIV-1-positive and AIDS patients from Puerto Rico and Canada, with a strong binding to the complete vaccine and the V3 component. Altogether, our results demonstrate that antibodies from seropositive patients infected with different HIV-1 clades recognize and bind to the HIV hypervariable epitope construct vaccine preparation and its individual components.

Rational engineering of a miniprotein that reproduces the core of the CD4 site interacting with HIV-1 envelope glycoprotein

Protein-protein interacting surfaces are usually large and intricate, making the rational design of small mimetics of these interfaces a daunting problem. On the basis of a structural similarity between the CDR2-like loop of CD4 and the beta-hairpin region of a short scorpion toxin, scyllatoxin, we transferred the side chains of nine residues of CD4, central in the binding to HIV-1 envelope glycoprotein (gp120), to a structurally homologous region of the scorpion toxin scaffold. In competition experiments, the resulting 27-amino acid miniprotein inhibited binding of CD4 to gp120 with a 40 microM IC(50). Structural analysis by NMR showed that both the backbone of the chimeric beta-hairpin and the introduced side chains adopted conformations similar to those of the parent CD4. Systematic single mutations suggested that most CD4 residues from the CDR2-like loop were reproduced in the miniprotein, including the critical Phe-43. The structural and functional analysis performed suggested five additional mutations that, once incorporated in the miniprotein, increased its affinity for gp120 by 100-fold to an IC(50) of 0.1-1.0 microM, depending on viral strains. The resulting mini-CD4 inhibited infection of CD4(+) cells by different virus isolates. Thus, core regions of large protein-protein interfaces can be reproduced in miniprotein scaffolds, offering possibilities for the development of inhibitors of protein-protein interactions that may represent useful tools in biology and in drug discovery.

Production and characterization of monoclonal antibodies against pigtailed macaque (Macaca nemestrina) cell adhesion molecules

Our previous in vitro studies indicate a significant role for cell adhesion molecules in the biology of HIV-1 and HTLV-1. Confirmation of the involvement of these molecules in the pathogenesis of retrovirus infection in vivo will require a suitable animal model. The SIV/pigtailed macaque (Macaca nemestrina) model of acquired immunodeficiency syndrome (AIDS) is an ideal system in which to study adhesion molecules and viral pathogenesis. The monoclonal antibodies (MAbs) against human adhesion molecules previously produced in our laboratory either do not react with or fail to block function of pigtailed macaque adhesion molecules. We have used papiovirus-transformed pigtailed macaque B cells as immunogen to generate murine MAbs against macaque adhesion molecules including ICAM-1, VCAM-1, and LFA-1. The specificity of the MAbs was confirmed by immunoprecipitation from lysates of vectorially iodinated cells, flow cytometry analysis of transfected cell lines and primary cells, binding assays on recombinant soluble human VCAM-1 and ICAM-1, and by inhibition of adhesion functions. MAbs against ICAM-1 and VCAM-1 showed positive staining of fixed tissue in immunohistochemistry studies. The same antibodies also blocked the function of these two adhesion molecules. The new MAbs can be used to study the tissue expression of adhesion molecules in SIV-infected animals as well as to test the involvement of these molecules in virus infection. Thus they should prove invaluable as probes of the role of cell adhesion molecules in AIDS pathogenesis in an animal model.

Role of immune responses against the envelope and the core antigens of simian immunodeficiency virus SIVmne in protection against homologous cloned and uncloned virus challenge in Macaques

We previously showed that envelope (gp160)-based vaccines, used in a live recombinant virus priming and subunit protein boosting regimen, protected macaques against intravenous and intrarectal challenges with the homologous simian immunodeficiency virus SIVmne clone E11S. However, the breadth of protection appears to be limited, since the vaccines were only partially effective against intravenous challenge by the uncloned SIVmne. To examine factors that could affect the breadth and the efficacy of this immunization approach, we studied (i) the effect of priming by recombinant vaccinia virus; (ii) the role of surface antigen gp130; and (iii) the role of core antigens (Gag and Pol) in eliciting protective immunity. Results indicate that (i) priming with recombinant vaccinia virus was more effective than subunit antigen in eliciting protective responses; (ii) while both gp130 and gp160 elicited similar levels of SIV-specific antibodies, gp130 was not as effective as gp160 in protection, indicating a possible role for the transmembrane protein in presenting functionally important epitopes; and (iii) although animals immunized with core antigens failed to generate any neutralizing antibody and were infected upon challenge, their virus load was 50- to 100-fold lower than that of the controls, suggesting the importance of cellular immunity or other core-specific immune responses in controlling acute infection. Complete protection against intravenous infection by the pathogenic uncloned SIVmne was achieved by immunization with both the envelope and the core antigens. These results indicate that immune responses to both antigens may contribute to protection and thus argue for the inclusion of multiple antigens in recombinant vaccine designs.

Enhanced expression, native purification, and characterization of CCR5, a principal HIV-1 coreceptor

Seven-transmembrane segment, G protein-coupled receptors (GPCRs) play important roles in many biological processes in which pharmaceutical intervention may be useful. High level expression and native purification of GPCRs are important steps in the biochemical and structural characterization of these molecules. Here, we describe enhanced mammalian cell expression and purification of a codon-optimized variant of the chemokine receptor CCR5, a GPCR that plays a central role in the entry of the human immunodeficiency virus-1 (HIV-1) into immune cells. CCR5 could be solubilized in its native state as determined by its ability to be precipitated by 2D7, a conformation-dependent anti-CCR5 antibody, and by the HIV-1 gp120 envelope glycoprotein. The 2D7 antibody recognized immature and mature forms of CCR5 equally, whereas gp120 preferentially recognized the mature form, a result that underscores a role for posttranslational modification of CCR5 in its HIV-1 coreceptor function. The methods described herein contribute to the analysis of CCR5 and are likely to be applicable to many other GPCRs.

The anti-HIV pseudopeptide HB-19 forms a complex with the cell-surface-expressed nucleolin independent of heparan sulfate proteoglycans

The HB-19 pseudopeptide 5[Kpsi(CH(2)N)PR]-TASP, psi(CH(2)N) for reduced peptide bond, is a specific inhibitor of human immunodeficiency virus (HIV) infection in different CD4(+) cell lines and in primary T-lymphocytes and macrophages. Here, by using an experimental CD4(+) cell model to monitor HIV entry and infection, we demonstrate that HB-19 binds the cell surface and inhibits attachment of HIV particles to permissive cells. At concentrations that inhibit HIV attachment, HB-19 binds cells irreversibly, becomes complexed with the cell-surface-expressed nucleolin, and eventually results in its degradation. Accordingly, by confocal immunofluorescence microscopy, we demonstrate the drastic reduction of the cell-surface-expressed nucleolin following treatment of cells with HB-19. HIV particles can prevent the binding of HB-19 to cells and inhibit complex formation with nucleolin. Such a competition between viral particles and HB-19 is consistent with the implication of nucleolin in the process of HIV attachment to target cells. We show that another inhibitor of HIV infection, the fibroblast growth factor-2 (FGF-2) that uses cell-surface-expressed heparan sulfate proteoglycans as low affinity receptors, binds cells and blocks attachment of HIV to permissive cells. FGF-2 does not prevent the binding of HB-19 to cells and to nucleolin, and similarly HB-19 has no apparent effect on the binding of FGF-2 to the cell surface. The lack of competition between these two anti-HIV agents rules out the potential involvement of heparan sulfate proteoglycans in the mechanism of anti-HIV effect of HB-19, thus pointing out that nucleolin is its main target.

Leishmania: fine mapping of the Leishmanolysin molecule's conserved core domains involved in binding and internalization

The Leishmanolysin molecule's role in the uptake of Leishmania parasites by the human U937 pro-myelocytic cell line was studied, using synthetic peptides representing the complete Leishmania (Viannia) guyanensis Leishmanolysin protein amino acid sequence. The particular peptides present in two protein's core domains efficiently impaired the internalization of promastigotes from four different Leishmania species and modified the kinetics of the binding of heterologous recombinant Leishmanolysin protein. The functional domains which exhibited this property represent a highly conserved portion of the sequence among different Leishmania species. The peptides' inhibitory activity correlated with their ability to bind molecules present on the surface of the human cell line. One of the two functional core domains identified involves the previously described adhesive sequence (SRYD) and the putative zinc-binding motif (HExxH). The second functional core domain includes a third histidine residue coordinated with zinc which determines the molecule's structural features. These findings indicate that the molecular interactions between Leishmanolysin's conserved domains and the macrophage surface molecules efficiently contribute to the parasite's internalization. Induction of neutralizing immune responses, which impair the early parasite-host interaction described here, may be an important alternative in designing synthetic subunit human leishmaniasis vaccines.

Activation conditions determine susceptibility of murine primary T-lymphocytes to retroviral infection

BACKGROUND

Genetically modified T-lymphocytes are potential therapeutic agents for the treatment of various disorders. Successful retroviral infection of primary murine T-lymphocytes is a prerequisite to the study of adoptive cell therapies in a small animal model. The definition of factors controlling retroviral infection of T-lymphocytes would also be useful to better understand retroviral diseases. However, retroviral-mediated gene transfer into murine primary T-cells has remained elusive.

METHODS

In order to define the requirements for stable and efficient gene transfer in primary murine T-lymphocytes, we investigated factors capable of affecting retroviral infection. These include activation conditions (using mitogens or monoclonal antibodies), culture conditions (including media composition and cytokine addition), timing of viral exposure, retroviral receptor selection (ecotropic, amphotropic, or vesicular stomatitis virus G (VSV-G) glycoprotein receptor), and viral titer.

RESULTS

We show that efficient gene transfer can be achieved in murine T-lymphocytes, provided that a number of favorable conditions are met, in particular optimized T-cell activation conditions, optimal timing of infection, adequate interleukin-2 concentration and T-cell density, and a high viral titer. On a particulate basis, we find that ecotropic particles are more effective than amphotropic or VSV-G-pseudotyped particles, and recommend the use of a specifically selected retroviral packaging cell line. CD4+ T-cells are equally or more infectible than CD8+ lymphocytes, depending on the activation conditions. The Th1 and Th2 subsets are comparably susceptible to retroviral infection, in contrast to what has been reported in some instances of human T-cell infection by human immunodeficiency virus (HIV).

CONCLUSIONS

We establish conditions that enable efficient retroviral-mediated gene transfer in murine primary T-lymphocytes. T-lymphocytes are not uniformly susceptible to retroviral infection depending on the mode of T-cell activation. These findings have implications for devising approaches to the study of T-cell biology, adoptive cell therapies, and the pathophysiology of retroviral diseases in mouse models.

Roles of CD4 and coreceptors in binding, endocytosis, and proteolysis of gp120 envelope glycoproteins derived from human immunodeficiency virus type 1

Infections by human immunodeficiency virus type 1 (HIV-1) involve interactions of the viral envelope glycoprotein gp120 with CD4 and then with a coreceptor. R5 isolates of HIV-1 use CCR5 as a coreceptor, whereas X4 isolates use CXCR4. It is not known whether coreceptors merely trigger fusion of the viral and cellular membranes or whether they also influence the energetics of virus adsorption, the placement of the membrane fusion reaction, and the metabolism of adsorbed gp120. Surprisingly, the pathway for metabolism of adsorbed gp120 has not been investigated thoroughly in any cells. To address these issues, we used purified (125)I-gp120s derived from the R5 isolate BaL and from the X4 isolate IIIB as ligands for binding onto human cells that expressed CD4 alone or CD4 with a coreceptor. The gp120 preparations were active in forming ternary complexes with CD4 and the appropriate coreceptor. Moreover, the cellular quantities of CD4 and coreceptors were sufficient for efficient infections by the corresponding HIV-1 isolates. In these conditions, the kinetics and affinities of (125)I-gp120 adsorptions and their subsequent metabolisms were strongly dependent on CD4 but were not significantly influenced by CCR5 or CXCR4. After binding to CD4, the (125)I-gp120s slowly became resistant to extraction from the cell monolayers by pH 3.0 buffer, suggesting that they were endocytosed with half-times of 1-2 h. Within 20-30 min of endocytosis, the (125)I-gp120s were proteolytically degraded to small products that were shed into the media. The weak base chloroquine strongly inhibited (125)I-gp120 proteolysis and caused its intracellular accumulation, suggesting involvement of a low pH organelle. Results supporting these methods and conclusions were obtained by confocal immunofluorescence microscopy. We conclude that the energetics, kinetics, and pathways of (125)I-gp120 binding, endocytosis, and proteolysis are determined principally by CD4 rather than by coreceptors in cells that contain sufficient coreceptors for efficient infections. Therefore, the role of coreceptors in HIV-1 infections probably does not include steerage or subcellular localization of adsorbed virus.

The channel-forming toxin aerolysin neutralizes human immunodeficiency virus type 1

Aerolysin is a channel-forming toxin secreted by Aeromonas spp. that binds to glycosyl phosphatidylinositol (GPI)-anchored proteins, such as Thy-1, on sensitive target cells. Receptor binding is followed first by oligomerization of the toxin and then by insertion of the oligomers into the membrane to form stable channels that disrupt the permeability barrier. Human immunodeficiency virus type 1 (HIV-1) produced from T cells is known to incorporate Thy-1 and other GPI-anchored proteins into its membrane. Here, we show that aerolysin is capable of neutralizing HIV-1 in a dose-dependent manner and that neutralization depends upon the presence of these proteins in the viral envelope. Pretreatment with phosphatidylinositol-specific phospholipase C to remove GPI-anchored proteins greatly reduced HIV-1 sensitivity to the toxin, and virus originating from a mutant cell line that lacks GPI-anchored proteins was not neutralized. Aerolysin variants with single amino acid changes that prevent oligomerization or insertion of the toxin were unable to inactivate the virus, implying that channel formation is necessary for neutralization to occur. These findings represent the first evidence that a pathogenic human virus can be neutralized by a bacterial toxin.

Compartmentalization of the IgG immune response to HIV-1 in breast milk

OBJECTIVE

To evaluate the IgG immune response to HIV-1 in colostrum.

METHODS

Paired serum and colostrum were collected from 16 asymptomatic HIV-1-infected women. IgG to gp160 and to four peptides (gp41 immunodominant DI domain, gp41/Id; EDLKWA epitope of DIII domain, gp41/K; gp120 C-terminus, gp120/Ct; V3 loop, gp120/V3) were evaluated in all samples. Functional activity of purified IgG was assessed for the ability to block transcytosis of cell-associated HIV-1 through a tight monolayer of endometrial epithelial cell line HEC1.

RESULTS

IgG antibody to gp160 and to the four env-encoded synthetic peptides were detected in all specimens. The mean specific activity of IgG to gp41/K was 4.2 fold higher in colostrum than in paired serum. In contrast, mean specific activities of IgG to gp160 and gp41/Id were twofold higher in serum than in paired colostrum. Mean specific activities of IgG to gp120/V3 and to gp120/Ct were similar in systemic and milk compartments. Functional activity of IgG was evaluated in six paired serum and colostrum: in two women, serum IgG was 3.0 and 7.6 fold more efficient in blocking transcytosis than colostrum IgG; in one patient, colostrum IgG exhibited a 28 fold higher inhibitory capacity than serum IgG; in the remaining patients, serum and colostrum IgG demonstrated similar inhibitory activities against transcytosis of HIV.

CONCLUSION

These features are consistent with a compartmentalization of the humoral IgG immune response to HIV within the mammary gland. Some HIV-1 antigens are able to induce a strong humoral mucosal immune response which may be of relevance for the design of a mucosal vaccine against HIV-1.

HIV and SIV gp120 binding does not predict coreceptor function

Interaction of HIV and SIV Envelope (Env) proteins with viral coreceptors is a critical step in viral entry. By using a sensitive and specific gp120 binding assay, we have identified a discordance between the ability of a coreceptor to support Env-mediated membrane fusion and high-affinity binding of gp120. Direct binding of gp120 from the dual-tropic HIV-1 strain 89.6 was not detectable for any coreceptor that it uses for fusion, while detectable binding of gp120s from the R5 HIV-1 strains JRFL and CM235 and the SIV strain 239 was not measurable for many CCR5 chimeras and mutants that function efficiently as viral coreceptors. In comparison, binding of chemokines to these same mutants was highly predictive of their ability to signal. Thus, gp120 is more sensitive than chemokines to perturbations of CCR5 structure. We conclude that while chemokine binding to CCR5 is a good predictor of chemokine receptor function, gp120 binding does not always predict coreceptor function.

Genetic determinants of HIV-1 infection and its manifestations

The human immunodeficiency virus type 1 (HIV-1), which has become pandemic within a single generation, has encountered an immune system in which genetically encoded elements have evolved gradually under different environmental pressures in diverse populations. Important heritable differences in genes that alter susceptibility to HIV-1 infection or the rate of deterioration of immunity, or both, have been discovered in cohorts carefully defined for intensity of exposure to the virus, viral subtype characteristics, and onset and course of infection. For the highly polymorphic human leukocyte antigen (HLA) antigen processing and presenting system, the principle that small contributions of multiple interactive HLA marker combinations (primarily in the class I pathway) significantly modulate the course of HIV-1 infection has now been confirmed in several independently evaluated groups of patients. Variants of HLA genes probably also play some role in the acquisition of infection by the various routes of transmission. Genes for an elaborate set of circulating chemokine molecules and their cell-surface receptors clearly regulate cell attachment and penetration of HIV. Certain allelic forms of one, the CCR5 gene, alter susceptibility to infection and the rate of progression of disease; in the homozygous state, a deleted form (Delta32 CCR5) strongly protects against infection, and in infected heterozygotes, it slows the disease process somewhat. Mutants in genes of other chemokine system components further differentiate the response to infection, and frequencies of these forms vary between and within races. Work relating additional genetic markers to HIV infection or disease is at earlier stages. Dissecting the effects of multiple variants in complex gene systems will clearly require organized comprehensive approaches in considerably larger populations than have typically been assembled.

Both neutralization resistance and high infectivity phenotypes are caused by mutations of interacting residues in the human immunodeficiency virus type 1 gp41 leucine zipper and the gp120 receptor- and coreceptor-binding domains

Neutralization resistance of human immunodeficiency virus type 1 (HIV-1) is a major impediment to vaccine development. We have found that residues of HIV-1 MN strain in the C terminus of gp120 and the leucine zipper (LZ) region of gp41 viral envelope proteins interact cooperatively to determine neutralization resistance and modulate infectivity. Further, results demonstrate that this interaction, by which regions of gp120 are assembled onto the LZ, involves amino acid residues intimately related to those which participate in the binding of the envelope to its receptor and coreceptor. Variations in this critical assembly structure determine the concordant, interdependent evolution of increased infectivity efficiency and neutralization resistance phenotypes of the envelopes. The results elucidate important structure-function relationships among epitopes that are important targets of vaccine development.

Chemokine receptors as HIV-1 coreceptors: roles in viral entry, tropism, and disease

In addition to CD4, the human immunodeficiency virus (HIV) requires a coreceptor for entry into target cells. The chemokine receptors CXCR4 and CCR5, members of the G protein-coupled receptor superfamily, have been identified as the principal coreceptors for T cell line-tropic and macrophage-tropic HIV-1 isolates, respectively. The updated coreceptor repertoire includes numerous members, mostly chemokine receptors and related orphans. These discoveries provide a new framework for understanding critical features of the basic biology of HIV-1, including the selective tropism of individual viral variants for different CD4+ target cells and the membrane fusion mechanism governing virus entry. The coreceptors also provide molecular perspectives on central puzzles of HIV-1 disease, including the selective transmission of macrophage-tropic variants, the appearance of T cell line-tropic variants in many infected persons during progression to AIDS, and differing susceptibilities of individuals to infection and disease progression. Genetic findings have yielded major insights into the in vivo roles of individual coreceptors and their ligands; of particular importance is the discovery of an inactivating mutation in the CCR5 gene which, in homozygous form, confers strong resistance to HIV-1 infection. Beyond providing new perspectives on fundamental aspects of HIV-1 transmission and pathogenesis, the coreceptors suggest new avenues for developing novel therapeutic and preventative strategies to combat the AIDS epidemic.

Neutralizing antibody responses in Africa green monkeys naturally infected with simian immunodeficiency virus (SIVagm)

This study assessed the magnitude and cross-reactivity of the neutralizing antibody response generated by natural SIV infection in wild-caught African green monkeys. Neutralizing antibodies of variable potency, sometimes exceeding a titer of 1:1,000, were detected in 20 of 20 SIV-seropositive African green monkeys in Kenya. Detection of those neutralizing antibodies was dependent on the strain of virus and the cells used for assay, where the most sensitive detection was made with SIVagm1532 in Sup T1 cells. Potent neutralization of SIVagm1532 was seen with contemporaneous autologous serum. Potent neutralization was also detected with laboratory-passaged SIVmac251 and SIVsmB670, but not with SIVsmE660 and two additional strains of SIVagm. Serum samples from rhesus macaques (Macaca mulatta) experimentally infected with either SIVmac251 or SIVsmE660 were capable of low-level neutralization of SIVagm. These results indicate that natural infection with SIV can generate strain-specific neutralizing antibodies in African green monkeys. They also indicate that some neutralization determinants of SIVagm are partially shared with SIV strains that arose in sooty mangabys and were subsequently transmitted to rhesus macaques.

IL-12 Delivery from Recombinant Vaccinia Virus Attenuates the Vector and Enhances the Cellular Immune Response Against HIV-1 Env in a Dose-Dependent Manner

To develop vaccination strategies against HIV-1 infection aimed to specifically enhance the cell-mediated immunity (CMI), we have engineered vaccinia virus (VV) recombinants expressing HIV-1 Env (rVVenv) and murine IL-12 (rVVlucIL-12) genes or coexpressing both genes (rVVenvIL-12). In mice inoculated with rVVlucIL-12 there is a rapid clearance of the virus, and this correlates with the induction of high levels of IL-12 and IFN-γ in serum and spleen early after infection. Enzyme-linked immunospot analysis of mice inoculated with rVVlucIL-12, revealed a nearly 2-fold increase in the number of specific anti-VV CD8+ T cells compared with that in mice given control rVV, and the serum Ab response was biased in favor of a Th1 response. An enhancement of about 2-fold in the number of anti-gp160 IFN-γ-secreting CD8+ T cells was observed in mice inoculated with rVVenvIL-12, when a dose of 1 × 107 PFU/mouse was used, but this enhancement was not observed when mice were given 5 × 107 PFU. This variation with virus dosage was confirmed in mice immunized simultaneously with different multiplicities of rVV expressing singly the env or IL-12 genes. The highest specific CMI was obtained in mice coadministered a low dose (2 × 104 PFU) of rVVlucIL-12 and 1 × 107 PFU of rVVenv. Our findings provide evidence for specific enhancement of the CMI to HIV-1 Env by the differential expression of IL-12 and env genes delivered from VV recombinants. This approach can be of wide vaccination interest as a means to improve immune responses to other Ags.

Role of the HIV type 1 glycoprotein 120 V3 loop in determining coreceptor usage

Macrophage (M)-tropic HIV-1 isolates use the beta-chemokine receptor CCR5 as a coreceptor for entry, while T cell line-adapted (TCLA) strains use CXCR4 and dual-tropic strains can use either CCR5 or CXCR4. To investigate the viral determinants involved in choice of coreceptor, we used a fusion assay based on the infection of CD4+ HeLa cells that express one or both coreceptors with Semliki Forest virus (SFV) recombinants expressing the native HIV-1 gp160 of a primary M-tropic isolate (HIV-1BX08), a TCLA isolate (HIV-1LAI), or a dual-tropic strain (HIV-1MN). We examined whether the V3 region of these glycoproteins interacts directly with the corresponding coreceptors by assaying coreceptor-dependent cell-to-cell fusion mediated by the different recombinants in the presence of various synthetic linear peptides. Synthetic peptides corresponding to different V3 loop sequences blocked syncytium formation in a coreceptor-specific manner. Synthetic V2 peptides were also inhibitory for syncytium formation, but showed no apparent coreceptor specificity. A BX08 V3 peptide with a D320 --> R substitution retained no inhibitory capacity for BX08 Env-mediated cell-to-cell fusion, but inhibited LAI Env-mediated fusion as efficiently as the homologous LAI V3 peptide. The same mutation engineered in the BX08 env gene rendered it able to form syncytia on CD4+CXCR4+CCR5-HeLa cells and susceptible to inhibition by SDF-1alpha and MIP-1beta. Other substitutions tested (D320 --> Q/D324 --> N or S306 --> R) exhibited intermediate effects on coreceptor usage. These results underscore the importance of the V3 loop in modulating coreceptor choice and show that single amino acid modifications in V3 can dramatically modify coreceptor usage. Moreover, they provide evidence that linear V3 loop peptides can compete with intact cell surface-expressed gp120/gp41 for CCR5 or CXCR4 interaction.

Host-specific modulation of the selective constraints driving human immunodeficiency virus type 1 env gene evolution

To address the evolution of human immunodeficiency virus type 1 (HIV-1) within a single host, we analyzed the HIV-1 C2-V5 env regions of both cell-free genomic-RNA- and proviral-DNA-derived clones. Sequential samples were collected over a period of 3 years from six untreated subjects (three typical progressors [TPs] and three slow progressors [SPs], all with a comparable length of infection except one. The evolutionary analysis of the C2-V5 env sequences performed on 506 molecular clones (253 RNA- and 253 DNA-derived sequences) highlighted a series of differences between TPs and SPs. In particular, (i) clonal sequences from SPs (DNA and RNA) showed lower nucleotide similarity than those from TPs (P = 0. 0001), (ii) DNA clones from SPs showed higher intra- and intersample nucleotide divergence than those from TPs (P < 0.05), (iii) higher host-selective pressure was generally detectable in SPs (DNA and RNA sequences), and (iv) the increase in the genetic distance of DNA and RNA sequences over time was paralleled by an increase in both synonymous (Ks) and nonsynonymous (Ka) substitutions in TPs but only in nonsynonymous substitutions in SPs. Several individual peculiarities of the HIV-1 evolutionary dynamics emerged when the V3, V4, and V5 env regions of both TPs and SPs were evaluated separately. These peculiarities, probably reflecting host-specific features of selective constraints and their continuous modulation, are documented by the dynamics of Ka/Ks ratios of hypervariable env domains.

Cyanovirin-N binds to gp120 to interfere with CD4-dependent human immunodeficiency virus type 1 virion binding, fusion, and infectivity but does not affect the CD4 binding site on gp120 or soluble CD4-induced conformational changes in gp120

Cyanovirin-N (CV-N), an 11-kDa protein isolated from the cyanobacterium Nostoc ellipsosporum, potently inactivates diverse strains of human immunodeficiency virus type 1 (HIV-1), HIV-2, and simian immunodeficiency virus. While it has been well established that the viral surface envelope glycoprotein gp120 is a molecular target of CV-N, the detailed mechanism of action is of further interest. We compared matched native and CV-N-treated virus preparations in a panel of assays that measure viral replication, assessing successive stages of the viral life cycle. CV-N-treated virions failed to infect cells as detected by p24 production and quantitative PCR for HIV-1 reverse transcription products, whereas treatment of the target cells did not block infection, confirming that CV-N acts at the level of the virus, not the target cell, to abort the initial infection process. Compared to native HIV-1 preparations, CV-N-treated HIV-1 virions showed impaired CD4-dependent binding to CD4(+) T cells and did not mediate "fusion from without" of CD4(+) target cells. CV-N also blocked HIV envelope glycoprotein Env-induced, CD4-dependent cell-cell fusion. Mapping studies with monoclonal antibodies (MAbs) to defined epitopes on the HIV-1 envelope glycoprotein indicated that CV-N binds to gp120 in a manner that does not occlude or alter the CD4 binding site or V3 loop or other domains on gp120 recognized by defined MAbs and does not interfere with soluble CD4-induced conformational changes in gp120. Binding of CV-N to soluble gp120 or virions inhibited subsequent binding of the unique neutralizing MAb 2G12, which recognizes a glycosylation-dependent epitope. However, prior binding of 2G12 MAb to gp120 did not block subsequent binding by CV-N. These results help clarify the mechanism of action of CV-N and suggest that the compound may act in part by preventing essential interactions between the envelope glycoprotein and target cell receptors. This proposed mechanism is consistent with the extensive activity profile of CV-N against numerous isolates of HIV-1 and other lentiviruses and supports the potential broad utility of this protein as a microbicide to prevent the sexual transmission of HIV.

Differential inhibition of human immunodeficiency virus type 1 fusion, gp120 binding, and CC-chemokine activity by monoclonal antibodies to CCR5

The CC-chemokine receptor CCR5 mediates fusion and entry of the most commonly transmitted human immunodeficiency virus type 1 (HIV-1) strains. We have isolated six new anti-CCR5 murine monoclonal antibodies (MAbs), designated PA8, PA9, PA10, PA11, PA12, and PA14. A panel of CCR5 alanine point mutants was used to map the epitopes of these MAbs and the previously described MAb 2D7 to specific amino acid residues in the N terminus and/or second extracellular loop regions of CCR5. This structural information was correlated with the MAbs' abilities to inhibit (i) HIV-1 entry, (ii) HIV-1 envelope glycoprotein-mediated membrane fusion, (iii) gp120 binding to CCR5, and (iv) CC-chemokine activity. Surprisingly, there was no correlation between the ability of a MAb to inhibit HIV-1 fusion-entry and its ability to inhibit either the binding of a gp120-soluble CD4 complex to CCR5 or CC-chemokine activity. MAbs PA9 to PA12, whose epitopes include residues in the CCR5 N terminus, strongly inhibited gp120 binding but only moderately inhibited HIV-1 fusion and entry and had no effect on RANTES-induced calcium mobilization. MAbs PA14 and 2D7, the most potent inhibitors of HIV-1 entry and fusion, were less effective at inhibiting gp120 binding and were variably potent at inhibiting RANTES-induced signaling. With respect to inhibiting HIV-1 entry and fusion, PA12 but not PA14 was potently synergistic when used in combination with 2D7, RANTES, and CD4-immunoglobulin G2, which inhibits HIV-1 attachment. The data support a model wherein HIV-1 entry occurs in three stages: receptor (CD4) binding, coreceptor (CCR5) binding, and coreceptor-mediated membrane fusion. The antibodies described will be useful for further dissecting these events.

Hypervariable region 3 residues of HIV type 1 gp120 involved in CCR5 coreceptor utilization: therapeutic and prophylactic implications

Crystallographic characterization of a ternary complex containing a monomeric gp120 core, parts of CD4, and a mAb, revealed a region that bridges the inner and outer domains of gp120. In a related genetic study, several residues conserved among primate lentiviruses were found to play important roles in CC-chemokine receptor 5 (CCR5) coreceptor utilization, and all but one were mapped to the bridging domain. To reconcile this finding with previous reports that the hypervariable region 3 (V3) of gp120 plays an important role in chemokine coreceptor utilization, elucidating the roles of various V3 residues in this critical part of the HIV type 1 (HIV-1) life cycle is essential. Alanine-scanning mutagenesis was carried out to identify V3 residues critical for CCR5 utilization. Our findings demonstrated that several residues in V3 were critical to CCR5 utilization. Furthermore, these residues included not only those conserved across HIV-1 subtypes, but also those that varied among HIV-1 subtypes. Although the highly conserved V3 residues may represent unique targets for antiviral designs, the involvement of variable residues raises the possibility that antigenic variation in the coreceptor binding domain could further complicate HIV-1 vaccine design.

HIV-1 attachment: another look

HIV-1 attachment to host cells is generally considered to take place via high-affinity binding between CD4 and gp120. However, the binding of virion-associated gp120 to cellular CD4 is often weak, and most cell types that are permissive for HIV-1 infection express little CD4. Thus, other interactions between the virion and the cell surface could dominate the attachment process.

Characterization of a neutralization-escape variant of SHIVKU-1, a virus that causes acquired immune deficiency syndrome in pig-tailed macaques

A chimeric simian-human immunodeficiency virus (SHIV-4) containing the tat, rev, vpu, and env genes of HIV type 1 (HIV-1) in a genetic background of SIVmac239 was used to develop an animal model in which a primate lentivirus expressing the HIV-1 envelope glycoprotein caused acquired immune deficiency syndrome (AIDS) in macaques. An SHIV-infected pig-tailed macaque that died from AIDS at 24 weeks postinoculation experienced two waves of viremia: one extending from weeks 2-8 and the second extending from week 18 until death. Virus (SHIVKU-1) isolated during the first wave was neutralized by antibodies appearing at the end of the first viremic phase, but the virus (SHIVKU-1b) isolated during the second viremic phase was not neutralized by these antibodies. Inoculation of SHIVKU-1b into 4 pig-tailed macaques resulted in severe CD4(+) T cell loss by 2 weeks postinoculation, and all 4 macaques died from AIDS at 23-34 weeks postinoculation. Because this virus had a neutralization-resistant phenotype, we sequenced the env gene and compared these sequences with those of the env gene of SHIVKU-1 and parental SHIV-4. With reference to SHIV-4, SHIVKU-1b had 18 and 6 consensus amino acid substitutions in the gp120 and gp41 regions of Env, respectively. These compared with 10 and 3 amino acid substitutions in the gp120 and gp41 regions of SHIVKU-1. Our data suggested that SHIVKU-1 and SHIVKU-1b probably evolved from a common ancestor but that SHIVKU-1b did not evolve from SHIVKU-1. A chimeric virus, SHIVKU-1bMC17, constructed with the consensus env from the SHIVKU-1b on a background of SHIV-4, confirmed that amino acid substitutions in Env were responsible for the neutralization-resistant phenotype. These results are consistent with the hypothesis that neutralizing antibodies induced by SHIVKU-1 in pig-tailed macaque resulted in the selection of a neutralization-resistant virus that was responsible for the second wave of viremia.

The V3 loop-mimicking pseudopeptide 5[Kpsi(CH2N)PR]-TASP inhibits HIV infection in primary macrophage cultures

The V3 loop-mimicking pseudopeptide 5[Kpsi(CH2N)PR]-TASP [psi(CH2N) representing a reduced peptide bond], which presents pentavalently the tripeptide Kpsi(CH2N)PR, is a potent inhibitor of HIV entry. By its capacity to bind specifically protein components on the cell surface, 5[Kpsi(CH2N)PR]-TASP blocks the attachment of virus particles to permissive CD4+ cells. Here, the inhibitory effect of 5[Kpsi(CH2N)PR]-TASP was investigated in monocyte-derived macrophages (MDMs) infected by the monocytotropic HIV-1(Ba-L) isolate. We show that 5[Kpsi(CH2N)PR]-TASP inhibits HIV-1(Ba-L) infection in a dose-dependent manner, with more than 90% inhibition at 2 microM concentration. On the other hand, the control 5[QPQ]-TASP construct and the monovalent Kpsi(CH2N)PR tripeptide have no effect even at high concentrations. Under such experimental conditions, the biotin-labeled 5[Kpsi(CH2N)PR]-TASP, but not the Kpsi(CH2N)PR construct, binds specifically to the surface of MDMs and forms a stable complex with the cell surface-expressed nucleolin, as has been demonstrated to be the case in peripheral blood mononuclear cells. Infection of MDMs by HIV-1(Ba-L) could also be inhibited by beta-chemokines RANTES and MIP-1beta. Interestingly, association of low concentrations of 5[Kpsi(CH2N)PR]-TASP and beta-chemokines results in a synergistic inhibitory effect on HIV infection compared with the effect observed with each reagent alone. The inhibitory effect of 5[Kpsi(CH2N)PR]-TASP in primary macrophage cultures point out its potential as an anti-HIV drug in cells, which are the natural viral targets.

Structural biology of HIV

The human immunodeficiency virus (HIV) genome encodes a total of three structural proteins, two envelope proteins, three enzymes, and six accessory proteins. Studies over the past ten years have provided high-resolution three-dimensional structural information for all of the viral enzymes, structural proteins and envelope proteins, as well as for three of the accessory proteins. In some cases it has been possible to solve the structures of the intact, native proteins, but in most cases structural data were obtained for isolated protein domains, peptidic fragments, or mutants. Peptide complexes with two regulatory RNA fragments and a protein complex with an RNA recognition/encapsidation element have also been structurally characterized. This article summarizes the high-resolution structural information that is currently available for HIV proteins and reviews current structure-function and structure-biological relationships.

Dendritic cells: a link between innate and adaptive immunity

Dendritic cells (DC) constitute a unique system of cells able to induce primary immune responses. As a component of the innate immune system, DC organize and transfer information from the outside world to the cells of the adaptive immune system. DC can induce such contrasting states as active immune responsiveness or immunological tolerance. Recent years have brought a wealth of information regarding DC biology and pathophysiology, that shows the complexity of this cell system. Although our understanding of DC biology is still in its infancy, we are now in a position to use DC-based immunotherapy protocols to treat cancer and infectious diseases.

HIV gp120: double lock strategy foils host defences

The recent determination of the structure of a complex formed between the HIV-1 glycoprotein gp120, CD4 and an antibody fragment has revealed new mechanisms for viral evasion of the immune response and shed light on how the virus enters target cells. The results of this work, together with related biochemical studies, may assist in the future design of therapeutic strategies against HIV-1 infection.