Exam 1 Super Deck Flashcards
What is vaccination?
- Inoculation of healthy individuals with weakened or attenuated strains of disease causing agents to prevent disease
What is adaptive immunity?
- The production of antibodies in response pathogens as a result of adaptation to infections
- Requires response time
What is innate immunity?
- Nonspecific defense against infections
- Rapid response
ex. Macrophage (wbc) engulfs and digests microorganism
What is an antigen?
- A substance that stimulates antibody generation and is recognized by adaptive immunity
ex. proteins, glycoproteins, polysacs of pathogens, metals, organic chemicals, and drugs
What is the origin of most leukocytes? What is the exception?
- Most come from bone marrow where many develop and mature
- Certain tissue resident macrophages and lymphocytes (microglia of CNS) originate from yolk sac or fetal liver during embryo development
What is the lymphatic system?
- How immune cells travel through peripheral tissue
- Used to drain extracellular fluid and immune cells from tissue and transported as lymph
- lymph drains into the blood stream
What two progenitors can be produced from a multipotent hematopoietic stem cell?
- Common lymphoid progenitor
- B cells, T cells, NK cells
- Adaptive immunity, develops in lymphoidal organs
- Common myeloid progenitor
- All other leukocytes, dendrites, and platelets
- Innate immunity, develop in bone marrow
What are the four disease-causing microorganisms?
- Viruses
- Bacteria and archaea
- Fungi
- Parasites
What is a microbiome?
- Colonies of microbial communities that are found on skin, mucosa, gastrointestinal tract and have a symbyotic relationship with the host
- Parasites can often get through mucosa and harm cells
What is the order of the body’s defenses against pathogens?
- Anatomical barriers: Skin, mucosa, intestine, respiratory epithelium
- Compliment/antimicrobial proteins: Chemical and enzymatic response near epithelial tissue
- Innate immune cells: Macrophages, granulocytes, NK cells
- Adaptive immunity: B cells and T cells
Outline the steps involved in the immune system being activated
-
Inflammatory inducers indicate the presence of a pathogen or tissue damage
- Bact. lipopolysac, ATP, Urate crystals
-
Sensor cells detect signal via receptors and defend or propagate immune response
- Macrophages, Neutrophils, dendritic cells
-
Mediators act on target tissue
- Cytokines, cytotoxicity
- Target tissue produces antimicrobial proteins to kill infected cells
Why does adaptive immunity take longer to respond compared to innate immunity?
- Adaptive immunity takes longer because it involves making B cells and T cells with a specific antigen receptor to target the pathogen. Additionally memory cells ensure prolonged protection if the same pathogen invades
What is hematopoiesis?
- The development of blood cells, Both red and white, from hematopoietic stem cells
- In bone marrow
- T cells then mature in thymus
- Mast cells then mature in peripheral tissue
- Macrophages then mature in tissue
What is a macrophage? What are its functions?
- Leukocyte
- Found in tissue
- Circulates (immature) as monocyte
- Involved in Adaptive and innate
Functions:
- Phagocytosis (ingestion)
- Induce inflammation by releasing mediators o recruit immune cells
- Antigen presentation to activate T cells
- Scavenge, clear old/dead cells and debris
What is a neutrophil? What is its function?
- Leukocyte
- Also called PMNs
- Most common WBC
- First responder to infections + injuries
- Major player in innate
Functions:
- Phagocytosis (main), and granules are released to digest bacteria
- Cytokine signaling (can signal to other immune cells)
What is an Eosinophil? What is its function?
- Leukocyte
- Less abundant than neutrophils
- Granules contain enzymes and toxic proteins
Functions:
- Defend against parasitic infections
- When pathogen is too large to be engulfed it is instead broken down by granules
What are Basophils and Mast cells? What is their function?
Both:
- Leukocyte
- Less abundant than neutrophils
Functions:
- Allergic response
- Defend against parasites via granule secretion
Basophils:
- Circulate in blood
Mast cells:
- Found in peripheral tissue
- Skin, intestines, airway mucosa and they’re early sensors of infection or injury
What are Natural Killer Cells? What is their function?
- Leukocytes
- Share many functions with T cells
- Cytotoxic molecules (release lytic granules)
Functions:
- Recognize and destroy tumor and infected cells
- Determined by activating and inhibiting ligands for NK’s innate receptors
- Tumor cells: ligand expression decreases which NK cells recognize as non-self
What are innate lymphoid cells? What is their function?
- Leukocyte
- In peripheral tissue
Functions:
- Secrete cytokines that regulate immune cells
- Mirror T-cell functions
What are dendritic cells? What is their function?
- Leukocyte
- In tissue
- Bridge b/t innate and adaptive immunity
Functions:
- Activate T-cells
- Phagocytosis (innate immune receptors)
- Control response of innate immune cells
What are T-cells? What is their function?
- Leukocyte
- Mature in Thymus
- Naive T-cell (immature) has not been exposed to antigen
Functions:
- Effector cells
- Helper T-cells (CD4+)
- Activate immune cells
- Express CD4 co-receptor
- Cytotoxic T-cells (CD8+)
- Release cytotoxins to target cells to cause apoptosis and membrane proliferation
- Regulatory T-cells
- Control immune reaction + prevent autoimmunity
- Inhibit T-cells
- Memory cells
- Have memory of receptors which can be used upon reinfection
What is a B-cell? What is its function?
- Leukocyte
- Matures in bone marrow
- Humoral immunity
- Circulation via blood, mucus, tears etc.
- Native B-cells (immature) have not been exposed to antigen
Functions:
- Effector cells
- Plasma cells (produce antibodies)
- Memory B-cells
- Function as antigen presenter
What is an antigen? What is Immunogenicity? What is Antigenicity?
- Any molecule that can specifically to an antibody or generate peptide fragments that are recognized by a T-cell receptor
- Proteins, glycoproteins, polysac, lipids, drugs, metals, chemicals
- Epitope is a region by which an antigen can be recognized by antigen receptors or antibodies
- The ability to induce humoral/cell-mediated immune response
- The ability of an antigen to bind/interact w/ B or T cell receptors
What are the 6 types of antibody function?
- Agglutination - aggregation
- Neutralization
- Opsonization - recognition and phagocytosis of microbes
- Cytotoxicity
- Degranulation
- Compliment activation
What are cytokines?
- Proteins made by a cell to illicit response
- Cell signaling
- Often called interleukins (IL-n)
Identify the different leukocytes.
What are the different types of microbes?
- Bacteria - single celled, prokaryotes
- Viruses - genetic material surrounded by protein coat
- Fungi - single to multicellular, surrounded by protein coat
- Parasites - Protozoans (single cell euk) or helminths (worms)
What are three mechanisms pathogens use to damage tissue?
- Exotoxin production
- Proteins produced inside the pathogenic bacteria and secreted into surrounding
- Endotoxin
- Constitutive elements of bacteria membrane that are released when bacteria die
- Direct cytopathic effect
- Pathogen harms its host cell
What are the two types of bacteria?
- Gram positive (GP)
- One membrane and thick layer of peptidoglycan
- Gram negative (GN)
- Two membranes + thin peptidoglycan layer
- Lipopolysaccharides (LPS) outer membrane
What can bacteria do to spread?
- Release toxin from outside of cells
- Invade the tissue
- Mix of both
Where can extracellular infection come from? Where can intracellular infection come from?
- Respiratory system, Renal system, CNS
- Immune cells, Epithelial cells, Mesenchymal cells
What are the two types of viruses?
Naked:
- Protein coat (Capsid)
- Spike (membrane protein involved in entry to host)
Envelope:
- Additional membrane envelope
Both:
- RNA and DNA contents can be single or double stranded
What are the general steps of viral infection?
What are fungi?
- Eukaryotes
- Unicellular (yeast, budding) or multicellular (molds, asexual spores)
- Most fungi are opportunistic
- Dimorphic fungi are true pathogens
- Can invade extra or intracellularly
What are parasites?
- Protozoans
- Grouped by locomotion
- Intra or extra
- Helminths
- Worms66
- Round, flat, segmented
What are the three strategies against microbial threat?
Avoidance:
- Anatomical barriers
- Behaviors to avoid diseased animals (ex. hygiene)
Tolerance:
- Protecting tissues against toxins
- Repairing damages
Resistance:
- Activating innate and adaptive immunity
- Stimulating inflammation
What are the four phases of the immune response?
- Anatomical barriers
- Skin mucosa, epithelium, intestine
- Complement/ antimicrobial proteins
- C3, defensins, RegIIIγ
- Innate immune cells
- Macrophages, granulocytes, NK, epithelial cells
- Adaptive immunity
- B cells, antibodies, T cells
Compare the timing of the innate immune response and adaptive immune response .
Innate:
- First line of defense
- Try to prevent spread
- Fast response
Adaptive:
- Recognizes and eliminates pathogens quickly
- May take days and be too late if innate cannot reduce spread effectively
Outline the steps and purpose of the compliment system.
- Pathogen must overcome physical barriers
- Complements are a set of plasma proteins
- Can attach to pathogen so it can be more easily be identified by macrophage
- Can form pores in pathogen promoting inflammation
How is the immune system activated?
- Activation via inflammatory inducers which indicate presence of pathogens or damaged tissue
- Sensor cells detect inducers using innate recognition receptors
- Mediators then amplify immune response
What type of cells comprise most of the innate immune response?
- Myeloid lineage
What are sensor cells? What are pattern recognition receptors?
- Sensor cells initially screen for self and non self via recognition receptors
- Pattern recognition receptors (PRRs) are innate receptors that recognize pathogen-associated molecular patters (PAMPs)
- PAMPs are not present in all microorganisms, conserved in evolution, identified on cell wall
What are TLR and NOD receptors?
- They’re PRRs
- Toll-like receptors (TLR)
- Nucleotide-binding and oligomerization domain (NOD) - like receptors
Other types of receptors: Mannose, glycan, scavenger
How do sensor cells induce an inflammatory response? What is a chemokine?
- Via production of mediators such as chemokines and cytokines
- A specialized subgroup of secreted proteins that act as a chemoattractant. Neutrophils and monocytes are attracted by their chemokine receptors
- Accumulation of fluid and while blood cells causes redness, swelling, heat, and pain known as inflammation
Outline the steps involved in inflammation.
- Macrophage encounters microorganism = cytokine/chemokine release
- Response to chemokines is increase permeability and dilation of blood vessels (vasodilation) results in inflammation
- Chemokines from macrophages attract neutrophils + WBC to infection
- As epithelial adhesive properties alter, pain is caused
What is a lymphocyte? Why are they important in an immune response?
- A type of WBC usually B,T, or NK cell
- They’re important due to high variability in antigen receptors on surface
- Antigens can always be recognized due to unique antigen receptors
What happens when a Naïve B-cell encounters an antigen?
- BCR on the naïve B cell encounter an antigen, receptor binds
- B cell to proliferate and differentiate into a plasma cell as a response
- Effector form (plasma cell) will secrete antibodies (Ig)
- Antigen then becomes the target of the B cell’s progeny
What happens when a TCR on a T cell binds to an antigen?
- T cell encounters antigen, receptor binds
- T cell proliferates and differentiates into an effector T lymphocyte
- Helper T cells (CD4+)(coreceptor 4) Activates immune B cells
- Cytotoxic T-cells (CD8+)(coreceptor 8) releases cytotoxins to induce apoptosis and membrane proliferation to kill infected cells
- Regulatory T cells control immune reaction to avoid autoimmunity
What is an MHC? What is its purpose?
- Major histocompatibility complex
- Two classes
- Class I - antigen recognized by cytotoxic T cell (expressed in all nucleated cells)
- Class II - antigen recognized by T-helper cells (expressed in antigen presenting cells (dendritic, macro, B cells)
- The purpose of MHC is to bind the epitope peptide and present it to the T cell
How are lymphocytes able to have so many unique receptors?
- Antigen receptor genes are assembled by somatic gene rearrangements of incomplete gene segments
What is clonal expansion?
- When an antigen binds and activates a B cell, proliferation will create many identical progeny which develop into effector (plasma) cells which release identical antibodies
- Gene rearrangement is irreversible so progeny will have antibodies identical to surface receptors
What do helper T cells do to help B-cells?
- When a B-cell is activate, the antigen is bound to MHCII which presents the antigen to a helper T cell
- Helper T cell then activates releasing cytokines which help in the proliferation (replication)of the B-cells and differentiation (creating plasma cells)
- Helper T cells also aid in formation of B-memory cells
Outline the steps in Clonal Selection Theory.
- Each lymphocyte is unique with its own specificity
- Interaction of B-cell with antigen results in activation
- Differentiated effector cell will produce antibodies identical to parent B cell
- Lymphocytes that were self antigens are removed thus absent from mature lymphocyte inventory
What happens to self reactive lymphocytes?
- Normally eliminated or functionally inactivated
- Eventually self reactive antigens produced are eliminated before maturation (clonal deletion)
- Immature lymphocytes that receive too much or little signal removed by apoptosis
- Induction of anergy (inactivation of immature B-cells)
Where do lymphocytes mature? What are primary lymphoid organs? What are secondary lymphoid organs?
- Mature in bone marrow or thymus (T cells)
- Circulate in blood and lymph system
- Where lymphocytes develop from non functional precursors (bone marrow and thymus)
- Where mature naive lymphocytes are maintained and adaptive immune responses are carried out
- Lymph nodes, spleen, mucosal lymphoid tissue of gut, and other mucosa
Outline the steps involved in adaptive immune responses leading up to antigen presentation.
- PRR on dendritic cells bind to PAMPs on pathogen leading to engulfment
- Bacteria is engulfed by macrophage encased in vacuole
- Lysosomes fuse and digest bacteria
- Antigen from digested bacteria are presented by MHCII on the surface of the macrophage
How do dendritic cells activate naive T cells?
- Immature dendritic cell in peripheral tissue migrates via lymphatic vessels to lymph node where naive T cells are found and activated
- Surface proteins (co-stimulatory molecules) are expressed on dendritic cells and stimulate proliferation and differentiation
- Dendritic cell or other APC (antigen presenting cell) will display peptide fragment to naive T cell to activate
- Other APC can be macrophages and B cells but dendritic cells are main presenters
Why are dendritic cells important?
- They bridge innate and adaptive immunity
How can infections be detected by lymphocytes in peripheral tissue or other areas?
- Lymphocytes circulate and hen encountering a pathogen release chemokines to attract more WBC
- Free antigens circulating may also end up in lymph nodes and be picked up by APC (antigen presenting cells)
- Once lymphocyte differentiates into effector cell it will leave lymphoid organ and enter blood to reach infection
What are lymphoid organs?
- Aggregates of lymphocytes that trap antigens and APC from sites of infection to present antigens to lymphocytes which causes an adaptive response
- Spleen, lymph nodes, and mucosal-associated lymphoid tissue
What is the structure and function of a lymph node?
- Follicles made up of B-cells
- Paracortical area made of T cells (also where APC are)
- Medullary cords made of macrophages and plasma cells
- Activate B and T cells move close so T cells can carry out helper function
- B cells proliferate and differentiate creating germinal centers expanding the lymph node
What is the function of the spleen?
- Not connected to lymphatic system and relies on circulation
- Filters and detects antigens and pathogens in blood
- Filters old RBC and stores platelets
- Lymphoid tissue in white pulp, platelets in red pulp
Describe the structure of white pulp.
- Periarteriol lymphoid sheath made of T cells
- Follicles made of B cells (also form germinal center)
- Marginal zone made of Unique B cells, macrophages, dendritic cells
Describe the structure of Peyer’s patches in GALT.
- Follicle-B-cells
- T-cell dependent areas
- Subepithelial dome - T,B, dendritic cells
- Microfold (M) cells are specialized cells that collect and transport pathogens from lumen to the dome
- Tonsils, adenoids, appendix, Peyer’s patches in small intestine
What happens to lymphocytes after an infection is overcome?
- Effector cells die and antibody levels decline
- Long lived plasma cells can remain for years to lifetime
- A significant number of progeny B and T cells will remain as clonal cells which can be reactived
Compare the primary vs secondary response to a pathogen.
Primary:
- Naive B cell
- 4-7 days, peak at 7-10
- Lower antigen affinity
Secondary:
- Memory B cell
- 1-3 days, peak 3-5 days
- Higher antigen affinity
Why are vaccines so important?
- Vaccines mimic primary response for disease in which adaptive immune system may respond too late
- Most effective way to control infectious diseases
What is the significance of effector mechanisms in innate and adaptive immunity?
- For each of the four major lymphocytes there is a corresponding type of T cell with generally similar functional characteristics
What are the three effector mechanisms of immunity?
- Neutralization
- Antibodies bind to bacterial toxins/viruses and are then ingested by macrophages
- Opsonization
- Bacteria is coated with antibodies to facilitate phagocytosis
- Complement
- antibodies attach to bacteria which result in bacteria being lysed and digested
What is the function of MHC I?
- CD8 (cytotoxic) T-cells recognize MHC I
- Infected cells display antigen in the MHC I complex
- CD8 cells recognize the MHC I molecule and kill the infected cell
What is the function of MHC II?
- CD4 (helper) T-cells recognize MHC II
- Activates macrophage causing lysosomes to fuse to vesicle then kill bacteria
What do TH2 cells produce
Interleukin-4,5,13
- Recruit eosinophils activates Mast and Basophils
What does TH 17 do?
Produces Interleukin-17
- Recruit neutrophils
- Works against Bacteria and Fungi
What does TFH do?
Regulates antibody production
What is the order of the innate immune system’s defenses?
Anatomic barriers: Skin, mucosa
Complement/antimicrobial proteins: C3, defensins, RegIIIy
Innate immune cells: Macrophages, granulocytes, NK cells, epithelial cells
Adaptive immune response: B cells/antibodies, T cells
What are the three mechanisms by which bacteria can directly damage tissue?
- Exotoxin Production
- Proteins secreted by bacteria
- Endotoxin
- Proteins that are liberated when bacteria dies
- Direct cytopathic effect
What are the three mechanisms by which bacteria can indirectly damage tissue?
- Immune complexes
- Formation of complex can lead to complement activation and inflammation leading to tissue damage
- Anti-host antibodies
- Cell-mediated immunity
- When an immune response is triggered a Helper T cell (CD8+) may kill infected cells
How does infection by a pathogen usually occur?
- Pathogen usually enter the body via mucosa
- Can sometimes enter via epithelium after cut or damage
Outline the general steps of an immune response
- Pathogen must attach, infect, or cross epithelium
- Local immune response tries to eliminate pathogen or stop spread via inflammation (recruiting WBC and effectors)
- If innate immune system cannot eliminate, adaptive immune response will kick in
Describe the features of anatomical barriers and initial chemical defenses.
- Epithelial surfaces are the first barrier held by tight junctions allowing for (Mech, chem, micro)
Mechanical barriers: - Cilia move mucus trapping pathogens
- Airflow pushes mucus away
- Peristalsis also pushes microbes away
Chemical: - Mucus secretion
- Antimicrobial peptides or enzymes
Microbiological: - Normal flora excrete antimicrobial compounds and cause competition
Why is skin important? Explain the layers of epithelium.
- It is the first line of defense and is multilayered
- Stratum corneum: dead layer that sheds periodically
- Stratum spinosum and granulosum: secrete lamellar bodies to form watertight lipid layer which secrete antimicrobial molecules
How are pores, sweat glands, and hair follicles protected from infection?
Sebum (oil) - has antimicrobial compounds
Sweat glands - release antimicrobial compound (Dermcidin)
Commensal microbes
How is the epidermis in the lungs protected? (Bronchial epithelium)
- Goblet cells and mucous glands create mucous layer
- Cilia transport mucus out of lung
- Alveolar cells produce defensins
- Commensal microbes provide their own microbial protection
What is the difference in a normal airway vs the airway of someone with cystic fibrosis?
- Normal airway has watery mucus while a cystic fibrosis patient will have sticky mucus which traps germs
Describe the features of the gut epithelium.
- Mucus layer (colon has two layers)
- Paneth cells secrete antimicrobial molecules
- Commensals (microbiome makes antibiotic proteins)
- Peristalsis moves pathogens trapped in mucus
- Bile salts and digestive enzymes
- Acid and pepsin in stomach
What are two important enzymes that are produced by epithelial cells and phagocytes?
Lysosomes
- Degrade bacterial cell wall by cleaving peptidoglycan layer
Phospholipase A2
- Hydrolyze phospholipids in there cell membrane
What are the two types of antimicrobial peptides and how are they activated?
Membrane targeting peptides
- Amphipathic
- Disrupt plasma membrane by forming pores
Non-membrane targeting proteins
- Inhibit enzymes or other biomolecules
- Bind and sequester nutrients from pathogens
- Activated by proteolysis to release peptide
What are defensins? What are cathelicidins?
- Effective (broad spectrum) against bacteria, fungi, and enveloped viruses
- α-defensins - produced in neutrophils (primary granule) and Panth cells
- β-defensins - produced in epithelial cells
- Broad spectrum
- Produced by macrophages and neutrophils
- Propeptide is found in secondary granules of neutrophils
What are histatins? What is RegIIIα?
- Effective against yeast
- Promotes wound healing
- Produced in oral cavity
- C-type lectin (carb binding protein)
- Produced in intestines
- Effective against gram-positive bacteria
What are the nomenclature rules of complement proteins?
- Most designated with letter C
- When cleaved, a smaller fragment(a) and larger fragment(b) is formed C2 is exception
- ex C3a, C3b
- C1 is composed of proteins Q, R, S
- Other complement proteins include MBL, ficolins, B, Factor D, MASP
What are the outcomes of the complement system
- Inflammation
- Phagocytosis
- Membrane attack
What is the central and most important step in the complement system?
- All three pathways converge and make C3 convertase
- C3a is mediator of inflammation
- C3b is main effector molecule
- C3b can bind to C3 convertase to make C5 convertase which leads to MAC (membrane attack complex)formation to disrupt cell membrane
What happens to C3b if it does not attach to its target surface? Why?
- If it does not find its target surface it will be deactivated via hydrolysis
-It is deactivated because if it isn’t it may attack the wrong target and it is a highly reactive thioester bond
How is the lectin pathway triggered? What are mannose-binding lectins? What are ficolins?
- Pathway uses soluble receptors to recognize microbial surfaces. Begins with recognition of PAMPs by PRRs (MBL and ficolins)
MBL
- Made in the liver
- 3 monomers = 1 triomer, each MBL has 2-6 triomers
- Associated w/ MASP-1, -2, -3, MAp19, and MAp44
- Binds to mannose, fructose, Nacetylglucosamine (peptidoglycan) residues
Ficolins
- Produced by neutrophils, liver, lungs
- Similar structure to MBL
- Three types
- Ficolin-1, -2: bind to acetylated sugars (N-acetylglucosamine)
- Bind to D-fructose and D-galactose
- No binding to mannose
Outline the steps of the lectin pathway.
- PRR binds to pathogen surface enabling MASP-1 to cleave and activate MASP-2
- MASP-2 cleaves C2 and C4
a. C4b covalently binds to surface of pathogen
b. C2a + C4b = C3 convertase (C4b2a) - C3 convertase cleaves C3
a. C3a initiates inflammation
b. C3b binds to microbial surface as opsonin
c. C3b binds to C3 convertase to make C5 convertase and bind to C5 - MASP-3 is regulatory to control MASP-2 immune response
Outline the steps of the classical pathway.
- C1q binding activates C1r which cleaves C1s
- C1s cleaves C2 and C4
- C3 convertase (C4b2a) is assembled the same way as the lectin pathway
How is the classical pathway triggered? What is the main function?
- Begins when pathogen sensor,C1, binds to pathogen surface, C-reactive protein, or an antibody (main function)
Describe the structure of C1 complex.
- Composed of C1q, C1r, C1s
- Hexamer of trimers
- C1r, C1s are serine proteases (similar to MASP-2)
How does the complement system ensure it does not harm the wrong cell? (2 mechanisms)
- C4b and C3b are both inactivated by hydrolysis if covalent bond is not formed on pathogen surface
- C2 can only be cleaved if bound to C4
What is the purpose of the alternative pathway?
- It is an amplification loop for C3b formation that is accelerated by properdin when pathogens are present
- Can be activated by C3b produced by lectin or classical
- can be activated spontaneously
Outline the steps of the alternative pathway.
- C3b from previous pathway binds to factor B
- Factor D cleaves factor B into Ba and Bb
- Bb binds to C3b forming C3 convertase (C3bBb)
- C3b production is amplified
How does spontaneous activation of the alternative pathway occur?
- Hydrolysis of the thioester bond in C3 must occur to form C3(H2O)
- Factor B binds to C3(H2O) and factor D cleaves factor B = C3 convertase C3(H2O)Bb
What is Factor P? What is its purpose?
- Properdin - made of neutrophils and binds to pathogen surface
- Convertase of alternative pathway only last 90 seconds but properdin allows a x5-10 extension
What do complement regulator proteins do?
- Prevent convertase from forming or promote rapid dissociation
- DAF competes w/ factor B to bind to C3b
- Factor 1 cleaves C3b to iC3b(inactive)
- CR1 (compliment receptor 1) and factor H inhibit convertase formation
What happens when the compliment system is activated and C3b coats the pathogen?
- C5 convertase is generated
What are the two possible C5 convertases?
- C4b2a3b - Lectin and Classical
- C3b2Bb - Alternative
How do phagocytes recognize compliment proteins?
- Phagocytes recognize complement proteins by their compliment receptors
- C3b is main opsinin, CR1 main receptor
- C5a activates macrophage –> phagocytosis
- Minor opsinin is C4b and cleaved forms of C3b
What happens to the small fragments of compliment proteins when they’re cleaved?
- They initiate a local inflammatory response
What is a MAC? What are the steps in its formation?
- Membrane attack complex
- When compliment proteins assemble and insert themselves in a membrane creating a pore
- C5b binds C6,7
- Complex binds to surface
- C8 binds and inserts into surface
- C9 binds and polymerizes creating pore
What are PAMPs? What are DAMPs? How are they recognized?
- Pathogen associated molecular patterns
- Damage associated molecular patterns
- Recognized by PRRs
What are the four main groups of PRRs?
- Free receptors in serum
- Mannose binding proteins and ficolins
- Membrane-bound phagocytic receptors
- Membrane-bound signaling receptors
- Cytoplasmic signaling receptors
What are three types of phagocytic receptors?
C-lectin type like
- Dectin-1 (recognize fungi)
- Mannose receptors (fungi, bacteria, viruses)
Scavenger receptors
- Class A
- receptors have collagenous structure
- recognize Recognize bacterial components
- Class B
- Bind to fatty acids
Complement and FC receptors
What is phagocytosis? Is it the same in neutrophils?
- Pathogens and opsinins bind to receptors
- Pathogen is internalized and fuzed with lysosomes
- Acidic, antimicorbial peptides, Auperoxide and nitic oxide radicals
Neutrophils: Phagosomes fuse w/ primary and secondary granules
What is an important primary granule in a neutrophil?
- Myeliperoxidase
What type of receptor do phagocytes express to stimulate antimicrobial killing?
- G protein coupled receptor
- Activates intracellular GTP-binding proteins
- Direct response to PAMPs, anaphylatoxins (C5a), leukotrienes and chemokines
- fMet-Leu-Phe (fMLF) receptor and C5a receptor
How does GPCR activation result in ROS generation?
- Before ligand binds, G protein is not associated
- Binding of ligand allows association with G protein by replacing GDP w/ GTP
- G protein dissociates and forms alpha and beta-gamma subunits to activate other proteins
- GTP cleavage allows for the formation of G protein once more
How does the production of ROS result in the destruction of bacteria? What happens in neutrophils?
- Rapid production of superoxide anions by NADPH oxidase results in increase O2 consumption known as respiratory burst
- Superoxide anions converted into hydrogen peroxide
- In neutrophils, hydrogen peroxide is converted by myeloperoxidase from the primary granule to hypochlorite
What is the role of Rac in phagocytosis?
- fMLF binds to GPCR which signals RAC to induce assembly of NADPH oxidase
- NADPH oxidase leads to the generation of O2-
What are NETs?
- Neutrophil extracellular traps
- meshwork of nuclear chromatin release in extracellular space and undergo apoptosis trapping bacteria which enhances phagocytosis
What are the three main functions of pathogen recognition by innate immune cells?
- Deliver effector molecules(compliment) and cells( neutrophils, monocytes, etc.)
- Induce local blood clotting to prevent spread
- Promote repairment of injured tissue after infection
What are the four types of changes that can occur in local blood vessels during inflammation?
Vasodialation
- Increase/decrease vascular diameter to slow or speed up blood flow
- redness and heat
Activated endothelial cells (cell-adhesion molecule expression)
- Promotes binding of leukocytes (neutrophils first then monocytes)
- Monocytes can become either inflammatory monocytes or dendritic cells, and release inflammatory cytokines
- Tissue damage and pain
Activated endothelial cells (increase vascular permeability)
-Allow fluid and effector molecules (compliment proteins) to cross
- Cause swelling/edema and pain
Clotting of micro vessels
- Activation of platelets or coagulation cascade to limit spread of infection
What are TLRs? What do they do? What do they look like? Where are they found?
- Toll like receptors (similar to toll in flies)
- Induce expression of host-defense mechanisms
- Horse shoe shaped transmembrane proteins on the surface or inside of endosome/phagosome
- Found in leukocytes, stromal cells, and epithelial cells
What happens when a ligand binds to a TLR?
- Dimer forms
- Either Homo or hetero dimer
How do TLRs become functional?
- TLR-3,-7,-8,-9,-11,-12,-13 all require transmembrane transport protein UNC93B1 to reach endosome
- Nucleic acid sensing TLRs are cleaved in endosome to become functional
- This mechanism is to prevent host’s nucleic acids from activating the receptor
Are TLRs all able to independently recognize bacterial lipopolysaccharides?
- Some are able to bind directly to ligand while others may require accessory proteins
Ex. TLR-4 requires LPS, CD14, and MD-2
What does LPS look like?
- Multiple fatty-acyl chains linked to glycan head
- Binds to MD-2
- Free chain is able to bind to complex of another T:LR during dimerization
Describe in general terms how cytokine genes are produced.
- Dimerization of TLRs results in activation of downstream ligase
- Phosphorylation of a complex leading to the release of NFkB
- NFkB enters nucleus inducing expression of cytokine genes
What does RIG-1 do?
- Detect cytoplasmic viral RNAs and activate MAVS (Adaptor proteins)
- Induction of T1 interferons and pro-inflammatory cytokines
- Activation of RIG-1 results in IRF3 and NFkB
What does activation of cGAS-STING result in?
- Production of type 1 interferon
What is an important function NLRs (intracellular sensors) carry out?
- Activate NFkB, release different pro-inflammatory cytokines which trigger pyroptosis
How do NLR proteins react to infection and/or cellular damage?
- Form inflammasome that induce cell death and secretion of inflammatory cytokines
What are the three types of cytokine signaling that can occur?
Autocrine, paracrine, endocrine
How is cytokine signaling regulated?
Pleitropy - One cyto can have different effects on different cells
Redundancy - Many cyto have same effect
Synergy - Cyto cooperate to enhance signaling
Antagonism - Cyto inhibit actions of others
What happens in the JAK-STAT pathway?
- 4 components of JAK, 7 STATs
What type of cytokines can amacrophage secrete?
- IL1 Beta, TNF,IL-6, CXCL8,IL-12
What do CC and CXC chemokines do?
CC: promote migration of monocytes,lymphocytes, and other cell types
CXC: Promote migration of neutrophils
What do cell adhesion molecules control?
- Control interactions bt leukocytes and endothelial cells during inflammatory response
- Selectins on activated endothelial cells initiate leukocyte-endothelial cell interaction by binding to ligands of leukocyte (cause rolling)
- Intracellular adhesion molecules (ICAMs) on activated endo cells bind to integrins on leukocyte forming tighter adhesion which arrests rolling cell
What drives endothelial activation? What is it? How long does it take? What does TNF do?
- Driven by macrophage produced cytokines (especially TNF)
- When granules are rapidly externalized from endo cells when TNF, C5a, leukotriene B4, or histamine bidn to receptor
- Occurs within minutes
- Induces transcription of E-selectin and later other adhesion molecules
How do neutrophils manage to cross the blood vessel and enter the inflamed tissue?
- Selectin-mediated adhesion is weak which allows for leukocyte rolling
- Chemokines cause conf change which allows for tight binding as a result of activated integrins
- Diapedesis (CD31) is when the neutrophil starts to make its way into the tissue and eventually fully migrates in
What can result if TNF is systematically released?
- Cytokine can trigger local containment but systemic release induces shock
What is acute phase response?
- TNF, IL1B, IL-6 (cytokines)initiate acute phase response
- Change in blood proteins during inflammation, infection, trauma
- Acute phase proteins increase in blood while certain proteins decrease in []
What are additional biological activities of TNF, IL-1B, IL-6?
- Induce fever and mobilize neutrophils
What are the two sources for neutrophils? What happens if there is an increase in neutrophils circulating in blood pool?
- Bone marrow
- Production of neutrophils and main reserve
- Marginating Pool
- Neutrophils attached to cells throughout the body
- Increase in neutrophils that enter circulating pool leads to leukocytosis/neutrophilia
What are type I and type III interferons?
Type I:
- IFN A and B
- Produced in almost all cell types
- Produced mainly by palasmacytoid dendritic
- Autocrine and paracrine signaling
- STAT1 + STAT2 + IRF9 = TF ISGF3 to promote ISRE
Type II:
- Interferon - ƛ
- Receptors restricted to largely epithelial cell (mucosal immunity)
- Similar signal pathway and ISG as Type I
What are type II interferons?
- IFN-γ
- Defend against intracellular pathogens
- tuberculosis
- Produced by CD4 TH1 cells, CD* T cells, NK cells
- Activate macrophages
What are B cells? Where do they mature? What types are there? What is additional function they can serve?
- Lymphocyte that matures in bone marrow
- involved in hummoral immunity
- Naive B cells (have not encountered antigen)
- Effector cells (plasma cells that produce Ig)
- Memory B cells
- Also function as antigen presenter cell
Where do T cells mature? How do they recognize antigens? What are the types of T cells?
- Thymus
- TCR recognize antigens presented on a MHC molecule
- Naive has not encountered antigen
Effector cells - Helper (CD4+): cytokines, recog MHC II
- Cytotoxic (CD8+): cytotoxins, recog MHC I
- Regulatory: control immune reaction, inhibit T cells
Memory T cells
What are four effector modules and what do they do?
- Cytotoxicity, Intracellular immunity, Mucosal barrier, Extracellular immunity
What do ILCs do?
- Function as effector cells in innate immunity to amplify signals delivered by innate recognition
- Similar to T cells
- Cytokines signal for innate lymphoids which make effector molecules to carry out a function
What do cytokines and interferons do?
- Activate NK cells
- IL-12,-18 stimulate NK cells to release IFN- γ
- Activation of NK cells contains virus while adaptive immune response makes antigen specific T cells and antibodies
- Also produce TNF, GM-CSF, chemokines
How do NK cells kill?
- Release cytotoxic granules similar to T cells containing perforin and granzymes which induce apoptosis
- Antibody dependent cell mediated cytotoxicity (ADCC) is when NK FC receptors recognize antibodies which induce the release of granules
- TRAIL (Tumor necrosis factor related apoptosis inducing ligand) pathway
- TRAIL inceracts with TNFR (DR4 DR5) which activate caspase 8 leading to apoptosis
How do NK cells differentiate between healthy and infected cells?
- Activating (activate cytotoxic activity) and inhibitory (suppress cytotoxic) receptors
- NK cells recognize the balance between receptors and act accordingly
What are the four types of recognition NK cells can make?
- Normal healthy cells
- Express activating ligands (inhibitory > activating)
- high MHC I
- Missing self
- Absence of MHC I
- Activating signal not suppressed (inhibitory < activating)
- Stress induced self
- Increased expression of activating (inhibitory «activating)
- Infectious non-self
- Expression of activating ligands encoded by infectious agents (inhibitory < activating)
What is the structure of an antibody?
- Immunoglobins (Ig) are Y shaped proteins
- 2 heavy chains, 2 light chains connected by disulfied bonds
- 2 regions, Variable varries, constant is relatively conserved bt Ig
- Hinge links Fc and Fab
Describe the domains of an immunoglobulin.
Heavy chain: 4 domains
- 3 constant (CH1-3) and 1 variable (VH)
Light chain: 2 domains
- 1 Constant (CL) and one variable (VL)
- Each domain made of two folded beta sheets and covalent bonded by disulfied bond
What can papain or pepsin do to an antibody?
- Can be readily cleaved into functionally distinct fragments
What role does the hinge region play in flexibility?
- Hinge region allows for binding to multiple antigens
- Hinge flexibility enables Fab region to move freely
How are different classes of immunoglobulins distinguished?
- Distinguished by the structure of thei heavy-chain constant region
- 5 major isotypes/classes: IgG,M,A,D,E
- IgG: IgG1,2,3,4 IgA: IgA1,A2
- Differ by number of C domains, location and # of disulfied bonds,# N linked carbs
- Fc receptors may say which antibody it binds to
- Fcγ binds IgG
What do the constant regions of an antibody confer with? What do IgM and G do? What are the effector functions of Fc region
- Constant regions confers function specialization
- IgM: first antibody from B cell IgG: most common in serum (crosses placenta)
- Fc receptor binding, complement activation, regulation of secretion
How are antibodies transported?
- Fc portion binds to receptors that actively transport antibody through cell
- IgA into mucous, tears, breast milk (pIgR)
- IgG mother -> fetus (FcRn)
What is the importance of hypervariability?
- Forms antigen binding site
- Rest of V domain is framework region
- Sequenced variability in three segments
- Heavy: 30-36, 49-65, 95-103
- Light: 28-35, 50-56, 91-98
What is a benefit of hypervariability sequences being closely positioned to one another?
- Allow for complementary determing regions (CDRs) which compliment the antigen
How are antibodies able to bind to antigens?
- Complementary determining regions (CDRs) complement size and shape of antigen
What role do non-covalent interactions play in antibody binding?
- Electrostatic, H, Van der Waals, Hydrophobic, Cation pi
Describe the T cell receptor.
- Similar to Fab of Ig
- Heterodimer (alpha beta) bound by disulfied bonds
- Contain 3 CDRs in Variable domain, fourth in hypervariability domain (away from antigen)
- TCR dimer forms multiprotein complex with 6 polypeptides that do signal transduction (CD3, chain)
Describe three properties of TCRs.
- One antigen binding region
- Never secreted
- Basic residues required for assembly
What relationship do TCR and MHC have with each other?
- TCR recognize antigens presented to them by MHC receptors
Describe MHC I.
- 2 Polypeptides
- Alpha chain(3 domains)
- Beta - microglobulins
- Peptide binding cleft
Describe MHC II
- 2 polypeptides
- alpha domain (2 chains)
- beta domain (2 chains)
- Peptide binding cleft
Discuss polygeny in MHC molecules?
- MHC genes on chromo 6 (HLA)
- Different genes code for different MHC molecules of same class (isotype)
- 3 MHC I genes = 3 different MHC I isotypes (HLA-A,-B,-C)
- 3 MHC II genes =3 different isotypes (DQ,DP,DR)
How many alleles are there for HLA? What is codominance?
- There are two alleles, one maternal and one paternal
- MHC alleles are polymorphic (many diff in the population)
- Both alleles are expressed equally and 3 are from dad 3 are from mom
What is cross combination?
- When alpha and beta chains from different alleles combine leading to MHC II combinations
- DQ from mom can swap with DQ from dad
How do peptides binding to MHC molecules affect it?
- Peptides bond stability and stabilize cell surface
Describe the kind of binding that can occur for MHC class I molecules.
- Bind short peptides (8-10 residues)
- stabilizes complex by binding peptide
- Anchor residues on peptide allow for specificity (No anchor, no binding)
Are anchor residues the same amino acids as those in different MHC I molecules?
- Anchor residues differ in amino acids and position of different MHC I variants
- Similar it peptides within the same variant
Describe the binding constraints by MHC class II molecules.
- 13< aa length, usually 13-17
- Peptide ends not bound
- Peptide binds along the length of the binding groove
- Also anchor residues like MHC I
Do T cell receptors always bind in the same orientation to MHC complex?
- THe usually do bind in a similar orientation with the TCR over the peptide
How do CD4 and CD8 proteins send a signal to MHC to elicit a response?
- CD4 CD8 surface proteins (from T cell) come in direct contact with MHC to respond to antigen
-Increase sensitivity of T cells to antigen x100 - CD8 recognizes MHC I
- CD4 recognizes MHC II
- Coreceptors bind to MHC away from the peptide binding site
Describe the structure of CD4 and how it binds to MHC II.
- Single chain protein
- D(omain)1,2 are tightly packed like a rod, Hinge lings D1,2 to D3,4
- Lateral face of D1 binds to hydrophobic crevice at junction of A2B2 domains of MHC II
What is the effect of interferons on MHC molecule expression?
- Type I increases the expression of MHC I
- Type II increases the expression of MHC II
What do surface markers on leukocytes do?
Help identify:
- Cell lineage and subsets within lineage
- Stage of maturation
- State of cell activation
- Markers can have receptor, ligand, or structural functions
- Cells can gain or lose markers
Why do we have so many antibodies?
- Increase chance of one antibody being able to bind weakly or strongly to antigen
- BCR (B cell recptor/antibody) binding will activate B-cell to undergo clonal expansion
- Somatic hypermutation results in higher affinity
What is class switching?
- First antibody produced in immature B cell is IgM
- IgD is later produced in mature naive B cells
- Other isotypes can be produced by switching IgM constant region
Why is BCR able to recognize so many antigens?
- Because complete immunoglobulin genes are generated by somatic recombination of segments
- 2 segments make up the light chain variable region
- 3 segments make up heavy chain variable region
How are segments selected for V, D, and J segments?
- Segments are randomly selected leading to enormous number of combinations
- Not all segments will be functional or stable
- Different C regions in heavy = different isotypes
What 4 processes leads to diversity of the immunoglobulin repitoire?
- Combination diversity (main)
- Junctional diversity - recomb = adding/subtracting of nucleotides ar joint segments
- Variable heavy and light chain combination diversity
- Somatic hypermutation (after activation of B cell)
Do IgM and IgD come from the same mRNA? How are different isotypes produced? How does IgM turn to IgD?
- Derived from same pre mRNA, and both expressed on surface of mature B cell
- Clevage, polyadenylation and splicing makes alternate mRNA
- Spliceosome protein ZFP318 promotes splicing yeilds IgD and only expressed on mature activated B cell
What allows for Heavy chains to be processed into transmembrane and secreted Ig?
- Alternative processing allows for transmembrane and sectereted Ig derived from the same heavy chain
How are T-cell receptor gene segments arranged? How do alpha and beta chains recombine?
- Arranged in a pattern similar to that of Ig gene segments and are rearranged by same enzymes
- Alpha chain recombines V,J
- Beta chain recombines V,D,J
What two things does Antigen presentation do? What do dendritic cells do?
- Arm effector T cells and trigger their effector function
Dendritic cells activate CD8 and CD4 T cells (bridge bt innate and adaptive)
What do INF-γ and IL-21 do?
- Generate alpha T-cell receptor
How are antigens processed? Where can the antigen be acquired?
- Intracellular degredation of foreign protein into a peptide that can bind to MHC molecules for presentation to T cells
- Cytosol or vesicular system (ER, golgi, lysosome, endosome)
What happens when MHC I has an antigen attached?(cytosolic)
- CD8 cytotoxic T cell will kill it (direct presentation)
What happens when MHC II has an antigen attached? (endosome of phago or macro)
- MHC II presents to a CD4 (NOT helper) T cell to activate cytokine production activating the macrophage
What happens when an extracellular pathogen attaches to a cell surface receptor and enters via endocytosis?
- Antigens are presented by MHC II to CD4 Helper T cells which stimulate B cells to make antibodies
Outline the steps in cytosolic and vesicular processing of MHCs.
Cytosolic:
- Virus enters proteosome and is broken down into peptides where it enters ER and attached to MHC I which can be sent to cell surface to present to CD8
Vesicular:
- antigen is uptaken by endosome which breaks down peptide and MHC II from ER attaches to peptide to present on surface to CD4
How are peptide fragments formed?
- Proteosome complex combines in cyotosol
- Protein is ubiquinated and recognized by the proteosome
- Different proteosomes form different peptide fragments
- Protein is degraded in catalytic core and released into cytosol
What happens to the peptide fragments released into the cytosol? WHat do interferons do?
- Peptides transported by TAP into ER where it is further processed before binding to MHC I
- Expression enhanced by interferons
- Transport 8-16 amino acids in length
What is Calnexin? How can peptides bind if they’re low affinity?
- Chaperone protein plays a central role in the assembly of immunological proteins
- Associates with TCR, MHC II, antibodies
- Peptides with low binding affinity are replaced with high binding affinity (peptide editing)
How are MHC II complexes generated?
- Made in acidified endocytic vesicles from proteins (obtained by phago,autophagy,endo)
- Peptide fragments generated by various proteases that are active at low pH
What does the invariant chain do? What happens when it binds to MHC II? What is left after the degredation
- Directs the MHC II molecule to acidified vesicle
- Invariant chain prevents the binding of peptides and unfolded proteins to MHC II
- Once invariant chain is fully assembled, MHC II is released from Calnexin and transported out of ER
- After degradation, CLIP remains
How are HLA-DM, HLA-DO related to CLIP and MHC II?
- HLA-DM,DO regulate the exchange of CLIP for other peptides
- HLA-DM binding to MHC II releases CLIP
- HLA-DO is negative regulator that binds to HLA-DM until endocytic compartment is acidified
Can autophagy deliver cytosolic antigens to MHC II?
- Autophagy can deliver cytosolic antigens for presentation by MHC II molecules
How is a dendritic cell used? What is cross-presentation?
- Dendritic cells use cross-presentation to present exogenous proteins on MHC I molecule to prime CD8 T cell
- Cross presentation is a process in which extracel. proteins taken up by dendritic cells which can give rise to peptides presented by MHC I molecules
- T cell activation (cross priming)
What is a superantigen? What does it cause?
- Illicits T cell response. Bind independently to MHC II and TCR
- They’re not processed
- Cause a massive production and release of cytokines from CD4 cells
- systematic cytotoxicity and supress adaptive immune
What do signals from antigen receptors stimulate?
- Reorganization of the cytoskeleton
- Activation of transcription factors
- Synthesis of new proteins
- Results in nondividing naive lymphocyte
How does an extracellular signal illicit a response inside the cell?
- Transmembrane receptors convert extracel. signal into intracellular biochemical event
- Intracellular multiprotein receptor complexes transmit the signal through many pathways
What is the order of lymphocyte signal transduction?
- Tyrosine kinase is activated
- Adaptor proteins are modified recruiting lipid kinases to receptor complexes
- Signal amplification by activating GTPase,serine/threonine kinases, protein phosphatases
- Transcription factor activation, cytoskeletal changes, increase in cellular adhesion and metabolism which all Activate T and B cells
How does signal strength relate to TCR signaling?
Signal strength determines magnitude of cellular responses
- Affinity of receptor for ligand
- Abundance of ligand
- [intracellular signaling components]
- Positive and Negative feedback pathways
What do tyrosine kinases do? What happens to ITAMs?
- Lymphocyte receptors have no intrinsic enzymatic activity
- By associating with intracellular tyrosine kinases they can initiate signals
- TKs phosphorylate tyrosine residues called ITAMs (immunoreceptor tyrosine-based activation motifs)
How do coreceptors work with tyrosine kinases?
- TCRs and co-receptors combo with MHC complexes by binding peptides recruits Lck to phosphorylate ITAMs
- TK ZAP-70 binds to phosphorylated ITAMs through SH2 allowing ZAP-70 to be phosphorylated by Lck
- ZAP-70 then phosphorylates other intracellular signaling molecules
What happens when phosphorylation of ITAMs occurs by Src family kinases?
- Members of Src: Fyn, Blk, Lyn
- Syk binds to phosphorylated ITAMs of Ig beta chain
- Syk are close and thus via transphosphorylation initiate further signaling
How is intracellular signaling mediated?
- Large multiprotein signaling complexes
- Assembly of multi-subunit signaling complexes involves the binding of signal molecules via protein interaction domains
What do adaptor and scaffold proteins do? What are some example proteins?
- Lack enzyme activity but recruit signal proteins into complex before being phosphorylated
- Ex. LAT, SLP-76, Gads, ADAP
What four signaling molecules can be activated by phosphorylation of LAT and SLP-76?
- Akt, PLC-gamma, VAV, ADAP
Can PIP2 recruit signaling molecules? What does Ca2+ do?
- PIP2 + PI3-kinase = PIP3 which can recruit signaling molecules leading to a modification of membrane lipids
- Ca is secondary messenger, amplifies signal, activates proteins
What does GTPase do?
- Bound to plasma membrane and functions to recruit signal proteins and activate them
- Often a signal will travel and activate a molecule to turn on or off a transcription factor
What do kinases and phosphatases do?
- Propagate a signal by activating or inactivating components of a pathway
What does recruitment of ADAP to LAT: Gads:SLP-76 do?
- Activates integrin adhesion and aggregation
What proteins are associated with B-cell?
- Coreceptors: CD19, CD21, CD81 enhance the signal
- SLP-65 adaptor protein
- PI3K-lipid kinase
What does recruitment of Vav to the LAT:Gads:SLP-76 complex result in?
- Induces activation of Cdc42 leading to actin polymerization
How is a signal terminated? Why must it be terminated?
- Protein phosphatases shut down signaling
- Phosphorylated proteins recruit ubiquinated ligases (Cbl) targeting them for degredation
- Membrane receptors are ubiquinated then transported to lysosome for destruction
- Must be terminated to avoid autoimmunity or cancer
How do inhibitory receptors on lymphocytes down-regulate immune response?
- Recruiting protein or lipid phosphatases
How can B cells monitor the level of antigen specific antibodies in the environment?
- Immune complexes are poor at activating naive B cells, because of coengagement with inhibitory Fc receptor
- Recruitment of phosphatase SHIP results in PIP3->PIP2 which can recruit Vav,Btk, and PLC-gamma
What are B lymphocytes derived from?
- Hematopoietic stem cells give rise to B, T, or innate lymphoid cells
- Regulation via network of transcription factors
What are two major cell groups in bone marrow?
Hematopoietic cells
- Immature blood cells
Stromal cells
- Supporting cells help hematopoiesis through soluble and membrane bound cytokines
- Adipocytes, mature blood cells, cells involved in bone remodeling
How is the progress of gene rearrangement monitored?
- Progress is monitored through production of a protein chain
- Serves as signal for cell to progress to next stage
- Only successful arrangements produce protein chain
- Determination of B or T cell dependent of transcription factor network
What does early B cell development depend on? How is development initiated?
- Dependent on bone marrow stromal cells
- Intiated by activation of transcription factor E2A
How does B cell development occur?
- Begins by rearrangement of heavy chain locus
- Initiated in pro-B cell when E2A and EBF induce expressions of proteins (RAG-1,-2)
What rearrangement occurs in Early pro-B cell development? Late pro B?
- First rearrangement is joining D gene to J segment at immunoglobulin heavy chain locus
- V to DJ arrangement
- successful rearrangement leads to production of intact heavy chain required for pre-B receptor
- Produces surrogate light chains
- Pro B that do not produce chains are eliminated
- 45% of pro-B cells are lost
What is a large pre-B cell? Small pre-B cell?
- Express pre-B receptor
- Contains surrogate light chains
- Signaling promotes heavy chain-allelic exclusion
- Prevents heavy chain rearrangement stimulates proliferation if bound IL-17
- Rearrangement of light chain locus
- RAG proteins are produced again
What is an immature B cell?
- Fully assembled BCR (IgM)
- Allelic exclusion and isotypic exclusion
- Cells that don’t produce fully functional BCR are eliminated (2nd checkpoint)
What happens before B cells leave the bone marrow? What happens if is strongly reactive?
- Tested for autoreactivity (central tolerance)
- If strongly reactive, cell development is arrested and cell undergoes receptor editing
- Some weakly reactive B cells will be kept. Some autoimmunity is always present
What happens if after receptor editing, a receptor is still auto reactive?
- Self reaction leads to expression of recomb. protein RAG
- Light chain is replaced until non reactive receptor is produced or no segments available to recombine
- **If new receptor is still autoreactive, it undergoes apoptosis (clonal deletion)
- Defects contribute to rheumitoid arthritis and lupis
What is a transitional B cell? What happens to most transitional B cells
- Immature B cells that exit the bone marrow to spleen to continue final steps of maturation. Undergo peripheral tolerance
- Most transitional B cells die due to competition for access to follicles in spleen. Follicles provide cytokines necessary for B-cell survival and maturation
- Follicles favor entry of mature B cells
What are the two stages before reaching a naive B cell? WHat happens after the transitional stage?
- T1 is when low levels of IgD and high IgM
- T2 is when ther is increased levels of IgD
- Maturity defined by presence of B cell coreceptor CD21
- B cells enter long lived peripheral pool as follicular B cell (B-2) or marginal zone B cell
- Divergence at T2 stage, BCR specificity determines type of B cell
What is a B-1 B cell? What does it recognize? How is it generated? What is an FO and MZ B cell?
- Major producer of natural antibodies
- Does not require T cells
- Antibodies recognize capsular polysaccharide antigens
- Generated by progenitor cells in fetal liver
How and where do precursors commit to the T cell lineage? How are T cells developed and distributed across the body?
- Precusors commit to T cell lineage after Notch signaling in thymus, initiate TCR gene rearrangement
- T cell progenitors develop in bone marrow then sent to thymus to complete development. Then mature T cells migrate to peripheral lymphoid organs and activated T cells migrate to sites of infection
Describe how T cells develop in the thymus.
- Thymocytes originate from bone marrow
- Cortex consists of mostly thymocytes and a few macrophages
- Medulla consists of more mature thymocytes, a few B cells, macrophages, dendritic cells
- Thymic epithelial cells (TEC) plays a major role in T cell development
Why do 98% of thymocytes die from apoptosis?
- Intensive screening for the ability to recognize self peptide: Self MHC complexes and for self tolerance
Describe the development of T cells in terms of expression of TCR and cell-surface proteins.
- Thymocyte will initially be negative for CD3,4,8
- Thymocyte can then become either CD3+ or CD3,4,8 large (double positive)+
- CD3+ sent to periphery
- CD3,4,8 large positive becomes small resting (double positive)
- Will then become either CD4+ or CD8+
- Exported to periphery
What do early thymic progenitors(ETP) do?
- Also known as DN1 arrive to thymus from bone marrow
- Express SCF receptor Kit
- Receive signal from TECs that is transduced by notch receptor
- Initiate T cell specific gene expression and commitment to T cell lineage
- TCF1 + GATA3 express genes encoding component of CD3 and RAG proteins
- IL-7 receptor expression
- Notch signaling continuity is important
What happens to Pro-T (DN2)?
- Express CD25
- Reduction in Kit expression
- Cells become more committed to T lineage
- BclllB TF
- TCR beta chain gene rearrangement begins
- D and J rearrangement
What happens to small pre-T (DN3)?
- Fully committed
- Lost Kit expression
- TCR beta chain rearrangement complete
- Expression of beta chain
-
Express invariant chain pTalpha and CD3 to form pre-TCR
- Signaling doesnt require ligand
- Allelic exclusion
- Proliferation
What happens to large pre-T (DN4)?
- Loss of CD25
- Expression of coreceptors CD4 + CD8 begin
- Rapid cell proliferation into double positive cells
What is a double positive or single positive T lymphocyte?
Double positive
- Majority of thymocytes
- Express CD4 and CD8
- Alpha chain locus rearrangement
- Expression of TCR
- Cells begin to undergo positive selection
Single positive
- Express one of the coreceptors
- Undergo negative selection
What order does T cell development follow?
ETP-> DN2-> DN3-> DN4-> DP-> SP
Where in the thymus do the different stages of T cell development occur?
- DN1 enters thymus from blood stream via venules near cortico-medullary junction
- Ligands that interact with Notch receptor and on immigrant cells
- Medulla contains mature single positive T cells which eventually leave the thymus
What is positive selection? What is negative selection?
Positive:
- T cells must be able to recognize self-MHC molecules
- Failure results in apoptosis
- Determination of CD4 or CD8 lineage and transcriptual programming
- Mediated by cortical epithelial cells
Negative:
- T cells that react strongly to Self-MHC or self-MHC + antigen complex = elimination
- Mediated by antigen presenting cells and medullary epithelial cells
Describe positive selection.
- TCR recognizing peptides: MHC II complexes receive strong and persistent signaling leading to upregulation of TF ThPOK
- ThPOK induces maturation of CD4 and promotes expression of Helper t cell genes
- TCR recognizing peptide: MHC I complex receives weaker signaling and w/ cytokines induces expression of TF Runx3
- **Runx3 induces the maturation of CD8 and promotes expression of cytotoxic genes
What happens if a T cell alp[ha chain is not positively selected?
- T cell alpha genes undergo successive rearrangements until positive selection or cell death
Describe what happens in negative selection.
- Occurs in the thymus and continues to periphery
- Purges T cells with self reactive receptors
- ** Medullary epithelial cells express tissue-specific proteins found outside of thymus present in T cells
- Controlled by AIRE
- APC presents self antigens obtained by other cells or their environment
- The signals a T cell receives during inflammation prevents T cell from undergoing apoptosis when it binds to a foreign antigen
What do regulatory T cells do?
- Function to maintain self tolerance
- Develop from CD4 thymo. whose TCR have moderately high affinity for self peptides:slef MHC molecules
- Upregulate TF FoxP3 and CD25
- Dev. dependent on IL-2
