Baseline resistance of primary human immunodeficiency virus type 1 strains to the CXCR4 inhibitor AMD3100

TCR-CXCR4 signaling stabilizes cytokine mRNA transcripts via a PREX1-Rac1 pathway: implications for CTCL

As with many immunopathologically driven diseases, the malignant T cells of cutaneous T-cell lymphomas (CTCLs), such as Sézary syndrome, display aberrant cytokine secretion patterns that contribute to pathology and disease progression. Targeting this disordered release of cytokines is complicated by the changing cytokine milieu that drives the phenotypic changes of CTCLs. Here, we characterize a novel signaling pathway that can be targeted to inhibit the secretion of cytokines by modulating either CXCR4 or CXCR4-mediated signaling. We demonstrate that upon ligation of the T-cell antigen receptor (TCR), the TCR associates with and transactivates CXCR4 via phosphorylation of S339-CXCR4 in order to activate a PREX1-Rac1-signaling pathway that stabilizes interleukin-2(IL-2), IL-4, and IL-10 messenger RNA (mRNA) transcripts. Pharmacologic inhibition of either TCR-CXCR4 complex formation or PREX1-Rac1 signaling in primary human T cells decreased mRNA stability and inhibited secretion of IL-2, IL-4, and IL-10. Applying this knowledge to Sézary syndrome, we demonstrate that targeting various aspects of this signaling pathway blocks both TCR-dependent and TCR-independent cytokine secretion from a Sézary syndrome-derived cell line and patient isolates. Together, these results identify multiple aspects of a novel TCR-CXCR4-signaling pathway that could be targeted to inhibit the aberrant cytokine secretion that drives the immunopathogenesis of Sézary syndrome and other immunopathological diseases.

Macrophage Migration Inhibitory Factor-CXCR4 Receptor Interactions: EVIDENCE FOR PARTIAL ALLOSTERIC AGONISM IN COMPARISON WITH CXCL12 CHEMOKINE

An emerging number of non-chemokine mediators are found to bind to classical chemokine receptors and to elicit critical biological responses. Macrophage migration inhibitory factor (MIF) is an inflammatory cytokine that exhibits chemokine-like activities through non-cognate interactions with the chemokine receptors CXCR2 and CXCR4, in addition to activating the type II receptor CD74. Activation of the MIF-CXCR2 and -CXCR4 axes promotes leukocyte recruitment, mediating the exacerbating role of MIF in atherosclerosis and contributing to the wealth of other MIF biological activities. Although the structural basis of the MIF-CXCR2 interaction has been well studied and was found to engage a pseudo-ELR and an N-like loop motif, nothing is known about the regions of CXCR4 and MIF that are involved in binding to each other. Using a genetic strain of Saccharomyces cerevisiae that expresses a functional CXCR4 receptor, site-specific mutagenesis, hybrid CXCR3/CXCR4 receptors, pharmacological reagents, peptide array analysis, chemotaxis, fluorescence spectroscopy, and circular dichroism, we provide novel molecular information about the structural elements that govern the interaction between MIF and CXCR4. The data identify similarities with classical chemokine-receptor interactions but also provide evidence for a partial allosteric agonist compared with CXCL12 that is possible due to the two binding sites of CXCR4.

Resistance against inhibitors of HIV-1 entry into target cells

ABSTRACT 

HIV resistance against currently approved entry inhibitors, the chemokine receptor-5 (CCR5) antagonist maraviroc and the fusion inhibitor enfuvirtide (T-20), manifests in a complex manner that is distinct from the resistance patterns against other classes of antiretroviral drugs. Several attachment and fusion inhibitors are currently under various stages of development. Whereas CCR5 co-receptor antagonists have been widely studied until now, because patients who lack CCR5 are healthy and protected to some extent from HIV-infection, CXCR4-antagonist development has been slower, due to limited antiviral activity and potential toxicity given that CXCR4 may have essential cellular functions. Novel fusion inhibitor development is focusing on orally available small-molecule inhibitors that might replace T-20, which needs to be administered by subcutaneous injection.

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

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

Mechanism of HIV antiretroviral drugs progress toward drug resistance

The rapid replication rate of HIV-1 RNA and its inherent genetic variation have led to the production of many HIV-1 variants with decreased drug susceptibility. The capacity of HIV to develop drug resistance mutations is a major obstacle to long-term effective anti-HIV therapy. Incomplete suppression of viral replication with an initial drug regimen diminishes the clinical benefit to the patient and may promote the development of broader drug resistance that may cause subsequent treatment regimens to be ineffective. The increased clinical use of combination antiretroviral treatment for HIV-1 infection has led to the selection of viral strains resistant to multiple drugs, including strains resistant to all licensed nucleoside analog RT inhibitors and protease inhibitors. Therefore, it is important to understand the influence of such mutations on viral properties such as replicative fitness, fidelity, and mutation rates. Although research continues to improve our understanding of resistance, leading to refined treatment strategies and, in some cases, improved outcome, resistance to antiretroviral therapy remains a major cause of treatment failure among patients living with HIV-1.

Phenotypic and immunologic comparison of clade B transmitted/founder and chronic HIV-1 envelope glycoproteins

Sexual transmission of human immunodeficiency virus type 1 (HIV-1) across mucosal barriers is responsible for the vast majority of new infections. This relatively inefficient process results in the transmission of a single transmitted/founder (T/F) virus, from a diverse viral swarm in the donor, in approximately 80% of cases. Here we compared the biological activities of 24 clade B T/F envelopes (Envs) with those from 17 chronic controls to determine whether the genetic bottleneck that occurs during transmission is linked to a particular Env phenotype. To maximize the likelihood of an intact mucosal barrier in the recipients and to enhance the sensitivity of detecting phenotypic differences, only T/F Envs from individuals infected with a single T/F variant were selected. Using pseudotyping to assess Env function in single-round infectivity assays, we compared coreceptor tropism, CCR5 utilization efficiencies, primary CD4(+) T cell subset tropism, dendritic cell trans-infections, fusion kinetics, and neutralization sensitivities. T/F and chronic Envs were phenotypically equivalent in most assays; however, T/F Envs were modestly more sensitive to CD4 binding site antibodies b12 and VRC01, as well as pooled human HIV Ig. This finding was independently validated with a panel of 14 additional chronic HIV-1 Env controls. Moreover, the enhanced neutralization sensitivity was associated with more efficient binding of b12 and VRC01 to T/F Env trimers. These data suggest that there are subtle but significant structural differences between T/F and chronic clade B Envs that may have implications for HIV-1 transmission and the design of effective vaccines.

Engineering HIV-resistant human CD4+ T cells with CXCR4-specific zinc-finger nucleases

HIV-1 entry requires the cell surface expression of CD4 and either the CCR5 or CXCR4 coreceptors on host cells. Individuals homozygous for the ccr5Δ32 polymorphism do not express CCR5 and are protected from infection by CCR5-tropic (R5) virus strains. As an approach to inactivating CCR5, we introduced CCR5-specific zinc-finger nucleases into human CD4+ T cells prior to adoptive transfer, but the need to protect cells from virus strains that use CXCR4 (X4) in place of or in addition to CCR5 (R5X4) remains. Here we describe engineering a pair of zinc finger nucleases that, when introduced into human T cells, efficiently disrupt cxcr4 by cleavage and error-prone non-homologous DNA end-joining. The resulting cells proliferated normally and were resistant to infection by X4-tropic HIV-1 strains. CXCR4 could also be inactivated in ccr5Δ32 CD4+ T cells, and we show that such cells were resistant to all strains of HIV-1 tested. Loss of CXCR4 also provided protection from X4 HIV-1 in a humanized mouse model, though this protection was lost over time due to the emergence of R5-tropic viral mutants. These data suggest that CXCR4-specific ZFNs may prove useful in establishing resistance to CXCR4-tropic HIV for autologous transplant in HIV-infected individuals.

Macrophage migration inhibitory factor is secreted by rhabdomyosarcoma cells, modulates tumor metastasis by binding to CXCR4 and CXCR7 receptors and inhibits recruitment of cancer-associated fibroblasts

The overexpression of macrophage migration inhibitory factor (MIF) has been observed in many tumors and is implicated in oncogenic transformation and tumor progression. MIF activates CXCR2 and CD74 receptors and, as recently reported, may also bind to the stromal-derived factor-1 (SDF-1)-binding receptor CXCR4. Here, we report that human rhabdomyosarcoma (RMS) cell lines secrete MIF and that this chemokine (a) induces phosphorylation of mitogen-activated protein kinase (MAPK) p42/44 and AKT, (b) stimulates RMS cell adhesion, (c) enhances tumor vascularization, but surprisingly (d) decreases recruitment of cancer-associated fibroblasts (CAF). Because RMS cells used in our studies do not express CXCR2 and CD74 receptors, the biological effects of MIF on RMS cells depend on its interaction with CXCR4, and as we report here for the first time, MIF may also engage another SDF-1-binding receptor (CXCR7) as well. Interestingly, downregulation of MIF in RMS cells inoculated into immunodeficient mice led to formation of larger tumors that displayed higher stromal cell support. Based on these observations, we postulate that MIF is an important autocrine/paracrine factor that stimulates both CXCR4 and CXCR7 receptors to enhance the adhesiveness of RMS cells. We also envision that when locally secreted by a growing tumor, MIF prevents responsiveness of RMS to chemoattractants secreted outside the growing tumor (e.g., SDF-1) and thereby prevents release of cells into the circulation. On the other hand, despite its obvious proangiopoietic effects, MIF inhibits in CXCR2/CD74-dependent manner recruitment of CAFs to the growing tumor. Our data indicate that therapeutic inhibition of MIF in RMS may accelerate metastasis and tumor growth.

Macrophage migration inhibitory factor is secreted by rhabdomyosarcoma cells, modulates tumor metastasis by binding to CXCR4 and CXCR7 receptors and inhibits recruitment of cancer-associated fibroblasts

The overexpression of macrophage migration inhibitory factor (MIF) has been observed in many tumors and is implicated in oncogenic transformation and tumor progression. MIF activates CXCR2 and CD74 receptors and, as recently reported, may also bind to the stromal-derived factor-1 (SDF-1)-binding receptor CXCR4. Here, we report that human rhabdomyosarcoma (RMS) cell lines secrete MIF and that this chemokine (a) induces phosphorylation of mitogen-activated protein kinase (MAPK) p42/44 and AKT, (b) stimulates RMS cell adhesion, (c) enhances tumor vascularization, but surprisingly (d) decreases recruitment of cancer-associated fibroblasts (CAF). Because RMS cells used in our studies do not express CXCR2 and CD74 receptors, the biological effects of MIF on RMS cells depend on its interaction with CXCR4, and as we report here for the first time, MIF may also engage another SDF-1-binding receptor (CXCR7) as well. Interestingly, downregulation of MIF in RMS cells inoculated into immunodeficient mice led to formation of larger tumors that displayed higher stromal cell support. Based on these observations, we postulate that MIF is an important autocrine/paracrine factor that stimulates both CXCR4 and CXCR7 receptors to enhance the adhesiveness of RMS cells. We also envision that when locally secreted by a growing tumor, MIF prevents responsiveness of RMS to chemoattractants secreted outside the growing tumor (e.g., SDF-1) and thereby prevents release of cells into the circulation. On the other hand, despite its obvious proangiopoietic effects, MIF inhibits in CXCR2/CD74-dependent manner recruitment of CAFs to the growing tumor. Our data indicate that therapeutic inhibition of MIF in RMS may accelerate metastasis and tumor growth.

Stabilized HIV-1 envelope glycoprotein trimers lacking the V1V2 domain, obtained by virus evolution

The envelope glycoproteins (Env) are the focus of HIV-1 vaccine development strategies based on the induction of humoral immunity, but the mechanisms the virus has evolved to limit the induction and binding of neutralizing antibodies (NAbs) constitute substantial obstacles. Conserved neutralization epitopes are shielded by variable regions and carbohydrates, so one strategy to increase their exposure and, it is hoped, their immunogenicity is to delete the overlying variable loops. However, deleting the variable regions from Env trimers can be problematic, because hydrophobic patches that are normally solvent-inaccessible now become exposed, causing protein misfolding or aggregation, for example. Here, we describe the construction and characterization of recombinant gp140 trimers lacking variable domains 1 and 2 (ΔV1V2). The design of the trimers was guided by HIV-1 evolution studies that identified compensatory changes in V1V2-deleted but functional Env proteins (Bontjer, I., Land, A., Eggink, D., Verkade, E., Tuin, K., Baldwin, C., Pollakis, G., Paxton, W. A., Braakman, I., Berkhout, B., and Sanders, R. W. (2009) J. Virol. 83, 368-383). We now show that specific compensatory changes improved the function of ΔV1V2 Env proteins and hence HIV-1 replication. The changes acted by reducing the exposure of a hydrophobic surface either by replacing a hydrophobic residue with a hydrophilic one or by covering the surface with a glycan. The compensatory changes allowed the efficient expression of well folded, soluble gp140 trimers derived from various HIV-1 isolates. The evolved ΔV1V2 Env viruses were extremely sensitive to NAbs, indicating that neutralization epitopes are well exposed, which was confirmed by studies of NAb binding to the soluble ΔV1V2 gp140 trimers. These evolved ΔV1V2 trimers could be useful reagents for immunogenicity and structural studies.

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

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

The role of stromal-derived factor-1--CXCR7 axis in development and cancer

Cancer metastasis is a major clinical problem that contributes to unsuccessful therapy. Augmenting evidence indicates that metastasizing cancer cells employ several mechanisms that are involved in developmental trafficking of normal stem cells. Stromal-derived factor-1 (SDF-1) is an important alpha-chemokine that binds to the G-protein-coupled seven-transmembrane span CXCR4. The SDF-1-CXCR4 axis regulates trafficking of normal and malignant cells. SDF-1 is an important chemoattractant for a variety of cells including hematopoietic stem/progenitor cells. For many years, it was believed that CXCR4 was the only receptor for SDF-1. However, several reports recently provided evidence that SDF-1 also binds to another seven-transmembrane span receptor called CXCR7, sharing this receptor with another chemokine family member called Interferon-inducible T-cell chemoattractant (I-TAC). Thus, with CXCR7 identified as a new receptor for SDF-1, the role of the SDF-1-CXCR4 axis in regulating several biological processes becomes more complex. Based on the available literature, this review addresses the biological significance of SDF-1's interaction with CXCR7, which may act as a kind of decoy or signaling receptor depending on cell type. Augmenting evidence suggests that CXCR7 is involved in several aspects of tumorogenesis and could become an important target for new anti-metastatic and anti-cancer drugs.

Host cell factors in HIV replication: meta-analysis of genome-wide studies

We have analyzed host cell genes linked to HIV replication that were identified in nine genome-wide studies, including three independent siRNA screens. Overlaps among the siRNA screens were very modest (<7% for any pairwise combination), and similarly, only modest overlaps were seen in pairwise comparisons with other types of genome-wide studies. Combining all genes from the genome-wide studies together with genes reported in the literature to affect HIV yields 2,410 protein-coding genes, or fully 9.5% of all human genes (though of course some of these are false positive calls). Here we report an “encyclopedia” of all overlaps between studies (available at http://www.hostpathogen.org), which yielded a more extensively corroborated set of host factors assisting HIV replication. We used these genes to calculate refined networks that specify cellular subsystems recruited by HIV to assist in replication, and present additional analysis specifying host cell genes that are attractive as potential therapeutic targets.

Resistance to CCR5 inhibitors caused by sequence changes in the fusion peptide of HIV-1 gp41

We have investigated the mechanism of resistance of a HIV type 1 (HIV-1) R5 primary isolate, D1/85.16, to the small molecule CCR5 inhibitor, vicriviroc (VVC). Unlike other viruses resistant to this class of compound, D1/85.16 lacks sequence changes in the V3 region of the gp120 surface glycoprotein. Inspection of env sequences from D1/85.16 compared with those derived from the parental, inhibitor-sensitive virus, CC1/85, revealed a cluster of 3 conservative changes in the fusion peptide (FP) of the gp41 transmembrane glycoprotein that tracked with the resistance phenotype. Studies with engineered Env-chimeric and point-substituted viruses confirmed that these 3 FP residues were substantially responsible for VVC resistance without altering coreceptor usage, as assessed in both peripheral blood mononuclear cells and the TZM-bl cell line. VVC resistance is manifested differently in the 2 cell types, and there are assay-dependent complexities to the dose-response curves for the engineered resistant viruses. To explain them, we created a model for resistance and generated theoretical VVC inhibition curves that closely mimic the experimental data for the resistant viruses. The basis for the model is the existence of distinct forms of CCR5, with varying affinities for small molecule CCR5 inhibitors that are presumed to be present in different proportions on different cell types, and are used selectively by resistant HIV-1 variants when ligated with an inhibitor. Together, the experimental results and theoretical model may help understand how HIV-1 uses CCR5 to enter target cells under various conditions.