HLA-DM sensitivity is reversible by HLA-DO. (A) Histograms are shown for class II–associated invariant chain peptide (CLIP) surface expression on MJS wt (open) and MJS cells transduced with HLA-DOα/β (filled) as measured by flow cytometry. (B) The influence of HLA-DO on presentation of the DM-sensitive antigen PI4K2B and DM-resistant antigen PTK2B was analyzed by testing T-cell recognition of MJS wt (open bars) and MJS transduced with HLA-DOα/β (closed bars). MJS melanoma cells endogenously express the invariant chain and HLA-DM but lack expression of HLA-DO. The HLA-DRB3*0101 restriction allele for PTK2B is endogenously expressed by MJS cells and the HLA-DQB1*0603/DQA1*0103 restriction allele for the HLA class II epitope from PI4K2B was introduced retrovirally. To induce antigen expression, MJS cells were loaded with recombinant proteins in 2 different concentrations. Mean ± SD release of IFN-γ in duplicate wells is shown. (C) MJS-DQ6 (open bars) and MJS-DQ6-DO (closed bars) cells were transduced with retroviral vectors encoding PI4K2B and PTK2B to test the influence of HLA-DO on endogenously expressed antigens. T-cell recognition was tested against titrated numbers of target cells (10 000 and 3000), and mean ± SD release of IFN-γ in duplicate wells is shown.