ENGAGEMENT OF CD99 INDUCES UP-REGULATION OF TCR AND MHC CLASS-I AND CLASS-II MOLECULES ON THE SURFACE OF HUMAN THYMOCYTES

CD99 is a cell surface molecule involved in the aggregation of lymphoc ytes and apoptosis of immature cortical thymocytes. Despite its high l evel expression on immature cortical thymocytes, the functional roles of this molecule during thymic selection are only now being elucidated . Examination of the effects of CD99 engagement on the expression kine tics of the TCR and MHC class I and II molecules, which are involved p rimarily in thymic positive selection, revealed a marked up-regulation of these proteins on the surface of immature thymocytes, This increas e was the result of accelerated mobilization of molecules stored in cy tosolic compartments to the plasma membrane, rather than increased RNA and protein synthesis. Confocal microscopic analysis revealed the cha nges in subcellular distribution of these molecules. When CD99 was eng aged, TCR and MHC class I and II molecules were concentrated at the pl asma membrane, particularly at cell-cell contact sites. The TCRlow sub population of immature double positive thymocytes was much more respon sive to CD99-mediated up-regulation than was the TCRhigh population. T hese findings suggest that CD99-dependent up-regulation may have possi ble implication in positive selection during thymocyte ontogeny.