CD99 regulates HLA class I externalization. (A) Internalization kinetics of surface-bound anti–HLA class I mAb on untreated (−) or 72-hour IFN-γ–treated (+) CD99_neg, CD99_pos, and CD99_wt Jurkat T cells, as described in “Methods.” Results are expressed as a percentage of remaining HLA class I molecules at the cell surface. All values are means plus or minus SD of 3 independent experiments. (B) Kinetics of cell-surface expression of HLA class I molecules of untreated (−) or 72-hour IFN-γ–treated (+) CD99_neg, CD99_pos, and CD99_wt Jurkat T cells. Surface levels of HLA class I molecules labeled with an FITC-conjugated “W6/32” mAb were analyzed by flow cytometry and reported as relative Ag intensity (RAI), as described in “Kinetics of HLA I expression by flow cytometry.” All values are means plus or minus SD of 3 independent experiments.