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The immune system is permanently confronted with mutated and self-, microbe-, and tumor-derived neoantigens–as well as other,“unknown” antigens–and has to differentiate between self or nonself. These antigenic molecules (protein or lipid based) must be phagocytosed, processed, and/or presented in the respective major histocompatibility complex (MHC) molecules on the cell surface in recognizable form to train immune cells such as effector T cells, leading to their specific activation. These “trainers” are so-called antigen-presenting cells (APCs), which can be divided into professional (eg, dendritic cells [DCs], B cells, and macrophages) and nonprofessional APCs (eg, fibroblasts and hepatocytes). While all nucleated human cells can present peptide fragments of endogenous proteins using the MHC class I pathway and display them on the surface to CD8+ cytotoxic T lymphocytes [1-3], only professional APCs …