Hans S. L. M. Nottet

DC-SIGN, a Dendritic Cell–Specific HIV-1-Binding Protein that Enhances trans-Infection of T Cells

Dendritic cells (DC) capture microorganisms that enter peripheral mucosal tissues and then migrate to secondary lymphoid organs, where they present these in antigenic form to resting T cells and thus initiate adaptive immune responses. Here, we describe the properties of a DC-specific C-type lectin, DC-SIGN, that is highly expressed on DC present in mucosal tissues and binds to the HIV-1 envelope glycoprotein gp120. DC-SIGN does not function as a receptor for viral entry into DC but instead promotes efficient infection in trans of cells that express CD4 and chemokine receptors. We propose that DC-SIGN efficiently captures HIV-1 in the periphery and facilitates its transport to secondary lymphoid organs rich in T cells, to enhance infection in trans of these target cells.

Role of the pro-inflammatory cytokines TNF-alpha and IL-1beta in HIV-associated dementia.

Human immunodeficiency virus‐1 (HIV‐1)‐infected and immune‐activated macrophages and microglia secrete neurotoxins. Two of these neurotoxins are the pro‐inflammatory cytokines tumour necrosis factor‐α (TNF‐α) and interleukin‐1β (IL‐1β), which are thought to play a major role in inducing neuronal death. Both TNF‐α and IL‐1β increase the permeability of the blood–brain barrier, through which subsequently HIV‐infected monocytes can enter the brain. They both induce over‐stimulation of the NMDA‐receptor via several pathways, resulting in a lethal neuronal increase in Ca2+ levels. Additionally, TNF‐α co‐operates with several other proinflammatory mediators to enhance their toxic effects. Although most research has focused on the neurotoxic effects of TNF‐α and IL‐1β in HAD, there is also evidence that these cytokines can be neuroprotective. In this paper the effect of TNF‐α and IL‐1β on neuronal life and death in HAD is discussed.

Amyloid-β-induced chemokine production in primary human macrophages and astrocytes

In Alzheimers disease (AD), chemotaxis might be responsible for attracting glial cells towards the neuritic plaque. Using primary monocyte-derived macrophages and primary adult astrocytes as a model, amyloid-beta (Abeta) (1-42) was able to stimulate the production, as measured by RT-PCR, of MIP-1alpha and MIP-1beta mRNA in macrophages and MCP-1 in astrocytes. Cocultures showed in unstimulated as well as in Abeta-stimulated cells an increase in MIP-1alpha, MIP-1beta and MCP-1 mRNA. ELISAs of supernatant samples of stimulated macrophages and astrocytes also showed an increase in MIP-1alpha and MIP-1beta in macrophages and MCP-1 in astrocytes. Stimulated cocultures showed an increase in MIP-1alpha, MIP-1beta and MCP-1 protein levels in contrast to unstimulated cocultures.

Activation and Cell Cycle Antigens in CD4+ and CD8+ T Cells Correlate with Plasma Human Immunodeficiency Virus (HIV-1) RNA Level in HIV-1 Infection

The relationship between T cell activation and human immunodeficiency virus type 1 (HIV-1) replication was studied in HIV-infected subjects, 20 with and 10 without anti-HIV treatment. Expression of Ki-67 proliferation-associated antigen was increased in CD4+ and CD8+ T cells and correlated with HLA-DR. In subjects without anti-HIV treatment, the plasma HIV-1 RNA level correlated with HLA-DR in CD4+ T cells, with Ki-67 in CD8+ T cells, and with expression of CD38 in both T cell subsets. A proportion of treated subjects had increased T cell activation despite 4 months of highly active antiretroviral treatment (HAART). In subjects receiving HAART, a high percentage of HLA-DR+ CD4+ T cells was associated with signs of opportunistic infections. This work supports the concept that, in the natural course of HIV-1 infection, HIV replication itself leads to general T cell activation and that opportunistic infections generate additional CD4+ T cell activation and HIV replication.

Unraveling the neuroimmune mechanisms for the HIV-1-associated cognitive/motor complex

Infection of the brain with human immunodeficiency virus 1 (HIV-1) often leads to the devastating loss of mental faculties. Surprisingly, HIV-1 elicits such brain dysfunction without significantly infecting neurons, astrocytes and oligodendroglia. The target for HIV-1 in the brain is the macrophage, which usually functions as a phagocytic, antigen-presenting and immune-regulatory cell. How can these cells produce such serious cognitive and motor brain impairments? Here, Hans Nottet and Howard Gendelman propose that HIV-1 penetrates the blood-brain barrier inside differentiating macrophages, which become immune activated once inside the brain, and secrete high levels of neurotoxins. Chronic, subclinical disease results by astrocyte regulation of macrophage effector functions. Ultimately, endogenous control mechanisms break down, leading to motor and mental impairments in some affected subjects.

Multiple effects of HIV‐1 trans‐activator protein on the pathogenesis of HIV‐1 infection

The HIV‐1 trans‐activator (Tat) protein is proposed as an important factor in the complex HIV‐induced pathogenesis of AIDS. In this paper, multiple effects of this viral protein are described. Originally discovered as an intracellular activator of HIV‐1 transcription, Tat was found to regulate viral reverse transcription as well. Trans‐activator was found to be secreted by HIV‐infected cells and taken up by neighbouring cells. In this way, Tat is able to affect both infected and uninfected cells. Intracellularly, Tat can deregulate the expression of several heterologous cellular and viral genes. Extracellular Tat can contribute to the spreading of HIV‐1 and immunosuppression of uninfected cells. Finally, there is evidence that exogenous Tat is involved in AIDS‐associated pathologies such as Kaposis sarcoma and HIV‐associated dementia. These capacities together accelerate the progression towards AIDS and make Tat an interesting candidate as a constituent of an anti‐AIDS vaccine.

Cytokine-Stimulated, But Not HIV-Infected, Human Monocyte-Derived Macrophages Produce Neurotoxic Levels of l-Cysteine

Approximately one-quarter of individuals with AIDS develop neuropathological symptoms that are attributable to infection of the brain with HIV. The cognitive manifestations have been termed HIV-associated dementia. The mechanisms underlying HIV-associated neuronal injury are incompletely understood, but various studies have confirmed the release of neurotoxins by macrophages/microglia infected with HIV-1 or stimulated by viral proteins, including the envelope glycoprotein gp120. In the present study, we investigated the possibility that l-cysteine, a neurotoxin acting at the N-methyl-d-aspartate subtype of glutamate receptor, could contribute to HIV-associated neuronal injury. Picomolar concentrations of gp120 were found to stimulate cysteine release from human monocyte-derived macrophages (hMDM) in amounts sufficient to injure cultured rat cerebrocortical neurons. TNF-α and IL-1β, known to be increased in HIV-encephalitic brains, as well as a cellular product of cytokine stimulation, ceramide, were also shown to induce release of cysteine from hMDM in a dose-dependent manner. A TNF-α-neutralizing Ab and an IL-1βR antagonist partially blocked gp120-induced cysteine release, suggesting that these cytokines may mediate the actions of gp120. Interestingly, hMDM infected with HIV-1 produced significantly less cysteine than uninfected cells following stimulation with TNF-α. Our findings imply that cysteine may play a role in the pathogenesis of neuronal injury in HIV-associated dementia due to its release from immune-activated macrophages but not virus-infected macrophages. Such uninfected cells comprise the vast majority of mononuclear phagocytes (macrophages and microglia) found in HIV-encephalitic brains.

Interactions between macrophages and brain microvascular endothelial cells: role in pathogenesis of HIV-1 infection and blood - brain barrier function

Monocytes have been shown to infiltrate in brain tissue during various neurological disorders including AIDS dementia complex. The presence of an excess of activated macrophages in brain tissue is accompanied by tissue damage resulting in a loss in neuronal function and viability. Therapeutic options against such neurological disorders could therefore be aimed at the prevention of monocyte infiltration across the blood - brain barrier. Therefore, a better understanding of these processes is needed. Recent insights in cellular processes between monocytes/macrophages and brain microvascular endothelial cells in the neuropathogenesis of HIV-1 infection demonstrate that monocytes roll on endothelial cells via the inducible endothelial adhesion molecule E-selectin. Binding of these cells are mainly mediated via the endothelial adhesion molecule vascular cell adhesion molecule-1. The transmigration through the blood - brain barrier is facilitated by both endothelial and monocyte/macrophage-derived nitric oxide and by the increased production of gelatinase B activity by HIV-infected monocytes/macrophages. Chemokines produced within the brain regulate the traffic of the infiltrating monocytes through the brain parenchyma. In addition, endothelial cells also produce monocyte attracting chemokines during their first interactions with HIV-infected monocytes/macrophages thus promoting additional influx of phagocytes into the brain. Furthermore, excessive infiltration of monocytes is accompanied by endothelial damage resulting in the loss of tight junctions. Thus, in toto, brain microvascular endothelial cells might contribute to the neuropathogenesis of HIV-1 infection.

Inhibition of Human Immunodeficiency Virus Type 1 Replication in Human Mononuclear Blood Cells by the Iron Chelators Deferoxamine, Deferiprone, and Bleomycin

Replication of human immunodeficiency virus type 1 (HIV-1) can be influenced by iron. Hence, decreasing the availability of iron may inhibit HIV-1 replication. Deferoxamine and deferiprone, both forming catalytically inactive iron-chelator complexes, and bleomycin, by use of which iron catalyzes oxidative nucleic acid destruction, were investigated. Expression of p24 antigen in human monocyte-derived macrophages and peripheral blood lymphocytes (PBL) was reduced by all 3 iron chelators. In PBL, p24 reduction was mirrored by a decrease in proliferation after incubation with deferoxamine or deferiprone, suggesting that viral inhibition is closely linked to a decrease in cellular proliferation. In contrast, clinically relevant bleomycin concentrations reduced p24 levels by approximately 50% without affecting proliferation. When deferoxamine and the nucleoside analogue dideoxyinosine were used in combination, they acted synergistically in inhibiting HIV-1 replication. These observations suggest that iron chelators with different mechanisms of action could be of additional benefit in antiretroviral combination therapy.

Interactions of human immunodeficiency virus-1 proteins with neurons : possible role in the development of human immunodeficiency virus-1-associated dementia

Human immunodeficiency virus‐1 (HIV‐1)‐associated dementia is a severe neurological complication of HIV‐1 infection that affects 15–20% of the patients in the late stages of acquired immunodeficiency syndrome. HIV‐1‐associated dementia is most probably a consequence of HIV‐1 infection of the brain rather than of an opportunistic pathogen. The exact mechanism by which the virus causes this disorder, however, is not completely understood. A number of HIV‐1 proteins have been shown to be released from HIV‐1‐infected cells and/or to be present in the extracellular milieu in the HIV‐1‐infected brain. Moreover, these proteins have been shown to posses neurotoxic and/or neuromodulatory features in vitro. This review describes the possible direct interactions of the HIV‐1 proteins gp120, gp41, vpr, tat, rev, vpu and nef with neurons, which might play a role in the development of HIV‐1‐associated dementia in vivo.