Week 1 Lecture 1 - Immune System Cells

Question 1

Recognition by Innate Immunity

Answer 1

Pathogen-associated molecular patterns and damage-associated molecular patterns
Encoded germline and not produced by somatic recombination => less diverse
Non-clonally distributed => identical receptors on all cells
T and B cell clones recognise a unique receptor for a specific antigen

Question 2

Neutrophils

Answer 2

Mediate first phases of inflammatory reactions
Phagocytose microbes -> phagolysosome
- phagolysosome kills bacteria by:
- enzymes that break bacterial wall e.g. lysosome, proteases, phospholipases
- cationic peptides: defensins and cathelicidins
- reactive oxygen species: superoxide and hydrogen peroxide
Also kill bacteria extracellularly by releasing neutrophil extracellular traps (NETs): trapping microbes and stopping spread

Question 3

High Local Concentration Molecules in NETs

Answer 3

Neutrophil Elastase (NE)
Cathepsin G
Lysozyme
MPO
Proteases
Ion chelators
Histones

These kill gram positive and negative bacteria as well as parasites and fungi

Question 4

Main Components of Pus

Answer 4

Neutrophils surrounded by NETs

Question 5

Mechanism of NET Formation

Answer 5

NETosis: neutrophil cell death leading to NET formation
Triggered by infections with bacteria, fungi, parasites and viruses
1. Activated neutrophils flatten and adhere to substrate
2. Production of ROS, migration of NE and MPO from granules to the nucleus
3. PHOX (phagocytic oxidase) at the cell and phagolysosome membrane -> superoxide -> hydrogen peroxidase (substrate for MPO)
4. Nuclear envelop disaggregates, nucleoplasm and cytoplasm form a homogenous mass
5. Cell membrane ruptures and cell interior is ejected

Question 6

Late Suicidal NETosis: Steps

Answer 6

Activation of NADPH-oxidase, ROS produced and PAD4 is activated -> chromatin decondensation
NE and MPO translocated to the nucleus -> more chromatin unfolding, disruption of nuclear membrane -> chromatin in cytosol decorated with protein
NETs released by disruption of cell membrane and cell dies

Question 7

Early Vital NETosis

Answer 7

Triggered by TLR4 signalling, or complement receptors acting with TLR2 -> PAD4 activation induces chromatin decondensation
NE translocated to the nucleus, nuclear membrane disruption
Protein decorated chromatin expelled via vesicles and neutrophil retains function

Question 8

Where are Macrophages Dervied from in Adult Tissue

Answer 8

Embryo yolk sac
Foetal liver
Bone marrow

Question 9

M1 Macrophages

Answer 9

Classical M1 Activation: stimulated by microbial product and inflammatory cytokines (IFN-y and/or LPS and TNFa or GM-CSF)
M1 Phenotype:
- high antigen presentation
- express proinflammatory cytokines (IL-1, IL-6, IL-12, IL-23 and TNFa)
- reactive
- nitrogen and oxygen intermediates
- strong microbicidal and tumoricidal activity

Question 10

M2 Macrophages

Answer 10

Alternative M2 Activation: stimulated by IL-4, IL-10 or IL-12
M2 Phenotype:
- high IL-10 and low levels of IL-12 expression
- increased secretion of anti-inflammatory cytokines
- enhanced scavenging of cellular debris
- promotion of tissue remodelling and repair and parasite containment

Question 11

Classical Dendritic Cells (cDCs)

Answer 11

Found in blood, lymph nodes, spleen and non lymph tissue including skin, liver, lung and gut
Specialised Ag processing cells - efficiently present endogenous and exogenous Ag in both MHC 1 and 2 contexts
Strategically positioned at body barriers and organ entry points and migrate to T cell zones within lymphoid organs

Question 12

Plasmacytoid Dendritic Cells (pDCs)

Answer 12

Named because of their non-dendritic, plasma cell-like morphology when inactive
Circulate in blood and LN compartments
Secrete large amounts of type 1 interferons in response to viral infections
In steady state:
- have a poor capacity to stimulate CD4+ T cells
- low MHC class 2 and co-stimulatory receptor expression
- limited phagocytosis of antigens
When activated:
- dendritic morphology
- up regulate HLA-DR and co-stimulatory receptors
- differentiate into functional APC
- activate naïve CD4+ T cells

Question 13

Inflammatory Dendritic Cells (infDCs)

Answer 13

DCs derived from monocytes in an inflammatory setting
Recruited to sites of inflammation and produce TNFa and iNOS
Promote early pathogen specific T cell responses, preferentially induce Th1-type responses
Initiate inflammation and support the function of cDCs

Question 14

Outcomes of T Cell Encounters with MHC-peptide on cDCs

Answer 14

T cell proliferation
Th cell polarisation
Memory cell formation
3 signals are required for these outcomes and all come from the cDCs:
1. MHC peptide - T cells interact with cell associated Ag
2. Co-stimulatory signals (induced by microbes) - T cells respond to peptides from microbes and not harmless molecules
3. Instructing Cytokines - direct polarisation

Question 15

Cytokines Essential for the Survival of Naïve B and T Cells

Answer 15

IL-7 for survival and low-level cycling of naïve T cells
- produced by stromal cells in the thymus and periphery
- promotes upregulation of pro-survival/anti-apoptotic molecules BCL-2 and MCL1
- involved in lymphopoiesis and T cell maintenance
B cell-activating factor (BAFF), a TNF family member, for naïve B cell survival
- BAFF can bind via 3 receptors, one of which is the BAFF-receptor
- binding of BAFF to BAFF-R controls B cell maturation and survival
- elevated levels of BAFF are detected in serum of patients with various B cell mediated autoimmune disorders and B cell lymphomas

Question 16

Naïve T cell Differentiation and the Cytokines that Drive Them

Answer 16

Th1 cells
- driven by IL-12
Th2 cells
- driven by IL-4
iTreg cells
- driven by TGFβ
- induced Tregs (iTreg) develop outside the thymus and require TGFβ
Th17
- driven by TGFβ and IL-6 (needs both)
Without pro-inflammatory signals, priming of naïve CD4 t cells in an environment rich in TGFβ -> iTregs and with pro-inflammatory signals -> Th17

Question 17

Th17 Cells

Answer 17

Important in maintaining barrier immunity at mucosal surfaces, such as the gut and lungs as well as in the skin
Key role of Th17 cells is to promote host defence
Secretes IL-17, IL-17F, IL-22, GM-CSF and TNFα
Act as a bridge between adaptive and innate immunity where they promote neutrophil activation, immunity to pathogens and inflammation

Question 18

Treg cells

Answer 18

Critical for maintenance of immune cell homeostasis as they suppress potentially deleterious activities of CD4+, CD8+ T cells, DCs, B cells, macrophages, NK cells etc.
Treg cells are activated via the TCR to become suppressive thus:
- Treg-cell activation is antigen specific
Once activated:
- do not require re-stimulation via the TCR to suppress
- suppression is not MHC restricted
They secrete TGFβ, IL-4 and IL-10 contributes to suppression
They suppress APC activity:
- CTLA-4 on Tregs down regulates or prevents expression of CD80 and CD86 on APCs thus decreasing co-stimulation

Question 19

Innate Lymphoid Cells

Answer 19

ILC lack rearranged Ag-receptors, lack pattern recognition receptors
Activated by stress signals, microbial compounds and cytokines, not Ag
Express effector cytokines normally associated with T helper cells and regulate Th1, Th2 and Th17 cells
Also regulate fat metabolism and body temp

Question 20

ILC2 and CD4+ T Cell Interactions

Answer 20

ILC2s are rapidly activated by alarm signals from epithelia
- release of IL-4 and IL-13 induce Th2 responses and DC migration into lymph nodes and T cell zones
Activated ILC2s secrete amphiregulin (AREG)
- critical for Treg functions and is involved in tissue repair
Interaction of ILC2s and CD4+ T cells via MHC2 Ag presentation, co-stimulatory signals and cytokines -> help amplify ILC2 and Th2 T cell responses

Question 21

ILC3s and CD4+ T Cell Interactions

Answer 21

ILC3s alter recruitment of CD4+ Th cells to mucosal sites
- released LTα increases homing of CD4+ Th cells to gut -> functional T cell subsets
- released IL-22 decreases chemokine production by epithelial cells -> less Th2 cell recruitment
Splenic ILC3 induce Ag-specific CD4+ T cell responses and memory CD4+ T cell survival
- splenic ILC3 express co-stimulatory CD30-ligand and OX40L -> interact with cells in adaptive immune responses
Some ILC3 cells internalise, process and present foreign Ag to CD4+ Tc cells