Abstract
Using cultivated human umbilical vein endothelial cells (HUVEC) as a model, we investigated a selection of basic biochemical and cell physiological functions with relevance to antigen processing and presentation, namely pinocytosis and phagocytosis, the presence and activity of proteinases and the expression of MHC class II molecules. Antigen presentation (tetanus toxoid) to HLA-matched T lymphocytes with addition of proteinase inhibitors (E-64 for cysteine, pepstatin for aspartate proteinases) to HUVEC during the antigen handling phase resulted in a strongly reduced T lymphocyte stimulation by this antigen. This is taken as the first reported evidence for an antigen processing step taking place in endothelial cells. The involvement of cysteine as well as aspartate proteinases in antigen processing has been found also for other antigen presenting cells like macrophages or B lymphoblastic cells, therefore antigen processing in HUVEC most probably follows the consensus pathway formulated for these cells: uptake of antigen by pino/phagocytosis, endo/lysosomal degradation of antigen and expression of epitopes bound to MHC class II molecules on the cell surface for presentation to Th lymphocytes.