Cellular caspase-8 (FLICE)-like inhibitory protein (c-FLIP) is an important regulator of death receptor-induced apoptosis and plays an essential role in thymocyte maturation. Two major isoforms of c-FLIP derived from alternative mRNA splicing, c-FLIPL and c-FLIPS, have been identified in mouse T lymphocytes. Our previous studies have demonstrated that conditional deletion of both c-FLIP isoforms in T lymphocytes results in an almost complete lack of mature T cells and increased apoptosis of single positive (SP) thymocytes. To further define the roles played by the c-FLIPL and c-FLIPS isoforms in thymocyte maturation and peripheral T cell function, we have generated mice specifically lacking the c-FLIPL (c-FLIPL-/-) or c-FLIPS (c-FLIPS-/-) isoform. Surprisingly, we found that expression of c-FLIPS but not c-FLIPL in c-FLIP conditional knockout mice rescued thymocyte development. Our studies further demonstrate that c-FLIPL is essential for mature T cell proliferation, as T cells from c-FLIPL-/- mice fail to develop into effectors after Listeria monocytogenes infection. Although accumulating evidence suggests that c-FLIP has both anti-apoptotic and cell signaling functions, the mechanisms by which c-FLIP regulates thymocyte maturation and mature T cell homeostasis remain unknown. Based on our preliminary results, we hypothesize that c-FLIP has three major functions mediated through c-FLIPL and c-FLIPS in the T cell compartment: 1. c-FLIPS protects mature SP thymocytes from TCR-induced apoptosis in the thymic medulla. 2. Both c-FLIP isoforms are essential in maintaining mature T cell homeostasis by promoting survival and proliferation. 3. c-FLIPL regulates T cell proliferation through its cleaved form c-FLIPp43. In this proposal, we will test these three hypotheses using several c-FLIP genetic models we have generated. The results will not only provide important insights into the mechanisms by which c-FLIP regulates thymocyte maturation and T cell homeostasis but also provide a better understanding of general T lymphocyte biology. Furthermore, determining the role of c-FLIP in regulating effector T cell survival may improve strategies for immunization and vaccine design. Narrative: c-FLIP is an important protein that protects T lymphocytes from death and is essential for T lymphocyte to develop. Our proposed studies will provide important information on how c-FLIP protects T cells and when it will protect T cells from death. Results from this study will improve our understanding of immunodeficiency and the regulation of immune response to microbial pathogen infections.

National Institute of Health (NIH)
National Institute of Allergy and Infectious Diseases (NIAID)
Research Project (R01)
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Cellular and Molecular Immunology - B Study Section (CMIB)
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Lapham, Cheryl K
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Duke University
Schools of Medicine
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