Lecture 1: Immune effectors and immune response

Question 1

What the first barriers in protection against pathogens?

Answer 1

Barries to infection: skin, mucus, stomach acid, tears, sweet: prevent pathogens from entering the body.

Question 2

What are the Primary Lymphoid tissues?

Answer 2

Thymus, Bone marrow

Question 3

What are the secondary lymphoid tissues?

Answer 3

Lymphaticvessels (network of vessels that run through the body), Lymph nodes, adenoids (Nose, throat and armpits)

Question 4

How does the innate immune system generally work?

Answer 4

Acts quickly on infections

All the same receptors encoded from your genome

• Receptors do not alter
• Responses do not alter with repeat exposure

Ivolves:
- Neutrophils

• Macrophages
• NK cells
• Dendritic cells

Question 5

What is the Adaptive Immune response?

Answer 5

Whereas the innate immune responses are general defense reactions, the adaptive immune responses are:

Highly specific to the particular pathogen that induced them,
Provide long lasting protection
4 to 5 days before immune response
Secondary response much faster
Slow
Receptors are remarkably diverse
Memory! Secondary response
Is faster and larger upon re-exposure
B cells (make antibodies)
T cells (CD8, CD4)
Dendritic cells

Question 6

Why is An innate immune response an essential pre-requisite to a primary adaptive immune response?

Answer 6

Cells of the innate system pass on important signals in the form of co-stimulatory molecules and cytokines
And tailor it towards the type of pathogen encounterd.

Because stimulatory molecules induced on cells of the innate immune system during their response to microbes are essential for the activation of antigen-specific lymphocytes

Cellular = T cells

Humoral = B cells

Innate Immunity ->Adaptive Immunity -> T cells (Cellular response) -> CD8 or CD4
Innate Immunity ->Adaptive Immunity ->B cells (Humoral response) >Antibodies

Question 7

How do Dendritic Cells link innate and adaptive immune responses?

Answer 7

The activation of a cell response can be summarised in this slide that you’ve already seen a few times.

Pathogenic microbe enters across a wound in the epithelium

Local innate immune resp (not shown) helps to contain the infection, initiates inflammatory response

Microbes are phagocytosed by by DCs, delivering antigen to the DC

Antigen-loaded DCs move to the lymphatic system and into draining LNs

Present antigen to the T cells which are activate and undergo clonal expansion

Eventually migrate out into the blood and traffic to the site of infection.

Question 8

Where are dendritic cells found?

Answer 8

Some DCs found in the blood

But most are in the tissues, at the interfaces between our body and the environment

Act as Sentinels, scanning our tissues for infection or damage

Question 9

Name some blood dendritic cells

Answer 9

MDCs (CD11c+): CD1c, CD141, CD16
PDC’s (BDCA-2+, BDCA-4+): CD2high, CD2low

Question 10

Name some Skin Dendritic cells

Answer 10

LCs
Dermal CD14+ DCs
Dermal CD1a+ DCs

Question 11

Explain Phagocytosis and the activation of innate immune cells

Answer 11

Cytokine/chemokine release

Recruitment of immune cells

Initiation of inflammatory response

Antigen uptake by DCs

Question 12

Briefly explain Pattern recognition receptors and their history

Answer 12

1989 Charles Janeway:
“certain characteristics or patterns common on infectious agents, but absent from the host”

Led to the discovery of Pattern Recognition Receptors (PRRs)

- Germline encoded
- Do not alter
- Enable rapid response

Widely expressed on innate immune cells

- Macrophages
- Neutrophils
- Dendritic cells

PRRs recognise molecules or motifs present on microbes but not in the host organism

Question 13

What are Toll-like receptors (TLRs)?

Answer 13

Family of single membrane-spanning receptors

10 identified in humans

Expressed on different cell types

Found in different cellular locations

Question 14

What are Pathogen-associated molecular patterns (PAMPs)?

Answer 14

PAMPs:

Common to entire classes of pathogens

Essential for the survival of the pathogen

Distinguishable from “self”

PAMPs include

Components of bacterial and yeast cell wall

LPS/endotoxins

Viral envelope proteins

DNA/dsRNA

Question 15

Where are TLR cellularly located and why?

Answer 15

Different TLR receptors are needed for specific pathogens

Internal TLR receptors are needed to recognise because Viruses are intercellular pathogens

I.e: TLR6 and TLR2 bind to parasites (GPI anchor) and TLR3 bind to dsRNA receptor viruses

Question 16

How does the Innate system recognise stress?

Answer 16

Damage-associated Molecular Patterns (DAMPs)

Danger signals arising from damage/stress to the host

Self-molecules that are normally hidden from the PRRs

Released from necrotic cells or upregulated on stressed cells

DAMPs include:

Proteoglycans

Heat shock proteins

Nucleic acids

Compartmentalisation in the cell or sequestration in the EM

Question 17

What happens after TLR’s bind to their respective PAMPS?

Answer 17

After recognising their respective PAMPs, TLRs activate signalling pathways that provide specific immune responses tailored to the microbes expressing that PAMP.

Resulting in different repertoires of genes being activated. This means that the innate immune system allows cells to react with an immediate and balanced response to infection depending on the infectious organism

The combination of PAMPs/DAMPs expressed functions as a fingerprint

Triggering a specific set of PRRs

Tailoring both the innate and subsequent adaptive immune response to that specific pathogen/injury

Question 18

Explain the DC activation and migration to lymphoid system

Answer 18

Binding of PRRs leads to: Phagocytosis and direct killing of pathogens

Release of pro-inflammatory cytokines

Activation of DCs

The activation of a cell response can be summarised in this slide that you’ve already seen a few times.

Pathogenic microbe enters across a wound in the epithelium

Local innate immune resp (not shown) helps to contain the infection, initiates inflammatory response

Microbes are phagocytosed by by DCs, delivering antigen to the DC

Antigen-loaded DCs move to the lymphatic system and into draining LNs

Present antigen to the T cells which are activate and undergo clonal expansion

Eventually migrate out into the blood and traffic to the site of infection.

Migration of DCs to the lymphoid system:

Immature DC: Antigen uptak antigen processin -> Maturation signals -> Mature DC: Antigen presentation, Co-stimulation and T cell activation

Migration of DCs to the draining lymph node:

The activation of a cell response can be summarised in this slide that you’ve already seen a few times.

Pathogenic microbe enters across a wound in the epithelium

Local innate immune resp (not shown) helps to contain the infection, initiates inflammatory response

Microbes are phagocytosed by by DCs, delivering antigen to the DC

Antigen-loaded DCs move to the lymphatic system and into draining LNs

Present antigen to the T cells which are activate and undergo clonal expansion

Eventually migrate out into the blood and traffic to the site of infection.

Question 19

Explain T cell priming

Answer 19

T cells enter lymph node cortex from the blood via endothelial venules (HEV)

Adaptive immune responses initiated in the peripheral lymphoid organs: lymph nodes, spleen and Peyer’s patches

Antigen-loaded DCs entering in afferent lymph from the tissues

Meet naïve T cells entering from the blood through high endothelial venules (HEVs)

Naïve cells express CD62L (adhesion molecule) and CCR7 (homing receptor), enabling entry through HEVs.

Adaptive immune responses are initiated in the peripheral lymphoid organs- LNs Spleen and Payers patches in Gut

Body has 2x10^12 lymphocytes

Rare antigen specific T cells must

Question 20

Explain the second stage of T cell priming

Answer 20

T cells not activated by antigen presented by dendritic cells exit the lymph node via the cortical sinuses

Become activated to proliferate and differentiate into effector T cells

T cells activated by antigen presented by dendritic cells start to proliferate and lose the ability to exit the lymph node

Activated T cells differentiate to effector cells and exit the lymph node

DC deliver a number of signals to activate T cells and shape the immune response

Question 21

Explain Signal 1: ‘which T cells respond’- antigen specificity

Answer 21

T cells recognise short peptide fragments presented in MHC molecules

Antigen specificity determined by the T cell receptor (see immune receptors lecture)

Estimated that the naïve T cell repertoire contains ~25x106 unique TCRs

Specificity of the response generated determined by antigenic peptides presented in MHC molecules by APCs.

Question 22

How are dangerous self recognising T cells prevented?

Answer 22

During T cell development, T cells undergo thymic selection

98% of T cells will die in the thymus because they

i) bind too strongly (self reactive), or

ii) bind too weakly (no use)

You’re left with billions of T cells, each with a unique receptor
able to recognise non-self.

Hemopoietic stem cells -> Common lymphoid progenitor cell (in the hemopoietic tissue) -> Thymus -> Thymocyte -> Peripheral lymphoid organs -> T cell -> (antigen introduced) -> T-cell mediated immune response

Question 23

What do the CD8 and CD4 molecules determine?

Answer 23

Both CD8 and CD4 T-cells have T cell receptors

The CD8 and CD4 molecules determine whether a T-cell binds to MHC class I or MHC class II.

CD8 = MHC I (Every Nucleated cell has MHC I
CD4 = NHC II (Only certain cells have MHC II, i.e: Antigen presenting cells)

T helper:

Target cell might be:

B-cell: eg B-cell has BCR that binds bacterial invader. Binding at cell surface, bacteria is ingested, degraded and processed and bacterial proteins presented by class II molecules at cell surface. Th cell that recognise these foreign peptides in MHC cII will stimulate only B-cells with appropriate BCRs to divide, thereby specifically amplifying the response.

Or macrophage: stimulate to increase bactericidal mechanisms.

CTL

Killing of virus-infected cell.

Discrimination of self from foreign peptides. B and T cells that recognise self peptides are dangerous and are eliminated during T cells development in thymus. Critical !

Stress that CD8 T cells recognise class I MHC, CD4 T cells recognise class II MHC.

Question 24

What are the different types of MHC class pathways

Answer 24

MHC Class I pathway:

Endogenous antigen -> Proteasome -> Protein fragments pass through TAP and into the endoplasmic reticulum -> MHC class I -> Passes through the Golgi -> Peptide MHC Class I ( Endogenous)

MHC Class II pathway:

Endogenous antigen or Exogenous genous -> Endocytic route -> MHC Class II enters into the endocytic route from the endoplasmic reticulum -> Peptide- MHC class II (endogenous and exogenous)

CD8+ DCs only cross presentation:

Peptide- MHC Class I (exogenous)

Question 25

What is the important structural differences between MHC class I and II molecules

Answer 25

MHC I is typically 9-11 amino acids long while MHC II i 12 to 20 amino acids lon

Question 26

T cells are highly specific, meaning?

Answer 26

Requires both the ‘correct’ MHC allele and the ‘correct’ peptide

Question 27

Explain T cell co-stimulation

Answer 27

T cell activation is highly regulated by a number of interactions called checkpoints

Co-stimulatory molecules stimulate T cell priming/effector functions
Specific signal and co-stimulator -> APC -> MHC class II -> Antigen -> T- Cell receptor -> T cell -> Activates T cell

Inhibitory molecules attenuate T cell functions
Specific signal and co-stimulator -> APC -> T cell -> Activates T cell

Co-stimulation:
APC ->
Costimulatory molecule binds to Costimulatory receptor -> T cell -> Signal 2 co-stimulation
Peptide-MHC antigen binds to T cell antigen receptor complex -> T cell -> Signal 1 (Antigen recognition)
-> Activated T cells

Absence of Co-stimulation:
APC: Immature/unactivated
No costimulatory molecule
Peptide-MHC antigen -> binds to T cell antigen receptor complex (Signal 1) (Antigen recognition)
Unresponsive (Anergic T cells) Or apoptotic T cells

Inhibitory signal:
APC ->
Coinhibitory molecule binds to Coinhibitory receptor (inhibitory signal)
Peptide-MHC antigen -> binds to T cell antigen receptor complex (Signal 1) (Antigen recognition)
-> Unresponsive (anergic) or apoptotic T cells

Question 28

Why have these checkpoint controls evolved?

Answer 28

Inhibitory checkpoints are critical to dampen down T cell responses

ctla-4 gene k/o mice die from massive autoimmunity at a young age

Mutations in CTLA-4 gene in humans associated with Type I diabetes, multiple sclerosis, celiac disease, systemic lupus erythematosus

Pd-1 gene k/o mice develop SLE-like symptoms after 1 year

PD-L1/PD-L2 : PD-1

CTLA4 : CD80/CD86

Emerged as important checkpoint controls in cancer immunology

Question 29

What process respondst Bacteria pathogens entering the skin?

Answer 29

Nature of the T cell response can be determined by events occurring upstream of T cell activation
…different invaders…. Different responses

Bacteria -> Enters the skin - intercepted by a dendritic cell -> Dendritic cell breaks down the pathogen -> Peptide from the pathogen is placed in the groove of a class II MHC protein -> Dendritic cell enters the peripheral lymphoid organ -> Activated dendritic cell -> presents the pathogen peptide to a naive helper T cells, while co-stimulatory occurs -> The activated dendritic cell produces IL12 which determines the type of T cell the naive helper T cell becomes -> effect Th1 cell -> moves to site of infection

Parasite -> Enters the gut -> intercepted by a dendritic cell -> Dendritic cell breaks down the pathogen -> Peptide from the pathogen is placed in the groove of a class II MHC protein -> Dendritic cell enters the peripheral lymphoid organ -> Activated dendritic cell -> presents the pathogen peptide to a naive helper T cells, while co-stimulatory occurs -> Jagged bond (Notch) -> effector Th2 cell -> presents pathogen to naive or memory B cell -> Effect B cell -> Anti-parasite antibodies - expulsion of parasites

Question 30

How does the T cell know where the infection site is?

Answer 30

When the dendritic cell binds to the Naive T cell and determines the type of T cell needed, the dendritic cell produces a specific molecule which guides the T cell to its location, for example:

Intestinal dendritic cells produce Retinoic acid which converts the T cell into a Gut-homing T cell by producing CLA-, Alpha4beta7+ and CCR9+

Or Skin dendritic cells produce skin homing polarizing signals, causing the new skin homing T cell to produce CLA+, Alpha4Beta7-, CCR4+ and CCR10+

Question 31

Once a T cell has been primed by a DC to become an effector cell, it can?

Answer 31

recognise cells presenting its specific peptide:MHC

Question 32

Once a T cell has been primed by a DC to become an effector cell, it can recognise target cells presenting its specific peptide:MHC. This leads to?

Answer 32

Cytokine production: E.g: Interferon gamma (IFNy)

Effector cytotix or helper T cell -> IFNy -> IFNy attaches to IFNy receptor-> Virus-infected host cell -> binding leads to gene activation - virus replication is blocked -> MHC proteins increase, proteasome subsunits increases, Peptide transporter increase -> Increased susceptibility of host cell to T cell killing

Question 33

Explain Cytotoxicity

Answer 33

Protect against intracellular pathogens such as viruses and some bacteria, and pathogens that multiply in cytoplasm where they are sheltered from Ab attack

Kill cell before pathogen can multiply and escape to infect more cells.

Once activated by an APC, it can kill any target cell presenting same epitope.

Figure 25–47 Two strategies by which effector cytotoxic T cells kill their target cells. In both cases, the T cell has to contact the target cell to kill it, and a single cytotoxic T cell can kill multiple target cells in sequence.

(A) The cytotoxic T cell (TC) releases perforin and proteolytic enzymes onto the surface of an infected target cell by localized exocytosis. The high concentration of Ca2+ in the extracellular fluid causes the perforin to assemble into transmembrane channels in the target cell plasma membrane. The channels are thought to allow the proteolytic enzymes to enter the target cell cytosol. One of the enzymes, granzyme B, cleaves the Bid protein to produce the truncated form tBid, which releases cytochrome c from mitochondria to initiate a caspase cascade leading to apoptosis.

(B) The homotrimeric Fas ligand on the surface of the cytotoxic T cell binds to and activates the Fas protein on the surface of a target cell. The cytosolic tail of Fas contains a death domain, which, when activated, binds to an adaptor protein, which in turn recruits a specific procaspase (procaspase-8). Clustered procaspase-8 molecules become activated and initiate a proteolytic caspase cascade leading to apoptosis (see Figure 18–6).

Question 34

How does CD4+ T cell help CD8+ cells?

Answer 34

Enhances the quality of CD8+ T cell responses

Improves CD8+ T cell memory generation

Induces ‘better quality’ CD8+ T cell responses.

Better memory generation.

CD40 on CD4 binds to CD8 to help improve CD8 quality