Antigen presenting cells and antigen presentation

Contents Antigen presenting cells Antigen presentation

Antigen presenting cell, APC A variety of cell types which carry antigen in a form that can stimulate lymphocytes. It is also termed accessory cell. professional APC:MΦ,DC,B non-professional APC

APCs fall into two categories: professional or non-professional. To help distinguish between the two types of APCs, those that express MHC class II molecules are often called professional antigen-presenting cells. These professional APCs are very efficient at internalizing antigen. A non-professional APC does not constitutively express the Major histocompatibility complex proteins required for interaction with naive T cells; these are expressed only upon stimulation of the non-professional APC by certain cytokines such as IFN-γ.

The 3 types of professional APCs there are 3 main types of professional APCs At certain development stages they grow branched projections, the dendrites, that give the cell its name Constitutively express a high level of MHC II and the co-stimulatory protein,B7. the most effective APC must be activated by the process of phagocytosis before expressing class II MHC and B7. Constitutively express class II MHC but must be activated to produce B7.

I. Dendritic cells (DC) Dendritic cells were first described by Paul Langerhans (Langerhans cells) in the late nineteenth century. It wasn't until 1973, however, that the term "dendritic cells" was coined by Ralph M. Steinman and Zanvil A. Cohn. In 2007 Steinman was awarded the Albert Lasker Award for Basic Medical Research for his discovery.

Marks Mouse 33D1,NLDC145 Rat OX62 Human CD1a,CD11c,CD83 Other marks:MHCⅡ,CD80,CD86

Sources of DC HSC Myeloid progenitor Lymphoid progenitor DC Mo PMN DC GM-CSF TNF-a IL-4 HSC Myeloid progenitor Lymphoid progenitor DC Mo PMN DC DC macrophage

Dendritic Cell Maturation                                                                                                                                                               LPS IL-1,TNFα

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Functions of DC: antigen presentation, immune activation, immune tolerance                                                                                                                                                                            DC通过FcR和C3bR吞噬被抗体和补体包被的抗原；通过甘露糖受体吞噬甘露糖化的抗原，受体介导的内吞句有选择性、高效性和饱和性。 DC非特异性吞噬颗粒抗原能力很弱。 FDC表面捕获抗原，无吞噬处理功能，直接提呈给B细胞识别

Antigen presenting cells mononuclear phagocyte system, MPS 1.Differentiation and distribution 2. Surface markers 3. function

Differentiation and distribution of MPS Bone marrow Blood tissues HSC monocyte macrophage Myeloid progenitor Pre-monocyte monocyte

Mononuclear phagocyte system, MPS 1. Surface markers: MHC-I/Ⅱmolecules CKR: M-CSFR CAM: LFA-1,ICAM-1,B7,CD40 FcR, CR1/3/4 2. secretion： -enzymes: lysosome , myeloperoxidase -cytokines (IL-1,TNF,IL-12) -complement:C1～C9, Bf -coagulation factor, PG, LTs, ACTH, etc. 分泌功能： -产生溶酶体酶、髓过氧化物酶、活性氧等，与杀伤和抗原处理有关 -分泌细胞因子、补体、凝血因子等，与炎症反应有关； -产生前列腺素、白三烯、ACTH、内啡肽等，与免疫调节有关。

Biological functions of MPS Phagocytosis antitumor：indirect or direct killing, ADCC; participating in immune response： -Ag presenting, providing the second signal； -CMI participating in immune regulation: -positive regulation：secrete IL-1/12，TNF-α -negative regulation：PG, TGF-β Mediating inflammation: phagocytosis, secrete inflammation medium 促炎细胞因子：TNF-alpha, IL-1, IL-6, IL-8, 参与局部和全身的炎症反应。

phagocytosis

Antigen presenting cells B cells BCR摄取抗原，在抗原浓度低的情况下，有效摄入和递呈，显示浓缩抗原效应。主要摄取可溶性抗原。

The properties of various APCs

Ag presentation Definition APC Ag presentation -MHC class I molecule pathway -MHC class II molecule pathway

Antigen capturing Phagocytosis Pinocytosis Receptor-mediated endocytosis

Antigen processing The conversion of an antigen into a form in which it can be recognized by lymphocytes.

Antigen presentation The process by which certain cells in the body (APC) express antigen peptide-MHC molecule complex on their cell surface in a form recognize by lymphocytes.

Recourses of antigens * exogenous antigen * endogenous antigen 递呈抗原的来源与抗原处理和递呈途径及诱导的免役应答类型密切相关。

The site of pathogen replication or mechanism of antigen uptake determines the antigen processing pathway used EXTRACELLULAR OR ENDOSOMAL REPLICATION Y Y Vesicular Compartment Contiguous with extracellular fluid Exogenous processing (Streptococcal, Mycobacterial antigens) INTRACELLULAR REPLICATION The intracellular replication takes place in cytosolic compartment and these pathogens are processed by endogenous processing pathway. The pathogens are for example viral antigens.The tumor antigens also use this pathway for processing.. extracellular replication or endosomal replication take place in extracellular fluid or intracellular vesicles. These infectious agents can be processed by exogenous processing pathway. For example streptococcal bacteria replicate in extracellular fluid and are taken up by phagocytosis. The mycobacteria replicate in vesicular compartment, such as endosome or lysosome and they are processed by exogenous processing pathway. The immune system has different strategies for eliminating pathogens from these two sites. Cells infected with viruses or with bacteria that live in the cytosol are eliminated by cytotoxic T cells; Pathogens and their products in the vesicular compartments of cells are detected by a different class of CD4 T cell. Cytosolic compartment Endogenous processing (Viral antigens) Distinct mechanisms of antigen generation are used to raise T cells suited to the elimination of endogenous or exogenous pathogens

Pathogens in Cytosol/Vesicles

The pathway of MHC I -associated endogenous Ag presentation endogenous antigen（such as virus Ag, tumor Ag) cytoplasm degraded by proteasome antigen peptide（8-13 AA） 　 transported to endoplasmic reticulum by TAP Peptide/MHC-I molecule complex to surface of APC submit to CD8+T

Degradation in the proteasome Cytoplasmic cellular proteins, including non-self proteins are degraded continuously by a multicatalytic protease of 28 subunits Proteins in cells are continually being degraded and replaced with newly synthesized proteins. Much cytosolic protein degradation is carried out by a large, multicatalytic protease complex called the proteasome. The proteasome is a large cylindrical complex of some 28 subunits. Proteins to be degraded are introduced into the core of the proteasome and are there broken down into short peptides. Two subunits of the proteasome, called LMP2 and LMP7, are encoded within the MHC near the TAP1 and TAP2 genes. A third subunit, MECL-1, which is not encoded within the MHC, is also induced by interferons and also displaces a constitutive proteasome subunit. These three inducible subunits and their constitutive counterparts are thought to be the active proteases of the proteasome. LMP: low molecule peptide MECL-1: 内源性蛋白通过蛋白酶体的孔道，被降解为6-30aa The components of the proteasome include MECL-1, LMP2, LMP7 These components are induced by IFN- and replace constitutive components to confer proteolytic properties. LMP2 & 7 encoded in the MHC Proteasome cleaves proteins after hydrophobic and basic amino acids and releases peptides into the cytoplasm

Transporters associated with antigen processing (TAP1 & 2) ER membrane Lumen of ER Cytosol ER membrane Lumen of ER Cytosol ATP-binding cassette (ABC) domain Hydrophobic transmembrane domain Peptide antigens from proteasome TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide Transporter has preference for >8 amino acid peptides with hydrophobic C termini.

Maturation and loading of MHC class I Endoplasmic reticulum TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide TAP-1 TAP-2 Peptide Calnexin: 钙联蛋白 calreliculin ：钙网蛋白 Calnexin binds to nascent class I chain until 2-M binds B2-M binds and stabilises floppy MHC Tapasin, calreticulin, TAP 1 & 2 form a complex with the floppy MHC Cytoplasmic peptides are loaded onto the MHC molecule and the structure becomes compact

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The pathway of MHC II -associated exogenous Ag presentation Exogenous antigen newly synthesised MHC class II molecule （in the endoplasmic reticulum ） early endosome li binds in the groove of MHC class II molecule lysosome protease MIIC late endosome li degrade protease Degrade into 1318AA peptide + MHC class II molecule Ag peptide/MHC class II molecule complex transport to the surface of APC, recognized by CD4+T Phagocytosis, pinocytosis, FcR-phagocytosis 新合成的MHCII在内质网与li结合, li为三聚体,每一个亚单位与一个分子的MHCII抗原结合槽结合,形成九链聚合体.其功能有两个: (1) 防止无关肽与MHCII结合; (2) 引导MHCII离开内质网, 运至含有特异性肽的内体. 当MHCII小泡与内体融合, 酸性环境下,蛋白酶将li降解, 仅留CLIP(class II -associated invariant-chain peptide，II类相关恒定链肽)仍结合在抗原结合槽内. HLA-DM可促进CLIP从抗原槽内解离, 并促进抗原肽结合进去。

Uptake of exogenous antigens Membrane Ig receptor mediated uptake Y Y Phagocytosis Complement receptor mediated phagocytosis Y Pinocytosis 巨噬细胞：吞噬、胞饮和受体介导的胞吞作用。 树突状细胞：以胞饮和受体介导的胞吞作用为主， B细胞：以BCR特异性摄取抗原为主；非特异性胞饮为辅。 opsonization Y Fc receptor mediated phagocytosis

Exogenous pathway Protein antigens Uptake In endosome Endosomes Increase in acidity Protein antigens In endosome Endosomes To lysosomes Cathepsin B, D and L proteases are activated by the decrease in pH Proteases produce ~24 amino acid long peptides from antigens Drugs that raise the pH of endosomes inhibit antigen processing

The functions of Ii: CLIP：class II-associated invariant chain peptide involve in the assembling and folding of MHC class II molecule; Block the groove of MHC class II molecule; Lead the assembled class II molecule to MⅡC. CLIP:II类相关恒定链肽段 CLIP：class II-associated invariant chain peptide

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Cross-presentation Class I MHC molecules present exogenous Ags to CD8+ T cells Cross-presentation of Ags by DC plays an important role in anti-viral infection and anti-tumor immunity. Refers to the presentation of exogenous antigens on MHC class I molecules. This pathway is thought to play an important role in activating CD8 T cells to respond to virus-infected cells and to some tumors.