Control of SIV rebound through structured treatment interruptions during early infection

In a randomized controlled trial with acute simian immunodeficiency virus (SIV)-infected macaques, both highly active antiretroviral therapy (HAART) and HAART with fixed-schedule structured treatment interruption (STI-HAART; alternating 3 weeks on and 3 weeks off therapy) suppressed viral load. In the STI-HAART group, T cell virus-specific immune response (VIR) and control of viral rebound increased concurrently during subsequent interruptions. In contrast, VIR did not increase and SIV rebounded after permanent treatment withdrawal in all animals on continuous HAART. Fixed-schedule STI-HAART appears to be an effective alternative to continuous HAART for the early treatment of retroviral infection.     

Use of CD134 as a primary receptor by the feline immunodeficiency virus

Feline immunodeficiency virus (FIV) induces a disease similar to acquired immunodeficiency syndrome (AIDS) in cats, yet in contrast to human immunodeficiency virus (HIV), CD4 is not the viral receptor. We identified a primary receptor for FIV as CD134 (OX40), a T cell activation antigen and costimulatory molecule. CD134 expression promotes viral binding and renders cells permissive for viral entry, productive infection, and syncytium formation. Infection is CXCR4-dependent, analogous to infection with X4 strains of HIV. Thus, despite the evolutionary divergence of the feline and human lentiviruses, both viruses use receptors that target the virus to a subset of cells that are pivotal to the acquired immune response.     

Epitopes of the CD4 antigen and HIV infection

The CD4 (or T4) surface antigen of human T lymphocytes is an important part of the receptor for the human immunodeficiency virus (HIV). After binding to the receptor, the HIV may enter the T cell and induce the formation of syncytia. In an attempt to identify the receptor site more closely, monoclonal antibodies (Mab's) to CD4 were tested for their ability to block HIV infection in a syncytium formation assay, and the CD4 epitopes so identified were mapped by antibody cross-blocking. The antibodies that showed strong inhibition of HIV fell into two main families while a third group of Mab's blocked syncytia formation weakly or not at all. Several different isolates of HIV as well as the laboratory strain CBL1 grown in CEM cells were used to induce the syncytia. The data indicate that only some epitopes of CD4 are important for virus binding and imply that the virus-binding site for CD4 is conserved in different isolates of HIV with substantially divergent env gene sequences. Preliminary studies of patients suggest that polymorphism of these epitopes does not play a role in determining susceptibility to infection.     

The human immunodeficiency virus: infectivity and mechanisms of pathogenesis

Infection with the human immunodeficiency virus (HIV) results in a profound immunosuppression due predominantly to a selective depletion of helper/inducer T lymphocytes that express the receptor for the virus (the CD4 molecule). HIV also has tropism for the brain leading to neuropsychiatric abnormalities. Besides inducing cell death, HIV can interfere with T4 cell function by various mechanisms. The monocyte serves as a reservoir for HIV and is relatively refractory to its cytopathic effects. HIV can exist in a latent or chronic form which can be converted to a productive infection by a variety of inductive signals.     

CD8+ lymphocytes can control HIV infection in vitro by suppressing virus replication

Lymphocytes bearing the CD8 marker were shown to suppress replication of human immunodeficiency virus (HIV) in peripheral blood mononuclear cells. The effect was dose-dependent and most apparent with autologous lymphocytes; it did not appear to be mediated by a cytotoxic response. This suppression of HIV replication could be demonstrated by the addition of CD8+ cells at the initiation of virus production as well as after several weeks of virus replication by cultured cells. The observations suggest a potential approach to therapy in which autologous CD8 lymphocytes could be administered to individuals to inhibit HIV replication and perhaps progression of disease.     

Sexual transmission and propagation of SIV and HIV in resting and activated CD4+ T cells

In sexual transmission of simian immunodeficiency virus, and early and later stages of human immunodeficiency virus-type 1 (HIV-1) infection, both viruses were found to replicate predominantly in CD4(+) T cells at the portal of entry and in lymphoid tissues. Infection was propagated not only in activated and proliferating T cells but also, surprisingly, in resting T cells. The infected proliferating cells correspond to the short-lived population that produces the bulk of HIV-1. Most of the HIV-1-infected resting T cells persisted after antiretroviral therapy. Latently and chronically infected cells that may be derived from this population pose challenges to eradicating infection and developing an effective vaccine.     

Induction of CD4+ human cytolytic T cells specific for HIV-infected cells by a gp160 subunit vaccine

Cytolytic T lymphocyte (CTL) responses were evaluated in humans immunized with recombinant human immunodeficiency virus type 1 (HIV) envelope glycoprotein gp160. Some vaccinees had gp160-specific CTLs that were shown by cloning to be CD4+. Although induced by exogenous antigen, most gp160-specific CTL clones also recognized gp160 synthesized endogenously in target cells. These clones lysed autologous CD4+ T lymphoblasts infected with HIV. Of particular interest were certain vaccine-induced clones that lysed HIV-infected cells, recognized gp160 from diverse HIV isolates, and did not participate in "innocent bystander" killing of noninfected CD4+ T cells that had bound gp120.     

In vivo activity against HIV and favorable toxicity profile of 2',3'-dideoxyinosine

The purine analog 2',3'-dideoxyinosine (ddI), which has anti-retroviral activity in vitro was administered for up to 42 weeks to 26 patients with acquired immunodeficiency syndrome (AIDS) or severe AIDS-related complex (ARC). Ten of these individuals were AZT-intolerant. Eight dose regimens were studied. The drug was orally bioavailable and penetrated into the cerebrospinal fluid (CSF). Comparatively little evidence of an effect against human immunodeficiency virus (HIV) was seen at the lowest four doses. However, patients in the four highest dose groups (ddI at 1.6 milligrams per kilogram intravenously and then greater than or equal to 3.2 milligrams per kilogram orally at least every 12 hours or higher) had increases in their circulating CD4+ T cells (P less than 0.0005), increased CD4/CD8 T cell ratios (P less than 0.01), and, where evaluable, more than an 80% decrease in serum HIV p24 antigen (P less than 0.05). The patients also had evidence of improved immunologic function, had reduced viremic symptomatology, and gained a mean of 1.6 kilogram with these comparatively infrequent dosing schedules (every 8 or 12 hours). The most notable adverse effects directly attributable to ddI administration at the doses used in this study included increases in serum uric acid (due to hypoxanthine release) and mild headaches and insomnia. These results suggest that serious short-term toxicity at therapeutic doses is not an inherent feature in the profile of agents with clinical anti-HIV activity. Further controlled studies to define the safety and efficacy of this agent may be worth considering.     

Antiretroviral drugs for tuberculosis control in the era of HIV/AIDS

Human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) has dramatically increased the incidence of tuberculosis (TB) in subSaharan Africa, where up to 60% of TB patients are coinfected with HIV and each year 200,000 TB deaths are attributable to HIV coinfection. Now HIV threatens control of TB in Asia, Eastern Europe, and Latin America. Antiretroviral (ARV) drugs can prevent TB by preserving immunity, but cohort analysis shows that early therapy, plus high levels of coverage and compliance, will be needed to avert a significant fraction of TB cases. However, ARV drugs could enhance the treatment of TB, and TB programs provide an important entry point for the treatment of HIV/AIDS.     

Dextran sulfate suppression of viruses in the HIV family: inhibition of virion binding to CD4+ cells

The first step in the infection of human T lymphocytes by human immunodeficiency virus type 1 (HIV-1) is attachment to the target cell receptor, the CD4 antigen. This step may be vulnerable to attack by antibodies, chemicals, or small peptides. Dextran sulfate (molecular weight approximately 8000), which has been given to patients as an anticoagulant or antilipemic agent for more than two decades, was found to block the binding of virions to various target T lymphocytes, inhibit syncytia formation, and exert a potent inhibitory effect against HIV-1 in vitro at concentrations that may be clinically attainable in human beings. This drug also suppressed the replication of HIV-2 in vitro. These observations could have theoretical and clinical implications in the strategy to develop drugs against HIV types 1 and 2.     

Synergism between HIV gp120 and gp120-specific antibody in blocking human T cell activation

The human immunodeficiency virus (HIV) binds to CD4-positive cells through interaction of its envelope glycoprotein (gp120) with the CD4 molecule. CD4 is a prominent immunoregulatory molecule, and chronic exposure to antibody against CD4 (anti-CD4) has been shown to cause immunodeficiency in mice. T cell-dependent in vitro immune responses can also be inhibited by anti-CD4. Experimental findings reported here indicate that CD4-bound gp120 attracts gp120-specific antibodies derived from the blood of HIV-seropositive individuals to form a trimolecular complex with itself and CD4. Thus targeted to CD4, the gp120-specific antibody functions as an antibody to CD4; it cross-links and modulates the CD4 molecules and suppresses the activation of T cells as measured by mobilization of intracellular calcium (Ca2i+). The synergism between gp120 and anti-gp120 in blocking T cell activation occurs at low concentrations of both components. Neither gp120 nor anti-gp120 inhibits T cell activation by itself in the concentrations tested.     

HIV with reduced sensitivity to zidovudine (AZT) isolated during prolonged therapy

The drug sensitivities of human immunodeficiency virus (HIV) isolates from a group of patients with acquired immunodeficiency syndrome (AIDS) or AIDS-related complex (ARC) who were receiving zidovudine (3'-azido-3'-deoythymidine, AZT) therapy were tested by means of a newly developed plaque assay in CD4+ HeLa cells. Fifty percent inhibitory dose (ID50) values of 18 isolates from untreated individuals ranged between 0.01 microM and 0.05 microM. In contrast, most isolates from patients who had received zidovudine for 6 months or more exhibited decreased sensitivity characterized by changes in ID50 or ID95 values (or both), with isolates from several patients (5/15) showing 100-fold increases in ID50. The latter isolates were also insensitive to 3'-azido-2',3'-dideoxyuridine; however, the isolates were still sensitive to 2',3'-dideoxycytidine, 2',3'-dideoxy-2',3'-didehydrothymidine, or phosphonoformate. It cannot be determined from this small sample of patients whether development of a less sensitive virus phenotype results in clinical resistance. Appearance of such variants was not associated with a consistent increase in viral p24 concentrations in patient plasma and did not herald any sudden deterioration in clinical status. More extensive studies are required to determine the clinical significance. Thus, it would be premature to alter any treatment protocols for HIV-infected individuals at present.     

Alterations in T4 (CD4) protein and mRNA synthesis in cells infected with HIV

Cells infected with the human immunodeficiency virus (HIV) show decreased expression of the 58-kilodalton T4 (CD4) antigen on their surface. In this study, the effect of HIV infection on the synthesis of T4 messenger RNA (mRNA) and protein products was evaluated in T-cell lines. Metabolically labeled lysates from the T4+ cell line Sup-T1 were immunoprecipitated with monoclonal antibodies to T4. Compared with uninfected cells, HIV-infected Sup-T1 cells showed decreased amounts of T4 that coprecipitated with both the 120-kilodalton viral envelope and the 150-kilodalton envelope precursor molecules. In four of five HIV-producing T-cell lines studied, the steady-state levels of T4 mRNA were also reduced. Thus, the decreased T4 antigen on HIV-infected cells is due to at least three factors: reduced steady-state levels of T4-specific mRNA, reduced amounts of immunoprecipitable T4 antigen, and the complexing of available T4 antigen with viral envelope gene products. The data suggested that the T4 protein produced after infection may be complexed with viral envelope gene products within infected cells. Retroviral envelope-receptor complexes may thus participate in a general mechanism by which receptors for retroviruses are down-modulated and alterations in cellular function develop after infection.     

AIDS. Origins of HIV

Establishing the date when immunodeficiency viruses were transmitted from nonhuman primates to humans should provide a clue to the origin of the AIDS pandemic. In his Perspective, Hillis discusses an important analysis of the molecular divergence of SIV and HIV genes (Korber et al.). This analysis establishes 1931 as the date of origin of the HIV-1 M-group viruses (the principal cause of the AIDS pandemic). Hillis discusses three possible hypotheses of immunodeficiency virus transmission that are consistent with this date.     

Human CD4 binds immunoglobulins

T cell glycoprotein CD4 binds to class II major histocompatibility molecules and to the human immunodeficiency virus (HIV) envelope protein gp120. Recombinant CD4 (rCD4) bound to polyclonal immunoglobulin (Ig) and 39 of 50 (78%) human myeloma proteins. This binding depended on the Fab and not the Fc portion of Ig and was independent of the light chain. Soluble rCD4, HIV gp120, and sulfated dextrans inhibited the CD4-Ig interaction. With the use of a panel of synthetic peptides, the region critical for binding to Ig was localized to amino acids 21 to 38 of the first extracellular domain of CD4. CD4-bound antibody (Ab) complexed with antigen approximately 100 times better than Ab alone. This activity may contribute to the Ab-mediated enhancement of cellular HIV interaction that appears to depend on a trimolecular complex of HIV, antibodies to gp120, and CD4.     

The reservoir for HIV-1 in human peripheral blood is a T cell that maintains expression of CD4

Human immunodeficiency virus type 1 (HIV-1) selectively infects cells expressing the CD4 molecule, resulting in substantial quantitative and qualitative defects in CD4+ T lymphocyte function in patients with acquired immunodeficiency syndrome (AIDS). However, only a very small number of cells in the peripheral blood of HIV-1-infected individuals are expressing virus at any given time. Previous studies have demonstrated that in vitro infection of CD4+ T cells with HIV-1 results in downregulation of CD4 expression such that CD4 protein is no longer detectable on the surface of the infected cells. In the present study, highly purified subpopulations of peripheral blood mononuclear cells (PBMCs) from AIDS patients were obtained and purified by fluorescence-automated cell sorting. They were examined with the methodologies of virus isolation by limiting dilution analysis, in situ hybridization, immunofluorescence, and gene amplification. Within PBMCs, HIV-1 was expressed in vivo predominantly in the T cell subpopulation which, in contrast to the in vitro observations, continued to express CD4. The precursor frequency of these HIV-1-expressing cells was about 1/1000 CD4+ T cells. The CD4+ T cell population contained HIV-1 DNA in all HIV-1-infected individuals studied and the frequency in AIDS patients was at least 1/100 cells. This high level of infection may be the primary cause for the progressive decline in number and function of CD4+ T cells in patients with AIDS.     

Second conserved domain of gp120 is important for HIV infectivity and antibody neutralization

Rabbit antisera were raised against three overlapping synthetic peptides with sequence homology to the second conserved domain of the external envelope glycoprotein (gp120) of the human immunodeficiency virus (HIV). All of the antisera immunoprecipitated the envelope glycoprotein. In particular, an antiserum directed against amino acids 254 to 274 of env was efficient in neutralizing three different isolates of HIV in vitro, without affecting the binding of the virus to CD4-positive cells. Therefore, this conserved region of gp120 appears to be critical in a postbinding event during virus penetration and may represent a target for antibody neutralization of HIV. These findings may be applicable in the design of a vaccine for the acquired immunodeficiency syndrome.     

The Fc and not CD4 receptor mediates antibody enhancement of HIV infection in human cells

Antibodies that enhance human immunodeficiency virus (HIV) infectivity have been found in the blood of infected individuals and in infected or immunized animals. These findings raise serious concern for the development of a safe vaccine against acquired immunodeficiency syndrome. To address the in vivo relevance and mechanism of this phenomenon, antibody-dependent enhancement of HIV infectivity in peripheral blood macrophages, lymphocytes, and human fibroblastoid cells was studied. Neither Leu3a, a monoclonal antibody directed against the CD4 receptor, nor soluble recombinant CD4 even at high concentrations prevented this enhancement. The addition of monoclonal antibody to the Fc receptor III (anti-FcRIII), but not of antibodies that react with FcRI or FcRII, inhibited HIV type 1 and HIV type 2 enhancement in peripheral blood macrophages. Although enhancement of HIV infection in CD4+ lymphocytes could not be blocked by anti-FcRIII, it was inhibited by the addition of human immunoglobulin G aggregates. The results indicate that the FcRIII receptor on human macrophages and possibly another Fc receptor on human CD4+ lymphocytes mediate antibody-dependent enhancement of HIV infectivity and that this phenomenon proceeds through a mechanism independent of the CD4 protein.     

HIV-1-infected T cells show a selective signaling defect after perturbation of CD3/antigen receptor

The binding of antigen or monoclonal antibody to the T cell receptor for antigen or the closely associated CD3 complex causes increases in the concentration of intracellular ionized calcium and subsequent cell proliferation. By measuring second messenger production in primary cultures of human immunodeficiency virus (HIV-1)--infected T cells stimulated with monoclonal antibodies specific for either CD3 or CD2, a specific impairment of membrane signaling was revealed. The HIV-1--infected T cells were unable to mobilize Ca2+ after stimulation with anti-CD3, whereas CD2-induced calcium mobilization remained intact. Furthermore, the HIV-1--infected cells proliferated poorly after CD3 stimulation, although the cells retained normal DNA synthesis in response to interleukin-2 stimulation. These results show that the signals initiated by CD2 and CD3 can be regulated independently within the same T cell; uncoupling of signal transduction after antigen-specific stimulation provides a biochemical mechanism to explain, in part, the profound immunodeficiency of patients with HIV-1 infection.