Adaptive immune response Flashcards
Implications of an adaptive immune system (based on protein antigens)?
Proteins are infinitely diverse and a complementary diversity of receptors if therefore required
There is significant likelihood of cross-reactivity between prokaryotic proteins and their eukaryotic homologues
What gives antibody diversity (given there is not sufficient genes to code for each one individually)?
Gene rearrangement
How are heavy and light chains split up (genes)?
H and L regions each split into a V, D, J and C encoded by distinct gene segment
How is combinational diversity for antibodies generated?
Multiple gene segments for each region can be mixed and matched in any combination for significant combinational diversity
What enzyme catalyses somatic gene rearrangement?
V(D)J recombinase
What are the 4 levels of antibody variation at the genome level?
Multiple copies of V, D, J and C gene segments may be randomly recombined
D region genes may be transcribed in multiple reading frames
Imprecise joining may occur during rearrangement of genes and excision of the intervening DNA
Nucleotides are randomly inserted or deleted from the region flanking the sites where joining occurs
When can further specificity and variation in antibody structure occur?
At the antigen binding stage/during the immune response.
What does somatic hypermutation produce
Somatic hypermutation introduces mutations - producing closely related B-cell clones that differ subtly in specificity and antigen recognition.
Where does somatic hypermutation occur?
Dark zones of germinal centre of lymph node
How are mutations induced in somatic hypermutation?
Mutations are induced by Activation induced deaminase (AID) - this enzyme deaminates cytosine to uracil at the hypervariable hotspots.
Error prone DNA repair pathways create double strand breaks and introduce mutations
Most mutations (somatic hypermutations) are … ? What happens to these cells ?
Negative on B-cells ability to bind to the original antigen - these cells become apoptotic in the germinal centres and are engulfed by macrophage.
What happens if mutation is positive for antigen binding?
Antibody selected for - cells have increased survival rate.
These cells may successfully present a complimentary antibody to ap T follicular helper cells/ dendritic cell in light zone, which then signal them to re-enter the dark zone to accumulate further mutations
What T cells do developing B cells present antigens to?
Tfh cells
Where do developing B cells interact with Tfh cells?
The light zone
How to Tfh cells keep B cell alive?
CD40 ligand, IL-21 secretion
What is class switching, where does it occur?
Light zone, this involves the changing of the Fc region (depending on the stage of the immune response).
Initially, all are IgM/D, but later can convert to IgA/E/G.
Occurs after antigen activation
Class switch recombination underpinned by switch regions
Initial antibodies produced
IgM, IgD
MHC I structure
Heterodimers, with a polymorphic heavy α-subunit (gene within MHC locus) and a β2 microglobulin subunit (gene located outside of it).
In humans, there are 3 class I α genes HLA-A, HLA-B and HLA-C.
This α-unit is composed of three domains (α1, α2, α3), α1 and α2 form the deep peptide-binding groove
MHC II structure
Heterodimers have polymorphic α and β subunits (HLA-DR, HLA-DP and HLA-DQ).
The peptide-binding groove of MHC-II molecules is formed by both α1 and β1 subunits of the heterodimer, unlike MHC-I molecules, where only one chain is involved.
MHC restriction
MHC restriction implicates that antigen recognition by T-cells depends on the MHC genotype.
T-cell receptor binding prerequisite being for both antigen and MHC – heightening the specificity of the response.
MHC I location (infection speciality)
Nearly all somatic cells, except erythrocytes, express MHC I molecules on their surface.
This allows the identification of intracellular infection and damage as HLA acts as a ‘window’ into the cell, by the presentation of epitopes from the internal environment.
MHC II location (infection speciality)
MHC II molecules only reside on the cell surface of ‘professional antigen presenting cells’ such as dendritic cells and macrophages
This allows fragments of extracellular pathogens to be presented due to these cells specific abilities to phagocytose or undergo receptor-mediated endocytosis.
MHC I cell activates
CD8+
MHC II cell activates
CD4+
Endogenous vs exogenous pathways for MHC
Endogenous (MHC I, expresses intracellular proteins degraded by the proteasome)
Exogenous (MHC II, expresses extracellular proteins degraded by phagocytosis in professional APCs)
Why can dendritic cells cross present?
Cytotoxic T-cells recognise viral peptides bound to MHC I, but target cell lysis requires their prior activation.
Research suggests that dendritic cells are central in this process, and must also express MHC I which binds to the same peptide to activate naïve CD8 cells.
But, not all pathogens will infect dendritic cells, restricting the extent to which these cells can generate peptide fragments and MHC I.
To remedy this, dendritic cells exhibit cross presentation, which suggests that they can present extracellularly sourced peptides on MHC I molecules.
What are CD4/CD8 receptors and what do they do
The co-receptors CD8 and CD4 stabilize the interaction between the TCR and either MHC class I or class II
TCR diversity
Multiple copies of V, D, J and C gene segments may be randomly recombined
D region genes may be transcribed in multiple reading frames
Imprecise joining may occur during rearrangement of genes and excision of the intervening DNA
Nucleotides may be randomly inserted or deleted from the regions flanking the sites where joining occurs
Any a chain may pair with any possible b chain to generate 10^9 – 10^16 TCRs with different specificity
T cell activation
Requires three independent signals.
- Peptide-MHC complex presented
- Co-stimulatory molecules CD40/CD80: promote survival and expansion of T cells
- Pro-inflammatory cytokines (i.e IL-12): direct differentiation of T cell
Primary lymphoid tissues
Primary lymphoid organs include the thymus and bone marrow and are the tissue sites where antigenic receptor repertoires of T and B cells respectively, are selected.
Bone marrow site of
Creation of T cells and the production and maturation of B cells (from common lymphoid progenitor) - primary site of haematopoiesis.
What is the role of the osteoblastic niche?
The osteoblastic niche Protects the HSC (haemopoietic stem cell) pool
Maintains quiescence among HSC to prevent their exhaustion
The endosteal surface offers physical protection from trauma and toxins
Bone absorbs environmental radiation, preventing DNA damage
Significant distance from a blood supply ensures a low O2 tension, reducing exposure to oxidative stress
What happens when blood cells leave the osteoblastic niche?
Expansion and differentiation of progenitors
B cells immediately join the circulatory system and travel to secondary lymphoid organs in search of pathogens.
T cells travel from the bone marrow to the thymus, where they develop further and mature
What happens in the thymus?
T cells mature from thymocytes, proliferate, and undergo a selection process in the thymic cortex before entering the medulla to interact with epithelial cells.
Thymic stromal cells allow for the selection of a functional and self-tolerant T cell repertoire (central tolerance).
What is the role of the secondary lymphoid tissues?
Secondary tissues sample antigens from different body compartments, accumulation of components of the adaptive immune system.
Three types of secondary lymphoid tissue
The spleen is highly vascularised
Peyer’s patches serve the gut
Lymph nodes drain tissue fluid from interstitial tissues
Spleen structure
Specialised to capture antigens that enter the blood stream (vascular network by branches of central arteriole)
Lymphoid tissue component called white pulp
Separated from red pulp by marginal sinus
T and B cells initially delivered to the marginal sinus (rich in macrophages and marginal zone B cells)
T cells migrate to periarteriolar lymphoid sheath (PALS)
B cells to follicles (some germinal centers where hypermutation occurs)
Follicular dendritic cells present - specialised to present antigens
Lymph node structure
Specialised to analyse extracellular fluid for antigens
T and B cell areas
B cell follicles located just under outer capsule, these have germinal centres where B cells mature
T cell zones surround follicles in paracortical areas
Connections to both lymphatic system (afferent, efferent ducts) and the bloodstream (artery and vein/ high endothelial venules HEV)
MALT structure
Associated with bodies epithelial surface.
Peyer’s patches are lymph node like structures under the surface of the mucosa
They have B cell follicles and T cell zones
Epithelium overlying contains M cells (adapted to pathogens directly from gut)
B cells become committed to IgA.
High endothelial venules
HEV are found within the paracortex of lymph nodes and act as portals for the entry and exit of cell types (naïve T-cells from thymus)
HEV constitutively express the adhesion molecules GlyCAM-1, CD34 and ICAM-1
HEV mimic inflamed endothelium at sites of infection
Binding of L-selectin on naïve T cells to GlyCAM-1 and CD34 induces rolling along the luminal surface
Activation of LFA-1 results in tight binding to ICAM-1 and diapedesis
Dendritic cells at lymph nodes
Means presentation of foreign antigen to naïve T cells in the paracortical T cell zones.
The paracortex represents the highest density of naïve T cells
A single DC can simultaneously interact with up to 200 T cells in order to identify rare clones with a complementary TCR
Functional ligation of the TCR leads to T cell activation and clonal expansion
CD4 cells differentiate into
Th1, Th2, Th17, Tfh and Treg
Th1
IFNγ (intracellular pathogens), activates macrophages, CTLs and b cells
Th2
IL-4, IL-13 (extracellular parasites) mainly activates b cells
Th17
IL-17a, IL-17f (extracellular bacteria and fungi)
Tfh
Help B-cells and humoral immunity to extracellular pathogens by promoting germinal centres
Tfh cells activate antigen-specific B cells through interaction of CD40L with the co-stimulatory molecule CD40
Secretion of IL-21 sustains B cell proliferation and formation of germinal centres
What causes Tfh cells to migrate to primary follicles?
Expression of CXCR5 confers on Tfh cells the capacity to migrate from the paracortex to the primary follicles
Memory B cells vs Naive cells
Memory cells are a subset of B-cells, naïve cells are different from memory cells in several ways
Memory cells have already undergone class switching and are, therefore, capable of producing an isotype appropriate for the pathogen
Memory cells show constitutive down-regulation of cell cycle inhibitors enabling them to rapidly undergo clonal expansion
Somatic mutation ensures that the affinity of antibodies is many orders of magnitude higher than that of naïve B cells
3 types of B cells?
Short lived plasma cells, long lived plasma cells and memory b cells
Difference between long lived plasma cells and memory B cells?
Long-lived plasma cells (LLPCs) constitutively secrete antibody to neutralize antigen immediately upon reinfection (reside in bone marrow), whereas memory B cells (MBCs) produce antibody only upon restimulation by specific antigen
T memory cells
CD4+ and CD8+ T cells also develop into memory cells of two types:
Effector memory T cells (TEM):
Generated early in the immune response
Central memory T cells (TCM):
Generated late in the immune response
These can persist for decades in the body but need continuous replication - mechanism not known
How are autoreactive B cell clones eliminated?
Recognition of self-antigens by naïve B cells in the bone marrow drives clonal deletion to eliminate autoreactive clones. This ensures the B-cell population is tolerant to self antigens.
What bone marrow cells express self antigens?
Stromal cells in the bone marrow express Fc receptors for ‘natural’ IgM antibodies that bind a spectrum of self-proteins for presentation to emerging B cells.
If the BCR can bind strongly to self-antigen, then the B cell undergoes one of which four fates
Clonal deletion (cell death by apoptosis)
Receptor (L chain) editing (remove self receptor)
Anergy (migrate to periphery and die as don’t receive survival signals)
Ignorance (binding so weak can’t normally activate cell)
Limitations of B Cell Clonal Deletion, what helps this?
Although initial self reactive B cells removed in BM, confounded by somatic hypermutation of immunoglobulin genes during the course of the immune response
Another level of tolerance is, therefore, imposed at the level of Th cells since deprivation of T cell help inhibits B cell activation
Stromal cell types in the thymus
Stromal cell types include macrophages, DC and thymic epithelial cells (TEC), either medullary (mTEC) or cortical (cTEC).
T cell development from new thymocytes to CD4+/CD8+ thymocytes
Thymocytes (when enter the thymus)
α:β and γ:β are the first two subtypes of T cell.
α:β/ CD4-/8- type differentiates into CD4/8 cells
Interactions with thymic stroma stimulate this development
pre-TCR (newly rearranged B chain)
Ligation of the pre-TCR halts further b chain gene rearrangement - alpha chain locus accessible to the V(D)J recombinase - rearrangement of the alpha chain locus
Up-regulation of CD4 and CD8 leading to the ‘Double Positive’ phenotype
Drives thymocyte proliferation as a prelude to positive selection
What are pro/pre T cells called?
CD4-8- (absence of CD4/8)
What is a pre-T cell receptor composed of?
Composed of a newly rearranged b (somatic gene rearrangement) chain with an invariant pTa chain
Positive T cell selection benefit and important cells
Positive selection rescues thymocytes from cell death and produces a lineage commitment.
Cortical Thymic Epithelial Cells (cTEC) are very important in this process
How are cTEC adapted to interacting with thymocytes?
cTEC form a 3-dimensional network with no basement membrane
The stellate morphology of cTEC ensures maximum interaction with developing thymocytes
cTEC constitutively expresses both SELF MHC class I and class II allowing thymocytes to assess the restriction specificity of their newly rearranged TCRs
What happens if double positive thymocytes cannot recognise a self antigen?
Small double positive thymocytes express small amounts of TCR, most of these cannot recognise a self peptide: self MHC complex, they will fail positive selection and die.
What happens to the cells that can recognise self peptide: self MHC complexes?
Positively selected, mature and express more TCR.
Those that are MHC I restricted stop expressing CD8 co-receptor - CD4+ formed
Those that are MHCII restricted stop expressing the CD4 co-receptor - CD8+ formed
Where is the site of T cell negative selection?
Medulla of thymus
What underpins self antigen expression in the thymus?
Dendritic cells are migratory and patrol the body acquiring self antigens which are then transported to the thymus
mTEC exhibit patterns of ‘promiscuous gene expression’ permitting expression of diverse self proteins (not required by them for their own function).
Promiscuous gene expression is mediated by the transcription factor AIRE and casts an immunological self shadow within the thymus
When self peptide MHC binds strongly to TCR in thymus…
T-cell undergoes apoptosis.
How can recognition of the same ligand by the same TCR lead to the opposite outcomes of survival and maturation versus death by apoptosis?
Avidity of antigen recognition is a key factor in determining positive versus negative selection
Thresholds of avidity define a narrow window of avidity for the emergence of mature T cells from the thymus and their colonization of secondary lymphoid organs
What are the limitations of negative T cell selection?
Inadequate provision of self-antigens - in the thymus may makes negative selection incomplete (AIRE does not code for everything)
Developmental antigens - Ectopic re-expression of developmental antigens in later life may activate self-reactive T cells that were never subject to negative selection
Molecular mimicry - among T cell epitopes may stimulate self-reactive T cells, (i.e self peptide may be very similar to microbial peptide)
As well as negative selection what other mechanisms are useful for preventing autoimmunity?
Anergy: T cell anergy helps restrict the impact of autoreactive T cells that evade negative selection
Immune privilege: Some tissues and organs display immune privilege in order to limit collateral damage
Treg cells: A repertoire of Treg cells polices the immune system to reinstate self-tolerance
What induces T cell anergy?
T cell anergy can be induced by self-antigen recognition due to AIRE expression in the periphery.
Recognition of antigen by CD4+ Th cells presented to them by AIRE+ APC, induces profound anergy and lack of subsequent responsiveness to conventional stimuli
The anergic state is highly stable and resistant to perturbation
Immune privilege
The capacity to repel an autoimmune response.
Immune privileged tissues include
The central nervous system
The anterior chamber of the eye
The testis
Foetus in pregancy
Mechanisms of immune privilege
Physical barriers to sequester tissue-specific antigens from the immune system
Lack of lymphatic drainage isolating the tissue from the adaptive immune system
Expression of inhibitory receptors such as FasL that induce apoptosis of leukocytes (i.e NK cells)
Induction of a local anti-inflammatory environment due to secretion of TGF-b
Depletion of essential amino acids to deprive T cells of the nutrients they require (e.g. tryptophan)
Acquired Immune Privilege of Tissues
Established tumours may actively exploit the principles of immune privilege in order to avoid immune surveillance
Recruitment of M2 macrophages and secretion of anti-inflammatory cytokines such as IL-10 and TGF-b may counteract the local inflammatory response
Down-regulation of MHC class I renders tumour cells invisible to CTL
Treg cell main role
The role of Treg cells is conferring immune privilege on any tissue
Key transcription factor that defines Treg lineage?
FoxP3 -defines commitment to Treg lineage
Natural Treg cells (nTreg)
Become committed to the lineage during repertoire selection in the thymus
Induced Treg cells (iTreg)
Naïve CD4+ T cells recognizing antigen under specific circumstances in the periphery may become polarised towards induced Treg cells (iTreg).
Incomplete chronic stimulation of naïve CD4+ T cells is the main stimulus for induction of induced Treg
nTreg vs iTreg
Both require expression of FoxP3 as a master transcription factor
iTreg modulate responses to harmless foreign antigens
nTreg are more phenotypically stable and inhibit responses to self antigens
How do Treg cells compare in avidity to effector T cells?
Treg cells have even higher avidity for self antigens than effector T-cells.
Mechanisms of action of Treg
Direct inhibition of the activation of neighbouring T cells through cytokine release
Inhibition of DC maturation through secretion of IL-10
Since immature DC lack the costimulatory molecules required for full T cell activation, they induce further Treg cells, perpetuating the tolerant state
All B lymphocytes are derived from haematopoietic stem cells - T/F
True
In a humoral immune response to a previously unencountered antigen IgM expression always precedes IgG expression T/F
True
Memory B cells never leave secondary lymphoid organs- T/F
False
Pre-B cells do not secrete immunoglobulin - T/F
True
The increased affinity of IgG antibodies in a recall antigen response is due to somatic hypermutation and
clonal selection - T/F
True
Are primary lymphoid organs are also sites of haematopoiesis?
No (only bone marrow)
Are primary lymphoid organs are sites of lymphocyte production and activation?
No, only production, not activation (secondary lymphoid organs)
Within which part of a mesenteric lymph node do B cells undergo isotype switching and affinity maturation?
Germinal centre
Dendritic cells carrying recently acquired antigen enter the mesenteric lymph node via which structure?
Afferent lymphatic
Rheumatic fever is caused by what?
Cross reactive antibodies to Streptococcus pyogenes
Increased susceptibility to pyogenic infection is most likely to be caused by which one of the following?
Deficiency of all B cells
A Th1 T cell response is characterised by which of the following?
Secretion of cytokines that activate macrophages
These proteins, which are secreted by CD4+ Th2 type T helper cells, have important effects on B lymphocyte growth and Ig class switching.
Interleukin-4 (IL-4) and Interleukin-5 (IL-5)
What happens in the nine days post infection that accounts for the dramatic improvement a patient’s condition to Streptococcus pneumoniae?
Generation of high titre capsule specific IgM antibodies
Which class of secretory cells of the immune system are principally responsible for regulating the functions of other leukocytes?
CD4+ T helper cells
What surface glycoprotein is a “marker” for these cells?
CD4
Which cytokine, produced by CD4 cells is a growth factor for T lymphocytes?
IL-2
Which cytokine, produced by these cells, promotes Th2 differentiation and proliferation?
IL-4/ IL-2
Which cytokine, produced by CD4 cells, activates macrophages?
IFN gamma
List three surface molecules that characterize T-cells.
CD4/CD8
MHC I/II
TCR-CD3
Give three mechanisms by which T-cells avoid responding to self-antigens.
Positive selection: T cells failing to respond to MHC will undergo apoptosis
Negative selection: AIRE expressed by mTEC cells, if T cells are autoreactive they are induced to anergy/apoptosis
Treg cells
If B cell encounter cognate antigen before reaching primary follicle,
They are activated to become short lived plasma cells which migrate to medullary cords of lymph node where they secrete IgM
What do short lived plasma cells secrete?
IgM
If cognate antigen encounters B cell in primary follicle
Proliferation and clonal expansion forms germinal centre
What is the germinal centre site of?
Somatic hypermutation and affinity maturation
Ig class switching
What do cells in the germinal centre develop into?
Plasma cells (migrate to bone marrow, secondary lymphoid organs) and memory B cells in germinal centre remain quiescent and respond to same infection
Immune responses to blood-borne antigens occur primarily
In the spleen
Naive T lymphocytes enter the spleen via high-endothelial venules
False
Germinal centres are conspicuous in non-antigen-stimulated lymph nodes
True
The medulla of an antigen-stimulated lymph node contains numerous plasma cells
True
Afferent lymphatics carry antigens from the intestinal lumen to Peyer’s patches
False
What do medullary cords contain?
Antibody-secreting plasma cells
