A293T cells [73] were maintained as previously described [74] and CIITA-stable A293T cell lines were generated by cotransfecting PVU I linearized pDNA3.FLAG.CIITA8 [75] and pCMV4His, a mammalian selection vector which encodes the histidinol dehydrogenase gene under control of the CMV promoter into the cells. Stable clones were selected in DMEM medium containing 5 mM L-histidinol (Sigma-Aldrich., St. Louis, MO) and cloned by limiting dilution assay. GHOST cell medium was supplemented with 0.2 mg/ml G418, 0.1 mg/ml hygromycin B and 1 mg/ml puromycin (Sigma) as previously described [76] .