Basic Principles - Teeling Flashcards

1
Q

What is the hematopoietic stem cell?

A

All immune cells are derived from a common progenitor known as the hematopoietic stem cell (HSC)

The HSC develops into two progenitor types:
- Lymphoid
- Myeloid

Most lymphoid cells contribute to adaptive immunity, while most myeloid cells contribute to innate immunity.

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2
Q

What cells are under the lymphoid lineage?

A

B Cells
T Cells
NK Cells

These are the cells of the adaptive immune system

Responsible for fighting antibody/cell mediated immune responses

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3
Q

What cells are under the myeloid lineage?

A

Neutrophil
Eosinophil
Mast Cell
Basophil

Monocyte
- Dendritic cell
- Macrophage

These are the cells of the innate immune system

Responsible for first interaction with pathogens

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4
Q

Role and structure of the innate immune system?

A

Immediate response
Non Specific
No immunological memory

Humoral:
- Pattern receptors
- Complement
- Enzymes
- Cytokines

Cellular:
- Phagocytes
- Natural Killer Cells

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5
Q

Role of the adaptive immune system?

A

Long term
Specific to antigen
Lag time from exposure to response
Immunological memory after exposure

Humoral:
- Antibodies
- Cytokines

Cellular:
- T Cells
- B Cells

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6
Q

What is the role of the immune system?

A

Defence (of host) against threat of disease by pathogenic infectious organisms (pathogen)

IS complexity due to range of organisms encountered – evolved to deal with many challenges

IS protects against tumours (some)

Vaccines offer exciting new hopes for infections and cancer therapy

Chronic immune responses can cause disease, for example sepsis, autoimmunity, Type-2 Diabetes

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7
Q

What are some innate physical barriers?

A

Anatomic:
- Skin
- Mucous membranes

Physiological:
- Temperature
- Low pH
- Chemical mediators

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8
Q

What is inflammation?

A

It is a complex biological response to harmful stimuli like pathogens or injury

Swelling, redness, heat, pain and immobility

Fluids leak from blood vessels and collect in tissues

Leukocytes will also begin to exit from blood vessels to investigate the local tissue in a process known as extravasation

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9
Q

What are the signs of inflammation?

A

Cardinal signs of inflammation:
- Rubor (redness)
- Tumor (swelling)
- Calor (heat)
- Dolor (pain)
- Functio Laesa (loss of function)

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10
Q

What are PPR (Pattern Recognition Receptors)

A

PRRs are a class of proteins expressed by cells of the innate immune system to recognise specific molecular patterns associated with pathogens

They detect the presence of foreign pathogens or danger signals and initiate innate immune responses

Examples:
- Toll-like receptors (TLRs)
- NOD-like receptors (NLRs)
- RIG-I-like receptors (RLRs)
- C-type lectin receptors (CLRs)

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11
Q

What are PAMPs (Pathogen-Associated Molecular Patterns)

A

PAMPs are conserved molecular structures commonly found on pathogens but not on host cells

They are recognised by PRRs as signals of microbial invasion

Activation of PRRs by PAMPs triggers immune responses such as inflammation, phagocytosis, and the release of antimicrobial molecules

It also activates innate responses such as:
- Chemokines / cytokines to recruit cells
- Phagocytosis of pathogens
- Lysis of pathogens by antimicrobial peptides

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12
Q

What are DAMPs (Damage-Associated Molecular Patterns)

A

DAMPs are endogenous molecules released by damaged or dying host cells in response to tissue injury or stress

They alert the immune system to tissue damage and activate immune responses

DAMPs signal danger and trigger inflammation, repair processes, and the recruitment of immune cells to the site of tissue damage

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13
Q

What are TLR (Toll-like Receptors)

A

TLRs are a class of PRRs present on the surface or within cells of the innate immune system

They recognize specific PAMPs and DAMPs, initiating immune responses

They trigger an intracellular signalling cascade

Activation of TLRs leads to the production of cytokines, chemokines, andother molecules that promote inflammation and coordinate immune responses against pathogens.

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14
Q

What is the TLR signalling cascade in mammals?

A

MyD88 – Myeloid differentiation primary response gene (88) is a universal TIR adaptor (except TLR3)

MyD88 interacts via a TIR domain

MyD88 promotes association of IRAK1 & 4 kinases (interleukin-1 receptor-associated kinase)

IRAK4 phosphorylates IRAK1 creating a docking site for TRAF6 (TNF-receptor-associated factor 6)

TRAF-6-IRAK1 dimer complex dissociates

Complexes with TAK1 (+other proteins) (TGF-beta activated kinase 1) causing kinase activation

TAK1 is pivotal as it activates both NFkappaB and Map kinase pathways

Increased transcription of target genes

TAK1 activates IKK which phosphorylates IkappaB causing activation of NFkappaB

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15
Q

What does TLR activation in macrophages result in?

A

TLR activation in macrophages results in oxidative burst (phagocytosis) and inflammatory cytokine/chemokine release

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16
Q

What does TLR activation in dendritic cells result in?

A

TLR activation in dendritic cells results in maturation, antigen presentation, and cytokine production- bridge to adaptive immunity

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17
Q

What is a cytokine?

A

Small proteins released by cells

Have a specific effect on the interactions and communications between cells

Regulate the immune response

Can promote or inhibit inflammation

Are crucial in fighting infections and in immune responses to disease

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18
Q

What are natural killer cells?

A

Natural Killer cells are a type of lymphocyte (a white blood cell) in the immune system

They play a major role in the host-rejection of both tumours and virally infected cells

NK cells are known for their ability to kill without prior immunisation to the target

They release cytotoxic (cell-killing) granules that lead to the destruction of the target cell

Unlike T-cells, NK cells do not need antigen recognition to attack their targets

They are part of the innate immune system and provide a rapid response to virally infected cells and tumour formation

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19
Q

What are dendritic cells?

A

Dendritic cells are immune cells that form part of the mammalian immune system

They are antigen-presenting cells (APCs) which means they process antigen material and present it on the cell surface to the T cells of the immune system

Dendritic cells act as messengers between the innate and the adaptive immune systems

They have a unique ability to capture and present antigens, leading to the activation of T cells

Dendritic cells can be found in peripheral tissues such as the skin (where they are often referred to as Langerhans cells) and the inner lining of the nose, lungs, stomach, and intestines

They play a crucial role in initiating and modulating the immune response, especially in the context of infection, cancer, and vaccination

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20
Q

What are macrophages?

A

Macrophages are large white blood cells found in almost all tissues

They are part of the immune system and play a key role in the initiation, maintenance, and resolution of inflammation

Macrophages are involved in the detection, phagocytosis (engulfing and then digesting), and destruction of bacteria and other harmful organisms

They can also present antigens to T cells and initiate inflammation by releasing molecules known as cytokines

Macrophages play a role in wound healing and tissue repair by removing dead cells and stimulating the repair of damaged tissues

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21
Q

What do cytokines and interferons do?

A

Released from cell after immune response sensing and signal transduction pathway

They are able to:
- Signal neighbouring cells to ‘put up barriers’
- Signal infected cells to die
- Recruit white blood cells to stimulate long lasting immunity

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22
Q

What is the JAK-STAT pathway?

A

The JAK-STAT pathway is a signaling mechanism used by a group of cell surface receptors for cytokines and growth factors

It involves the activation of Janus Kinases (JAKs) and Signal Transducers and Activators of Transcription (STATs)

This pathway is essential for transmitting information from extracellular signals to the cell nucleus, influencing gene expression

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23
Q

What are Janus Kinases (JAKs)?

A

JAKs have two functional sites:
- A binding site to associate with the cytokine R
- A catalytic site, which when activated, has tyrosine kinase activity

JAKs bind a receptor and tyrosine phosphorylation creates a binding site for the SHa domain of STATs, firstly on the receptor and then on the Stat itself- this leads to dissociation & dimerisation

WATCH THE VIDEO (QUICK AND SIMPLE)

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24
Q

What are Signal transducer and activator of transcription (STATs)?

A

It is a transcription factor (TF) that binds
DNA sequence-specifically and promotes transcription from GAS element in response to cytokine stimulation

STATs may operate with other TFs

WATCH THE VIDEO (QUICK AND SIMPLE)

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25
Q

Effect of IL-1Beta cytokine

A

Local Effects:
- Activates vascular endothelium
- Activates lymphocytes
- Local tissue destruction
- Increases access of effector calls

Systemic Effects:
- Fever
- Production of IL-6

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26
Q

Effect of TNF-alpha cytokine?

A

Local Effects:
- Activates vascular endothelium and increases vascular permeability, which leads to increased entry of IgG, complement, and cells to tissues and increased fluid drainage to lymph nodes

Systemic Effects:
- Fever
- Mobilisation of metabolites
- Shock

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27
Q

Effects of IL-6 cytokines?

A

Local Effects:
- Lymphocyte activation
- Increased antibody production

Systemic Effects:
- Fever
- Induces acute-phase protein production

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28
Q

Effects of CXCL8 chemokines?

A

Local Effects:
- Chemotactic factor recruits neutrophils, bascphils, and T calls to site of infection

No systemic effects

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29
Q

Effects of IL-12 cytokines?

A

Local Effects:
- Activates NK cals
- Induces the differentiation of CD4 T-cells into TH1 cells

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30
Q

What is the complement system?

A

It is a part of the immune system that enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and attack the pathogen’s cell membrane

It consists of a series (around 40) of small proteins found in the blood, generally synthesised by the liver, and within tissues throughout the body

These proteins work together in a cascade fashion, where the activation of one component leads to the activation of the next, resulting in a rapid response to threats

The system is part of the innate immune response but also bridges with the adaptive immune system

It can be activated through three main pathways:
- The classical pathway
- The lectin pathway
- The alternative pathway

Each initiated in different ways but converging on the same terminal pathway that leads to the destruction of pathogens

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31
Q

What is complement activation?

A

It is the process by which complement proteins are activated in the blood, leading to a series of immune responses aimed at defending against pathogens

Activation can occur via three pathways:
- The classical pathway (triggered by antibody-antigen complexes)
- The lectin pathway (initiated by mannose-binding lectin binding to pathogen surfaces)
- The alternative pathway (activates spontaneously on pathogen surfaces)

Upon activation, a series of proteolytic reactions ensue, resulting in the production of key components that enhance inflammation, opsonisation (marking of pathogens for destruction), and the formation of the membrane attack complex (MAC) that can lyse pathogen cell membranes

The activation cascade is tightly regulated by complement control proteins to avoid damage to host cells

The outcome of complement activation includes direct killing of pathogens, facilitation of phagocytosis, and enhancement of the inflammatory response

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32
Q

What is the compliments classical activation pathway?

A
  • Stimulated by an antigen bound to an antibody (antigen/antibody immune complex)
  • Interacts with C1q-C1r-C1s
  • Then recruits further components
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33
Q

What is the compliment systems alternative activation pathway?

A
  • Activated by direct presence of bacteria, fungi, virus
  • C3 (protein in blood) is hydrolysed in presence of stimuli
  • C3 changes conformation
  • C3 recruits proteins B, D and P to form complex which is then cleaved into C3a
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34
Q

Merging of different compliment pathways

A

All pathways merge at the key event, the proteolytic activation of the central C3 to C3b

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35
Q

What are the basic functions of the compliment system?

A
  • Lysis of cells, bacteria and viruses
  • Opsonisation which promotes phagocytosis of antigen
  • Binding to specific compliment receptors triggering cell activation

-Immune complex clearance

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36
Q

Neutrophil overview

A

Also called polymorphonuclear leucocyte (PMN)

Short lived cell

First line of defence – recruited to infection (IL-8, C5a)-chemokine

Detect and phagocytose pathogens

Effector mechanisms to kill pathogens (lysosomal killing mechanisms)

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37
Q

What are C3a and C5a known as, and what are their main functions in the complement system?

A

Known as anaphylatoxins

Potent inflammatory mediators

Increase vascular permeability, leading to edema
Attract and activate phagocytes (neutrophils, macrophages)

C5a is particularly potent in attracting neutrophils

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38
Q

What role does C3b play in the complement system?

A

Functions as an opsonin

Binds to pathogen surfaces, marking them for destruction (opsonization)

Involved in the amplification loop of the alternative pathway

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39
Q

How does C4b contribute to the complement system?

A

Plays a role in the classical and lectin pathways

Binds to surfaces and forms a complex with C2a to create the C3 convertase

C3 convertase cleaves C3 into C3a and C3b

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40
Q

What is the role of C5b in the complement system?

A

Initiates the assembly of the membrane attack complex (MAC)

Associates with C6, C7, C8, and multiple C9 molecules

Forms the MAC, which creates pores in the pathogen cell membrane, leading to lysis and death

41
Q

What is neutrophil extravasation and describe its steps?

A

Neutrophil extravasation is the recruitment of neutrophils from the blood stream into tissues

Chemokines at vessel wall cause chemoattraction of the receptors on the neutrophil towards the infection site

The neutrophil will then interact with the vessel wall and start rolling over the cells

They do this as they are able to interact with receptors that are either expressed on the neutrophil or expressed on the endothelial cells

Endothelial cells express selectins (adhesion molecules)

Neutrophils express a glycan (in leukocytes, it is a sialyl lewis x glycan)

The glycans are able to recognise the selectin and able to bind and cause the slowing down of the neutrophil

As they are slow, they can then move through gap junctions in the cells

42
Q

How does tethering and rolling occur in neutrophil migration?

A

Neutrophils express glycan structures (sialyl lewis x as pictured) which are able to interact with either P or E selectins

Neutrophils also express integrins called LFA-1, which also interacts with the selectins

This leads to transient adhesion and slowing down of the neutrophils

43
Q

How are the neutrophils ‘stopped’ in neutrophil migration

A

Chemokines that are expressed at the site of infection are able to activate the neutrophil

The activation of the endothelial cells increases the expression of adhesion molecules such as ICAM-1

Conformation change of the LFA-1 integrins on the activated neutrophils allows tight binding to ICAM-1 (adhesion molecule) on the endothelial cells

This causes firm adhesion and arrest of the neutrophil

44
Q

How do neutrophils undergo extravasation in neutrophil migration?

A

After stopping, the neutrophil squeezes between endothelial cells into tissue attracted by chemokines at the site of infection

45
Q

What are some types of tissue macrophages?

A

Kupffer cell (liver)

Microglia (brain)

Osteoclast (bone)

Alveolar macrophage (lung)

All play an important role in maintaining homeostasisB

46
Q

Basic macrophage mechanism?

A

Chemotaxis and adherence of microbe to phagocyte

Ingestion of microbe by phagocyte

Formation of a phagosome

Fusion of the phagosome with a lysosome to form a phagolysosome

Digestion of ingested microbe by enzymes

Formation of residual body containing indigestible material

Discharge of waste materials

47
Q

What are the mechanisms of a macrophage that destroy the pathogen?

A

Acidification:
- pH of 3.5-4.0 reduces bacterial function

Toxic oxygen-derived products:
- Creates highly reactive oxygen species that are damaging to for pathogens (also damages host)

Toxic nitrogen oxides:
- Nitric oxide NO

Antimicrobial peptides:
- Defensins and cationic proteins

Enzymes:
- Lysozyme - Dissolves cell walls of some gram positive bacteria
- Acid hydrolases - Further digest bacteria

Competitors:
- Lactoferrin (binds Fe) and Vitamin B12- binding protein

48
Q

B cells overview

A

Humoral Immunity

B cells arise in the bone marrow of adult mammals

B cells recognize native, (extracellular) antigen via BCR

B cells produce antibodies and cytokines

49
Q

T Cells overview

A

Cell-Mediated Immunity

T cells arise in the bone marrow but mature in the thymus.

They do not produce antibody molecules but have surface receptors that are structurally related to Ig (TCR)

T cells recognise peptide fragments of antigens complexed with MHC glycoproteins (Antigen presenting cells)

50
Q

CD4+ T Cell overview?

A

CD4+ T cells, also known as helper T cells, play a critical role in orchestrating the immune response by helping other cells of the immune system respond to pathogens

They are involved in activating and directing other immune cells, such as B cells, other T cells, and macrophages

Receptors:
- Αβ TCR, CD3, CD4

Secretions:
- IFN-γ, IL-2, IL-4, IL-5, IL-13, IL-10 (cytokines)

Functions:
- Immunity against intracellular bacteria and parasites
- Provide help to B cells and CD8 T cells
- Promote humoral immune responses

51
Q

CD8+ T Cell

A

CD8+ T cells, also known as cytotoxic T lymphocytes (CTLs), are primarily responsible for killing infected cells and cancer cells

They recognise and destroy cells infected with pathogens and cells that have become cancerous

Receptors:
- Αβ TCR, CD3, CD8

Secretions:
- IFN-γ, perforin, granzyme

Functions:
- When activated they become cytotoxic
- Kill virally infected cells
- Kill tumour cells

52
Q

How do antibodies and T-cell receptors differ in the way they recognize antigens?

A

Antibodies bind directly to epitopes on the surface of antigens

T-cell receptors (TCRs) do not recognise free-floating antigen

Instead, they recognise epitopes (part of an antigen that is recognised by the immune system) that are often hidden and must be presented on MHC molecules

53
Q

What steps are involved in the processing of antigens for T-cell recognition?

A

The antigen is internalised by an antigen-presenting cell (APC)

It is then broken down into peptide fragments

The epitope peptide binds to self molecules known as Major Histocompatibility Complex (MHC) molecules

54
Q

What is the role of MHC molecules in T-cell recognition?

A

MHC molecules present the epitope peptides on the surface of the antigen-presenting cell

T-cell receptors recognise the complex formed by the MHC molecule and the epitope peptide

55
Q

T cell receptor structure

A

Heterodimer composed of an α and a β chain

Each chain has two domains, one variable and one constant domain

56
Q

How do TCR produce a signal?

A

The TCR associates with CD3 molecules

CD3 allows a signal to be fed into the cell

57
Q

TCR specificity

A

Each T cell has a specific cell surface receptor (TCR)

Specificity is determined prior to exposure to antigen

There are >10^12 different T cell clones

There are only limited number of genes that encode TCRs

58
Q

How do T cells generate diversity to recognise many pathogens?

A

Answer lies in VDJ (variable–diversity–joining) recombination

Many genes that make up the TCR locus

These genes rearrange to make many receptors

The recombination occurs at recombination signal sequences (RSS)

It is catalysed by enzymes called VDJ recombinase

The most important are RAG1 and RAG2

59
Q

Describe the process of VDJ recombination in T cells

A

VDJ recombination is the process of rearranging variable (V), diversity (D), and joining (J) gene segments

This recombination generates diverse T cell receptors (TCRs) that are essential for the immune response

Occurs at the TCR alpha (α) and beta (β) loci during T cell development in the thymus

60
Q

What is the function of recombination signal sequences (RSS) in VDJ recombination?

A

RSS are short DNA sequences that flank V, D, and J gene segments

Serve as recognition sites for the recombination machinery

Consist of a conserved heptamer and nonamer separated by a spacer of either 12 or 23 base pairs, following the 12/23 rule

61
Q

What enzymes are used in VDJ recombination?

A

Enzymes known as VDJ recombinase

Most important are RAG1 and RAG2

These are expressed in lymphocytes

62
Q

How does VDJ recombination generate diversity?

A

Combinational diversity:
- From the different combinations of gene segments

Junctional diversity:
- From the addition of nucleotides when recombination occurs

63
Q

What are the steps of VDJ recombination?

A

Recognition of Signal Sequences:
- VDJ recombination is initiated at specific DNA sequences called recombination signal sequences (RSS)
- The RSS is recognized by the recombination activating gene (RAG) complex

Cleavage by RAG Complex:
- The RAG complex, composed of RAG1 and RAG2 proteins, binds to the RSS
- It cleaves the DNA to form hairpin structures at the ends of the V, D, and J segments

Hairpin Opening:
- The hairpin structures formed are opened by the RAG complex with the help of Artemis, making the ends of DNA single-stranded (P-nucleotides)

Addition of N-Nucleotides:
- Terminal deoxynucleotidyl transferase (TdT) adds random nucleotides (N-nucleotides) at the junctions, increasing diversity.

Joining of Segments:
- The ends are then brought together and repaired by DNA repair enzymes to form a coding joint, completing the recombination process

64
Q

What are major histocompatibility complexes MHC?

A

MHC = Major Histocompatibility Complex

MHC molecules bind to proteins from viruses and bacteria and present them to T cells

They also bind to self peptides from the cell

This is called Antigen Presentation

65
Q

Class 1 vs Class 2 MHC

A

Class I MHC
- Found on most nucleated cells (not RBC)
- Present endogenous antigens (intracellular, internal)
- Display self proteins, virus proteins, intracellular pathogens
- Present antigen to cytotoxic T cells (CD8)

Class II MHC
- Found primarily on Antigen Presenting Cells
- Present exogenous antigens (extracellular, external)
- Phagocytosis, receptor mediate endocytosis
- Present antigen to helper T cells (CD4)

66
Q

How are naive T cells activated against a pathogen?

A

Dendritic cells take up pathogens and traffic to the lymph node

In the lymph node they interact with naive T cells – activating T helper cells and killer T cells

67
Q

How are naive CD4+ T cells activated

A

Three signals are involved in activation of naïve CD4+ T cells by APC’s

1) MHCII + foreign peptide

2) Co-stimulation signal (CD28-B7)

3) cytokines

68
Q

How do CD28 and CTLA-4 regulate T cell activation and downregulation?

A

CD28:
- Initiates T cell activation by binding to B7 molecules on APCs
- Necessary for the second signal in T cell activation, leading to IL-2 production and T cell proliferation

CTLA-4:
- Counteracts CD28’s actions by binding to B7 with higher affinity
- Leads to T cell down regulation and tolerance, preventing over activation

69
Q

How do various cytokines influence T helper cell differentiation? (T helper subsets)

A

Th1 Cells:
- Differentiation driven by IFN-γ and IL-12
- Produce IFN-γ, which activates macrophages and promotes cell-mediated immunity

Th2 Cells:
- IL-4 drives differentiation
- Produce IL-4, IL-5, IL-9, and IL-13, which are important for humoural immunity and defense against extracellular parasites

Th17 Cells:
- Differentiation requires IL-6 and TGF-β; IL-23 stabilises the subset
- Produce IL-17 and are involved in defense against extracellular bacteria and fungi, as well as in inflammatory and autoimmune diseases

Treg Cells (Regulatory T Cells):
- Differentiation is driven by TGF-β
- Produce IL-10 and TGF-β, which suppress immune responses and inflammation

70
Q

How are naive CD8+ T cells activated

A

CD8 T cell activation requires additional help

CD4 effector T cells amplify the activation of naïve CD8 T cells by further activating the APC

B7 expressed by DCs first activates the effector CD4 T cells to express IL-2 and CD40L

CD40L binds CD40 on the DC, delivering an additional signal that increases the expression of B7 and 4-1BBL by DCs, which in turn provides additional co-stimulation to the naïve CD8 T cell

The IL-2 produced by the activated CD4 T cells also acts to promote effector CD8 T cell differentiation

71
Q

What do CD8 T cells kils?

A

They kill virally infected cells

CD8 T cells interact with APCs and detect pathogen peptides on MHC molecules

These T cells then become activated

Differentiate into cytotoxic T cells

72
Q

How do CD8+ t cells kill?

A

Perforin:
- Aids in delivering contents of granules into the cytoplasm

Granzymes:
- Serine proteases, initiate apoptosis once injected into cytoplasm

Granulysin:
- Antimicrobial

73
Q

What are the types of antigen presenting cells?

A

Three types of ‘professional’ APCs that express both MHC-I and MHC-II:
- Dendritic cells
- Activated macrophages
- Activated B cells

Cells other than APCs that express MHC-I are ‘target’ cells:
- Target can be virus infected, malignant and aging cells, or cells from a graft

74
Q

Dendritic cells overview

A
  • Detect danger signals at site of infection
  • Transport antigen to lymph node
  • Present antigen to naïve T cell
  • Uses both MHC-I and MHC-II
  • Major role: activation of naïve T cells
75
Q

Activated macrophages overview

A
  • Detects danger signal at site of infection
  • Major role: present antigen to T cells that
    arrive at the site of infection
  • Re-stimulation of T cells: keep the response going
76
Q

Activated B cells overview

A
  • B cells must be activated by Th cells to act as APCs
  • BCR has very high affinity for antigens
  • Major role: late in initial infection or early in subsequent infection by the same pathogen
77
Q

Where does T cell development take place?

A

T cell development occurs in the thymus

78
Q

Antibody structure

A

General Structure
- Y-shaped molecule
- Two identical heavy (H) chains
- Two identical light (L) chains

Regions
- Variable (V) region: Antigen-binding site
- Located at the tips of the Y-arms
- Determines antigen specificity
- Constant (C) region: Effector function
- Forms the base of the Y
- Dictates the antibody’s class

Chains
- Heavy chains: Define isotype (IgG, IgM, etc.)
- Light chains: Two types (kappa and lambda)

Domains
- Immunoglobulin (Ig) domains: Structural units
- Found in both V and C regions

Disulfide Bridges
- Stabilize the antibody structure
- Connect H chains together and H to L chains

Flexibility
- Hinge region: Allows flexibility
- Facilitates binding to multiple antigens

79
Q

What happens during a B cell immune response?

A

Activation:
- B cells are activated by encountering their specific antigen
- They recognize antigens through membrane-bound immunoglobulins (B cell receptors or BCRs)
- Activation often requires help from T helper cells, which interact through the antigen presented by
- Major Histocompatibility Complex II (MHC II) on B cells.

Differentiation:
- Once activated, B cells can
differentiate into either plasma cells or memory B cells

Plasma cells:
- Secrete large amounts of antibodies specific to the antigen.
- These antibodies help neutralize pathogens and mark them for destruction by other immune cells.
- Plasma cells are primarily involved in the immediate defense during an infection.

Memory B cells:
- Do not secrete antibodies immediately.
- Persist in the body for years or even decades.
- Rapidly respond to subsequent exposures to the same antigen by quickly differentiating into plasma cells or proliferating.
- Provide long-lasting immunity and are the basis for the effectiveness of vaccines.

80
Q

Specificity of B cells?

A

Each B cell has a specific antibody as a cell surface receptor (BCR)

Specificity is determined prior to exposure to antigen

There are >10^9 different antibody molecules, thus 109 different B cell clones

Only limited number of genes that encode antibodies

81
Q

How is antibody diversity achieved?

A

There are three different genetic loci that make up the light chains and heavy chains

The genes that make up antibody molecules come in different segments

These segments are not next to each other in the genome

Segments are joined in different combinations to generate antibody diversity

This occurs through VDJ recombination

Recombination occurs at Recombination Signal Sequences (RSSs)

RSSs are made up of a heptamer – 12/23 - nonamer

82
Q

How does VDJ joining occur?

A

VDJ joining occurs by DNA recombination

Ig genes rearrange by looping out intervening DNA.

First heavy chain D-J , followed by V-DJ, then light chain V-J

RAG1/2: unable to initiate recombination
SCID: unable to join correctly

83
Q

What are the number of gene segments in human immunoglobulin loci

A
84
Q

B cell development and maturation

A

Very early on B cells express BCR

Heavy chain is the first part of the BCR that needs to be generated and tested for specificity

First functional heavy chain prevents rearrangement of the other chromosome in a process called allelic exclusion designed to prevent expression of two antibodies by the same B cell

The same is true for light chain

Cells exit bone marrow as naïve, IgM/IgD positive B cells with as single antigenic specificity

85
Q

B cell generation of specificity and diversity summary

A

BONE MARROW
- Antigen independent:
- Paring of different heavy and light chain
- Recombination of V, D and J segments
- Variability on the joins of the recombined gene segments
- P- and N region nucleotide addition

PERIPHERY
- Antigen dependent:
- Somatic hypermutation
- Class switching and affinity maturation

86
Q

What is somatic hypermutation?

A

Further diversifies antibodies to generate a more specific antigen response

Introduces point mutations in the V region of the light and heavy chain

AID is a cytidine deaminase that introduces nicks in the DNA that are ‘repaired’

Requires a single strand of DNA

Targets DNA that is being transcribed ei. Ig genes

Somatic hypermutation allows for generation of higher affinity BCRs

87
Q

B cell activation overview?

A

After exit from bone marrow, B cell activation,
proliferation and differentiation occurs in the periphery
and requires antigen

In the absence of antigen induced activation, B cells have a
short life span

B cell can be activated by two different pathways
- Thymus dependant (The B cell requires T helper cells)
- Thymus independent (No T helper cells required)

T-cell help is required for B cells that respond to peptide
antigens, but not for polysaccharide antigen

88
Q

B cell activation

A

Antigen cross-links membrane bound Ig, generating signal, which leads to increased expression of class II MHC and co-stimulatory B7

Antigen-antibody complexes are internalised by receptor-mediated endocytosis and degraded to peptides, some of which are bound by class II MHC and presented on the membrane as peptide-MHC complexes

TH cell recognises antigen-class I| MHC on B-cell membrane. This plus co-stimulatory signal activates Th cell

  1. Th cell begins to express CD40L
  2. Interaction of CD40 and CD40L provides second signal
  3. B7-CD28 interactions provide co-stimulation to the Th cell.
  4. B cell begins to express receptors for various cytokines
  5. Binding of cytokines released from Th cell in a directed fashion sends signals that support the progression of the B cell to DNA synthesis and to differentiation
89
Q

Where does B cell proliferation occur?

A

B cell proliferation occurs in the germinal centers of the lymph nodes

90
Q

Lymph node structure

A

Primary Lymphoid Follicle:
- Small, solid, spherical structures without a germinal centre; predominately contains inactive B cells.

Secondary Lymphoid Follicle:
- Contains a germinal centre; primarily active in immune response, where B cells proliferate and differentiate.

Germinal Center:
- Central area in secondary follicles where B cells rapidly divide and differentiate into plasma cells or memory B cells.

High Endothelial Venule (HEV):
- Specialised blood vessels that enable lymphocytes to enter the lymph node from the bloodstream

T-Cell Area (Paracortex):
- Area adjacent to the follicles, rich in T cells and important for initiating immune responses

Medullary Cords:
- Extensions of lymphatic tissue within the medulla that contain B cells, T cells, and plasma cells

Efferent Lymphatic Vessel:
- Carries lymph, now filtered of antigens, out of the lymph node to the venous circulation.

91
Q

B Cell route through lymph node?

A

Naive B cells travel to the lymph node via the bloodstream and leave via the efferent lymph

B cells that encounter antigen at the T-cell-B-cell border become activated

They form primary foci in the medullary cords

Some cells then migrate to the primary follicle, forming a germinal centre

Plasma cells migrate to the medullary cords or leave via the efferent lymphatics

Plasma cells migrate to the bone marrow

92
Q

How long does it take for germinal centres to arise?

A

It takes roughly 7-10 days for germinal centres to arise after exposure to an antigen

93
Q

What is the major outcome of the germinal center?

A

It is to generate higher affinity B cells from B cells of lower affinity

In general, affinity maturation and memory cell formation require germinal centers, however, some class switching and significant plasma cell formation occur outside germinal centers

94
Q

What 3 differentiation events take place in germinal centres?

A

Three important B cell differentiation events take place in germinal centres:
- Affinity maturation
- Class Switching
- Formation of plasma cells and memory B cells

95
Q

What are the types of antibodies in humans?

A

There are five main classes of antibodies (immunoglobulins) in humans, which are distinguished based on the type of heavy chain they contain. These classes are:

IgM: Often the first antibody produced in response to an infection; primarily found in blood and lymph fluid

IgD: Functions mainly as a receptor on B cells that have not been exposed to antigens

IgG: The most common type of antibody found in the circulation; important for virus and bacteria defense, and is the only antibody that can cross the placenta to provide protection to the fetus

IgE: Involved in allergic responses and defense against parasitic infections

IgA: Primarily found in mucous membranes lining the respiratory and gastrointestinal tracts, as well as in saliva and tears

Within the IgG class, there are four subclasses named IgG1, IgG2, IgG3, and IgG4. Each subclass is encoded by a different heavy chain gene (Gamma1, Gamma2, Gamma3, and Gamma4, respectively) and differs in its biological properties and roles in the immune response.

96
Q

Rewatch 20th feb lecture from 24 mins to 32 mins

A
97
Q

What molecules drive the differentiation of each type of antibody (IgM, IgD, IgG, IgE, IgA)?

A

IgM Differentiation
- Molecules: BAFF, APRIL, Activation through BCR
- Key Info: Early class switching, initial immune responses

IgD Differentiation
- Molecules: Co-expression with IgM during B cell development
- Key Info: Less defined role, part of B cell maturation

IgG Differentiation
- Molecules: IL-21, IFN-γ, TGF-β, CD40L-CD40 interaction
- Key Info: Promoted by Th1 cells, important for bacterial and viral defence

IgE Differentiation
- Molecules: IL-4, IL-13
- Key Info: Driven by Th2 cells, crucial in allergic responses and parasitic infections

IgA Differentiation
- Molecules: TGF-β, APRIL, BAFF
- Key Info: Supported by mucosal environment, important for gut and respiratory tract defense

98
Q
A