2 - Adrian kelly Flashcards
Components of innate immune system
Anatomical barriers
Physiological barriers
Phagocytic cell
Inflmmation
Important feature of innate immunity
No memory of encounter with foreign antigen
Activation of B lymphocytes
Differentiation into plasma cells Secrete antibody (soluble version of BCR)
Antibodies interact with …
Epitopes
Linear epitopes
Conformational epitopes (protein in native structure)
Antibody regions
Variable - bind antigen
Constant - recruit effector function (macrophage,neutrophil,basophil,mast cell, complement)
Antibodies work in 3 ways;
Neutralisation - block activity
Opsonisation - enhanced phagocytosis
Complement activation - recruited to antibody coated antigen, cause lysis
Gene rearrangement for TCR occurs
When they mature in the thymus
T cell receptor
Only recognises degraded proteins (peptides) complexes with MHC, presented by APCs
MHC
Major histocompatability conplex
APC
Antigen presenting cell
MHC class 1 molecules
Expressed on most cells
Associate with peptides produced in cytosol (internal protein antigen)
T cells express co receptor CD8, mature into cytotoxic T cells
MHC class II
T cells express CD4 co receptor
Mature into t helper cells
External protein antigens
Central lymphoid organs
Bone marrow and thymus
Mature or naive cells
T and B cells with successfully rearranged receptors, not yet exposed to particular antigen
Professional APC
First time naive T cell sees peptide-MHC complex it must be presented by professional APC
Lymph nodes and spleen are designed to
Optimise interaction between APC and B and T lymphocytes
High endothelial venules
Specialised endothelia that lymphocytes enter node through
Dendritic cells with captured antigen
Enter T cell area
Naive T cells survey antigen-MHC on dendritic cells
Retained if have complementary receptors
Stimulated to differentiate into effector T cells
FDCs
Follicular dendritic cells
B cells concentrate around, in B cell areas
FDCs trap antigen/antibody/c3d complex
Affinity maturation
FDCs trap antigen/antibody/c3d complex
Hold antigen to be screened by B cells
Internalise, process and present it to T helper cells
Activate B cell to produce antibody
Cells with highest affinity receive T help and are preferentially induced to proliferate
No. Of epitopes on antigen
Can have many different, recognised by different antibodies
Fab
Fragment of antigen binding
Fc
Fraction crystallisable
Binds receptor
Different light chains
K kappa or lambda
Changing Fc
Different antibody, so different effector funvtion
Immunoglobulin fold domains
Light chain has one variable and one constant
Heavy chain has one variable and three constants
Which antibodies have four heavy constant domains?
IgM
IgE
Hinge region
Flexibility to interact with antigen
Flexibility also at V-C junctions
CDR
Three Complementarity determining regions
CDR3 is most variable
Appear as loops at surface
Somatic recombination
Gene segments rearrange during B cell development
65 variable gene segments
27 diversity gene segments
6 junctional segments
CDR1 and CDR2 variation
By variable gene segments
CDR3 variation
By diversity and junction all gene segments
In junctions between V-D-J
Order of heavy chain rearrangement
1- D –> J
2- V –> DJ
Allelic exclusion
Rearrangement on both chromosomes
But if a functional heavy chain is generated, rearrangement on other chromosome is prevented
Light chain rearrangement
No diversity segments
K chain first, if unproductive, lambda chain next
Rearrangement controlled by
Recombinases that recognise conserved heptamer and nonomer sequences adjacent to VD+J sequences
Generation of antibody diversity (4 processes)
Different heavy and light combinations
Different heavy VD+J segments
Junctional diversity
Somatic hypermutations
Junctional diversity
Addition and loss of nucleotides at VDJ junctions
Somatic hypermutation
Point mutations in variable regions
Increased affinity selected by affinity maturation
B cells initially expressed with
Mu and sigma constant regions
Produce IgM and IgD antibodies
Coexpressed and generated by alternative splicing
Isotope/class switching
B cell can change constant region isotype
Generation of BCR and soluble antibody
Alternative splicing (multiple exons) Different polyadenylation sites
Five immunoglobulin isotypes
IgM IgG IgA IgD IgE
Different Fc receptors
Fc gamma RI - High affinity, bind monovalent
FcgRII and III - Low affinity bind multivalent antigen/antibody complexes
Macrophages and neutrophils express what type of receptor?
Fc gamma
Binding of antigen/antibody by C3b receptors on RBCs…
Delivers complexes to liver and spleen for removal by phagocytes
NK cells FcgRIII receptor
Ligation by antigen/antibody complex releases cytoplasmic granules, lytic enzymes
(ADCC)
ADCC
Antibody dependent cell mediated cytotoxicity
Functions mediated by Fc domains of antibodies
Opsonisation ADCC Mobilise inflammatory mediators Active antibody transport Regulate antibody production
Fc binds to mast cells and basophils ..
Release of inflammatory mediators
Allergic reactions - allergen binds to
IgE on mast cells
Transport across epithelial cell
A receptor for polymeric Ig recognises j chain region of IgA and transports
B cells negative feedback system
B cells specific for same antigen can bind by FcgammaRIIB and BCR
Negative signal that terminates B cell response
Affinity
Interaction between single antibody binding site and single epitope
Avidity
Strength if interaction due to recognition of polyvalent epitopes
MHC
Major histocompatability complex
Gene locus on chromosome 6
Codes for MHC1 and MHC2 molecules
MHC molecules
Glycoproteins
Present peptides to T cells
MHC Class 1
Transmembrane a (a1,a2,a3)
Non covalently linked to B2-microglobulin
a1 and a2 form peptide binding groove (8-9aa)
Groove base is Bpleated sheet, sides are a helices
MHC Class 2 molecules
2 similar sized transmembrane chains, a and B
Peptide binding groove formed by a1 and B1
Open ended - can bind 13-25aa
Polygeny of MHC molecules
Expression of multiple independent loci 3 isotypes 1- HLA-A, HLA-B, HLA-C 2- HLA-DP, HLA-DQ, HLA-DR HLA=human leukocyte antigens
Polymorphism of MHC
Many alternative forms of same gene
250 alleles
Extensive allelic polymorphism is pathogen driven
How are MHC genes inherited?
As haplotypes
Co-dominantly expressed (maternal and paternal)
Heterozygotes can present more peptides
MHC restriction
T cell sees a combination of a particular MHC molecule associated with a particular peptide
Anchor residues
Conserved amino acids at certain points on the peptide, to make contact with pockets in binding groove
Anchor residues determine the …
Peptide binding motif for that MHC molecule
Endogenous proteins are presented on…
MHC class 1 molecules
Exogenous proteins are presented on …
MHC class 2 molecules
MHC class 1 antigen processing pathway:
Proteins in cytosol degraded by proteasome
Peptides moved into ER by TAP
Suitable peptides loaded onto class 1 molecules
MHC peptide complexes released from chaperones, pass through Golgi, secretory pathway to cell surface
Presented to CD8 cytotoxic cells
TAP
Transporter associated with antigen processing
MHC class 2 antigen processing pathway
Class2 a/B chains bind with invariant chain Ii (prevent binding peptides in ER)
ER to Golgi to endocytic pathway
Ii partially removed, leaving CLIP in groove
Antigens degraded by proteases
In MIIC peptide loading compartments CLIP is removed
Peptides loaded onto class 2 molecules
Cell surface for presentation to CD4 helper T cells
Invariant chain
MHC class 2 pathway
- blocks peptide binding groove
- folding chaperone
- targets II/Ii complexes to endocytic pathway
TCR structure
Related to single Fab fragment 2 polypeptides, with constant and variable region Monovalent Membrane bound No somatic hypermutation Antigen recognition-no effector function
TCR rearrangement
B chain
B chain - D-->J - V-->DJ Surface expression with surrogate a chain Stops B chain rearrangement Proliferation CD4-CD8- double negative
TCR rearrangement
a chain
a chain
- V–>J
CD4+CD8+ double positive
Which region of TCR is most variable?
CDR3
Due to VDJ junctions
Makes major contact with peptide in groove
T cells are selected for:
Successful B chain rearrangement
Positive selection
Negative selection
Positive selection
Tested against self MHC/peptide
Moderate affinity - positive signal to continue maturation
Low affinity - death by neglect
CD4+ single positive selected against MHC class1/peptide
CD8+ single positive selected against MHC class2/peptide
Negative selection
Auto reactive thymocytes
High affinity for MHC/self peptide
Eliminated by apoptosis
Can only eliminate cells recognising combinations expressed in the thymus
Cells mediating positive selection
Cortical epithelial cells
In cortex of thymus
In cortex, thymocytes rearrange…
a chain, giving multiple opportunities for +ve selection
Lack of +ve selection –> cell death
Cells triggering negative selection
Dendritic cells
Macrophages - remove thymocytes that fail to mature
Mature naive T cells
Have survived selection
Activated by professional APC to become functional
T cell activation - 2 signal hypothesis
Signal 1 - TCR engagement
Signal 2 - co-stimulatory molecules
T cell co-stimulatory molecules
T cell:
CD28 interacts with B7.1 and B7.2 (CD80 and CD86) on APC
CD40L interacts with CD40 on APC
Defining feature of professional APC
Expression of co-stimulatory molecules
How do naive T cells enter lymphoid organs?
From blood through High Endothelial Venules
Binding of naive T cell to professional APC
Naive T cell proliferates and differentiates into a population of armed effector T helper cells
Maturation of naive B cells…
Bind and internalise antigen (signal 1)
Present peptide on surface as MHC2/peptide complex
Complex is recognised by T helper
Co-stimulatory molecules (CD28/B7 and CD40L/CD40)
Cross linking CD40 (signal 2)
Cross linking of CD40 on B cell promotes…
B cell proliferation
B cell survival signals
Immunoglobulin class switching
B-T cell cooperation
Interaction with co-stimulatory molecules leads to mutual activation
After activation, B cells bind to
Follicular dendritic cells (present in B cell follicles)
Enable B cell to form germinal centres
Thymus independent antigens
TI antigens
Stimulate naive B cells without B-T cell interaction
Naive CD4 cells differentiate into
TH1/TH2 bias is self reinforcing
Depends on cytokines present during proliferation
Cytokines released by dendritic cells, macrophages, NK cells, focus immune response
TH1
Cell mediated immunity
TH1 - produce cytokines, which activate macrophages, NK cells, CTLs, intracellular pathogens
Stimulated by abundant antigen and high affinity TCR interactions
TH2
Humoral pathway
TH2 - cytokines activate B cells (antibodies) (IL4, IL5, IL6)
IgG, igA, IgE, eosinophils and mast cells
Stimulated by low peptide abundance or weak affinity TCR
What cytokines promote B cell growth?
IL4
IL6
What cytokine causes production of
IgG?
IgA?
IgE?
IL4
IL5
IL4
What cytokines stimulate
Eosinophils?
Mast cells?
IL5
IL4
