Our work mainly focuses on cellular and molecular aspects
of the replication of HIV and Zika virus,
and on the interplay between viruses and the host.
Our work focuses on the mechanisms of HIV-1 replication, and on the interplay between the virus and the host immune system. We are also investigating how other pandemic pathogenic viruses, such as Chikungunya and Zika viruses, spread and interact with their target cells.
Three close and complementary axes of research characterize our scientific activities. We are currently studying:
- HIV replication and interaction with the immune system
- Zika virus multiplication and cytopathic effects
- Strategies to visualize and eliminate the HIV-1 reservoir
An artistic view of cells and viruses drawings by Fabrice HYBER
When scientists meet artists: http://www.organoide-pasteur.fr
Virus and Immunity Unit
More than 35 years after its appearance, the HIV/AIDS epidemic represents more than ever a public health threat worldwide. About 37 million people are living with HIV/AIDS as of the end of 2015, most of them originating from emerging countries. Our work focuses on cellular and molecular aspects of HIV-1 replication, and on the mechanisms of the interaction of HIV-infected cells with the immune system. Three close and complementary axes of research characterize our scientific activities.
We are studying viral cell-to-cell spread. Direct cell-to-cell transfer represents a potent and rapid mode of viral propagation, which involves the formation of virological synapses between infected donor cells and uninfected recipients. We are examining the role of cellular and viral proteins which facilitate viral spread. We are deciphering the underlying mechanisms.
HIV cell-to-cell transfer in lymphocytes (HIV in green, target cell in blue)
HIV budding at the contact zone between one infected and one target cell
We are also studying the role of the viral protein Nef, which facilitates viral replication by interfering with the antiviral activity of cellular proteins (SERINC proteins), and which also modulates the trafficking of proteins, such as MHC-I that are involved in immune responses.
MHC-I intracellular localization in non-infected (right) and Nef-expressing lymphocytes
We are also examining how cellular proteins may inhibit viral spread. These proteins are called restriction factors and are often induced by type-I Interferons. We are studying for instance the role of IFITMs (Interferon induced Transmembrane proteins), which impair viral fusion and then access of incoming virions to the cytoplasm. Our aim is to understand how IFITM proteins inhibit HIV-1 replication and how the virus counteracts this effect.
Movie: Real time imaging of HIV-1 cell-to-cell transmission (from https://www.ncbi.nlm.nih.gov/pubmed/19369333).
The recent Zika virus (ZIKV) epidemics in South East Asia, French Polynesia, the Caribbean islands and the Americas, and its association with neurological disorders including Guillain-Barré syndrome and microcephaly and other defects in newborns have triggered a global public health response. ZIKV infection mainly occurs after a bite by infected Aedes mosquitoes, through maternal–fetal transmission, and less frequently by sexual transmission. In 2017, evidence of vector-borne ZIKV transmission have been reported in about 85 countries or territories.
The innate immune response controls viral spread and disease development in most of infected individuals, through mechanisms that are not clearly understood. We are studying the cytopathic effect of the virus, that is to say the morphological changes of the cell, to understand what happens once the virus replicates in the cell. We recently observed that the infected cell reacts by forming massive intracellular vacuoles which leads to the death of the cell. This phenomenon of cellular destruction is mainly visible in the absence of the protein IFITM3. These observations are made in the cells naturally targeted by the virus, including epithelial cells, fibroblasts of the skin and brain astrocytes.
Our aim is to understand how Zika virus replicates despite this massive cytopathic effect, and to further describe the protective innate response of the host.
We are using video microscopy electron microscopy and other techniques to address these questions.
Zika Virus particles accumulating in infected cells, visualized by electron microscopy. Scientific image by Université François Rabelais, Tours and Institut Pasteur, Paris, France
Cell infected with Zika virus (cytopathic effect), with massive vacuoles (holes). In red: viral protein. In green: cell. In blue: nucleus. Immunofluorescence microscopy (Institut Pasteur).
ZIKV modifies the morphology of the cells and makes them implode. Example of HeLa cells.
ZIKV modifies the morphology of the cells and makes them implode. Example of primary human skin fibroblasts.
Despite the success of anti-retroviral therapy (ART) to treat HIV-1-infected individuals, the persistence of a viral reservoir remains the major obstacle to a cure. Characterization of the antibody repertoire in patients led to the identification of broadly neutralizing antibodies (bNAbs) targeting the viral envelope glycoproteins and suppressing HIV-1 infectivity with unprecedented potency. Passive administration of bNAbs in animal models or in infected humans revealed their capacity to decrease viral loads and to delay viral rebound after ART cessation. bNAbs also activate the immune system and mediate functions that go well beyond neutralization, such as killing of infected cells or enhancement of T and B cell responses. Immune cells with cytotoxic, phagocytic or immunomodulatory activities express Fc receptors (FcR) that recognize the constant Fc region of antibodies. The complement cascade is also initiated through binding to the Fc. The molecular and cellular mechanisms underlying the activation of Fc-dependent effector functions of antibodies, including bNAbs, are incompletely understood.
We have shown, in collaboration with Laboratory of Humoral Response to Pathogens (Institut Pasteur/CNRS), led by Hugo Mouquet, together with Olivier Lambotte’s team (Bicêtre Hospital), we have demonstrated that they act in complementary ways. Firstly, the bNAbs neutralize the spread of the virus, either as cell-free virions, but also when the virus is directly transmitted from one infected cell to a novel target cell. But the most effective ones are also capable of directly recognizing infected cells and triggering their destruction by Natural Killer (NK) cells We have shown that exposure of the viral envelope varies considerably on the surface of infected cells, and depends on the HIV strain, therefore modulating immune responses.
We have also shown that bNAbs may bind reactivated latently infected cells and thus may potentially eliminate the viral reservoir of patients.
Example of HIV-1-infected cell killing by NK cells: An HIV-1 infected cell (green) and NK (smaller dark cells) are incubated with a bNAb and trapped in a microwells. A picture is taken every 5 minutes. A blue dye reveals the dying cell (Bruel et al. 2016).
It has been recently reported by Monsef Benkirane (IGM, Montpellier) that latently infected cells carry specific biomarkers. The main molecule is CD32a, which provides an invaluable tool to analyze the origin, composition and localization of the viral reservoir in patients, and to define strategies of eradication.
Our current aims are to study the mechanisms of antibody-dependent cellular cytotoxicity (ADCC) by bNAbs and to further assess the ability of bNAbs to clear the latent HIV-1 reservoir, using ultrasensitive methods to detect latently infected cells in samples from patients under successful ART. We also wish to determine whether CD32a represents a molecular target to eliminate latently infected cells.
Overall, this project should provide new insights into the activity of antibodies during HIV-1 infection. More generally, it will also help define the mechanisms of antibody-based therapeutic strategies.