CTE1 - host-microbe interaction

Question 1

bacterial metabolites

Answer 1

• Short-chain fatty acids –> origin is indigestible carbs
• Trimethylamine N-oxide (TMAO) –> fermentation by the gut microbiota of dietary nutrients such as choline and carnitine (red meat), which are transformed to trimethylamine (TMA) and converted into TMAO in the liver by FMO1 and FMO3
• LPS –> Gram negative cell wall
• Peptidoglycan –> part of bacterial cell wall
• Flagellin –> bacterial flagellum
• Indole –> intercellular signal molecule
• Secondary bile acids –> primary bile acids are produced by the liver as bile salts which are deconjugated by bacteria in the gut to secondary bile acids

Question 2

AMPs

Answer 2

• defensins (activated by tripsin in lumen)
• cathelecidin LL-37
• C-type lectins
• lyzozyme

Question 3

defensins

Answer 3

• Defensins are small, cationic peptides containing disulphide bonds necessary to damage the bacterial cell membrane and eradicate bacteria
  o These small peptides are further classified as α- and β-defensins depending on the position of their disulphide bonds
   The α-defensins comprise four human neutrophil peptides (HNPs 1–4) present in neutrophils and two human α-defensins (HD-5 and HD-6) that are mainly made by Paneth cells
   β-Defensins are expressed by many types of epithelial cells, including enterocytes
• eukaryotic cells have no negative charge on outer side of membrane so the positve charged defensins will not be attracted like they are to bacetrial membranes

Question 4

activation of ILCs

Answer 4

• IL12, IL15, IL18 –> ILC1
• IL25 and IL33 –> ILC2
• IL1b and IL23 –> ILC3

Question 5

effector cytokines of ILCs and functions

Answer 5

ILC1: IFNy –> intracellular
ILC2: IL5 and IL13 –> parasites / allergies
ILC3: IL17 and IL22 –> intestinal barrier function

Question 6

commensals and mucosal integrity

Answer 6

1. SCFA from commensals bind GPR109a
2. signal transduction
3. pro-IL-18
4. cleaved by caspase 1 (inflammasome)
5. IL18 secreted
6. paracrine action –> IL18R
7. mucus and AMPs production

LPS from commensals cause inflammasome formation (and pro-IL18 transcription) which can cleave the pro-IL18

Question 7

microbiota and myeloid cell development

Answer 7

• Germ-free mice have a profoundly altered innate immune system –> myeloid-cell development in the bone marrow is reduced (SFCA-related)
• The influence of the microbiota on myelopoiesis begins before birth
• The microbiome also influences the maturation of myeloid cells after haematopoiesis.
  o The continuous presence of microbiota-derived TLR ligands drives the ageing of leukocytes, regulated gene expression and drives division

Question 8

microbiota and ILCs

Answer 8

• different from myeloid cells
• develop normally in sterile environment but do not function properly
• commensal signal drive maturation and differentiation of ILCs
• DCs can respond to PAMPs by activating ILCs

Question 9

transcriptional reprogramming - microbiome

Answer 9

• in response to PRR ligands and bacterial metabolites
• influence expression of AMPs, barrier function, cytokine/chemokine production

Question 10

epigenetic reprogramming

Answer 10

commensal metabolites (e.g. SFCAs) can regulate methylation and cause gene expression/repression
TLR4 is highly expressed in sterile mice –> commensals might repress PRR expression and cause tolerance in the lumen

Question 11

IgA production - T cell dependent

Answer 11

• Bacteria that can invade the inner mucous layer of the intestine and colonize regions in proximity to epithelial cells induce high-affinity T-cell-dependent IgA response
• T-cell-dependent IgA class-switch recombination takes place mostly in Peyer’s patches,
  1. Dendritic cells that are located near to the surface of the epithelium capture antigens from microbes (SFB) that are transferred by M cells.
  2. Dendritic cells induce the differentiation of CD4-expressing T cells into the T follicular helper (TFH) cell subset.
  3. CD40 ligand (CD40L) and IL-21 from TFH cells induce the expression of activation-induced cytidine deaminase (AID) in B cells and promote IgA class-switch recombination

Question 12

IgA production

Answer 12

• T-cell-independent IgA class-switch recombination occurs predominantly in the lamina propria and isolated lymphoid follicles (ILFs), where B-cell activating factor (BAFF) and its homologue APRIL, which are derived from dendritic cells, promote the induction of AID expression in B cells.
• Transforming growth factor β (from dendritic cells and stromal cells), retinoic acid (from dietary vitamin A) and ILC3 (and their products) play important parts in both T-cell-dependent and T-cell-independent pathways

Question 13

microbiota and Th17 development

Answer 13

• The microbiota are the most prominent influence from the environment on the differentiation of TH17 cells
  o In germ-free mice, TH17 cells are scarce in the lamina propria of the intestines as well as in the skin
  o Segmented filamentous bacteria are especially important for the development of Th17 cells (the physical adhesion to gut epithelium drives differentiation by inducing amyloid A expression)
   The mechanism through which serum amyloid A stimulates TH17 cells has yet to be resolved.

Question 14

Tregs in the intestine - function

Answer 14

• Intestinal Treg cells play an important part in maintaining immune tolerance to dietary antigens and the gut microbiota as well as in suppressing tissue damage inflicted by immune responses against pathogenic bacteria