CMB2004- Immune system Flashcards
Adaptive/ specific immunity
induced by exposure to particular infection
highly specific
exhibits memory
specific immunity is mediated by
B/T lymphocytes
clonal selection theory
- removal of self-reactive immature lymphocytes from the repertoire
- pool of immature lymphocytes for foreign antigens
- proliferation & differentiation of specific lymphocytes -> clone of effector cells
BCR =
B cell receptor
- expressed by B lymphocytes
antibodies are secreted when….
B cell is activated
TCR =
T cell receptor
- expressed by T lymphocytes
TCR will only recognise…(1) bound to…(2)
- peptide fragments of antigen
- MHC expressed by APC
Antibody functions
- help with infection by encapsulated bacteria
- activate complement system
- activation of effector cells
antibody structure
4 polypeptides each with variable and constant regions
- heavy chain and light chains
Fab region on antibody
Fragment antigen binding
Fc region on antibody
fragment constant
5 antibody classes/isotopes
IgM, D, A, G, E
isotype is determined by…
C region heavy chain
Domains =
patterns present in many other proteins in the immune system
how many domains does the L chain have
2
how many domains does the H chain have
4/5
what links L chain and H chain
disulphide bridge
Hypervariable regions
concentrated region of variability
3 in VH, 3 in VL (HV1-3)
CDR =
complementary-determining regions
- determine specificty and affinity of ab for ag
epitope
bit of antibody that recognises antigen
antibody and antigen form… interactions
non-covalent
MHC class 1 in humans
HLA-A, -B, -C
MHC class I expressed on…
all nucleated cells
MHC I structure
- heterodimer: alpha chain and beta2 microglobulin
- a1 and a2 domains fold -> b-sheet -> peptide binding site
- a3 domain + b2 microglobulin fold -> Ig-like domains
MHC 2 expressed on…
APCs + cytokine activated cells
MHC II in humans
HLA-DP, HLA-DQ, HLA-DR
MHC II structure
heterodimers a and b chaims are both transmembrane
- a2 and b2 domains are Ig-like
- grooves are more open than in class 1, bind longer peptides
H chain and TCRb V region encoded by
3 gene segments:
V, D, J
L chain and TCRa encoded by
V, J
NHEJ
non-homologous end joining region
- genes rearranging during B cell development to form a functional gene
steps of NHEJ
- DNA breaks between V+J
-> brings together a V chain and a C chain -> light chain genome - V, D,J segment join together in a single B cell -> V region of the heavy chain
hierarchy of gene rearrangements
- first heavy chain genes: D-J then V-D
- then light chain genes: kappa then V-J
- if kappa rearrangement is unsuccessful -> genes rearrange
H chain chromosome
14
kappa chain chromosome
2
kappa chain chromosome
2
delta chain chromosome
22
RSS
Recombination Signal Sequences
- special sequences flanking the VDJ regions -> guide rearrangement
RSS enzyme complex
V(D)J recombinase - recombination activating gene (RAG)
RAG1 and RAG2 genes encode…
lymphoid specific components of the V(D)J recombinase
mutations -> immunodeficiency
Allelic exclusion
single B cell = only one allele of H and L is expressed
- single B cell expresses kappa or delta, never both
- light chain isotype exclusion
-> individual B cell only produces one specific antibody
antibody combinational diversity
different VDJ segments recombine to produce different segments
antibody junctional diversity
- imprecise joining
- N regions: random nucleotide addition of nucleotides at V-D and D-J junctions by terminal transferase
somatic hypermutation
- mutation frequency in antibody VH gene is higher than normal spontaneous mutation rate
- occurs in germinal centres as B cells recognise Ag and proliferate/activate
- involves AID enzyme
AID enzyme
Activation-induced deaminase
acts on DNA to convert cytosil to uracil -> recognise by error prone DNA repair pathways -> mutations
constant region of each heavy chain is encoded by…
a different C region gene segment
IgM heavy chain gene
C upsilon (u)
IgD heavy chain gene
C delta
IgG heavy chain gene
C gamma (y)
IgA heavy chain gene
C alpha (a)
IgE heavy chain gene
IgE epsilon
Ig class switching
requires further DNA recombination, guided by switch regions
- involves the AID enzyme
pathpgen -> cytokine -> switch
TBR receptor genes
encoded by VDJ segment rearrangement
TCR generating diversity
- similar to BCR ( combinational, junctional diversity)
- NO somatic hypermutation
MHC genes
No gene rearrangement
- genes located within MHC (HLA in humans - chromosome 6)
- extremely polymorphic
MHC polymorphism
allows binding of a wide range of peptides to T cells -> clear evolutionary advantage
downside: increases risk of immune mediated diseases, makes organ donation complicated and inefficient
how do peptides get to the surface of cells with MHC I molecules
- intracellular antigen processing to peptides in proteasome
- peptide transport into ER
- peptide binding by MHC I
- MHC class I presents peptide at cell surface
TAP
component of multi-protein assembly
- peptide loading complex - includes tapasin and calreticulin
Antigen processing and presentation by MHC II molecules
- Ag endocytosed into IC vesicles
- protein cleaved by acid proteases in vesicles -> peptides
- vesicles fuse with visicles containing MHC II
- peptides bind MHC II
- complex is transported in vesicles to cell surface
MHC class 1 accessory molecules
TAP and LMP
MHC class 2 accessory molecules
HLA-DM
B cells develop from… that express….
haemopoietic stem cells
PAX5 transcription factor
stages of B cell development
- Generation of B cell receptors in bone marrow
- negative selection (self-reactive removed)
- migration of B cells through blood to lymphoid organs -> B-cell activation by foreign antigen
- antibody secretion and memory cells in bone marrow and lymphoid tissue
what happens if a B cell doesn’t encounter an antigen
it dies
formation of pre-B cell receptor
Early pro-B cell -> Vh, DJh rearrangements occur -> large pre-B cell -> stop heavy chain gene rearrangements -> light chain continues rearranging in small pre-B cells -> immature B cell -> stops light-chain rearrangement -> mature B cell
pre-BCR signal
- Turns off RAG-1 and -2 genes
-> 5-6 rounds of cell division
-> surrogate light chain expression stops
-> RAG genes turned back on
-> L chain rearrangement starts
immature B cells only express…
IgM
immature B cells that bind multivalent self-antigens…
- clonal deletion -> apoptosis
- receptor editing -> further light chain gene rearrangements (give it another chance)
immature B cells that bind soluble self-antigen
cell becomes unresponsive (anergic)
T cells develop in the…
Thymus
steps of T cell development
- notch signalling -> precursors commit to T-cell lineage, self reactive are removed
- T cells activated by foreign antigens, migrate to peripheral lymphoid organs
- activated T cells migrate to sites of infection -> proliferate and eliminate
once T cell precursors reach thymus they develop into…
thymocytes
thymocytes
- rearrange TCR genes (b first) and express TCR
- Aquire other markers e.g. CD3, 4, 8
- positive and negative selection
Thymus
bi-lobed organ in ant. mediastinum
cells: lymphoid cells, epithelial cells, macrophages, dendritic
T-cell maturation in thymus
- pro-thymocytes enter cortex from bone marrow
- rearrange TCR, 1st TCRb then pre-TCRa -> pre-TCR
- cells proliferate then rearrange TCRa genes
- express TCR together with CD3
- also express CD4 and CD8 - peripheral T cells express one or the other
TCR expression requires…
CD3 complex
- CD£ transmits a signal to T cell nucleus after TCR recognition
T cells expressing a randomlt rearranged abTCR may:
- recognise forgeign Ag - immunity
- recognise self-Ag - autoimmunity
- not be able to recognise self-MHC - useless
- need to keep 1 and eliminate 2/3
T cell positive selection
- occurs when CD4+/CD8+ T cells recognise MHC on cortical epithelial cells in thymus
- not recognised -> apoptosis
T cell negative selection
on dendritic cells/macrophages with high affinity
- TCR binding self-peptide -> apoptosis
naive T cells
blood -> lymph node via HEV -> move to T cell area -> APCs present antigen and deliver other activation signals (like cytokines)
CAM
Cell Adhesion Molecules
- chemokine receptors on T cell surface bind chemokines expressed/released by other cells
CAMs mediating cell-cell interactions
different CAMs mediate cell-cell interactions
- naive T cell with HEV
- T cell with APC
- Effector T cell with target cells
T cell contact with APC
- T cells contact APCs using CAMs
- TCR scans APC-MHC complexes
no recognition -> disengages
recognition -> CD3 signal from TCR complex
- increases affinity for CAM interactions
-> T cell divides -> differentiate into effector cells -> exit lymph nodes
-> T cell- mediated response
Signals T-cells require to be activated
LFA-1: Leukocyte Function associates Antigen
ICAM-1: InterCellular Adhesion Molecule
three signals needed for T-cell activation
Signal 1: from TCR contacting MHC/ peptide on APC
Signal 2: APC also express co-stimulatory molecules that bind CD28
Signal 3: APCs release cytokines -> bind to cytokine receptors
T cells activated by 3 signals ->
proliferate and express ICOS and CTLA-4
- ICOS binds ICOL on APC -> cytokine secretion
- CTLA-4 binds B7.1/2 on APC -> negative signal to activated T cell -> dampens down/limits T cell response
PRR
Pattern Recognition Receptor
- Expressed by APCs
- binding -> activates APC
APCs
express MHC class I and II molecules
Dendritic cells-> present Ag, activate naive T cells
Macrophages and B cells
Myeloid cells
(coventional DC 2/3)
- potent APC involved in activation of naive T cells
- dont express co-stim molecules until active
- mature and migrate to lymph node after danger signal
Plasmacytoid
(pDC, DC6)
- viral infection
secrete a and b interferons
express TLR 7 and 9 -> snese viral antigens
Maturation of dendritic cells
- immature dendritic cells are activated by PAMPs
- TLR signalling induces CCR7, enhances antigen processing
- CCR7 directs migration into lymphoid tissues, augments expression of co-stim and MHC molecules
- mature dendritic cell in T cell zone primes naive T cell
IL-2
key cytokine for T cell survival - potent autocrine T cell growth factor
IL binding to IL2R on activated T cells
-> T cell proliferation
After activation by APC, T cells differentiate into:
effector T cells
- CD8+ cells acquire cytotoxic activity
- CD4+ cells function by secreting cytokines
Effector T cells
display effector function when TCR engaged
- no longer require co-stim
- change expression of adhesion molecules
no longer enter lymoh nodes
but still enter tissues
-> migrate to where they are needed
activation of CD8+ T cells
requires high levels of co-stim activity
- CD8+ T cells can be activated directly by infected/cross-presenting APC
- may require additional help from CD4+ T cells
antibody functions
- neutralisation - prevents bacterial adherence
- opsonisation - promotes phagocytosis
- complement activation
BCR signal 1
crosslinking of BCR -> activates intracellular kinases
enhancing B cell signal 1
- if it has activated complement cascade -> lots of C3b
- complement receptor 2 on B = CD21
- CR2/CD19/CD81 -> form complex -> augments the signal
BCR TI Ag signal 2
Thymus-independent Ag
- signal 2 is provided by the antigen itself or by BCR cross-linking
BCR TD Ag signal 2
Thymus dependant Ag
- signal 2 provided by CD4+ T cells
TI antigen
Thymus independent
- antibody production without needing T cell involvment
TI-1 Ag
Binds BCR etc -> signal 2
- in high concs, act as mitogens (polyclonal activators) for B cells
- the two signals (1 from BCR, 1 from TLR) -> B cell activation/proliferation/Ab secretion
TI-2 Ag
- repeated epitopes
- cross link many BCR on same cell
- induce B cell activation
Don’t develop until 5 years old
TD Ag
Requires presence of CD4+ cells
- Ab responses much better than Ti
- CD4+ cells -> recieve signal 2 via CD40 interaction
- can help induce Ig class switching
Way to improve efficency of a vaccine
converting TI Ag to TD Ag (conjugate vaccine)
Germinal centre
where lots of B cells are proliferating
-> centroblasts
-> centrocytes
B cell zone
- in lymph node
dark zone: loads of cells proliferating
light zone: less cells
FDC: follicular dendritic cells
B cells withing a GC
- Differentiate into plasma cells
- form long-lived memory cells + recirculate
- Die within lymphoid tissue if BCR no longer binds antigen
FDCs
Follicular dendritic cells
- not derived from bone marrow
- capture intact Ag for centrocytes to bind via BCR
Follicular T helper cells
CD4+ cells in B cell folicles of lymph node
- secrete Th1 and Th2 cytokines
role of CD40 on B cells
- protects centrocytes from apoptosis
- induces isotype switching
why do we need immunological tolerance?
- random BCR/TCR repertoire -> some self reactive
- no tolerance -> autoreactivity -> serious pathology
AIRE
autoimmune reglator protein
- transcription factor, key role in tolerance induction
deficiency -> major autoimmune syndrome
Tolerance through clonal anergy
lymphocytes that regognise self-ag can become unresponsive
- important for generating peripheral resistance
immunological tolerance
many Ag not presented at sufficent levels to activate T cells
Regulatory T cells
CD4+ subset that suppress immune responses
- crucial for preventing autoimmune responses
- arise in thymus from T cells with high affinity receptors for self -Ag
- produce IL-10/TGF-b -> inhibit self-reactive cells
Regulatory B cells
B cells that secrete IL-10
- prevent autoimmunity
Th1
activation of macrophages, NK cells, cytotoxic T cells
respond to IC pathogens
Th2
promote eosinophil/mast cell mediated/ antibody responses - esp IgE
- respond to EC parasites
Th17
recruit neutrophils against fungi
secrete IL-17
Tfh
helps B cells
can produce Th1,2,17 cytokine
CD4+ Th1 can kill…
chronically infected macrophages
via Fas ligand
-> bacteria is released and destroyed by other, healthy macrophages
CD4+ Treg
CD4+/CD25+
nTreg = from thymus
iTreg = from circulation
- secrete suppressive cytokines
gram positive bacteria
thick layer of peptidoglycan
gram negative bacteria
thinner peptidoglycan layer, outer membrane
binding of PAMP to TLR ->
promote inflammation
promote dendritic cell maturation
influence differentiation of t cells
activate B cells
virus infected host cells
type one interferons - IFN-a, IFN-b
- prevents viral nucleic acid production + replication
IFN induces synthesis of
2,5-oligoadenylate synthetase -> degrades viral mRNA
protein kinase -> inhibits protein synthesis
type 2 interferon
IFNy - secreted by T cells and NK cells
- recruits Th2 response, promotes Th1
- recruits macrophages
Natural killer cells
- innate lymphoid cells
- recognises stressed cells in absence of Igs and MHC
what do NK cells kill with
perforin and granzyme
- EC mechanism
perforin
cytotoxic granule - polymerises in membrane
granzymes
cytotoxic protease that enters cell
killing by cytotoxic T cell
- secretion of cytotoxic granules: perforin and granzymes
- Fas ligang on T cell interacts with Fas on target -> death
-> apoptosis
4 mechanisms pathogens use to evade immunity
- concealment of antigens
- antigenic variation
- immunosuppression
- interference with effector mechanisms
concealment of antigens
some viruses inhibit antigen presentation by MHC I via privileged sites and uptake of host molecules (cloak effect)
4 mechanisms of antigenic variation
- large number of antigenic types
- mutation = antigenic drift
- recombination = antigenic shift
- gene switching
streptoccocus pneumoniae
-> otitis media, sinusitis, bronchitis, pneumonia
gram positive
antibodies to the capsule opsonise bacteria and protect
- large number of different capsular types
pneumovax
vaccine for s.pneumoniae
- polysaccharide vaccine - antigens to all 23 capsules
- not effective in children under 2 or immunocompromised people -> just B cell IgM
Prenvar 13
vaccine for s.pneumoniae
- conjugate vaccine - only 13 capsule antigens bound to diptheria toxin - highly immunogenic but non-toxic
- T cell and B cell (all Ig) response
- converts TI-2 ag -> TD ag == young children can respond
B&C influenza virus
- RNA virus w/ -ve sense segmented genome
- major surface antigens: haemagglutin and neuraminidase
- can undergo antigenic drift and shift
antigenic drift ->
mild epidemic
antigenic shift ->
major pandemic
trypansoma brucei
protozial parasite -> african sleeping sickness
- genetic rearrangement -> changes in major surface antigens of trypanosome
immunosuppression
- infection of immune cells
- induction of regulatory T cells
Helicobacter pylori
gram negative -> gastric/duodenal ulcers -> gastric adenocarcinomas
- Treg cells may be involved -> persistent infection
measles virus
RNA virus -> disease w/ rash/malaise/resp distress
infects dendritic cells -> increased apoptosis, decreased t cell stimulation, decreased IL-12 production
molecules interfering with antibody function
IgA proteases
Fc-binding molecules
molecules interfering with complement
pseudomonas -> enzymes that break down C3a/C5a
molecules inhibiting complement activation
IL-1b effects
local: activates vascular endothelium and lymphocytes, local tissue destruction, increased acces of effector cells
systemic: fever, IL-6 production
TNFa effects
Local: increased vascular permeability -> increased entry of IgG complement and cells to tissues, increased lymph drainage
Systemic: fever, shock, mobilization of metabolites
IL-6 effects
local: lymphocyte activation, increased antibody production
systemic: fever, induces acute phase protein production
CXCL8 effects
Local: chemotactic factor, recruits neutrophils, basophils and T cells to site of infection
IL-12 effects
Local: activates NK cells, inducing CD4+ cells -> Th1 cells
local infection with gram-negative bacteria
-> macrophage secrete TNFa into tissue
-> release of plasma proteins and lymphocyte migration, increased platelet adhesion to blood vessel wall
-> bacteria phagocytosis, local vessel occlusion, plasma and cells drain to local lymph node
-> removal of infection, adaptive immunity
Sepsis
overreaction of immune system
systemic infection -> sepsis -> mass cytokine release -> death
cytokine storm
shed glycoprotein from virus binds macrophage and dendritic cells -> cytokine release -> increased vascular permeability
immuno pathology
- cytokine storm
- coagulation cascade
disseminated invtravascular coagulation -> death
passive immunity
- short lived half life ~ 3wk
Active immunity
- exploits immunological memory
- secondary response is faster and greater than primary response
requirements of a safe and effective vaccine
safe
high level of lasting protection
right type of response
low cost
stable
easy administration
minimal side effects
inactivated vaccines
dead organisms
attenuated vaccine
live but virulence disabled
e.g. yellow fever and BCG vaccine, MMR, vaccina, sabin
subunit vaccine
protein fragments
e.g. hepatitis B
Toxoid
bacterial toxin
e.g. tetanus
conjugate vaccine
something with low antigenic property covalently bound to something with high
e.g. s.pneumoniae + diptheria
pros and cons of live vaccines
pros: single dose, may be given by natural route, may induce local/systemic immunity, may induce right type of response
cons: reversion to virulence, contamination possible, susceptible to inactivation, causes disease if host is immunocompromised
Adjuvants
= substance administered with an antigen to promote immune response
- pure antigens only elicit a weak response, adjuvants enhance these responses
ways that adjuvants can act
- activate and promote uptake by dendritic cells
- release endogenous danger signals
- stimulate release of cytokines/chemokines
- promote cross-penetration of exogenous antigens
animal adjuvants
freud adjuvants: oil in water emulsion
freud complete adjuvants: also contains mycobacteria
human adjuvants
- aluminium most commonly used
- aluminium hydroxide/phosphate
- better Ab responses than cell mediated immunity
antiserum
serum from an immunized person/animal
- contains antibodies that will bind the Ag
limitations of using antisera
- would be v useful if purified, but contains a mixture of Abs
