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Home > Annual Report > 2001-2002 | |||
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Introduction The Lausanne Branch has continued to study the molecular mechanisms of key events that occur during an immune response and to apply recent progress in basic immunology to the development of cancer vaccines. In addition, the Branch has been actively involved in the implementation of the National Center for Competence in Research (NCCR) program entitled : "Molecular Oncology : from basic research to therapeutic approaches." This program has been selected by the Swiss Federal Science and Research Group among 230 proposals submitted to the Swiss National Science Foundation. The objectives of the program are the development of new translational projects in cancer research in Switzerland under the leadership of the three research institutions based in the Biomedical Research Center in Epalinges (the Swiss Institute for Experimental Cancer Research, the Institute of Biochemistry and the Lausanne Branch of the Ludwig Institute).
Developmental Immunology Group The Developmental Immunology group has continued its analysis of the role played by the signaling molecule Notch1 in T cell development in the thymus. It is know that Notch1 is indispensable for the initial specification of T lineage precursors. Previous studies from the group have ruled out the possibility that Notch1 plays an essential role in the differentiation of thymocytes into either CD4+ or CD8 mature T cells. These findings were obtained by using conditional knockout mice in which the Notch1 gene was inactivated in thymocytes that had already undergone some early differentiation within the thymus. To further delineate the stage of T cell development that depends on Notch1, the group produced other conditional knockout mice in which the Notch1 gene was inactivated at a very early thymic stage. In contrast to the lack of effect of Notch1 in CD4/CD8 lineage commitment, analysis of these mice led to the identification of novel and non-redundant functions for Notch1, including an important role in the development of T cells expressing TCRab without any effect on T cells expressing TCRgd. In addition to Notch1, there are many signaling molecules that influence the fate of T cells developing in the thylamus. Indeed, it is now evident that T-cell development in the thylamus proceeds through two phases of cell proliferation, which are both followed by a developmental checkpoint. For example, after successful rearrangement of the TCR b gene CD4- CD8-, thymocytes undergo a series of divisions and differentiate into CD4+ CD8+ cells. Thymocytes with nonproductive TCR b gene rearrangements at this checkpoint undergo programmed cell death. Signals from the TCR are essential for survival, proliferation and differentiation of thymocytes undergoing TCR rearrangements. However, these signals are not sufficient and additional signals provided by other surface receptors are needed. The group has now determined that Wnt-frizzled-induced activation of the transcription factor T cell factor 1 is critical during TCR gene rearrangements in thymocytes.
Viral Immunology Group The Viral Immunology group has identified a novel pathway in the regulation of MHC class II expression. MHC class II molecules present short peptide fragments on the cell surface that can be recognized by CD4+ T cells. In the thymus, constitutive expression of MHC class II by cortical epithelial cells is pivotal to the induction of the so-called positive selection of CD4+ T cells. Regulation of MHC class II expression at the transcriptional level is complex. Previous studies have shown that the class II transactivator (CIITA) plays a key role in this process. Regulation of CIITA expression at the transcriptional level is also complex and involves differential usage of at least 3 independent promoters in distinct cell types. By generating mice lacking a defined promoter of CIITA, the group has shown that the pathway leading to the initiation of transcription in cortical thymic epithelial cells differs from that induced by interferon gamma in other epithelial cells. In addition, these studies provide definitive evidence that differential CIITA promoter usage plays an important physiological role in the induction of immune responses.
Molecular Immunology Group In the Molecular Immunology group, the analysis of the molecular interactions that elicit or inhibit activation of CD8+ CTL has led to new crucial information regarding the key role played by the CTL coreceptor CD8. This molecule is expressed on the CTL surface as a ab heterodimer. The cytoplasmic tail of CD8a interacts with lck, a Src-family kinase that is essential for CTL activation. By contrast, the function of CD8b has remained controversial. As reported previously by the group, the CD8b has a short cytoplasmic tail that is palmitoylated . This palmitoylation enables CD8 to partition into the membrane microdomains known as lipid rafts. Recent studies from the group indicate that the CD8b cytoplasmic and transmembrane portions enable constitutive association of CD8 with the TCR-CD3 complex, thus recruiting it to lipid rafts. As it is known that lipid rafts are privileged sites for the induction of TCR-associated signal transduction cascade, it thus appears that the primary role of the CD8 coreceptor in CTL activation may involve the recruitment of TCR to such domains.
Molecular Tumor Immunology Group In the Molecular Tumor Immunology group, substantial progress has been made in the identification of peptides derived from tumor antigens that are able to be recognized by CD8+ T cells. An approach commonly used to identify such peptides is based on the prediction of peptide sequences displaying the appropriate MHC class I binding motif and on the subsequent in vitro generation of CTL against the predicted peptides. However, this approach does not take into account the limitations imposed by the intracellular degradation of tumor antigens by the proteasome that is required for the production of peptides. To overcome this problem, the group has developed an in vitro proteasomal degradation assay that can be used to restrict the number of candidates among peptides predicted to bind to defined HLA molecules. The usefulness of this approach was validated by the successful identification of an HLA-A2-restricted peptide derived from a recently described tumor antigen, SSX-2, which is expressed in a wide variety of tumors.
Clinical Tumor Immunology Group Studies carried out by the Clinical Tumor Immunology group include the characterization of tumor antigenic peptide analogues with improved immunogenicity. Previous studies led to the identification of a potent peptide analogue of the tumor antigen Melan-A. To understand the molecular basis of improved recognition by specific CTL, the group has recently determined the crystal structure of the HLA-A2/Melan-A peptide analogue. While this is the first high resolution crystal structure of a CTL-defined human tumor antigen, the insight gained from this collaborative study has important implications for the understanding of the conformation adopted by the Melan-A peptide when it is associated to HLA-A2. A major focus of the patient-oriented research program carried out by the group is the detailed analysis of tumor-specific CTL responses that develop spontaneously in cancer patients. While previous studies indicated that more than 50% of melanoma patients develop spontaneously CTL responses against antigens expressed by their tumor cells, further analysis of such responses has revealed an unexpected diversity in the CTL TCR usage among individual patients. Moreover, direct comparison of the Melan-A-specific CTL responses in two patients showed that the TCR usage was not overlapping, thus emphasizing the complexity of TCR-mediated tumor antigen recognition. Another major activity of the group is the design and execution of early-phase clinical trials of peptide-based immunotherapy. In particular, an ongoing trial is testing different vaccine preparations based on Melan-A peptides. As of December 31, 2001, 42 HLA-A2 patients with Melan-A positive melanoma had been included consecutively in the different groups. The vaccine-induced immune response of each patient was monitored by ex vivo analysis of antigen-specific CD8+ T cells in peripheral blood, using fluorescent tetramers of HLA-A2/Melan-A peptide complexes. Significant differences in the immune response elicited by the different vaccine preparations were observed, thus allowing a preliminary evaluation of their efficacy.
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