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Role of CXCR4 in cell-cell fusion and infection of monocyte-derived macrophages by primary human immunodeficiency virus type 1 (HIV-1) strains: two distinct mechanisms of HIV-1 dual tropism.

Dual-tropic human immunodeficiency virus type 1 (HIV-1) strains infect both primary macrophages and transformed T-cell lines. Prototype T-cell line-tropic (T-tropic) strains use CXCR4 as their principal entry coreceptor (X4 strains), while macrophagetropic (M-tropic) strains use CCR5 (R5 strains). Prototype dual tropic strains use both coreceptors (R5X4 strains). Recently, CXCR4 expressed on macrophages was found to support infection by certain HIV-1 isolates, including the dual-tropic R5X4 strain 89.6, but not by T-tropic X4 prototypes like 3B. To better understand the cellular basis for dual tropism, we analyzed the macrophage coreceptors used for Env-mediated cell-cell fusion as well as infection by several dual-tropic HIV-1 isolates. Like 89.6, the R5X4 strain DH12 fused with and infected both wild-type and CCR5-negative macrophages. The CXCR4-specific inhibitor AMD3100 blocked DH12 fusion and infection in macrophages that lacked CCR5 but not in wild-type macrophages. This finding indicates two independent entry pathways in macrophages for DH12, CCR5 and CXCR4. Three primary isolates that use CXCR4 but not CCR5 (tybe, UG021, and UG024) replicated efficiently in macrophages regardless of whether CCR5 was present, and AMD3100 blocking of CXCR4 prevented infection in both CCR5 negative and wild-type macrophages. Fusion mediated by UG021 and UG024 Envs in both wild-type and CCR5-deficient macrophages was also blocked by AMD3100. Therefore, these isolates use CXCR4 exclusively for entry into macrophages. These results confirm that macrophage CXCR4 can be used for fusion and infection by primary HIV-1 isolates and indicate that CXCR4 may be the sole macrophage coreceptor for some strains. Thus, dual tropism can result from two distinct mechanisms: utilization of both CCR5 and CXCR4 on macrophages and T-cell lines, respectively (dual-tropic R5X4), or the ability to efficiently utilize CXCR4 on both macrophages and T-cell lines (dual-tropic X4).

Anti-HIV Agents↗

CD4-Chemokine receptor hybrids in human immunodeficiency virus type 1 infection.

Most human immunodeficiency virus (HIV) strains require both CD4 and a chemokine receptor for entry into a host cell. In order to analyze how the HIV-1 envelope glycoprotein interacts with these cellular molecules, we constructed single-molecule hybrids of CD4 and chemokine receptors and expressed these constructs in the mink cell line Mv-1-lu. The two N-terminal (2D) or all four (4D) extracellular domains of CD4 were linked to the N terminus of the chemokine receptor CXCR4. The CD4(2D)CXCR4 hybrid mediated infection by HIV-1(LAI) to nearly the same extent as the wild-type molecules, whereas CD4(4D)CXCR4 was less efficient. Recombinant SU(LAI) protein competed more efficiently with the CXCR4-specific monoclonal antibody 12G5 for binding to CD4(2D)CXCR4 than for binding to CD4(4D)CXCR4. Stromal cell-derived factor 1 (SDF-1) blocked HIV-1(LAI) infection of cells expressing CD4(2D)CXCR4 less efficiently than for cells expressing wild-type CXCR4 and CD4, whereas down-modulation of CXCR4 by SDF-1 was similar for hybrids and wild-type CXCR4. In contrast, the bicyclam AMD3100, a nonpeptide CXCR4 ligand that did not down-modulate the hybrids, blocked hybrid-mediated infection at least as potently as for wild-type CXCR4. Thus SDF-1, but not the smaller molecule AMD3100, may interfere at multiple points with the binding of the surface unit (SU)-CD4 complex to CXCR4, a mechanism that the covalent linkage of CD4 to CXCR4 impedes. Although the CD4-CXCR4 hybrids yielded enhanced SU interactions with the chemokine receptor moiety, this did not overcome the specific coreceptor requirement of different HIV-1 strains: the X4 virus HIV-1(LAI) and the X4R5 virus HIV-1(89. 6), unlike the R5 strain HIV-1(SF162), infected Mv-1-lu cells expressing the CD4(2D)CXCR4 hybrid, but none could use hybrids of CD4 and the chemokine receptor CCR2b, CCR5, or CXCR2. Thus single-molecule hybrid constructs that mimic receptor-coreceptor complexes can be used to dissect coreceptor function and its inhibition.

Animals↗

Use of inhibitors to evaluate coreceptor usage by simian and simian/human immunodeficiency viruses and human immunodeficiency virus type 2 in primary cells.

We have used coreceptor-targeted inhibitors to investigate which coreceptors are used by human immunodeficiency virus type 1 (HIV-1), simian immunodeficiency viruses (SIV), and human immunodeficiency virus type 2 (HIV-2) to enter peripheral blood mononuclear cells (PBMC). The inhibitors are TAK-779, which is specific for CCR5 and CCR2, aminooxypentane-RANTES, which blocks entry via CCR5 and CCR3, and AMD3100, which targets CXCR4. We found that for all the HIV-1 isolates and all but one of the HIV-2 isolates tested, the only relevant coreceptors were CCR5 and CXCR4. However, one HIV-2 isolate replicated in human PBMC even in the presence of TAK-779 and AMD3100, suggesting that it might use an undefined, alternative coreceptor that is expressed in the cells of some individuals. SIV(mac)239 and SIV(mac)251 (from macaques) were also able to use an alternative coreceptor to enter PBMC from some, but not all, human and macaque donors. The replication in human PBMC of SIV(rcm) (from a red-capped mangabey), a virus which uses CCR2 but not CCR5 for entry, was blocked by TAK-779, suggesting that CCR2 is indeed the paramount coreceptor for this virus in primary cells.

Amides↗

CXC-chemokine receptor 4 is not a coreceptor for human herpesvirus 7 entry into CD4(+) T cells.

Human herpesvirus 7 (HHV-7) is a T-lymphotropic virus which utilizes the CD4 receptor as its main receptor to enter the target cells. Hence, HHV-7 can interfere with human immunodeficiency virus type 1 (HIV-1) infection in CD4(+) T cells. It was recently suggested that the CXC chemokine receptor 4 (CXCR4), which was found to be a crucial coreceptor for T-tropic HIV-1 strains, may also play a role in the HHV-7 infection process. However, the results presented here demonstrate that CXCR4 is not involved in HHV-7 infection. The natural ligand of CXCR4, SDF-1alpha, was not able to inhibit HHV-7 infection in SupT1 cells or in CD8(+) T-cell-depleted peripheral blood mononuclear cells. Also, AMD3100, a specific CXCR4 antagonist with potent antiviral activity against T-tropic HIV strains (50% inhibitory concentration ¿IC(50), 1 to 10 ng/ml), completely failed to inhibit HHV-7 infection (IC(50), >250 microg/ml). Thus, two different agents known to specifically interact with CXCR4 were not able to inhibit HHV-7 infection. Other T-lymphoid cell lines, expressing both CD4 and CXCR4 (e.g., HUT-78 and MT-4) could not be infected by HHV-7. In addition, the CD4-transfected cell lines HOS. CD4 and U87.CD4 and the CD4/CXCR4 double-transfected cell lines HOS. CD4.CXCR4 and U87.CD4.CXCR4 were not infectable with HHV-7. Also, we found no down-regulation of surface-bound or intracellular CXCR4 in HHV-7-infected CD4(+) T cells. As compared to uninfected SupT1 cells, stromal cell-derived factor 1alpha (SDF-1alpha)/CXCR4-mediated intracellular calcium flux was unchanged in SupT1 cells that were acutely or persistently infected with HHV-7. All these data argue against CXCR4 as a receptor involved in the HHV-7 infection process.

Antigens, Viral↗

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.

Amino Acid Sequence↗

Identification of a subset of human immunodeficiency virus type 1 (HIV-1), HIV-2, and simian immunodeficiency virus strains able to exploit an alternative coreceptor on untransformed human brain and lymphoid cells.

The chemokine receptors CCR5 and CXCR4 are the major coreceptors for human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV). At least 12 other chemokine receptors or close relatives support infection by particular HIV and SIV strains on CD4(+) transformed indicator cell lines in vitro. However, the role of these alternative coreceptors in vivo is presently thought to be insignificant. Infection of cell lines expressing high levels of recombinant CD4 and coreceptors thus does not provide a true indication of coreceptor use in vivo. We therefore tested primary untransformed cell cultures that lack CCR5 and CXCR4, including astrocytes and brain microvascular endothelial cells (BMVECs), for naturally expressed alternative coreceptors functional for HIV and SIV infection. An adenovirus vector (Ad-CD4) was used to express CD4 in CD4(-) astrocytes and thus confer efficient infection if a functional coreceptor is present. Using a large panel of viruses with well-defined coreceptor usage, we identified a subset of HIV and SIV strains able to infect two astrocyte cultures derived from adult brain tissue. Astrocyte infection was partially inhibited by several chemokines, indicating a role for the chemokine receptor family in the observed infection. BMVECs were weakly positive for CD4 but negative for CCR5 and CXCR4 and were susceptible to infection by the same subset of isolates that infected astrocytes. BMVEC infection was efficiently inhibited by the chemokine vMIP-I, implicating one of its receptors as an alternative coreceptor for HIV and SIV infection. Furthermore, we tested whether the HIV type 1 and type 2 strains identified were able to infect peripheral blood mononuclear cells (PBMCs) via an alternative coreceptor. Several strains replicated in Delta32/Delta32 CCR5 PBMCs with CXCR4 blocked by AMD3100. This AMD3100-resistant replication was also sensitive to vMIP-I inhibition. The nature and potential role of this alternative coreceptor(s) in HIV infection in vivo is discussed.

Animals↗

The CCR5 and CXCR4 coreceptors are both used by human immunodeficiency virus type 1 primary isolates from subtype C.

Human immunodeficiency virus type 1 (HIV-1) subtype C viruses with different coreceptor usage profiles were isolated from 29 South African patients with advanced AIDS. All 24 R5 isolates were inhibited by the CCR5-specific agents, PRO 140 and RANTES, while the two X4 viruses and the three R5X4 viruses were sensitive to the CXCR4-specific inhibitor, AMD3100. The five X4 or R5X4 viruses were all able to replicate in peripheral blood mononuclear cells that did not express CCR5. When tested using coreceptor-transfected cell lines, one R5 virus was also able to use CXCR6, and another R5X4 virus could use CCR3, BOB/GPR15, and CXCR6. The R5X4 and X4 viruses contained more-diverse V3 loop sequences, with a higher overall positive charge, than the R5 viruses. Hence, some HIV-1 subtype C viruses are able to use CCR5, CXCR4, or both CXCR4 and CCR5 for entry, and they are sensitive to specific inhibitors of entry via these coreceptors. These observations are relevant to understanding the rapid spread of HIV-1 subtype C in the developing world and to the design of intervention and treatment strategies.

Acquired Immunodeficiency Syndrome↗

Profile of resistance of human immunodeficiency virus to mannose-specific plant lectins.

The mannose-specific plant lectins from the Amaryllidaceae family (e.g., Hippeastrum sp. hybrid and Galanthus nivalis) inhibit human immunodeficiency virus (HIV) infection of human lymphocytic cells in the higher nanogram per milliliter range and suppress syncytium formation between persistently HIV type 1 (HIV-1)-infected cells and uninfected CD4(+) T cells. These lectins inhibit virus entry. When exposed to escalating concentrations of G. nivalis and Hippeastrum sp. hybrid agglutinin, a variety of HIV-1(III(B)) strains were isolated after 20 to 40 subcultivations which showed a decreased sensitivity to the plant lectins. Several amino acid changes in the envelope glycoprotein gp120, but not in gp41, of the mutant virus isolates were observed. The vast majority of the amino acid changes occurred at the N glycosylation sites and at the S or T residues that are part of the N glycosylation motif. The degree of resistance to the plant lectins was invariably correlated with an increasing number of mutated glycosylation sites in gp120. The nature of these mutations was entirely different from that of mutations that are known to appear in HIV-1 gp120 under the pressure of other viral entry inhibitors such as dextran sulfate, bicyclams (i.e., AMD3100), and chicoric acid, which also explains the lack of cross-resistance of plant lectin-resistant viruses to any other HIV inhibitor including T-20 and the blue-green algae (cyanobacteria)-derived mannose-specific cyanovirin. The plant lectins represent a well-defined class of anti-HIV (microbicidal) drugs with a novel HIV drug resistance profile different from those of other existing anti-HIV drugs.

Amino Acid Substitution↗

In vivo evolution of X4 human immunodeficiency virus type 1 variants in the natural course of infection coincides with decreasing sensitivity to CXCR4 antagonists.

CXCR4-using (X4) human immunodeficiency virus type 1 (HIV-1) variants evolve from CCR5-restricted (R5) HIV-1 variants. Early after their first appearance in vivo, X4 HIV-1 variants additionally use CCR5. The ability to use CCR5 in addition to CXCR4 is generally lost late in infection. Here we studied whether this evolution of the coreceptor repertoire is also reflected in a changing sensitivity of X4 variants to CXCR4 antagonists such as peptide T22 and the synthetic compound AMD3100. We observed differences in the concentrations of CXCR4 antagonists needed to suppress replication of X4 HIV variants from different patients. In general, late X4 HIV variants were less sensitive to AMD3100 than were early R5X4 HIV variants. The differences between early R5X4 HIV variants and late X4 variants were less pronounced for T22-mediated inhibition. These results suggest an ongoing evolution of X4 virus variants toward more efficient usage of the cellular entry complex.

Anti-HIV Agents↗

Preferential feline immunodeficiency virus (FIV) infection of CD4+ CD25+ T-regulatory cells correlates both with surface expression of CXCR4 and activation of FIV long terminal repeat binding cellular transcriptional factors.

Previously, we have characterized feline CD4+ CD25+ T-regulatory (Treg) cells with regard to their immune regulatory properties and ability to support feline immunodeficiency virus (FIV) replication in vitro and in vivo. Our studies showed that while CD4+ CD25+ cells were capable of replicating FIV in the presence of interleukin-2 (IL-2) alone, CD4+ CD25- cells harbored a latent infection that required a strong mitogenic stimulus to activate virus replication. In the present study, we investigated the mechanisms governing the preferential replication of FIV in highly purified CD4+ CD25+ Treg cells compared to their CD4+ CD25+ counterparts. Studies aimed at elucidating mechanisms regulating infection of these cells revealed that CD4+ CD25- cells were less susceptible to FIV binding and entry than CD4+ CD25+ cells, which correlated with increased surface expression of FIV coreceptor CXCR4. In addition, the number of CD4+ CD25+ cells that expressed the primary receptor CD134 was greater than for CD4+ CD25- cells. Although increased permissiveness to FIV infection of CD4+ CD25- cells following mitogenic stimulation correlated strongly with upregulation of surface CXCR4, it did not correlate with CD134 expression. Further, study of intracellular factors regulating FIV replication revealed that CD4+ CD25+ but not CD4+ CD25- T cells showed constitutive and IL-2-responsive transactivation of activating transcription factor, CAAT enhancer binding protein, and activating protein 1 transcription factors that are important for FIV replication. These factors were upregulated in CD4+ CD25- T cells following ConA stimulation, which correlated with FIV replication. This is the first report elucidating the mechanisms that allow for productive lentiviral infection of CD4+ CD25+ Treg cells.

Animals↗

Potassium-induced cell swelling in Necturus gallbladder epithelium.

In Necturus gallbladder epithelium, elevation of mucosal K+ to 95 mM in the presence of 10 mM Na+ resulted in cell swelling at a rate of 3.2% original volume per minute, followed by volume-regulatory shrinking. When Na+ was completely removed from or when amiloride (10(-4) M) was added to the mucosal medium, K+-induced cell swelling was abolished. In the presence of 10 mM Na+, 1 mM Ba2+ abolished and substitution of mucosal Cl- by NO-3 had no effect on K+-induced swelling. Thus solute entry following elevation of mucosal K+ is effected by separate K+ and Cl- pathways. Furthermore, substitution of 95 mM K+ for Na+ in the mucosal bathing medium leads to the development of a Cl- conductance in the basolateral membrane as long as some Na+ remains in the medium. However, cell swelling induced by mucosal dilution does not lead to the appearance of a Cl- conductance. Thus the activation of this conductance requires both swelling and membrane depolarization. These results show that 1) high mucosal K+ leads to cell swelling due to the entry of Cl- along with K+ and the Cl- can enter across either membrane, 2) the Cl- pathways require the presence of mucosal Na+, and 3) cell volume regulation is activated by an increase in volume per se, i.e., a hyposmotic exposure is not required for volume regulation to occur.

Animals↗

Origin of electrical PD's in hamster thin ascending limbs of Henle's loop.

Microelectrodes with 2-micronm-tip diameters arranged to record differentially between this ascending limbs (ALH) and ascending vasa recta (AVR), gave values of 1.95 +/- 0.17 mV, ALH positive, in hydropenia with mineral oil bathing the kidney. Average values remained in the range of 1-2 mV when the ALH and AVR values were obtained sequentially, when the kidney was bathed in Ringer solution, when 5-8 Momega Ling-Gerard microelectrodes were used in the ALH, or when the hamsters were in saline diuresis. These results contradict reports of earlier studies with high impedance Ling-Gerard electrodes that the PD was -9 mV, ALH negative. Perfusion of ALH in saline diuresis with solutions of various compositions provided estimates of ionic transport numbers: tNa+ = 0.33 +/- 0.01, tK+ = 0.00 +/- 0.02, tC1- = 0.67 +/- 0.02. When the perfusion solution was designed to have the same Na+, K+, and C1- concentrations as AVR plasma, the PD was 1.36 +/- .20 mV; when ouabain or furosemide were included (10(-5) M), the PD declined 1.35 +/- 0.21 mV and 1.41 +/- 0.28 mV, respectively. The results suggest that active C1- transport is mainly responsible for the PD, but that diffusion potentials can contribute.

Animals↗

Isolated perfused salamander proximal tubule. II. Monovalent ion replacement and rheogenic transport.

Early proximal tubules of the salamander kidney (Ambystoma tigrinum) were isolated and perfused in vitro. Transepithelial and basolateral electrical potential differences, transepithelial resistances, and intracellular ionic activities were measured during removal of Na+, K+, or Cl- from the lumen, the bath, or both lumen and bath. The effects of these external ionic replacements are interpreted in terms of an equivalent circuit that represents the renal epithelium as a network of passive ionic resistances, ionic diffusion potentials, and active transport current sources. Results indicate that rheogenic transport across the basolateral membrane is substantially diminished by removal of Na+ from either lumen or bath or by removal of K+ from the bath. On the other hand, bilateral chloride removal produces an increase in transepithelial resistance but almost no change in the calculated rate of basolateral rheogenic transport. This suggests that the source of the basolateral rheogenic ion flux ia a Na-K-ATPae that actively transports an excess of outward Na+ over inward K+.

Animals↗

CXCR4 regulates growth of both primary and metastatic breast cancer.

The chemokine receptor CXCR4 and its cognate ligand CXCL12 recently have been proposed to regulate the directional trafficking and invasion of breast cancer cells to sites of metastases. However, effects of CXCR4 on the growth of primary breast cancer tumors and established metastases and survival have not been determined. We used stable RNAi to reduce expression of CXCR4 in murine 4T1 cells, a highly metastatic mammary cancer cell line that is a model for stage IV human breast cancer. Using noninvasive bioluminescence and magnetic resonance imaging, we showed that knockdown of CXCR4 significantly limited the growth of orthotopically transplanted breast cancer cells. Mice in which parental 4T1 cells were implanted had progressively enlarging tumors that spontaneously metastasized, and these animals all died from metastatic disease. Remarkably, RNAi of CXCR4 prevented primary tumor formation in some mice, and all mice transplanted with CXCR RNAi cells survived without developing macroscopic metastases. To analyze effects of CXCR4 on metastases to the lung, an organ commonly affected by metastatic breast cancer, we injected tumor cells intravenously and monitored cell growth with bioluminescence imaging. Inhibiting CXCR4 with RNAi, or the specific antagonist AMD3100, substantially delayed the growth of 4T1 cells in the lung, although neither RNAi nor AMD3100 prolonged overall survival in mice with experimental lung metastases. These data indicate that CXCR4 is required to initiate proliferation and/or promote survival of breast cancer cells in vivo and suggest that CXCR4 inhibitors will improve treatment of patients with primary and metastatic breast cancer.

Animals↗

Role of the CXCL12/CXCR4 axis in peritoneal carcinomatosis of gastric cancer.

Peritoneal carcinomatosis is a frequent cause of death in patients with advanced gastric carcinoma. Because chemokines are now considered to play an important role in the metastasis of various malignancies, we hypothesized that they may be involved in the development of peritoneal carcinomatosis by gastric carcinoma. Human gastric carcinoma cell lines, which were all highly efficient in generating malignant ascites in nude mice upon i.p. inoculation, selectively expressed CXCR4 mRNA and protein. In particular, NUGC4 cells expressed CXCR4 mRNA at high levels and showed vigorous migratory responses to its ligand CXCL12. CXCL12 enhanced proliferation and rapid increases in phosphorylation of protein kinase B/Akt and extracellular signal-regulated kinase of NUGC4 cells. We also showed that AMD3100 (a specific CXCR4 antagonist) effectively reduced tumor growth and ascitic fluid formation in nude mice inoculated with NUGC4 cells. Additionally, we examined human clinical samples. Malignant ascitic fluids from patients with peritoneal carcinomatosis contained high concentrations of CXCL12 (4.67 ng/mL). Moreover, immunohistochemical analysis showed that 22 of 33 primary gastric tumors with peritoneal metastasis were positive for CXCR4 expression (67%), whereas only 4 of 16 with other distant metastasis were positive (25%). Notably, 22 of 26 CXCR4-expressing primary tumors developed peritoneal metastases (85%). CXCR4 positivity of primary gastric carcinomas significantly correlated with the development of peritoneal carcinomatosis (P < 0.001). Collectively, our results strongly suggest that the CXCR4/CXC12 axis plays an important role in the development of peritoneal carcinomatosis from gastric carcinoma. Thus, CXCR4 may be a potential therapeutic target for peritoneal carcinomatosis of gastric carcinoma.

Animals↗