Antigens PDF

Antigens MB.Ch.B Dr Safeen Othman Mahmood FKBMS / Clinical Microbiology-infectious diseases Immune system module Module Advisor Dr Dana M. tofiq Learning objectives By the end of this session you should be able to know: • Definition of antigen • Factors influencing immunogenicity • Biological c...

Antigens MB.Ch.B Dr Safeen Othman Mahmood FKBMS / Clinical Microbiology-infectious diseases Immune system module Module Advisor Dr Dana M. tofiq Learning objectives By the end of this session you should be able to know: • Definition of antigen • Factors influencing immunogenicity • Biological classes of antigens Immune System Module Antigen Antigen is defined as any substance that satisfies two distinct immunologic properties, immunogenicity and antigenicity. • Immunogenicity … to induce • Antigenicity… to react Immune System Module Immunogenicity Immunogenicity is the ability of a substance to induce immune response in the body Substance that satisfies this property i.e. immunogenicity is more appropriately called as ‘immunogen’ rather than ‘antigen’. Immune System Module Antigenicity Antigenicity (immunological reactivity) is the ability of an antigen to combine specifically with antibodies and/or T cell-surface receptors. All molecules having immunogenicity property, also show antigenicity, but the reverse is not true. For example, hapten is antigenic but not immunogenic, so it can react but cant induce immune response alone. However, in most writings the term antigen is used to describe either antigen or immunogen!! Immune System Module Epitope An epitope (antigenic determinant) is smallest unit of antigenicity. Structurally it is a area present on the antigen comprising of few (four to five) amino acids or monosaccharide residues, that is capable of sensitizing T and B cells and reacting with specific site of T cell receptor or an antibody. Specific site of an antibody that reacts with the corresponding epitope of an antigen is called as “paratope”. Immune System Module Epitope Diagram of sperm whale myoglobin showing locations of five sequential (linear) epitopes, Antigenic determinants (epitopes) required by antibodies (left) and linear sequence of peptides recognized by T cells (right). Immune System Module Types of Epitope Sequential (linear) epitope is present as a single linear sequence of few amino acid residues. Conformational (non sequential) epitopes found on the flexible region of complex antigens having tertiary structures. Formed by bringing together the surface residues from different sites of the peptide chain during its folding into tertiary structure. T cells recognize sequential epitopes, while B cells bind to the conformational epitopes. Immune System Module Hapten Hapten are low molecular weight molecules that lack immunogenicity (cannot induce immune response) but retain antigenicity or immunological reactivity (i.e. can bind to their specific antibody or T cell receptor). Hapten can become immunogenic when combined with a larger protein molecule called ‘carrier’. The hapten-carrier complex is capable of inducing immune response inImmune the System body. Module Hapten It is observed that animals immunized with such a hapten-carrier conjugate produce antibodies specific for: • Epitopes of hapten • Unaltered epitopes on the carrier protein • New epitopes formed by combined parts of both the hapten and carrier Immune System Module Classification of Hapten Based on complexity, hapten can be: • Complex hapten, it contain two or more epitopes • Simple hapten, it contain only one epitope (univalent) Immune System Module Antigen-host Relationship Based on the antigen-host relationship, antigens can be grouped into two groups: • Self or autoantigens • Non-self or foreign antigens Non-self Self Immune System Module Antigen-host Relationship, autoantigens Self or autoantigens belong to the host itself and are normally not immunogenic. Hosts do not react to their own antigens by exhibiting a mechanism called immunological tolerance. Sometimes, the self-antigens are biologically altered (e.g. as in cancer cells) and can become immunogenic. Immune System Module Antigen-host Relationship, non-self antigens Non-self antigen or foreign antigen are of three types based on their phylogenetic distance to the host. 1. Alloantigens are species specific. Tissues of all individuals in a species contain species-specific antigens. 2. Isoantigens are type of antigens which are present only in subsets of a species, e.g. blood group antigens and histocompatibility antigens. The histocompatibility antigens are highly specific as they are unique to every individual of a species. Immune System Module Antigen-host Relationship, non-self antigens 3. Heteroantigens: Antigens belonging to two different species are called as heteroantigens, e.g. antigens of plant or animal or microorganisms etc. A heterophile antigen is a type of heteroantigen that exists in unrelated species. Immune System Module Heterophile antigens Heterophile antigens are a type of heteroantigens that are present in two different species; but they share epitopes with each other. Forssmann antigen is universal heterophile antigen. It is a lipid carbohydrate complex present in all animals, plants and bacteria, but absent in rabbits. Hence, anti-Forssmann antibody can be prepared in rabbits. Immune System Module Diagnostic Application of Heterophile Antigens Heterophile antigens can be used in various serological tests. Antibody against one antigen can be detected in patients serum by employing a different antigen which is heterophile (cross reactive) to the first antigen. For example, Weil-Felix reaction is done for typhus fever. Antibodies against rickettsial antigens are detected by using cross reacting Proteus antigens. Immune System Module Diagnostic Application of Heterophile Antigens Paul-Bunnell test is used to diagnose infectious mononucleosis (Epstein-Barr virus). Here, sheep RBC antigens are used to detect cross-reacting antibodies in patient's sera. Cold agglutination test and Streptococcus MG test are done for primary atypical pneumonia. Here, antibodies against Mycoplosma pneumoniae are detected by using human O blood group RBC and Streptococcus MG antigens respectively. Streptococcus MG: Streptococcus milleri group Immune System Module Factors Influencing Immunogenicity Many factors determine immunogenicity: • • • • • • • • • • Molecular size Chemical–Structural Complexity Susceptibility of immunogen to tissue enzymes Foreignness to the host Host genetic factor Optimal dose of antigen Route of antigen administration Repeated Number of doses of antigens Multiple antigens Effect of prior administration of antibody Immune System Module Factors Influencing Immunogenicity, Molecular size Molecular size determine immunogenicity, the larger is the size; the more potent is the molecule as an immunogen. Molecules of > 10,000 Dalton molecular weight only can induce immune response (e.g. hemoglobin). Immune System Module Factors Influencing Immunogenicity, Chemical–Structural Complexity Certain amount of chemical complexity is required (e.g., amino acid homopolymers are less immunogenic than heteropolymers containing two or three different amino acids). Proteins are stronger immunogens than carbohydrates followed by lipid and nucleic acids. Immune System Module Factors Influencing Immunogenicity, Susceptibility to tissue enzymes Only substances that are susceptible to the action of tissue enzymes are immunogenic. Degradation of the antigen by the tissue enzymes produces several immunogenic fragments having more number of epitopes exposed. Immune System Module Factors Influencing Immunogenicity, Foreignness to the host Key factor which determines immunogenicity is foreignness Higher is the phylogenetic distance between the antigen and the host; more is the immunogenicity. Immune System Module Factors Influencing Immunogenicity, Host genetic factor Different individuals of a given species show different types of immune responses towards the same antigen. • Responders- are the individuals who produce antibody faster • Slow responders- are the individuals who produce antibody slowly and may need repeated antigenic exposures • Non-responders - are the individuals who do not produce antibody in spite of repeated antigenic exposures Immune System Module Factors Influencing Immunogenicity, Optimal dose of antigen An immunogen is immunologically active only in the optimal dose range. A too little dose fails co elicit immune response, and a too large dose leads to development of immunological tolerance, a phenomenon previously designated as immunological paralysis. Immune System Module Factors Influencing Immunogenicity, Route of antigen administration The immune response is better induced following parenteral administration of an antigen; however it also depends on the type of antibody produced. Immunoglobulin A (IgA) are better induced following oral administration of antigens. Inhalation of pollen antigens induces IgE synthesis; whereas the same antigens given parenterally lead to formation of IgG antibodies. The hepatitis B vaccine is more immunogenic following deltoid injection than gluteal injection. This may be due to the rareness of antigen presenting cells (APCs) in gluteal fat. Immune System Module Factors Influencing Immunogenicity, Repeated doses of antigens Repeated doses of antigens over a period of time are needed to generate an adequate immune response. This is due to the role of memory cells in secondary immune response. However, after a certain doses of antigens, no further increase in antibody response is seen. Immune System Module Factors Influencing Immunogenicity, Multiple antigens When two or more antigens (immunogen) are administered simultaneously, the effects may vary. Antibody response to one or the other antigen may be equal or diminished (due to antigenic competition) or enhanced (due to adjuvant-like action). Immune System Module Factors Influencing Immunogenicity, Adjuvant The term "adjuvant" refers to any substance that enhances the immunogenicity of an antigen. They are usually added to vaccines to increase the immunogenicity of the vaccine antigen. Examples of adjuvant include: • Alum (aluminium hydroxide or phosphate) • Mineral oil (liquid paraffin) • Freund's incomplete adjuvant- It is a water-in-oil emulsion containing a protein antigen in the aqueous phase Immune System Module Factors Influencing Immunogenicity, Adjuvant • Freund's complete adjuvant is the mixture of Freund's incomplete adjuvant & suspension of killed tubercle bacilli in the oil phase • Lipopolysaccharide (LPS) fraction of Gram-negative bacilli • Other bacteria or their products such as Mycobacterium bovis, toxoid (diphtheria toxoid and tetanus toxoid act as adjuvant for Haemophilus influenzae-b vaccine.) • Nonbacterial products: Silica particles, beryllium sulfate, squalene, and thimerosal. Immune System Module Factors Influencing Immunogenicity, Mechanism of Adjuvant Action Adjuvants augment the immune response appear to be through one or more of the following effects: Delaying the release of antigen: Adjuvant on mixing, precipitate the antigen which is then released slowly from the site of administration, thus prolonging the antigenic exposure. By activating phagocytosis: The adjuvant-antigen precipitate is of larger size, thus increases the likelihood of phagocytosis. The MDP (muramyl dipeptide) component of tubercle bacilli can activate the macrophages directly. Immune System Module Factors Influencing Immunogenicity, Mechanism of Adjuvant Action By activating TH cells: Activated macrophages release interleukin-11 (IL-11) and express higher level of MHC-II; thus promoting helper T (TH) cell activation which in turn activates B cells to produce specific antibodies. By granuloma formation: Certain adjuvants such as Freund's complete adjuvant causes chronic inflammation and granuloma formation at the inoculation site (hence not suitable for human use). Activated phagocytes in granuloma continue to enhance TH cell activation. Immune System Module Factors Influencing Immunogenicity, Effect of prior administration of antibody The immune response against a particular antigen is suppressed if its corresponding antibody was administered prior to that. The primary immune response is more susceptible to get suppressed than the secondary immune response. As a therapeutic application, in Rh -ve women carrying an Rh +ve fetus, the anti-Rh globulin is administered immediately following delivery (within 72 hours ), this prevents Rh sensitization in Rh negative women by a negative feedback mechanism. Immune System Module Biological Classes of Antigens Depending on the mechanisms of inducing antibody formation, antigens are classified as: • T cell-dependent (TD) antigens • T cell-independent (TI) antigens • Superantigen Immune System Module T-dependent (TD) Antigens Most of the normal antigens are T cell-dependent, they are processed and presented by antigen-presenting cells (APCs) to T cells which leads to T cell activation. Activated T cells then secrete cytokines that in turn stimulate the B cells to produce antibodies. Immune System Module T-independent (TI) Antigens TI-antigens like bacterial capsule, flagella and LPS (lipopolysaccharide) do not need the help of T cells and APCs. They directly bind to immunoglobulin receptors present on B cells and stimulate B cells polyclonally and leads to increased secretion of non-specific antibodies (hypergamma-globulinemia). TI-antigens can activate both mature and immature B cells. B cells can only differentiate into activated cells. Immune System Module T-independent (TI) Antigens No or less memory cells formation? Activated B cells do not undergo affinity maturation and class switch over (both properties are unique to TD antigen stimulated B cells); thus such an activated B cell can produce only limited classes of antibodies such as IgM and IgG3. Immune System Module T-Independent Antigen Vs T-dependent antigens T-Independent Antigen T-dependent Antigen Structurally simple- LPS, capsular polysaccharide, flagella Dose dependent Immunogenicity Structurally complex- protein in nature No or less memory Memory present No antigen processing Antigen processing step is needed Slowly metabolized Rapidly metabolized Immunogenic over wide range of dose Immune System Module T-Independent Antigen Vs T- dependent antigens T-Independent Antigen T-dependent Antigen Activate B cells polyclonally Activate B cells monoclonally Activate both mature and immature B cells B cells stimulated against T independent antigen do not undergo∙ Affinity maturation ∙ Class switch over Antibody response is restricted to IgM and IgG3 Activate mature B cells only B cells stimulated against T dependent antigen undergo ∙ Affinity maturation ∙ Class switch over Antibodies of all classes can be produced Immune System Module Superantigens Superantigens are the third variety of biological class of antigens, recently described in the last decades. Unique feature of superantigen is, they can activate T cells directly without being processed by antigen presenting cells (APCs). The variable β region of T cell receptor (vβ of TCR) appears to be the receptor for superantigen. Immune System Module Superantigens Superantigen directly bridge non-specifically between major histocompatibility complex (MHC)-II of APCs and T cells. Non-specific activation of T cells leads to massive release of cytokines which can activate B cell polyclonally, which leads to increased secretion of non-specific antibodies (hypergammaglobulinemia) Immune System Module Superantigens Immune System Module Superantigens Bacterial superantigen Staphylococcal toxin: Toxic shock syndrome toxin-1 (TSST-1); Exfoliative toxin; Enterotoxins Streptococcal pyrogenic exotoxin (SPE)-A and C Mycoplasma arthritidis mitogen-I Yersinia enterocolitica Yersinia pseudotuberculosis Viral superantigen Epstein-Barr virus associated superantigen Cytomegalovirus associated superantigen Rabies nucleocapsid HIV encoded superantigen (Nef protein (Negative regulatory factor)) Fungal superantigen Malassezia furfur Immune System Module Disease associated with superantigens Conditions associated with staphylococcal toxins are as follows: • Toxic shock syndrome • Food poisoning • Scalded skin syndrome Rare conditions such as atopic dermatitis, Kawasaki syndrome, psoriasis, acute disseminated encephalomyelitis. Immune System Module