Antigen Recognition in Adaptive Immune System PDF

Antigen Recognition in the Adaptive Immune System: Structure of Lymphocyte Antigen Receptors and Development of Immune Repertoires  Main Study Points qHow do the an+gen receptors of lymphocytes recognize extremely diverse an+gens and transmit ac+va+ng s...

Antigen Recognition in the Adaptive Immune System: Structure of Lymphocyte Antigen Receptors and Development of Immune Repertoires  Main Study Points qHow do the an+gen receptors of lymphocytes recognize extremely diverse an+gens and transmit ac+va+ng signals to the cells? qWhat are the differences in the recogni+on proper+es of an+gen receptors on B cells and T cells? qHow is the vast diversity of receptor structures in the lymphocyte repertoire generated?  Antigen Receptors of Lymphocytes ØVariable region(V):an+gen recognizing domains. ØConstant region(C) : conserved domain. ØHypervariable regions(complementarity- determining regions (CDRs)) : short stretches in variable region contains parts of the an+gen receptor. ØB cell receptor (BCR) complex /T cell receptor (TCR) complex: vSet of plasma membrane an+gen receptor and signaling molecules. ØFunc+ons of lymphocyte receptor: vAn+gen recogni+on. vSignal transduc+on causing lymphocyte to divide, differen+ate and perform effector func+ons.  Antibodies qThree hypervariable regions(CDRs) present: ØCDR3:located at the junc+on of the V and C regions, contains the greatest variability and contributes most to an+gen binding. qFragment, an+gen-binding(Fab): The fragment of an an+body that contains a whole light chain aPached to the V and first C domains of a heavy chain. qFc (fragment, crystalline): The fragment in the heavy-chain C domains responsible for the biologic ac+vity and effector func+ons of the an+bodies. q Hinge region: linking Fab and Fc regions of most an+body molecules, allows the two Fab regions of each an+body molecule to move independent , that are separated from one another.  Features of the major isotypes (classes) of antibodies qFive types of Ig heavy chains: Øμ, δ, γ, ε, and α. qTwo types of light chains: Øκ and λ differ in the C region. v κ or λ remains fixed throughout the life of each B cell clone, regardless of whether or not heavy-chain class switching has occurred. vLight chains do not par+cipate in effector func+ons, except binding and neutralizing microbes and toxins.  Binding of Antigens to Antibodies qEpitopes(an+genic determinants): Ø Parts of an+gens that are recognized by an+bodies. ØAn+gen may be linear or conforma+onal.  Binding of Antigens to Antibodies qAffinity of interac+on: ØStrength with which one an+gen-binding site of an an+body binds to one epitope. qDissocia+on constant (Kd): ØMolar concentra+on of an an+gen required to occupy half the available an+body molecules in a solu+on. The lower the Kd, the higher the affinity. qAffinity matura+on : ØThe increase in an+gen-binding strength. qAvidity of the interac+on: ØCumula+ve binding strength of an an+body. ØCross-reac+on: ØAn+bodies produced against one an+gen may bind other, structurally similar an+gens.  Generation of Hybridomas and Monoclonal Antibodies qMonoclonal an+bodies: ØUsed as therapeu+c agents and diagnos+c reagents for many diseases in humans. vMouse monoclonal an+bodies induce immune response when injected into human. ØGene+c engineering: the an+gen- binding V regions of the mouse are retained and the rest of the an+body is replaced with human Ig. ØRecombinant DNA technology: monoclonal an+bodies are generated using recombinant DNA technology to clone the DNA encoding human an+bodies of desired specificity.  T Cell Receptors for Antigens ØTCR has three hypervariable, or complementarity-determining regions. vCDR3 is the most variable among different TCRs. ØTCR interacts with both the MHC molecule and the pep+de presented. ØTCR ac+va+on requires: v CD3 and ζ chains: crucial for the ini+a+on of signaling when the TCR recognizes an+gen. vCD4 or CD8: coreceptor which recognize nonpolymorphic por+ons of MHC molecules. Features of Antigen Recognition by Immunoglobulins and T cell Antigen Receptors  Development of T and B Lymphocytes Ø 107–109 lymphocyte different clones arise before encounter with an+gen. v Each clone has dis+nct specificity. vGrowth factors by stromal cells in the bone marrow and thymus s+mulate survival and prolifera+on of earliest lymphocyte precursors. vIL-7 maintains and expands the number of T lymphocyte progenitors before they express an+gen receptors. Ø Posi+ve selec+on: immature T cells are selected to survive only if they have some affinity for MHC molecules in the thymus. ü Ensures that complete mature cells will be able of recognizing microbial pep+des displayed by the same MHC molecules on APCs. Ø Nega+ve selec+on: an+gen receptors that strongly recognize pep+des of self pep+des bound to self MHC, or strongly recognize self MHC regardless of the pep+de displayed are eliminated. vPrevent the development of autoimmune responses.  Production of Diverse Antigen Receptors: Germline Organization of Antigen Receptor Gene Loci qHematopoie+c stem cells in bone marrow and early lymphoid progenitors contain Ig and TCR genes. vAn+gen receptor gene loci contain V, J, and C gene segments. vIg heavy chain and TCR β chain loci also contain D gene segments. vVariable region (V) gene segments are about 30 to 45, vOne or a few constant region (C) genes present.  Production of Diverse Antigen Receptors: Somatic Recombination and Expression of Immunoglobulin (Ig) Genes ØIg heavy chain expression involves two gene recombina+on events (D-J joining, followed by joining of a V region to the complex. vThe recombined gene is transcribed, and the VDJ complex is spliced onto the C region exons of the first heavy-chain RNA (which is μ), to give rise to μ messenger RNA (mRNA). v The mRNA is translated to produce the μ heavy-chain protein. Ø The recombina+on of Ig light chain and the T cell receptor (TCR) α and β chains —follows the same sequence, except lacking D segments (Ig light chains and TCR α) vV gene recombines directly with a J gene segment.  Production of Diverse Antigen Receptors: Diversity in antigen receptors ØSoma+c recombina+on is mediated by a lymphoid specific enzyme (VDJ recombinase): vIt is expressed only by immature B&T lymphocytes. vComposed of recombina+on ac+va+ng gene 1 and 2 (RAG-1 and RAG-2) proteins. vBrings two Ig or TCR gene segments close together and cleaves the DNA at specific sites. vCombinatorial diversity is limited by the number of available V, D, and J gene segments vJunc+onal diversity maximizes the varia+ons in the CDR3 regions. ØDNA breaks repaired by ligases, produce full- length recombined VJ or VDJ exon without the intervening DNA segments.  Maturation and Selection of B Lymphocytes ØLarge pre-B cells are selected to survive and proliferate if they successfully rearrange an Ig heavy- chain gene and assemble a pre-BCR. v Pre-BCR consists of a membrane- associated Ig μ protein aPached to two other proteins called surrogate light chains. ØSmall pre-B cells ini+ate Ig light-chain gene rearrangement. Ø Immature B cells assemble a complete membrane IgM receptor. ØMature B cells coexpress IgD. B-Cell Differen-a-on Heavy μ chains of IgM class accumulate in cytoplasm Combine with short Ig-α and Ig-β chains to form a pre-B cell receptor (pre-BCR) on the B-cell surface  Role of the Pre-BCR Complex in B Cell Maturation qPre-BCR expression and B cell matura+on: ØBruton’s tyrosine kinase (Btk) encoded on the X chromosome vIt is ac+vated and required for delivery of signals from downstream of pre-BCR that mediate survival, prolifera+on, and matura+on. v Muta+ons in Btk in boys results in the failure of pre-B cells to survive, and failure or absence B cell matura+on causing X- linked agammaglobulinemia(XLA). ØPre-BCR regulates further rearrangement of Ig genes in two ways: vAllelic exclusion: Individual B cell expresses an Ig heavy chain protein encoded by only one of the two inherited alleles. ü Ensures that every B cell will express a single receptor, maintaining clonal specificity. v S+mula+ng κ light chain gene rearrangement: Signals from the pre-BCR complex shut off expression of the surrogate light- chain genes and open up the Ig κ light-chain locus making it available for recombina+on.  Mature B-cells qSelec+on of Mature B-cells: vPosi+ve selec+on: mature B cell express complete an+gen receptors. vNega+ve Selec+on: immature B cells bind self an+gens(ubiquitous) in bone marrow with high affinity, may re-express the VDJ recombinase enzyme, undergo addi+onal light-chain V-J recombina+on, generate a different light chain, and change specificity of the an+gen receptor by receptor edi+ng process. Maturation and Selection of Major Histocompatibility Complex (MHC)–Restricted T Lymphocytes ØPro-T cells : double-nega+ve T cells expand in the number by influence of thymic IL7. ØThe T cell receptor (TCR) β chain is first expressed at the double-nega+ve pre-T cell stage, mediated by VDJ recombinase. ØSynthesized TCR β is expressed on cell surface in associa+on with an invariant protein pre-Tα, to form the pre-TCR. ØComplete T cell receptor is expressed in double-posi+ve cells. ØMatura+on culminates in the development of CD4+ and CD8+ single- posi+ve T cells. vFailure to express an+gen receptors at any stage leads to death of the cells by apoptosis. T-Cell Differen-a-on: Double-Nega-ve (DN) Stage Thymocytes undergo rearrangement of genes coding for the beta chain of the T-cell receptor (TCR). Gene segments are selected and joined together to produce a unique anIgen-binding site. T-Cell Differen-a-on: Double-Nega-ve (DN) Stage CD3/TCR complex Alpha and beta chains recognize anIgen. Six other chains (in three pairs) assist with intracellular signaling aQer anIgen binds to TCR. Positive and negative selection of thymocytes in the thymus Pos. Selec-on= MHC Restric-on Neg. Selec-on= Self Tolerance Thanks