Regulation of HLA class I surface expression requires CD99 and p230/golgin-245 interaction

By presenting antigenic peptides on the cell surface, human leukocyte antigen (HLA) class I molecules are critical for immune defense. Their surface density determines, to a large extent, the level of CD8(+) T cell-dependent immune reactions; their loss is a major mechanism of immune escape. Therefore, powerful processes should regulate their surface expression. Here we document the mechanisms used by CD99 to mediate HLA class I modulation. Up-regulation of HLA class I by IFN-gamma requires CD99. In the trans Golgi network (TGN), and up to the cell surface, CD99 and HLA class I are physically associated via their transmembrane domain. CD99 also binds p230/golgin-245, a coiled-coil protein that recycles between the cytosol and buds/vesicles of the TGN and which plays a fundamental role in trafficking transport vesicles. p230/golgin-245 is anchored within TGN membranes via its Golgin-97, RanBP1, IMh1p, P230 (GRIP) domain and the overexpression of which leads to surface and intracellular down-modulation of HLA class I molecules.