Immunology 146:474

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Immunology 146474

• Tu, Fri 1st period (840-1000 AM)
• Serc 118
• Dr. Lori Covey-Office hrs 1-3 Thursday
• Dept. of Cell Biology Neuroscience
• Nelson Hall, B314
• covey_at_biology.rutgers.edu
• Class web site http//lifesci.rutgers.edu/covey/
  Immuno/index.htm

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Figure 4-21 part 2 of 2
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Membrane and secreted form

• Each heavy chain gene has a membrane-coding (MC)
  and a secretion-coding (SC) exon.
• Each mRNA transcript has 2 cleavage sites for
  polyadenylation (pAm and pAs)
• Membrane form-25 amino acid hydrophobic tail
• Secreted form- has a hydrophilic COOH end

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(No Transcript)
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All isotypes are expressed as a membrane and
secreted form
Membrane form-specific cleavage and
polyadenylation
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Membrane and secreted form, contd

• Resting, naïve B cells express membrane IgM and
  IgD.
• In late-stage plasma cells-almost all the
  expression is secreted form

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Somatic hypermutations create diversity

• Occurs later after B cells are in secondary
  lymphoid organs
• Introduces single nt mutations into germline DNA
• Results in increased binding affinity of antibody
  for antigen

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Somatic hypermutations create diversity
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Figure 4-9
Important enzyme involved in this process called
activation induced cytidine deaminase or AID
Requires transcription
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Antigen Processing

• Generation of peptides from intact proteins to be
  displayed by MHC Class I and Class II molecules
• Antigen Presentation

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2 major intracellular compartments

• Cytosolic which includes nucleus.2. Vesicular
  system ER, golgi, endosomes, lysosomes

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Infectious agents can replicate in two distinct
compartments of the cell
Viruses and some bacteria replicate in cytosol
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Two functional classes of CD4 T cells TH1
cell--gtactivate macrophage to kill intravesicular
bacteria within macrophageTH2 cells--gtactivate
B cells to make antibodies
Th1 Th2
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Peptides that bind MHC class I are transported
from the cytosol to the endoplasmic reticulum (ER)
The MHC class I molecules are synthesized in the
cytoplasm and transported through ER
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Transporters associated with antigen
processing--gt TAP1 and TAP2
Form a heterodimer that is a peptide transporter
in the ER membrane--gtrequires ATP hydrolysis
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Figure 5-3
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Generation of peptides in the cytosol
Carried out by the proteosomeLarge,
multicatalytic protease complex.
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ProteosomeCylindrical complex of 28 subunits,
arranged in four stacked rings, each of 7
subunits. Hollow core thats exposed to the
catalytic sites. Proteins enter hollow core and
then broken down into short peptides and released
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Horizontal cross-section
Longitudinal section
Proteins unfold and enter proteosome
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Three proteosome subunits--gtb1i, b2i and
b5i Expression is induced by interons ? and ?
Substitute for 3 subunits of proteosome
subunits---gt immunoproteasome
Three inducible subunits, and counterparts--gt
active proteases of proteosome or immunoproteasome
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Difference between proteasome and
immunoproteasome?
1. Change protease preference produces peptides
with carboxy-terminal residues that are preferred
anchor residues for binding to MHC class I
molecules
2. Preferred peptides for binding by TAP1/2
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PA28 proteasome-activator molecule
Induced by IFN-? Binds to both ends of
proteasome increase rate at which peptides can
be released ?
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Endoplasmic reticulum aminopeptidase associated
with antigen processing ERAAP
Peptides transported into lumen of ER, enzyme
trims peptides from amino end that dont fit into
MHC class I molecules ?
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MHC class I molecules are assembled in the ER
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MHC class I molecules are assembled in the ER
Association of MHC class I ? chain with calnexin.
Binding of ?2microglobulin displaces
calnexinMHC class I ? chain and ?2m binds to
calreticulin and tapasin
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Tapasin binds to TAP1 and TAP2
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Figure 5-6 part 2 of 2
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Viruses have evolved mechanisms for interfering
with MHC class I expression
Herpes simplex virus--gt produces proteins that
bind TAP prevents movement of peptides into ER
Adenovirus--gtencode a protein that binds to MHC
class I molecules and retains them in the ER
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Mechanisms, contd
Cytomegalovirus accelerates retrograde
translocation of MHC class I molecules back into
the cytosol--gtdegraded.
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Parotozoan parasite LeishmaniaMycobacteria
that causes leprosy and tuberculosis replicate
inside intracellular vesicles in macrophages.
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Peptides generated for loading into MHC class II
molecules
Intracellular pathogens that replicate in
intracellular vesiclesEndocytosis--gtendosomesPh
agocytosis--gtphagosomesReceptor-mediated
endocytosis, i.e recognition of antigen by
antibody
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Peptides generated for loading into MHC class II
molecules
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Cathepsins important proteases for processing
vesicular antigens
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MHC-class II-associated Invariant chain
Forms trimers with each subunit binding
noncovalently to an MHC class II ?? heterodimer
Part of polypeptide chain lies within the
peptide-binding groove
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Invariant chain forms trimers
Assembled in ER
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Only after transport into acidic vesicle,
invariant chain is cleaved
Protected from binding peptides in ER
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CLIP protein binds to the groove
MHC class II molecules cant bind other peptides
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Figure 5-10 part 1 of 2
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Loading of MHC class II molecules with peptide
HLA-DM--gt found in the MIIC compartmentBinds to
MHC class II molecules catalyzes the release of
CLIP
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Binding of peptide to MHC molecule is irreversible
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Figure 3-19