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adaptive immunity

  1. ADAPTIVE IMMUNITY By Prof Dr Asghar Javaid
  2. ADAPTIVE IMMUNITY • The next line of host defense is the adaptive immune system, which is composed of lymphocytes (also called lymphoid cells) and their secreted factors. • The immune response is specifically tailored against different microbes.
  3. ADAPTIVE IMMUNITY • This is achieved by first generating an enormous number of diverse lymphocytes, each with a unique antigen specificity. • Before they see their antigen, these lymphocytes are called naïve .
  4. ORIGIN OF LYMPHOID CELLS • All white and red blood cells originate from stem cells in the fetal liver and yolk sac during embryonic life and in the bone marrow after birth • . The common lymphoid progenitor is a type of stem cell that gives rise to lymphocytes of the adaptive immune system, including B cells and T cells
  5. ORIGIN OF LYMPHOID CELLS • The common lymphoid progenitor is also the source of innate lymphocytes, such as natural killer (NK) cells. • The process by which common lymphoid progenitors develop into lymphocytes depends on cytokines, and mutations in the genes encoding the receptors of these cytokines are often the cause of severe combined immunodeficiency, a complete absence of mature lymphocytes
  6. ORIGIN OF LYMPHOID CELLS • The ratio of T cells to B cells is approximately 3:1. • Often T cells are named by markers we can detect on their cell surface, called “cluster of differentiation” (CD) markers: • helper T cells are CD4-positive (CD4+), whereas cytotoxic T cells are CD8-positive (CD8+).
  7. ORIGIN OF LYMPHOID CELLS • All vertebrates produce enormously diverse pools of antigen receptors; • in humans, this pool is estimated to comprise 100 million different specificities, protecting us from millions of potential pathogens.
  8. T-cell Development • Prior to entering the thymus, stem cells lack antigen receptors and lack CD3,CD4,and CD8 proteins on the surface • The stem cell which initially express neither CD4 nor CD8(double negative) ,first differentiate to express both CD4 and CD8(double positive) and then proceed to express either CD4 or CD8
  9. T-cell Development • A double positive cell will differentiate into a CD4- positive cell if it contacts a cell bearing class II MHC proteins but will differentiate into a CD8-positive cell if it contacts a cell bearing class I MHC proteins
  10. T-cell Development • The double negative and the double positive cells are located in the cortex of thymus ,whereas the single positive cells are located in medulla. • Within thymus two very important processes called thymic education occur
  11. T-cell Development • Clonal deletion (NEGATIVE SELECTION) CD4+,CD8+ cells bearing antigen receptors for self proteins are killed by a process of programmed cell death called apoptosis (survival of cells with TCRs that do not recognize self antigen)
  12. T-cell Development • Positive selection ---- CD4+,CD8+ cells bearing antigen receptors that do not react with self MHC proteins are also killed (survival of cells with TCRs that react with self MHC)
  13. Why is this important? • For a T cell to function, it is essential that its TCR can interact strongly with an appropriate MHC molecule, and positive selection guarantees that the T cells that eventually exit the thymus have functional MHC binding TCRs
  14. Why is this important? • The removal of self-reactive cells ensures that the naïve T cells that eventually exit the thymus are not specific for self-antigens, and this self-tolerance is one of the key ways the immune system discriminates between what is self and what is foreign
  15. Why is this important? • Positive and negative selection remove all but the T- cell clones that react weakly with self-peptides presented in complexes with MHC proteins. • The same MHC proteins that are required for initial thymic selection of T-cell precursors later become the critical signals for activating T cells through their TCRs
  16. T-cell Development • During their passage through thymus each double positive T-cell synthesizes a different highly specific antigen receptor called T-cell receptor • T-cell progenitors differentiate under the influence of thymic hormones (THYMOSIN and THYMOPOITEN)into T-cell subpopulations.
  17. T-cell Development • These cells have surface glycoproteins CD3,CD4,CD8 • T-cell are divided into two categories CD4 or CD8 Naïve T- lymphocytes are found in blood and constitute 60% to 70%, and in T-cell zones of peripheral lymphoid organs such as the paracortical areas of lymph nodes and periarteriolar sheaths of the spleen.
  18. TCR • Each T-cell is genetically programmed to recognize a specific cell bound antigen by means of an antigen – specific T-cell receptor • Each T cell expresses TCR molecules of one structure and specificity
  19. TCR • TCR consists of a disulfide –linked heterodimer made up of an α & a β polypeptide chain, each having a variable (antigen-binding) and a constant region, minority composed of γ and δ polypeptide chain
  20. TCR • αβ TCR recognizes peptide antigens that are displayed by MHC molecules, γ δ TCR recognizes peptides ,lipids and small molecules without a requirement for display by MHC proteins • Each TCR is noncovalentally linked to CD3 (γ,δ,ε ) and dimer of ξ (zeta) chain (5 transmembrane proteins)
  21. CD4 lymphocytes • 1-Help B-cell to develop into antibody –producing plasma cells • 2-Help CD8 T cells to become activated cytotoxic T- cells • 3-Help macrophage to effect delayed hypersensitivity Functions performed by two populations of CD4 cells
  22. CD4 lymphocytes Th-1 cells – a) activate cytotoxic T-cells by producing IL-2, b) initiate the delayed hypersensitivity response by producing IL-2 and gamma interferon Th-2 cells -perform B-cell helper function by producing IL-4 and IL-5
  23. Comparison of Th-1cells and Th-2 cells • IL-2 and gamma interferon yes • IL-4,IL-5,IL-6,IL-10 no • Cell mediated immunity and DTH yes • Antibody production no • Stimulated by IL-12 yes • Stimulated by IL-4 no • IL-2 and gamma interferon no • IL-4,IL-5,IL-6,IL-10 yes • Cell mediated immunity and DTH no • Antibody production yes • Stimulated by IL-12 no • Stimulated by IL-4 yes
  24. Regulator of balance between Th-1 and Th-2 cell • IL-12 produced by macrophages ,increases Th-1 cells • Gamma interferon produced by Th-1 cells activate macrophage and inhibits the production of Th-2 cells • IL-10 produced by Th-2 cells inhibit IL-12 production by macrophages
  25. CD8 lymphocytes Perform cytotoxic functions • Kill virus infected cell • Kill tumor cells • Kill allograft cells They kill by two mechanisms Release of perforins Apoptosis
  26. Activation of T-cells • Ingestion of foreign protein into APC • Cleaved small peptides in cytoplasm • Associate with class II MHC proteins (viral with MHC-I protein ) • Complex is transported to surface of APC • Antigen associated with MHC-II on surface presented to receptor on the CD4+cell ( associated with MHC-I to CD8+ cell )
  27. Activation of T-cells • T-cell recognizes on polypeptides in association with MHC molecules • Class I proteins present endogenously synthesized antigens (cleaved by a proteosome ) viral proteins • Class II proteins present antigens of extracellular organisms (cleaved within endosome)
  28. Two Signals are required to activate T-cells • 1-interaction of antigen and MHC protein with T-cell receptor specific for antigen • CD4 protein interacts with MHC class II protein • LFA lymphocyte function associated antigen on T- cells (both CD4+,CD8+) binds to ICAM intercellular adhesion molecule on APC
  29. Two Signals are required to activate T- cells • 2-Costimulatory signal B-7 on APC interacts CD28 on Helper T-cell Production of costimulatory protein depends on activation of toll – like receptor on APC
  30. Activation of T-cell • Signal is transmitted by CD3 protein complex • Activates phosphokinases, activate phospholipase C • Which cleaves Phosphoinositide to produce inositol triphosphate • Opens calcium channels
  31. Activation of T-cell • Calcium activates calcineurin • Calcineurin moves to nucleus • Activation of genes for IL-2 and IL_2 receptor • Stimulates T-cell to multiply into clone of antigen specific T-cell
  32. INHIBITION OF T-CELL ACTIVATION • CTLA-4 appears on T-cell • Binds with B-7 by displacing CD28 • Interaction inhibits T-cell activation by blocking IL-2 synthesis
  33. Effector functions of T-cells • Delayed Hypersensitivity antigens of intracellular organisms e.g,Histoplasma, Mycobacterium Mediated by CD4 cells (Th-I) macrophage Important cytokines gamma interferon , macrophage activation factor, macrophage migration inhibition factor Role of IL-12 ---- gamma interferon axis
  34. Effector functions of T-cells • Cytotoxicity • CD8 cells recognize viral antigen and MHC class I molecules on the surface of infected cells • Cytotoxic T cell must be activated by IL_2 produced by helper T-cell • Helper T-cells recognize viral antigens bound to class molecules on an APC
  35. Effector functions of T-cells • Activated T-helper cell secrete IL-2 which stimulates the virus specific cytotoxic cell to form a clone of activated cytotoxic cell • perforins form channels • granzymes are proteases that degrade proteins in cell membrane • Activate caspases that initiate apoptosis
  36. Cytotoxic T Cells Lyse Infected Cells
  37. Fas-Fas Ligand interaction • Fas is a protein displayed on the surface of many cells • When a cytotoxic T-cell receptor recognizes an epitope on the surface of a target cell,FasL is induced in the cytotoxic cells. • When Fas and FasL interacts apoptosis of the target cell occurs
  38. Immune surveillance • Many tumor cells develop new antigens on their surface. • These antigens bound to class I proteins are recognized by cytotoxic T-cells which are stimulated to proliferate by IL-2 • The resultant clone of cytotoxic T-cells can kill the tumor cells phenomenon called Immune surveillance
  39. Response to allograft • Cytotoxic T-cells recognize the class I molecules on the surface of the foreign cells • Helper cells recognize the foreign class II molecules on certain cells in the graft e.g,macrophages and lymphocytes • The activated helper cell secrete IL-2 which stimulates the cytotoxic cell to form a clone of cells.These kill the cells in allograft
  40. Regulatory functions of T-cells • Antibody production • Cell mediated immunity • Suppression of certain immune responses Suppressor T-cells----CD25 ,CD4
  41. Central Role of Helper T Cells
  42. The Th-17 cell subset • The Th-17 cell subset is closely related to the Th-1 cell subset but is generated by high levels of IL-1, IL- 6, and IL-23 at the time of initial activation by DCs. • Th-17 cells express the signature transcription factors RORC and STAT3, which are reinforced by autocrine signaling from the cytokine IL-21.
  43. The Th-17 cell subset • Th-17 cells also produce the cytokines IL-17 (the source of their name), which stimulates phagocytes and mucosal epithelial cells to increase the production of IL-1, IL-6, and neutrophil-attracting chemokines.
  44. The Th-17 cell subset • Th-17 cells also make IL-22, which stimulates mucosal epithelial cells to increase the production of antimicrobial defensins and tight junction proteins. • Together, the cytokines from Th-17 cells and the neutrophils they recruit defend the barrier tissues against bacterial and fungal infections.
  45. B cells • During embryogenesis,B-cells precursors are recognized in the fetal liver 7-8th week of gestation • They do not require the thymus for maturation • They migrate to bone marrow
  46. B cells • 10 -20% of circulating peripheral lymphocyte population • Also present in peripheral lymphoid organs such as ,lymph nodes ,spleen ,tonsils and extralymhatic organs such as gastrointestinal tract
  47. B cells • Lymh node superficial cortex • Spleen white pulp • B –cells are located in follicles • Maturation of B-cell has two phases Antigen independent phase Antigen dependent phase
  48. B cells Stem cell, pro-B cell---ü chain (cytoplasm),CD10,CD19, pre-B-cell--- ü chain +light chain,CD10,CD19 immature B-cell----CD10,CD19 ,CD20,SIg(IgM) mature B-cell , Activated B-cell memory B cell plasma cell
  49. B CELLS • B cells perform two important functions: • (1) they differentiate into plasma cells that produce antibodies (also called immunoglobulins) and • (2) they can become long-lived memory B cells that can rapidly respond to a reinfection.
  50. B CELLS • The immunoglobulin on the B-cell surface is its antigen receptor (B-cell receptor or BCR) and the ability of a B-cell precursor to make this antigen receptor determines whether it is allowed to develop into a mature B cell.
  51. B CELLS • B-cell precursors first arise from stem cells in the fetal liver, but by the time of birth, these stem cells migrate to the bone marrow, which is their main location during childhood and adult life. • Unlike T cells, B cells do not require the thymus for maturation.
  52. B CELLS • The maturation of B cells has two phases: • the first is the antigen-independent phase, which consists of stem cells, pre-B cells, and B cells, and it is during this phase that the B cell recombines its immunoglobulin genes to make a unique antigen receptor.
  53. B CELLS • For pre-B cells to differentiate into B cells, a functional immunoglobulin must be present on the cell surface. • A protein called Bruton’s tyrosine kinase (BTK) detects this immunoglobulin and signals to the cell to continue to divide and differentiate.
  54. B CELLS • A mutation in the gene encoding this protein causes X-linked agammaglobulinemia, a condition in which cells cannot progress to the pre-B cell stage and no antibodies are made
  55. B CELLS • During the second phase, which is the antigen- dependent phase, mature B cells with functional antigen receptors interact with antigens. • The immunoglobulin (Ig), or BCR, of a mature B cell is an IgM molecule with an additional region at the end of its heavy chain that tethers it to the B-cell surface.
  56. B CELLS • Approximately 109 B cells are produced each day, but only a small fraction of these make it from the bone marrow into the circulation, and unless they are activated through their antigen receptors, circulating B cells have a short life span (i.e., days or weeks).
  57. B cells Markers- (BCR) IgM or IgD, Igα/Igβ –(hetrodimer of nonpolymorphic transmembrane proteins,) donot bind antigen like CD3 but essential for signal transduction through antigen receptor MHC class I,MHC class II,B7,CD40, CD10,CD19, surface receptors for FcγR, complement receptor(CD21)
  58. Activation of B-cell • B-lymphocytes may be activated by protein and non protein antigens • Antigens do not have to be presented to B-cells in association with class II MHC proteins, unlike T-cells • Antigen receptors on B-cell (IgM or IgD) recognize many different types of molecules, such as peptides, polysacharides,nucleic acids, and small molecules
  59. Activation of B-cell • Antigen is processed by APC (B-cell) • Epitopes appear on the surface in association with MHC class II protein • This complex is recognised by helper T-cells with TCR
  60. Two costimulatory interactions • CD28 on T-cell must react with B7 on B-cell, required for activation of T- cell to produce IL-2 • CD40L on the T-cell must interact with CD40 on the B-cell, required for class switching from IgM to IgG • LFA on T-cell interacts with ICAM -1 on B-cell
  61. Activation of B-cell • T-cell produces cytokines (IL-2,IL-4 B-cell growth factor,IL-5 B-cell differentiation factor). • Cytokines stimulate growth and differentiation of the B-cell .
  62. Activation of B-cell • Production of many plasma cells that produce large amounts of immunoglobulins specific for the epitope • Memory cells are also produced which remain quiescent for long periods but are capable of being activated rapidly upon re exposure to antigen
  63. Functions of B-cell • They differentiate into plasma cells and produce antibodies • They can present antigen to helper T-cell
  64. Immunoglobulins Structure Types Characters
  65. Antibodies • Antibodies are globulin proteins (immunoglobulins) that react specifically with that antigen that stimulate their production. • They make up about 20 % of the protein in blood plasma.
  66. Antibodies • Blood contain three types of globulins, alpha, beta and gamma, based on their electrophoretic migration rate. • Antibodies are gamma globulins.
  67. Antibodies • There are five classed of anitibodies: • IgG, IgM, IgA, IgD and IgE. • Antibodies are subdivided into these five classes based on difference in their heavy chains.
  68. Antibodies • H chains are distinct for each of the five immunoglobulin classes and are designated γ,α,µ,ε,and δ.
  69. IMMUNOGLOBULIN STRUCTURE • Immunoglobulins are glycoproteins made up of light (L) and heavy (h) polypeptide chains. • The terms “light” and “heavy” refers to molecular weight
  70. IMMUNOGLOBULIN STRUCTURE • light chains have a molecular weight of about 25,000 whereas heavy chains have a molecular weight of 50,000-70,000
  71. IMMUNOGLOBULIN STRUCTURE • Heavy chain gene present on chromosome 14 • Light chain lambda gene are present on chromosome 2 • Light chain kappa gene are present on chromosome 22
  72. IMMUNOGLOBULIN STRUCTURE • The simplest antibody molecule has a Y shape and consists of four polypeptide chains: • two H chains and two L chains. • An individual antibody molecule always consists of identical H chains and identical L chains
  73. IMMUNOGLOBULIN STRUCTURE • The four chains are lilnked by disulfide bonds. • Interchain = H-H,H-L,L-L • Intrachain =L chain has 2 bonds γ,α,δhave 4 bonds µ,ε have 5 bonds
  74. IMMUNOGLOBULIN STRUCTURE • L and H chains are subdivided into variable and constant regions. • The regions are composed of three dimentionally folded repeating segment called domains.
  75. IMMUNOGLOBULIN STRUCTURE • An L chain consist of one variable (VL) and one constant (CL) domain. • Most H chains consist of one variable (VH) and three constant (CH) domains. (IgG and IgA have three CH domains, whereas IgM and IgE have four).
  76. IMMUNOGLOBULIN STRUCTURE • Each domain is approximately 110 amino acids long. • The constant region of the light chain has no known biologic function.
  77. IMMUNOGLOBULIN STRUCTURE • The variable regions of both the light and heavy chains are responsible for antigen-binding, whereas the constant region of the heavy chain is responsible for various biologic functions e.g , complement binding site is in the CH2 domain.
  78. IMMUNOGLOBULIN STRUCTURE • STRUCTURE OF THE VARIABLE REGION • A. Hypervariable (HVR) or complementarity determining regions (CDR) • The variable regions of both L and H chains have three extremely variable (hypervariable) amino acid sequences at the amino-terminal end that form the antigen-binding site.
  79. IMMUNOGLOBULIN STRUCTURE • Only 5-10 amino acids in each hypervariable region form the antigen-binding site. • These small portions are called complimentarity determining regions(CDR)
  80. IMMUNOGLOBULIN STRUCTURE • L chains belong to one of two types, k( kappa) or λ (lambda), on the basis of amino acid differences in their constant regions. • Both types occurs in all classes of immunoglobulins (IgG, IgM etc) but any one immunoglobulin molecule contains only one type of L chain.
  81. IMMUNOGLOBULIN STRUCTURE • Framework regions • The regions between the complementarity determining regions in the variable region are called the FRAMEWORK REGIONS • The remarkable specificity of antibodies is due to these hypervariable regions.
  82. IMMUNOGLOBULIN STRUCTURE • Antigen-antibody binding involves electrostatic and vander Waal’s forces and hydrogen and hydrophobic bonds rather than covalent bonds.
  83. IMMUNOGLOBULIN STRUCTURE • The amino-terminal portion of each L chain participates in the antigen-binding site. • The amino-terminal portion of each H chains participates in the antigen-binding site. • The carboxy terminal portion forms the Fc fragment, which has the biological activities.
  84. IMMUNOGLOBULIN STRUCTURE • If an antibody molecule is treated with a proteolytic enzyme such as papain, peptide bond in the “hinge” region are broken, producing two identical fragments.
  85. IMMUNOGLOBULIN STRUCTURE • Fab fragments, which carry the antigen-binding site, and • one Fc fragment crystalisable which is involved in placental transfer, complement fixation, attachment site for various cells, and other biological activities.
  86. IMMUNOGLOBULIN STRUCTURE • Hinge Region This is the region at which the arms of the antibody molecule forms a Y. • It is called the hinge region because there is some flexibility in the molecule at this point.
  87. IMMUNOGLOBULIN STRUCTURE • Oligosaccharides Carbohydrates are attached to the CH2 domain in most immunoglobulins. • However, in some cases carbohydrates may also be attached at other locations
  89. IgG 75% of the total immunoglobulin, IgG is the major Ig in extra vascular spaces • 1000mg/dl in blood • Molecular wt is 150x1000 • monomer
  90. IgG • Each IgG molecule consists of two L chains and two H chains linked by disulfide bonds (molecular formula H2L2). • Because it has two identical antigen-binding sites, it is said to be divalent
  91. IgG • There are four subclasses. IgG1-IgG4, Based in antigenic differences in the H chains and on the number and location of disulfide bonds.
  92. IgG • IgG1 makes up most (65%) of the total IgG. • IgG2 antibody is directed against polysaccharide antigens and is an important host defense against encapsulated bacteria.
  93. IgG • IgG is the predominant antibody in the secondary response and constitutes an important defense against bacteria and viruses. • IgG is one of the two immunoglobulins that can activate complement; IgM is the other.
  94. IgG IgG is the only antibody to cross the placenta; only its Fc portion binds to receptors on the surface of placental cells . It is therefore the most abundant immunoglobulin in newborns.
  95. IgG • IgG is the immunoglobulin that opsonizes. • It can opsonize; i.e enhance phagocytosis, because there are, receptors for the γH chain on the surface of phagocytes. • IgM does not opsonize directly, because there are no receptors on the phagocyte surface for the uH chain.
  96. IgG • IgM activates complement, and the resulting C3b can opsonize because there are binding sites for C3b on the surface of phagocytes.
  97. IgA • IgA is the main immunoglobulin in secretions such as colostrum, saliva, tears and respiratory, intestinal and genital tract secretions. • It prevents attachment of microorganisms e.g bacteria and viruses, to mucous membranes
  98. IgA • 15% of total immunoglobulin • Mol.wt 400,000 • Each secretary IgA molecule consists of two H2L2 units plus one molecule each of J chain and secretary component
  99. IgA • The two heavy chains in IgA are α heavy chains. • It exists as monomer and dimer form • In serum, some IgA exists as monomeric 80%
  100. IgA • The secretory components is a polypeptide synthesized by epithelial cells that provide for IgA passage to the mucosal surface. • It also protects IgA from being degraded in the intestinal tract.
  101. IgM • IgM is the main immunoglobulin produced early in the primary response. • 10% of total immunoglobulin • Mol.wt. 900,000 • 120mg/dl in blood
  102. IgM • It is present as a monomer on the surface of virtually all B cells, where it function as an antigen-binding receptor. • In serum, it is pentamer composed of five H2L2 units, plus one molecule of joining chain. • IgM has a u heavy chain.
  103. IgM • Because the pentamer has 10 antigen-binding sites, it is the most efficient immunoglobulin in agglutination, complement fixation (activation) and other antibody reactions and is important in defense against bacteria and viruses.
  104. IgM • It can be produced by the fetus in certain infections. • It has the highest avidity of the immunoglobulin; • its interaction with antigen can involve all 10 of its binding sites.
  105. IgD • The immunoglobulin has no known antibody function but may function as an antigen receptor; • it is present on the surface of many B lymphocytes.
  106. IgD • It is present in small amounts in serum. • Exists as monomer • Mol.wt.180,000 • 0.2% of total immunoglobulin • Cannot cross placenta
  107. IgE • Exists as monomer • Mol.wt 190,000 • Four CH domains • The Fc region of IgE binds to the surface of mast cells and basophils. • Bound IgE serves as a receptor for antigen (allergen).
  108. IgE • IgE is medically important for two reasons. • (1) it mediates immediate (anaphylactic) hypersensitivity • (2) it participates in host defenses against certain parasites e.g helminthes (worms).
  109. IgE • When the antigen-binding sites of adjacent IgEs are cross-linked by allergens, several mediators are released by the cells and immediately (anaphylactic) hypersensitivity reactions occur.
  110. IgE • Although IgE is present in trace amounts in normal serum (approx.0.004%), persons with allergic reactivity has greatly increased amounts, and IgE may appear in external secretions.
  111. IgE • IgE does not fix complement • and does not cross the placenta. • It binds to FcεR1 ,which is high affinity receptor present on mast cell and basophil
  112. IgE • IgE is the main host defense against certain important helminthes (worms) infections such as strongyloides, Trichinella, Ascaris and the hookworms, Necator and Ancylostoma. • The serum IgE level is usually increased in these infections.
  113. IgE • Because worms are too large to be ingested by phagocytes, they are killed by eosinophils that release worm-destroying enzymes. • IgE specific for worm protein bind to receptors on eosinophils, triggering the antigen-dependant cellular cytotoxicity (ADCC) response
  114. 1-ISOTYPES • Isotypes are antigenic determinants that characterize classes and subclasses of heavy chains and types and subtypes of light chains. • Each class, subclass, type and subtype of immunoglobulin has its unique set of isotypic determinants.
  115. 1-ISOTYPES • These are defined by antigenic (amino acids) differences in their constant regions. • Although different antigenically, all isotypes are found in all normal humans.
  116. 1-ISOTYPES • For example, IgG and IgM are different isotypes; • the constant region of their H chains (γ & u) is different antigenically ( the five immunoglobulin classes-IgG, IgM, IgA, IgD and IgE are different isotypes, their H chains are antigenically different).
  117. 1-ISOTYPES • The IgG isotype is subdivided into four isotypes, IgG1, IgG2 ,IgG3, and IgG4, based on antigenic difference of their heavy chains. • Similarly, IgA1 and IgA2 are different isotypes (the antigenicity) of the constant region of their H chains is different.) and k and λ chains are different isotypes.
  118. 2-ALLOTYPES • Allotypes, on the other hand, are additional antigenic features of immunoglobulins that vary among individuals. • Allotypes are antigenic determinants specified by allelic forms of the Ig genes.
  119. 2-ALLOTYPES • They vary because the genes that code for the L and H chains are polymorphic and individuals can have different alleles. • All allotypes are not found in all members of the species. • The prefix Allo means different in individuals of a species
  120. 2-ALLOTYPES • For example, the γH chain contains an allotype called Gm, which is due to a one or two amino acid difference that provides a different antigenicity to the molecule. • Each individual inherits different allelic genes that code for one or another amino acid at the Gm site.
  121. 3-IDIOTYPES These are the antigenic determinants formed by the specific amino acids in the hypervariable region. Definition - Unique antigenic determinants present on individual antibody molecules or on molecules of identical specificity
  122. 3-IDIOTYPES • Identical specificity means that all antibodies molecules have the exact same hypervariable regions. • Each idiotype is unique for the immunoglobulin produced by a specific clone of antibody-producing cells.
  123. 3-IDIOTYPES • Anti-idiotype antibody reacts only with the hypervariable region of the specific immunoglobulin molecule that induced it.
  125. IMMUNOGLOBULIN GENES • To produce very large number of different immunoglobulin molecule (106-109) without requiring excessive number of genes, special genetic mechanism e.g DNA rearrangement and RNA splicing, are used. • The DNA rearrangements are performed by recombinases.
  126. recombination-activating genes • Two important genes that encode recombinase are RAG-1 and RAG-2 (recombination-activating genes). • Mutation in these genes arrest the development of lymphocytes and result in severe combined immunodeficiency.
  127. IMMUNOGLOBULIN GENES • Each of the four immunoglobulin chains consists of two distinct regions: • a variable and a constant.
  128. IMMUNOGLOBULIN GENES • For each type of immunoglobulin chain, i.e., kappa light chain (kL), lambda light chain (λL) and the five heavy chains (γH, α H,µ H. εH and δH), there is a separate pool of gene segments located on different chromosomes.(2,22,14)
  129. IMMUNOGLOBULIN GENES • The genes encoding the H and L chains are arranged in exons separated by intervening introns. • Each pool contains a set of different V gene segments widely separated from the D (diversity, seen only in H chains), J (Joining) and C (constant) gene segment
  130. IMMUNOGLOBULIN GENES • In the synthesis of an H chain, for example a particular V region is translocated to lie close to a D segment, several J segments and a C region.
  131. IMMUNOGLOBULIN GENES • These genes are transcribed into mRNA and all but one of the J segment are removed by splicing the RNA.
  132. IMMUNOGLOBULIN GENES • During B cell differentiation, the first translocation bring a VH gene near a Cu gene, leading to the formation of IgM as the first antibody produced in a primary response. • Note that the joining gene does not encode the J chain found in IgM and IgA.
  133. IMMUNOGLOBULIN GENES • The V region of each L chain is encoded by two gene segments (V+J). • The V region of each H chain is encoded by three genes segments (V+D+J). • These various segments are united into one functional V gene by DNA rearrangements.
  134. IMMUNOGLOBULIN GENES • Each of these assembled V genes is then transcribed with the appropriate C genes and spliced to produce an mRNA that codes for the complete peptide chain.
  135. IMMUNOGLOBULIN GENES • L and H chains are synthesized separately on polysomes and then assembled in the cytoplasm by means of disulfide bond to form H2L 2 units.
  136. IMMUNOGLOBULIN GENES • Finally, an oligosaccharide is added to the constant region of the heavy chains and the immunoglobulin molecule is released from the cell.
  137. Antibody diversity • Antibody diversity depends on (1) multiple gene segments, (2) their rearrangement into different sequences, (3) the combining of different L and H chains in the assembly of immunoglobulin molecules, and (4) mutations. 5)A fifth mechanism called junctional diversity applies primarily to the antibody heavy chain.
  138. Antibody diversity • Junctional diversity occurs by the addition of new nucleotides at the splice junctions between the V-D and D-J gene segments
  140. IMMUNOGLOBULIN CLASS SWITCHING (ISOTYPE SWITCHING) • Initially, all B cells carry IgM specific for an antigen and produce IgM antibody in response to exposure to that antigen. • Later, gene rearrangement permits the elaboration of antibodies of the same antigenic specificity but of different immunoglobulin classes
  141. IMMUNOGLOBULIN CLASS SWITCHING (ISOTYPE SWITCHING • Antigenic specificity remains the same for the lifetime of the B cell and plasma cell because the specificity is determined by the variable region genes (V,D and J genes on the heavy chains and V and J genes on the light chain) no matter which heavy-chain constant region is being utilized.
  142. CLASS SWITCHING • In class switching, the same assembled VH gene can sequentially associate with different CH genes so that the immunoglobulin produce later (IgG, IgA or IgE) are specific for the same antigen as the original IgM but have different biologic characteristics.
  143. CLASS SWITCHING • A different molecular mechanism is involved in the switching from IgM to IgD. • In this case, a single mRNA consisting of VDJ, CuCδ is initially transcribed and is then spliced into separate VDJ, Cu and VDJ Cδ and mRNAs. • Mature B cells can, in this manner, express both IgM and IgD, .
  144. CLASS SWITCHING • Class switching occurs only with heavy chains; • light chains do not undergo class switching. • Switch recombinase is the enzyme that catalyze the rearrangement of VDJ genes during class switching.
  145. CLASS SWITCHING • The control of class switching is dependant on at least two factors. • One is the concentration of various interleukins. • For example, IL-4 enhances the production of IgE, whereas 1L-5 increases IgA.
  146. CLASS SWITCHING • If a Tfh cell recognizes the B cell’s peptide antigen, the Tfh cell provides two key signals: • the CD40 ligand (CD40L) molecules on the Tfh cell bind to CD40 on the B cell, • and the Tfh cells produce the cytokine interleukin (IL)-21.
  147. CLASS SWITCHING • Together, these signals have three important effects on the B cells: • (1) they begin to proliferate rapidly; • (2) they start to class switch, changing from using the Cμ segment to using one of the other heavy chain CH segments (Cγ, Cε, or Cα) • and (3) they start a process of somatic hypermutation.
  148. CLASS SWITCHING • The other is the interaction of the CD40 protein on the B cell with CD40 ligand protein on the helper T Cell. • In HYPER IgM SYNDROME, the failure to interact properly results in an inability of the B cell to switch to the production of IgA or IgE. • Therefore, only IgM is made.
  149. ALLELIC EXCLUSION • A single B cell expresses only one L chain gene (either κ or λ) and one H chain gene. • In theory, a B cell could express two sets of immunoglobulin genes, a maternal set and a paternal set. But this is not what happens.
  150. ALLELIC EXCLUSION • Only one set of genes is expressed, either maternal or paternal, and the other set is silent (i.e., it is excluded). This is called allelic exclusion
  151. ALLELIC EXCLUSION • Each individual contains a mixture of B cells, some expressing the paternal genes and others the maternal ones. • The mechanism of this exclusion is unknown.
  152. CATALYTIC ANTIBODY • Antibody can act as an enzyme to catalyze the synthesis of ozone (O3) that has microbicidal activity. • Antibody can take the singlet oxygen produced by neutrophils and react it with water to produce hydrogen peroxide and O3.
  153. EFFECTOR FUNCTIONS OF ANTIBODIES • (3) stimulate immune cells through their Fc receptors to lyse a target cell, also called antibody- dependent cellular cytotoxicity (ADCC); and • (4) bind and neutralize toxins and viruses.
  154. EFFECTOR FUNCTIONS OF ANTIBODIES • The primary function of antibodies is to protect against infectious agents or their products . • Antibodies provide protection because they can • (1) activate complement to cause direct lysis of cell membranes and promote inflammation ; • (2) opsonize bacteria, which can occur with or without complement;
  155. THE PRIMARY RESPONSE • The primary response occurs the first time that an antigen is encountered. This usually involves T-cell– dependent activation of B cells. • . Most of the B cells activated from an initial exposure undergo class switching and affinity maturation and differentiate into plasma cells
  156. THE PRIMARY RESPONSE • Plasma cell produce large amounts of antibody specific for the epitope recognized by their immunoglobulin receptor. • Plasma cells secrete thousands of antibody molecules per second for a life span that lasts from a few days to months
  157. THE PRIMARY RESPONSE • The first antibodies are detectable in the serum after around 7 to 10 days in a primary response but can be longer depending on the nature and dose of the antigen and the route it takes to the secondary lymphoid organ (e.g., bloodstream or draining lymphatics).
  158. THE PRIMARY RESPONSE • The serum antibody concentration continues to rise for several weeks and then declines and may drop to very low levels. • The first antibodies to appear in the primary response are IgM, followed by IgG, IgE, or IgA, as more class- switched plasma cells are generated. • IgM levels decline earlier than IgG levels because these plasma cells have shorter life spans
  159. THE SECONDARY RESPONSE • Although most activated B cells become plasma cells, a small fraction of activated B cells become memory B cells, which can remain quiescent in the B-cell follicles for long periods but are capable of being activated rapidly upon reexposure to antigen. • Most memory B cells have surface IgG that serves as the antigen receptor, but some have IgM
  160. THE SECONDARY RESPONSE • When there is a second encounter with the same antigen or a closely related (or cross-reacting) antigen months or years after the primary response, the secondary response is both more rapid (the lag period is typically only 3–5 days) and generates higher levels of antibody than did the primary response
  161. THE SECONDARY RESPONSE • The memory B cells, which underwent some degree of affinity maturation during the primary response, now proliferate to form a new germinal center and repeat the process of competing for Tfh help (i.e., affinity maturation).
  162. THE SECONDARY RESPONSE • With each succeeding exposure to the antigen, the antibodies tend to bind antigen with higher affinity because the memory B cells are subjected to further rounds of affinity maturation . • During the secondary response, the amount of IgM produced is similar to that after the first contact with antigen
  163. THE SECONDARY RESPONSE • The secondary response also generates IgG plasma cells that are greater in number and longer-lived than those in the primary response, meaning secondary IgG levels are higher and tend to persist longer
  164. THE SECONDARY RESPONSE • This concept is medically important because the protection afforded by the first administration of a vaccine is less than that afforded by a booster shot. • Furthermore, booster doses of vaccines improve antibody binding by inducing new rounds of affinity maturation.
  165. RESPONSE TO MULTIPLE ANTIGENS ADMINISTERED SIMULTANEOUSLY • When two or more antigens are administered at the same time, the host reacts by producing antibodies to all of them. • Combined immunization is widely used (e.g., the diphtheria, tetanus, pertussis [DTP] vaccine and the measles, mumps, rubella [MMR] vaccine)
  166. ANTIBODIES IN THE FETUS • In general, the fetus and the newborn have an underdeveloped immune system that responds weakly to infections and vaccines. • Antibodies in the fetus are primarily IgG acquired by transfer of maternal IgG across the placenta. • This is why it is important to confirm the mother’s vaccine history to ensure the newborn will be protected.
  167. ANTIBODIES IN THE FETUS • Some antibodies can be made by the fetus if infection occurs, such as in congenital syphilis. • After birth, newborn infants can make IgG (and other isotypes, such as IgM and IgA) to certain protein antigens. • For example, the vaccine against hepatitis B that contains hepatitis B surface antigen is effective when given to newborns
  168. ANTIBODIES IN THE FETUS • Most vaccines are delayed by several weeks or months after birth to ensure the newborn’s immune system has developed enough to respond. After birth, maternal IgG declines, and protection from maternal IgG is lost by 3 to 6 months.
  169. ANTIBODIES IN THE FETUS • After 6 months, with the loss of maternal antibody, the risk of pyogenic infections from organisms such as Haemophilus influenza begins to increase, particularly in infants with B-cell deficiencies
  170. MONOCLONAL ANTIBODIES • Antibodies that arise in an animal in response to typical antigens are heterogeneous, because they are formed by several different clones of B cells (i.e., they are polyclonal). • Antibodies that arise from a single clone of cells are homogeneous, or monoclonal
  171. MONOCLONAL ANTIBODIES • A plasma cell tumor, or multiple myeloma, is a malignancy of a mature plasma cell clone. • This disease can present with high levels of a monoclonal immunoglobulin, usually IgG
  172. MONOCLONAL ANTIBODIES • The names of chimeric antibodies end with the suffix – ximab, such as infliximab (anti–tumor necrosis factor [TNF]) and rituximab (anti-CD20). • The names of humanized antibodies end with the suffix – zumab, such as omalizumab (anti-IgE) and pembrolizumab (anti–PD-1) • The names of fully human antibodies end with the suffix – umab, such as adalimumab (anti-TNF) and ipilimumab (anti– CTLA-4)

Notes de l'éditeur

  1. DAMPs: Damage-associated molecular patterns; PAMPS; Pathogen-associated molecular patterns; PD1: Programmed death-1; PDL1: Programmed death ligand 1; PRR: Pattern recognition receptor. Velazquez-Soto H, Real F, Jiménez-Martínez MC. Historical evolution, overview, and therapeutic manipulation of co-stimulatory molecules.