Lecture 12: (Part II) B cell activation and antibody production Flashcards Preview

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Flashcards in Lecture 12: (Part II) B cell activation and antibody production Deck (54):

_______ of a primary RNA transcript results in the formation of mRNA for the membrane or secreted form of the u heavy chain

Alternative processing 


B cell differentiation results in an increasing fraction of the u protein produced as the _____ form

  • secreted
    • Not the polyadenylation sites 
      • one by the TP (Tail piece)
      • one by the TM (transmembrane) and CY (cytoplasmic)


which two Igs have a CH4

IgE and IgM


The CH2 of IgM and IgG has a binding site for 



Isotype switching primarily ocurs in the the germinal centers and is driven by ____

  • TFH cells
  • (not that some may occur in extrafollicular B cells and this is driven by extrafollicular Th cells )


Abs specificity is unaltered in class switching. Why?


  • Class switching involves chaing the constant regions of the heavy chains 
    • this leaves the light chains unaltered and they are what determines Ab specificity


B cells activated by TFH cell signals (CD40L, cytokines) undergo switching to different Ig isotypes. What cytokines lead to the switching of each antibody class?


IFN-gamma from ____ induces IgG switching and IL-4 from ____ induces IgE switching

  • IFN-gamma from Th1
  • IL-4 from Th2


CD40 with CD40L induces ____, which both isotype switching and affinity maturation are criticaly dependent on

activation induced deaminase 


Immunodeficiencies related to the CD40-CD40L axis, the Ab response to protein Ag is dominated by ___ Abs


IgM (this is becaue there is limited isotype switching because inability to activate AID)


The molecular mecahnism of isotype switching is a process called switch recombination, in whihc the Ig heavy chain DNA in B cells is cut and recombined such that a previously formed VDJ exon that encodes the V domain is placed adjacent to a downstream C region, and the intervening DNA is deleted. These DNA recombination events involve nucleotide sequencs called switch regions, all of the heavy chains genes except ___ have this 5' ends of each locus. 

Delta (which codes for IgD) 


Activation-induced cytosine deamninase (AID) function

  • deaminates cytosines in single-standed DNA templates, converting C residues to uracil (U) residues
    • (note then uracil N-Glycosylase (UNG) removes U residues to generate abasic sites where the APE1 endonuclease creates nicks that lead to a double-stranded break) 


Affinity maturation is observed only in Ab responses to 

  • TD Ags
    • (TFH cells and CD40:CD40L interactions are required for somatic hypermutation of Ig V genes to be initiated) 


Are there fair more mutations in the IgG than IgM Abs



Mutations in V genes increase with 

time after immunization and with repeated immunization


B cells that have undergone somatic mutation migrate into the 

FDC-rich light zone of the germinal center


B cells with high-affinity receptors for the Ags are best able to bind the Ag when it is present at low concentrations by follicular DCs, and these B cells survive preferentially because of several mechanisms: 

  • Ag Recognition by itself induces expression of anti-apoptotic proteins of the Bcl-2 family 
  • High-affinity B cells will preferentially endocytose and present the Ag for TFH cells in the GC which signal via CD40L to promote the survival of the B cells 
  • High-affinity B cells may activate endogenous inhibitors of Fas when their BCRs recognize Ag and thus be protected from death, while low-affinity B cells are killed
  • Soem TFH cells express FasL, which can recognize Fas on B cells and induce apoptosis 


Many tumros of B cells, B cell lymphomas develop from 

  • germinal center B cells
    • (not this is caused by chromosomal translocations of various oncogenes into Ig gene loci) 
    • The DNA breaks associated with somatic hypermutation and isotype switching facilitate inorporation of oncogenes



Short-lived plasma cells 

  • are generated during T-independent responses
  • may well be generated early during T-dependent responses in extrafollicular B cell foci
  • are generally found in secondary lymphoid organs and in peripheral non-lymphoid tissues 


Long-lived plasma cells

  • are generated in T-dependent germinal center responses to protein Ag
  • Are generated by signals from the BCR and IL-21 via a stage of their precursors called plasmablasts


Long-lived plasma cells are maintained by ____ allowing the cells to survive for long periods, often as long as the life span of the host



Typically ____ weeks after immunization with T cell-dependent Ag, the long-lived plasma cells of the bone marrow beome a major site of Ab production

2-3 weeks 


It is estimated that almost ___% Ab in the blood of a healthy adult is produced by long-lived plasma cells and is specific for Ags that were encountered in the past



Memory B cells are generated mainly in the 

germinal centers for T-dependent protein Ags


Memory B cells express high levels of 

the anti-apoptotic protein Bcl-2


Memory cells typically express 

high-affinity (mutated) BCRs and Ig molecules of switched isotypes (other than IgM/or IgD)


Many non-protein Ags, such as polysaccharide, lipids, and nucleic acids, stimulate Ab production in the absence of Th cells, and these Ags and the response they elicit are termed thymus independent or TI. The Abs that they produce are 

generally lower affinity and consist mainly of IgM, with limited isotype switching to some IgG subtypes and also to IgA


Why are most TI Ags multivalent, being composed of repeated identical Ag epitopes

  • such multivalent Ags may induce maximal cross-linking of the BCR complex on specific B cells, leading ot activation without a requirement for cognate T cell help 


The ____ and ___ cells are especially important for Ab responses to TI Ags

  • Marginal zone (MZ)
    • are distince population of B cells in the spleen that mainlly respond to polysaccharides
  • B-1 cells 
    • Represent another lineage of B cells that respond readily to TI Ags mainly in the peritoneum and in mucosal site
  • (note after activation, these cells differentiate into short-lived plasma cells that produce mainly IgM) 


TI Ags may persist for prolonged periods on the surfaces of MZ macrophages in the spleen where they are recognized by specific _____

B-2 cells 


Why are TI Ags not recognized by CD4+ T helper cells

  • Most TI Ags are polysaccharides, glycolipids, and nucleic acids
  • They cannot be processed and presented in association with MHC molecules


Many polysaccharides activate complement system by 

the alternative pathway, generating C3b, which binds to the Ag and is recognized by CR2 on B cells


Can TI responses be facilitated by signals derived from microbial products that activate TLRs on B cells 



What is the dominant Ab class induced by Pneumococcal capsular polysaccharide



Explain how TI Antigens can lead to isotype switching

  • TLR ligand-activated DCs secrete BAFF, APRIL and TGF-beta which can induce IgA class switching in the B cell 


____ immunity is a major mechanism of host defense against infections by encapsulated bacteria 

  • humoral immunity 
    • individuals with immunodeficiencies of humoral immunity are especially susceptible to life-threatening infections with pneumococcus , meningococcus, and Haemophilus 


Most natural Abs are 

  • Low-affinity anti-carbohydrate Abs (This is why TI Ags contibute to the generation of natural Abs)
  • postulated to be produced by peritoneal B-1 cells stimulated by bacteria that colonize the GI tract and by MZ B cells in the spleen 


Effective Vaccines against microbes and microbial toxins must induce 

  • both affinity maturation and memory B cell formation 
    • (note thus they must be able to activate T helper cells) 


How is it possible to design a vaccine for capsular polysaccharides

  • To make memory B cells you have to activate helper T cells 
  • Capsular Polysaccharides are incapable of activating helper T cells
    • thus the polysaccharide is covalently linked to a foreign protein to form the equivalent of a Hapten-Carrier conjugate 
      • These vaccines are called conjugate vaccines



A polymorphism in the FcgammaRIIB gene has been linked to susceptibility to autoimmune disease 

systemic lupus erythematosus in humans 


It is not clear under which circumstances secreted Abs provide complement-mediated amplification or Fc receptor-mediated inhibition.  But a likely scenario is 

IgM which activates complement but do not bind to FcgammaRII is involved in amplificaiton, whereas increasing production of IgG leads to feedback inhibition 


  • Concerning the Fab region of an antibody molecule, which of the following is correct?
    • A.) 1 mole of a fab fragment has the capacity to bind 2 moles of antigen
    • B.) The fab includes the entire V and C regions of the light chain
    • C.) Only the hypervariable sequences of the heavy chain constitute the actual antigen contact residues
    • D.) The binding strength of the Fab region for antigen is termed avidity

  • B.) The Fab includes the entire V and C regions of the Light chain


  • Many cell surace molecules are important in the activation of B lympocytes to produce antibody. Which of these is not however involved during the actual binding of B cells to the T helper Cell surace?
    •  Ig
    • MHC class II
    • CD4

  • Ig


  • If you could analyze at the molecular level, a plasma cell making IgA antibody, you would find all of the following except?
    • A DNA sequence for V,D, and J genes translocated near the Calpha DNA exon 
    • mRNA specific for either K or l light chains
    • mRNA specific for J chains
    • mRNA specific for u heavy chains
    • a DNA sequence specific for TCR

  • mRNA specific for u heavy chains
  • A DNA sequence specific for TCR


  • Within the germinal centers of secondary lymphoid organs?
    • Follicular dendritic cells present antigen associated with MHC to T helper Cells
    • B cells proliferate  in the light zone to both T dependent and T independent antigens
    • B cell expressing affinity for antigen are rescued from cell death by binding to antigen-antibody complexes on follicular dendritic cells 

  • B cell expressing affinity for antigen are rescued from cell death by binding to antigen-antibody complexes on follicular dendritic cells 


  • The mechanism most responsible for co-expression of IgM and IgD on the surface of a B cell surface is?

  • Alternative RNA splicing 


  • The switch from membrane bound to the secreted form of Ig involves:
    • polyadenylation of mRNA which excludes the transmembrane coding sequecnes found in the genomic DNA
    • alternative splicing 
    • Both 
    • neither

  • Both


Multiple Myeloma is a cancer which has arisen form a 

plasma cell


  • which of the following is true about effector functions of immunoglobulins:
    • IgM binds to vascular epithelial Fc receptors which transport it into tissues and extravascular spaces
    • If B cells membrane Ig binds to soluble complexes of antigen and antibody, B cells may be down regulated 
    • Neonates are protected form infection by placental transferred IgM antibody
    • IgG antibodies are highly effective against pathogens in the gut mucosa, due to their transport across mucosal tissue via the FcRB receptor 

  • If B cells membrane Ig binds to soluble complexes of antigen and antibody, B cells may be down regulated 


  • Complementarity detemining regions (CDR's) of an immunoglobulin:
    • Act as binding sites for complement component C1q
    • show little amino acid sequence diversity
    • are all coded for by a single V gene segment for the heavy chain
    • Are situated on the Fc complementary fragment portion of an immunoglobulin
    • determine antibody specificity and are found on both the heavy and the light chains

  • determine antibody specificity and are found on both the heavy and the light chains


If you want to check for presence of memory B cells form your vaccine subjects by taking some B cells and re-stimulating them in vitro with antigen the best place ot get some memory cells would be from the 

  • Bone marrow 


The CDR (complementarity-determining region) is composed of 

Variable region of light and heavy chains 


Which of the CDRs (complementarity-determining regions) is most varibale 

CDR3s of both the Vsegment and the Vsegment are the most varibale of the CDRs