Antigens and Immunogens Lecture Notes PDF

Antigens and Immunogens Randal Gregg, PhD Medical Immunology COM5850 LECTURE 6 harry potter symbol - Bing images defense against the dark arts - Bing images 1 Learning Objectives 1) Recognize factors relating to the host and the antigen that affect immunogenicity. 2) Differentiate between B cel...

Antigens and Immunogens Randal Gregg, PhD Medical Immunology COM5850 LECTURE 6 harry potter symbol - Bing images defense against the dark arts - Bing images 1 Learning Objectives 1) Recognize factors relating to the host and the antigen that affect immunogenicity. 2) Differentiate between B cell and T cell epitopes. 3) Define the classes of antigens and which classes activate B cells and T cells. 4) Define the role of an adjuvant and their role in immunogenicity. 5) Differentiate between affinity and avidity. 2 Before we speak about B and T cells – we must define what these cells respond to and discuss some of the characteristics of such stimuli 3 Microbe components serve as immunogens which incorporate epitopes Cutting up proteins into peptides is part of a process called antigen processing Epitope 3 Epitope 2 Epitope 1 Antigen (Ag) – any substance (proteins, carbohydrates, etc) that can bind to components of the immune response (receptors or products); some Ags can bind to immune receptors but not induce responses (haptens) Immunogens – Any substance that can induce an immune response; all immunogens are antigens Epitope – specific region(s) of an immunogen or Ag that interacts directly with the immune receptor 4 T cells are activated by recognizing antigen displayed on MHC APC = antigen presenting cells MHC = major histocompatibility complex Ag presentation and activation of T cells - Search Images (bing.com) 5 B cells can recognize antigen directly BCR = B cell receptor B cell recognition of antigen - Search Images (bing.com) 6 Types of immunogens • Alloantigens o Usually associated with transplantations o Antigens derived from different individuals from the same species Transplantation rejection • Autoantigens o Self-antigens; responses against these antigens produce autoimmunity • Heterophile antigens o Derived from a different species but have some shared epitopes across species o Streptococcus bacteria M protein has similar epitopes with cardiolipin associated with the human heart (rheumatic fever) alloantigens and transplant rejection - Search Images (bing.com) 7 Types of immunogens • Superantigens o Antigens that can activate B and T cell receptors regardless of the receptor specificity o Polyclonal activation occurs (cells with multiple antigen specificities are activated) • Mitogens o Antigens that can activate B and T cells but do not stimulate through the antigen receptors o Polyclonal activation occurs as with superantigens superantigens - Search Images (bing.com) 8 Host factors that affect antigen immunogenicity Genetics (related to antigen presentation) • Proteosome in the cytosol operates to cut proteins into peptides which can determine the binding affinity of peptides to immune receptors • Once peptides are generated in the cytosol – peptides are transported through TAP-1 and TAP-2 into the endoplasmic reticulum for loading onto major histocompatibility complexes (MHC); TAPs can regulate what peptides gain access to the MHC • T cells express T cell receptors (TCR) that can bind to the peptides displayed on the cell surface by MHC • MHC alleles have differences between individuals and can bind peptides with different affinities for interaction with TCR • TCR and B cell receptors (BCR) likewise bind the peptides with different affinity which can impact the nature of the immune reactivity by T and B cells Immunogenicity = ability of an antigen to bind to an antigen receptor and induce signaling leading to a response APC T cell 9 Host factors that affect antigen immunogenicity Previous exposures to antigen • Following an immune response – T and B cells generate memory populations which have a high affinity for antigen • Responses to antigen can be determined by the repertoire of these memory T and B cells that an individual possesses from previous encounters Status of the immune system • Chronic conditions or current infections of an individual can divert responses from new antigens or reduce availability of T and B cells which can alter responses (including memory) to an antigen 10 Characteristics of immunogens Homopolymer 1) Chemical complexity • Complex molecules are more readily distinguished as foreign rather than self Heteropolymers • Heteropolymers or side chains make better immunogens • Homopolymers are less immunogenic 11 Characteristics of immunogens 2) Degradability • Antigens need to be degraded (or processed) for presentation to T cells • If an antigen is not easily degraded like medical devices, then these are generally not immunogenic • Some implants can cause an inflammatory reaction over time 3) Chemical properties • Charged molecules more immunogenic than neutral molecules • Hydrophilic molecules more immunogenic than hydrophobic molecules 12 Characteristics of immunogens 4) Epitope density • More epitopes = more T and B cell responses • More sites to bind can improve the amount of complement fixation – can lead to better opsonization 5) Foreignness • Increased difference from self-antigens = more immunogenicity • To discuss later – tolerance suppresses immune responses to self-antigens and can interfere with reactivity if foreign antigens mimic self-antigens 13 Characteristics of immunogens 6) Size of the antigens • Larger antigens tend to be more immunogenic • Proteins > 10,000 daltons are more immunogenic • Polysaccharides > 100,000 daltons can be immunogenic for B cells • This is the main characteristic that keeps medical therapeutics from being immunogenic  Drugs and antibiotics tend to range around 100 Daltons 14 Antigenicity and exposure Dosage • High dosages of antigen can induce anergy (a state of nonresponsiveness) • Low dosages of antigen results in failure to induce sufficient receptor engagement to mediate an effective immune response o At least 100 TCR interactions required to induce sufficient signaling for T cell reactivity Route of administration • Site of antigen encounter determines which immune cells will be activated in what organs • Subcutaneous or intramuscular entry usually elicit the strongest immune response although may not be the site of microbial encounter • Intravascular (IV), respiratory or gastrointestinal entry elicits the poorest response (more about this route later – the mucosal immune response) 15 Epitopes Antigen • Small parts of the larger antigen (pizza) • Component actually bound by the BCR and TCR • Antigens can have a large number of epitopes – large proteins in particular (a reason why these are more immunogenic) • Same epitope will be present in multiple copies in many antigens bacteria often have 1,000 or more copies of a protein on the bacterial surface Epitopes (olives) (pepperoni) (ham) 1 antigen; multiple epitopes on the antigen; each epitope is present in multiple copies 16 B cell epitopes • BCR on B cells binds soluble antigen (no display by MHC required) • Epitopes recognized by BCR are generally conformational (discontinuous) but some can be linear (continuous) • May be protein, polysaccharide, lipid or nucleic acid • Protein epitopes may be in the denatured form 17 T cell epitopes • TCR on T cells binds antigen displayed by MHC molecules • Antigens are processed and epitopes revealed • Can only be protein coming from any source (intracellular and extracellular) Anchor residues of antigenic peptide interact with residues of MHC to hold the peptide in place Antigen residues of the peptide that interact with the TCR residues sufficient to drive cell signaling represent the antigenic epitope for the TCR (essence of T cell specificity) 18 Immunodominance • A single large protein can have as many as 20 different epitopes • Some of the epitopes will elicit a better immune response than others • Those epitopes that are preferential for B and T cell activation are known as immunodominant epitopes • Thus injecting an antigen into several different individuals will generally provoke B and T cell responses in all that are primarily directed against the same immunodominant epitopes 19 Haptens (type IV hypersensitivity) • Small organic molecules with a simple structure • Responses occur most often when the hapten is conjugated to a carrier protein • Hapten-carrier responses require: o Hapten-specific B cells o Carrier protein-specific T cells (help the B cells respond) o Conjugation of the hapten and carrier protein • B cells bind the hapten via the BCR • Hapten carrier is endocytosed by B cells and carrier protein peptides are presented to T cells on MHC • T cells are activated and help activate B cells to produce anti-hapten antibodies 20 Haptens • Sometimes when a drug is administered it will become attached to a host protein and then the drug can be recognized by B and T cells o EX – penicillin attached to red blood cells • The anti-hapten antibodies are the mediators of drug hypersensitivity when this occurs • Drug-induced autoimmune hemolytic anemia = antibodies are generated to bind to penicillin, activate complement and MAC lysis occurs as well as opsonization of the red blood cells 21 Proteins Classes of antigens • Most immunogenic; Size of >10 kDa • Complexity in charge and side chains • Thousands of repeat antigens on the microbe surface Polysaccharides • Immunogenic due to their repetitive structure (epitope density) – repetitions improve B cell activation • More immunogenic when associated with protein (peptidoglycans) or lipids (glycolipids) • Not processed or presented • Do not activate T cells 22 Nucleic acids Classes of antigens • Immunogenicity increases when bound to proteins  In systemic lupus erythematosus antibodies are made against our own DNA • Certain bacterial and viral DNA and RNA are recognized by TLR Lipids • Not immunogenic unless associated with protein or polysaccharide • Glycolipids and lipoproteins are some of the most potent stimulators of the immune system 23 Superantigens • Antigens that can activate T cells regardless of the specificity of a particular T cell • Microbial antigens activate as few as 0.001% of T cells during an encounter • Superantigens are released by pathogenic bacteria and can activate up to 20% of all T cells • Superantigen directly binds to the TCR, regardless of specificity, and triggers cell signaling leading to T cell activation • Primary TCR region bound by superantigen is V region in domain 3 of the TCR beta chain 24 Superantigens 25 Mitogens • Can activate B cells or T cells regardless of antigen specificity leading to polyclonal (multiple antigen specificities) lymphocyte activation • More potent stimulatory signal than superantigens • Activate through aggregation of non-specific stimulatory receptors such as TLR rather than BCR/TCR • NOTE – when mitogens stimulate B or T cells there is little memory populations generated (i.e. must signal through the BCR or TCR to generate memory) • All B cells above activated and produce antibodies (driven by mitogens) • Only the “red” BCR expressing B cell is specific for the pathogen 26 Adjuvants • Adjuvants help antigens to become more immunogenic • These molecules can be immunogenic themselves but often operate to increase the immune response in general by stimulating antigen presenting cell function • Adjuvants do not form stable complexes with the antigen in the mix – in contrast to haptens which do form stable complexes with a carrier protein • Adjuvants are most often included in vaccine formulations to enhance immunity to the vaccine targets • Approved adjuvants for use in the US:  Aluminum compounds – amorphous aluminum hydroxyphosphate sulfate, aluminum hydroxide, aluminum phosphate and potassium aluminum sulfate (or alum)  Monophosphoryl lipid A + aluminum salt (AS04), oil in water composed of squalene (MF59), monophosphoryl lipid A + a tree bark compound (AS01B) and cytosine phosphoguanine (CpG 1018) 27 Adjuvants 28 Adjuvants Mechanisms of action • Increase the half-life of the antigen = slow, continuous release of the antigen over time (EX – oil emulsion) • Increase the production of local inflammatory cytokines to activate immunity = adjuvant targets PRR • Improve antigen delivery, processing and presentation by antigen presenting cells = precipitate antigen (EX – salts) which is more easily phagocytosed by dendritic cells • Main target of adjuvants is dendritic cells • Induces the maturation process (increases MHC and accessory molecules to optimize antigen presentation to T cells) • Promotes dendritic cell migration from tissues to lymph nodes 29 Affinity vs avidity of antigen receptors Affinity • Strength of interaction of a single antigenic epitope with a single binding site of a BCR or TCR • The higher the affinity the more specific the BCR or TCR is to its antigenic epitope Avidity • Strength of interaction of all the antigen binding sites with the target epitopes • Relates to the strength of tendency of further interactions to occur once a single epitope has been bound (or if a single interaction is broken – how strong is the other interactions to prevent the antigen from diffusing away?) 30