Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Switch regions
Document related concepts
Transcript
Diversification of antibodies after B-cells encounter antigen Alternative splicing Somatic hypermutation Ig. class switch The surface and secreted forms of an immunoglobulin are derived from the same heavy-chain gene by alternative RNA processing. SOMATIC HYPERMUTATION VL J gene product V gene product CDR1 CDR2 CDR3 Complementary Determining Region = hypervariable region STRUCTURE OF THE VARIABLE REGION Hypervariable (HVR) or complimentarity determining regions (CDR) Framework regions (FR) Somatic hypermutation is targeted to the rearranged gene segments that encode immunoglobulin V regions. AID: activation induced cytidine deaminase AID Cytosine to uracil change….. UNG Uracil-DNA Glycosilase, abasic nucleotide is excised by the nuclease APE1 Base excised and replaced with any nucleotide MUTATION!!!! The almost random variation produced by somatic hypermutation allows selection of variant immunoglobulins with improved antigenbinding sites Day 0. Ag Day7 7nap CDR1 CDR2 CDR3 CDR1 CDR2 IgM PRIMARY Day 21 21 nap 1 2 3 4 5 6 7 8 Plasma cell clones AFFINITY MATURATION immune response Day 14. 14 Ag nap CDR3 IgM/IgG IgG SECONDARY Immune response Hypervariable regions 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 ISOTYPE SWITCH embrionális Embryonal L1 V1 L2 V2 Ln Vn D1 - 12 DNS DNA 5' szomatikus rekombináció Somatic recombination D– D-J kapcsolódás átrendeződött Rearranged DNA DNS L1 V1 Ln Vn C Cδ C3 CM CD CG3 J1-4 J CA1Cε2 C1 CG1 CE2C1 CE1 C2 CG 2 CG 4 Cε1 CA2 C 1 C4 D1D2 J1J2-4 C CM Cδ CD 5' 3' szomatikus rekombináció Somatic recombination V -D-J kapcsolódás CM Cδ CD D2J1 J2-4 C 3' L1 V1 5' Primerprimer RNA transcript RNS-átirat mRNA mRNS 3’ Transcription transzkripció 5' L1 V1D2 J1 J2-4 C CM Cδ CD IgM Cγ1 IgG Cγ2 IgG Cγ3 IgG Modification Cγ4 IgG AAAA transzláció Translation L V DJ C C polipeptid Ig ISOTYPES Cµ 3' Processing átalakítás C L1 V1 D2 J1 CM naszcens Nascent polypeptide módosítás V C V–D–J NEHÉZL ÁNC (M ) Heavy chain IgM Cα IgA Cε IgE Antibody isotype switching Throughout the immune response the specificity of an antibody will be essentially the same (notwithstanding affinity maturation) The effector function of antibodies throughout a response needs to change drastically as the response progresses. Antibodies are able to retain Variable regions whilst exchanging Constant regions that contain the structures that interact with cells. Organisation of the functional human heavy chain C region genes J regions C Cd C3 C1 C1 C2 C4 Ce C2 Switch regions C S Cd C3 S3 C1 S1 C1 S1 C2 S2 C4 S4 Ce Se C2 S2 • Upstream of C regions are repetitive regions of DNA called switch regions. (The exception is the Cd region that has no switch region). • The S consists of 150 repeats of [(GAGCT)n(GGGGGT)] where n is between 3 and 7. • Switching is mechanistically similar in many ways to V(D)J recombination. • Isotype switching does not take place in the bone marrow, however, and it will only occur after B cell activation by antigen and interactions with T cells. Switch recombination C Cd C3 C1 C1 C2 C4 Cd Ce Cd C2 S3 C3 C3 C S1 C C1 V23D5J4 C3 V23D5J4 C1 V23D5J4 C1 V23D5J4 C3 V23D5J4 C1 V23D5J4 C1 IgG3 produced. Switch from IgM IgA1 produced. Switch from IgG3 IgA1 produced. Switch from IgM At each recombination constant regions are deleted from the genome An IgE - secreting B cell will never be able to switch to IgM, IgD, IgG1-4 or IgA1 C L VDJ Cδ Rearranged DNA in C2 C4 Ce C 3' 5' S S S S IgM-producing cell S Switch regions C Cδ, C2, C4 ISOTYPE SWITCH L VDJ 3' 5' Ce C L VDJ 5' 3' Rearranged DNA in IgE-producing cell L VDJ Primary RNA 3' transcript 5' AAAA e-Heavy chain Ce mRNA All isotype switch recombination is productive Different recombination signal sequences and enzymes from VDJ rearrangement Happens after antigenic stimulation Regulated by external signals, not random Hyper IgM syndrome Type 2. Activation Induced Cytidine Deaminase NO HYPERMUTATION AND ISOTYPE SWITCH ANTIBODY MEDIATED EFFECTOR FUNCTIONS • Neutralization – binding of the antibody inhibits the binding of the pathogen to the cell surface, entry to the cell or multiplication • Opsonization – binding of the antibody triggers complement activation and binding to the cell surface by complement (CR1) and IgG (FcR) receptors • Cytophylic property - antibody isotypes have distinct complement activating and FcR binding activity SECRETED ANTIBODIES BIND TO THE ANTIGEN COMPLEMENT ACTIVATION IMMUNE COMPLEX OPSONIZATION Ig Fc region Conformational change? Association? FcR CR PHAGOCYTOSIS Macrophage COMPLEMENT ACTIVATION – classical pathway BINDING TO CELLS – cytophilic property DEGRADATION ISOTYPE DEPENDENT IgG1 and IgG3 >> IgG2 és IgG4 EFFECTOR FUNCTIONS OF ANTIBODIES INHIBITION Binding of bacteria to epithelial cells Binding of viruses to receptor Binding of bacterial toxins to target cells NEUTRALIZATION Small proportion of antibodies OPSONIZATION COMPLEMENT ACTIVATION Binding of antibody increases phagocytosis PLAZMA CELL FcR Opsonization by C3b Complement C3b FcR FcR CR1 PHAGOCYTES ENGULFMENT, DEGRADATION T – CELLS PROMOTE B – CELL DIFFERENTIATION T-CELL ANTIGEN CYTOKINES B -CELL PLASMA CELL ISOTYPE SWITCH AND AFFINITY MATURATION OCCURS IN COLLABORATION WITH T – CELLS ONLY HOW T – CELLS RECOGNIZE ANTIGENS? T-cell development and differentiation A T-cell precursors migrate from the bone marrow to thymus T-cell markers are induced after thymocytes interact with the thymic epithelial cells A Notch-1receptor and its cytoplasmic region, acting as a transcription factor is required for the development of the T-cell lineage THE RESULT OF SOMATIC GENE REARRANGEMENTS 1. Combination of gene segments results in a huge number of various variable regions of the heavy and light chains expressed by different B-cells SOMATIC GENE REARRANGEMENT 2. Successful somatic rearrangement in one chromosome inhibits gene rearrangement in the other chromosome ALLELIC EXCLUSION 3. One B-cell produces only one type of heavy and one type of light chain COMMITMENT TO ONE TYPE OF ANTIGEN BINDING SITE 4. The B-cell pool consist of B-cells with differently rearranged immunoglobulin genes INDEPENDENT OF ANTIGEN OCCURS DURING B-CELL DEVELOPMENT IN THE BONE MARROW The αβ and γδ T-cell lineages develop from a common precursor A TCR-rearrangement—similar to BCR A β-chain rearrangement (like Ig heavy chain) Efficiency of beta chain rearrangement is about 80% RAG-1 RAG-2 genes become are inactivated Timocyte proliferation CD4, CD8 expressionó TCR-rearrangement is similar to that of the BCR The α-Chain rearrangement (Ig light chain) B- AND T-CELL RECEPTORS SHARE BASIC STRUCTURE mIg H mIg L TCR T cell receptor TCR V TCR C TCR = + The variable region of the -chain is generated by gene rearrangements of the V – D – J gene segments analogous to the generation of IgH diversity The variable region of the -chain is generated by the recombination of V and J analogous to IgL T-CELL Single binding C site No somatic mutation ESTIMATED VARIABILITY OF IMMUNOGLOBULIN AND T-CELL RECEPTOR GENES GENES/ KAPCSOLÓDÁS IMMUNOGLOBULIN T CELL RECEPTOR H / VARIABLE (V) 65 70 52 ~70 DIVERZITY (D) 27 0 2 0 rare - OFTEN - JOINING (J) 6 5/4 13 61 JOINING + P + N 2 1 50% 2 1 D (3 frame) V GENE PAIRS 3.4x106 5.8x106 JOINING ~3x107 ~2x1011 TOTAL ~1014 SOMATIC HYPERMUTATON 1018 NO CHARACTERISTICS OF T-CELL ANTIGEN RECOGNITION 1. The TCR is not able to interact directly with soluble or cell-bound antigen 2. T-cell activation can be induced by antigen in the presence of acessory cells, only 3. T-cells recognize virus-infected cells ACCESSORY CELL ANTIGEN BINDING NO INTERACTION V T-CELL ACTIVATION C Antigen receptor B-CELL / T-CELL Shaping the T-cell repertoire. Positive and negative selection Thymus Few TCR reacts with the MHC (about 2%) most T-cells die of neglect. ( no survival signals) α-chain rearrangement can continue until the assembly of a functional αβ receptor has been assembled. Selection of developing T-cells in the thymus Bare lymphocyte syndrome MHCI vagy MHCII deficiency Lack of CD8+ or CD4+ cells Role of co-receptors in the development of single + T-cells DC Macrophage in medulla of Thymus. Special transcription factor expressed… AIRE. Tissue spec. Antigens expressed AIRE mutaton: Autoimmune polyendocrinopathy -candidiasis-ectodermal dystrophy Multiple stages of T-cell development in the thymus CD25+ FoxP3+ cells FoxP3-deficiency: autoimmune disease IPEX: immune dysregulation polyendocrinopathy, enteropathy, X-linked syndrome
