Our Science – Bosselut Website

Remy Bosselut, M.D., Ph.D.

Laboratory of Immune Cell Biology Senior Investigator Building 37, Room 3032A, MSC 4259 9000 Rockville Pike Bethesda, MD 20892 Phone:   301-402-4849 Fax:   301-402-4844 E-Mail:   remy@helix.nih.gov

Biography

Dr. Remy Bosselut trained at the Institut Curie in Paris, France. He earned his M.D. degree in 1992 from the Xavier Bichat School of Medicine and his Ph.D. degree in 1993 from the University Denis Diderot, both in Paris. Dr. Bosselut obtained postdoctoral training at the NCI Experimental Immunology Branch and joined the Laboratory of Immune Cell Biology in 2000.

Research

Genetic analysis of intrathymic T-cell development

T lymphocytes constitute an essential part of the adaptive immune system, as dramatically illustrated by the severity of human T-cell immunodeficiencies, either genetic or induced by agents such as the Human Immunodeficiency Virus HIV. T-cells recognize peptide antigens through a specific T-cell receptor (TCR) and generally belong to either of two lineages: CD4 T-cells, which provide help to other components of the immune system such as B-cells or macrophages, and CD8 T-cells which are cytotoxic and able to directly kill cells expressing their target antigen. T-cells derive from bone-marrow precursors and complete their development in the thymus in a process that can be schematically divided into two phases. The first phase is characterized by the expansion of the precursors that entered the thymus and by the rearrangement of genes encoding TCR chains. The second phase, known as T-cell selection, can be seen as an “audition� process, in which T-cell precursors (thymocytes) that receive appropriate signals through their TCR are allowed to survive and to differentiate into mature T-cells.

Our group has focused on this selection step, and more specifically on the choice between CD4 or CD8 lineages. Since CD4 and CD8 T-cells develop in the thymus from precursors that express both CD4 and CD8 molecules, the question arises as to how these CD4+CD8+ thymocytes choose either the CD4 or the CD8 lineage. We and others have suggested that this lineage decision is determined by the kinetics or duration of TCR signals, with persistent signals promoting CD4 choice and transient signals promoting CD8 choice. To test this hypothesis directly in vivo, we undertook to manipulate the duration of TCR signaling during intrathymic T-cell development. We used a genetic approach to confine expression of Zap70, a tyrosine kinase required for TCR signaling, to the CD4+CD8+ stage, thereby limiting the duration (understood as the developmental window) of intrathymic TCR signaling. These limited signals allow thymocytes to initiate their differentiation but not to complete it, causing a developmental block before the generation of mature CD4 or CD8 T-cell populations. By assessing lineage choice in these arrested cells, we found that they choose the CD8 but not the CD4 lineage, even when signaled by intrathymic TCR ligands that normally promote CD4 lineage choice (that is, self peptides complexed to class II major histocompatibility complex molecules). Thus, these experiments provide direct in vivo support for the 'duration of signal' (or 'kinetic signaling') hypothesis of lineage choice.

Ongoing studies aim at identifying genes involved in T-cell selection. As one of the few developmental programs that continue throughout life, intrathymic T-cell development is a privileged model to study cell death/survival and differentiation decisions in mammals. Indeed, only those CD4+CD8+ thymocytes whose avidity for self ligands is within an 'appropriate' window differentiate into mature T-cells; avidities outside of this window result in thymocyte death because of insufficient or excessive signaling intensity. To gain mechanistic insight into these issues, we are using high-throughput gene expression analyses to identify genes whose expression increases or decreases during intrathymic T-cell selection and which are therefore potentially involved in differentiation or survival decisions. We are currently using in vivo genetic approaches to assess select candidate genes for their function in these events.

Another approach we are exploring is to control T-cell differentiation in vivo through the use of conditionally active proteins. This goal here is to introduce rapid changes in protein activity in vivo, in order to understand the dynamic of cell and organism responses to internal or external challenges. This is of interest non only to T-cell development biology but also to the broader goal of identifying the mechanisms that maintain cell (or organism) homeostasis and of understanding how their disruption can lead to disease. One approach to these issues has been to target specific genes for time controlled expression, using ligand (e.g. tetracycline)-regulated expression systems; however, the temporal resolution of this method is limited by the unavoidable time lag required to achieve sufficient protein concentrations through de novo gene expression. To by-pass this limitation, our strategy is to target protein function rather than gene expression, by developing proteins with ligand-controlled activity. Ongoing experiments are directed at validating this approach using transient expression in cell lines before using it in vivo.

This page was last updated on 6/11/2008.