Entry of R5X4 and X4 human immunodeficiency virus type 1 strains is mediated by negatively charged and tyrosine residues in the amino-terminal domain and the second extracellular loop of CXCR4

Mesenchymal Stromal Cells: a Possible Reservoir for HIV-1?

The introduction of antiretroviral therapy (ART) and highly active antiretroviral therapy (HAART) has transformed human immunodeficiency virus (HIV)-1 into a chronic, well-managed disease. However, these therapies do not eliminate all infected cells from the body despite suppressing viral load. Viral rebound is largely due to the presence of cellular reservoirs which support long-term persistence of HIV-1. A thorough understanding of the HIV-1 reservoir will facilitate the development of new strategies leading to its detection, reduction, and elimination, ultimately leading to curative therapies for HIV-1. Although immune cells derived from lymphoid and myeloid progenitors have been thoroughly studied as HIV-1 reservoirs, few studies have examined whether mesenchymal stromal/stem cells (MSCs) can assume this function. In this review, we evaluate published studies which have assessed whether MSCs contribute to the HIV-1 reservoir. MSCs have been found to express the receptors and co-receptors required for HIV-1 entry, albeit at levels of expression and receptor localisation that vary considerably between studies. Exposure to HIV-1 and HIV-1 proteins alters MSC properties in vitro, including their proliferation capacity and differentiation potential. However, in vitro and in vivo experiments investigating whether MSCs can become infected with and harbour latent integrated proviral DNA are lacking. In conclusion, MSCs appear to have the potential to contribute to the HIV-1 reservoir. However, further studies are needed using techniques such as those used to prove that cluster of differentiation (CD)4⁺ T cells constitute an HIV-1 reservoir before a reservoir function can definitively be ascribed to MSCs.

Genetic variations in the host dependency factors ALCAM and TPST2 impact HIV-1 disease progression

OBJECTIVES

Recently, the activated leukocyte cell adhesion molecule (ALCAM) and tyrosylprotein sulfotransferase 2 (TPST2) have been identified as important host dependency factors (HDFs) for in-vitro HIV-1 replication. To determine whether these genes play a role in HIV-1 pathogenesis, we analysed whether naturally occurring genetic variations were associated with the clinical course of infection.

DESIGN/METHODS

Single nucleotide polymorphisms (SNPs) in ALCAM and TPST2 were analysed in a cohort of 304 HIV-1-infected men who have sex with men and survival analysis was used to determine their effect on the outcome of untreated HIV-1 infection. Flowcytometry was used to determine the effect of SNPs on CD4 T-cell activation prior to HIV-1 infection and 1 and 5 years after infection. In-vitro HIV-1 infections were performed to analyse the effect of the SNPs on HIV-1 replication.

RESULTS

We observed that the minor allele of rs1344861 in ALCAM was associated with accelerated disease progression, whereas the minor allele of rs9613199 in TPST2 was associated with delayed disease progression. In-vitro infection assays did not demonstrate any differences in HIV-1 replication associated with rs9613199. However, the increase in CD4 T-cell immune activation levels during HIV-1 infection was less pronounced in infected individuals homozygous for rs9613199, which is in agreement with delayed disease progression.

CONCLUSION

Our data demonstrate that ALCAM and TPST2 play a role in HIV-1 pathogenesis. SNPs in these genes, without known functional implications, had a major effect on disease progression, and therefore, these HDFs may be attractive and effective targets for new treatment strategies.

CXCR4-targeting nanobodies differentially inhibit CXCR4 function and HIV entry

The chemokine receptor CXCR4 and its ligand CXCL12 contribute to a variety of human diseases, such as cancer. CXCR4 is also a major co-receptor facilitating HIV entry. Accordingly, CXCR4 is considered as an attractive therapeutic target. Drug side effects and poor pharmacokinetic properties have been major hurdles that have prevented the implementation of CXCR4-directed inhibitors in treatment regimes. We evaluated the activity of a new and promising class of biologics, namely CXCR4-targeting nanobodies, with the purpose of identifying nanobodies that would preferentially inhibit HIV infection, while minimally disturbing other CXCR4-related functions. All CXCR4-interacting nanobodies inhibited CXCL12 binding and receptor-mediated calcium mobilization with comparable relative potencies. Importantly, the anti-HIV-1 activity of the nanobodies did not always correlate with their ability to modulate CXCR4 signaling and function, indicating that the anti-HIV and anti-CXCR4 activity are not entirely overlapping and may be functionally separated. Three nanobodies with divergent activity profiles (VUN400, VUN401 and VUN402) were selected for in depth biological evaluation. While all three nanobodies demonstrated inhibitory activity against a wide range of HIV (X4) strains, VUN402 poorly blocked CXCL12-induced CXCR4 internalization, chemotaxis and changes in cell morphology. Each of these nanobodies recognized distinct, although partially overlapping epitopes on CXCR4, which might underlie their distinct activity profiles. Our results demonstrate the potential of CXCR4-targeting nanobody VUN402 as a novel lead and starting point for the development of a more potent and selective anti-HIV agent.

Design and evaluation of a CXCR4 targeting peptide 4DV3 as an HIV entry inhibitor and a ligand for targeted drug delivery

The feasibility of utilizing the cell surface chemokine receptor CXCR4 for human immunodeficiency virus (HIV) entry inhibition and as an intracellular portal for targeted drug delivery was evaluated. Novel DV3 ligands (1DV3, 2DV3, and 4DV3) were designed, synthesized and conjugated to various probes (fluorescein isothiocyanate (FITC) or biotin) and cargos with sizes ranging from 10 to 50 nm (polyethylene glycol (PEG), streptavidin, and a polymeric nanoparticle). 4DV3 conjugated probes inhibited HIV-1 entry into the CXCR4-expressing reporter cell line TZM-bl (IC₅₀ at 553 nM) whereas 1DV3 and 2DV3 did not. 4DV3 also inhibited binding of anti-CXCR4 antibody 44,708 to TZM-bl cells with nanomolar potency, while the small-molecule CXCR4 antagonist AMD3100 did not. Molecular modeling suggested simultaneous binding of a single 4DV3 molecule to four CXCR4 molecules. Differences in CXCR4-binding sites could explain the discrete inhibitory effects observed for 4DV3, the 44,708 antibody and AMD3100. In the Sup-T1 cell chemotaxis assay, the 4DV3 ligand functioned as a CXCR4 allosteric enhancer. In addition, 4DV3 ligand-conjugated cargos with sizes ranging from 10 to 50 nm were taken up into CXCR4-expressing Sup-T1 and TZM-bl cells, demonstrating that CXCR4 could serve as a drug delivery portal for nanocarriers. The uptake of 4DV3 functionalized nanocarriers combined with the allosteric interaction with CXCR4 suggests enhanced endocytosis occurs when 4DV3 is the targeting ligand. The current results indicate that 4DV3 might serve as a prototype for a new type of dual function ligand, one that acts as a HIV-1 entry inhibitor and a CXCR4 drug delivery targeting ligand.

Synthesis and anti-HIV activities of unsymmetrical long chain dicarboxylate esters of dinucleoside reverse transcriptase inhibitors

A series of 11 unsymmetrical dicarboxylate conjugates of dinucleoside reverse transcriptase inhibitors were synthesized. Three dicarboxylic acids, succinic acid, suberic acid and 1,14-tetradecandioc acid, were diesterified with either 3'-azido-2',3'-dideoxythymidine (AZT), 3'-fluoro-2',3'-dideoxythymidine (FLT), 2',3'-dideoxy-3'-thiacytidine (3TC), or 5-fluoro-2',3'-dideoxy-3'-thiacytidine (FTC). The anti-HIV activity of synthesized compounds was evaluated against HIV-1 X4 (IIIB) and R5 (BaL) viral strains in single-round infection assays. Results indicated that the tetradecandioate esters of nucleosides were more active against HIV than the corresponding parent nucleosides and nucleoside conjugates. The tetradecandioate conjugate of FLT and FTC (5) was found to be the most potent compounds with EC50 values of 47 and 75nM against X4 and R5 HIV-1 strains, respectively, while the EC50 values for the parent analogs, FLT and FTC, ranged from 700 to 3300nM.

A genome-wide CRISPR screen identifies a restricted set of HIV host dependency factors

Host proteins are essential for HIV entry and replication and can be important nonviral therapeutic targets. Large-scale RNA interference (RNAi)-based screens have identified nearly a thousand candidate host factors, but there is little agreement among studies and few factors have been validated. Here we demonstrate that a genome-wide CRISPR-based screen identifies host factors in a physiologically relevant cell system. We identify five factors, including the HIV co-receptors CD4 and CCR5, that are required for HIV infection yet are dispensable for cellular proliferation and viability. Tyrosylprotein sulfotransferase 2 (TPST2) and solute carrier family 35 member B2 (SLC35B2) function in a common pathway to sulfate CCR5 on extracellular tyrosine residues, facilitating CCR5 recognition by the HIV envelope. Activated leukocyte cell adhesion molecule (ALCAM) mediates cell aggregation, which is required for cell-to-cell HIV transmission. We validated these pathways in primary human CD4⁺ T cells through Cas9-mediated knockout and antibody blockade. Our findings indicate that HIV infection and replication rely on a limited set of host-dispensable genes and suggest that these pathways can be studied for therapeutic intervention.

Ligand selectivity of a synthetic CXCR4 mimetic peptide

The chemokine receptor CXCR4 belongs to the family of seven-transmembrane G-protein coupled receptors (GPCRs). It is activated by its natural ligand SDF-1α. In addition, CXCR4, along with CCR5, serve as coreceptors during HIV-1 entry into its target cell. Recently, we introduced a CXCR4 mimetic peptide, termed CX4-M1, which presents the three extracellular loops (ECLs) of the receptor. CX4-M1 was shown to selectively bind to gp120 of X4-tropic, that is, CXCR4 using, HIV-1, as well as to peptides that present the V3-loops of these gp120 proteins. Furthermore, CX4-M1 selectively inhibits infection of cells with X4-tropic HIV-1. We have now adapted the sequence of the ECLs presented by CX4-M1 to the recently published crystal structure of CXCR4. The binding behavior, as well as the effect on HIV-1 infection, of the resulting peptide (CX4-Mc) was very similar to CX4-M1, validating retrospectively the original design of CX4-M1. A peptide presenting the ECLs of CCR5 (CR5-M), on the other hand, did neither bind to gp120 from X4-tropic HIV-1, nor did it inhibit infection of cells with X4-tropic HIV-1. Furthermore, we could show that CX4-M1, as well as CX4-Mc, but not CR5-M, are selectively recognized by anti-CXCR4 antibodies, bind to SDF-1α, and also inhibit SDF-1α signaling, extending the scope of selective functional CXCR4 mimicry through CX4-M1.

Functional roles of evolutionary conserved motifs and residues in vertebrate chemokine receptors

Chemokine receptors regulate cell migration and homing. They belong to the rhodopsin-like family of GPCRs. Their ancestor genes emerged in the early stages of vertebrate evolution. Since then, the family has been greatly expanded through whole and segmental genome duplication events. During evolution, many amino acid changes have been introduced in individual chemokine receptors, but certain motifs and residues are highly conserved. Previously, we proposed a nomenclature system of the vertebrate chemokine receptors based on their evolutionary history and phylogenetic analyses. With the use of this classification system, we are now able to confidently assign the species orthologs of vertebrate chemokine receptors. Here, we systematically analyze conserved motifs and residues of each group of orthologous chemokine receptors that may play important roles in their signaling and biologic functions. Our present analysis may provide useful information on how individual chemokine receptors are activated upon ligand binding.

Molecular recognition of CXCR4 by a dual tropic HIV-1 gp120 V3 loop

HIV-1 cell entry is initiated by the interaction of the viral envelope glycoprotein gp120 with CD4, and chemokine coreceptors CXCR4 and CCR5. The molecular recognition of CXCR4 or CCR5 by the HIV-1 gp120 is mediated through the V3 loop, a fragment of gp120. The binding of the V3 loop to CXCR4 or CCR5 determines the cell tropism of HIV-1 and constitutes a key step before HIV-1 cell entry. Thus, elucidating the molecular recognition of CXCR4 by the V3 loop is important for understanding HIV-1 viral infectivity and tropism, and for the design of HIV-1 inhibitors. We employed a comprehensive set of computational tools, predominantly based on free energy calculations and molecular-dynamics simulations, to investigate the molecular recognition of CXCR4 by a dual tropic V3 loop. We report what is, to our knowledge, the first HIV-1 gp120 V3 loop:CXCR4 complex structure. The computationally derived structure reveals an abundance of polar and nonpolar intermolecular interactions contributing to the HIV-1 gp120:CXCR4 binding. Our results are in remarkable agreement with previous experimental findings. Therefore, this work sheds light on the functional role of HIV-1 gp120 V3 loop and CXCR4 residues associated with HIV-1 coreceptor activity.

V3-independent competitive resistance of a dual-X4 HIV-1 to the CXCR4 inhibitor AMD3100

A CXCR4 inhibitor-resistant HIV-1 was isolated from a dual-X4 HIV-1 in vitro. The resistant variant displayed competitive resistance to the CXCR4 inhibitor AMD3100, indicating that the resistant variant had a higher affinity for CXCR4 than that of the wild-type HIV-1. Amino acid sequence analyses revealed that the resistant variant harbored amino acid substitutions in the V2, C2, and C4 regions, but no remarkable changes in the V3 loop. Site-directed mutagenesis confirmed that the changes in the C2 and C4 regions were principally involved in the reduced sensitivity to AMD3100. Furthermore, the change in the C4 region was associated with increased sensitivity to soluble CD4, and profoundly enhanced the entry efficiency of the virus. Therefore, it is likely that the resistant variant acquired the higher affinity for CD4/CXCR4 by the changes in non-V3 regions. Taken together, a CXCR4 inhibitor-resistant HIV-1 can evolve using a non-V3 pathway.

Mimicking Protein-Protein Interactions through Peptide-Peptide Interactions: HIV-1 gp120 and CXCR4

We have recently designed a soluble synthetic peptide that functionally mimics the HIV-1 coreceptor CXCR4, which is a chemokine receptor that belongs to the family of seven-transmembrane GPCRs. This CXCR4 mimetic peptide, termed CX4-M1, presents the three extracellular loops (ECLs) of the receptor. In binding assays involving recombinant proteins, as well as in cellular infection assays, CX4-M1 was found to selectively recognize gp120 from HIV-1 strains that use CXCR4 for cell entry (X4 tropic HIV-1). Furthermore, anti-HIV-1 antibodies modulate this interaction in a molecular mechanism related to that of their impact on the gp120-CXCR4 interaction. We could now show that the selectivity of CX4-M1 pertains not only to gp120 from X4 tropic HIV-1, but also to synthetic peptides presenting the V3 loops of these gp120 proteins. The V3 loop is thought to be an essential part of the coreceptor binding site of gp120 that contacts the second ECL of the coreceptor. We were able to experimentally confirm this notion in binding assays using substitution analogs of CX4-M1 and the V3 loop peptides, respectively, as well as in cellular infection assays. These results indicate that interactions of the HIV-1 Env with coreceptors can be mimicked by synthetic peptides, which may be useful to explore these interactions at the molecular level in more detail.

Escape from human immunodeficiency virus type 1 (HIV-1) entry inhibitors

The human immunodeficiency virus (HIV) enters cells through a series of molecular interactions between the HIV envelope protein and cellular receptors, thus providing many opportunities to block infection. Entry inhibitors are currently being used in the clinic, and many more are under development. Unfortunately, as is the case for other classes of antiretroviral drugs that target later steps in the viral life cycle, HIV can become resistant to entry inhibitors. In contrast to inhibitors that block viral enzymes in intracellular compartments, entry inhibitors interfere with the function of the highly variable envelope glycoprotein as it continuously adapts to changing immune pressure and available target cells in the extracellular environment. Consequently, pathways and mechanisms of resistance for entry inhibitors are varied and often involve mutations across the envelope gene. This review provides a broad overview of entry inhibitor resistance mechanisms that inform our understanding of HIV entry and the design of new inhibitors and vaccines.

Function, diversity and therapeutic potential of the N-terminal domain of human chemokine receptors

Chemokines and their receptors play fundamental roles in many physiological and pathological processes such as leukocyte trafficking, inflammation, cancer and HIV-1 infection. Chemokine-receptor interactions are particularly intricate and therefore require precise orchestration. The flexible N-terminal domain of human chemokine receptors has regularly been demonstrated to hold a crucial role in the initial recognition and selective binding of the receptor ligands. The length and the amino acid sequences of the N-termini vary considerably among different receptors but they all show a high content of negatively charged residues and are subject to post-translational modifications such as O-sulfation and N- or O-glycosylation. In addition, a conserved cysteine that is most likely engaged in a receptor-stabilizing disulfide bond delimits two functionally distinct parts in the N-terminus, characterized by specific molecular signatures. Structural analyses have shown that the N-terminus of chemokine receptors recognizes a groove on the chemokine surface and that this interaction is stabilized by high-affinity binding to a conserved sulfotyrosine-binding pocket. Altogether, these data provide new insights on the chemokine-receptor molecular interplay and identify the receptor N-terminus-binding site as a new target for the development of therapeutic molecules. This review presents and discusses the diversity and function of human chemokine receptor N-terminal domains and provides a comprehensive annotated inventory of their sequences, laying special emphasis on the presence of post-translational modifications and functional features. Finally, it identifies new molecular signatures and proposes a computational model for the positioning and the conformation of the CXCR4 N-terminus grafted on the first chemokine receptor X-ray structure.

Critical role in CXCR4 signaling and internalization of the polypeptide main chain in the amino terminus of SDF-1α probed by novel N-methylated synthetically and modularly modified chemokine analogues

The replication of human immunodeficiency virus type 1 (HIV-1) can be profoundly inhibited by the natural ligands of two major HIV-1 coreceptors, CXCR4 and CCR5. Stromal cell-derived factor-1α (SDF-1α) is a natural ligand of CXCR4. We have recently developed a synthetic biology approach of using synthetically and modularly modified (SMM)-chemokines to dissect various aspects of the structure-function relationship of chemokines and their receptors. Here, we used this approach to design novel SMM-SDF-1α analogues containing unnatural N-methylated residues in the amino terminus to investigate whether the polypeptide main chain amide bonds in the N-terminus of SDF-1α play a role in SDF-1α signaling via CXCR4 and/or receptor internalization. The results show that SDF-1α analogues with a modified N-methylated main chain at position 2, 3, or 5 retain significant CXCR4 binding and yet completely lose signaling activities. Furthermore, a representative N-methylated analogue has been shown to be incapable of causing CXCR4 internalization. These results suggest that the ability of SDF-1α to activate CXCR4 signaling and internalization is dependent upon the main chain amide bonds in the N-terminus of SDF-1α. This study demonstrates the feasibility and value of applying a synthetic biology approach to chemically engineer natural proteins and peptide ligands as probes of important biological functions that are not addressed by other biological techniques.

Regulation of CXCR4 conformation by the small GTPase Rac1: implications for HIV infection

The chemokine receptor CXCR4 is a critical regulator of cell migration and serves as a coreceptor for HIV-1. The chemokine stromal cell derived factor-1, also known as CXCL12, binds to CXCR4 and exerts its biologic functions partly through the small guanosine triphosphate hydrolase (GTPase) Rac1 (ras-related C3 botulinum toxin substrate 1). We show in different cell types, including CD34(+) hematopoietic stem and progenitor cells, that inhibition of Rac1 causes a reversible conformational change in CXCR4, but not in the related receptors CXCR7 or CCR5. Biochemical experiments showed that Rac1 associates with CXCR4. The conformational change of CXCR4 on Rac1 inhibition blocked receptor internalization and impaired CXCL12-induced Gα(i) protein activation. Importantly, we found that the conformation adopted by CXCR4 after Rac1 inhibition prevents HIV-1 infection of both the U87-CD4-CXCR4 cell line and of primary peripheral blood mononuclear cells. In conclusion, our data show that Rac1 activity is required to maintain CXCR4 in the responsive conformation that allows receptor signaling and facilitates HIV-1 infection; this implies that Rac1 positively regulates CXCR4 function and identifies the Rac1-CXCR4 axis as a new target for preventing HIV-1 infection.

Biology and clinical relevance of chemokines and chemokine receptors CXCR4 and CCR5 in human diseases

Chemokines and their receptors are implicated in a wide range of human diseases, including acquired immune deficiency syndrome (AIDS). The entry of human immunodeficiency virus type 1 (HIV-1) into a cell is initiated by the interaction of the virus's surface envelope proteins with two cell surface components of the target cell, namely CD4 and a chemokine co-receptor, usually CXCR4 or CCR5. Typical anti-HIV-1 agents include protease and reverse transcriptase inhibitors, but the targets of these agents tend to show rapid mutation rates. As such, strategies based on HIV-1 co-receptors have appeal because they target invariant host determinants. Chemokines and their receptors are also of general interest since they play important roles in numerous physiological and pathological processes in addition to AIDS. Therefore, intensive basic and translational research is ongoing for the dissection of their structure - function relationships in an effort to understand the molecular mechanism of chemokine - receptor interactions and signal transductions across cellular membranes. This paper reviews and discusses recent advances and the translation of new knowledge and discoveries into novel interventional strategies for clinical application.

Conformational HIV-1 envelope on particulate structures: a tool for chemokine coreceptor binding studies

The human immunodeficiency virus type 1 (HIV-1) external envelope glycoprotein gp120 presents conserved binding sites for binding to the primary virus receptor CD4 as well as the major HIV chemokine coreceptors, CCR5 and CXCR4. Concerted efforts are underway to understand the specific interactions between gp120 and coreceptors as well as their contribution to the subsequent membrane fusion process. The present review summarizes the current knowledge on this biological aspect, which represents one of the key and essential points of the HIV-host cell interplay and HIV life cycle. The relevance of conformational HIV-1 Envelope proteins presented on Virus-like Particles for appropriate assessment of this molecular interaction, is also discussed.

Synthesis, antiviral and contraceptive activities of nucleoside-sodium cellulose sulfate acetate and succinate conjugates

Chemical conjugates between sodium cellulose sulfate (CS), displaying contraceptive and HIV-entry inhibiting properties, and nucleoside reverse transcriptase inhibitors (NRTIs) (3'-azido-2',3'-dideoxythymidine (AZT), 3'-fluoro-2',3'-dideoxythymidine (FLT), or 2',3'-dideoxy-3'-thiacytidine (3TC)) were designed to simultaneously provide contraceptive and anti-HIV activity. Two linkers, acetate and succinate, were used to conjugate the nucleoside analogs with CS. The conjugates containing cellulose sulfate-acetate (CSA) (e.g., AZT-CSA and FLT-CSA) were found to be more potent than CS and other conjugates (e.g., AZT-succinate-CS, and FLT-succinate-CS). The presence of both sulfate and the acetate groups on cellulose were critical for generating maximum anti-HIV activity. In addition to showing equal potency against wild-type and multidrug resistant HIV-1, the AZT-CSA conjugate displayed significant contraceptive activity in an animal model, providing the initial proof-of-concept for the design and synthesis of dual-activity compounds based on these combinations.

Molecular determinants of HIV-1 subtype C coreceptor transition from R5 to R5X4

The molecular mechanism(s) underlying transition from CCR5 to CXCR4 usage of subtype C viruses remain largely unknown. We previously identified a subtype C HIV-1 infected child whose virus demonstrated CXCR4 usage along with CCR5 upon longitudinal follow-up. Here we delineated the molecular determinants of Env involved in expanded coreceptor usage. Residue changes in three positions of Env V3 domain are critical for the dual tropic phenotype. These include: substitution of arginine at position 11, MG or LG insertion between positions 13 and 14, and substitution of threonine at the position immediately downstream of the GPGQ crown. Introducing these mutations into V3 region of a heterologous R5 virus also conferred dual tropism. Molecular modeling of V3 revealed a possible structural basis for the dual tropic phenotype. Determining what defines a subtype C X4 virus will lead to a better understanding of subtype C HIV-1 pathogenesis, and will provide important information relevant to anti-retroviral therapy.

A maraviroc-resistant HIV-1 with narrow cross-resistance to other CCR5 antagonists depends on both N-terminal and extracellular loop domains of drug-bound CCR5

CCR5 antagonists inhibit HIV entry by binding to a coreceptor and inducing changes in the extracellular loops (ECLs) of CCR5. In this study, we analyzed viruses from 11 treatment-experienced patients who experienced virologic failure on treatment regimens containing the CCR5 antagonist maraviroc (MVC). Viruses from one patient developed high-level resistance to MVC during the course of treatment. Although resistance to one CCR5 antagonist is often associated with broad cross-resistance to other agents, these viruses remained sensitive to most other CCR5 antagonists, including vicriviroc and aplaviroc. MVC resistance was dependent upon mutations within the V3 loop of the viral envelope (Env) protein and was modulated by additional mutations in the V4 loop. Deep sequencing of pretreatment plasma viral RNA indicated that resistance appears to have occurred by evolution of drug-bound CCR5 use, despite the presence of viral sequences predictive of CXCR4 use. Envs obtained from this patient before and during MVC treatment were able to infect cells expressing very low CCR5 levels, indicating highly efficient use of a coreceptor. In contrast to previous reports in which CCR5 antagonist-resistant viruses interact predominantly with the N terminus of CCR5, these MVC-resistant Envs were also dependent upon the drug-modified ECLs of CCR5 for entry. Our results suggest a model of CCR5 cross-resistance whereby viruses that predominantly utilize the N terminus are broadly cross-resistant to multiple CCR5 antagonists, whereas viruses that require both the N terminus and antagonist-specific ECL changes demonstrate a narrow cross-resistance profile.

Distinct molecular pathways to X4 tropism for a V3-truncated human immunodeficiency virus type 1 lead to differential coreceptor interactions and sensitivity to a CXCR4 antagonist

During the course of infection, transmitted HIV-1 isolates that initially use CCR5 can acquire the ability to use CXCR4, which is associated with an accelerated progression to AIDS. Although this coreceptor switch is often associated with mutations in the stem of the viral envelope (Env) V3 loop, domains outside V3 can also play a role, and the underlying mechanisms and structural basis for how X4 tropism is acquired remain unknown. In this study we used a V3 truncated R5-tropic Env as a starting point to derive two X4-tropic Envs, termed DeltaV3-X4A.c5 and DeltaV3-X4B.c7, which took distinct molecular pathways for this change. The DeltaV3-X4A.c5 Env clone acquired a 7-amino-acid insertion in V3 that included three positively charged residues, reestablishing an interaction with the CXCR4 extracellular loops (ECLs) and rendering it highly susceptible to the CXCR4 antagonist AMD3100. In contrast, the DeltaV3-X4B.c7 Env maintained the V3 truncation but acquired mutations outside V3 that were critical for X4 tropism. In contrast to DeltaV3-X4A.c5, DeltaV3-X4B.c7 showed increased dependence on the CXCR4 N terminus (NT) and was completely resistant to AMD3100. These results indicate that HIV-1 X4 coreceptor switching can involve (i) V3 loop mutations that establish interactions with the CXCR4 ECLs, and/or (ii) mutations outside V3 that enhance interactions with the CXCR4 NT. The cooperative contributions of CXCR4 NT and ECL interactions with gp120 in acquiring X4 tropism likely impart flexibility on pathways for viral evolution and suggest novel approaches to isolate these interactions for drug discovery.

HIV-1 resistance to CCR5 antagonists associated with highly efficient use of CCR5 and altered tropism on primary CD4+ T cells

We previously reported on a panel of HIV-1 clade B envelope (Env) proteins isolated from a patient treated with the CCR5 antagonist aplaviroc (APL) that were drug resistant. These Envs used the APL-bound conformation of CCR5, were cross resistant to other small-molecule CCR5 antagonists, and were isolated from the patient's pretreatment viral quasispecies as well as after therapy. We analyzed viral and host determinants of resistance and their effects on viral tropism on primary CD4(+) T cells. The V3 loop contained residues essential for viral resistance to APL, while additional mutations in gp120 and gp41 modulated the magnitude of drug resistance. However, these mutations were context dependent, being unable to confer resistance when introduced into a heterologous virus. The resistant virus displayed altered binding between gp120 and CCR5 such that the virus became critically dependent on the N' terminus of CCR5 in the presence of APL. In addition, the drug-resistant Envs studied here utilized CCR5 very efficiently: robust virus infection occurred even when very low levels of CCR5 were expressed. However, recognition of drug-bound CCR5 was less efficient, resulting in a tropism shift toward effector memory cells upon infection of primary CD4(+) T cells in the presence of APL, with relative sparing of the central memory CD4(+) T cell subset. If such a tropism shift proves to be a common feature of CCR5-antagonist-resistant viruses, then continued use of CCR5 antagonists even in the face of virologic failure could provide a relative degree of protection to the T(CM) subset of CD4(+) T cells and result in improved T cell homeostasis and immune function.

Adaptive mutations in a human immunodeficiency virus type 1 envelope protein with a truncated V3 loop restore function by improving interactions with CD4

We previously reported that a human immunodeficiency virus type 1 (HIV-1) clade B envelope protein with a severely truncated V3 loop regained function after passage in tissue culture. The adapted virus, termed TA1, retained the V3 truncation, was exquisitely sensitive to neutralization by the CD4 binding site monoclonal antibody b12 and by HIV-positive human sera, used CCR5 to enter cells, and was completely resistant to small molecule CCR5 antagonists. To examine the mechanistic basis for these properties, we singly and in combination introduced each of the 5 mutations from the adapted clone TA1 into the unadapted envelope. We found that single amino acid changes in the C3 region, the V3 loop, and in the fusion peptide were responsible for imparting near-normal levels of envelope function to TA1. T342A, which resulted in the loss of a highly conserved glycosylation site in C3, played the primary role. The adaptive amino acid changes had no impact on CCR5 antagonist resistance but made virus more sensitive to neutralization by antibodies to the CD4 binding site, modestly enhanced affinity for CD4, and made TA1 more responsive to CD4 binding. Specifically, TA1 was triggered by soluble CD4 more readily than the parental Env and, unlike the parental Env, could mediate entry on cells that express low levels of CD4. In contrast, TA1 interacted with CCR5 less efficiently and was highly sensitive to antibodies that bind to the CCR5 N terminus and ECL2. Therefore, enhanced utilization of CD4 is one mechanism by which HIV-1 can overcome mutations in the V3 region that negatively affect CCR5 interactions.

Characterization of a human immunodeficiency virus type 1 V3 deletion mutation that confers resistance to CCR5 inhibitors and the ability to use aplaviroc-bound receptor

The human immunodeficiency virus type 1 (HIV-1) V3 loop is essential for coreceptor binding and principally determines tropism for the CCR5 and CXCR4 coreceptors. Using the dual-tropic virus HIV-1(R3A), we previously made an extensive panel of V3 deletions and identified subdomains within V3 that could differentially mediate R5 and X4 tropism. A deletion of residues 9 to 12 on the N-terminal side of the V3 stem ablated X4 tropism while leaving R5 tropism intact. This mutation also resulted in complete resistance to several small-molecule CCR5 inhibitors. Here, we extend these studies to further characterize a variant of this mutant, Delta9-12a, adapted for growth in CCR5(+) SupT1 cells. Studies using coreceptor chimeras, monoclonal antibodies directed against the CCR5 amino terminus (NT) and extracellular loops, and CCR5 point mutants revealed that, relative to parental R3A, R5-tropic Delta9-12a was more dependent on the CCR5 NT, a region that contacts the gp120 bridging sheet and V3 base. Neutralization sensitivity assays showed that, compared to parental R3A, Delta9-12a was more sensitive to monoclonal antibodies b12, 4E10, and 2G12. Finally, cross-antagonism assays showed that Delta9-12a could use aplaviroc-bound CCR5 for entry. These studies indicate that increased dependence on the CCR5 NT represents a mechanism by which HIV envelopes acquire resistance to CCR5 antagonists and may have more general implications for mechanisms of drug resistance that arise in vivo. In addition, envelopes such as Delta9-12a may be useful for developing new entry inhibitors that target the interaction of gp120 and the CCR5 NT.

Specific interactions between the viral coreceptor CXCR4 and the biguanide-based compound NB325 mediate inhibition of human immunodeficiency virus type 1 infection

The present studies were conducted to better define the mechanism of action of polyethylene hexamethylene biguanide (PEHMB) (designated herein as NB325), which was shown in previous studies to inhibit infection by the human immunodeficiency virus type 1 (HIV-1). Fluorescence-activated flow cytometric analyses of activated human CD4(+) T lymphocytes exposed to NB325 demonstrated concentration-dependent reductions in CXCR4 epitope recognition in the absence of altered recognition of selected CD4 or CD3 epitopes. NB325 also inhibited chemotaxis of CD4(+) T lymphocytes induced by the CXCR4 ligand CXCL12. However, NB325 did not cause CXCR4 internalization (unlike CXCL12) and did not interfere with CXCL12 binding. Additional flow cytometric analyses using antibodies with distinct specificities for extracellular domains of CXCR4 demonstrated that NB325 specifically interfered with antibody binding to extracellular loop 2 (ECL2). This interaction was confirmed using competitive binding analyses, in which a peptide derived from CXCR4 ECL2 competitively inhibited NB325-mediated reductions in CXCR4 epitope recognition. Collectively, these results demonstrate that the biguanide-based compound NB325 inhibits HIV-1 infection by specifically interacting with the HIV-1 coreceptor CXCR4.

Inhibitory role of CXCR4 glycan in CD4-independent X4-tropic human immunodeficiency virus type 1 infection and its abrogation in CD4-dependent infection

CXCR4 functions as an infection receptor of X4 human immunodeficiency virus type 1 (HIV-1) . CXCR4 is glycosylated at the N-terminal extracellular region, which is important for viral envelope (Env) protein binding. We compared the effects of CXCR4 glycan on the CD4-dependent and -independent infections in human cells by X4 viruses. We found that transduction mediated by Env proteins of CD4-independent HIV-1 strains increased up to 5.5-fold in cells expressing unglycosylated CXCR4, suggesting that the CXCR4 glycan inhibits CD4-independent X4 virus infection. Co-expression of CD4 on the target cell surface or pre-incubation of virus particles with soluble CD4 abrogates the glycan-mediated inhibition of X4 virus infection, suggesting that interaction of Env protein with CD4 counteracts the inhibition. These findings indicate that it will be advantageous for X4 HIV-1 to remain CD4-dependent. A structural model that explains the glycan-mediated inhibition is discussed.

Effects of partial deletions within the human immunodeficiency virus type 1 V3 loop on coreceptor tropism and sensitivity to entry inhibitors

The human immunodeficiency virus type 1 (HIV-1) V3 loop is critical for coreceptor binding and principally determines tropism for the CCR5 and CXCR4 coreceptors. The recent crystallographic resolution of V3 shows that its base is closely associated with the conserved coreceptor binding site on the gp120 core, whereas more distal regions protrude toward the cell surface, likely mediating interactions with coreceptor extracellular loops. However, these V3-coreceptor interactions and the structural basis for CCR5 or CXCR4 specificity are poorly understood. Using the dual-tropic virus HIV-1(R3A), which uses both CCR5 and CXCR4, we sought to identify subdomains within V3 that selectively mediate R5 or X4 tropism. An extensive panel of V3 mutants was evaluated for effects on tropism and sensitivity to coreceptor antagonists. Mutations on either side of the V3 base (residues 3 to 8 and 26 to 33) ablated R5 tropism and made the resulting X4-tropic Envs more sensitive to the CXCR4 inhibitor AMD3100. When mutations were introduced within the V3 stem, only a deletion of residues 9 to 12 on the N-terminal side ablated X4 tropism. Remarkably, this R5-tropic Delta9-12 mutant was completely resistant to several small-molecule inhibitors of CCR5. Envs with mutations in the V3 crown (residues 13 to 20) remained dual tropic. Similar observations were made for a second dual-tropic isolate, HIV-1(89.6). These findings suggest that V3 subdomains can be identified that differentially affect R5 and X4 tropism and modulate sensitivity to CCR5 and CXCR4 inhibitors. These studies provide a novel approach for probing V3-coreceptor interactions and mechanisms by which these interactions can be inhibited.

Regulation of CXCR4 signaling

The chemokine receptor CXCR4 belongs to the large superfamily of G protein-coupled receptors, and is directly involved in a number of biological processes including organogenesis, hematopoiesis, and immune response. Recent evidence has highlighted the role of CXCR4 in a variety of diseases including HIV, cancer, and WHIM syndrome. Importantly, the involvement of CXCR4 in cancer metastasis and WHIM syndrome appears to be due to dysregulation of the receptor leading to enhanced signaling. Herein we review what is currently known regarding the regulation of CXCR4 and how dysregulation contributes to disease progression.

Distinct functional sites for human immunodeficiency virus type 1 and stromal cell-derived factor 1alpha on CXCR4 transmembrane helical domains

The entry of human immunodeficiency virus type 1 (HIV-1) into the cell is initiated by the interaction of the viral surface envelope protein with two cell surface components of the target cell, CD4 and a chemokine coreceptor, usually CXCR4 or CCR5. The natural ligand of CXCR4 is stromal cell-derived factor 1alpha (SDF-1alpha). Whereas the overlap between HIV-1 and SDF-1alpha functional sites on the extracellular domains of CXCR4 has been well documented, it has yet to be determined whether there are sites in the transmembrane (TM) helices of CXCR4 important for HIV-1 and/or SDF-1alpha functions, and if such sites do exist, whether they are overlapping or distinctive for the separate functions of CXCR4. For this study, by employing alanine-scanning mutagenesis, (125)I-SDF-1alpha competition binding, Ca(2+) mobilization, and cell-cell fusion assays, we found that the mutation of many CXCR4 TM residues, including Tyr(45), His(79), Asp(97), Pro(163), Trp(252), Tyr(255), Asp(262), Glu(288), His(294), and Asn(298), could selectively decrease HIV-1-mediated cell fusion but not the binding activity of SDF-1alpha. Phe(87) and Phe(292), which were involved in SDF-1alpha binding, did not play a significant role in the coreceptor activity of CXCR4, further demonstrating the disconnection between physiological and pathological activities of CXCR4 TM domains. Our data also show that four mutations of the second extracellular loop, D182A, D187A, F189A, and P191A, could reduce HIV-1 entry without impairing either ligand binding or signaling. Taken together, our first detailed characterization of the different functional roles of CXCR4 TM domains may suggest a mechanistic basis for the discovery of new selective anti-HIV agents.

Suppression of Multiclade R5 and X4 Human Immunodeficiency Virus Type-1 Infections by a Coreceptor-Based Anti-HIV Strategy

A cyclic chimeric dodecapeptide (cCD) mimicking the conformation-specific domains of CCR5 and CXCR4 was prepared in which Gly-Asp links the amino and carboxyl termini of two combined pentapeptides (S169-G173 of CCR5; E179-R183 of CXCR4) derived from human immunodeficiency virus type-1 (HIV-1) coreceptors. The immunization of Balb/c mice with cCD conjugated with a multiple-antigen peptide (cCD-MAP) induced seven cCD-specific monoclonal antibodies (mAbs, CPMAb-I to -VII) that reacted with native CCR5 and CXCR4. Among the tested mAbs, CPMAb-I and -II potently inhibited the infection of both the R5 and X4 laboratory strains. CPMAb-III and -VI were effective against only R5 laboratory strains, and also against some X4 and R5 primary isolates. CPMAb-IV and -V had potent antiviral activities against the R5 and X4 primary isolates. In particular, CPMAb-VII was protective against not only R5 and X4 laboratory strains, but also most of the R5 and X4 primary isolates. Moreover, cCD-MAP immunization also induced antibodies that were effective against R5 and X4 multiclade HIV-1 isolates in vitro in two of three cynomolgus monkeys. Taken together, the results suggest that cCD-MAP is a candidate multiclade immunogen that can be used to block multiclade R5 and X4 HIV-1 infections.

Anti-CXCR4 monoclonal antibodies recognizing overlapping epitopes differ significantly in their ability to inhibit entry of human immunodeficiency virus type 1

CXCR4 is one of two physiologically relevant human immunodeficiency type 1 (HIV-1) entry coreceptors. Studies of CXCR4 mutants have not clearly identified the determinants of coreceptor function and specificity. We therefore used a panel of monoclonal antibodies to further elucidate CXCR4 expression, structure, and function. Our findings show the existence of conformational subpopulations of CXCR4 that are in equilibrium on the cell surface but are not cell type specific as previously reported. HIV-1 X4 isolates can interact with multiple CXCR4 conformations in order to gain entry into target cells.

Molecular mapping of epitopes for interaction of HIV-1 as well as natural ligands with the chemokine receptors, CCR5 and CXCR4

OBJECTIVE

Mapping coreceptor epitopes used by the prototypic R5 and X4 strains, HIV-1BaL and HIV-1IIIB, in comparison with epitopes involved in the activation and signaling induced by the natural ligands, RANTES and SDF-1beta.

DESIGN

Receptor hybrids between CCR5 and CXCR4 were constructed.

METHODS

Using single-overlap and extension PCR, increasing portions of CCR5 were replaced with corresponding parts of CXCR4. Viral interaction with these constructs was monitored in infection experiments using stably transfected cell lines, and ligand-induced activation of cells transiently expressing the constructs was measured in terms of calcium fluxes.

RESULTS

SDF-1beta required an essentially complete CXCR4, whereas RANTES demanded both the N terminus and the first two extracellular loops of CCR5. HIV-1 infection experiments emphasized the importance of the CCR5 N terminus for infection with HIV-1BaL, whereas HIV-1IIIB was less demanding in its use of CXCR4.

CONCLUSION

This study, for the first time monitoring CCR5 and CXCR4 ligand activation and HIV-1 interaction concomitantly, indicates that ligands and virus use different receptor epitopes which, in turn, vary between the two receptors. One particular chimera (FC-4b), having its junctional region close to the conserved cysteine in ECL2, functioned as coreceptor for both HIV-1BaL and HIV-1IIIB, but was not activated with RANTES or SDF-1beta. The results provide a basis for tailoring drugs that block viral entry through the two major coreceptors without interfering with their physiological function.

Contrasting use of CCR5 structural determinants by R5 and R5X4 variants within a human immunodeficiency virus type 1 primary isolate quasispecies

Macrophagetropic R5 human immunodeficiency virus type 1 (HIV-1) isolates often evolve into dualtropic R5X4 variants during disease progression. The structural basis for CCR5 coreceptor function has been studied in a limited number of prototype strains and suggests that R5 and R5X4 Envs interact differently with CCR5. However, differences between unrelated viruses may reflect strain-specific factors and do not necessarily represent changes resulting from R5 to R5X4 evolution of a virus in vivo. Here we addressed CCR5 domains involved in fusion for a large set of closely related yet functionally distinct variants within a primary isolate swarm, employing R5 and R5X4 Envs derived from the HIV-1 89.6(PI) quasispecies. R5 variants of 89.6(PI) could fuse using either N-terminal or extracellular loop CCR5 sequences in the context of CCR5/CXCR2 chimeras, similar to the unrelated R5 strain JRFL, but R5X4 variants of 89.6(PI) were highly dependent on the CCR5 N terminus. Similarly, R5 89.6(PI) variants and isolate JRFL tolerated N-terminal CCR5 deletions, but fusion by most R5X4 variants was markedly impaired. R5 89.6(PI) Envs also tolerated multiple extracellular domain substitutions, while R5X4 variants did not. In contrast to CCR5 use, fusion by R5X4 variants of 89.6(PI) was largely independent of the CXCR4 N-terminal region. Thus, R5 and R5X4 species from a single swarm differ in how they interact with CCR5. These results suggest that R5 Envs possess a highly plastic capacity to interact with multiple CCR5 regions and support the concept that viral evolution in vivo results from the emergence of R5X4 variants with the capacity to use the CXCR4 extracellular loops but demonstrate less-flexible interactions with CCR5 that are strongly dependent on the N-terminal region.

HIV coreceptors: role of structure, posttranslational modifications, and internalization in viral-cell fusion and as targets for entry inhibitors

The human immunodeficiency virus (HIV) envelope glycoprotein forms trimers on the virion surface, with each monomer consisting of two subunits, gp120 and gp41. The gp120 envelope component binds to CD4 on target cells and undergoes conformational changes that allow gp120 to interact with certain G-protein-coupled receptors (GPCRs) on the same target membranes. The GPCRs that function as HIV coreceptors were found to be chemokine receptors. The primary coreceptors are CCR5 and CXCR4, but several other chemokine receptors were identified as "minor coreceptors", indicating their ability support entry of some HIV strains in tissue cultures. Formation of the tri-molecular complexes stabilizes virus binding and triggers a series of conformational changes in gp41 that facilitate membrane fusion and viral cell entry. Concerted efforts are underway to decipher the specific interactions between gp120/CD4, gp120/coreceptors, and their contributions to the subsequent membrane fusion process. It is hoped that some of the transient conformational intermediates in gp120 and gp41 would serve as targets for entry inhibitors. In addition, the CD4 and coreceptors are primary targets for several classes of inhibitors currently under testing. Our review summarizes the current knowledge on the interactions of HIV gp120 with its receptor and coreceptors, and the important properties of the chemokine receptors and their regulation in primary target cells. We also summarize the classes of coreceptor inhibitors under development.

Identification of gp120 binding sites on CXCR4 by using CD4-independent human immunodeficiency virus type 2 Env proteins

Human immunodeficiency virus (HIV) and simian (SIV) immunodeficiency virus entry is mediated by binding of the viral envelope glycoprotein (Env) to CD4 and chemokine receptors, CCR5 and/or CXCR4. CD4 induces extensive conformational changes that expose and/or induce formation of a chemokine receptor binding site on gp120. CD4-independent Env's of HIV type 1 (HIV-1), HIV-2, and SIV have been identified that exhibit exposed chemokine receptor binding sites and can bind directly to CCR5 or CXCR4 in the absence of CD4. While many studies have examined determinants for gp120-CCR5 binding, analysis of gp120-CXCR4 binding has been hindered by the apparently lower affinity of this interaction for X4-tropic HIV-1 isolates. We show here that gp120 proteins from two CD4-independent HIV-2 Env's, VCP and ROD/B, bind directly to CXCR4 with an apparently high affinity. By use of CXCR4 N-terminal deletion constructs, CXCR4-CXCR2 chimeras, and human-rat CXCR4 chimeras, binding determinants were shown to reside in the amino (N) terminus, extracellular loop 2 (ECL2), and ECL3. Alanine-scanning mutagenesis of charged residues, tyrosines, and phenylalanines in extracellular CXCR4 domains implicated multiple amino acids in the N terminus (E14/E15, D20, Y21, and D22), ECL2 (D187, R188, F189, Y190, and D193), and ECL3 (D262, E268, E277, and E282) in binding, although minor differences were noted between VCP and ROD/B. However, mutations in CXCR4 that markedly reduced binding did not necessarily hinder cell-cell fusion by VCP or ROD/B, especially in the presence of CD4. These gp120 proteins will be useful in dissecting determinants for CXCR4 binding and Env triggering and in evaluating pharmacologic inhibitors of the gp120-CXCR4 interaction.

Infection of cells expressing CXCR4 mutants lacking N-glycosylation at the N-terminal extracellular domain is enhanced for R5X4-dualtropic human immunodeficiency virus type-1

BACKGROUND

Infection with human immunodeficiency virus type-1 (HIV-1) requires binding of the viral envelope gp120 to CD4 and to the CXCR4 coreceptor. Both, gp120 and CXCR4 are subject to N-glycosylation. Here we investigated the influence of the N-linked glycans g1 and g2 present on CXCR4 for HIV-1 infection.

METHODS

The two CXCR4 N-glycosylation sites g1 (NYT) and g2 (NVS) were mutated by changing the first or third amino acids N or T/S to Q and A respectively (g1; N11Q or T13A; g2, N176Q or S178A). Human osteosarcoma cells (GHOST) expressing human CD4 and the various CXCR4 glycosylation mutants were tested for infection using NL4-3-based viruses with X4, R5 or R5X4-tropism differing only in the V3 loop region.

RESULTS

All constructed cell lines expressing the various CXCR4 glycomutants showed similar permissiveness for the X4-monotropic virus and no change in the coreceptor specificity that allows infection of a CCR5-dependent R5-monotropic virus. Interestingly, the removal of glycan g1 significantly enhanced the permissiveness of GHOST cells for the R5X4 dualtropic virus. GHOST cells expressing the CXCR4-g1 or CXCR4-g1/2 mutants were infected at higher rates by the R5X4-dualtropic virus compared to cells expressing CXCR4-wt or CXCR4-g2 coreceptors.

CONCLUSION

Our present observations underscore a role for glycans present on the CXCR4 coreceptor in the entry process of HIV-1. The data will help to better understand the multifaceted mechanism of HIV infection and the selective forces which drive HIV-1 evolution from mono- to dual-tropism.

Identification of conserved and variable structures in the human immunodeficiency virus gp120 glycoprotein of importance for CXCR4 binding

CD4 and the chemokine receptors, CXCR4 and CCR5, serve as receptors for human immunodeficiency virus type 1 (HIV-1). Binding of the HIV-1 gp120 envelope glycoprotein to the chemokine receptors normally requires prior interaction with CD4. Mapping the determinants on gp120 for the low-affinity interaction with CXCR4 has been difficult due to the nonspecific binding of this viral glycoprotein to cell surfaces. Here we examine the binding of a panel of gp120 mutants to paramagnetic proteoliposomes displaying CXCR4 on their surfaces. We show that the gp120 beta19 strand and third variable (V3) loop contain residues important for CXCR4 interaction. Basic residues from both elements, as well as a conserved hydrophobic residue at the V3 tip, contribute to CXCR4 binding. Removal of the gp120 V1/V2 variable loops allows the envelope glycoprotein to bind CXCR4 in a CD4-independent manner. These results indicate that although some variable gp120 residues contribute to the specific binding to CCR5 or CXCR4, gp120 elements common to CXCR4- or CCR5-using strains are involved in the interaction with both coreceptors.

Susceptibility of diverse primary HIV isolates with varying co-receptor specificity's to CXCR4 antagonistic compounds

The chemokine receptors CCR5 and CXCR4 are an obvious target for HIV therapies. Two compounds, T-22 and AMD-3100, have been shown to inhibit infection of CXCR4-using HIV-1 isolates. The specificity of T-22 and AMD-3100 was further confirmed by their ability to block entry of HIV-1 in GHOST-CXCR4 transfected cells with no effect on viral entry in the GHOST-CCR5 cells. The ability of T-22 to block replication of diverse HIV-1 isolates (group M, subtypes A, B, D, E, and F as well as group O) and HIV-2 primary isolates with varying coreceptor specificities ranging from exclusive CCR5 usage to multiple coreceptor usage was examined in detail. T-22 was found to be highly effective (>90%) at blocking infection of diverse HIV-1 (subtypes A-F, and group O) and HIV-2 isolates that use multiple coreceptors in human PBMCs homozygous for a 32-bp deletion in CCR5 (CCR5-/-), but less effective in CCR5 +/+ PBMCs. Additionally, sequential primary HIV-1 isolates obtained from a longitudinal cohort who had switched from single coreceptor usage to a broad range of multiple receptors could be blocked effectively by both T-22 and AMD-3100 in CCR5-/- PBMCs. Our data suggest that CXCR4 antagonistic compounds are highly effective in blocking the entry of X4-tropic HIV-1, and that these compounds could be a useful additive to current anti-retroviral therapy for clinical management of HIV disease.

Coreceptor usage and biological phenotypes of HIV-1 isolates

The discovery of chemokine receptors as HIV-1 entry molecules or "coreceptors" has lead to a greater understanding of how HIV-1 infects human cells. This has provided insight into the biological properties of HIV-1 isolates and unravelled the meaning of the syncytium-inducing and non-syncytium-inducing phenotypes. Understanding how HIV-1 exploits these coreceptors has given way to novel approaches to controlling HIV. As a result a new class of drugs has emerged that are being tested to prevent virus infection and to act as an alternative, or adjunct, to existing anti-retroviral drugs for HIV-infected individuals.

Characterization of the anti-HIV effects of native lactoferrin and other milk proteins and protein-derived peptides

In a search for natural proteins with anti-HIV activity, we screened a large set of purified proteins from bovine milk and peptide fragments thereof. Because several charged proteins and peptides are known to inhibit the process of virus entry, we selected proteins with an unusual charge composition or hydrophobicity profile. In contrast with some chemically modified (strongly negative) milk proteins, unmodified alpha(s2)-, beta- and kappa-casein, as well as several negatively and positively charged fragments thereof, did not show significant inhibition of virus replication. In fact, HIV-1 replication was elevated in the presence of beta-casein or amphiphilic fragments thereof. Bovine lactoferrin (bLF), a milk protein of 80 kDa, showed considerable inhibitory activity against HIV-1 with an IC50 of 0.4 microM. Modest inhibition was obtained with lactoferricin, a highly positively charged loop domain of bLF, indicating that other domains within the native bLF protein may also be required for inhibition. bLF blocked HIV-1 variants that use either the CXCR4 or the CCR5 coreceptor. In order to obtain further insight into the mechanism of action of this antiviral protein, we selected a bLF-resistant HIV-1 variant. The bLF-resistance phenotype is mediated by the viral envelope protein, which contains two interesting mutations that have previously been associated with an altered virus-host interaction and a modified receptor-coreceptor interaction. These results demonstrate that bLF targets the HIV-1 entry process.

Exploring the stereochemistry of CXCR4-peptide recognition and inhibiting HIV-1 entry with D-peptides derived from chemokines

Chemokine receptor CXCR4 plays an important role in the immune system and the cellular entry of human immunodeficiency virus type 1 (HIV-1). To probe the stereospecificity of the CXCR4-ligand interface, d-amino acid peptides derived from natural chemokines, viral macrophage inflammatory protein II (vMIP-II) and stromal cell-derived factor-1alpha (SDF-1alpha), were synthesized and found to compete with (125)I-SDF-1alpha and monoclonal antibody 12G5 binding to CXCR4 with potency and selectivity comparable with or higher than their l-peptide counterparts. This was surprising because of the profoundly different side chain topologies between d- and l-enantiomers, which circular dichroism spectroscopy showed adopt mirror image conformations. Further direct binding experiments using d-peptide labeled with fluorescein (designated as FAM-DV1) demonstrated that d- and l-peptides shared similar or at least overlapping binding site(s) on the CXCR4 receptor. Structure-activity analyses of related peptide analogs of mixed chiralities or containing alanine replacements revealed specific residues at the N-terminal half of the peptides as key binding determinants. Acting as CXCR4 antagonists and with much higher biological stability than l-counterparts, the d-peptides showed significant activity in inhibiting the replication of CXCR4-dependent HIV-1 strains. These results show the remarkable stereochemical flexibility of the CXCR4-peptide interface. Further studies to understand the mechanism of this unusual feature of the CXCR4 binding surface might aid the development of novel CXCR4-binding molecules like the d-peptides that have high affinity and stability.

Structure, function, and inhibition of chemokines

Chemokines are the largest family of cytokines in human immunophysiology. These proteins are defined by four invariant cysteines and are categorized based on the sequence around the first two cysteines, which leads to two major and two minor subfamilies. Chemokines function by activating specific G protein-coupled receptors, which results in, among other functions, the migration of inflammatory and noninflammatory cells to the appropriate tissues or compartments within tissues. Some of these proteins and receptors have been implicated or shown to be involved in inflammation, autoimmune diseases, and infection by HIV-1. The three-dimensional structure of each monomer is virtually identical, but the quaternary structure of chemokines is different for each subfamily. Structure-function studies reveal several regions of chemokines to be involved in function, with the N-terminal region playing a dominant role. A number of proteins and small-molecule antagonists have been identified that inhibit chemokine activities. In this review, we discuss aspects of the structure, function, and inhibition of chemokines.

CCR5 and CXCR4 usage by non-clade B human immunodeficiency virus type 1 primary isolates

CCR5 and CXCR4 usage has been studied extensively with a variety of clade B human immunodeficiency virus type 1 (HIV-1) isolates. The determinants of CCR5 coreceptor function are remarkably consistent, with a region critical for fusion and entry located in the CCR5 amino-terminal domain (Nt). In particular, negatively charged amino acids and sulfated tyrosines in the Nt are essential for gp120 binding to CCR5. The same types of residues are important for CXCR4-mediated viral fusion and entry, but they are dispersed throughout the extracellular domains of CXCR4, and their usage is isolate dependent. Here, we report on the determinants of CCR5 and CXCR4 coreceptor function for a panel of non-clade B isolates that are responsible for the majority of new HIV-1 infections worldwide. Consistent with clade B isolates, CXCR4 usage remains isolate dependent and is determined by the overall content of negatively charged and tyrosine residues. Residues in the Nt of CCR5 that are important for fusion and entry of clade B isolates are also important for the entry of all non-clade B HIV-1 isolates that we tested. Surprisingly, we found that in contrast to clade B isolates, a cluster of residues in the second extracellular loop of CCR5 significantly affects fusion and entry of all non-clade B isolates tested. This points to a different mechanism of CCR5 usage by these viruses and may have important implications for the development of HIV-1 inhibitors that target CCR5 coreceptor function.

Evidence for common structural determinants of human immunodeficiency virus type 1 coreceptor activity provided through functional analysis of CCR5/CXCR4 chimeric coreceptors

Human immunodeficiency virus type 1 (HIV-1) infection in vivo is dependent upon the interaction of the viral envelope glycoprotein gp120 with CC chemokine receptor 5 (CCR5) or CXC chemokine receptor 4 (CXCR4). To study the determinants of the gp120-coreceptor association, we generated a set of chimeric HIV-1 coreceptors which express all possible combinations of the four extracellular domains of CCR5 and CXCR4. Stable U87 astroglioma cell lines expressing CD4 and individual chimeric coreceptor proteins were tested against a variety of R5, X4, and R5X4 envelope glycoproteins and virus strains for their ability to support HIV-1-mediated cell fusion and infection, respectively. Each of the cell lines promoted fusion with cells expressing an HIV envelope glycoprotein, except for U87.CD4.5455, which presents the first extracellular loop (ECL1) and flanking sequences of CXCR4 in the context of CCR5. However, all of the chimeric coreceptors allowed productive infection by one or more of the viral strains tested. Viral phenotype was a predictive factor for the observed activity of the chimeric molecules; X4 and R5X4 HIV strains utilized a majority of the chimeras, while R5 strains were limited in their ability to infect cells expressing these chimeric molecules. The expression of CCR5 ECL2 within the CXCR4 backbone supported infection by an R5 primary isolate, but no chimeras bearing the N terminus of CCR5 exhibited activity with R5 strains. Remarkably, the introduction of any CXCR4 domain into the CCR5 backbone was sufficient to allow utilization by multiple X4 strains. However, critical determinants within ECL2 and/or ECL3 of CXCR4 were apparent for all X4 viruses upon replacement of these domains in CXCR4 with CCR5 sequences. Unexpectedly, chimeric coreceptor-facilitated entry was blocked in all cases by the presence of the CXCR4-specific inhibitor AMD3100. Our data provide proof that CCR5 contains elements that support usage by X4 viral strains and demonstrate that the gp120 interaction sites of CCR5 and CXCR4 are structurally related.

Structural and functional characterization of human CXCR4 as a chemokine receptor and HIV-1 co-receptor by mutagenesis and molecular modeling studies

The human CXC chemokine receptor 4 (CXCR4) is a receptor for the chemokine stromal cell-derived factor (SDF-1alpha) and a co-receptor for the entry of specific strains of human immunodeficiency virus type I (HIV-1). CXCR4 is also recognized by an antagonistic chemokine, the viral macrophage inflammatory protein II (vMIP-II) encoded by human herpesvirus type VIII. SDF-1alpha or vMIP-II binding to CXCR4 can inhibit HIV-1 entry via this co-receptor. An approach combining protein structural modeling and site-directed mutagenesis was used to probe the structure-function relationship of CXCR4, and interactions with its ligands SDF-1alpha and vMIP-II and HIV-1 envelope protein gp120. Hypothetical three-dimensional structures were proposed by molecular modeling studies of the CXCR4.SDF-1alpha complex, which rationalize extensive biological information on the role of CXCR4 in its interactions with HIV-1 envelope protein gp120. With site-directed mutagenesis, we have identified that the amino acid residues Asp (D20A) and Tyr (Y21A) in the N-terminal domain and the residue Glu (E268A) in extracellular loop 3 (ECL3) are involved in ligand binding, whereas the mutation Y190A in extracellular loop 2 (ECL2) impairs the signaling mediated by SDF-1alpha. As an HIV-1 co-receptor, we found that the N-terminal domain, ECL2, and ECL3 of CXCR4 are involved in HIV-1 entry. These structural and mutational studies provide valuable information regarding the structural basis for CXCR4 activity in chemokine binding and HIV-1 viral entry, and could guide the design of novel targeted inhibitors.

Mutation of Asp(171) and Asp(262) of the chemokine receptor CXCR4 impairs its coreceptor function for human immunodeficiency virus-1 entry and abrogates the antagonistic activity of AMD3100

The bicyclam AMD3100 is a highly potent and selective CXCR4 antagonist with strong antiviral activity against human immunodeficiency virus (HIV)-1 and HIV-2, which use CXCR4 as coreceptor for host cell entry. Here, we investigated the interaction of AMD3100 with CXCR4 at the molecular level by mutational analysis. We established a set of stably transfected U87.CD4 cell lines expressing different mutant forms of CXCR4 (i.e., CXCR4[WT], CXCR4[D171N], CXCR4[D262N], CXCR4[D171N,D262N], and CXCR4[H281A]), to compare the activity of the compound against mutated versus wild-type CXCR4. We found that the antagonistic action of AMD3100 against CXCR4--as assessed by the inhibitory effects of the compound on stromal cell-derived factor (SDF-1) binding to its receptor and on SDF-1-induced intracellular calcium signaling, and by displacement of the CXCR4-specific antibody, clone 12G5--was greatly reduced by substitution of Asp(171) and/or Asp(262) by neutral asparagine residue(s). Both aspartates, but most particularly Asp(262), also proved essential for the anti-HIV-1 activity of AMD3100 against the viruses NL4.3, IIIB, and HE. In contrast, substitution of His(281) by a neutral alanine potentiated the antagonistic and antiviral effects of the compound in the different assay systems. Importantly, compared with the wild-type receptor, CXCR4[D262N] was much less effective, whereas CXCR4[D171N,D262N] completely failed as a coreceptor for infection by HIV-1 NL4.3. Thus, the negatively charged aspartate residues at positions 171 and 262, located in transmembrane domains 4 and 6 of the 7-transmembrane receptor, respectively, may represent crucial sites for electrostatic interaction of the positive charges of the bicyclams, as well as for the highly basic V3 loop of the gp120 envelope protein of certain HIV-1 strains.

Cooperation of the V1/V2 and V3 domains of human immunodeficiency virus type 1 gp120 for interaction with the CXCR4 receptor

Human immunodeficiency virus type 1 (HIV-1) entry is triggered by the interaction of the gp120 envelope glycoprotein with a cellular chemokine receptor, either CCR5 or CXCR4. We have identified different mutations in human CXCR4 that prevent efficient infection by one HIV-1 strain (NDK) but not another (LAI) and sought to define these strain-dependent effects at the gp120 level. The lack of activity toward the NDK strain of the HHRH chimeric CXCR4 in which the second extracellular loop (ECL2) derived from the rat CXCR4 and of CXCR4 with mutations at an aspartic acid in ECL2 (D193A and D193R) was apparently due to the sequence of the third variable loop (V3) of gp120, more precisely, to its C-terminal part. Indeed, substitution of the LAI V3 loop or only its C-terminal part in the NDK gp 120 context was sufficient to restore usage of the HHRH, D193A, and D193R receptors. The same result was achieved upon mutation of a single lysine residue of the NDK V3 loop to alanine (K319A) but not to arginine (K319R). These results provide a strong case for a direct interaction between the gp120 V3 loop and the ECL2 domain of CXCR4. By contrast, V3 substitutions had no effect on the inability of NDK to infect cells via a mutant CXCR4 in which the amino-terminal extracellular domain (NT) is deleted. In experiments with a set of chimeric NDK-LAI gp120s, the V1/V2 region from LAI gp120 was both necessary and sufficient for usage of the NT-deleted CXCR4. Different variable domains of gp120 can therefore cooperate for a functional interaction with CXCR4.