Figure 3. Genes involved in antigen processing and presentation and T cell regulation are common targets of positive selection in mammals.
All pathway components are designated using official gene names (excluding the major histocompatibility complex (MHC) and T cell receptor (TCR)) and are highlighted in red if they are targets of positive selection in mammals or primates25,66,67. The molecular components of different antigen processing and presentation pathways are shown (details from Refs 107,108) to provide a general overview of the extent of positive selection and to highlight the function of positively selected genes, as most of their protein products directly interact with the antigen. Thus, the figure is not meant to show all molecules involved in the process or to convey mechanistic insights. Also, some genes may show tissue-specific expression or may be induced under specific circumstances: their products are nonetheless included for the sake of completeness. As for T cell regulatory molecules, the representation does not reflect the stoichiometry of binding (for example, CD28 functions as a dimer). Notably, the same molecule may be expressed by different populations of T cells, although here each molecule is shown on one T cell type only (to avoid redundancy). The dashed arrows and '?' indicate steps that lack clear molecular definition or are only inferred. The orange circles, and red and blue shapes at the bottom of the figure represent proteolytic fragments. B2M, β2-microglobulin; BLMH, bleomycin hydrolase; CALR, calreticulin; CD40LG, CD40 ligand; CTLA4, cytotoxic T lymphocyte protein 4; CTS, cathepsin; CYB, cytochrome b; ERAP, endoplasmic reticulum aminopeptidase; HAVCR2, hepatitis A virus cellular receptor 2; HLA-DM, major histocompatibility complex, class II, DM; ICOS, inducible T cell co-stimulator; ICOSLG, ICOS ligand; IFI30, interferon-γ-inducible protein 30; iNKT, invariant natural killer T; iTCR, invariant TCR; LGMN, legumain; LNPEP, leucyl-cystinyl aminopeptidase; NCF, neutrophil cytosol factor; NPEPPS, puromycin-sensitive aminopeptidase (also known as PSA); NRD1, nardilysin; PDCD1, programmed cell death 1; PDCD1LG2, programmed cell death 1 ligand 2; PDIA3, protein disulfide-isomerase A3; ROS, reactive oxygen species; TAP, antigen peptide transporter; TAPBP, TAP-binding protein (also known as tapasin); THOP1, thimet oligopeptidase 1; TPP2, tripeptidyl-peptidase 2.