Epigenetic regulation of immunity

Question 1

What are the 2 distinct functions of the immune system?

Answer 1

• Adaptive immune cells
• Innate immune cells

Question 2

What are adaptive immune cells? (2)

Answer 2

• Antigen specific
• B and T lymphocytes

Question 3

What are innate immune cells? (2)

Answer 3

• Antigen non-specific
• Granulocytes, monocytes, dendritic cells, macrophages

Question 4

What is immune memory? (2)

Answer 4

• When the secondary response to a foreign antigen is larger than the primary response
• A feature of both the innate and adaptive immune systems

Question 5

What is immune training? (3)

Answer 5

• First response is smaller than the ‘trained’ response due to functional reprogramming
• Feature of the innate immune system
• Involves metabolic and epigenetic reprogramming and enhanced effector functions

Question 6

How do monocytes respond to 2 different pathogens in vitro? (5)

Answer 6

• Fungal beta-glucan is recognised as foreign by monocytes in culture
• Causes activation and differentiation into macrophages
• Produce cytokines to activate the rest of the immune system
• Start to degrade the fungal beta-glucan
• Subsequent exposure to bacterial lipopolysaccharide (different pathogen, S. aureus) amplifies the response

Question 7

What are the features of monocytes? (2)

Answer 7

• Slight proliferative capacity
• Precursor to differentiated macrophages

Question 8

What are the features of macrophages? (3)

Answer 8

• Phagocytic
• Non-proliferative (post-mitotic)
• Digest foreign material and present protein to T cells via MHC to trigger adaptive immune response

Question 9

What is metabolic-epigenetic crosstalk in macrophages? (2)

Answer 9

• Stimulation of macrophages by foreign pathogen causes increased synthesis of enzymes in glycolysis and TCA cycle (boost metabolism to attack pathogen)
• Consequently increased production of epigenetic co-factors/substrates which cause transcription of cytokines needed to mobilise innate and adaptive immune response

Question 10

What are examples of experimentally-induced immune training? (6)

Answer 10

• Exposure to fungal beta-glucan protects against Staphylococcus aureus infection
• Bacterial peptidoglycan-derived muramyl-dipeptide protects against protozoan Toxoplasma
• Oligonucleotides with unmethylated CpGs (TLR9 ligands) protects against E. coli-induced sepsis/meningitis
• Bacterial flagellin protects Streptococcus pneumonia and rotavirus infection
• Immunisation of mice with BCG vaccine (Mycobacterium tuberculosis) protects against fungal Candida albicans and protozoan Schistosoma mansoni infection
• Immunity to Herpes virus protects against bacterial pathogens Listeria monocytogenes and Yersinia pestis

Question 11

Why do oligonucleotides with unmethylated CpGs cause an immune response?

Answer 11

Large tracts of unmethylated CpGs present in the bacterial genome so oligonucleotides are recognised as pathogenic fragments

Question 12

What is the impact of immunisation of SCID mice with the BCG vaccine? (3)

Answer 12

• BCG vaccine contains attenuated Mycobacterium bovis
• Protects against a lethal inoculum of fungal Candida albicans
• Primes induction of TNFalpha (pro-inflammatory cytokine) which boosts the rest of the immune system

Question 13

What are SCID mice? (2)

Answer 13

• Severe combined immunodeficiency (SCID)
• No adaptive immune system (no lymphocytes) but innate immune system is intact

Question 14

What is the impact of BCG immunisation in humans? (2)

Answer 14

• White blood cells are long-term primed for TNFalpha release when exposed to candida albicans
• BCG vaccination boosts the immune system to reduce morbidity of many cancers (e.g. bladder, melanoma, leukaemia, lymphoma)

Question 15

What is the impact of single and double LPS exposure on gene expression in macrophages? (2)

Answer 15

• Small increase in expression of trained immunity genes above baseline after first exposure
• Major boost in expression increase after second exposure

Question 16

What are the epigenetic modifications behind the 3 steps of immune training? (3)

Answer 16

• Stimulated macrophage: active promoters of trained immunity genes have H3K4me3 and H3K27ac (active gene transcription)
• Resting macrophage: transcription drops off, H3K27ac and H3K4me3 at the promoter lost, enhancers have H3K4me1 (poised)
• Re-stimulated macrophage: active promoters have H3K4me3 and H3K27ac again, active enhancers have H3K4me1 which amplifies gene expression above first response

Question 17

How does CBP respond to LPS exposure? (2)

Answer 17

• CBP (HAT) recruitment to LPS-inducible genes is enhanced
• Causes increased transcription e.g. TNFalpha

Question 18

What is the effect of immune training on epigenetic modifications?

Answer 18

H3K27ac, H3K4me3 and H3K4me1 (enhancers) are enhanced after immune training (after second exposure)

Question 19

What is Set7? (2)

Answer 19

• H3K4me1 methyltransferase
• Exposure is induced by first exposure and boosted by second exposure alongside enhanced release of cytokines (e.g. TNFalpha and IL-6)

Question 20

What is the impact of Set7 knockout? (2)

Answer 20

• Lose the trained immunity enhanced cytokine production response
• First and second response are the same size

Question 21

What is cyproheptadine (CPH)? (2)

Answer 21

• Set7-specific small molecule inhibitor
• Prevents trained immunity enhanced cytokine production response

Question 22

What happens to metabolic pathways in response to pathogen exposure? (2)

Answer 22

• Increased glycolysis and oxidative phosphorylation (training response accompanies the trained immunity response)
• Requires Set7 function

Question 23

How is the mitochondria involved in immunity? (3)

Answer 23

• Pathogen exposure causes increased metabolism (glycolysis and oxidative phosphorylation) to mobilise the cell against the pathogen
• Boosts production of epigenetic co-factors and substrates
• Boosts epigenetic processes (e.g. Set7 activity) which further increases metabolic activity (requires Set7)