Lecture 14 (Matuschewski)

Question 1

Phases of humoral immune response

Answer 1

Phases of humoral immune response
- takes 1 week for a single b-cell to produce 5000 antibody-secreting cells
- each plasma cell can secrete 2000 antibody molecules per second

Question 2

Germinal Center Reaction

Answer 2

Germinal Center Reaction
- Activated B cells migrate to the germinal center within secondary lymphoid organs
- Lymphnotes contain Dark and Light zones
- Dark zones contain germinal center where B cells rapidly divide and undergo somatic hypermutation to improve antibody affinity
- In the light zone, only B cells with high-affinity antibodies survive, undergo isotype switching, and prepare for differentiation
- Tfh & dendritic cells are antigen presenting cells
- High-affinity B cells become either Plasma or Memory cells

Question 3

Ig heavy chain isotype switching (CSR)

General

Answer 3

Ig heavy chain isotype switching (CSR)
- Class switching is the process by which a B cell changes the antibody class (isotype) it produces, switching from IgM or IgD to IgG, IgA, or IgE.
- This is a non-reversible process involving recombination between switch (S) regions, which are specific DNA sequences located upstream of each constant region gene.
- During recombination, the DNA between the active S region (e.g., Sμ) and the target S region (e.g., Sγ) is excised, bringing the downstream constant (C) region gene into alignment with the expressed VDJ segment allowing the antibody to retain its antigen specificity while changing its effector function.
- This process involves only the constant (C) region genes, while the original VDJ segment remains unchanged and continues to define antigen specificity.

Question 4

Ig heavy chain isotype switching (CSR)

Mechanism

Answer 4

Ig heavy chain isotype switching (CSR)
- B cells are activated by antigen recognition and signals from cytokines and CD40-CD40L interaction.
- AID (Activation-Induced Deaminase) deaminates cytosine bases in the switch (S) regions, converting them to uracil.
- Uracil bases are recognized as damaged DNA, triggering repair mechanisms like base excision repair, leading to double-strand breaks in the S regions.
- DSBs in two different S regions (e.g., Sμ and Sγ) are joined by non-homologous end joining (NHEJ).
- The intervening DNA, including the original constant region, is excised and removed.
- The V(D)J region is joined to a new constant region (e.g., Cγ for IgG or Cα for IgA).
- The B cell produces antibodies with the same antigen specificity but a new isotype, determining its immune function. (VDJ stays unchanged)

Mechanism

Question 5

Somatic Hypermutation

Answer 5

Somatic Hypermutation
- Introduces point mutations in the VDJ sequence to improve the affinity of the antibody for its antigen
- RNA Hybrid pairs with the DNA at GC-rich regions (weak DNA-DNA interactions)
- AID (activation-induced deaminase) converts cytosine bases to uracil by deamination in the DNA of the variable (V) region (NH2 ⟶ O)
- UNG (Uracyl N-glycosylase) removes the untypical U bases and creates abasic sites
- APE1 (endonuclease) generates nicks (ss-breaks) first in the non-template strand and after the RNA fell off also in the template strand
- A Ligase does non-homologous end-joining

Question 6

Affinity Maturation

Answer 6

Affinity Maturation
- progressive increase in the affinity of the antibodies due to:
-Somatic hypermutation: Creates diversity in the antigen-binding site of BCRs.
-Selection of high-affinity variants: Ensures that only B cells with improved antigen affinity are retained.

Question 7

Pathogen Recognition Receptors

Types

Answer 7

Pathogen Recognition Receptors
- Innate immunity is triggered by PRRs, which recognize highly conserved microbe-associated molecular patterns
- Pathogen-Associated Molecular Patterns (PAMPs) are Molecules unique to pathogens, like gramnegative bacterial LPS (Lipopolysaccharide), Flagellin, Mannan or viral ss or ds RNA
- Damage-Associated Molecular Patterns (DAMPs) are Endogenous signals from damaged or dying cells, like HSPs (stress induced protein), histones (found outside nucleus) or ATP (found outside mitochondria)
- Classes: Endosomal receptors, Cytosolic receptors, Membrane bond

Question 8

NOD-like receptors

Answer 8

NOD-like receptors (NLR)
- located in the cytoplasm (cytosolic receptor)
- They detect pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs).
- Recognize a distinct set of bacterial molecules, mostly peptidoglycans
- Examples include NOD1, NOD2, and other inflammasome-related NLRs.
- Dysregulation of NLR activity can contribute to chronic inflammation and autoimmune diseases.

Question 9

Toll-like receptor

General

Answer 9

Toll-like receptor
- humans have 9 TLR (other species differ)
- evolutionarily conserved
- dimer formation possible
- found in the Plasmamembrane and endosomal membranes of cells (e.g. b-cells) (membrane bond & endosomal receptor)
- Ligands are microbial molecules (e.g. bacterial LPS, peptidoglycans, viral nucleic acids)

Question 10

Toll Signaling pathway

Answer 10

Toll Signaling pathway
- Once a TLR is activated by Cytokine Spätzle (in insects) or an Interferon (a MAMP) (in humans) it undergoes conformational changes and interacts with adapters (Tube, Pelle), that relay the signal from Toll to downstream components
- A kinase complex IKK phosphorylates Cactus/ IkB, targeting it for degradation
- Tf NF-κB is released from its inhibitor, enabling it to enter the nucleus and activate genes for inflammation and innate immunity