Antigen recognition in innate and adaptive immunity

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Antigen recognition in innate
and adaptive immunity
Diversity in antigen recognition
receptors
Immune
system
The ability to
distinguish self
from non-self
Innate immunity
• Very old. Every organism has some form
of innate defence system.
• Does not adapt. Must rely of unique
molecular features of pathogens.
• Must develop a large number of different
receptor molecules, each with the capacity
to distinguish some component unique to
a pathogen.
Combination of innate and ad
Adaptive immunity
•
•
•
•
Only found in jawed fish and above.
Happened 500 million years ago.
Uses VDJ recombination.
Small number of similar receptor
molecules (Ig, TcR and MHC) but the
genes have been duplicated and
segmented to allow recombination.
Produces vast repertoire of “binding
molecules”.
• TcR is unique because it has to bind MHC.
Adaptive immunity
• Humoral
– B cells make soluble immunoglobulin.
– 5 classes beginning with IgM that switch to other classes
depending on site, or function
• Cellular
 a/b T cells
• Helper CD4 cells make cytokines to drive B-cells and T cells to
develop
• Cytotoxic CD8 killer cells have mechanism to kill infected cells
• NK T cells – unique TcR receptor for recognising non-peptide
antigens
 g/d T cells have limited repertoire. Important for recognising nonpeptide antigens and non-MHC antigens.
Did the adaptive immune
system evolve from the innate
system?
Two orders of agnathans - Hagfish and lamprey – the most
disgusting animal (parasite) on the planet
Agnathans
•
•
•
•
•
Diverse haematological cells – heterogenous leukocytes
Produce opsonins and agglutinins
Allograft rejection
DO NOT have MHC, Ig, TcR or RAG-1/RAG-2 genes
Have their own adaptive immune receptors VLR-A (T
cell like) and VLR-B (B-cell like).
• LRR receptors that can rearrange somatically.
• Mechanism is unknown, but probably a transposase.
• 35 VLR-A molecules and 38 VLR-B molecules
Agnathan VLR genes, transcripts and phylogeny.
Pancer Z et al. PNAS 2005;102:9224-9229
©2005 by National Academy of Sciences
Within 40 million years during the Cambrian period
2 radically distinct adaptive immune systems (agnathans
and gnathostomes) evolved to generate a system for
providing a set of diverse receptors.
Is this convergent evolution or ancestral evolution??
i.e. do the VLRs represent the forerunners to the Ig and
TcR or did they evolve independently?
The molecules
• Innate immunity
– Toll-like (TLR), CTL (C-type lectins) and NOD-like
(NLR) receptors
– Invariant TcR (NK recognition) (Rapid)
– Non-polymorphic MHC (i.e. CD1)
– Non-classical MHC
• Adaptive immunity
– T cell Receptor (a/b and g/d)
– Immunoglobulins
– MHC – pathogen fragment presentation
Effective immune response
• Requires combination of both adaptive and
innate response.
• Don’t get commitment without innate signals
• Adjuvants promote formation of a protective
immune response to antigen. Merely molecules
that trigger innate receptors e.g. killed
mycobacterium.
• Whole new field has opened up using the
various innate PAMPs as potential vaccine
adjuvants.
Types of immune responses
•
•
•
•
Complement – direct killing
Phagocytosis – complement opsonins
Defensins – secreted bactericidal compounds
Antibodies – block and direct receptor mediated
phagocytosis and complement killing.
• T cells provide cellular immunity – kill virally
infected cells. Also essential in driving B cell and
T cell maturation – cytokines
Central Cell
• Dendritic cell or primary antigen presenting cell.
Found in many forms – at the very heart of the
naïve immune response.
Classic cellular immunity
• Primary function is to respond to antigens
presented by the APC.
• Antigens are fragments of intracellular or
extracellular pathogens.
• Complex processing of antigens inside the APC
and loading of MHC class I and class II in
specific vesicles.
• Binding of peptides governed by motifs. Anchor
residues in peptides that interact with floor and
walls of the peptide groove.
polymorphic
HLA A2
a2
a3
a1
b2m
polymorphic
HLA DR1
a1
b1
a2
b2
Separate roles for Class I and
Class II and co-receptors
MHC
Class I
MHC
Class II
Peptide
length
Peptide
source
Pathogen
Responding
T cells
Effector
function
8-10
Intracellular
Viruses
CD8
Cytotoxic
Extracellular
Bacteria,
fungi,
protozoa
CD4
Helper
function
8-30
CD4 CD8 binding sites
CD8
SITE
CD4 site
Innate receptors
• Now a vast array of these discovered.
• Absolutely critical in the immune response
• LPS is an extremely powerful activator of B cells. First
discovered when mouse strain resistant to LPS. Found
to have a defect in the CD14 molecule.
• Later evidence found presence of genes similar to
drosophila Toll for pattern recognition in development.
• Found that these recognized various microbial
componds.
• Extremely important to vaccine development and
adjuvants.
• Appears they are also important in autoimmune disease.
Diversity of pattern recognition
receptors
CD1 antigen presentation
• CD1 is a non-polymorphic MHC class I like
molecule.
• Comes in 4 forms CD1a, b, c, d.
• Similar architecture to classical MHC class
1.
• Cleft is much narrower and deep with
hydrophobic residues. Binds things other
than peptides
CD1 ligands
 a-galactosyl ceramide (sponge)
• Phophatidyl choline
• Lipoarabinomannan (LAM)
• Sulphatide
• Many lipids (see review paper)
• Mostly associated with mycobacteria
• Some self antigens such as Ganglioside
GM1 – significance to autoimmunity.
CD1d structure with a GALCer
Recognized by iNK-T cells
Gal
Ceramide
CD1d structure with a GALCer
Recognized by iNK-T cells
SIDE
TOP
iNK-T cell
•
•
•
•
•
•
Semi-invariant T cell Receptor
Va24-Ja18 with Vb11 in humans
Va14-Ja18 with Vb8.2, 4 and 2 in mice.
Restricted by CD1
Recognises a-galactosyl ceramide.
Abundant in liver(~20%), BM (~3%)
spleen(~2%)
• Massive release of IFN-g, IL-4, TNF-a, IL-13,
GMCSF, IL-2. Facilitates T cell and NK cell
activation and expansion.
Innate immunity in plants flies and
mammals is conserved – Toll/TLR
Specificity of TLR ligands
Basic structure of TLR
• Characterised by a LRR (leucine rich
repeat external domain) and an
intracellular TIR (Toll/IL-1 receptor)
intracellular domain.
• LRR are a diverse set of proteins with
consensus sequence in the domain
• L(X2)LXL (X2) NXL(X2)L(X7)L(X2)
Signalling of innate receptors
Typical domain
structure of PRRs
Critical TIR domain for
surface receptor
(Toll/IL-1 receptor).
Typical LRR structure
• Common horseshoe structure
• Concave surface formed by parallel bstrands
• Convex surface formed by loops and 310
helices
• LRR are 20-30 amino acids long
• Binding region is typically the concave
surface but not always
Ectodomain of TLR2 showing Leu rich repeat
Mouse TLR3 homodimer with dsRNA
Pam3CSK4 is a potent PAMP
Triacylated lipopetide. 3 hydrophobic acyl chains and a charged
head group
Activates
through
TLR1/TLR2
No activation
From: Jin et al, 2007 Crystal structure of the TLR1-TLR2 heterodimer induced by binding if a tracylated lipopeptide Cell 130 1071-1082
TLR2 with Pam3CSK4 bound
From: Jin et al, 2007 Crystal structure of the TLR1-TLR2 heterodimer induced by binding if a tracylated lipopeptide Cell 130 1071-1082
Mechanism of TLR dimerization
From: Jin et al, 2007 Crystal structure of the TLR1-TLR2 heterodimer induced by binding if a tracylated lipopeptide Cell 130 1071-1082
Lipopolysaccharide (LPS)
Extremely potent Gram negative PAMP
This is the main
immunogenic
component
LPS – MD2 – TLR4 structure
Intracellular
PRRs
A vast array of NLR
specificities
Inflammasomes
Intracellular sensors of stress and infection
NLRP3 contains 3
proteins
1. ASC
2. Caspase-1
3. NLRP3
NLRP3 is a member of
the NOD (nucleotide
binding domain LRR
family of proteins
(NLRP1-14).
Formation of NLRP3
induces caspase 1
mediated apoptosis.
NOD proteins have been
shown to be very
important in
inflammatory diseases
i.e. NOD mouse is
diabetic.
Known activators of NLRP3
Classical peptide recognition
• How does a T cell “see” such a small amount of peptide.
• Each APC has ~105 MHC molecules on its surface.
• Majority are filled with self peptides that do not elicit and
immune response.
• Infection only likely to produce a small number of
relevant peptide-MHC complexes
• These are likely to be spread across the surface, not
clustered.
• How does the TcR “find” these.
• How many peptide-MHC complexes.
Thymic +ve and –ve selection
• TcRs are selected on self MHC and self
peptides in the thymus. 99% of all T cells
never make it out of the thymus.
• Antagonist peptides positively select.
Agonist peptides negatively select T cell.
• Same self peptides are required to keep T
cells alive in the periphery.
• T cells normally tolerant to self peptides.
The immunological Synapse
•
•
•
•
•
•
•
•
•
•
The interface between APC and T cell.
A lot going on over a small area in a short period of time.
Many molecules involved
Adhesion molecules (ICAM-1, LFA-1, B7.1/2 CD80,
CD86)
Antigen receptor
MHC
Co-receptors (CD4 & CD8)
Intracellular signalling molecules
Dynamic region that forms rapidly to concentrate events
into a tight junction between cells.
Clustering of intracellular kinases or phosphatases
Recent studies
• With more TcR/pepMHC systems studied, apparent that
½-life does not directly dictate agonist potential. In fact
certain cases, there is no correlation. Antagonists have
longer ½ lifes than their agonists in some cases.
• Now apparent that there is some steric change in TcR.
• Latest is a two step twist-cap model of engagement
• 1. Weak binding of TcR to MHC
• 2. Change to fit peptide by TcR to strengthen the
binding.
Involvement of self-peptides in T
cell Recognition
• Surprising recent finding is that agonist peptide
is enhanced by the inclusion of endogenous self
peptide MHC complexes.
• Model is now a pseudo-dimer where activation
involves dimerization of agonist peptide MHC
and self peptide MHC together.
• Can help to explain the role of self peptides in
thymic learning and requirement in periphery to
keep T cells alive.
Pseudo dimer model
Summary
• Multiple forms of immune recognition.
• Invariant MHC (CD1) present non-peptides such as
lipids to g/d T cells and special NK T cells.
• Innate receptors – vast complex patterns
• Classical T cell activation involves intricate interaction of
T cell with pepMHC and “understanding” minor
differences between agonist (pathogen) vs self-peptide
on which is was selected.
• New model is one where TcR sees combination of nonself and self peptide. Critical is the co-receptor CD4 and
CD8.
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