MHC class II antigen internalization

MHC II typically presents antigens derived from exogenous proteins internalized by APCs such as macrophages, B cells or dendritic cells. Different types of antigen use different routes of internalization. Endocytosis may be specific, mediated by a range of receptors expressed on APC, or may occur by nonspecific mechanisms such as phagocytosis, macropinocytosis or autophagy. Antigens are first loaded into endocytic vesicles and progress along the early endosomes (EE)-late endosomes (LE) lysosomal axis. Antigens are exposed to increasingly acidic, more denaturing and proteolytic conditions (Doherty & McMahon 2009, Underhill & Ozinsky 2002).

MHC II型分子通常展示由抗原呈递细胞（如巨噬细胞、B细胞或树突状细胞）内化的外源蛋白质来源的抗原。不同类型的抗原采用不同的内化途径。内吞作用可能是特异的，由抗原呈递细胞上表达的一系列受体介导，或者通过非特异性机制如吞噬作用、巨胞饮作用或自噬作用发生。抗原首先被装载入内吞小泡，并沿早期内体（EE）-晚期内体（LE）-溶酶体轴进展。抗原在此过程中逐渐暴露于更加酸性、更加变性化及蛋白水解作用的条件下（Doherty & McMahon 2009, Underhill & Ozinsky 2002）。