BECs constitutively present defined self-antigens in the context of MHC class II. (A) Specific detection of self-peptide:MHC class II complexes on antigen-presenting cells. B cells from HLA-DRB1*0101⁺HLA-A*0201⁺ donors (top) but not B cells from HLA-DRB1*0101⁺HLA-A*0201⁻ donors (bottom) react with mAb UL-5A1 specific for a MHC class II complex composed of HLA-DRB1 and a peptide of HLA-A2 [amino acids 103-117; DR1:A2(aa103-117); x-axis]. Peripheral blood B cells were identified by anti-CD19 immunoreactivity in mononuclear cell preparations from healthy donors (y-axis). (B) Comparable levels of self-peptide-MHC complexes are constitutively displayed by BECs and DCs in normal human skin. Dermal single-cell suspensions from HLA-DRB1*0101⁺HLA-A*0201⁺ (left panels) and HLA-DRB1*0101⁻HLA-A*0201⁺ donors (right panels) were labeled with mAb UL-5A1-FITC (top) or L243-FITC (bottom) together with anti–VE-cadherin-PC5, anti–PDPN-PE, anti–CD1a-APC, and anti–CD14-PC7. BECs (red histograms) and CD1a⁺ DDCs (blue histograms) were gated and analyzed for surface expression of DR1:A2(aa103-117) complexes (x-axis, upper panels) and total HLA-DR–peptide complexes (x-axis, lower panels). Gray histograms show the reactivity of BECs and CD1a⁺ DDCs with FITC-labeled control Abs. (C) Efficient presentation of self-peptide–MHC complexes by BECs. The relation between DR1:A2(aa103-117) and total DR:peptide complex display was calculated by forming the ratio between the respective measured fluorescence intensities for BECs, CD1a⁺ DDCs, and CD14⁺ DDCs (y-axis). Data are expressed as mean values (+ SDs) obtained in experiments with dermal cells from 2 unrelated HLA-DRB1*0101⁺HLA-A*0201⁺ donors.