ECs in metastatic melanoma, as normal BECs, express the MHC class II:peptide complexes along with T-cell activation–inhibiting costimulatory molecules. (A) Detection of MHC class II:peptide complexes on tumor ECs. Immunofluorescence double labeling using Abs to MHC class II:peptide complexes (red) and VE-cadherin (blue) were performed on 7-μm snap-frozen sections from freshly excised cutaneous melanoma metastases. Expression of MHC class II:peptide complexes by VE-cadherin⁺ tumor vessel ECs is shown by purple coloration of these vessels in the overlay exposure. Results are representative for experiments with metastases from 5 donors. (A-C) Normal and tumor ECs express similar amounts of mRNAs coding for proteases relevant for antigen processing and peptide-loading in MHC class II (A), MHC class II and MHC class II-associated molecules (B), as well as T-cell activation–inhibiting costimulatory molecules (C). Normalized expression data were derived from purified TuECs (n = 8), normal BECs (n = 5), and CD1a⁺ DDCs (n = 3) as obtained by GeneChip analysis. Mean mRNA expression values and standard errors of means are shown for cathepsins B, C, D, E, F, G, H, K, L, O, S, W, and Z (B); HLA-DR, HLA-DP, HLA-DQ, HLA-DM, and invariant chain/CD74 (C); and the costimulatory molecules B7.1, B7.2, B7-H1, B7-H2, B7-H3, and B7-H4 (D).