Effects of human immunodeficiency virus and colony-stimulating factors on the production of interleukin 6 and tumor necrosis factor alpha by monocyte/macrophages

The cytokine/chemokine response in Leishmania/HIV infection and co-infection

HIV infection progressively weakens the immune system by infecting and destroying cells involved in host defense. Viral infection symptoms are generated and aggravated as immunosuppression progresses, triggered by the presence of opportunistic infections: among these is leishmaniasis, a disease caused by the intracellular parasite Leishmania. The incidence of this co-infection is growing progressively due to the geographic distribution overlap. Both pathogens infect monocytes/macrophages and dendritic cells, although they can also modulate the activity of other cells without co-infecting, such as T and B lymphocytes. Leishmania/HIV co-infection could be described as a system comprising modulations of cell surface molecule expression, production of soluble factors, and intracellular death activities, leading ultimately to the potentiation of infectivity, replication, and spread of both pathogens. This review describes the cytokine/chemokine response in Leishmania/HIV infection and co-infection, discussing how these molecules modulate the course of the disease and analyzing the therapeutic potential of targeting this network.

CNS inflammation and macrophage/microglial biology associated with HIV-1 infection

Human immunodeficiency virus type 1 (HIV-1) infection of the central nervous system (CNS) can result in neurological dysfunction with devastating consequences in a significant proportion of individuals with acquired immune deficiency syndrome. HIV-1 does not infect neurons directly but induces damage indirectly through the accumulation of activated macrophage/microglia (M/M) cells, some of which are infected, that release neurotoxic mediators including both cellular activation products and viral proteins. One mechanism for the accumulation of activated M/M involves the development in infected individuals of an activated peripheral blood monocyte population that traffics through the blood-brain barrier, a process that also serves to carry virus into CNS and establish local infection. A second mechanism involves the release by infected and activated M/M in the CNS of chemotactic mediators that recruit additional monocytes from the periphery. These activated M/M, some of which are infected, release a number of cytokines and small molecule mediators as well as viral proteins that act on bystander cells and in turn activate them, thus amplifying the cascade. These viral proteins and cellular products have neurotoxic properties as well, both directly and through induction of astrocyte dysfunction, which ultimately lead to neuronal injury and death. In patients effectively treated with antiretroviral therapy, frank dementia is now uncommon and has been replaced by milder forms of neurocognitive impairment, with less frequent and more focal neuropathology. This review summarizes key findings that support the critical role and mechanisms of monocyte/macrophage activation and inflammation as a major component for HIV-1 encephalitis or HIV-1 associated dementia.

HIV-1-mediated apoptosis of neuronal cells: Proximal molecular mechanisms of HIV-1-induced encephalopathy

The induction of neuronal cell death in vivo has been recognized as a prominent feature of HIV type I (HIV-1) infection leading to HIV-1-induced encephalopathy. Viral and host cell products, released from HIV-1-infected cells, have been implicated as inducers of neuronal cell apoptosis. It is unclear which is more important in this process. Neuronal cells were treated with media bearing HIV-1 virions derived from infected T cells and macrophage or the same set of media depleted of virions. T cell media bearing virus induced high levels of apoptosis, whereas that depleted of virions did not. In contrast, neurons treated with media from infected macrophages induced cell death whether virions were present or depleted by ultracentrifugation. The former initiated a repeatedly and significantly higher degree of apoptosis. These data suggest that exposure of neurons to viral products is critical for the induction of apoptosis, in addition to putative host factors released from virally infected cells. Protein-array analyses identified host cell factors up-regulated from infected macrophages, versus their uninfected counterparts, and these host cell factors may be prime candidates for contributing to neuronal apoptosis. Gene-array analyses also identified mRNAs up-regulated in human neurons after treatment with purified HIV-1 gp120 envelope protein or virus-containing media from HIV-1-infected macrophages. These analyses suggest molecular mechanisms for the induction of apoptosis relating to the exposure of viral and host cell factors and rationally designed approaches toward neuroprotection.

Cytokines and HIV-1: interactions and clinical implications

Cytokines play an important role in controlling the homoeostasis of the immune system. Infection with HIV results in dysregulation of the cytokine profile in vivo and in vitro. During the course of HIV-1 infection secretion of T-helper type 1 (Th1) cytokines, such as interleukin (IL)-2, and antiviral interferon (IFN)-gamma, is generally decreased, whereas production of T helper type 2 (Th2) cytokines, IL-4, IL-10, proinflammatory cytokines (IL-1, IL-6, IL-8) and tumour necrosis factor (TNF)-alpha, is increased. Such abnormal cytokine production contributes to the pathogenesis of the disease by impairing cell-mediated immunity. A number of cytokines have been shown to modulate in vitro HIV-1 infection and replication in both CD4 T lymphocytes and cells of macrophage lineage. HIV-inductive cytokines include: TNF-alpha, TNF-beta, IL-1 and IL-6, which stimulate HIV-1 replication in T cells and monocyte-derived macrophages (MDM), IL-2, IL-7 and IL-15, which upregulate HIV-1 in T cells, and macrophage-colony stimulating factor, which stimulates HIV-1 in MDM. HIV-suppressive cytokines include: IFN-alpha, IFN-beta and IL-16, which inhibit HIV-1 replication in T cells and MDM, and IL-10 and IL-13, which inhibit HIV-1 in MDM. Bifunctional cytokines such as IFN-gamma, IL-4 and granulocyte-macrophage colony-stimulating factor have been shown to have both inhibitory and stimulatory effects on HIV-1. The beta-chemokines, macrophage-inflammatory protein (MIP)-1alpha, MIP-1beta and RANTES are important inhibitors of macrophage-tropic strains of HIV-1, whereas the alpha-chemokine stromal-derived factor-1 suppresses infection of T-tropic strains of HIV-1. This review outlines the interactions between cytokines and HIV-1, and presents clinical applications of cytokine therapy combined with highly active antiretroviral therapy or vaccines.

Differences among HIV-1 variants in their ability to elicit secretion of TNF-alpha

HIV-1 infection of human PBMC has been shown to elicit secretion of several different cytokines. TNF-alpha secretion induced by this virus has been of particular interest because it has been associated with the development of HIV-1 dementia and because TNF-alpha increases viral replication by enhancing NF-kappaB interaction with the viral promoter, the HIV-1 long terminal repeat. Thus, an autocrine pathway is potentially created in which HIV-1 stimulates its own replication. Conflicting reports exist, however, on the ability of HIV-1 to induce TNF-alpha secretion in vitro or in vivo. Using experimental protocols that controlled for potential bacterial endotoxin-induced TNF-alpha secretion, the current study demonstrates significant differences in TNF-alpha-eliciting properties among primary and laboratory obtained HIV-1. The relative TNF-alpha-inducing ability of different variants is conserved when tested using PBMC from different individuals. Elicitation of TNF-alpha secretion was not blocked by exposure of cells to zidovudine, indicating that viral integration was not required to induce secretion. Rather, the interaction between the virus and cell surface is critical for TNF-alpha induction, as Abs against CD4 or CCR5 blocked the induction of TNF-alpha synthesis by PBMC when added before virus exposure. Furthermore, the ability to induce TNF-alpha secretion mapped to a region of the HIV-1 env gene that includes the third hypervariable domain. Differences in the ability of different HIV-1 variants to elicit TNF-alpha may account for individual differences in HIV-1 disease course.