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UBC Theses and Dissertations

Involvement of CD45 in early thymocyte development Lai, Jacqueline Cheuk-Yan


CD45 is a protein tyrosine phosphatase that is expressed on all nucleated hematopoietic cells. The major substrates of CD45 in thymocytes and T cells are the Src family kinases Lck and Fyn. The role of CD45 in thymocyte development and T cell activation via its regulation of Src family kinases in T cell receptor signaling has been studied extensively. However, the role of CD45 in processes that affect thymocyte development prior to the expression of the T cell receptor has not been explored. The overall hypothesis of this study was that CD45 is a regulator of spreading, migration, proliferation, and differentiation of early thymocytes during development in the thymus and the absence of CD45 would alter the outcome of thymocyte development. The first aim was to determine how CD45 regulates CD44-mediated signaling leading to cell spreading. The interaction between CD44 and Lck was first examined. CD44 associated with Lck in a zinc-dependent and a zinc-independent manner. Mutation analysis localized the zinc-dependent interaction to the membrane proximal region of CD44, but did not involve individual cysteine residues on CD44. CD44 and Lck co-localized in microclusters upon CD44-mediated cell spreading. CD45 co-localized with Lck and CD44 in microclusters and with F-actin in ring structures. The recruitment of CD45 to microclusters may be a mechanism of how CD45 negatively regulates CD44-mediated spreading. The second specific aim was to determine the role of CD45 in migration, proliferation, and progression and differentiation of early thymocytes. CD45 negatively regulated CXCL12-mediated migration, and positively regulated the proliferation and progression of CD117- DN1 thymocytes. Absence of CD45 led to an altered composition of thymic subsets. The CD45-/- thymus contained decreased numbers of ETPs and an aberrant CD117- DN1 population that lacked CD24, TCRbeta, and CCR7 expression. There were also increased thymic NK and gamma/delta T cells, but decreased NKT cells. In addition, a novel intermediate between DN1 and DN2 that required Notch for progression was identified. Overall, this study identified new roles for CD45 in early thymocytes and provided a better picture of how the development of T cells, a central component of the immune system, is regulated.

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