Immunology Flashcards
Major Histocompatibility Complex (MHC)
- Fxn: present antigens non-covalently bound to MHC Class I and II to specific T cells
Class I MHC
- On most uncleared cells
- Restricts antigen recognition by CD8 T cells
- Antigen binding cleft
A. Peptides 8-10 aa
B. Anchor residues: specific AAs needed to bind peptide- Allows for variability if similar AAs at defined positions
C. Binding cleft closed (baseball glove)
- Allows for variability if similar AAs at defined positions
Class II MHC
- One APCs
- Restricts antigen recognition by CD4 T cells
- Antigen binding cleft
A. Peptides 10-13 AAs
B. Cleft open both ends (hot dog on a bun)
C. Peptides have internal conserved motifs instead of anchor residues
D. Generally only APCs
Antigen capture
- Pathogens enter
- Professional APCs in tissues take up and present antigen
- APCs migrate -> lymph nodes
- Lymph nodes = grand central station
- MHC-TCR engagement -> T cells proliferate, activate, and migrate to infection site via efferent vessel
- Exception: blood Bourne pathogens filter thru spleen
Dendritic cell migration
Cytokines (IL-1, IL-6, TNF-alpha) trigger dendritic cells
- Lose adhesiveness for epithelium
- Expresses CCR7: chemokine receptor released by lymph node containing presenting APCs
Cells in lymph nodes
- Dendritic cells: inc MHC class II
- Macrophages: must be activated (IFN-gamma) for inc MHC class II
- B cells: constitutively express MHC class II
A. Activated by cytokines for co-stimulatory molecules
MHC structure
- Peptide binding cleft at amino terminal end
- Beta 2-microglobin: MHC class I
- MHC class II: alpha and beta chains
- Antigen binding cleft
A. Doesn’t have fine specificity
B. 2,000 peptides/MHC allele
C. MHC class I
D. MHC class II
MHC class I peptides loading
Cytosolic/endogenous pathway
- Proteins ubinquitinated -> targets for proteosomes
- Proteosomes chop proteins -> peptides
- TAP (transporter associated w/ antigen processing) uses ATP to translocations peptides -> ER lumen
- Tapasin: links empty class I (structure held by chaperones) to TAP for easy access to peptides
- Fitting peptide releases chaperones
- Stable MHC-peptide complex -> surface via golgi
MHC class II peptide loading
Exogenous/endocytic pathway
1. MHC class II synthesis in ER constantly
A. Alpha and beta chains joined by chaperones and invariant chain (Ii)
2. class II w/ Ii thru golgi -> MHC class II compartment (MIIC)
3. EC proteins taken up by APC into endosomes
4. Proteins digested by proteosomes in endosome when pH drops
5. Peptides fuse w/ MIIC
6. Enzymes digest Ii -> CLIP (class II invariant chain peptide)
7. HLA-DM assists replacement of CLIP w/ antigenic peptide
8. Stable MHC II-peptide -> cell surface for TCR to recognize
Proteosomes inhibitors
Lead to protein, P53 accumulation -> apoptosis 1. Used in cancer tx A. Bortezomid B. Ixazomid C. Carfilzomid
Cross presentation
Peptide from intra or extracellular pathogen -> cytosolic pathway for MHC class I 1. DC digest both to stimulate helper and cytotoxic T cells in close proximity
HLA (human leukocyte antigen)
- Class I: HLA-A,B, and C
2. Class II: DQ, DR, and DP
MHC genetics
- Codominant expression
- Extremely polymorphic
- Family members don’t share unless ID twins
Rheumatoid arthritis HLA allele
DR4
Insulin-dependent diabetes mellitus HLA allele
DR3/DR4
MS, Goodpasture’s HLA allele
DR2
SLE HLA allele
DR2/DR3
Psoriasis, ankylosing spondylitis, IBS, reactive arthritis (PAIR) HLA allele
B27
Celiac HLA allele
(I 8 2 much gluten at DQ)
DQ2 or DQ8
Graves, myasthenia gravis HLA allele
B8
B cell surface markers
CD19, 20, 21
T cell (all) surface markers
CD3
Cytotoxic T cell surface markers
CD8
Helper T cell surface marker
CD4
NK cell surface marker
CD16, 56
Alpha-beta T cells
- 90% T cells
- Secondary lymphoid tissues
- Peptide-MHC
Gamma-beta T cells
- Exception to rule
- Can recognize free antigens or processed (lipids or peptides)
- <10% T cells
- Possible role in pathogenesis of psoriasis and RA
TCR gene rearrangement
Allows diversity
1. Beta chain rearranges before alpha chain
T cell maturation
1. Pro-T cell A. Proliferation B. Initiation somatic recombination of beta chains 2. Pre-T cell A. Initiation somatic recombination of alpha chains 3. Immature T cells A. Expression TCR B. Expression CD4 and CD8 = double (+) 4. Mature T cell A. Single (+) = either CD4 or CD8 B. Released into circulation
T cell selection
Goldilocks Principle
1. (+) selection: weak recognition MHC molecule presenting self-peptide -> survival and down regulation either CD4 or CD8
A. Recognize MHC but not self-antigens -> advance
2. (-) selection: too strong/no recognition of self-peptide w/in MHC molecules -> apoptosis
T cell signal transduction
- CD3, zeta chains, and ITAMS
- Activation: CD28 costimulation
- Inhibition: CTLA-4 and PD-1
T cell stabilizing adhesions
- CD4/CD8
2. LFA-1
T cell activation
- Secondary lymphoid organs
- Two activation signals
A. From interaction naive T cells w/ APCs
B. 1st = binding TCR or T cell to MHC on APC
C. 2nd = binding B7 on APC w/ CD28 on T cell
D. Need both to activate - 2 signals -> 3 transcription factors
A. NFAT
B. NF-KB
C. AP-1
D. Affect T cell fxn, activation state, and metabolism - Transcription factors -> transcription IL-2 gene
A. IL-2 necessary for proliferation (activates JAK/STAT pathway) - Inhibiting activation
A. Drugs prevent NFAT generation and prevents IL-2 transcription- Cyclosporin
- Tacrolimus
B. Immunosuppressive therapies
T cell proliferation/differentiation
- 1-2 days after activation
- Proliferate -> clone of antigen specific T cell
- Differentiate -> 1 of 2 types
A. Effector T cell: primary immune response, short-lived- Helper
- Cytotoxic
B. Memory T cells: respond subsequent exposures w/ inc sensitivity - Secondary immune response
- Long-lived
- Inhibition
A. After several rounds, T cell -> CTLA-4
B. CTLA-4 binds B7 inhibits proliferation
Effector helper T cells (Th) (CD4)
- Th1: control cell-mediated rxns
- Th2: control humoral rxns
- Th17: inflam disorders and some microbial defense
- Treg: contraction and inhibition immune response
Th1 cells
Control cell-mediated rxns 1. Intracellular pathogens 2. Bacteria 3. Viruses 4. Fungi 5. Role in chronic inflammation 6. Differentiation A. IL-12 and IFN-gamma from intracellular activated dendritic and NK cells activate Th1 cells 7. Cytokines A. IFN-gamma B. IL-2 C. TNF-alpha 8. Synapse: CD40/CD20L
Th2 cells
Control humoral rxns 1. Extracellular pathogens 2. Parasites 3. Allergens 4. Encourage inc IgE 5. Differentiation A. IL-4 secreted by Th2 1. Autocrine growth factor in absence of IL-12 from APC B. IL-4 also secreted from mast cells and eosinophils w/ parasites and allergens 6. Cytokines A. IL-4 B. IL-5 C. IL-13 7. Inhibits Mi macrophages and activates M2
Th17 cells
Inflammatory disorders and some microbial defense
1. Extracellular pathogens
2. Role in autoimmune disorders
3. Differentiation
A. TGF-beta = immunoregulatory cytokine
B. Cooperation inflammatory and anti-inflammatory cytokines -> T cell production/inc IL-21
C. IL-21 = autocrine cytokine -> Th17 differentiation
4. Cytokines
A. IL-17
B. IL-21
C. IL-22
5. Inc inflammatory processes
A. Inc acute phase proteins
B. Inc chemokines attract WBCs
C. Stimulate inflammatory cytokines from other cells
Effector cytotoxic T cells (CTLs) (CD8)
1. Differentiation A. Th1 cells -> cytokines -> naive Tc cells -> CTLs 2. Fxn: kill target cells w/ intracellular pathogens A. Requires cell-to-cell contact B. Mechanisms 1. Granzymes/perforin 2. Fas/FasL 3. Cytotoxic cytokines
Fas/FasL
CTL killing mechanism
- Fas expressed by all nucleated cells
- Only activated CTLs express FasL
- Only binds and -> apoptosis if CTL binds and holds MHC class I
Cytotoxic cytokines
CTL killing mechanism
- TNF
- LT (lymphotoxin)
- Produced after binding MHC and TNFR1 on target cell
Superantigens
Hold any MHC to any T cells ->
- Massive T cell activation and expansion
- Cytokine storm: INF-gamma
- Kill healthy cells
- Staph
- Group A strep
CMI response to intracellular bacteria
- Phagocytes -> IL-12 -> Th1 cell response -> IFN-gamma and CD40L -> activate macrophages to inc ROS
- If macrophage can’t kill pathogen after IFN-gamma activation
A. Pathogen presented on MHC I to CTLs
B. CTL kills affected cells
CMI response to viruses
- IFN-alpha and beta initiate rxn
A. Activate antiviral mechanism in neighboring cells- IFNs land on receptors
- Activate RNAse
- RNAse chops viral RNA
- Viral replication stopped
B. Inc MHC class I expression -> inc adaptive response
C. Stimulate NK cells - IL-12 -> NK secrete IFN-gamma and TNF
- IL-15 -> NK proliferation and activation
Memory T cells
- Last years to lifetime
- More rapid and amplified response to subsequent exposures
- Mediate secondary response
- Maintenance: IL-7 blocks apoptosis cascade and supports low-level proliferation
- Types
A. Central memory T cells (Tcm)
B. Effector memory T cells (Tem)
Central memory T cells
- Secondary lymphoid tissue
2. Proliferate quickly on secondary exposure
Effector memory T cells
- Peripheral tissues
2. Secrete cytokines but don’t proliferate as much
Regulatory T cells
- Suppression CD4+ FoxP3+ cell
- Natural or induced
- Dec immune response
- Active in contraction phase
- Inhibit auto-reactive T cell
- Mechanism
A. Express CTLA-4
B. Anti-inflam cytokines -> healing- IL-10
- TGF-beta
C. Inc IL-2 receptors -> “hog” IL-2 and prevents it from activating other T cells
- IPEX syndrome
Allogeneic
Genetically dissimilar
Alloimmunization
Shows alloantigen to immune system
Blood type H
Bombay blood
- Super rare
- Can only receive H
- Can give to others
Whole blood
- Collected in anti-coagulant
- Shelf life: 35 days
- At 24 hrs
A. Granulocytes dysfunctional
B. Plasma coag factor lose functionality - Indications
A. Trauma
B. Massive blood loss - Advantages
A. Colloid osmotic pressure and coag factors
B. Doesn’t expose pt to multiple donors - High military use
RBC transfusion
- “Packed RBCs/RBC concentrates”
- Platelet-rich plasma removed
- WBCs remain
- Shelf life: 42 days w/ preservatives
- Type O used in pts w/ unknown blood type and emergent situations
Leukocyte-reduced RBCs (LRRCs)
- WBCs removed
- Indications
A. Prevent febrile nonhemolytic transfusion rxn
B. Mitigate rxn to MHC/HLA antigens
C. Prevent transfusion-transmitted CMV infection
D. Prevent transfusion-related immunomodulation (TRIM) - Very expensive to make
Washed RBCs
- Plasma proteins removed
- Indications
A. Pt has had allergic rxn to transfusion
B. IgA deficient pts- Exposure to IgA -> anaphylaxis
Irradiated RBCs
- Very expensive
- Indications
A. Prevent transfusion-associated GVHD- Rare, but fatal
- Neonates
- Pts w/ blood cancers
- Stem-cell transplant pts
- Immunodeficiency pts
- Can’t replicate
Frozen RBCs
1. Destroys most constituents A. RBCs and few WBCs saved 2. Frozen storage up to 10 yrs 3. Use w/in 24 hrs after thawing 4. Stock pile of rare donor types 5. Autologous donations for later use 6. Expensive
Nucleic acid amplification testing
- MP-NAT: minipool NAT - pooled samples from up to 16 donors
- Multiplex NAT: tests for multiple pathogens
- ID-NAT: individual donor NAT
Pathogen reduction technology for transfusion
- Filters
- UV light
- Culturing tech
Blood cell subsititutes
- Nothing can replace whole blood
- RBC substitutes
A. Perfluorcarbons
B. Hemoglobin-based O2 carriers
C. Stem cell derived
B1 cells
- Super rare
- Dev in fetal liver
- Self-renewing
- Body cavities
- Respond carb antigens on bacteria
- Secrete IgM
- Limited V-region repertoire (innate-like receptor)
- Membrane IgM+IgD-
- Thymus-independent response
Conventional B2 cells
- Develop in bone marrow
- Respond protein antigens
- Secretes IgM first, then switches
- Diverse V-region repertoire
- Membrane IgM+IgD+ (IgMlow, IgDhigh)
- Thymus dependent response
B2 cell development
1. CLP proliferates -> Pro-B/T cells A. Failure to express CD19 -> apoptosis B. Germline DNA C. Ig expression: none 2. Pre-B cells A. Recombined H chain gene B. Ig expression: Pre-BCR and cytoplasmic u C. Failure to express antigen receptor -> apoptosis 3. Immature B cells A. Recombined H chain B. Ig expression: IgM C. Weak antigen recognition -> muture D. Strong antigen recognition -> apoptosis E. Functional 4. Mature B cells A. Ig expression: IgM and IgD
B cell rearrangements
*T cells don’t do this
1. Pro-B cell
A. H-chain rearrangement
2. Pre-B cell
A. L-chain rearrangement
3. Immature B cells
A. Rearrangement stops
Self-reactive B cells
- Random generation B and T cell receptors -> receptors specific for self
- Clonal deletion
A. Both B and T cells go thru (-) selection -> eliminate self-reactive receptors
Anergy
- B cells w/ weaker rxn to self-antigens -> Anergic (unresponsive)
- Dec IgM expression
- Can’t respond to antigen
- Don’t know why maintained in blood
BCR editing
- “do over” option for B cells that react to self
- More light chain rearrangement
- Retest (-) selection
- Need RAG
Clonal ignorance
Self-reactive but do’t respond during development 1. Not “taught” which antigen to target A. Antigen not encountered B. Low concentration C. Binds too weakly 2. Not anergic 3. Exist w/ low app invite for antigen
Mature B cell CD markers
- IgM+, IgD+
- CD19+, CD20+: used to ID B cells
- CD21-: present when activated by signal from spleen
B cell (+) selection
- If don’t enter follicle -> die (~3 days)
- Survival signal in follicle -> CD21 expression in mature cells
- Competition to enter follicles
A. Memory and mature cells favored - B cells leave follicle if no antigen encounter
Marginal zone B cells (MZ B cells)
- Develop in bone marrow
- Resident in spleen - don’t recirculate
- Respond carb antigens on bacteria
- Limited V-region repertoire (innate-like receptor)
- Membrane IgM+IgD-
- Thymus-independent response
Mature B cell outcomes
- Antibody secretion (IgM first)
- Isotype switching
- Affinity maturation
- Memory B cell
B cell antigen recognition in periphery
- Recognition -> lymphocyte activation and differentiation
A. Includes plasma and memory cells - Peripheral tolerance: mechanism to eliminate self-reactive lymphocytes in periphery