REGULATED EXPRESSION OF LY-6A.2 IS IMPORTANT FOR T-CELL DEVELOPMENT

Ly-6A.2 is a surface protein on T cells that may play a role in lympho cyte activation. The regulation of Ly-6A.2 expression during T cell ly mphopoiesis has been intriguing. It is one of the earliest markers exp ressed on pluripotent hemopoietic stem cells and is present on both pr imitive and mature T cells, but its expression is extinguished in the thymus during key developmental stages. To determine whether Ly-6A.2 i s active on developing T cells, as well as the significance of its dev elopmental regulation, Ly-6A.2 was expressed throughout T cell develop ment under control of the T cell-specific human CD2 enhancer in transg enic mice. The constitutive overexpression of Ly-6A.2 in vivo led to a marked impairment in the generation of thymocytes. Development was ar rested at the time in thymic development when Ly-6A.2 expression is no rmally turned off. These results indicate that the regulated expressio n of Ly-6A.2 in thymocytes may be important for normal development. Mo reover, these findings demonstrate that Ly-6A.2 is active in the thymi c microenvironment.