Unit: Virus and immunity

Director: SCHWARTZ Olivier

We are studying virological and immunological interactions between HIV-1 and its target cells. We are analyzing viral entry pathways in dendritic cells, the mechanisms of viral transmission from dendritic cells to lymphocytes, as well as the consequences of viral infection on the biology of the cell and on the trafficking of cellular proteins. From an immunological point of view, we are characterizing the pathways of MHC-restricted presentation of viral epitopes in dendritic cells.

Three close and complementary axes of research characterize our scientific activities.

The first axis concerns the molecular mechanisms of HIV-1 infectivity. We are studying early steps of the viral cycle. We intend to characterize entry pathways of the virus into target cells and to analyze the intracellular routing of reverse-transcription or pre-integration complexes. Previous studies on HIV entry have been mostly performed using established cell lines and have shown that HIV is a pH-independent virus which enters target cells by fusion at the plasma membrane. The process of viral entry in primary macrophages and in DC is less well characterized, and might be different since these cells display various active internalization pathways, including endocytosis, phagocytosis and macropinocytosis. We wish to characterize HIV entry in primary monocyte-derived macrophages and in DCs. Nef is also a positive factor of viral infectivity, required for efficient viral replication and acts at the early stages of the viral cycle. We will analyze the replicative default of nef-deleted viruses, with a special attention to the entry and post-entry steps. We have also studied the early intracellular defences against endogenous and exogenous retroviruses. We have identified an important role for APOBEC3 proteins, which protect the mamalian genome against retrotransposition of endogenous retroviruses. These studies are performed by Cécile Esnault, Arnaud Moris, Nathalie Sol-Foulon, Fréderic Delebecque, Nicoletta Casartelli, Françoise Porrot.

The second axis concerns the study of the impact of HIV-1 infection on the biology of the cell and on protein trafficking. We particularly focus on the HIV-1 Nef protein. We have demonstrated that Nef down-regulates the surface expression of MHC-I. We wish now to document further how Nef interacts with the cellular trafficking machinery to alter surface expression of MHC-I and CD4. So far, most of the studies concerning Nef and cellular trafficking have been performed using permanent cell lines. We will thus analyze perturbation of trafficking pathways induced by HIV-infection in the natural targets of the virus, i.e. lymphocytes, macrophages and dendritic cells (DC). We are also analyzing the effects of HIV infection on lymphocyte activation. These studies are performed by Nathalie Sol-Foulon, Cécile Esnault, Marion Souriceau, Fréderic Delebecque, Fabien Blanchet.

The aim of the third axis is a better understanding of the mechanisms of the recognition of HIV-1-infected cells by specific cytotoxic T lymphocytes (CTLs). In most cells, epitopes presented by MHC-I derive from endogenously synthesized proteins. In professional APCs, such as dendritic cells (DCs) and macrophages, evidence exists for an alternative pathway presenting peptides derived from extracellular antigens. We have observed that HIV-1 Gag epitopes derived from incoming virions are presented through the MHC-I-restricted exogenous pathway in APCs, leading to CTL activation in the absence of viral protein neosynthesis. Exogenous presentation of HIV-1 epitopes occurs efficiently in immature DCs and to a lower extent in macrophages. Cross-presentation requires interaction between the HIV-1 envelope protein and its receptors and fusion of viral and cellular membranes. Our aim is to extend these initial observations. We are characterizing viral entry pathways and post-entry steps leading to an efficient exogenous presentation of virion antigens. We are currently studying the pathways leading to MHC-II presentation of HIV epitopes. These studies are performed by Arnaud Morris, Florence Guivel-Benhassine and Nicoletta Casartelli.

Photo Legend. Close contact between a lymphocyte (yellow) and a dendritic cell (green) This interaction leads to the formation of a synapse between both cells, allowing lymphocyte activation and virus transfer from one type to another. Photo made at the electronic microscopy facility (Stéphanie Guadagnini et Marie-Christine Prevost)

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Activity Reports 2005 - Institut Pasteur
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