Antigen Processing and Presentation

Antigen-specific immune responses are constituted by two distinct manners: humoral and cellular immune responses. The major effector components of the humoral arm are antibodies and of the cellular arm are T lymphocytes. In general, antibodies recognize native antigen molecules free in solution especially their conformational structures, whereas antigen-specific T cells appear to see antigen in conjunction with molecules encoded by the major histocompatibility complex (MHC) on the surface of the target or antigen-presenting cells (APC)(4, 6, 42, 59 , 64). For such T cells, the antigen is usually not seen in its native form but only in fragmented form of the protein after processing by the APC (8, 15, 63). In relation to the processing, antigens are considered to be classified into two categories, and two distinct processing pathways are currently speculated (23). One is endogenous antigens, such as viral or newly synthesized cellular proteins like tumor cellderived proteins, that are synthesized inside of the cell, and the other is exogenous antigens, such as non-replicating protein like myoglobin or insulin that can be taken up by the APC from outside through endocytosis. The endogenous antigens are fragmented to be processed in the endoplasmic reticulum (ER) or cytoplasm and presented on the cell surface in association with class I MHC molecules, such as H-2K, -D, -L in mice and HLA-A, -B, -C in humans, and the processed form of antigen can be recognized by CD8 moleculebearing T cells like cytotoxic T lymphocytes (CTL). In contrast, the exogenous antigens are degraded to be processed in the endosome, transported to the cell surface, and presented to CD4 molecule-expressing T cells like helper cells (Th) in conjunction with class II MHC molecules, such as I-A or I-E in mice and HLA-DP, -DQ, -DR in humans. In this review, I would like to show and summarize recent progress and discoveries on the area of antigen processing and presentation and to discuss their biological meanings.