Inhibition of human immunodeficiency virus fusion by a monoclonal antibody to a coreceptor (CXCR4) is both cell type and virus strain dependent

Serum stromal-derived-factor-1 (CXCL12) and its alpha chemokine receptor (CXCR4) as biomarkers in neonatal sepsis

BACKGROUND

Neonatal sepsis remains one of the leading causes of morbidity and mortality both among term and preterm infants. Advances in neonatal care improved survival and reduced complications in preterm infants. Chemokines are chemotactic cytokines that give directional guidance for leukocyte migration during inflammatory process. The chemokine CXCL12 and its receptor CXCR4 are now known to play an important role in inflammatory states. However, its value as a biomarker in neonatal sepsis is unclear.

OBJECTIVES

To assess the value of measuring the serum levels of alpha-chemokine receptor type 4 (CXCR-4) and stromal-derived-factor-1 (CXCL12) in diagnosis of late onset neonatal sepsis.

SUBJECT AND METHODS

Serum levels of CXCL12 and CXCR4 were determined in 38 full term neonates, 23 cases of late onset sepsis (13 males and 10 female), and 15 healthy neonates as control (six males and nine females) by ELISA technique and flow-cytometry.

RESULTS

Serum levels of CXCR4 and CXCL12 were significantly higher in neonates with late onset sepsis compared with the non-septic ones. The sensitivity, the specificity, and the overall accuracy of CXCL12 were 100%. The sensitivity of CXCR4 was 87%; the specificity was 80% and the overall accuracy was 84%.

CONCLUSIONS

Serum CXCR4 and CXCL12 levels increase significantly in septic neonates and they are valuable marker in diagnosis of neonatal sepsis. Serum concentrations of both chemokines represent promising novel biomarkers for neonatal sepsis.

Affinity maturation of a novel antagonistic human monoclonal antibody with a long VH CDR3 targeting the Class A GPCR formyl-peptide receptor 1

Therapeutic monoclonal antibodies targeting G-protein-coupled receptors (GPCRs) are desirable for intervention in a wide range of disease processes. The discovery of such antibodies is challenging due to a lack of stability of many GPCRs as purified proteins. We describe here the generation of Fpro0165, a human anti-formyl peptide receptor 1 (FPR1) antibody generated by variable domain engineering of an antibody derived by immunization of transgenic mice expressing human variable region genes. Antibody isolation and subsequent engineering of affinity, potency and species cross-reactivity using phage display were achieved using FPR1 expressed on HEK cells for immunization and selection, along with calcium release cellular assays for antibody screening. Fpro0165 shows full neutralization of formyl peptide-mediated activation of primary human neutrophils. A crystal structure of the Fpro0165 Fab shows a long, protruding V_H CDR3 of 24 amino acids and in silico docking with a homology model of FPR1 suggests that this long V_H CDR3 is critical to the predicted binding mode of the antibody. Antibody mutation studies identify the apex of the long V_H CDR3 as key to mediating the species cross-reactivity profile of the antibody. This study illustrates an approach for antibody discovery and affinity engineering to typically intractable membrane proteins such as GPCRs.

Antibody epitopes on g protein-coupled receptors mapped with genetically encoded photoactivatable cross-linkers

We developed a strategy for creating epitope maps of monoclonal antibodies (mAbs) that bind to G protein-coupled receptors (GPCRs) containing photo-cross-linkers. Using human CXC chemokine receptor 4 (CXCR4) as a model system, we genetically incorporated the photolabile unnatural amino acid p-azido-l-phenylalanine (azF) at various positions within extracellular loop 2 (EC2). We then mapped the interactions of the azF-CXCR4 variants with mAb 12G5 using targeted loss-of-function studies and photo-cross-linking in whole cells in a microplate-based format. We used a novel variation of a whole cell enzyme-linked immunosorbent assay to quantitate cross-linking efficiency. 12G5 cross-linked primarily to residues 184, 178, and 189 in EC2 of CXCR4. Mapping of the data to the crystal structure of CXCR4 showed a distinct mAb epitope footprint with the photo-cross-linked residues clustered around the loss-of-function sites. We also used the targeted photo-cross-linking approach to study the interaction of human CC chemokine receptor 5 (CCR5) with PRO 140, a humanized mAb that inhibits human immunodeficiency virus-1 cellular entry, and 2D7. The mAbs produced distinct cross-linking patterns on EC2 of CCR5. PRO 140 cross-linked primarily to residues 174 and 175 at the amino-terminal end of EC2, and 2D7 cross-linked mainly to residues 170, 176, and 184. These results were mapped to the recent crystal structure of CCR5 in complex with maraviroc, showing cross-linked residues at the tip of the maraviroc binding crevice formed by EC2. As a strategy for mapping mAb epitopes on GPCRs, our targeted photo-cross-linking method is complementary to loss-of-function mutagenesis results and should be especially useful for studying mAbs with discontinuous epitopes.

Structural and pharmacological characterization of novel potent and selective monoclonal antibody antagonists of glucose-dependent insulinotropic polypeptide receptor

Glucose-dependent insulinotropic polypeptide (GIP) is an endogenous hormonal factor (incretin) that, upon binding to its receptor (GIPr; a class B G-protein-coupled receptor), stimulates insulin secretion by beta cells in the pancreas. There has been a lack of potent inhibitors of the GIPr with prolonged in vivo exposure to support studies on GIP biology. Here we describe the generation of an antagonizing antibody to the GIPr, using phage and ribosome display libraries. Gipg013 is a specific competitive antagonist with equally high potencies to mouse, rat, dog, and human GIP receptors with a K_i of 7 nm for the human GIPr. Gipg013 antagonizes the GIP receptor and inhibits GIP-induced insulin secretion in vitro and in vivo. A crystal structure of Gipg013 Fab in complex with the human GIPr extracellular domain (ECD) shows that the antibody binds through a series of hydrogen bonds from the complementarity-determining regions of Gipg013 Fab to the N-terminal α-helix of GIPr ECD as well as to residues around its highly conserved glucagon receptor subfamily recognition fold. The antibody epitope overlaps with the GIP binding site on the GIPr ECD, ensuring competitive antagonism of the receptor. This well characterized antagonizing antibody to the GIPr will be useful as a tool to further understand the biological roles of GIP.

Homogeneous time-resolved fluorescence assay to probe folded G protein-coupled receptors

Continued advances in G protein-coupled receptor (GPCR) structural biology and biochemistry depend in part on strategies to stabilize these polytopic membrane proteins in purified systems. New methods to measure properly folded GPCRs are needed to facilitate the identification of suitable conditions and ensure sample quality. Most GPCRs do not contain an intrinsic reporter on their functionality, so probes must be introduced. Here, we describe a fluorescence-based approach to quantitatively measure the chemokine receptor CCR5 with labeled antibodies. The assay is exceptionally sensitive and high-throughput. We detail procedures to label antibodies, characterize the system, and process data. We also describe several useful applications, including optimization of incorporation into nanoscale apolipoprotein bound bilayers (NABBs or nanodiscs), measurement of receptor stability, and competition binding assays.

Inhibition of HIV-1 infection by human α-defensin-5, a natural antimicrobial peptide expressed in the genital and intestinal mucosae

BACKGROUND

α-defensin-5 (HD5) is a key effector of the innate immune system with broad anti-bacterial and anti-viral activities. Specialized epithelial cells secrete HD5 in the genital and gastrointestinal mucosae, two anatomical sites that are critically involved in HIV-1 transmission and pathogenesis. We previously found that human neutrophil defensins (HNP)-1 and -2 inhibit HIV-1 entry by specific bilateral interaction both with the viral envelope and with its primary cellular receptor, CD4. Despite low amino acid identity, human defensin-5 (HD5) shares with HNPs a high degree of structural homology.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we demonstrate that HD5 inhibits HIV-1 infection of primary CD4(+) T lymphocytes at low micromolar concentration under serum-free and low-ionic-strength conditions similar to those occurring in mucosal fluids. Blockade of HIV-1 infection was observed with both primary and laboratory-adapted strains and was independent of the viral coreceptor-usage phenotype. Similar to HNPs, HD5 inhibits HIV-1 entry into the target cell by interfering with the reciprocal interaction between the external envelope glycoprotein, gp120, and CD4. At high concentrations, HD5 was also found to downmodulate expression of the CXCR4 coreceptor, but not of CCR5. Consistent with its broad spectrum of activity, antibody competition studies showed that HD5 binds to a region overlapping with the CD4- and coreceptor-binding sites of gp120, but not to the V3 loop region, which contains the major determinants of coreceptor-usage specificity.

CONCLUSION/SIGNIFICANCE

These findings provide new insights into the first line of immune defense against HIV-1 at the mucosal level and open new perspectives for the development of preventive and therapeutic strategies.

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.

Cellular and subcellular localization of CXCL12 and CXCR4 in rat nociceptive structures: physiological relevance

Initial studies implicated the chemokine CXC motif ligand 12 (CXCL12) and its cognate CXC motif receptor 4 (CXCR4) in pain modulation. However, there has been no description of the distribution, transport and axonal sorting of CXCL12 and CXCR4 in rat nociceptive structures, and their direct participation in nociception modulation has not been demonstrated. Here, we report that acute intrathecal administration of CXCL12 induced mechanical hypersensitivity in naive rats. This effect was prevented by a CXCR4-neutralizing antibody. To determine the morphological basis of this behavioural response, we used light and electron microscopic immunohistochemistry to map CXCL12- and CXCR4-immunoreactive elements in dorsal root ganglia, lumbar spinal cord, sciatic nerve and skin. Light microscopy analysis revealed CXCL12 and CXCR4 immunoreactivity in calcitonin gene related peptide-containing peptidergic primary sensory neurons, which were both conveyed to central and peripheral sensory nerve terminals. Electron microscopy clearly demonstrated CXCL12 and CXCR4 immunoreactivity in primary sensory nerve terminals in the dorsal horn; both were sorted into small clear vesicles and large dense-core vesicles. This suggests that CXCL12 and CXCR4 are trafficked from nerve cell bodies to the dorsal horn. Double immunogold labelling for CXCL12 and calcitonin gene related peptide revealed partial vesicular colocalization in axonal terminals. We report, for the first time, that CXCR4 receptors are mainly located on the neuronal plasma membrane, where they are present at pre-synaptic and post-synaptic sites of central terminals. Receptor inactivation experiments, behavioural studies and morphological analyses provide strong evidence that the CXCL12/CXCR4 system is involved in modulation of nociceptive signalling.

Susceptibility of HIV type 2 primary isolates to CCR5 and CXCR4 monoclonal antibodies, ligands, and small molecule inhibitors

Human immunodeficiency virus (HIV) entry into susceptible cells involves the interaction between viral envelope glycoproteins with CD4 and a chemokine receptor (coreceptor), namely CCR5 and CXCR4. This interaction has been studied to enable the discovery of a new class of antiretroviral drugs that targets the envelope glycoprotein-coreceptor interaction. However, very few data exist regarding HIV-2 susceptibility to these coreceptor inhibitors. With this work we aimed to identify this susceptibility in order to assess the potential use of these molecules to treat HIV-2-infected patients and to further understand the molecular basis of HIV-2 envelope glycoprotein interactions with CCR5 and CXCR4. We found that CCR5-using HIV-2 isolates are readily inhibited by maraviroc, TAK-779, and PF-227153, while monoclonal antibody 2D7 shows only residual or no inhibitory effects. The anti-HIV-2 activity of CXCR4-targeted molecules reveals that SDF-1α/CXCL12 inhibited all HIV-2 tested except one, while mAb 12G5 inhibited the replication of only two isolates, showing residual inhibitory effects with all the other CXCR4-using viruses. A major conclusion from our results is that infection by HIV-2 primary isolates is readily blocked in vitro by maraviroc, at concentrations similar to those required for HIV-1. The susceptibility to maraviroc was independent of CD4(+) T cell counts or clinical stage of the patient from which the virus was obtained. These findings indicate that maraviroc could constitute a reliable therapeutic alternative for HIV-2-infected patients, as long as they are infected with CCR5-using variants, and this may have direct implications for the clinical management of HIV-2-infected patients.

Direct measurement of thermal stability of expressed CCR5 and stabilization by small molecule ligands

The inherent instability of heptahelical G protein-coupled receptors (GPCRs) during purification and reconstitution is a primary impediment to biophysical studies and to obtaining high-resolution crystal structures. New approaches to stabilizing receptors during purification and screening reconstitution procedures are needed. Here we report the development of a novel homogeneous time-resolved fluorescence assay (HTRF) to quantify properly folded CC-chemokine receptor 5 (CCR5). The assay permits high-throughput thermal stability measurements of femtomole quantities of CCR5 in detergent and in engineered nanoscale apolipoprotein-bound bilayer (NABB) particles. We show that recombinantly expressed CCR5 can be incorporated into NABB particles in high yield, resulting in greater thermal stability compared with that of CCR5 in a detergent solution. We also demonstrate that binding of CCR5 to the HIV-1 cellular entry inhibitors maraviroc, AD101, CMPD 167, and vicriviroc dramatically increases receptor stability. The HTRF assay technology reported here is applicable to other membrane proteins and could greatly facilitate structural studies of GPCRs.

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.

Novel peptides based on HIV-1 gp120 sequence with homology to chemokines inhibit HIV infection in cell culture

The sequential interaction of the envelope glycoprotein of the human immunodeficiency virus type 1 (HIV-1) with CD4 and certain chemokine coreceptors initiates host cell entry of the virus. The appropriate chemokines have been shown to inhibit viral replication by blocking interaction of the gp120 envelope protein with the coreceptors. We considered the possibility that this interaction involves a motif of the gp120 that may be structurally homologous to the chemokines. In the amino acid sequences of most chemokines there is a Trp residue located at the beginning of the C-terminal α-helix, which is separated by six residues from the fourth Cys residue. The gp120 of all HIV-1 isolates have a similar motif, which includes the C-terminal part of a variable loop 3 (V3) and N-terminal part of a conserved region 3 (C3). Two synthetic peptides, derived from the relevant gp120 sequence inhibited HIV-1 replication in macrophages and T lymphocytes in sequence-dependent manner. The peptides also prevented binding of anti-CXCR4 antibodies to CXCR4, and inhibited the intracellular Ca²⁺ influx in response to CXCL12/SDF-1α. Thus these peptides can be used to dissect gp120 interactions with chemokine receptors and could serve as leads for the design of new inhibitors of HIV-1.

CXCR4 nanobodies (VHH-based single variable domains) potently inhibit chemotaxis and HIV-1 replication and mobilize stem cells

The important family of G protein-coupled receptors has so far not been targeted very successfully with conventional monoclonal antibodies. Here we report the isolation and characterization of functional VHH-based immunoglobulin single variable domains (or nanobodies) against the chemokine receptor CXCR4. Two highly selective monovalent nanobodies, 238D2 and 238D4, were obtained using a time-efficient whole cell immunization, phage display, and counterselection method. The highly selective VHH-based immunoglobulin single variable domains competitively inhibited the CXCR4-mediated signaling and antagonized the chemoattractant effect of the CXCR4 ligand CXCL12. Epitope mapping showed that the two nanobodies bind to distinct but partially overlapping sites in the extracellular loops. Short peptide linkage of 238D2 with 238D4 resulted in significantly increased affinity for CXCR4 and picomolar activity in antichemotactic assays. Interestingly, the monovalent nanobodies behaved as neutral antagonists, whereas the biparatopic nanobodies acted as inverse agonists at the constitutively active CXCR4-N3.35A. The CXCR4 nanobodies displayed strong antiretroviral activity against T cell-tropic and dual-tropic HIV-1 strains. Moreover, the biparatopic nanobody effectively mobilized CD34-positive stem cells in cynomolgus monkeys. Thus, the nanobody platform may be highly effective at generating extremely potent and selective G protein-coupled receptor modulators.

Fine definition of the CXCR4-binding region on the V3 loop of feline immunodeficiency virus surface glycoprotein

The chemokine receptor CXCR4 is shared by primary and laboratory-adapted strains of feline immunodeficiency virus (FIV) for viral entry. Our previous studies implicated a contiguous nine-amino-acid region of the V3 loop of the FIV envelope surface as important in CXCR4 binding and virus entry. The binding is specific for CXCR4 since it can be inhibited by AMD3100, a selective CXCR4 inhibitor. Additional site-directed mutagenesis was used to further reveal the key residues. Binding studies indicated that basic residues R395, K397, R399 as well as N398 are critical for CXCR4 binding. The effect of other amino acid residues on receptor binding depends on the type of amino acid residue substituted. The binding study results were confirmed on human CXCR4-expressing SupT1 cells and correlated with entry efficiency using a virus entry assay. Amino acid residues critical for CXCR4 are not critical for interactions with the primary binding receptor CD134, which has an equivalent role as CD4 for HIV-1 binding. The ELISA results show that W394 and W400 are crucial for the recognition by neutralizing anti-V3 antibodies. Since certain strains of HIV-1 also use CXCR4 as the entry receptor, the findings make the feline model attractive for development of broad-based entry antagonists and for study of the molecular mechanism of receptor/virus interactions.

ADS-J1 inhibits human immunodeficiency virus type 1 entry by interacting with the gp41 pocket region and blocking fusion-active gp41 core formation

We previously identified a small-molecule anti-human immunodeficiency virus type 1 (anti-HIV-1) compound, ADS-J1, using a computer-aided molecular docking technique for primary screening and a sandwich enzyme-linked immunosorbent assay (ELISA) as a secondary screening method. In the present study, we demonstrated that ADS-J1 is an HIV-1 entry inhibitor, as determined by a time-of-addition assay and an HIV-1-mediated cell fusion assay. Further mechanism studies confirmed that ADS-J1 does not block gp120-CD4 binding and exhibits a marginal interaction with the HIV-1 coreceptor CXCR4. However, ADS-J1 inhibited the fusion-active gp41 core formation mimicked by peptides derived from the viral gp41 N-terminal heptad repeat (NHR) and C-terminal heptad repeat (CHR), as determined by ELISA, native polyacrylamide gel electrophoresis, and circular dichroism analysis. Moreover, using a surface plasmon resonance assay, we found that ADS-J1 could bind directly to IQN17, a trimeric peptide containing the gp41 pocket region, resulting in the conformational change of IQN17 and the blockage of its interaction with a short D peptide, PIE7. The positively charged residue (K574) located in the gp41 pocket region is critical for the binding of ADS-J1 to NHR. These results suggest that ADS-J1 may bind to the viral gp41 NHR region through its hydrophobic and ionic interactions with the hydrophobic and positively charged resides located in the pocket region, subsequently blocking the association between the gp41 NHR and CHR regions to form the fusion-active gp41 core, thereby inhibiting HIV-1-mediated membrane fusion and virus entry.

Inhibition of envelope-mediated CD4+-T-cell depletion by human immunodeficiency virus attachment inhibitors

Human immunodeficiency virus type 1 (HIV-1) envelope (Env) binding induces proapoptotic signals in CD4(+) T cells without a requirement of infection. Defective virus particles, which represent the majority of HIV-1, usually contain a functional Env and therefore represent a potentially significant cause of such CD4(+)-T-cell loss. We reasoned that an HIV-1 inhibitor that prohibits Env-host cell interactions could block the destructive effects of defective particles. HIV-1 attachment inhibitors (AIs), which potently inhibit Env-CD4 binding and subsequent downstream effects of Env, display low-nanomolar antiapoptotic potency and prevent CD4(+)-T-cell depletion from mixed lymphocyte cultures, also with low-nanomolar potency. Specific Env amino acid changes that confer resistance to AI antientry activity eliminate AI antiapoptotic effects. We observed that CD4(+)-T-cell destruction is specific for CXCR4-utilizing HIV-1 strains and that the fusion blocker enfuvirtide inhibits Env-mediated CD4(+)-T-cell killing but is substantially less potent than AIs. These observations, in conjunction with observed antiapoptotic activities of soluble CD4 and the CXCR4 blocker AMD3100, suggest that this AI activity functions through a mechanism common to AI antientry activity, e.g., prevention of Env conformation changes necessary for specific interactions with cellular factors that facilitate viral entry. Our study suggests that AIs, in addition to having potent antientry activity, could contribute to immune system homeostasis in individuals infected with HIV-1 that can engage CXCR4, thereby mitigating the increased risk of adverse clinical events observed in such individuals on current antiretroviral regimens.

Derivation and characterization of a simian immunodeficiency virus SIVmac239 variant with tropism for CXCR4

Like human immunodeficiency virus type 1 (HIV-1), most simian immunodeficiency virus (SIV) strains use CCR5 to establish infection. However, while HIV-1 can acquire the ability to use CXCR4, SIVs that utilize CXCR4 have rarely been reported. To explore possible barriers against SIV coreceptor switching, we derived an R5X4 variant, termed 239-ST1, from the R5 clone SIVmac239 by serially passaging virus in CD4(+) CXCR4(+) CCR5(-) SupT1 cells. A 239-ST1 env clone, designated 239-ST1.2-32, used CXCR4 and CCR5 in cell-cell fusion and reporter virus infection assays and conferred the ability for rapid, cytopathic infection of SupT1 cells to SIVmac239. Viral replication was inhibitable by the CXCR4-specific antagonist AMD3100, and replication was abrogated in a novel CXCR4(-) SupT1 line. Surprisingly, parental SIVmac239 exhibited low-level replication in SupT1 cells that was not observed in CXCR4(-) SupT1 cells. Only two mutations in the 239-ST1.2-32 Env, K47E in the C1 domain and L328W in the V3 loop, were required for CXCR4 use in cell-cell fusion assays, although two other V3 changes, N316K and I324M, improved CXCR4 use in infection assays. An Env cytoplasmic tail truncation, acquired during propagation of 239-ST1 in SupT1 cells, was not required. Compared with SIVmac239, 239-ST1.2-32 was more sensitive to neutralization by five of seven serum and plasma samples from SIVmac239-infected rhesus macaques and was approximately 50-fold more sensitive to soluble CD4. Thus, SIVmac239 can acquire the ability to use CXCR4 with high efficiency, but the changes required for this phenotype may be distinct from those for HIV-1 CXCR4 use. This finding, along with the increased neutralization sensitivity of this CXCR4-using SIV, suggests a mechanism that could select strongly against this phenotype in vivo.

Immunoimaging of CXCR4 expression in brain tumor xenografts using SPECT/CT

Chemokine receptor 4 (CXCR4) is expressed in a variety of cancers, including breast, brain, ovarian, and prostate. CXCR4-CXCL12 interactions are critical for tumor development, growth, and metastasis. Compared with normal tissue, neoplastic tissue (including metastases) expresses high levels of CXCR4. Previous clinical and preclinical observations suggest that CXCR4 levels could be used as a predictive marker of metastatic potential. Here we report the results of SPECT/CT of CXCR4 expression levels in experimental brain tumors using (125)I-labeled anti-CXCR4 monoclonal antibodies (mAbs).

METHODS

hCXCR4 antibody 12G5 and control IgG(2A) antibody were radiolabeled. Radio-mAbs were obtained in 40%-60% yield, with 1.4-1.9 MBq/microg specific radioactivities and greater than 95% purity. Severe combined immunodeficient mice harboring U87 xenografts were used for ex vivo biodistribution and imaging studies. Surface CXCR4 expression levels on U87 tumor-derived cells were analyzed by flow cytometry.

RESULTS

Biodistribution and imaging studies showed a specific accumulation of (125)I-12G5 in U87 tumors, with tumor-to-muscle uptake ratios reaching 15 +/- 3 at 48 h after injection. The tumor-to-tumor uptake ratio for (125)I-12G5 and (125)I-IgG(2A) was 2.5 at 48 h after injection. Flow cytometry analysis of tumor-derived cells showed a 2- to 7-fold increase in CXCR4 expression relative to inoculums, accounting for the high mAb uptake observed in the tumors.

CONCLUSION

Our data demonstrate the feasibility of imaging CXCR4 expression in experimental brain tumors. The elevated CXCR4 levels observed may have been, in part, due to the hypoxic tumor microenvironment.

Analysis of chemokine receptor endocytosis and intracellular trafficking

Chemokine receptors are G protein-coupled receptors (GPCRs) that, through their ability to regulate chemotaxis by responding to small chemoattractant peptides termed chemokines, are involved in the development, maintenance, and functional activities of the immune system. In addition, members of the chemokine receptor family have been implicated in a number of other physiological and pathological processes, including human immunodeficiency virus infection and malaria. These activities are dependent on receptor expression at the cell surface and cellular events that reduce the cell-surface expression of chemokine receptors can abrogate these activities. Moreover, internalization of chemokine receptors by endocytosis is necessary for both receptor degradation and recycling, key regulatory processes that determine cell-surface expression levels. Here we provide detailed methods for the quantitative analysis of CCR5 endocytosis and recycling by flow cytometry, as well as fluorescence and electron microscopic procedures to analyze the endocytosis and intracellular trafficking of CCR5 by immunolabeling of cells or cryosections. In principle, the same approaches can be used for analyzing other chemokine receptors and other GPCR or non-GPCR cell-surface proteins.

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

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

Coreceptor tropism in human immunodeficiency virus type 1 subtype D: high prevalence of CXCR4 tropism and heterogeneous composition of viral populations

In human immunodeficiency virus type 1 (HIV-1) subtype B, CXCR4 coreceptor use ranges from approximately 20% in early infection to approximately 50% in advanced disease. Coreceptor use by non-subtype B HIV is less well characterized. We studied coreceptor tropism of subtype A and D HIV-1 collected from 68 pregnant, antiretroviral drug-naive Ugandan women (HIVNET 012 trial). None of 33 subtype A or 10 A/D-recombinant viruses used the CXCR4 coreceptor. In contrast, nine (36%) of 25 subtype D viruses used both CXCR4 and CCR5 coreceptors. Clonal analyses of the nine subtype D samples with dual or mixed tropism revealed heterogeneous viral populations comprised of X4-, R5-, and dual-tropic HIV-1 variants. In five of the six samples with dual-tropic strains, V3 loop sequences of dual-tropic clones were identical to those of cocirculating R5-tropic clones, indicating the presence of CXCR4 tropism determinants outside of the V3 loop. These dual-tropic variants with R5-tropic-like V3 loops, which we designated "dual-R," use CCR5 much more efficiently than CXCR4, in contrast to dual-tropic clones with X4-tropic-like V3 loops ("dual-X"). These observations have implications for pathogenesis and treatment of subtype D-infected individuals, for the association between V3 sequence and coreceptor tropism phenotype, and for understanding potential mechanisms of evolution from exclusive CCR5 use to efficient CXCR4 use by subtype D HIV-1.

Modest human immunodeficiency virus coreceptor function of CXCR3 is strongly enhanced by mimicking the CXCR4 ligand binding pocket in the CXCR3 receptor

The chemokine receptor CXCR3 can exhibit weak coreceptor function for several human immunodeficiency virus type 1 (HIV-1) and HIV-2 strains and clinical isolates. These viruses produced microscopically visible cytopathicity in U87.CD4.CXCR3 cell cultures, whereas untransfected (CXCR3-negative) U87.CD4 cells remained uninfected. Depending on the particular virus, the coreceptor efficiency of CXCR3 was 100- to >10,000-fold lower compared to that of CXCR4. A CXCR3 variant carrying the CXCR4 binding pocket was constructed by simultaneous lysine-to-alanine and serine-to-glutamate substitutions at positions 300 and 304 of the CXCR3 receptor. This mutant receptor (CXCR3[K300A, S304E]) showed markedly enhanced HIV coreceptor function compared to the wild-type receptor (CXCR3[WT]). Moreover, the CXCR4 antagonist AMD3100 exhibited antagonistic and anti-HIV activities in U87.CD4.CXCR3[K300A, S304E] cells but not in U87.CD4.CXCR3[WT] cells.

Variation of macrophage tropism among HIV-1 R5 envelopes in brain and other tissues

Human immunodeficiency virus (HIV)-positive individuals frequently suffer from progressive encephelopathy, which is characterized by sensory neuropathy, sensory myelopathy, and dementia. Our group and others have reported the presence of highly macrophage-tropic R5 variants of HIV-1 in brain tissue of patients with neurological complications. These variants are able to exploit low amounts of CD4 and/or CCR5 for infection and potentially confer an expanded tropism for any cell types that express low CD4 and/or CCR5. In contrast to the brain-derived envelopes, we found that envelopes from lymph node tissue, blood, or semen were predominantly non-macrophage-tropic and required high amounts of CD4 for infection. Nevertheless, where tested, the non-macrophage-tropic envelopes conferred efficient replication in primary CD4(+) T-cell cultures. Determinants of R5 macrophage tropism appear to involve changes in the CD4 binding site, although further unknown determinants are also involved. The variation of R5 envelopes also affects their sensitivity to inhibition by ligands and entry inhibitors that target CD4 and CCR5. In summary, HIV-1 R5 viruses vary extensively in macrophage tropism. In the brain, highly macrophage-tropic variants may represent neurotropic or neurovirulent viruses. In addition, variation in R5 macrophage tropism may also have implications (1) for transmission, depending on what role macrophages or cells that express low CD4 and/or CCR5 play in the establishment of infection in a new host, and (2) for pathogenesis and depletion of CD4(+) T cells (i.e., do highly macrophage-tropic variants confer a broader tropism among CD4(+) T-cell populations late in disease and contribute to their depletion?).

Identification of a d-amino acid decapeptide HIV-1 entry inhibitor

Entry of human immunodeficiency virus type 1 (HIV-1) virion into host cells involves three major steps, each being a potential target for the development of entry inhibitors: gp120 binding to CD4, gp120-CD4 complex interacting with a coreceptor, and gp41 refolding to form a six-helix bundle. Using a D-amino acid decapeptide combinatorial library, we identified peptide dC13 as having potent HIV-1 fusion inhibitory activity, and effectively inhibiting infection by several laboratory-adapted and primary HIV-1 strains. While dC13 did not block binding of gp120 to CD4, nor disrupt the gp41 six-helix bundle formation, it effectively blocked the binding of an anti-CXCR4 monoclonal antibody and chemokine SDF-1alpha to CXCR4-expressing cells. However, because R5-using primary viruses were also neutralized, the antiviral activity of dC13 implies additional mode(s) of action. These results suggest that dC13 is a useful HIV-1 coreceptor antagonist for CXCR4 and, due to its biostability and simplicity, may be of value for developing a new class of HIV-1 entry inhibitors.

Complex determinants in human immunodeficiency virus type 1 envelope gp120 mediate CXCR4-dependent infection of macrophages

Host cell range, or tropism, combined with coreceptor usage defines viral phenotypes as macrophage tropic using CCR5 (M-R5), T-cell-line tropic using CXCR4 (T-X4), or dually lymphocyte and macrophage tropic using CXCR4 alone or in combination with CCR5 (D-X4 or D-R5X4). Although envelope gp120 V3 is necessary and sufficient for M-R5 and T-X4 phenotypes, the clarity of V3 as a dominant phenotypic determinant diminishes in the case of dualtropic viruses. We evaluated D-X4 phenotype, pathogenesis, and emergence of D-X4 viruses in vivo and mapped genetic determinants in gp120 that mediate use of CXCR4 on macrophages ex vivo. Viral quasispecies with D-X4 phenotypes were associated significantly with advanced CD4+-T-cell attrition and commingled with M-R5 or T-X4 viruses in postmortem thymic tissue and peripheral blood. A D-X4 phenotype required complex discontinuous genetic determinants in gp120, including charged and uncharged amino acids in V3, the V5 hypervariable domain, and novel V1/V2 regions distinct from prototypic M-R5 or T-X4 viruses. The D-X4 phenotype was associated with efficient use of CXCR4 and CD4 for fusion and entry but unrelated to levels of virion-associated gp120, indicating that gp120 conformation contributes to cell-specific tropism. The D-X4 phenotype describes a complex and heterogeneous class of envelopes that accumulate multiple amino acid changes along an evolutionary continuum. Unique gp120 determinants required for the use of CXCR4 on macrophages, in contrast to cells of lymphocytic lineage, can provide targets for development of novel strategies to block emergence of X4 quasispecies of human immunodeficiency virus type 1.

Generation and properties of a human immunodeficiency virus type 1 isolate resistant to the small molecule CCR5 inhibitor, SCH-417690 (SCH-D)

We describe the generation of two genetically related human immunodeficiency virus type 1 (HIV-1) isolates highly (>20,000-fold) resistant to the small molecule CCR5 inhibitor, SCH-417690 (formerly SCH-D). Both viruses were cross-resistant to other small molecules targeting entry via CCR5, but they were inhibited by some MAbs against the same coreceptor on primary CD4+ T-cells. The resistant isolates remained sensitive to inhibitors of other stages of virus entry, and to replication inhibitors acting post-entry. Neither escape mutant could replicate detectably in peripheral blood mononuclear cells (PBMC) from two donors homozygous for the CCR5-Delta32 allele and both were insensitive to the CXCR4-specific inhibitor, AMD3100. Hence, the SCH-D escape mutants retained the R5 phenotype. One of the resistant isolates was, however, capable of replication in U87.CD4.CXCR4 cells and, after expansion in those cells, was sensitive to AMD3100 in primary CD4+ T-cells. Hence, some X4 variants may be present in this escape mutant swarm. A notable observation was that the SCH-D escape mutants were also cross-resistant to PSC-RANTES and AOP-RANTES, chemokine derivatives that are reported to down-regulate cell surface CCR5 almost completely. However, the extent to which CCR5 is down-regulated was dependent upon the detection MAb. Hence, the escape mutants may be using a CCR5 configuration that is only detected by some anti-CCR5 MAbs. Finally, two SCH-D-resistant clonal viruses revealed no amino acid changes in the gp120 V3 region relative to the parental viruses, in marked contrast to clones resistant to the AD101 small molecule CCR5 inhibitor that possess 4 such sequence changes. Several sequence changes elsewhere in gp120 (V2, C3 and V4) were present in the SCH-D-resistant clones. Their influence on the resistant phenotype remains to be determined.

Down-modulation of the CXCR4 co-receptor by intracellular expression of a single chain variable fragment (SFv) inhibits HIV-1 entry into primary human brain microvascular endothelial cells and post-mitotic neurons

Our laboratories previously demonstrated that expression of a single chain variable antibody fragment (SFv), anti-CXCR4 SFv, in human lymphoid cells suppresses surface display of the chemokine co-receptor CXCR4 and inhibits infectious entry of human immunodeficiency virus type I (HIV-1). We now sought to extend these results to two types of central nervous system (CNS) cells, primary isolated human brain microvascular endothelial cells (MVECs), and post-mitotic differentiated human neurons, both of which normally express significant levels of CXCR4. The anti-CXCR4 SFv expression construct was delivered using an HIV-1-based vector, and control cells received LacZ-expressing viral particles. Upon intracellular expression of the anti-CXCR4 SFv, immunostaining revealed a marked reduction in surface display of CXCR4 on both cell types. Consequently, post-mitotic neurons expressing the anti-CXCR4 SFv were significantly protected from HIV-1 infection, as measured by HIV-1 p24 antigen production, and partial protection was observed in human brain MVECs. The ability to selectively down-modulate the surface expression of CXCR4 in CNS cells may allow for the development of clinical molecular therapy strategies against HIV-1-related neurodegenerative disorders and neuroinvasion.

Theaflavin derivatives in black tea and catechin derivatives in green tea inhibit HIV-1 entry by targeting gp41

Theaflavin derivatives and catechin derivatives are the major polyphenols in black tea and green tea, respectively. Several tea polyphenols, especially those with galloyl moiety, can inhibit HIV-1 replication with multiple mechanisms of action. Here we showed that the theaflavin derivatives had more potent anti-HIV-1 activity than catechin derivatives. These tea polyphenols could inhibit HIV-1 entry into target cells by blocking HIV-1 envelope glycoprotein-mediated membrane fusion. The fusion inhibitory activity of the tea polyphenols was correlated with their ability to block the formation of the gp41 six-helix bundle, a fusion-active core conformation. Computer-aided molecular docking analyses indicate that these tea polyphenols, theaflavin-3,3'-digallate (TF3) as an example, may bind to the highly conserved hydrophobic pocket on the surface of the central trimeric coiled coil formed by the N-terminal heptad repeats of gp41. These results indicate that tea, especially black tea, may be used as a source of anti-HIV agents and theaflavin derivatives may be applied as lead compounds for developing HIV-1 entry inhibitors targeting gp41.

N-substituted pyrrole derivatives as novel human immunodeficiency virus type 1 entry inhibitors that interfere with the gp41 six-helix bundle formation and block virus fusion

A recently approved peptidic human immunodeficiency virus type 1 (HIV-1) fusion inhibitor, T-20 (Fuzeon; Trimeris Inc.), has shown significant promise in clinical application for treating HIV-1-infected individuals who have failed to respond to the currently available antiretroviral drugs. However, T-20 must be injected twice daily and is too expensive. Therefore, it is essential to develop orally available small molecule HIV-1 fusion inhibitors. By screening a chemical library consisting of "drug-like" compounds, we identified two N-substituted pyrroles, designated NB-2 and NB-64, that inhibited HIV-1 replication at a low micromolar range. The absence of the COOH group in NB-2 and NB-64 resulted in a loss of anti-HIV-1 activity, suggesting that this acid group plays an important role in mediating the antiviral activity. NB-2 and NB-64 inhibited HIV-1 fusion and entry by interfering with the gp41 six-helix bundle formation and disrupting the alpha-helical conformation. They blocked a d-peptide binding to the hydrophobic pocket on surface of the gp41 internal trimeric coiled-coil domain. Computer-aided molecular docking analysis has shown that they fit inside the hydrophobic pocket and that their COOH group interacts with a positively charged residue (K574) around the pocket to form a salt bridge. These results suggest that NB-2 and NB-64 may bind to the gp41 hydrophobic pocket through hydrophobic and ionic interactions and block the formation of the fusion-active gp41 core, thereby inhibiting HIV-1-mediated membrane fusion and virus entry. Therefore, NB-2 and NB-64 can be used as lead compounds toward designing and developing more potent small molecule HIV-1 fusion inhibitors targeting gp41.

Inhibition of human immunodeficiency virus replication by a dual CCR5/CXCR4 antagonist

Here we report that the N-pyridinylmethyl cyclam analog AMD3451 has antiviral activity against a wide variety of R5, R5/X4, and X4 strains of human immunodeficiency virus type 1 (HIV-1) and HIV-2 (50% inhibitory concentration [IC(50)] ranging from 1.2 to 26.5 microM) in various T-cell lines, CCR5- or CXCR4-transfected cells, peripheral blood mononuclear cells (PBMCs), and monocytes/macrophages. AMD3451 also inhibited R5, R5/X4, and X4 HIV-1 primary clinical isolates in PBMCs (IC(50), 1.8 to 7.3 microM). A PCR-based viral entry assay revealed that AMD3451 blocks R5 and X4 HIV-1 infection at the virus entry stage. AMD3451 dose-dependently inhibited the intracellular Ca(2+) signaling induced by the CXCR4 ligand CXCL12 in T-lymphocytic cells and in CXCR4-transfected cells, as well as the Ca(2+) flux induced by the CCR5 ligands CCL5, CCL3, and CCL4 in CCR5-transfected cells. The compound did not interfere with chemokine-induced Ca(2+) signaling through CCR1, CCR2, CCR3, CCR4, CCR6, CCR9, or CXCR3 and did not induce intracellular Ca(2+) signaling by itself at concentrations up to 400 microM. In freshly isolated monocytes, AMD3451 inhibited the Ca(2+) flux induced by CXCL12 and CCL4 but not that induced by CCL2, CCL3, CCL5, and CCL7. The CXCL12- and CCL3-induced chemotaxis was also dose-dependently inhibited by AMD3451. Furthermore, AMD3451 inhibited CXCL12- and CCL3L1-induced endocytosis in CXCR4- and CCR5-transfected cells. AMD3451, in contrast to the specific CXCR4 antagonist AMD3100, did not inhibit but enhanced the binding of several anti-CXCR4 monoclonal antibodies (such as clone 12G5) at the cell surface, pointing to a different interaction with CXCR4. AMD3451 is the first low-molecular-weight anti-HIV agent with selective HIV coreceptor, CCR5 and CXCR4, interaction.

Inhibition of HIV-1 fusion with small interfering RNAs targeting the chemokine coreceptor CXCR4

RNA interference (RNAi) is an evolutionarily conserved process by which plants and animals protect their genomes utilizing small, double-stranded RNAs to degrade target RNAs in a sequence-specific manner. Post-transcriptional gene silencing by these moieties can lead to degradation of both cellular and viral RNAs. It has recently been shown that double-stranded, small interfering RNAs (siRNAs) of 21-25 nucleotides can be transfected into relevant cells to target specific RNAs. This approach was utilized to inhibit human immunodeficiency virus type I (HIV-1) infection in human cells. siRNAs with homology to a motif in the mRNA that encodes for the HIV-1 chemokine coreceptor CXCR4 was utilized. Complementary studies via immunofluorescence microscopy and fluorescence-activated cell sorting demonstrated downregulation of CXCR4 from the surface of cells transfected with the specific siRNAs. As well, siRNAs without sequence homology to CXCR4 were used as controls and demonstrated no downregulation of CXCR4. siRNAs targeted to another chemokine coreceptor, APJ, showed specificity for downregulation of APJ but had no effects on CXCR4. Transfections with siRNAs targeting CXCR4 mRNA were shown to inhibit HIV-1 envelope fusion, which is relatively resistant to most viral inhibitors targeting chemokine coreceptors. The specificity of this effect was demonstrated by the inhibition of fusion by CXCR4-tropic and dual-tropic (CXCR4 and CCR5) envelope glycoproteins from HIV-1 on CXCR4+ indicator cells, but the lack of effects by siRNAs targeting CXCR4 mRNA on dual-tropic HIV-1 envelopes in CCR5+ indicator cells utilizing these fusion assays. Interestingly, siRNAs targeting CXCR4 selectively inhibited CXCR4-tropic cell-free virus infection of human cells but at only modest levels as compared to cell:cell fusion. siRNA may be a potential molecular therapeutic approach to alter a cellular cofactor critical for infection of human cells by relevant strains of HIV-1. The targeting of a cellular cofactor, rather than the HIV-1-specific mRNAs or genomic RNA, holds promise as the rapid mutational ability of the HIV-1 genome may obviate the potential clinical use of RNAi directly against this virus.

Prostratin Induces HIV Activation and Downregulates HIV Receptors in Peripheral Blood Lymphocytes

Induction of HIV expression through lymphocyte activation has been proposed as a strategy to purge latent reservoirs. Prostratin is a non-tumourogenic phorbol ester that delays HIV replication in vitro, but paradoxically activates HIV expression in latently infected cells. To get a better insight into the mechanisms of action of prostratin, we have analysed the effect of prostratin on HIV activation and HIV receptor and coreceptors’ surface expression in human lymphocytes. Peripheral blood mononuclear cells (PBMCs) were transfected with luciferase expression constructs under the control of wild type HIV-long terminal repeat (LTR) and consensus sequences for transcription factors involved in HIV-LTR transactivation (NF-κB, SP1, NFAT). Prostratin stimulates transactivation of LTR vectors, κB- and SP-1-driven luciferase constructs. In another set of experiments, PBMCs were transfected with a full-length infectious viral clone. Prostratin induced HIV transcription and viral expression as detected by luciferase activity in cellular extracts and p24 levels in culture supernatants, respectively. Expression of the HIV coreceptors CCR5 and CXCR4 was decreased by prostratin and, concomitantly, prostratin inhibited the infection of PBMCs with R5 and X4 strains. However, prostratin did not inhibit infection with a pseudotyped viral clone that enters into the cells independently of HIV receptors. These results help to explain the paradoxical effects of prostratin. On one hand, prostratin induces HIV activation in latently infected cells through the induction of NF-κB and Sp1. On the other hand, strong and persistent downregulation of HIV receptors decreases infection of new targets and delays HIV propagation. These data support the potential use of prostratin to activate HIV from latency and purge viral reservoirs.

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

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

Identification of CD4 and transferrin receptor antibodies by CXCR4 antibody-guided Pathfinder selection

To generate human antibodies against CXCR4, a seven-transmembrane chemokine receptor and a principal coreceptor for HIV-1, several rounds of Pathfinder and Step-back selection from a large phage display antibody library were performed on Jurkat cells. A mAb against CXCR4 or biotinyated phage antibodies were used as guide molecules. Over 100 pan-Jurkat-cell-positive antibodies were characterized, but none were CXCR4 specific. However, several antibodies against CD4 and the transferrin receptor were identified. Our results indicate that, although Pathfinder and Step-back selection can be used to select phage antibodies on whole cells, the successful selection of certain targets is still complex and limited. The reason is probably, in part, due to the inaccessibility of the targeted extracellular structures and the range of the horseradish peroxidase-labeled guide molecule. Refinements of these techniques are required to improve target specificity and selectivity.

The antiviral activity of the CXCR4 antagonist AMD3100 is independent of the cytokine-induced CXCR4/HIV coreceptor expression level

The chemokine receptor CXCR4 is the main coreceptor used by T-tropic X4 HIV-1 strains to infect its target T cells. It has been proven that the CXCR4 expression level in T cells is strongly up-regulated by interleukin (IL)-4, a Th2-type cytokine that is secreted preferentially in HIV-infected patients in a later stage of disease. This results in an enhancement of HIV-1 replication in CD4+ T-lymphocytes. We have now evaluated the potency of the CXCR4 antagonist AMD3100 in phytohemagglutinin (PHA)/IL-2- versus PHA/IL-4-activated T cells in order to determine whether the compound has comparable CXCR4-antagonistic and anti-HIV-1 effects under these different cytokine treatments. We analyzed the CXCR4 expression level and the dose-dependent inhibition of CXCR4 expression by AMD3100, by monitoring the binding of an anti-CXCR4 monoclonal antibody (clone 12G5). We also determined stromal cell-derived factor (SDF)-1-induced intracellular calcium signaling and HIV-1 replication in these cells in the absence and presence of AMD3100. The CXCR4 expression level in PHA/IL-4-stimulated cells was much higher than in PHA/IL-2-stimulated cells. However, the potency of the bicyclam AMD3100 to block anti-CXCR4 mAb binding, SDF-1-induced intracellular calcium signaling, and HIV-1 replication of the X4 NL4.3 strain and three primary isolates remained unchanged. Our data indicate that CXCR4 antagonists such as AMD3100 act independently of the HIV-1 coreceptor expression level. These compounds should therefore be useful in suppressing HIV-1 infection in all stages of the disease.

Development of a cell-based enzyme-linked immunosorbent assay for high-throughput screening of HIV type 1 entry inhibitors targeting the coreceptor CXCR4

CXCR4, a coreceptor for human immunodeficiency virus type 1 (HIV-1) X4 virus, plays an important role in virus entry into the target cells. Antiviral agents that bind to CXCR4 are expected to inhibit HIV-1 entry. A cell-based enzyme-linked immunosorbent assay (ELISA) was developed utilizing an anti-CXCR4 monoclonal antibody, 12G5, and cells expressing CD4 and CXCR4, U373-MAGI-CXCR4(CEM) cells. Using this assay, we demonstrated that 12G5 specifically binds to the CXCR4-expressing cells, but not to CCR5-expressing cells and cells without CXCR4 and CCR5, consistent with the results obtained by using flow cytometry. The well-characterized CXCR4 antagonists, T22, T14012 (a downsized analog of T-22), and AMD3100, effectively inhibited 12G5 binding to CXCR4-expressing cells, while HIV-1 entry inhibitors targeting CD4 and gp41 as well as HIV-1 reverse transcriptase and protease inhibitors did not block the binding of 12G5 to U373-MAGI-CXCR4(CEM) cells. The prepared plates containing the fixed cells could be stored at -80 degrees C for at least 5 months without affecting the cell reactivity with 12G5, which enhances the convenience of this method. This suggests that the cell-based ELISA is specific, sensitive, convenient, rapid, and economic. With a robotic sample processing system, this assay can be used for high-throughput screening of HIV-1 entry inhibitors targeted to the HIV-1 coreceptor CXCR4.

Identification and characterization of HIV-2 strains obtained from asymptomatic patients that do not use CCR5 or CXCR4 coreceptors

In vivo, human immunodeficiency virus type 2 (HIV-2) infection reveals several unique characteristics when compared to HIV-1 infection, the most remarkable of which is the extraordinarily long asymptomatic period. Here we describe two HIV-2 primary isolates, obtained from asymptomatic individuals, which do not infect any coreceptor-expressing cell lines tested. In those cells, we show that the absence of replication is directly related to cell entry events. Furthermore, productive infection observed in peripheral blood mononuclear cells (PBMC) was not inhibited by natural ligands and monoclonal antibodies directed to CCR5 and CXCR4. Finally, viral entry efficiency and viral progeny production of these viruses are markedly impaired in PBMC, indicating a reduced replicative fitness of both viruses. In conclusion, our data suggest that in some HIV-2 asymptomatic individuals, the circulating viruses are unable to use the major coreceptors to infect PBMC. This fact should have important implications in HIV-2 pathogenesis and transmission.

Leukocyte-endothelium interaction promotes SDF-1-dependent polarization of CXCR4

Chemokine-driven migration is accompanied by polarization of the cell body and of the intracellular signaling machinery. The extent to which chemokine receptors polarize during chemotaxis is currently unclear. To analyze the distribution of the chemokine receptor CXCR4 during SDF-1 (CXCL12)-induced chemotaxis, we retrovirally expressed a CXCR4-GFP fusion protein in the CXCR4-deficient human hematopoietic progenitor cell line KG1a. This KG1a CXCR4-GFP cell line showed full restoration of SDF-1 responsiveness in assays detecting activation of ERK1/2 phosphorylation, actin polymerization, adhesion to endothelium under conditions of physiological flow, and (transendothelial) chemotaxis. When adhered to cytokine-activated endothelium in the absence of SDF-1, CXCR4 did not localize to the leading edge of the cell but was uniformly distributed over the plasma membrane. In contrast, when SDF-1 was immobilized on cytokine-activated endothelium, the CXCR4-GFP receptors that were present on the cell surface markedly redistributed to the leading edge of migrating cells. In addition, CXCR4-GFP co-localized with lipid rafts in the leading edge of SDF-1-stimulated cells, at the sites of contact with the endothelial surface. Inhibition of lipid raft formation prevents SDF-1-dependent migration, internalization of CXCR4, and polarization to the leading edge of CXCR4, indicating that CXCR4 surface expression and signaling requires lipid rafts. These data show that SDF-1, immobilized on activated human endothelium, induces polarization of CXCR4 to the leading edge of migrating cells, revealing co-operativity between chemokine and substrate in the control of cell migration.

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.

Mutations at the CXCR4 interaction sites for AMD3100 influence anti-CXCR4 antibody binding and HIV-1 entry

The interaction of the CXCR4 antagonist AMD3100 with its target is greatly influenced by specific aspartate residues in the receptor protein, including Asp(171) and Asp(262). We have now found that aspartate-to-asparagine substitutions at these positions differentially affect the binding of four different anti-CXCR4 monoclonal antibodies as well as the infectivity of diverse human immunodeficiency virus type 1 (HIV-1) strains and clinical isolates. Mutation of Asp(262) strongly decreased the coreceptor efficiency of CXCR4 for wild-type but not for AMD3100-resistant HIV-1 NL4.3. Thus, resistance of HIV-1 NL4.3 to AMD3100 is associated with a decreased dependence of the viral gp120 on Asp(262) of CXCR4, pointing to a different mode of interaction of wild-type versus AMD3100-resistant virus with CXCR4.

Human Leydig cells are productively infected by some HIV-2 and SIV strains but not by HIV-1

OBJECTIVES

With the use of highly active antiretroviral therapy, the identification of HIV reservoirs within the body has become an important issue. However, the testis has been largely ignored despite representing a pharmacologic sanctuary which could act as a viral reservoir.

DESIGN

Because alterations in testosterone production have frequently been reported in HIV-infected individuals, we investigated whether the testosterone-producing Leydig cells could become directly infected by HIV-1, HIV-2 or SIV.

METHODS

Purified Leydig cells were infected with a panel of HIV-1, HIV-2 and SIV strains and examined for expression of HIV/SIV receptors. Additionally, the impact of CD4 transduction on Leydig cell infection was determined.

RESULTS

Leydig cells were unable to support productive infection of the seven HIV-1 isolates tested. No CD4, CXCR4 or CCR5 expression was evident on the surface of Leydig cells and transduction with a CD4 expressing adenovirus did not induce HIV-1 infection. In contrast, some primary and laboratory adapted CD4-independent HIV-2 and SIV strains were able to enter and replicate productively in Leydig cells.

CONCLUSIONS

Our results suggest that Leydig cells do not represent a target for HIV-1 infection within the testis. In contrast, Leydig cells support HIV-2 and SIV infection and thus represent a potential target for infection. Receptor use and significance of HIV-2/SIV infection of Leydig cells remain to be determined.

Identification of allosteric peptide agonists of CXCR4

The chemokine receptor CXCR4 is a co-receptor for T-tropic strains of HIV-1. A number of small molecule antagonists of CXCR4 are in development but all are likely to lead to adverse effects due to the physiological function of CXCR4. To prevent these complications, allosteric agonists may be therapeutically useful as adjuvant therapy in combination with small molecule antagonists. A synthetic cDNA library coding for 160,000 different SDF-based peptides was screened for CXCR4 agonist activity in a yeast strain expressing a functional receptor. Peptides that activated CXCR4 in an autocrine manner induced colony formation. Two peptides, designated RSVM and ASLW, were identified as novel agonists that are insensitive to the CXCR4 antagonist AMD3100. In chemotaxis assays using the acute lymphoblastic leukemia cell line CCRF-CEM, RSVM behaves as a partial agonist and ASLW as a superagonist. The superagonist activity of ASLW may be related to its inability to induce receptor internalization. In CCRF-CEM cells, the two peptides are also not inhibited by another CXCR4 antagonist, T140, or the neutralizing monoclonal antibodies 12G5 and 44717.111. These results suggest that alternative agonist-binding sites are present on CXCR4 that could be screened to develop molecules for therapeutic use.

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.

The macrophage response to HIV-1: Intracellular control of X4 virus replication accompanied by activation of chemokine and cytokine synthesis

During human immunodeficiency virus (HIV)-1 infection, T lymphocytes and macrophages play dual roles. They are the primary targets for virus replication, but they are also primary effector cells in acquired and innate immunity, respectively. The authors are now investigating how these roles come together in the response of human monocyte-derived macrophages (MDM) to certain HIV-1. The authors and others have previously shown that MDM permit entry of some X4 virus strains, but control viral replication intracellularly. In the present study, viral DNA synthesis, entry into the nucleus, and transcription to RNA were all observed in X4 virus-infected MDM. MDM arrested HIV-1 replication prior to expression of mature capsid antigen p24 and production of cell-free infectious viral particles. Cell-associated transmissible HIV-1 was detected by cocultivation of infected MDM and susceptible T lymphocytes. A second protective response of MDM to specific R5 as well as X4 HIV-1 was identified in rapid and extensive secretion of tumor necrosis factor-alpha, macrophage inflammatory protein-1alpha, and RANTES. These findings support the view that MDM act aggressively to control HIV-1 replication: X4 strains by severely limiting the progeny virus production and R5 strains by producing beta-chemokines competent to block virus entry into target cells. Optimizing these innate immune responses offers another means to control HIV-1 infection in the human host.

The influence of CD4 and CXCR4 on maedi-visna virus-induced syncytium formation

CD4 is the principal binding site for human and simian immunodeficiency virus (HIV/SIV) receptor interactions and the a chemokine receptor CXCR4 has been implicated as a primordial lentivirus receptor. This study sought to determine the relevance of CD4 and CXCR4 in virus-receptor interactions for the prototype lentivirus, maedi-visna virus (MVV) of sheep. Neither CD4 nor alpha/beta chemokine receptors represent principal receptors for MVV since human osteosarcoma cells devoid of these molecules were susceptible to productive infection. Interestingly, the presence of either CD4 and/or CXCR4 on indicator cells dramatically enhanced MVV-induced cell fusion (syncytium formation) for three independent virus strains. Syncytium formation results from virus-receptor interactions and can be inhibited by receptor ligands. However, neither SDF-la that binds CXCR4 nor recombinant gp120 (rgp120) that binds CD4 could specifically inhibit the observed enhancement of MVV-induced cell fusion under conditions that significantly reduced HIV-1-induced cell fusion. Our observations suggest that CD4 and CXCR4 may represent optional auxiliary components of an MVV receptor (or receptor complex) that facilitate MVV-mediated membrane fusion events, a feature important for virus entry. This potential accessory role for CXCR4 in MW receptor interactions may reflect the distant relationship between the ovine (MVV) and the human/feline lentiviruses (HIV/FIV).

XTT formazan widely used to detect cell viability inhibits HIV type 1 infection in vitro by targeting gp41

XTT can be metabolically reduced by mitochondrial dehydrogenase in viable cells to a water-soluble formazan product. Thus XTT has been widely used to evaluate cell viability and to screen anti-HIV agents and the cytotoxicity of these agents. The present studies demonstrated that XTT formazan derived from XTT in cell culture significantly inhibits the fusion of HIV-1-infected cells with uninfected cells. Synthetic XTT formazan effectively inhibited the replication of laboratory-adapted and primary HIV-1 isolates and cell-to-cell fusion with low cytotoxicity. It blocks the six-helix bundle formation between peptides derived from the N- and C-terminal heptad repeat regions of the gp41 ectodomain (designated N- and C-peptides, respectively). Analysis by a computer-aided docking program indicates that XTT formazan may bind to the highly conserved hydrophobic pocket on the surface of the central trimeric coiled coil of gp41. These results suggest that XTT formazan inhibits HIV-1 entry by targeting the alpha-helical coiled-coil domain of gp41. This small molecular nonpeptide antiviral compound can be used as a lead for designing more effective HIV-1 entry inhibitors targeting the fusion stage of HIV-1 infection. But because XTT formazan itself has anti-HIV-1 activity, caution should be exercised when XTT is used to evaluate HIV-1 infectivity.

The role of post-translational modifications of the CXCR4 amino terminus in stromal-derived factor 1 alpha association and HIV-1 entry

The chemokine receptor CXCR4 plays critical roles in development, immune function, and human immunodeficiency virus type 1 (HIV-1) entry. Here we demonstrate that, like the CC-chemokine receptors CCR5 and CCR2b, CXCR4 is posttranslationally modified by sulfation of its amino-terminal tyrosines. The sulfate group at tyrosine 21 contributes substantially to the ability of CXCR4 to bind its ligand, stromal derived factor 1 alpha. Tyrosine sulfation plays a less significant role in CXCR4-dependent HIV-1 entry than in CCR5-dependent HIV-1 entry. In some cell lines, CXCR4 is efficiently modified by a chondroitin sulfate chain at serine 18, but neither HIV-1 entry nor stromal derived factor 1 alpha binding was affected by loss of this glycosaminoglycan. These data demonstrate a functional role for tyrosine sulfate in the CXC-chemokine receptor family and underscore a general difference in HIV-1 utilization of CCR5 and CXCR4.