Week 3 Flashcards
Th0
benign, undecided precursor helper T cells
- begin as Th0 when they exit the bone marrow and travel to thymus before differentiating
- Differentiate/divide when correct antigen is brought to them by DC
- Previous experience of DC (conditions in periphery, when it was stimulated, what TLR were engaged, what cytokines/chemokines predominated) determine the Th0’s ultimate progeny
Th1
“hypersensitivity” T cells, urgent destruction of invaders
-Involved in inflammation
Secrete lymphokines: IFNy and IL-2 when activated by APC (DC or macrophages)
-Activate M1 angry macrophages (classical pathway)
IFNy is a lymphokine secreted by _______ and acts to activate ________. These then release ______ and _______.
Th1 cells
M1 macrophages
TNFa and IL-1 cytokines
IL-2 is a lymphokine secreted by _______ and acts to…
Th1 cells
help CTL get fully activated after they recognize antigen
Th2
- Involved in healing via M2 cells (debris removal, scar formation, walling off pathogens that M1 macrophages have failed to kill)
- Appear later in sites of inflammation
- Leave lymph node as Th1, and circulate in blood/lymph until they encounter their antigen in the tissues
- Secretes lymphokines IL-4, IL-5, and IL-13
IL-4, IL-5, and IL-13 are secreted by _______ and act to…
Th2 cells
attract macrophages via M2 alternative activation
Il-4 is chemotactic for _________
eosinophils (parasite killers)
Th17
involved in inflammation, implicated in many autoimmune diseases
- Makes inflammatory lymphokine IL-17
- Activates M1 macrophages as well
Tfh
help B cells differentiate
- Migrate into follicles of the cortex and help B cells that have recognized antigen become activated and differentiate into antibody secreting plasma cells
- Secrete cytokines and by DIRECT contact stimulate B cells to switch from IgM → IgG, IgA, etc.
Treg make transcription factor _________ and secrete ______ and _______. They have phenotype _________.
Foxp3
TGF-B
IL-10
CD4+/CD8+
Treg
suppress activation and function of all other Th cells
Very potent - can suppress 1000 Th cells
Respond specifically to corresponding antigen, but their suppression of other T cells is NOT antigen-specific → any nearby Th is suppressed
CTL
- immune surveillance
- Delivers “lethal hit” to cells telling it to undergo apoptosis → rapid DNA fragmentation and nuclear collapse
- Activated in lymph nodes after contact with antigen-bearing DC
Two pathways CTL can signal cell death:
1) bind _________ on target –> apoptosis
2) secrete ______ containing _______ and ________ –> __________ –> apoptosis
1) Death Receptor Fas (CD95)
2) secrete lytic granules
containing proteases (granzymes) and proteins (perforins)
–> penetration into target cell
CTL are activated by ______ + _______
CTL are converted into memory cells with the help of _______
Th1 and IL-2
IL-21
Memory cells
the small % of CTL left behind
Replace themselves, rapidly differentiate into effector (helper, killer) cells when re-exposed to low antigen concentrations
CD3
- on the surface of virtually all T cells
- Complex of molecules intimately associated with TCR
- T cell binds correct antigen + MHC with its TCR → CD3 transmits actual signal that turns T cell on
CD4
on T helpers
No reliable surface antigens to distinguish Th1 from Th2 (must look at lymphokines they make)
CD8
on CTL
Lymphokine
short range mediators made by LYMPHOCYTE that affects behavior of the same or another cell.
A subset of cytokines.
EX) IL-2, IFNgamma, IL-4, IL-5, IL-10
Chemokine
small short range mediators made by any cell that causes inflammation
EX) MIP-1 to -4, RANTES, CCL28, CXCL16
Cytokine
short range mediators made by any cell that affects behavior of the same or another cell
EX) IL-1, TNFalpha, IL-12.
How do Tfh cells help B cells get activated by antigen and switch Ig class?
6 steps
1) B cell binds epitope specific to B cell receptor
2) Bound molecule is endocytosed and broken down in endocytic vesicle
3) Peptide fragments bind to MHC Class II molecules brought in by other vesicles that fuse with the endosome and MHC-peptide complex moves to surface
4) B cell displays antigen + Class II MHC
5) Correct Tfh comes along and sees its epitope + Class II MHC on B cell → binds and focuses surface interactions and helper lymphokines on B cell
6) → B cell activated and can class switch
NOTE: epitope that T cell “sees” is not the same as the one B cell “saw”
Mitogen
protein that stimulates T cell (and sometimes B cell) division (a kind of lectin)
Mitogen doesn’t actually bind to antigen-binding site on T cell, like an antigen does - binds to CD3 signal transduction domain
- Can activate all T cells at the same time (can lead to cytokine storm!)
- Can be used in lab to make cells divide and specifically activate all helpers and CTL without regard to antigen specificity
Phyohemagglutinin (PHA) and Concanavalin A (Con A)
mitogen
stimulates T cells to divide by binding CD3
(Antibody to CD3 can be a T cell mitogen too)
Pokeweed mitogen (PWM)
mitogen
stimulates both B and T cells (non-specifically) to divide
Antigens are ______ while mitogens are _______
Specific
nonspecific
B cell antigen receptors __________, while T cell antigen receptors ___________
bind antigen directly with surface antibodies (can interact with FREE antigen)
T cells see antigen ONLY when it is complexed with cell-surface MHC molecules
(attention on cell surfaces, NOT free antigen)
T cell receptor structure
Made of 2 chains: alpha and beta (each has 1 common and 1 variable portion)
-each alpha and beta chain has 3 CDRs and a transmembrane domain
(unlike surface Ig where ONLY the heavy chain is trans-membrane domain)
-Genes: Alpha (V, J), Beta (V, D, J)
Antigen presenting cells include (3)
Dendritic Cell
Macrophage
B-cells
Th0 binds to a good APC, then in order for a T cell to be activated and begin proliferating / differentiating it requires 3 signals
1) Signal via TCR-MHC interaction (a certain number of receptors must interact with antigen and a certain number must interact with specific MHC → sufficient interaction for activation)
2) Accessory molecular interactions only provided by true APC, modify, enhance, or diminish activation
3) Cytokines secreted by APC activate, inhibit or modulate T cell activation
MHC restriction
T-cell is antigen specific and MHC restricted
- CTL does not see antigen alone, but only antigen presented to them on the surface of a genetically-identical cell
- MHC antigens are very variable - thousands of alleles in any population
MHC Class I is on surfaces of ________ and recognized by _______. MHC Class I molecules associated best with peptides from ___________
all nucleated cells
recognized by CTL
proteins synthesized WITHIN the cell itself (not taken up by endocytosis)
MHC class II is on surfaces of __________ and recognized by _________
dendritic, macrophage-type cells, B cells, and other APCs
recognized by T helpers
Extrinsic Pathway of T cell Activation (7 steps)
involves antigen from outside the APC
1) Antigen enters body→ infects locally and causes innate response
2) Breakdown products from innate response (or virus itself) ingested by DC
3) Viral proteins → peptides within endosome (broken down by lysozymes)
4) Endosome fuses with other vesicles with Class II MHC molecules embedded in their membrane (facing IN)
5) Some of the peptide associates with the MHC molecule
6) Endosome goes to cell surface and fuses with plasma membrane → MHC + antigenic peptide are exposed to outside world
7) Appropriate T cell (HELPER T’s) recognizes both MHC + antigen → turned on
Epitope that T cell sees is ALWAYS ________
continuous
vs. B cells which can see discontinuous segments on antigen epitope
Intrinsic Pathway of T cell activation (4 steps)
1) Protein made within cell→ chopped up into peptides→ fuses with vesicles containing Class I MHC molecules embedded in membrane
2) Some of the peptide associates with MHC molecules→ fuses with plasma membrane
3) MHC + peptide are exposed to outside world
4) Appropriate T cell (CYTOTOXIC) recognizes both MHC + antigen → turned on
Cross-Presentation
Allows peptides from antigens it’s eaten leak over to “intrinsic” pathway so it can present them on Class I AND Class II MHC at the same time
- Eats it, breaks it down in endosome→ present on class II (extrinsic)
- Eats some stuff, lets it leak out of endosome → presents on class I like it was made inside the cell (intrinsic)
CTL cells typically kill cells infected with ________ because they are activated by _________
viruses
MHC Class I, intrinsic pathway
T-independent antigens
- no T cell help required for antibody response
- Usually have same epitope repeated over and over (rare in proteins, but common in carbohydrates, ie: Streptococcus pneumoniae)
- response to T-independent antigens is almost all IgM without switching (T cells needed to switch from IgM to IgG, IgA or IgE)
If a person is deficient in T cells, they still can make antibody to carbohydrates.
With protein antigen—a little IgM and NO IgG is made without T cell help
A T cell must… (3)
1) Not recognize “self” - not bind so firmly to self structure (MHC alone, or MC loaded with a “self” peptide) that the T cell becomes activated
(This would be autoimmunity)
2) Not recognize free antigen (which is antibody’s job)
3) Recognize antigenic peptide plus self MHC
T cell maturation:
Th0 cells are _____/______ with activated ________ –> rearrange ________
–> _______/_______ –> ____________ –> mature phenotype (_____/_______) or (_______/_______)
CD4-/CD8- (double negative)
activated RAG DNA recombinases –> rearrange TCR V(D)J genes
–> (CD4+/CD8+) double positive
–> selection and turn off one gene
–> mature phenotype (single positive) CD4+/CD8- or CD4-/CD8+
Repertoire selection can be explained by 3 theories
Positive Selection
Negative Selection
Non-selection
Positive selection:
1) Thymocyte TCR encounters MHC (both class I and II) loaded with endogenous self peptides
2) LOW affinity for binding between TCR and MHC, but binds just enough to be told to mature
3) Affinity for SELF MHC, but NOT SELF PEPTIDE → high affinity for self MHC + foreign peptide in periphery
Explains MHC restriction - only recognize specific MHC from that person because they were positively selected for that
Negative Selection
TCR binds MHC bound to SELF peptide with HIGH affinity → activate T cell → cell dies by apoptosis or becomes Treg
Otherwise would result in autoimmunity
AIRE (autoimmune regulatory) gene
causes thymic stromal cells to express stuff that makes sure T cells come into contact with important “self” items → T cells bind → killed
Non selection
Random V(D)J gene segments generated → most resultant TCR has no affinity for particular MHC molecules in thymus → immature cell receives no stimulation through TCR → dies by apoptosis in 2-3 days
Major Histocombatibility Complex (MHC)
- family of genes, coded for on a single chromosome (chr 6)
- very high degree of allelic variability
HLA = human MHC
Class I MHC include HLA-__ and HLA-___.
These are on ______ cells and recognized by _____ cells
HLA-A, HLA-B
on ALL nucleated cells (and platelets)
recognized by cytotoxic killer T cells
Class II MHC include HLA-____.
These are on _______ cells and are recognized by ______ cells
antigen presenting cells only (dendritic, macrophages, B cells, etc.)
recognized by T helper cells
HLA (MHC) structure
Class I vs. Class II
glycoprotein + 2 polypeptide chains
Class I antigens: allelically variable chain + invariant chain (B2-microglobulin)
Class II antigens: two variable (alpha and beta) chains
Syngenic/isografts
grafts between genetically identical individuals
Allogenic/allografts
rafts between non-identical members of the same species
Xenogeneic/Xenografts
grafts between members of different species
Autografts
graft from one individual to himself
Haplotype and MHC genes
set of alleles at a group of linked loci
MHC gene set that you inherited from your parents is called a haplotype.
(Mom has 2 haplotypes, Dad has 2 haplotypes)
No recombination within a haplotype of MHC (linkage disequilibrium)
________ matching is the most important thing for donors because…
HLA-DR (class II)
T helper cells won’t be stimulated.
For Class I matching ______ and ________ are the most important thing because…
HLA-A and HLA-B
killer T cells won’t be stimulated but Th1 cells will be.
Who would be the best donor for someone?
- an identical twin, followed by a sibling, there is a 25% chance that two siblings will have identical haplotypes
- Parents will always be ½ different from their child so they are not good donors
- Children are ½ different from their parents, so they are not good donors.
- You can also use volunteer donor if they match via HLA.
One-way mixed leukocyte reaction (MLR)
- want to see how strongly recipient’s T cells recognize Class II of this potential donor versus that one?
1) WBCs from donor treated (DNA synthesis inhibitors, radiation) to prevent their division - only want to know if recipient can recognize donor’s MHC?
(Lymphocytes of donor killed, monocytes survive)
2) WBCs from donor and recipient are mixed together to see if recipient’s Th cells divide in response to the donor’s HLA-D (mostly DR, on monocytes).
3) Th of one person looking for class II will see class II on monocytes of other person → stimulated, divide, active, release cytokines
A strong reaction (burst of cell division) may preclude the transplant
If you’re immunologically similar then there will be very little reaction
HLA-D vs. HLA-DR -DP -DQ
HLA-D are all Class II MHC
HLA-DR has to do with transplantation
HLA-DQ and HLA-DP have to do more with autoimmune diseases.
If the donor and recipient are identical Class I but different at Class II…
Th1 activated, no CTL activated
Graft rejected slowly
If donor and recipient are different at Class I but identical at Class II…
No Th1 activated (no IL-2 produced), few CTL activated
→ Class II match is the most important thing
Hyperacute Rejection
Graft given to patient who has preexisting antibody (IgG or IgM) to it (either to MLA due to prior graft transfusions or in a mismatch to blood group antigens)
Ab binds endothelial cell of graft blood vessels → complement activated →
Organ may never become perfused with blood (“white graft”)
Graft Rejection: Th 1 cell recognizes foreign HLA-DR on graft cells –>
_______ proliferates (this is what the MLR measures) and secrete lymphokines (______ and _____) to attract a ________ inflammatory response and activate ______
Macrophages produce pro-inflammatory cytokines (_______), CTL active, –> graft is destroyed.
Th1 proliferates
secrete lymphokines (IFNgamma and IL-2)
attract macrophage inflammatory response and activate CTLs
pro-inflammatory cytokines (TNF-alpha)
CTL active –> graft is destroyed.
Graft Rejection: CTL’s recognize foreign HLA-A and HLA-B on all cells
This recognition is insufficient to activate them; they also require Th1-derived interleukins as a 2nd signal.
Once activated the CTL’s become highly cytotoxic
Graft Rejection
Exactly parallels what happens in normal immune response (like a virus) except that in a normal response a peptide + self-MHC is recognized, in rejection its foreign MHC.
T cells selected to recognize _______ but also recognize ________
why?
self MHC + antigen
foreign MHC (allorecognition)
The recognition of foreign MHC is a chance cross-reaction; the receptors are actually selected to recognize self-MHC and antigen
5% of T cells will bind a foreign MHC strong enough to cause activation.
Ankylosing spondylitis
arthritic condition, inflammation of the insertions of tendons into bones→calcification of joints and they become inflexible (ankylosed).
92% of people with this are HLA-B27 (90x greater risk if HLA-B27)
Strong association between HLA-DR3 and –DR4 and ____________
juvenile diabetes
Lymph node function
non-specific filters of debris, microorganisms, etc.
Key site for antigen presentation in adaptive immunity
Primary lymph organs
bone marrow and thymus gland
- Major sites of development of B lymphocytes and T lymphocytes, respectively
- Lymphopoiesis (differentiation of lymph cells from pluripotent progenitors)
Secondary Lymph organs
aggregates of lymphocytes found in close proximity to antigen presenting cells and can also furnish an adaptive immune response
Includes lymph nodes, tonsils, adenoids, Peyer’s patch and spleen
Seeded with cells from primary organs.
Mucosal-Associated Lymphoid tissue (5)
1) Tonsils (palatine, lingual and pharyngeal (adenoids)
2) Esophageal nodules
3) appendix
4) bronchial nodules
5) Large number of aggregations of lymphocytes in intestine
(Usually increase in size / abundance along intestine length until in colon → very abundant multiple groups of nodules both in mucosa and submucosa known as Peyer’s patches)
Distribution of lymph nodes
small organs all over the body, found individually or in chain/clusters