Structural basis of coreceptor recognition by HIV-1 envelope spike.

HIV-1 envelope glycoprotein (Env), which consists of trimeric (gp160)3 cleaved to (gp120 and gp41)3, interacts with the primary receptor CD4 and a coreceptor (such as chemokine receptor CCR5) to fuse viral and target-cell membranes. The gp120-coreceptor interaction has previously been proposed as the most crucial trigger for unleashing the fusogenic potential of gp41. Here we report a cryo-electron microscopy structure of a full-length gp120 in complex with soluble CD4 and unmodified human CCR5, at 3.9 Å resolution. The V3 loop of gp120 inserts into the chemokine-binding pocket formed by seven transmembrane helices of CCR5, and the N terminus of CCR5 contacts the CD4-induced bridging sheet of gp120. CCR5 induces no obvious allosteric changes in gp120 that can propagate to gp41; it does bring the Env trimer close to the target membrane. The N terminus of gp120, which is gripped by gp41 in the pre-fusion or CD4-bound Env, flips back in the CCR5-bound conformation and may irreversibly destabilize gp41 to initiate fusion. The coreceptor probably functions by stabilizing and anchoring the CD4-induced conformation of Env near the cell membrane. These results advance our understanding of HIV-1 entry into host cells and may guide the development of vaccines and therapeutic agents.

Open and closed structures reveal allostery and pliability in the HIV-1 envelope spike.

For many enveloped viruses, binding to a receptor(s) on a host cell acts as the first step in a series of events culminating in fusion with the host cell membrane and transfer of genetic material for replication. The envelope glycoprotein (Env) trimer on the surface of HIV is responsible for receptor binding and fusion. Although Env can tolerate a high degree of mutation in five variable regions (V1-V5), and also at N-linked glycosylation sites that contribute roughly half the mass of Env, the functional sites for recognition of receptor CD4 and co-receptor CXCR4/CCR5 are conserved and essential for viral fitness. Soluble SOSIP Env trimers are structural and antigenic mimics of the pre-fusion native, surface-presented Env, and are targets of broadly neutralizing antibodies. Thus, they are attractive immunogens for vaccine development. Here we present high-resolution cryo-electron microscopy structures of subtype B B41 SOSIP Env trimers in complex with CD4 and antibody 17b, or with antibody b12, at resolutions of 3.7 Å and 3.6 Å, respectively. We compare these to cryo-electron microscopy reconstructions of B41 SOSIP Env trimers with no ligand or in complex with either CD4 or the CD4-binding-site antibody PGV04 at 5.6 Å, 5.2 Å and 7.4 Å resolution, respectively. Consequently, we present the most complete description yet, to our knowledge, of the CD4-17b-induced intermediate and provide the molecular basis of the receptor-binding-induced conformational change required for HIV-1 entry into host cells. Both CD4 and b12 induce large, previously uncharacterized conformational rearrangements in the gp41 subunits, and the fusion peptide becomes buried in a newly formed pocket. These structures provide key details on the biological function of the type I viral fusion machine from HIV-1 as well as new templates for inhibitor design.

[Atomic force microscopy: a tool to analyze the viral cycle]. / Microscopie à force atomique pour l'étude du cycle viral - L'exemple du VIH-1.

Each step of the HIV-1 life cycle frequently involves a change in the morphology and/or mechanical properties of the viral particle or core. The atomic force microscope (AFM) constitutes a powerful tool for characterizing these physical changes at the scale of a single virus. Indeed, AFM enables the visualization of viral capsids in a controlled physiological environment and to probe their mechanical properties by nano-indentation. Finally, AFM force spectroscopy allows to characterize the affinities between viral envelope proteins and cell receptors at the single molecule level.

[A model of the structure of the receptor apparatus of HIV target cells (1)]. / Model' struktury retseptornogo apparata kletok-mishenei VICh (soobshchenie 1).

A model is proposed and well based of structure of the receptor apparatus of interacting HIV target cells: Langerhans cells (LC) and helper cells (HC)--designed to explain feasibility of realizing a double activation signal, together with some features of immune pathogenesis of HIV-infection (morphofunctional abnormalities in certain subpopulations of the immune system such as HC, suppressor cells, killer cells, natural killers, lymph cells, macrophages, LC), as well as immunogenetic predisposition to AIDS.

Interaction of human epidermal Langerhans cells with HIV-1 viral envelope proteins (gp 120 and gp 160s) involves a receptor-mediated endocytosis independent of the CD4 T4A epitope.

The CD4 molecule is known to be the preferential receptor for the HIV-1 envelope glycoprotein. Epidermal Langerhans cells are dendritic cells which express several surface antigens, among them CD4 antigens. To clarify the exact role of CD4 molecules in Langerhans cell infection induced by HIV-1, we investigated the possible involvement of the interactions between HIV-1 gp 120 or HIV-1 gp 160s (soluble gp 160) and Langerhans cell surface. We also assessed the expression of CD4 molecules on Langerhans cell membranes dissociated by means of trypsin from their neighbouring keratinocytes. The cellular phenotype was monitored using flow cytometry and quantitative immunoelectron microscopy. We reported that human Langerhans cells can bind the viral envelope proteins (gp 120 or gp 160s), and that this binding does not depend on CD4 protein expression. This binding is not blocked by anti-CD4 monoclonal antibodies. We show that a proportion of gp 120/gp 160s-receptor complexes enters Langerhans cells by a process identified as a receptor-mediated endocytosis. The amount of surface bound gp 120/gp 160s is not consistent with the amount of CD4 antigens present on Langerhans cell membranes. Gp 120/gp 160s binding sites on Langerhans cell suspensions appeared to be trypsin resistant, while CD4 antigens (at least the epitopes known to bind the HIV-1) are trypsin sensitive. A burst of gp 120 receptor expression was detected on 1-day cultured Langerhans cells while CD4 antigens disappeared. These findings lead to the most logical conclusion that binding of gp 120/gp 160s is due to the presence of a Langerhans cell surface molecule different from CD4 antigens.

[HIV in sperm]. / HIV im Sperma.

HIV in the semen is located in the mononuclear cell fraction, which contains CD4 lymphocytes and monocytes/macrophages, i.e. the classic HIV host cells. In addition, HIV is also present in cell-free seminal fluid. The results on HIV binding to spermatozoa are still controversial. Genital tract infections are a possible cofactor that might increase the risk of HIV transmission by way of the semen, because they lead to a marked increase in the number of potential HIV host cells in semen.

Structural analysis of the human immunodeficiency virus-binding domain of CD4. Epitope mapping with site-directed mutants and anti-idiotypes.

The CD4 molecule, a differentiation marker expressed primarily by T lymphocytes, plays an important role in lymphocyte activation. CD4 is also the receptor for HIV. A number of recent studies have localized the high affinity binding site of the HIV envelope glycoprotein, gp120, to the NH2-terminal (V1) domain of CD4, a region with sequence and predicted structural homology with Ig kappa chain V domains (V kappa). In this report, we show that V1 bears structural similarities with V kappa regions through detailed epitope mapping of 26 CD4 mAbs. The binding sites of these mAbs were initially defined relative to one another by crossblocking analysis and were then localized to specific domains of CD4 in blocking studies with truncated, soluble CD4 proteins. The epitopes within the V1 domain were mapped in detail with a panel of 17 substitution mutants, and the specificities of several mAbs that appear to recognize very similar epitopes were examined in crossblocking studies with anti-idiotype antibodies. The location of the epitopes is consistent with a V kappa-like structure of V1. Most of the epitopes lie within regions of predicted exposed loops. A number of these epitopes span discontinuous residues in the linear sequence that lies in close proximity in an Ig fold. Alignment of CD4 V1 with the Ig V kappa chains places these epitopes within stretches corresponding to the complimentarity-determining regions. This epitope analysis is relevant for a vaccine strategy for HIV based on anti-idiotype antibodies to CD4 mAbs and for studies with CD4 antibodies on the role of CD4 in T lymphocyte activation.

[The molecular bases of the pathogenesis of AIDS]. / Molekuliarnye osnovy patogeneza SPID.

The comparative study of the amino acid sequence of gp120 in human immunodeficiency virus (HIV) strains HTLV-III and ARV-2 and the amino end areas of the growth hormone receptors of human skin and the insulin receptors has been carried out, thus making it possible to predict the existence of two compact domains connected with an area of a peptide chain. This area is incapable of the formation of a compact globular structure due to a high content of the remnants of proline. The data obtained as the result of electron microscopic study in combination with image processing have confirmed the predicted three-dimensional structure of gp120. This study has also shown that the amino acid sequence of some regions in the domains of gp120 has a significant degree of homology with similarly located regions of the growth hormone and insulin receptors; in its turn, this amino acid sequence is homologous to the framework regions of the VH domain of immunoglobulin. Antibodies to this VH domain specifically react with recombinant HTLV-III antigen. On the basis of the data obtained in our experiments and from the analysis clinico-immunological information, we have come to the conclusion that AIDS is an autoimmune disease induced by HIV due to the structural homology of gp120 with highly important receptors of human cells.