Transfer of host T-cell membrane HLA-DR and CD25 to target cells by human retroviruses

Analysis of human immunodeficiency virus cytopathicity by using a new method for quantitating viral dynamics in cell culture

Human immunodeficiency virus (HIV) causes complex metabolic changes in infected CD4(+) T cells that lead to cell cycle arrest and cell death by necrosis. To study the viral functions responsible for deleterious effects on the host cell, we quantitated the course of HIV type 1 infection in tissue cultures by using flow cytometry for a virally encoded marker protein, heat-stable antigen (HSA). We found that HSA appeared on the surface of the target cells in two phases: passive acquisition due to association and fusion of virions with target cells, followed by active protein expression from transcription of the integrated provirus. The latter event was necessary for decreased target cell viability. We developed a general mathematical model of viral dynamics in vitro in terms of three effective time-dependent rates: those of cell proliferation, infection, and death. Using this model we show that the predominant contribution to the depletion of viable target cells results from direct cell death rather than cell cycle blockade. This allows us to derive accurate bounds on the time-dependent death rates of infected cells. We infer that the death rate of HIV-infected cells is 80 times greater than that of uninfected cells and that the elimination of the vpr protein reduces the death rate by half. Our approach provides a general method for estimating time-dependent death rates that can be applied to study the dynamics of other viruses.

Potential roles of cellular proteins in HIV-1

Human immunodeficiency virus type-1 particles contain host proteins, both on their surface and interior. This review summarises the cellular proteins found in these virions and covers some of their potential roles in the viral life cycle and pathogenesis. For most proteins studied, their role and function are either unknown or in the hypothesis stage. This reflects the relatively recent emphasis given to these proteins by the HIV-1 field as well as the incomplete understanding of their function in the cell. The study of cellular proteins in HIV-1 promises to help us better understand the interaction of this virus with the cell, the immune system, and the whole human host as well as to shed light on the nature of AIDS and suggest more targets for therapeutic intervention. Finally, many of the cell systems themselves are still poorly understood. The extensive study of HIV-1 has already brought increased attention to the fields of immunology and vaccine science and, in the same way, might assist our understanding of the cellular pathways themselves.

HIV-1 gag polyprotein rescues HLA-DR intracellular transport in a human CD4+ cell line

Major histocompatibility complex class II HLA-DR molecules are plasma-membrane integral heterodimers, constitutively expressed in antigen-presenting cells. Their expression is known to be upregulated in peripheral T lymphocytes upon cell activation and to be constitutive in T cell lines. In H78-C10.0, a subclone of the human CD4+ T cell line HUT-78, the transport of MHC class II HLA-DR molecules is blocked, resulting in their localization within internal vesicular compartments rather than at the cell surface. In this article, we show that HIV-1(HX10) infection of H78-C10.0 cells induces HLA-DR surface expression. Moreover, the produced infectious viruses harbor the heterodimer molecules in their envelopes. To define which of the viral proteins was involved in this phenomenon, we infected H78-C10.0 cells with recombinant vaccinia vectors containing either the gag-pro coding sequence or the entire env gene. Only gag expression was able to induce HLA-DR cell-surface localization in H78-C10.0 cells. RT-PCR analysis of the infected cells revealed no significant alteration in the amount of HLA-DRalpha-specific RNA compared to untreated cells. This implies that Gag acts on downstream events. When the env viral gene, coding for the precursor glycoprotein gp160, was expressed in H78-C10.0, the Env protein targeted to the cell surface was poorly processed to its final mature forms gp120 and gp41. However, coexpression of the env and gag genes led to restoration of this phenotype. Although the mechanism is unknown, the data compiled in this study strongly suggest that the viral Gag protein can interact with the cellular trafficking apparatus. Moreover, in a specific cell type as H78-C10.0 this interaction can even reverse intracellular transport defects.

Cellular factors influence the binding of HIV type 1 to cells

The goal of this study was to determine the importance of cellular factors for binding of HIV to cells. HIV primary isolates (PIs) produced in peripheral blood mononuclear cells (PBMCs) bound at relatively high levels to PBMCs but at low levels to cell lines, whereas T cell line-adapted (TCLA) virus produced in the H9 T cell line bound at high levels to both cell lines and PBMCs. Expression of CD4 in CD4-negative cells or blocking CD4 with antibody on CD4-positive cells did not affect virus binding. Blocking of gp120/gp41 with antibodies or a lack of expression of gp120/gp41 in virus particles also did not affect virus binding. However, the cell type from which virus was produced did affect virus binding. Thus, the binding pattern of TCLA virus shifted to that of a PI virus when produced in PBMCs. A PI binding pattern also occurred when a cloned TCLA virus (NL4-3) was produced in PBMCs, indicating that the virus-producing cell type has more of an effect on virus binding than the virus strain. These experiments show that both the virus-producing cell and the target cell have a major influence on HIV binding and suggest that host cell factors incorporated into virions are important for virus binding.

Expression of MHC class II in T cells is associated with increased HIV-1 expression

HIV-1 replicates in activated T cells at significantly higher levels than in resting cells. Thus, certain molecules up-regulated during T cell activation appear to be important for HIV-1 replication. In this study, we present evidence suggesting that expression of MHC class II (class II) molecules on CD4+ T cells facilitate HIV-1 replication. T cells that expressed class II supported greater virus replication than T cells lacking class II. The class II+ cells, when either infected with HIV-1 or transfected with an env-minus HIV-1 provirus plasmid, produced 10-20-fold greater virus expression than class II- cells. Anti-class II antibody markedly inhibited virus expression in class II+ cells (but not class II- cells) and also decreased the nuclear binding activity of AP-1, an inducible transcription factor important in T cell activation and HIV-1 expression. Most importantly, the induction of class II expression by transfection of the MHC class II transactivator (CIITA) stimulated HIV-1 replication in Jurkat T cells. Taken together, these data suggest that expression of MHC class II molecules and/or CIITA in T cells enhances HIV-1 transcription.

Evidence for budding of human immunodeficiency virus type 1 selectively from glycolipid-enriched membrane lipid rafts

A number of recent studies have demonstrated the significance of detergent-insoluble, glycolipid-enriched membrane domains or lipid rafts, especially in regard to activation and signaling in T lymphocytes. These domains can be viewed as floating rafts composed of sphingolipids and cholesterol which sequester glycosylphosphatidylinositol (GPI)-linked proteins, such as Thy-1 and CD59. CD45, a 200-kDa transmembrane phosphatase protein, is excluded from these domains. We have found that human immunodeficiency virus type 1 (HIV-1) particles produced by infected T-cell lines acquire the GPI-linked proteins Thy-1 and CD59, as well as the ganglioside GM1, which is known to partition preferentially into lipid rafts. In contrast, despite its high expression on the cell surface, CD45 was poorly incorporated into virus particles. Confocal fluorescence microscopy revealed that HIV-1 proteins colocalized with Thy-1, CD59, GM1, and a lipid raft-specific fluorescent lipid, DiIC(16)(3), in uropods of infected Jurkat cells. CD45 did not colocalize with HIV-1 proteins and was excluded from uropods. Dot immunoassay of Triton X-100-extracted membrane fractions revealed that HIV-1 p17 matrix protein and gp41 were present in the detergent-resistant fractions and that [(3)H]myristic acid-labeled HIV Gag showed a nine-to-one enrichment in lipid rafts. We propose a model for the budding of HIV virions through lipid rafts whereby host cell cholesterol, sphingolipids, and GPI-linked proteins within these domains are incorporated into the viral envelope, perhaps as a result of preferential sorting of HIV Gag to lipid rafts.

Increased infectivity of HIV type 1 particles bound to cell surface and solid-phase ICAM-1 and VCAM-1 through acquired adhesion molecules LFA-1 and VLA-4

HIV-1 incorporates a variety of host membrane proteins during budding. We have previously shown that adhesion molecules are acquired by the virus in their activated or functional states. Our studies and those of others indicate that adhesion molecules can have profound effects on virus infectivity and its resistance to neutralization by antiviral antibodies. In this study we have examined the effect on infectivity of immobilization or margination of HIV-1 through acquired integrins LFA-1 and VLA-4 onto nonsusceptible cells and solid-phase adhesion ligands (ICAM-1 and VCAM-1, respectively). LFA-1- and VLA-4-mediated HIV-1 binding was supported by ICAM-1 and VCAM-1 immunoglobulin Fc chimeras, respectively. Integrin-mediated HIV-1 binding was also supported by 293 cells transfected with ICAM-1. In both cases the specificity of binding was confirmed with the appropriate blocking monoclonal antibodies or soluble adhesion ligands. We used a sensitive single-cycle infection assay based on a cell line expressing an LTR-luciferase cDNA construct to compare the infectivity of bound virus with that of free virus. Our results show that the binding of HIV-1 to nonsusceptible cells or immobilized adhesion ligands through acquired integrins can increase its infectivity by as much as two orders of magnitude. These results have implications for in vivo dissemination and transmission of HIV-1 and may also explain the high level of virus replication seen in solid lymphoid organs.