1
Q
Cytokine general physical properties
A
- low molecular weight
- <30 kDa
- Mostly single polypeptide chains; can be in aggregated forms (eg TNF, a homotrimer in circulation)
- Potent. Effective at picomolar concentrations
2
Q
Cytokines regulate the intensity and duration of the immune response by… (6)
A
- (1) stimulating or inhibiting the activation, (2)proliferation and/or (3)differentiation and (4)migration of multiple cell types
- (5)regulating the synthesis and secretion of immunoglobulins and other cytokines
- in some cases by (6)inducing programmed cell death of a target cell
3
Q
Autocrine
A
binds receptors on the same cell that secretes the cytokine
4
Q
paracrine
A
binds to receptors on a nearby cell
5
Q
juxtacrine
A
binds a neighbouring cell
6
Q
endocrine
A
binds to receptors on distant target cells
7
Q
pleiotropy
A
a cytokine has different effects on different target cells
8
Q
redundancy
A
different cytokines have the same effect on target cell
9
Q
synergistic cytokines - means?
A
an effect greater than the additive effect of each cytokine alone
10
Q
antagonistic cytokines
A
opposing activities of cytokines
11
Q
cascade - cytokines
A
stimulating other cytokines, forming cytokine networks
12
Q
lymphokines
A
cytokines secreted by lymphocytes
13
Q
monokines
A
cytokines secreted by macrophages and monocytes
14
Q
chemokines
A
- Family of small polypeptides that selectively control (1)adhesion, (2)chemotaxis, and (3)activation of leukocytes
- Can be constitutively expressed and likely involved in homeostatis or developmental roles.
- Others expressed only after stim of the cell
15
Q
interferons
A
chemokines initially described as having anti-viral activity
16
Q
tumor necrosis factor
A
a more specific form of cytokine
17
Q
Colony stimulating factors
are?
A
A specific term for a cytokine.
18
Q
Interleukins
A
Cytokines with a role in communication between leukocytes.
19
Q
the six groups of cytokines and their receptors
A
- IL-1 family
- Hematopoietin family (Class I)
- Interferon Family (Class II)
- Tumor necrosis factor family
- IL-17
- Chemokines
20
Q
7 ways that nonspecific cytokines maintain the specificity of the immune response
A
- REgulated production of the cytokine.
- Of only one chain of a heterodimer is made
- Limited radius of effectiveness
- Short half-life
- Regulation of cytokine receptor expression (ex. lymphocytes need to interact with antigen to express particular cytokine receptors)
- Antagonism
21
Q
Antagonism of Cytokines (4)
A
There are multiple means of intercepting a cytokine.
- shedding receptors that will occupy a cytokine is solution, preventing it from binding another receptor.
- decoy receptors - that even in TM form do not transmit a signal
- Specific cytokine antagonists - eg IL-1 receptor antagonist and IL-36 receptor antagonist
- Binding proteins - bind the cytokine, preventing it from binding the receptor
22
Q
IL-1Ra
A
IL-1 (cytokine) receptor antagonist
An example of a specific cytokine antagonist. Binds the receptor, without inducing a signal
23
Q
Example of a binding protein that demonstrated antagonism of cytokines
A
IL-18 has a specific IL-18 binding protein that binds the cytokine preventing it from binding the receptor.
24
Q
Four classes of chemokines are based on
A
the position of two of four conserved cysteines.
Can have up to four cysteines
25
What are the four families of chemokines
1. CXC or alpha chemokines where X is a variable AA. SUbclasses: ELR motif (neutrophil chemoattractants) and without ELR motif (mononuclear - monocytes/lymphocytes - chemoattractants)
2. CC or beta chemokines - mononuclear cell chemoattractants
3. C or XC or gamma chemokines. Has two members
4. CX3C or delta chemokines, consist or a single member, FRACTALKINE - a neutrophil chemoattractant that can be membrane-bound
26
What is meant by, 'chemokines show considerable promiscuity'
Chemokines interact with a large number of receptors/cells
27
Subsets of T helper cells
arise from..
CD4+ lymphocytes.
Th1
Th2
Th17
They all arise from a common Th0 precursor.
28
Th1
- Induced by
- make
- Master gene regulator (transcription factor)
- roles (3)
- Induced by IL-12, IL-18, IFN-y
- Make: TNF, IFN-y
- T-Bet
1. Promote opsonizing Ig isotypes
2. cell-mediated immunity (intracell. parasites)
3. Differentiation of CD8+ cells to become cytotoxic
29
Th2
- Induced by
- Make
- Master gene regulator (transcription factor)
- Roles (2)
- Induced by IL-4
- Make IL-4, IL-5, IL-10, IL-13
- GATA-3
1. Allergy
2. B cell activation and antibody responses
30
Th17
- Induced by
- Makes
- Master gene regulator (transcription factor)
- Roles (2)
- Induced by TGF-beta, IL-6, IL-23
- Makes IL-17
- RORy
1. Contributes to inflammation, autoimmunity
2. Resistance to fungal infection
31
Cross-regulation by Th subsets (2)
Examples (3)
Cytokines that promote differentiation of one Th subset may also suppress the development of the alternate subset
Explains the inverse relationship bt classical cell-mediated and antibody responses leading to allergy
Th2 production is favoured - presence of IL-4, IL-12 and IFN-y will stim Th-2
IL-4 and L-10 indirectly downregulate Th1 cells
IFN-y inhibits Th2 proliferation (aside from when IL-4 is present)
32
Families of cytokine receptors
6 families based on conserved structural features
1. IL-1 (Immunoglobulin superfamily) receptors
2. Class I / hematopoietin receptor family
3. Class II / interferon receptor family
4. TNF receptor fmaily
5. Chemokine receptor family
6. IL-17
33
IL-2R
- cytokine receptor among the hematopoietin receptor family.
- ALpha, beta, gamma polypeptides
- Can exist in 3 forms - more chains, stronger affinity
- Low affinity = alpha chain
- Intermed affinity = beta and gamma chains
- high affinity = alpha, beta, gamma chains
- signal transduction requires both beta and gamma chains.
- gamma chain expressed constitutively on T cells
- alpha/beta expression enhanced by antigen stimulation
- NKT cells expression gamma/beta dimer constitutively (int. affinity)
34
IL-2R subunits - redundancy
- alpha chain is unique to IL-2R, but the gamma chain is the same for a lot of other Class 1 receptors and so their TM signals are similar for diff receptors.
35
IL-2 subunits - antagonism
Unique alpha subunits for members in CLass 1 receptors may compete for association with a limited number of Beta subunits so if there is more of one cytokine, that signal might win out because all of the B monomers will be paired up.
36
Soluble cytokine receptors
- no longer attached to a cell
| - soluble forms typically retain high affinity for cytokine and are thus able to bind cytokine in solution.
37
Soluble cytokine receptors: mechanisms that result in solubilization (2)
1. Proteolytic cleavage of the EC domain
- release receptor form PM
- Often a result of specific activation event in acting on the cell
2. Splicing out of the TM encoding exon of the primary RNA transcript resulting in a protein that is secreted.
38
Roles of soluble cytokine receptors (4)
1. Receptor down-regulation: can no longer signal in cell, limits cell response.
2. Soluble receptor can be a binding proteins that protects the ligand from being degraded or cleared: receptor no longer signals, but can deliver the ligand to other membrane-bound receptors.
3. Direct Antagonism: soluble receptor binds the cytokine, preventing it from binding membrane-bound receptors
4. Soluble alpha chain may bind to a ligand, thus conferring sensitivity to another cell that may have only the signalling chain. Expands the number and types of cells sensitive to the soluble receptor/ligand complex
39
Many Class 1 and Class 2 cytokine receptors lack _____
intrinsic tyrosine kinase domains
40
To a great extent, cytokine receptor signaling is achieved by:
phosphorylation of proteins already present int he cytoplasm, which results in a rapid pattern of alterations in multiple proteins
41
Signalling of Class II / Interferon-type receptors
- Subunits of the receptors associated with inactive protein tyrosine kinases
- alpha chain associated with JAK (Janus Kinase) even in the absence of bound cytokine
- In absence of bound ligand, JAKs lack protein tyrosine kinase activity
- Cytokine binding induces association of the alpha and beta subunit and activation of the JAKs (phosphorylated)
- Activated JAKs create docking sites for signal transducers and activators of transcription (STAT transcription factors) by phosphorylation of specific tyrosine residues on the receptors.
- Docking occurs between SH2 domain of STAT and phosphorylated tyrosine on the receptor
- JAK then phosphorylates the docked STAT
- Phosphoylated STATs translocate from the receptor as dimers, and go to nucleus to initiate transcription of specific genes.
- The genes that are transcribed are determined by specific DNA sequences in promotor regions that monomeric or dimeric STATs bind to.
- Multiple JAKs and STATs act in different permutations, providing specificity of the response.
42
Cross-regulation of Th subtype at level of transcription factors
Expression of TF, T-Bet drives cell to Th1 differentiation and suppresses Th2 differentiation.
Expression of TF, GATA-3 promotes development of Th2 but inhibits development of Th1
43
Dimerization of STATs
STATs typically form homodimers in the cytoplasm and go on to regulate transcription in the nucleus.
Heterodimers are possible due to simultaneous activation of different cytokine receptors that result in phosphorylation of different STATs
Formation of heterodimers can cause synergy or antagonism.
44
Three factors that determine the specificity of a cytokine effect
1. The particular JAK/STAT pathway
2. STAT-specific sequences int he promotor regions of genes
3. Only certain target genes can be activated in a particular cell type. Only a subset of potential target genes of a particular STAT may be permitted expression in any given cell type (some may be masked).
45
Chemokine receptor family signalling (4)
(1) Chemokine receptors are couple with large, heterotrimeric G proteins.
(2) The signal transduction process generates second messengers such as Ras (MAPK), Rho, PLCBeta, (3)Ultimately inducing AP-1 and NF-kappaB
(4) ALso able to signal through JAK-STAT phosphorylation events. Contingent on dimerization of chemokine receptors (homodimers or heterodimers)
46
NFkappaB mechanism
- Sequestered in the cytoplasm by IkappaB
- IkappaBalpha phosphorylated by IKK (IkappaBkinase) and is then ubiquitinated and destroyed by the proteosome
- The freed NFkappaB is then able to translocate to the nucleus where it can direct gene transcription.
47
mediation of NFkappaB signalling
NFkappaB makes its own inhibitor, IkappaBalpha, which serves to stop signalling
48
Why is NFkappaB so important
Activates a wide range of genes and so this transcription factor is an important mediator of inflammation.
Expression of cytokines, adhesion molecules and other inflammatory mediators are all promoted by NFkappaB.
49
NFkappaB (3)
- Nuclear Factor KappaB, heterodimer
- IL-1, TNF and IL-17 all have signalling pathways that ultimately lead to activation of NFkappaB
- Expression of cytokines, adhesion molecules and other inflammatory mediators are all promoted by NFkappaB.
- Makes its own inhibitor, ikappaBalpha, which serves to stop signalling.
50
MAPK (4)
- mitogen activated protein kinases
- serine/threonine kinases
- Three final pathway effectors: ERK, p38 and JNK each ultimately activating TFs that reg inflam and immune molecules
- Have become popular targets for therapies for a number of diseases.
51
IL-1, TNF and IL-17 receptors signal through mechanisms that differ from chemokine/hemaptopoietin/IFN receptors. These also involve ...
Serial phosphorylation of different proteins.
| Ultimately converge with the activation of NFkappaB and MAPK
52
IL-1 (immmunoglobulin superfamily) receptor family
A heterodimer consisting of IL-1RI and IL-1receptor accessory protein (IL-1RAcP) and the adaptor protein MyD88
53
TNF family receptors
become trimers on binding ligand (also a trimer) and some can lead to apoptosis.
54
The B Cell Antigen Receptor (BCR)
- Composed of four polypeptide chains: 2 identical light and 2 identical heavy chains
- Chains connected by disulphide bonds and spanning cell membrane.
- 2 types of light chains: lambda and happa
Heavy chains have one V regiona nd 3/4 C regions
- Multiple types of heavychains defined by their constant regions, representing the different subclasses (isotypes)
- The valency of the BCR is 2 (each V region from heavy and light chain)
55
The T Cell Receptor (TCR)
- Composed of two polypeptides, disulphide linked and spanning the membrane
- Two types of TCR: alpha/beta or gamma/delta heterodimers
- Each polypeptide has one V region and 1 C region so the receptor has a valency of 1
56
Two important fundamental differences between BCR and TCR
1. T cells do not secrete TCR while B cells secrete Ig molecules by deleting the TM-spanning exon during alternative splicing of the primary RNA transcript
2. The BCR is specific for native or intact antigen, but the TCR is now. Instead, antigen must be presented by self MHC molecules to the TCR.
57
Differences bt BCRs and TCRs: hypermutation
The BCR variable region undergoes hypermutation during affinity maturation, while the TCR variable regions do not hypermutate.
58
Differences bt BCRs and TCRs: Class-Switching
The constant regions of the BCR heavy chain can be replaced during class-switching, while the equivalent does not occur with the TCR
59
Receptor associated molecules in BCRs and TCRs
BCRs: Ig-alpha and Ig-beta associate with the BCR
TCR: associates with CD3 - a multimolecular complex comprised of 6 polypeptides
60
Know: Light chains show significant variability on the _______ end and constancy on the _______
amino, carboxy end sequence
61
Two possible models for variation in the light chain.
1. Germ-line theory: proposed there was a different gene for each immunoglobulin.
2. Somatic-Variation Theory: proposed the genome has few genes but that recombinations or mutations result in diversity.
62
Dryer and Bennet proposed that (Ig encoding) (3)
(1) Two genes encode a single immunoglobulin light or heavy chain
(2) and a recombination event explained their contributing to a single polypeptide.
(3) There was a great number of variable genes but few constant genes.
63
Tonegawa and Hozumi found evidence that (regarding immunoglobulin genetics)
- separate genes encode the V and C regions of immunoglobulins
- But even further complexity was evident
64
Signal Peptide
- AKA signal sequence, leader sequence, leader peptide
- A short (5-30 AA) peptide present at the N terminus of the majority of newly synthesized proteins that are destined for the secretory pathway (including those that reside in secretory organelles or in membranes)
65
Coding Joint
The splice site in each segment of DNA in immunoglobuline gene rearrangement.
66
IgH gene rearrangement
- IgH is the only Ig that has the D region
67
RSS elements
Recombination Specific Sequence
- Flanks the coding regions of the gene segments and flag for recombination.
- 2 forms with conserved palindromic heptamers and nonamers spaced by 2 different bp lengths (12 or 23)
- The two type are complementary and permit recombination
- recombination can only occur between the two different types of RSS elements
- Possible that the lengths are due to supercoiling of DNA (~12 bp to turn, and 23 bp would be two turns)
68
recombination vs permutation
recombination = moving regions around
permutation = recombination where order is required. Eg must be VDJ, VVV not possible
69
V(D)J Recombinase
A multimolecular complex.
- Includes RAG1 and RAG2. which are Recombination-Activating Genes that are lymphoid cell specific
- RAG1 and RAG2 expression limited to B cells and T cells (lymphocytes) and they are required for recombination of the TCR or BCR
- Remaining molecules in the complex are concerned with DNA repair and ligation
70
Summary of Recombination of Immunoglobulin DNA
1. RAG binds the RSS elements, leading to the hairpin coding end, which gets cleaved
2. P nucleotide additions
3. Exonuclease trimming (3' end)
4. N nucelotide additions
5. Ligation of the DNA
71
Palindrome
Reads the same backwards as it does forwards. Eg civic.
TGGATCCA
ACCTAGGT
72
Five mechanisms that contribute to the generation of antibody diversity
1. Multiple gene segments
2. P nucleotide addition
3. Exonuclease trimming
4. Non-templated (N) nucleotide addition
5. Combinatorial diversity - two chains combining into one antigen binding site
73
Combinatorial diversity
The variable resions of two polypeptides, the light chain and heavy chain in the case of Ig, and two two chains of the TCR, combine to make the antigen binding site.
74
Co-expression of IgM and IgD on B cells
- Due to alternative splicing of the primary RNA transcript.
- The 3' flank of the last exon for constant regions includes a polyadenylation site. Downstream of the polyA site are two more exons that both code for TM domains and a second polyA site.
- B cells splice the TM domain out to have an Ig secreted. Can also do this to remove the mu exons, thus allowing expression of the delta polypeptide
- So this isn't a true class switching event - rather just RNA splicing
75
Affinity maturation in B cells
- High mutation rates in the heavy chain V regions after the BCR binds antigen
- Somatic hypermutation occurs in the complementarity determining regions (CRDs) in V region DNA
- Slight alterations in the V region to improve affinity of the Ig for the antigen
- Increases the net affinity of the Ig population --> AFFINITY MATURATION
- As concentration of antigen decline, only B cells with high affinity BCRs will cont to be stimulated and their expansion is favoured.
- Possibility of coding a premature stop codon, but this cell will no longe rbe able to synthesize and secrete Ig.
76
Class switching recombination / isotype switching
(1) Changing constant region segments in Igs
(2) Changes effector role without changing V region specificity
(3) Driven by cytokine stimuli, depending on which Ig subclass qualities are most desirable
(4) Activation Induced adenosine Deaminase (AID) cuts at mu switch regions and the switch region upstream of the desired subclass
(5) Once the cell has switched to another class, it can't go back because those exons have been deleted from the chromosome
77
Cytokines driving Class Switching Recombination (CSR) example (IL-4)
IL-4 drives class switch to IgG1 and IgE - in this order. It must go through IgG first.
Gives T cells and other cytokine sources input into what isotype of Ig B cells make.
78
TCR gene rearrangements - similarities with BCR rearrangements (6)
Similar to BCR rearrangements with some exceptions.
(1) Beta and Delta genes have D regions like Ig heavy chains
(2) Alpha and Gamma sequences lack D regions (like light chains)
(3) Multiple copies of V, D and J regions available for recombination
(4) D to J recomb first and then V to D recomb
(5) N- and P- nucleotide additions occur in the the coding joint
(6) Two polypeptides combine their V regions to form the antigen binding sites
79
TCR gene rearrangements: how they differ from BCR gene rearrangements
- Alternative joining of D segments
- TCR gene segments have RSSs but the particular arrangement of 12 bp vs 23 bp RSS among the D regions allows for recomb between D regions and thus more than one D region could be included in the coding joint of delta chains
- TCR gene segments do not undergo somatic hypermutation
80
Chimeric Antibodies
- Assembly of Ig polypeptides by gene segment rearrangements inspired development of chimeric antibodies as therapeutics
- Manipulate mouse DNA to make mouse antibodies with human heavy chains that will react to human inflammatory cytokines.
- Will target human cytokines, which is useful because we won't produce antibodies that target self
81
Generally: Immunoglobulin deficiencies
Characterized by significantly lower levels of immunoglobulin, including of selected isotypes
82
X-linked agammaglobulinemia
| Bruton's hypogammaglobulinemia
- Mutation in Bruton's Tyrosine Kinase leads to no peripheral B cells (arrest at pro-B stage)
- Recurrent bacterial infections beginning at ~9 months
- extremely low IgG, lack of other classes
83
Hyper-IgM syndrome
- T cell defect. Fail to signal through CD40
- Deficiency of IgG, IgA, IgE, increased IgM due to T independent stimulation (lack of class switching)
- Symptoms: recurrent infections, particularly respiratory
84
Common Variable Immunodeficiency Disorders (CVIDs)
- Low numbers of antibody producing cells and antibody classes
- B cells fail to mature to plasma cells
- Multiple possible causes. Likely linked to possible causes
85
Hyper IgE Syndrome
- Frequently due to mutated STAT3 leading to dysregulated T helper activity - particularly Th17
- Multisystem disorder with variable presentation
- High IgE, skin abscesses, pneumonia, eczema
86
Selective IgA deficiency
- IgA deficiency most prevalent selected deficiency (others are rare)
- IgA+ B cells unable to differentiate to plasma cells
- Often asymptomatic, otherwise variable presentaions
87
Development of B cells
(1) When does it occur?
(2) Where does it occur
Steps (4)
(1) Occurs beginning in fetal life and continues throughout life.
(2) Occurs in the bone marrow through to stage of BCR rearrangement
- Entirely antigen dependent
1. Hematopoietic stem cells divide and the progeny become Common Lymphoid Progenitor
- Common Lymphoid Progenitors already express RAG1/2 + TdT
2. Pre-pro-B cells express B220 on their surface
3. B cells that successfully rearrange BCR and express IgM on surface undergo selection for self-reactive cells
4. B cells that aren't self-reactive leave bone marrow as naive B cells to circulate bt blood + lymph nodes(~15% of total pop.)
88
Stages of differentiation of B cells
Pre-pro-B cell: Ig genes in germ line; positive for B220
Pro-B cell: D-J rearrangement, then V-D rearrangement of heavy chain allele. Expression of Vpre-B and lambda5 begins (comprise surrogate light chain).
Pre-B cell: Express heavy chain with surrogate light chain, along with Ig-alpha and Ig-beta (Pre-BCR). Rearranges light chain with 4 alleles and starts dividing.
Immature B Cell: Short-lived; Light chain assembled with heavy chain into final IgM expressed on surface; negative selection of autoreactive cells
Mature B Cell: Express IgD and IgM on surface; differentiate following antigen stimulation
Plasma cells: no longer proliferate; secrete Ig; terminal stage
89
Surrogate light chain made up of
made up of lambda5 and Vpre-B
90
pre-BCR made up of
Heavy chain, surrogate light chain, Ig-alpha, Ig-beta
91
Pre-BCR capable of
Capable of signaling - cell receives signals including downregulation of TdT and Rag proteins to prevent rearrangement of second heavy chain chromosome (allelic exclusion)
92
Mice deficient in lambda4 /Ig-alpha/beta
have arrested B cell development
93
Allelic Exclusion
Pre-B cell has a pre-BCR which is capable of receiving transient signals that downregulate TdT and Rag proteins to prevent rearrangement of the second heavy chain chromosome.
94
Negative selection of self-reactive B cells
Immature B cells that are self-reactive are identified in the bone marrow and eliminated by APOPTOSIS or CLONAL DELETION
The result is CENTRAL TOLERANCE - most of the remaining cells do not react to self.
95
What about autoreactive B cells that aren't eliminated in negative-selection? (3)
- Some will attempt rearranging remaining light chain alleles in RECEPTOR EDITING
- Some will escape marrow but remain unresponsive to antigen: ANERIGIC
- Some may encounter self-antigen and be deleted by apoptosis while in the periphery (PERIPHERAL TOLERANCE)
96
How does BCR signal? (5)
(1) Ig-alpha and Ig-beta posess ITAMs
(2) Crosslinking of BCR brings it in close proximity src family tyrosine kinases
(3) src family TKs phosphorylate ITAMs
(4) Phosphorylated ITAMS become docking sites for syk kinase
(5) syk activated --> leads to multiple signalling events
97
2 ways by which a B cell can become stimulate to proliferate and produce antibody
1. Thymus (T)-Indiependent
| 2. T-dependent
98
T dependent antigens: stimulation of B cell
(Signal 1) An antigen crosslinks BCR
(Signal 2) T cell CD40 ligand makes contact with CD40 on B cell
B cell proliferates and produces antibody. Memory cells generated, cytokines modify the response.
99
cytokines in B cell stimulation
They are additional growth factors for B cells and specific cytokines control class switching to different subclasses in B cell.
100
T-Independent antigens: stimulation of B cells
Activate B cells independently of the specificity of their BCRs. The BCRs will crosslink regardless.
Type 1 polyclonal activators: some bacterial products, TLR stimulators.
Type 2: highly repetitious molecules that cross-link BCR and CD21. Only activate mature B cells.
Type 1 and 2 both provide signal 1 and 2 to promote B cell activation.
No memory cells made
101
T cell development generally (4)
(1) Occurs in thymus (a gland with cortical and medullary regions) - called thymocytes while here
(2) Maturation takes 1-3 weeks
(3) Hematopoietic stem cells are attracted to the thymus from BM by chemokines
(4) Development in thymus dependent on NOTCH LIGAND, secreted by thymic epithelial cells, binding to the receptor, NOTCH
102
Acute stress effects on thymus (2)
(1) Stress can downreg. T cell maturation --> rapid loss of cortical thymocytes
(2) Thought to be at least partly due to the effect of steroids such as glucocorticoid as the thymus is richly innervated
103
Thymus with age and pregnancy (2)
(1) Thymus involutes with age and pregnancy.
| (2) Aged thymic involution correlates with decreased T cell function.
104
Allelic exclusion in T cells
Successful rearrangement of a set of TCR genes suppresses further rearrangement of TCR genes on the second chromosome so that each cell only expresses TCR with a single specificity
105
gamma/delta T cells (5)
(1) A small population of DN T cells that are exported from thymus at defined periods of fetal and neonatal development
(2) migrate to and populate different epithelial-rich tissues: skin, intestinal epithelium + pulmonary epithelium
(3) Remain fixed in tissue sites where they are thought to be the first line of defense against infections and cancers
(4) not limited to self-MHC
(5) Encoding specific and limited Vgamma gene segments
(5)
106
Hypotheses for transformation from double-positives to single-positives.
1. Instructive model
2. stochastic model
3. kinetic signaling model
107
Instructive model
For transformation from double-positives to single positives.
Assumes multiple interactions between TCR, CD4 or CD8 co-receptors.
Class I or II MHC molecules instruct the cell through different signals to differentiate along one pathway.
108
Stochastic Model
For transformation from double-positives to single positives.
Suggests that one coreceptor (CD4 or CD8) is hut off randomly, independent of interactions with TCR
109
Kinetic Signalling Model
For transformation from double-positives to single positives.
Strong TCR signaling throughout the development favors CD4 expression, while weak TCR signalling or interrupted singnaling favors CD8 expression
110
Negative selection of thymocytes (3)
- thymocytes that survive positive selection physically interact with DCs and macrophages in the thymic medulla
- thymocytes with high affinity TCR for self-MHC/Self-antigen die by apoptosis
- Remaining thymocytes are self-tolerant and are released into the periphery
111
Double-negative alpha-beta TCR-bearing T cells (2)
- A small population of these does exist.
- May develop from single-positive cells following negative selection
- Another T cell subset (such as NKT)
112
Positive selection of thymocytes (5)
- A physical interaction between immature thymocytes and epithelial cells in the thymic cortex
- believed that this interaction results in a protective signal that prevents death by apoptosis
- Only thymocytes with low/int affinity for self-MHC survive
- Cells become CD4 or CD8 single-positives
- Most have affinity that is too low so they fail to be positively selected (death by neglect)
113
Stimulation of T cell through TCR (5)
(1) Engaging TCR + CD4 or CD8 leads to assembly of a signalling complex
(2) CD45 removes an inhibitory phosphate, activating Ick
(3) Ick phosphorylates ITAMS on CD3, to which ZAP -70 docks
(4) ZAP -70 becomes phosphorylated by Ick and activated
(5) ZAP-70 then catalyzes a number of other phosphorylations that lead to gene expression, differentiation and activation
114
Examples of aggregated homogenous cytokines in biological fluids
TNF = homotrimer
| IFNs sometimes homodimers
115
Th0 development (3)
Precursor to Th1, Th2, Th17
Capable of making cytokines from all subsets.
Differentiation determined by the cytokine environment during antigen activation.
116
Second Messengers: Chemokine receptor signalling (4)
PKC (survival)
Ras (MAP Kinase)
Rho (Cell motility)
PLCBeta (mediator gene expression)
117
Switch Regions
- highly repetitive DNA stretches
- lie 5' (upstream) of constant domain exons
- Delta doesn't have a switch region so it can't go through this process
