CXCR4 engagement is required for HIV-1–induced L-selectin shedding

The Role of L-Selectin in HIV Infection

HIV envelope glycoprotein is the most heavily glycosylated viral protein complex identified with over 20 glycans on its surface. This glycan canopy is thought to primarily shield the virus from host immune recognition as glycans are poor immunogens in general, however rare HIV neutralizing antibodies nevertheless potently recognize the glycan epitopes. While CD4 and chemokine receptors have been known as viral entry receptor and coreceptor, for many years the role of viral glycans in HIV entry was controversial. Recently, we showed that HIV envelope glycan binds to L-selectin in solution and on CD4 T lymphocytes. The viral glycan and L-selectin interaction functions to facilitate the viral adhesion and entry. Upon entry, infected CD4 T lymphocytes are stimulated to progressively shed L-selectin and suppressing this lectin receptor shedding greatly reduced HIV viral release and caused aggregation of diminutive virus-like particles within experimental infections and from infected primary T lymphocytes derived from both viremic and aviremic individuals. As shedding of L-selectin is mediated by ADAM metalloproteinases downstream of host-cell stimulation, these findings showed a novel mechanism for HIV viral release and offer a potential new class of anti-HIV compounds.

HIV-1 targets L-selectin for adhesion and induces its shedding for viral release

CD4 and chemokine receptors mediate HIV-1 attachment and entry. They are, however, insufficient to explain the preferential viral infection of central memory T cells. Here, we identify L-selectin (CD62L) as a viral adhesion receptor on CD4⁺ T cells. The binding of viral envelope glycans to L-selectin facilitates HIV entry and infection, and L-selectin expression on central memory CD4⁺ T cells supports their preferential infection by HIV. Upon infection, the virus downregulates L-selectin expression through shedding, resulting in an apparent loss of central memory CD4⁺ T cells. Infected effector memory CD4⁺ T cells, however, remain competent in cytokine production. Surprisingly, inhibition of L-selectin shedding markedly reduces HIV-1 infection and suppresses viral release, suggesting that L-selectin shedding is required for HIV-1 release. These findings highlight a critical role for cell surface sheddase in HIV-1 pathogenesis and reveal new antiretroviral strategies based on small molecular inhibitors targeted at metalloproteinases for viral release.

HIV binding is mediated via CD4 and chemokine co-receptors, but this does not explain the preferential infection of central memory CD4+ T cells. Here the authors show HIV targets L-selectin, induces shedding from the infected cell, and inhibition of L-selectin reduces HIV infection and release.

Brief Report: L-Selectin (CD62L) Is Downregulated on CD4+ and CD8+ T Lymphocytes of HIV-1-Infected Individuals Naive for ART

The expression of L-selectin (CD62L) in HIV-1 infection has not been extensively investigated. Here, we measured CD62L expression on T-cell subsets of HIV-1-infected individuals naive for antiretroviral therapy (ART-naive) or receiving therapy (ART), and seronegative control subjects (HIV-neg). We found reduced frequencies of CD62L(+) cells among CD4(+) and CD8(+) T cells from ART-naive as compared with ART and HIV-neg groups, particularly within naive and central memory subsets. CD62L expression on T cells inversely correlated with viral load and rapidly increased after ART initiation. Plasma sCD62L levels did not correlate with CD62L expression, being higher in all HIV-1-infected individuals as compared with HIV-neg subjects. Finally, CD62L downregulation was found associated with the expression of the CD38 activation marker in CD8(+) T cells, but not in CD4(+) T cells. We suggest that CD62L downregulation due to unconstrained HIV-1 replication may have important consequences for T-cell circulation and function and for disease progression.

HIV-1 Nef and Vpu Interfere with L-Selectin (CD62L) Cell Surface Expression To Inhibit Adhesion and Signaling in Infected CD4+ T Lymphocytes

Leukocyte recirculation between blood and lymphoid tissues is required for the generation and maintenance of immune responses against pathogens and is crucially controlled by the L-selectin (CD62L) leukocyte homing receptor. CD62L has adhesion and signaling functions and initiates the capture and rolling on the vascular endothelium of cells entering peripheral lymph nodes. This study reveals that CD62L is strongly downregulated on primary CD4(+) T lymphocytes upon infection with human immunodeficiency virus type 1 (HIV-1). Reduced cell surface CD62L expression was attributable to the Nef and Vpu viral proteins and not due to increased shedding via matrix metalloproteases. Both Nef and Vpu associated with and sequestered CD62L in perinuclear compartments, thereby impeding CD62L transport to the plasma membrane. In addition, Nef decreased total CD62L protein levels. Importantly, infection with wild-type, but not Nef- and Vpu-deficient, HIV-1 inhibited the capacity of primary CD4(+) T lymphocytes to adhere to immobilized fibronectin in response to CD62L ligation. Moreover, HIV-1 infection impaired the signaling pathways and costimulatory signals triggered in primary CD4(+) T cells by CD62L ligation. We propose that HIV-1 dysregulates CD62L expression to interfere with the trafficking and activation of infected T cells. Altogether, this novel HIV-1 function could contribute to virus dissemination and evasion of host immune responses.

IMPORTANCE

L-selectin (CD62L) is an adhesion molecule that mediates the first steps of leukocyte homing to peripheral lymph nodes, thus crucially controlling the initiation and maintenance of immune responses to pathogens. Here, we report that CD62L is downmodulated on the surfaces of HIV-1-infected T cells through the activities of two viral proteins, Nef and Vpu, that prevent newly synthesized CD62L molecules from reaching the plasma membrane. We provide evidence that CD62L downregulation on HIV-1-infected primary T cells results in impaired adhesion and signaling functions upon CD62L triggering. Removal of cell surface CD62L may predictably keep HIV-1-infected cells away from lymph nodes, the privileged sites of both viral replication and immune response activation, with important consequences, such as systemic viral spread and evasion of host immune surveillance. Altogether, we propose that Nef- and Vpu-mediated subversion of CD62L function could represent a novel determinant of HIV-1 pathogenesis.

Suppression of Foxo1 activity and down-modulation of CD62L (L-selectin) in HIV-1 infected resting CD4 T cells

HIV-1 hijacks and disrupts many processes in the cells it infects in order to suppress antiviral immunity and to facilitate its replication. Resting CD4 T cells are important early targets of HIV-1 infection in which HIV-1 must overcome intrinsic barriers to viral replication. Although resting CD4 T cells are refractory to infection in vitro, local environmental factors within lymphoid and mucosal tissues such as cytokines facilitate viral replication while maintaining the resting state. These factors can be utilized in vitro to study HIV-1 replication in resting CD4 T cells. In vivo, the migration of resting naïve and central memory T cells into lymphoid tissues is dependent upon expression of CD62L (L-selectin), a receptor that is subsequently down-modulated following T cell activation. CD62L gene transcription is maintained in resting T cells by Foxo1 and KLF2, transcription factors that maintain T cell quiescence and which regulate additional cellular processes including survival, migration, and differentiation. Here we report that HIV-1 down-modulates CD62L in productively infected naïve and memory resting CD4 T cells while suppressing Foxo1 activity and the expression of KLF2 mRNA. Partial T cell activation was further evident as an increase in CD69 expression. Several other Foxo1- and KLF2-regulated mRNA were increased or decreased in productively infected CD4 T cells, including IL-7rα, Myc, CCR5, Fam65b, S1P₁ (EDG1), CD52, Cyclin D2 and p21^CIP1, indicating a profound reprogramming of these cells. The Foxo1 inhibitor AS1842856 accelerated de novo viral gene expression and the sequella of infection, supporting the notion that HIV-1 suppression of Foxo1 activity may be a strategy to promote replication in resting CD4 T cells. As Foxo1 is an investigative cancer therapy target, the development of Foxo1 interventions may assist the quest to specifically suppress or activate HIV-1 replication in vivo.

Specific loss of cellular L-selectin on CD4(+) T cells is associated with progressive multifocal leukoencephalopathy development during HIV infection

HIV(+) progressive multifocal leukoencephalopathy (PML) patients had a significantly lower expression of CD62L on CD4(+) T cells (P < 0.001) when compared with HIV(+) patients who did not develop PML. CD62L expression on CD4(+) T cells did not correlate with parameters such as CDC stage, CD4(+) cell percentage (of total CD3(+) T cells), CD4(+) cell counts, virus count, or clinical parameters. Measurement of CD62L might provide a biomarker for PML risk and could prompt a treatment change and/or close monitoring.

Glycoproteomic analysis identifies human glycoproteins secreted from HIV latently infected T cells and reveals their presence in HIV+ plasma

Glycoproteins secreted into plasma from T cells infected with human immunodeficiency virus (HIV) latent infection may provide insight into understanding the host response to HIV infection in vivo. Glycoproteomics, which evaluates the level of the glycoproteome, remains a novel approach to study this host response to HIV. In order to identify human glycoproteins secreted from T cells with latent HIV infection, the medium from cultured HIV replication-competent T cells was compared with the medium from cultured parental A3.01 cells via solid phase extraction of glycopeptides (SPEG) and high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Using these methods, 59 human glycoproteins were identified as having significantly different abundance levels between the media from these two cell lines. The relevance of these 59 proteins to HIV infection in vivo was assessed in plasma from HIV+ and HIV- subjects. Comparison between T cell and plasma revealed that six glycoproteins (galectin-3-binding protein, L-selectin, neogenin, adenosine deaminase CECR1, ICOS ligand and phospholipid transfer protein) were significantly elevated in the HIV+ T cells and plasma studies. These findings suggest that the response of T cells harboring latent HIV infection contributed, in part, to the glycoprotein changes in HIV+ plasma. These proteins, once validated, could provide insight into host-HIV interaction.

HIV envelope protein gp120-triggered CD4+ T-cell adhesion to vascular endothelium is regulated via CD4 and CXCR4 receptors

Activation of T-lymphocytes is an important component of inflammatory and infectious processes, including HIV infection. It is regulated via the actions of various cell-surface receptors, including CD4 and CXCR4. We examined the roles of CD4 and CXCR4 in the adhesive interaction of CD4+T-cells with the vascular endothelium. CD4+Jurkat cells were incubated in the presence or absence of anti-CD4 to stimulate CD4, or with SDF-1 alpha, a cognate ligand of CXCR4. Stimulation of CD4 or CXCR4 each significantly enhanced cell adhesion. We next stimulated the two receptors together, using gp120, a component of HIV. This enhanced cell adhesion was greater than stimulation of CD4 or CXCR4 individually. Western blotting revealed that stimulation of CXCR4 by SDF-1 alpha significantly increased the phosphorylation of ERK1/2 in Jurkat cells. Treatment with anti-CD4 also activated ERK1/2, although to a lesser extent. When the expression of CD4 was reduced by siRNA transfection, both CD4-dependent adhesion and MAPK activation were diminished. Furthermore, pre-treatment with fluvastatin, significantly attenuated observed Jurkat cell adhesion. These findings indicate novel mechanisms of CD4+ T-cells recruitment to activated endothelium via CD4 and CXCR4, which are modulated by statin.

Apoptosis induced in HIV-1-exposed, resting CD4+T cells subsequent to signaling through homing receptors is Fas/Fas ligand-mediated

The hallmark of HIV-1 disease is the gradual disappearance of CD4+ T cells from the blood. The mechanism of this depletion, however, is still unclear. Evidence suggests that lymphocytes die in lymph nodes, not in blood, and that uninfected bystander cells are the predominant cells dying. Our and others' previous studies showed that the lymph node homing receptor, CD62 ligand (CD62L), and Fas are up-regulated on resting CD4+ T cells after HIV-1 binding and that these cells home to lymph nodes at an enhanced rate. During the homing process, signals are induced through various homing receptors, which in turn, induced many of the cells to undergo apoptosis after they entered the lymph nodes. The purpose of this study was to determine how the homing process induces apoptosis in HIV-1-exposed, resting CD4+ T cells. We found that signaling through CD62L up-regulated FasL. This resulted in apoptosis of only HIV-1-presignaled, resting CD4+ T cells, not normal CD4+ T cells. This homing receptor-induced apoptosis could be blocked by anti-FasL antibodies or soluble Fas, demonstrating that the Fas-FasL interaction caused the apoptotic event.

HIV Type 1 Glycoprotein 120 Inhibits Human B Cell Chemotaxis to CXC Chemokine Ligand (CXCL) 12, CC Chemokine Ligand (CCL)20, and CCL21

We analyzed the modulation of human B cell chemotaxis by the gp120 proteins of various HIV-1 strains. X4 and X4/R5 gp120 inhibited B cell chemotaxis toward CXCL12, CCL20, and CCL21 by 40-50%, whereas R5 gp120 decreased inhibition by 20%. This gp120-induced inhibition was strictly dependent on CXCR4 or CCR5 and lipid rafts but not on CD4 or V(H)3-expressing BCR. Inhibition did not impair the expression or ligand-induced internalization of CCR6 and CCR7. Our data suggest that gp120/CXCR4 and gp120/CCR5 interactions lead to the cross-desensitization of CCR6 and CCR7 because gp120 does not bind CCR6 and CCR7. Unlike CXCL12, gp120 did not induce the activation of phospholipase Cbeta3 and PI3K downstream from CXCR4, whereas p38 MAPK activation was observed. Similar results were obtained if gp120-treated cells were triggered by CCL21 and CCL20. Our results are consistent with a blockade restricted to signaling pathways using phosphatidylinositol-4,5-bisphosphate as a substrate. X4 and X4/R5 gp120 induced the cleavage of CD62 ligand by a mechanism dependent on matrix metalloproteinase 1 and 3, CD4, CXCR4, Galpha(i), and p38 MAPK, whereas R5 gp120 did not. X4 and X4/R5 gp120 also induced the relocalization of cytoplasmic CD95 to the membrane and a 23% increase in CD95-mediated apoptosis. No such effects were observed with R5 gp120. The gp120-induced decrease in B cell chemotaxis and CD62 ligand expression, and increase in CD95-mediated B cell apoptosis probably have major deleterious effects on B cell responsiveness during HIV infection and in vaccination trials.

CXCR4-tropic HIV-1 envelope glycoprotein functions as a viral chemokine in unstimulated primary CD4+ T lymphocytes

Interaction of HIV-1 envelope glycoprotein gp120 with the chemokine receptor CXCR4 triggers not only viral entry but also an array of signal transduction cascades. Whether gp120 induces an incomplete or aberrant set of signals, or whether it can function as a full CXCR4 agonist, remains unclear. We report that, in unstimulated human primary CD4(+) T cells, the spectrum of signaling responses induced by gp120 through CXCR4 paralleled that induced by the natural ligand stromal cell-derived factor 1/CXCL12. gp120 activated heterotrimeric G proteins and the major G protein-dependent pathways, including calcium mobilization, phosphoinositide-3 kinase, and Erk-1/2 MAPK activation. Interestingly, gp120 caused rapid actin cytoskeleton rearrangements and profuse membrane ruffling, as evidenced by dynamic confocal imaging. This coordinated set of events resulted in a bona fide chemotactic response. Inactivated HIV-1 virions that harbored conformationally intact envelope glycoproteins also caused actin polymerization and chemotaxis, while similar virions devoid of envelope glycoproteins did not. Thus gp120, in monomeric as well as oligomeric, virion-associated form, elicited a complex cellular response that mimicked the effects of a chemokine. HIV-1 has therefore the capacity to dysregulate the vast CD4(+) T cell population that expresses CXCR4. In addition, HIV-1 may exploit its chemotactic properties to retain potential target cells and locally perturb their cytoskeleton, thereby facilitating viral transmission.