Antigen Presentation (2023-2024) PDF

Block 1.2 (2023-2024) Doctor explanation Student explanation Antigen presentation Previous-years notes Del eted Done by : Key words Mawaddah Alnefaie Book Saleh Alobaid...

Block 1.2 (2023-2024) Doctor explanation Student explanation Antigen presentation Previous-years notes Del eted Done by : Key words Mawaddah Alnefaie Book Saleh Alobaid A bbreviations Title: Antigen Presentation CRN No: 15569(Male), 15581 (Female) Block: 1.2 Subject/Discipline: Immunology Expert: Dr.Sayed A.Quadri Block Coordinator: Dr.Sayed A.Quadri Learning objectives 14. Explain how T and B lymphocytes recognize antigens. 15. Why does the immune system require specialized system of antigen presentation for effective immune responses. 16. What kind of molecules plays a critical role in antigen presentation to T- lymphocytes. 17. What cell type (s) play important roles in the capture of invading microorganisms? Explain this role. 18. Explain how immune responses of CD8+ T- lymphocytes are initiated by dendritic cells. Define “cross presentation”. 19. Explain in general terms the molecular structure of MHC class I and class II molecules. Briefly discuss the major biological and functional characteristics of MHC molecules and the cells that express the various classes of MHC molecules 20. Describe in general terms how peptides originating from proteins outside the cell may be absorbed by MHC class II molecule ( briefly discuss the role of the CLIP peptide in this context) and explain how proteins of intracellular origin are presented by MHC class I molecule(Briefly discuss the role of TAT proteins in this process). 21. Explain the biological relevance of the extreme polymorphism of MHC molecules. 22. What molecules on T-lymphocytes contribute to their specificity of either class I or class II MHC associated peptide antigens. 23. Describe in what ways the two classes of MHC molecules play a role effector functions by CD4+ and CD8+ lymphocytes  Major = Dominant Terms to be familiar Histo = tissue Compatibility = how well the Adjustment take place It is also known as HLA = Human leukocyte antigen Complex = multiple genes MHC molecules (Major Histocompatibility Complex ) MHC molecules are important in Proteins present on the surface of cells. determining acceptance and rejection of transplanting organ. APC (Antigen Presenting Cells) Later on they discovered its psychological function : it is very Cells that process and display antigen to T-lymphocytes crucial in antigen presentation Naïve T-Lymphocytes - ‫ساذجة‬ Mature lymphocytes are capable to Also there is Naïve B-lymphocyte produce immune response and T-cells that haven't come in contact with antigen have encountered antigen but naïve lymphocytes does NOT Co-stimulation and Co-stimulatory signals have encountered antigen Additional signals required for T-cells activation Antigen and antigen receptor interaction is not sufficient for full stimulation of T-lymphocyte. T lymphocytes activation required additional signals and this signals are essential for full stimulation of T- lymphocytes and we call them as Co-stimulation and Co-stimulatory signals  Clonal expansion Clon = cells derived from single Proliferation (increase in number) of a particular clone of parental lymphocyte cell ( progenitor ) and Each clone is lymphocyte (antigen specific lymphocyte) specific for particular antigen Cytokines Proteins released by several types of cells that act on other cells -Primary or central lymphoid (adjacent – paracrine, or distant-endocrine) organs are the organs where lymphocyte maturation takes Also called the cell messengers place (E.g.: Bone marrow and thymus gland). Lymphoid organs -Secondary or peripheral Organs where lymphocytes mature (primary) or accumulate lymphoid organs are the organs (secondary) where mature lympho- cytes accumulate E.g. Spleen Lymphocyte recirculation and lymph node and mucosa- associated lymphoid tissue continuous movement of lymphocytes from – blood - peripheral lymphoid organs – lymph - blood Mature lymphocyte enter circulation when they reach peripheral lymphoid organs they stay for few days If they didn’t encountered antigen, they will exit  Lymphocyte differentiation Clonal proliferation culminates in formation of effector cells / memory cells. They differentiate to either: Effector cells Lymphocytes which produce an immune response – antibody producing plasma cells from B-Lymphocytes and Cytotoxic Lymphocytes(CTL) and T-helper cells (TH cells) from T-lymphocytes. We have distinct lymphocyte clones each for a specific antigen. Since we have thousands of antigens we will also have for each of them specific distinct lymphocyte clones Memory cells Some lymphocytes differentiates into long lived cells and are responsible for secondary immune response.  Lymphocyte repertoire - if one lymphocyte has a specificity to antigen X, Total collection of lymphocyte specificities. the other lymphocyte has a specificity to antigen Y. Characteristics of adaptive IS Specificity (one lock for one key), Ensures that immune responses are precisely targeted to microbial pathogens Diversity (several locks for different keys), Enables immune system to respond to a large variety of antigens Clonal Expansion, Non-reactivity to Self (no locks for self-keys). Prevents injury to the host during responses to foreign antigens Keys: Antigens Locks: Lymphocytes  Rule 1 Each lymphocyte clone has a distinct antigen receptor unique for a specific antigen.  The idea is the same here. The policeman(1) cannot arrest other than his criminal(1) and that goes for all of the other policemen. That’s how lymphocytes work so each lymphocyte will only recognize it’s sought antigen  Lymphocytes recognize antigens by antigen receptors on their surfaces Lymphocyte Antigen Antigen Receptor  Antigen recognition by B-lymphocytes B-lymphocytes recognize Antibodies and immunoglobulin are synonymous Recognize antigens by “membrane bound antibodies” on their surface. Recognize a variety of antigens (proteins, polysaccharides, fats and nucleic acids) There are two forms of antibodies: B-Lymphocytes recognize antigen directly -There are two closely associated 1/ bound to the membrane molecules with these membrane 2/ secreted antibodies which bound antibodies which called are produced by B-lymphocyte Ig-alpha and Ig-beta and they are and released in the surrounding important in signal transduction area - Signal transduction once the receptor bind to antigen it has to follow with certain events like cell proliferation ( clonal expansion ) Ig = Immunoglobulin and activation of the cell then differentiate into effector cells  Antigen recognition by T-lymphocytes T-lymphocytes recognize peptide antigen bound to and displayed by MHC on APCs- by their T-Cell Receptor (TCR) T-Lymphocytes recognize antigen in bond form The antigen is presented by the help of APC (antigen presenting cell). The antigen must be presented on the surface of APC and be bound to MHC. There are exceptions. There are T lymphocytes that can recognize antigens directly without the help of APC. But here we are talking about the most T lymphocytes (the majority)  Rule 2 T-lymphocytes recognize antigen when the antigen is presented by another cell on its surface with MHC. Why does the immune system require specialized system of antigen presentation for effective immune responses ? Why does the immune system require specialized system of antigen presentation for effective immune responses ? a. Antigen may enter through anybody site. b. Antigen specific lymphocyte has to come in contact with its homologous antigen (1 in 1 million)-----should locate & react ASAP. As soon as possible c. Different types of antigen may require different responses. (antigens in the cytosol and nucleus vs. antigens in the vesicle). IS = Immune system d. Sometimes IS needs to react to the same microbe in different ways depending on its stage of infection. Immune system has developed highly specialized system for capture and display of antigen. The antigen can be present in the cytosol ( Cytosolic antigen ) or inside the vesicle, such as : lysosome, phagosome, and endosome ( vesicular antigen)  Molecules THAT play a critical role in antigen presentation to T-lymphocytes MHC molecules present on cell surface. Antigen is bound with MHC molecule on cell surface. Antigen- MHC complex displayed on surface of APC. T-Lymphocytes recognize antigen only when it is displayed on surface of APC with MHC. Each individual’s MHC is unique. MHC polymorphism: That T-Lymphocytes recognize antigen only with MHC of means MHC in different same individual – MHC restriction. individuals are not the same, they are different. For example: T-Lymphocytes recognize antigen by TCR. Ahmed, Mohammed and Ali Each one of them has different MHC Ali’s T cell will recognize the antigen only when the antigen is presented with Ali’s MHC and so on What kind of molecules plays a critical role in antigen presentation to T-lymphocytes? The answer is: MHC 1&2 T-cell receptor T-cell epitope T-cell epitope, is a specific portion of an antigen that can be recognized by T-cells B cell’s antigen binding 2 population of T cells: receptor binds with the 1- T helper cell Co-stimulator antigen directly. It molecule: CD4. It has antigen produces antibody receptor called T cell receptor (TCR). 2- T cytotoxic cell Co-stimulator molecule: CD8. It has antigen receptor called T cell receptor (TCR).  What kind of molecules plays a critical role in antigen presentation to T-lymphocytes Mature T- lymphocyte cannot have both CD4 and CD8 CD3 present in all T lymphocytes on the cell surface and it is responsible for signal transduction of TCR Antigen TCR Capture Steps of antigen presentation: 1- Capture The antigen has to be captured by the antigen presenting cell. 2- Process Process The antigen has to be processed by antigen presenting cell. 3- Present The antigen has to be presented by the antigen presenting cell. Present  Microorganism will penetrate the mucosal surface and enter the body. Antigen capturing processing and presenting cells: They are located in the Dendritic cell will catch this organism then body site where capture process and presenting this the antigen can organism to T cell. get entry. Dendritic cells are important in antigen capture and presentation.  Function of Dendritic cells Binding to lectin receptor – phagocytosis/ Receptor- mediated endocytosis Capture antigen Stimulation through TLR & other PRR + Cytokines IL-1 & TNF Loose adhesiveness & Get activated express CCR7 1-Capture Binding of antigen to the dendritic cells in lectin receptor. Migrate to L.N The dendritic cell will ingest the antigen through receptor mediated endocytosis (macrophages will do it Present antigen to Lymphocytes through phagocytosis also dendritic cell can make it through phagocytosis). 2- Activation of dendritic cell The dendritic cells will get activated. Increase the production of MHC.  Function of Dendritic cells Binding to lectin receptor – phagocytosis/ Receptor- mediated endocytosis Capture antigen Stimulation through TLR & other PRR + Cytokines IL-1 & TNF 3- Migration Loose adhesiveness & Get activated The antigen has to be migrated express CCR7 to the lymph node through the chemokine receptor. Migrate to L.N The dendritic cells will express CCR7 (Chemokine receptor), it will express that after antigen capture. Present antigen to Lymphocytes It will attracted to move towards the lymph node. 4- Presentation Presentation of the antigen to lymphocyte in the lymph node. In the blood the dendritic cells present in the spleen are responsible for antigen capturing presentation because spleen is the site where blood are filtered. In the mucosal membrane in the skin, the antigen captured is by dendritic cells located in the Skin(sub- epithelial area). Explain in general terms the molecular structure of MHC class I and class II molecules. Briefly discuss the major biological and functional characteristics of MHC molecules and the cells that express the various classes of MHC molecules ‫تكبيل اليدين‬  MHC molecules M=Major H=Histo C=Compatibility HLA (Human Leukocyte Antigens) is a synonym of MHC molecules. Membrane proteins on APCs that display peptide antigen for recognition by T- lymphocytes. 2 types of MHC molecules – Class I & II.  structure of MHC class I structure of MHC class # It consist of : II - A large alpha polypeptide # It has alpha and beta chain polypeptide chains and - A small beta polypeptide they are almost equal in chain the size #It has three alpha domain: #It has two alpha Alpha 1, Alpha 2, and Alpha domain: 3 Alpha 1 and Alpha 2 and one beta domain : and two beta domain : Beta 2-microglobulin. Beta 1 and Beta 2 # between alpha 1 and 2 CD 8 CD 4 binding # peptide binding cleft there is depression called : binding is between alpha 1 and peptide binding cleft and beta 1 #peptide binding cleft is the Both of the area where peptide antigen polypeptide chains are is attached adhering to plasma The C terminal in alpha membrane chain is adhering to the. plasma membrane. But in beta it is not  #The amino acids sequence in the peptide binding cleft is not the same but remaining part is constant. # different people have different variance of MHC due to the amino acids. # As we discussed before: Each T-Lymphocyte CD 8 can have either CD4 or CD8 CD 4 and these molecules are binding binding essential for signal transduction CD8+T lymphocyte can recognize antigen that present with MHC class I And CD4+T-lymphocyte can recognize antigen that present with MHC class II Every individual have multiple MHC class I and II genes # class I have 3 genes : A,B, and C # class II have multiple genes and this is because in class I only alpha is responsible for peptide binding cleft, but in class II alpha and beta are responsible for that And this is in one chromosome, and in each chromosome we have different set of MHC genes # MHC expression is Each individual Codominant expression has different which means both variance and the alleles are expressed. parents determine the # For class I we have 6 genes ( one set alleles 3 from the first from mother and one and three from the the other from second, similarly we father ) have multiple DP alpha beta , DQ alpha beta, and so on. In this way the cell can express six variance of class I and multiple variance of class II MHC molecules  Rule 3 CD 4+ T-lymphocytes recognize antigen with MHC II molecules. And CD 8+ T-lymphocytes recognize antigen with MHC I molecules.  # Different variance of MHC molecules has implications of survival Biological and functional characteristics of MHC of individual and species. # Some of MHC variance may bind with certain peptide antigens more strongly and present antigen more efficiently For example : my MHC class I type 128 and this type is more strongly binding with antigen X and your MHC class I type is 238 and its binding less strongly. Therefore, the immune response in second type will be lower, so I will have the survival advantage This ensuring that individual in a population when the exposed to 1. MHC genes are highly polymorphic specific antigen at least few of individuals will survive because of efficient immune response against antigen  Rule 4 MHC molecule exhibit high level of polymorphism. (different variants of the genes in a population) And T-lymphocytes recognize antigen with self MHC molecules only : MHC restriction.  Codominant means all the genes will be expressed and all genes inherited from paternal will be expressed. That will increase the polymorphism. 2. 3.  4. 5. 6. This is the reason of why T- lymphocytes just recognize peptide antigens  7. Peptide antigen can be recognized by different type of lymphocytes. We have two types of T- lymphocytes: 1/CD4+T-lymphocyte MHC class II 2/CD8+T-lymphocyte MHC class I MHC class I attaches to cytosolic antigen and MHC class II attaches to vesicular antigen If no peptide fitted, they cannot be 8. expressed No immune response for self- 9. peptide antigens (that are normally present in our bodies) because of immunological tolerance, adaptive immune can distinguish Immunological tolerance is a complex series of mechanisms that impair the immune system to mount responses against self-antigens. Cells that express MHC MHC class II has restricted distribution MHC class I has wide distribution 21. Describe in general terms how peptides originating from proteins outside the cell may be absorbed by MHC class II molecule ( briefly discuss the role of the CLIP peptide in this context) and explain how proteins of intracellular origin are presented by MHC class I molecule(Briefly discuss the role of TAT proteins in this process). The class I MHC pathway converts proteins in the cytosol into peptides that bind to class I MHC molecules for recognition by CD8+ T cells. The class II MHC pathway converts protein antigens that are endocytosed into vesicles of antigen- presenting cells into peptides that bind to class II MHC molecules for recognition by CD4+ T cells. CTL, Cytotoxic T-lymphocyte; ER, endoplasmic reticulum; TAP, transporter associated with antigen processing. Packaging and loading of cargo Explanation in next slide The microbe is cytosolic antigen. It is not in a membrane bound vesicle. At first, the microbial proteins are folded so the first step is unfolding the microbial antigen, and this happens with a specific protein called Proteasome. Then after the antigens are generated, They need to be carried to the ER where MHC molecules are there. The transportation of the microbial peptide occurs through a protein known as TAP (transporter associated with antigen processing). Then it is carried to the MHC class 1 molecule to be presented on the surface of the cell. MHC molecule fit only the peptide antigen fragments of 10 to 15 amino acids sequence After the microbe is ingested inside the cell it will be in a membrane bound vesicle. This membrane bound vesicle will fuse with a phagosome containing lysosomes and ROS. Then the proteins of the microbe get degraded and peptide antigen fragments are generated. This antigen that entered the cell and became inside a membrane bound vesicle is the antigen presented by MHC class 2 and we call it vesicular antigen. We have MHC class 2 protein produced inside the ER and is fitted with a peptide called invariant chain. Then the protein produced in ER becomes inside a vesicle and this vesicle fuses with the vesicle containing the antigen. After they fuse, the invariant peptide bound with the MHC molecule is released and the peptide antigen is fitted into the MHC molecule and then is presented on the surface of the APC. Was discussed above  Endocytic vesicle Explain the biological relevance of the extreme polymorphism of MHC molecules Polymorphisms present in antigen binding groove. This determines ability of a MHC molecule to present a specific antigen. MHC polymorphism ensure at least few individuals have MHC that can present a particular antigen and ensure survival of the species  Describe in what ways the two classes of MHC molecules play a role effector functions by CD4+ and CD8+ lymphocytes. Explanation in next slide Describe in what ways the two classes of MHC molecules play a role effector functions by CD4+ and CD8+ lymphocytes. A, Protein antigens of microbes that are endocytosed from the extracellular environment by macrophages and B lymphocytes enter the class II MHC pathway of antigen processing. As a result, these proteins are recognized by CD4+ helper T lymphocytes, whose functions are to activate macrophages to destroy phagocytosed microbes and activate B cells to produce antibodies against extracellular microbes and toxins. B, Protein antigens of microbes that live in the cytoplasm of infected cells enter the class I MHC pathway of antigen processing. As a result, these proteins are recognized by CD8+ cytotoxic T lymphocytes, whose function is to kill infected cells.  Quiz Q1/ T-lymphocytes recognize antigens displayed on MHC Q3/How is the process of antigen presentation initiated molecules of the same individual only , this is called : for cytosolic antigens? A. MHC polymorphism A. Unfolding with TAP B. MHC restriction B. Direct presentation on the cell surface C. MHC expression C. Unfolding with Proteasome D. MHC homology D. Transport to the Golgi apparatus Q2/ which of the following cells are capable of displaying Q4/ Which of the following types of lymphoid organs is MHC class II Molecule? responsible for lymphocyte maturation? A. B-lymphocytes A. Primary lymphoid organs B. T-lymphocytes B. Secondary lymphoid organs C. NK cells C. Tertiary lymphoid organs D. Eosinophils D. Peripheral lymphoid organs 1/B – 2/A - 3/C - 4/A