Donor-matched BCL and a representative CD4+ T cell clone were activated with cross-linking αCD3/αCD28 and assayed for cell surface expression of (A) total class II MHC, (B) HLA-DR, and (C) HLA-DR:CLIP complexes. Isotype control shown in grey. Also shown, quantification of relative mRNA expression in BCL and CD4+ T cell clones by Taqman RT-PCR of (D) the class II transcriptional activator and (E) HLA-DRα chain in BCL and T cell clones. (F) Endogenous synthesis of HLA-DRα, HLA-DRα, and Ii was confirmed by metabolic [35S]-methionine labeling, followed by immunoprecipitation of HLA-DR complexes with the conformationally specific antibody Tü36, and SDS-PAGE under the conditions indicated (B, denatured by boiling; NB, non-boiled). Histogram and mean fluorescence intensity (MFI) of representative samples shown for A–D; for E–F, graphs represent mean±SEM, n = 4 donors.