Viral infection of immunocompetent cells always leads to disordered regulation of the immune system. Thus, infection by HIV1 (human immunodeficiency virus, type 1) of mononuclear phagocytes and lymphocytes is linked to the induction of the acquired immunodeficiency syndrome (AIDS). HIV1 replication in mononuclear phagocytes appears to be dependent on both the stage of maturation and on differentiation of mononuclear phagocytes. Because of the heterogeneity of the mononuclear phagocyte system, the U937 cell line provides a convenient model for studying the regulation of HIV1 replication in mononuclear phagocytes and the involvement of these cells in the immunopathogenesis of HIV1. We have shown that endogenous interferon alpha (IFN alpha) restricted viral replication in these promonocytic cells, probably by acting on proviral transcription and by interfering with transcriptional factors involved in the transactivation of the LTR (long terminal repeat) of HIV1. Indeed, addition of a monoclonal antibody to IFN alpha U937 cells cotransfected with a LTR HIV linked to the bacterial chloramphenicol acetyl transferase (CAT) gene, together with a tat expression vector, leads to an increase in CAT activity. Conversely, the addition of IFN alpha to cells cotransfected with the same vectors is followed by a decrease in CAT activity. Finally, recent experiments indicate that chronically HIV-1-infected U937 cells are more differentiated than uninfected U937 cells, suggesting that viral gene expression is able to trigger the maturation process of the promonocytic cells towards a stage where viral transcription may escape IFN alpha. These results suggest that the first replicative cycle of HIV1 in monocytes/macrophages could be a unique target for therapeutic strategies based on the use of IFN alpha.

Acquired Immunodeficiency Syndrome↗

PubMedDate not supplied

Trypsin-resistant gp120 receptors are upregulated on short-term cultured human epidermal Langerhans cells.

The CD4 molecule is known to be the preferential receptor for the HIV1 envelope glycoprotein. Epidermal Langerhans cells (LC) are dendritic cells which express several surface antigens, among them the CD4 antigens. LC infection was suggested when these cells were seen to present buddings coincident with membrane thickening of roughly 100 nm in size. These buddings were similar in ultrastructural aspect to HIV buddings on in vitro infected promonocytic cells (U937). To clarify the exact role of CD4 molecules in LC infection induced by HIV1, we investigated the possible involvement of between native and recombinant HIV1 gp120 and the LC surface. We also assessed the expression of CD4 molecules on LC membranes dissociated by means of trypsin from their neighbouring keratinocytes. The cellular phenotype was monitored using flow cytometry. We show that human LC can bind the viral envelope protein and that this binding does not depend on CD4 protein expression. The amount of surface bound gp120 was not consistent with the amount of CD4 antigens present on LC membranes. The gp120-binding sites on LC in suspension appear to be typsin-resistant while the CD4 antigens (at least the epitopes known to bind HIV1) are trypsin-sensitive. A burst of gp120 receptor expression was detected on 1-day cultured LC while the CD4 antigens disappeared. These findings lead to the logical conclusion that the binding of gp120 is due to the presence of a LC surface molecule which is different from CD4 antigens.

Antibodies, Monoclonal↗

PubMedDate not supplied

Interferon-regulated viral replication in chronically HIV1-infected promonocytic U937 cells.

To further document the role of interferons in the restriction of HIV1 replication in cells of the monocyte/macrophage lineage, the antiviral effects of alpha-interferon (IFN alpha) were studied in chronically HIV 1-infected promonocytic cells U937, in which IFN alpha was endogenously produced or to which recombinant IFN alpha was added. Protein analysis performed after immunoprecipitation of culture media revealed that the addition of anti-IFN alpha antibody led to an increase in the production of viral particles, whereas addition of IFN alpha caused its decrease in a dose-dependent manner, indicating that IFN alpha mainly affects viral assembly and virion release. However, the treatment of HIV 1-infected cells with IFN alpha did not cause an increase in the amount of intracellular viral proteins, suggesting that this cytokine might act on other steps in the viral life cycle. No decrease in the level of the 3 main types of viral RNA was observed by Northern blot analysis, indicating that proviral transcription was not restricted by IFN alpha. Furthermore, cotransfection experiments with TAR(trans-activation responsive)-element-containing expression plasmids demonstrated that viral replication appears to be restricted at either a post-transcriptional and/or a translational level. These experiments suggest that the double-stranded (ds) inverted repeat TAR sequence present in HIV1 leader RNA may inhibit viral protein synthesis by phosphorylating the dsRNA-activated protein kinase induced by IFN. These results provide an impetus for achieving antiretroviral therapy based on the constitutive expression of IFN in monocytes and macrophages.

Cell Line↗

PubMedDate not supplied

Expression of the Tat protein of HIV1 in human promonocytic U937 cells.

Numerous studies have shown that, upon HIV1 infection, human promonocytic U937 cells were induced to differentiate, as indicated, for example, by increased expression of adhesion molecules. One of the viral proteins involved in this process might be the Tat protein. Indeed, this viral protein, which is essential for productive infection, has also been shown to display growth-stimulating properties and immunomodulatory activities. In order to apprehend the role of the HIV1 tat gene in inducing the differentiation of HIV1-infected U937 cells, we have successfully introduced this gene into U937 cells by infecting them with retroviral particles transducing tat. The effect of the Tat protein constitutively expressed by these cells upon their differentiation was then evaluated by looking for the expression of the c-fos and of the c-fms proto-oncogenes which are linked to the differentiation of myelomonoblastic cells. Northern blot analysis revealed in these cells, an increase in the transcription of these two proto-oncogenes, and this increase was amplified after treatment with phorbol myristate acetate. No such increase was observed in control U937 cells. These results indicate that, among HIV1 gene products, the Tat protein appears to trigger monocytic differentiation, and suggests that this viral protein directs progenitors of the monocyte/macrophage lineage towards a differentiation stage in which production of viral antigens and virions might be more efficient.

Cell Differentiation↗

PubMedDate not supplied

Direct activation of resting T lymphocytes by human T-lymphotropic virus type I.

The mitogenic or antigenic stimulation of T lymphocytes in the presence of accessory cells results in the synthesis of interleukin-2 (IL-2), which, on interacting with de novo synthesized receptors on the membrane, induces the proliferation and differentiation of T cells. T lymphocytes are the preferential target cells of human T-lymphotropic virus type I (HTLV-I). This virus was isolated from leukaemic cells of patients with adult T-cell leukaemia. No viral transcription was detected in these leukaemic cells, indicating that consistent expression of HTLV-I is not needed for maintenance of the neoplastic state. After a short time in culture, these leukaemic cells produce viral particles which immortalize normal T cells. Hence, HTLV-I might be an important agent not only in the development, but also in the initiation, of this lymphoproliferative disease. This possibility was sustained by our previous observations indicating that normal T lymphocytes incubated with HTLV-I are able to form colonies in soft agar, seven days later, in the absence of exogenous IL-2. These results led us to explore further the immediate effects of viral infection on purified resting T lymphocytes. Here, we present evidence that HTLV-I is able to activate T lymphocytes. Indeed, binding of viral particles to these cells induces IL-2 production and IL-2 receptor expression, and triggers T-cell proliferation. This initial activation, which appears to be independent of accessory cells, may be relevant in understanding the role of HTLV-I in the aetiology of adult T-cell leukaemia.

Cell Transformation, Viral↗