MHC structure. Antigen processing and presentation
MHC restriction of T cells
CD4+ and CD8+ T cells can see peptides only when these peptides (MHC peptides) are displayed by that individual's MHC molecules. This property of T cells is called MHC restriction. Each TCR, and hence each clone of CD4+ or CD8+ T cells, recognizes one peptide displayed by one of the many MHC molecules in every individual.
structure of MHC molecules
Class I and class II MHC molecules are membrane proteins that each contain an extracellular peptide-binding cleft. The regions which show the higher variability among different HLA alleles are associated with the peptide-binding groove, ensuring that each allele present a different set of antigenic peptides
Binding of Peptides to Class II MHC Molecules
Class II MHC- expressing APCs constantly synthesize these MHC molecules in the ER. Each newly synthesized MHC class 2 contains an attached protein called invariant chain (Ii) which contains in turn a sequence called the class II invariant chain peptide (CLIP) that binds to the peptide-binding cleft of the class II molecule. This CLIP enables unspecific peptide binding to MHC class 2 and enables proteins that are supposed to bind to MHC class 1 from binding to MHC class 2. Later on this molecule with Ii gets transported to golgi stacks and instead of going directly to cell membrane, it goes afterwards to endosome and lysosomes where the Ii is degraded and CLIP is only left at antigen-binding cleft. Also in this vesicle, the foreign peptide/antigen is present as well. The vesicles also contain a class II MHC-like protein called DM, whose function is to exchange CLIP in the class II MHC molecule with other peptides that may be available in this compartment that can bind to the MHC molecule with higher affinity. -Basically the DM molecule facilitates the removal of CLIP and subsequent binding of the antigenic peptide
What is cross presentation?
Cross-presentation is a process that occurs when a professional APC takes phagocytosed material and presents it on MHC I. -This is the opposite of what we previously saw for the classical pathways that involved MHC II because if you phagocytose a foreign peptide, the cell will have MHC 2 instead of MHC 1. dendritic cells has the ability to ingest infected host cells, dead tumor cells, microbes, and microbial and tumor antigens and transport the ingested antigens into the cytosol, where they are processed by the proteasome. The antigenic peptides that are generated then enter the ER and bind to class I molecules, which display the antigens for recognition by CD8+ T lymphocytes
Class I MHC Molecules
Each class I MHC molecule consists of an α chain noncovalently associated with a protein called β2-microglobulin that is encoded by a gene outside the MHC. The α chain consists of three extracellular domains followed by transmembrane and cytoplasmic domains. The amino-terminal α1 and α2 domains of the α chain form two walls and a peptide-binding cleft, or groove; where the top part of the cleft make the contact with the T cell receptor and the bottom part contains amino acids residues (called anchor residues) that bind to the peptides to present to the T-cell. Also this alpha 1 and alpha 2 chain has polymorphism, meaning that those parts would differ from people to people, so that different MHC molecules to bind distinct sets of peptides. Expressed on all nucleated cells
Class 2 MHC
Each class II MHC molecule consists of two transmembrane chains, called α and β. Each chain has two extra-cellular domains, followed by the transmembrane and cytoplasmic regions. The nonpolymorphic α2 and β2 domains contain the binding site for the CD4 T cell coreceptor. Because CD4 binds to class II MHC molecules but not to class I, CD4+ T cells can only respond to peptides presented by class II MHC molecules. class II molecules are expressed ONLY on APCs such as dendritic cells, macrophages, and B lymphocytes naive CD4+ T-cells are MHC class 2 restricted meaning that they require MHC class 2 interaction to become activated. Upon activation naive T-cell become T-helper cell
pathogens have different strategies to escape MHC class 1
removing newly synthesized MHC molecules inhibiting the transcription of MHC gene blocking the transport of peptides by working on TAP by inhibiting MHC class 1 molecules, virus can nu evade immune system by reducing the presentation of its own antigens to CD8+ T cells
questions that antigen presenting can answer
How do the rare naive lymphocytes specific for any microbial antigen find that microbe, especially considering that microbes may enter anywhere in the body? How do different types of T cells recognize microbes in different cellular compartments?
Transport of Peptide-MHC class 2 Complexes to the Cell Surface
If a class II molecule binds a peptide with the right fit, the complex is stabilized and transported to the cell surface, where it can be recognized by a CD4+ T cell. Class II molecules that do not find peptides they can bind are eventually degraded by lysosomal proteases.
Binding of Peptides to Class I MHC Molecules
In order to form peptide-MHC complexes, the peptides must be transported into the endoplasmic reticulum (ER) The peptides produced by proteasomal digestion are in the cytosol, while the MHC molecules are being synthesized in the ER, and the two need to come together. This transport is achieved by a molecule called a transporter associated with antigen processing (TAP), which is located on the ER membrane. TAP actively pumps those peptides from cytosol to the inner side of ER. These pumped peptides get bound to the empty synthesized MHC class 1 receptors via a bridging protein called TAPASIN (acts as a transposon) !!! THIS REQUIRES ATP
MHC (HLA) proteins
MHC molecules are membrane proteins on APCs that display peptide antigens for recognition by T lymphocytes. MHC was discovered as the genetic locus that is the principal determinant of acceptance or rejection of tissue grafts exchanged between individuals, so basically if you had same MHC molecule, your body would accept the foreign/transplanted tissue. HOWEVER since tissue transplantation is not a naturally occuring process, MHC gene must have evolved to have other functions as well we know today that MHC molecules display peptides derived from microbial protein antigens to antigen-specific T lymphocytes as a first step in protective T cell-mediated immune responses to microbes.
Transport of Peptide-MHC Complexes to the Cell Surface
Once the peptide is bound to MHC class 1 molecule, they get stable and are delivered to the cell surface If MHC molecule on the surface cannot find the specific antigen it gets unstable and hence degraded in ER. One protein antigen may give rise to many peptides, only a few of which (perhaps only one or two from each antigen) can bind to the MHC molecules present in the individual and have the potential to stimulate immune responses in that individual. MHC class 1 (HLA type 2 in humans) are recognized by CD8+ T cells (killer T cells)
Presentation of lipid antigens
Presentation of lipids via the non polymorphic MHC class I-like molecule CD1 CD1 molecule is related to the class I MHC molecules, and are involved in the presentation of lipid antigens to T cells. -have very limited polymorphism -have 2 types; adaptive receptors and innate receptors
Cross-Presentation of Internalized Antigens to CD8+ T Cells
Some dendritic cells can present ingested antigens on class I MHC molecules to CD8+ T lymphocytes, which is not a typical pathway as we said that class 2 MHC molecules present internalized antigens to CD4+ T-cells. This pathway of antigen presentation is contrary to the general rule for APCs that most internalized proteins are displayed by class II MHC molecules to CD4+ T cells.
PROCESSING AND PRESENTATION OF PROTEIN ANTIGENS
The antigen presented via MHC class I or II is always a peptide (amino acid). Proteins in the cytosol of any nucleated cell are processed in proteolytic complexes called proteasomes and displayed by class I MHC molecules. (endogenous antigens) extracellular proteins that are internalized by specialized APCs (dendritic cells, macrophages, B cells) are processed in late endosomes and lysosomes and displayed by class II MHC molecules (MHC class II presents exogenous Ags (taken from the extracellular environment by endocytosis or phagocytosis) These different ways of presenting antigen are designed to sample all the proteins present in the extracellular and intracellular environments. The segregation of antigen-processing pathways also ensures that different classes of T lymphocytes recognize antigens from different compartments.
how is antigen presenting for B-cells?
The antigen receptors of B lymphocytes— namely, membrane-bound antibodies—can recognize a variety of macromolecules (proteins, polysaccharides, lipids, nucleic acids), in soluble form or cell surface- associated form, as well as small chemicals. Therefore, B cell-mediated humoral immune responses may be generated against many types of microbial cell wall and soluble antigens.
how is antigen presenting for T-cells?
The antigen receptors of most T lymphocytes, on the other hand, can see only peptide fragments of protein antigens, and only when these peptides are displayed on host cell surfaces bound to specialized proteins called major histocompatibility complex (MHC) molecules. Because the association of antigenic peptides and MHC molecules occurs inside cells, T cell- mediated immune responses may be generated only against protein antigens that are either produced in or taken up by host cells.
antigen presenting cells (APCs)
The cells that capture microbial antigens and dis-play them for recognition by T lymphocytes are called antigen-presenting cells (APCs) Naive T lymphocytes must see protein antigens presented by dendritic cells to initiate clonal expansion and differentiation of the T cells into effector and memory cells. Differentiated effector T cells again need to see antigens, which may be presented by various kinds of APCs besides dendritic cells, to activate the effector functions of the T cells in both humoral and cell-mediated immune responses.
How, then, are naive CD8+ T lymphocytes in lymph nodes able to respond to the intracellular antigens of infected cells? Similarly, tumors arise from many different types of cells, so how can diverse tumor antigens be presented to naive CD8+ T cells in lymph nodes by dendritic cells?
The initial response of naive CD8+ T cells, similar to CD4+ cells, requires that the antigens be presented by mature dendritic cells in lymph nodes through which the naïve T cells circulate. However, some viruses may infect only particular cell types and not dendritic cells, and these infected cells may not travel to lymph nodes or produce all the signals needed to initiate T cell activation.
ANTIGENS RECOGNIZED BY T LYMPHOCYTES
The majority of T lymphocytes recognize peptide anti- gens that are bound to and displayed by the MHC molecules of antigen-presenting cells (APCs) what is MHC (major histocompatibility complex)? The MHC is a genetic locus whose principal protein products function as the peptide display molecules of the immune system. The T cell receptor (TCR) recognizes some amino acid residues of the peptide antigen and simultaneously also recognizes residues of the MHC molecule that is displaying that peptide Also, some small populations of T cells recognize lipid and other nonpeptide antigens either presented by nonpolymorphic class I MHC-like molecules or without a requirement for a specialized antigen display system.
antigen presentation
When an antigen-presenting cell e.g. phagocyte displays foreign antigens on their own cell-surface membrane. Antigen presentation is performed by specialised proteins encoded by genes located within the Major Histocompatibility Complex (MHC) locus. B and T lymphocytes differ in the types of antigens they recognize.
Processing of Internalized Antigens for Display by Class II MHC Molecules
this pathway includes; 1. Internalization of the antigen, 2. proteolysis in endocytic vesicles, 3. association of peptides with class II molecules, 4. transport of peptide-MHC complexes to the cell surface
Why is cross presentation important?
allows immunity against most tumors and viruses that do not directly infect or damage the cells function. - kills body cells which are infected. Cross-presentation indicate that one type of cell, dendritic cells, can present the antigens of other infected or dying cells or cell fragments and prime (or activate) naive CD8+ T lymphocytes specific for these antigens. Once the CD8+ T cells have differentiated into CTLs, they kill infected host cells or tumor cells without the need for dendritic cells or signals other than recognition of antigen
Plasmacytoid dendritic cells
are named because of their morphologic resemblance to plasma cells; they are present in the blood and tissues. Plasmacytoid dendritic cells are also the major source of type I interferons in innate immune responses to viral infections
MHC contains two sets of highly polymorphic genes
called class I and class II MHC genes polymorphism refers to the presence of many variants of these genes in the population, meaning that several variants (defined as alleles) of the same gene exist in different members of the population. Polygenicity and polymorphism of MHC genes ensure that each individual can present a very broad repertoire of antigenic peptides to the T lymphocytes
Internalization and Proteolysis of Antigens in MHC class 2
different ways of internalization of antigens into the cell; -dendritic cells and macrophages may ingest extracellular microbes or microbial proteins by several mechanisms, including phagocytosis and receptor- mediated endocytosis -microbes might bind to specific receptors that are specific for that microbes products, or products of complement activation (opsonins) attached to the microbes. -B lymphocytes efficiently internalize proteins that specifically bind to the cells' antigen receptors -Certain APCs, especially dendritic cells, may also pinocytose proteins without any specific recognition event. WHAT HAPPENS AFTERWARDS? internationalized proteins enter acidic intracellular vesicles, called endosomes or phagosomes, which fuse with lysosomes where these proteins are broken down into different peptide chains that have different lengths and sequences.
MHC function (Physiological)
- The MHC complex is essential to help T cell recognition between self and non-self proteins, e.g. proteins from a microbe and proteins from are own cells -Presentation of self antigens via the MHC complex in the thymus is essential for T cell maturation ( negative selection by medullaryDC in medulla of thymus) -Each MHC molecule can present only one peptide at a time, because there is only one binding cleft, but each MHC molecule is capable of presenting many different peptides, which is essential for the antigen display function of MHC molecules, because each individual has only a few different MHC molecules that must be able to present peptides derived from a vast number and variety of protein antigens. -MHC molecules bind mainly peptides and not other types of antigens, only peptides have the structural and charge characteristics that permit binding to the clefts of MHC molecules. HOWEVER, The MHC is also involved in the reactions of T cells to some nonpeptide antigens, such as small molecules and metal ions. -MHC molecules acquire their peptide cargo during their biosynthesis, assembly, and transport inside cells. Therefore, MHC molecules display peptides derived from protein antigens that are inside host cells (pro- duced inside cells or ingested from the extracellular environment). This explains why MHC-restricted T cells recognize cell-associated microbes and not free antigens in the circulation, tissue fluids, or mucosal lumens. Class I MHC molecules acquire peptides from cytosolic proteins and class II molecules from proteins that are taken up into intracellular vesicles. -Only peptide-loaded MHC molecules are stably expressed on cell surfaces. The reason for this is that MHC molecules must assemble both their chains and bound peptides to achieve a stable structure, and empty molecules are degraded inside cells. This requirement for peptide binding ensures that only useful MHC molecules—that is, those displaying peptides—are expressed on cell surfaces for recogni- tion by T cells.
conventional dendritic cells (PROFESSIONAL ANTIGEN PRESENTING DENDRITIC CELLS)
-Professional antigen presenting cell (APC) -Picks up antigens in tissues and moves to secondary lymphoid tissues to activate T cells and initiate adaptive immune responses. The majority of dendritic cells in tissues and lymphoid organs belong to the classical subset.
Professional Antigen Presenting Cells (APC)
1. Efficient phagocytosis and/or internalization of microbial products 2. High expression of the restriction element at the cell surface (MHCI/II) 3. High levels of antigenic peptides production (antigen processing) 4. High expression of co-stimulatory molecules (CD28 binds to CD80 and CD86) 5. Expression of adhesion molecules to strengthen APC/T cell interaction (ICAM1, LFA3; CCR7)
How do T cells recognize an infected cell or foreign protein Ag?
1. Proteins broken down into peptides 2. Some peptides associate with class I or II MHC-peptide moves to surface of cell 3. Recognized by T cell with appropriate receptor -In presence of costimulatory signals, T cells are activated and mount a response 4. Responses are MHC-restricted (T cells do not recognize soluble antigens)
Both MHC I and MHC II are expressed codominantly
meaning that All vertebrates possess maternally and paternally inherited MHC loci, which include genes encoding the MHC proteins
why antigen presentation?
Adaptive immune responses are initiated by the recognition of antigens by antigen receptors of lymphocytes. An essential step for activation of T lymphocytes - very few naive lymphocytes are specific for anyone antigen and this small fraction of the body's lymphocytes need to locate and react rapidly to the antigen, wherever it is introduced - different types of adaptive immune responses are required to defend against different types of microbes.
dendritic cells are one of the antigen presenting cells
present in the T cell-rich areas of peripheral lymphoid organs and, in smaller numbers, in most other organs. There are two major populations of dendritic cells; - conventional (or classical) -plasmacytoid which differ in their locations and responses. while dendritic cells are capturing antigens, products of the microbes stimulate innate immune reactions by binding to Toll-like receptors (TLRs) and to other innate pattern-recognition receptors in the dendritic cells, tissue epithelial cells, and resident macrophages This results in the production of inflamma- tory cytokines such as tumor necrosis factor (TNF) and interleukin-1 (IL-1)
Processing of Cytosolic Antigens for Display by Class I MHC Molecules
Antigenic proteins may be; -produced in the cytoplasm from viruses that are living inside infected cells, -from some phagocytosed microbes that may leak from -be transported out of phagosomes into the cytosol, -from mutated or altered host genes that encode cytosolic or nuclear proteins, as in tumors. all of these proteins are targeted for proteolytic digestion by the ubiquitin-proteasome pathway. These proteins are tagged with a molecule called ubiquitin and later on threaded through a protein complex called the proteasome. What does proteasome do? The proteasomes degrade the unfolded proteins into peptides.
summary of this chapter
Antigens (Ags) presented to T cells via MHC class I/class II T cells recognize just a protein fragment (peptide), limited to primary structure (aa sequence) Some lipids are presented to T cells via the MHC class I-like CD1 molecules T cells recognize MHC/pep complex on the surface of APC (no soluble) The MHC haplotype determines against which epitopes the host T cells will respond to MHC is polygenic and polymorphic: ensure presentation of a broad spectrum of epitopes to host T cells Antigens that are recognized by these antibodies could be protein, lipid, carbohydrate or small molecules Antibodies can recognize linear or globular epitopes
CAPTURE OF PROTEIN ANTIGENS BY ANTIGEN-PRESENTING CELLS
Antigens are taken to peripheral lymphoid organs in two ways. Microbes or their antigens may enter the lymph or blood and circulate to lymph nodes or spleen, respectively, where they are captured by resident dendritic cells and presented to T cells. Other APCs may also capture antigens and display them to B cells in these organs. Dendritic cells in epithelia, connective tissues, and organs transport microbial antigens to lymphoid organs. This process involves a series of events following the encounter of dendritic cells with microbes capture of antigens, activation of the dendritic cells, migration of the antigen-carrying cells to lymph nodes, and display of the antigen to T cells.
