Concepts in immunity



Download 209.57 Kb.
Page3/3
Date28.07.2017
Size209.57 Kb.
#23911
1   2   3

ANTIBODY DIVERSITY

  • Nomenclature for Ig gene segments

    • Variable (V): exons encoded as a family of related sequences

    • Diversity (D): small exons that contribute to sequence variability; 3’ to V

    • Joining (J): small exons that contribute to sequence variability; 3’ to J

    • Constant (C): exons that define class and effector function of Ig; 3’ to VDJ

  • Antibody genes

    • 3 unlinked gene groups on diff chromosomes encode Ig

      • one for kappa L chains

      • one for lamba L chains

      • one for H chains

    • Recombination occurs within each group

      • Multiple coding exons recombine

    • Variable region (V)

      • Mature B cell or plasma cell: Vregion of H chain is continuous

      • Germ-line or non B cell: V region of H chain is interrupted by introns

      • V region of H chain has 3 segments

        • V segment

        • D segment

        • J segment

      • V region of L chains

        • V segment

        • J segment

    • Constant region (C)

      • In both L and H chains

      • 3’ to V genes but separated from them by unused J segments and non-coding DNA

      • 1 functional gene segment for each class and subclass

        • C regions of H chains

          • 1 segment each for M, D, G1-4, E, A1-2

          • also encodes transmembrane domain

        • C regions of L chains

          • 1 segment for kappa group, 7 segments for lambda group

  • Gene rearrangement/translocation

    • In development, B cell selects:

      • one V,D, J combo for H chain

      • one V, J combo for L chain

    • H chain group rearranges first

      • Selected D moved next to selected J

      • Selected V moved upstream of DJ  heavy chain V region

      • C genes are downstream, the closest one is M

      • Primary transcript has VDJ, intervening DNA, M

      • That is spliced to mRNA with VDJC  complete IgM heavy chain

    • L chain rearrangement next

      • Selected V from either kappa or lambda moves next to selected J  light chain V region

      • C region

        • Kappa chains: C region 3’ to V region with unused J between

        • Lambda chains: one of 7 lambda C genes associated with the J segment

      • Primary transcript has VJ, intervening DNA, C

      • That is spliced to mRNA with VJC  complete IgM light chain

    • Rearrangement requires recombination activating genes RAG-1, RAG-2

      • Only expressed in developing lymphocytes

      • Break and rejoin DNA during translocation

  • Synthesis and assembly of chains

    • L and H polypeptides combine in the ER to form Ig

    • Antigen-specific Ig transported to plasma membrane

    • Heavy chain has C terminal sequence that anchors Ig to plasma membrane

  • Ways to create diversity

    • Antigen independent mechanisms

      • Combinational diversity

        • Gene segments chosen at random in each B cell (number of possible VDJ, VJ combos)

        • Random selection and pairing of L and H chains in the ER

      • Junctional diversity

        • Imprecise joining of VDJ of H chain or VJ of L chain (variability in position of joining)

          • Last nt of V could be replace by first nt of J  change in antigen binding area

        • Addition or subtraction of nt during recombination

          • TdT adds nt’s to ends of gene segments, called N nucleotides

          • Overhangs during recombination may be filled in with P nucleotides

    • Antigen-dependent mechanisms

      • B cells stimulated by antigen and Th undergoes somatic mutation: changes in V regions of L and H chain that increase/decrease affinity

        • Affinity maturation: B cells that increase affinity of their Ig will become plasma cells with a higher affinity Ig

    • Lots of B cells are wasted, but excess diversity ensures response to wide array of antigens

  • Allelic exclusion

    • After successful rearrangement of VH and VL, other gene segments are suppressed  1 specificity

      • If rearrangement doesn’t work, another rearrangement will occur

    • Unique to B and T cell antigen receptors

  • Differential splicing and Ig receptor expression -------------

    • IgM expressed first on B cell

    • Then IgM and IgD

      • Have same VH and VL  same specificity

      • Mature naïve B cell

    • Primary RNA transcript is differentially spliced to yield two mRNA’s (IgM and IgD)

      • Primary has both M and D constant regions


  • Stages of B cell differentiation ---------------------------

    • In bone marrow

    • Signals: marrow stromal cells, cytokines induce

    • H chain rearranged first

    • Pre-BCR complex: M protein expressed on cell surface with 2 invariant chains (L-like)

      • Signal to stop H chain rearrangement and work on L chain

    • Successful H and L chain  functional IgM

      • = immature B cell

      • susceptible to tolerance induction

      • IgM is signal for survival and expansion

    • Then get coexpression of IgM and IgD

  • B cell receptor (BCR) complex ---------------------------------------------------------------------------

    • Ig alpha and beta are 2 TM polypeptides associated with Ig that signal for BCR

      • Ig are TM molecules but cytoplasmic domain is only 3 aa long  can’t signal

      • Signal prepares B cell to interact with Th cell (effects on transcription factors like Myc, AP-1)

      • Identical signaling for all B cells

    • B cell coreceptor complex: CD21/CR2, CD32, CD19, CD81

      • Enhances or inhibits BCR complex

      • If BCR and coreceptor both bind to antigen/complement, CD21 and 32 are engaged  CD19 and 81 influence signaling via Ig-alpha/beta complex

  • B cell tolerance

    • During B cell development

      • Negative selection: deletion or anergy (can’t respond) of self-reactive B cells which express receptors with high affinity for self or bind to membrane antigens

      • Receptor editing: recombination of L chain genes, making B cell with new specificity

        • Chance to change from self-reactivity and survive

    • After secondary stimulation of memory B cells

      • Susceptible to tolerance by epitopes presented multivalently if they don’t get T cell help

    • Self-reactive B cells can get activated if they express both self and non-self peptides at the same time (will get T cell help)

  • What to know

    • Know how antibody diversity is generated (recombination)

    • Understand how differential splicing is used to create immunoglobulin

    • Know what allelic exclusion is and why it is important (get 1 specificity!)

    • Understand the role of antigen in the generation of antibody diversity (no much of a role!)

    • Know stages of B cell development and when self-tolerance can occur (2 occurrences)

    • Know the different components of the B cell receptor

    • Understand B cell tolerance to self antigens


HUMORAL IMMUNITY

  • Antigen origin impacts response

    • Introduced in blood  spleen  splenic macs and CDs  presentation of antigenic determinants  T cells

    • Mucosal areas  B cells, macs, DCs below mucosa  presentation of antigenic determinants  T cells  B cell has antigen plus T cell help  humoral response  IgA released to mucosa

    • Tissues  travels through lymphatics to lymph tissue/node  B cells, macs, DCs  presentation of antigenic determinants to T cells  T cells in paracortical region, B cells in follicle and divide in germinal center

  • Primary response

    • Occurs within 5-8 days of exposure

    • Production of IgM followed by IgG or IgA by plasma cells

    • Durations depends on quantity of antigen and mode of entry

    • Ig reacts with antigen, making complexes or precipitates eliminated by phagocytosis

    • Ig production reaches peak after antigen is gone

    • One B cell makes one specific Ig, but lots of B cells get activated by diff antigenic determinants

  • B cell activation

    • Need to avoid activating non-specific B cells

    • B cells recognize antigenic determinant via Ig on membrane (affiliated with Ig-alpha-beta = BCR)

    • Antigen coated with complement interacts with CD21/CR2, CD19, CD81

    • Results in signaling pathway activation, transcription factor activation

    • Consequences of antigen binding

      • Proliferation

      • Upregulation of CD80/B7

      • Upregulation of cytokine receptors

      • Processing and presentation of antigen on MHC II

      • Downregulation of chemokine receptors so it can leave follicle

      • Low-level secretion of IgM

      • Now can encounter antigen-specific T cell

    • B-T interaction for activation of naïve B cells when they get to parafollicular zone (T cell zone)

      • B cell presents antigen to activated Th

      • Th has CD154 (CD40 ligand, reacts with CD40 on B) and cytokines

        • CD154-CD40 interaction required  ensures antigen-specific activation

      • B cells proliferate and become:

        • Memory cells

        • Plasma cells that secrete specific Ig

  • B cell differentiation after activation: occurs in germinal center

    • Ig isotype switching ------------------------------------------

      • B cells with IgM and IgD can switch to other H chain classes

        • E.g. to IgA or IgE in mucosal region

      • Requires stimulation via CD154-CD40 and cytokines (IFN-gamma, IL-4, IL-5)

      • Get translocation of VDJ 5’ to another H constant (C) region

    • Affinity maturation

      • Affinity of Ig for antigen increases with time

      • B cells increase in number, start competing  those w/ high affinity expand

      • Also, somatic mutation of VH and VL in B cells stimulated by antigen and T cells

      • Decrease in dissociation constant

  • Secondary (memory) response -------------------------------------

    • Reintroduction of antigen  faster, better response

    • Dec lag period for Ig presence due to memory cells

    • More plasma cells develop

    • Higher conc Ig in serum

    • More antigen-binding cells  those with highest affinity for follicular DC’s or Th win  results in plasma cells with high affinity

      • Competition = need higher affinity!!

    • Vaccination




  • T cell independent (TI) B cell response

    • Some antigens can activate B without Th = TI antigens

      • Resistant to degradation

      • Often repeated epitopes, so they cluster around the B cell and activate it

      • E.g. bacterial cell wall, poly-aa, dextran

    • Primary IgM response peaks a little early

    • But poor induction of memory B cells (secondary response is like primary)

    • No IgG response, prob bc they don’t induce cytokines/signaling that lead to isotype switching

  • Immunity in newborn: immature and takes a bit longer

    • Lymphocytes in newborn

      • Higher thank normal numbers of T and B (and NK)

      • But, limited ability to respond to some antigens

      • Age of exposure to antigen is important

        • Sequential expression of genes encoding for receptors for antigens

        • Immaturity of B and T cells

        • Immaturity of APCs

    • Ig in newborn

      • Maternal IgG crosses placenta (using Fc receptors), compensates for lack of IgG until infant makes it

      • IgM made during fetal development

      • IgA is made by 1-2 months age

        • Maternal IgA obtained via colostrum, milk gives mucosal immunity (passive)

      • Passive immunity from mother may interfere with development of active immunity  don’t respond to some antigens




Download 209.57 Kb.

Share with your friends:
1   2   3



The database is protected by copyright ©ininet.org 2024
send message
    Main page
different levels
unique features
basic nature