Functions of CD45 in TCR signaling in CD4§+CD8§+ double-positive thymocytes

grantor: University of TorontoT cell receptor (TCR) signals are essential for normal T cell development, and CD45 is thought to be essential for coupling TCR to the intracellular signaling machinery. However, T cell development is only partially compromised in CD45-deficient mice. This thesis describes a CD45-deficient CD4$\sp+$CD8$\sp+$ double-positive (DP) thymoma called 3T7. I show that in 3T7 cells and DP thymocytes CD45 is necessary for the TCR-induced protein tyrosine phosphorylation, as well as changes in CD5, RAG-1 and CD4/CD8 expression levels. However, co-aggregation of TCR and either CD4 or CD8 induced signaling events in a CD45-independent manner, providing a rationale for the developmental phenotype observed in CD45-deficient mice. Surprisingly, I found no differences in the overall phosphorylation state of Lck or Fyn in CD45-deficient cells versus CD45-positive cells. However, I show that loss of CD45 is accompanied by a hyperphosphorylation of TCR$\zeta$ in 3T7 cells and conversely a hypophosphorylation of TCR$\zeta$ in CD45-deficient thymocytes, providing one possible biochemical mechanism for the TCR signaling deficits described. Collectively, these observations provide a conceptual and experimental framework for understanding the role of CD45 in TCR signaling at the DP stage of T cell development.M.Sc