L06: Pattern recognition in innate immune system

Question 1

TCR and BCR

Answer 1

Have variable (antigen binding) and constant regions, millions of different specificities generated by recombination of gene segments. BCR is membrane bound antibody. TCR recognises antigens processed to peptides and presented on MHC molecules

Question 2

T cell activation by DCs

Answer 2

Requires 2 signals:
1. MHC-peptide: TCR
2. Costimulatory signal (e.g. B7 on DC: CD28 on T cell)
Activation of antigen specific T cell determined by costimulatory molecules on DC. Resting DC in tissues have none. During infections, DCs become activated and express costimulatory molecules which can be induced by recognition of PAMPs by the DC through pathogen-specific receptors (TLRs).

Question 3

Self/non-self hypothesis

Answer 3

1. Ag-specific cells which recognise self-molecules are destroyed before they mature, leading to tolerance of self
2. Only lymphocytes which recognise foreign Ag are selected for
3. Clonal selection replicates lymphocytes which recognises the Ag
4. Inconsistencies: activated DC are required to present Ags to T cells, requirement for adjuvant in vaccine

Question 4

Infectious non-self hypothesis

Answer 4

1. Distinction between self and foreign relies on recognition of characteristic microbial molecules (PAMPs) by the innate immune system
2. PAMPs are recognised by PRRs on innate immune cells (e.g. TLR)
3. PAMPs active DCs which upreguate the costimulatory molecules required for T cell activation
4. Other pathways other than PAMPs to activate DCs

Question 5

Danger hypothesis

Answer 5

1. Distinction between self and foregin relies on release of cellular molecules which indicate stress, damage or danger to cell
2. Self-molecules may be able to active cells through the same PRR that recognises PAMPs. Altered self-molecules are called damage associated molecular patterns (DAMPs).

Question 6

Evidence for DAMP - monosodium urate crystals

Answer 6

1. Cytoplasmic extract behaved as an adjuvant, activating DCs and generating specific CTL against the antigen
2. Cytoplasmic extract was fractionated to find an active component - uric acid active as monosodium urate crystals
3. Cells contain higher concentration of uric acid (increases in dying cells) due to breakdown of DNA/RNA
4. Extracellular monosodium urate crystals indicate damage which could be due to mechanical injury or infection
5. Once cell lyses uric acid is released, crystals can form due to high concentrations of sodium outside the cell (crystals = DAMPs)

Question 7

DAMP examples

Answer 7

1. Small molecules: monosodium urate, extracellular ATP
2. Intracellular proteins: HMGB1
3. Mitochondrial molecules: DNA from mitochondria, N-formyl peptides from mitochondria

Question 8

PAMP examples

Answer 8

1. Bacterial cell wall/surface molecules
2. Flagellin
3. Yeast cell wall
4. Nucleic aids: DNA - methylated in humans, unmethylated in bacteria; ssRNA; dsRNA - indication of viral infection

Question 9

Pattern recognition receptors (PRR)

Answer 9

PRRs mediate:

1. Phagocytosis
2. Chemoattraction
3. Production of chemokines and cytokines
4. DC activation which leads to activation of the acquired immune system