Unit: Dendritic Cells Immunobiology
Director: ALBERT Matthew
Apoptosis is widely recognized as the primary mechanism whereby physiologic and pathologic cell death occurs. For immunologists, it has become increasingly clear that death is not an endpoint, but the beginning of an immune responsedying cells influence the immune system through the delivery of antigen for MHC presentation and the modulation of cytokine production. In order to dissect the ways in which dying cells influence the regulation of immune activation versus tolerance, we must clearly define the path taken as a cell moves from among the living to its final resting place within a phagocyte. Specifically, our laboratory examines the dying cell as a source of antigen for dendritic cells to generate MHC I / peptide complexes, a pathway called cross-presentation.' Through experimentation in human and mouse models, we aim to uncover the distinct effector mechanisms that govern the dendritic cells' decision to trigger the cross-priming versus cross-tolerance of CD8+ T cells. This insight will result in the better understanding of how to harness the immune system for tumor immunotherapy as well as define the statregies by which tumors and viruses actively evade the immune system.
Theme 1. To define the molecular mechanism by which CD4+ helper T cells license DCs and permit the cross-priming of CD8+ T cells (D. Braun, R. Longman, H. Saklani). Using both mouse and human models for cross-presentation we are working to identify key points of regulation in the decision between CD8+ T cell activation and tolerance. Specifically, we have been investigating the role for CD40L / CD40 signaling at the CD4 T cell / DC interface and defining the signaling pathways responsible for licensing' DCs to prime a CD8+ T cell response. This work has also focused on the role for the immunosuppressive enzyme indoleamine 2,3-dioxygenase (IDO). Strikingly, IDO may be responsible for both DC mediated tolerance, as well as tumor mediated tolerance (e.g. breast and cervical cancer). With respect to DC biology, we have discovered that IDO is markedly upregulated in response to maturation signals; we believe that signals from the CD4+ T cell switches IDO to an inactive state, in turn permitting CD8+ T cell cross-priming. Thus, in the absence of tumor-reactive CD4+ T cells, IDO remains activate, and attempts to prime CD8+ T cells may instead result in the opposite immunologic effect. By defining a role for IDO we may not only gain insight into fundamental aspects of cross-priming, but we may also identify strategies to inhibit IDO while maintaining the necessary properties of a DC to achieve tumor immunity.
Theme 2. To define the antigen processing and presentation pathway(s) for loading MHC I with exogenous antigen (L. Walter, J. Decalf, C. Schilte). Apoptotic cells are an interesting and physiologically important source of information for the immune system. They are a complex source of antigen and contain the possessing machinery required for generating antigenic peptides. We have recently discovered that the dying cells play an active role in the transfer of antigen to DCs for the loading of MHC I / peptide complexes. We are also developing assay systems that will allow us to track the processed antigen from within the apoptotic cell into the DCs MHC I antigen presentation pathway. While this project is quite fundamental, information about the routes by which antigen accesses the DC may influence the design of tumor vaccine strategies. It is also influencing our effort on HCV pathogenesis and we have initiated an observational clinical study (collaboration with Prof Stanislas Pol, The Necker Hospital) to determine the role of DCs in the pathogenesis of chrionic HCV inflection. This effort will also be aimed at determining the source of antigen that is responsible for loading DCs with HCV protein, in turn revealing the route by which natural antigen is being presented to CD8+ T cells.
Theme 3. To demonstrate the therapeutic utility of apoptotic cells for the delivery of antigen to dendritic cells; and to identify surrogate markers for monitoring clinical immunotherapy trials (M. Albert, J. Decalf, A. Casrouge, M. Uhl). This is a long-term goal that involves investigators in the US, Egypt and France. In an interventional trial, we are testing DCs loaded with antigen derived from apoptotic cells in a Phase I study and aim to develop this methodology for clinical use in cancer patients (collaboration with Robert Darnell, USA). Simultaneously, we have extended out interest in examples of effective tumor immunity in hopes of using such positive control' patients for the development and validation of assays useful in the monitoring of clinical trials. Specifically, we are (i) investigating patients with superficial transitional cell carcinoma of the bladder in order to test our hypothesis that the BCG therapy they receive (which results in >70% cure rates) acts by initiating the cross-priming of tumor-reactive CD8+ T cells; (ii) screening tumors form patients who fail therapy (expected rate of clinical failure is ~30%), comparing the transcriptional profile of pre- and post-therapy tumors with the aim of identifying mechanisms by which tumor cells evade T cell immunity; and (iii) establishing and validating surrogate makers for clinically effective tumor immunity. This study is carried out in collaboration with Dr. Nicolas Thiounn at The Necker Hospital. Finally, we have been developing a cohort of chronically infected hepatitis C patients (collaboration with Pr. Stanislas Pol, Hospital Necker) and have begun a collaboration with investigators in Egypt on the analysis of acutely infected subjects (Dr. Mostafa Mohamed, Tropical Medicine Research Institute, Cairo Egypt). The goal is to evaluate the role of dendritic cells in HCV pathogenesis and to better define the immune mechanisms accounting for viral clearance.
Keywords: dendritic cells, apoptosis, hepatitis C virus, tumor immunity, cross-presentation