T lymphocytes comprise one arm of the adaptive immune system and are critical for immunity to neoplasia and infection. A full understanding of their development has important implications for the treatment of autoimmunity, immunodeficiency, and leukemias arising from T cell developmental intermediates. The Notch signaling pathway is already known to be absolutely required for T cell commitment and development, but its collaboration with other factors is poorly understood. Unfortunately, deficiency in many of the genes critical to hematopoiesis, including Notch, causes early embryonic lethality by disrupting multiple developmental processes. This complicates the study of such genes by in vivo models or ex vivo hematopoietic progenitors. To circumvent these difficulties, this thesis describes the use of in vitro-differentiated embryonic stem cell-derived T progenitors to examine the roles of two GATA family members during early T cell development. GATA-2, while not required for T cell development, is shown to act downstream of Notch signals to inhibit myelopoiesis. These findings both characterize a novel role for GATA-2, and demonstrate that T progenitor maturation and exclusion of non-T cell fates are distinct and separable events. GATA-3, in contrast to GATA-2, is absolutely required for T lymphopoiesis. However, the current literature does not distinguish between a requirement for GATA-3 in homing to the thymic environment, committing to the T cell fate, or surviving such a commitment event. This thesis demonstrates that GATA-3 is dispensable for commitment itself, but required to permit survival and proliferation after commitment. Taken together, the results presented in this thesis employ a novel model system to characterize the interactions of two important collaborators with Notch signals during T cell development, and further dissect the stages through which early T cell development is enacted.

T lymphocytes comprise one arm of the adaptive immune system and are critical for immunity to neoplasia and infection. A full understanding of their development has important implications for the treatment of autoimmunity, immunodeficiency, and leukemias arising from T cell developmental intermediates. The Notch signaling pathway is already known to be absolutely required for T cell commitment and development, but its collaboration with other factors is poorly understood. Unfortunately, deficiency in many of the genes critical to hematopoiesis, including Notch, causes early embryonic lethality by disrupting multiple developmental processes. This complicates the study of such genes by in vivo models or ex vivo hematopoietic progenitors. To circumvent these difficulties, this thesis describes the use of in vitro-differentiated embryonic stem cell-derived T progenitors to examine the roles of two GATA family members during early T cell development. GATA-2, while not required for T cell development, is shown to act downstream of Notch signals to inhibit myelopoiesis. These findings both characterize a novel role for GATA-2, and demonstrate that T progenitor maturation and exclusion of non-T cell fates are distinct and separable events. GATA-3, in contrast to GATA-2, is absolutely required for T lymphopoiesis. However, the current literature does not distinguish between a requirement for GATA-3 in homing to the thymic environment, committing to the T cell fate, or surviving such a commitment event. This thesis demonstrates that GATA-3 is dispensable for commitment itself, but required to permit survival and proliferation after commitment. Taken together, the results presented in this thesis employ a novel model system to characterize the interactions of two important collaborators with Notch signals during T cell development, and further dissect the stages through which early T cell development is enacted.