Lecture 13 - regulation of immune cell interactions in time and space

Which compartments must immune responses coordinate between?

• Peripheral tissues: sites of infection or antigen encounter

• Secondary lymphoid organs: sites of immune cell activation and communication

What triggers dendritic cell (DC) migration from tissue to lymph nodes?

PRR activation (e.g., TLRs) → upregulation of CCR7, MHC II, CD80/CD86, and cytoskeletal remodeling → migration along CCL21 gradient

Which chemokine–receptor pair guides DC entry into lymphatics?

CCL21 (lymphatic endothelial cells) binds CCR7 (on activated DCs)

Where do DCs enter lymph nodes, and what is their destination?

Enter via subcapsular sinus, migrate to paracortex (T cell zone) to present peptide–MHC II to naïve CD4⁺ T cells

How do naïve CD4⁺ T cells enter lymph nodes?

Via high endothelial venules (HEVs) using CCR7–CCL19/21 signaling

What is CD69's role in lymph node retention of activated T cells?

CD69 inhibits S1P₁ receptor, preventing early T cell egress from LN, allowing time for functional maturation

What is S1P and how does it regulate lymphocyte trafficking?

Sphingosine-1-phosphate: lipid mediator, high in blood/lymph, low in LNs
Binds S1P₁R → promotes T cell egress unless blocked by CD69

How do B and T cells position themselves during activation in LNs?

• Activated B cells upregulate CCR7, migrate to T–B border

• Activated CD4⁺ T cells upregulate CXCR5, migrate toward follicles

What chemokine receptors guide GC B cell movement in light and dark zones?

• CXCR4 → dark zone

• CXCR5 → light zone

What experiment showed CD69 is diluted through T cell division?

Swan et al., 2012: CFSE-labeled T cells tracked via flow cytometry → CD69 expression halves with each division → allows S1P₁R re-expression and LN exit

What is Fingolimod (FTY720) and how does it work?

S1P₁R agonist → causes receptor internalisation/desensitisation → traps T cells in LNs
Used in multiple sclerosis, trials in ulcerative colitis

What tissue-specific "addressin" guides T cell entry into the gut?

MadCAM-1, expressed on gut endothelium; binds α4β7 integrin on gut-homing T cells

What chemokine receptor and ligand direct gut T cells?

• CCR9 on T cells

• CCL25 secreted by gut epithelium → forms gradient for precise homing

How do gut DCs program gut-homing T cells?

Convert vitamin A to retinoic acid (RA) → induces CCR9 and α4β7 expression during T cell priming in mesenteric LNs

Which cell type secretes CCL25 in the gut and what is its role?

Gut epithelial cells → create local CCL25 gradient for CCR9⁺ T cell recruitment

How does Vedolizumab (Entyvio®) treat IBD?

Blocks α4β7–MadCAM-1 interaction → inhibits gut-specific T cell homing → reduces inflammation in ulcerative colitis and Crohn’s

What is Natalizumab and how does it differ from Vedolizumab?

Blocks α4β1 integrin → broader action than Vedolizumab; used in MS and Crohn’s, but with more systemic effects

What does dysregulated immune cell trafficking contribute to?

Pathogenesis of IBD, coeliac disease, and other chronic immune-mediated diseases

What are the key signals that coordinate immune cell migration across compartments?

• Chemokine gradients

• Adhesion molecules (e.g., integrins and addressins)

• Lipid mediators like S1P

What changes occur in dendritic cells upon PRR activation?

• ↓ Phagocytosis

• ↑ MHC II, CD80/CD86, CCR7

• ↑ Cytoskeletal remodeling for motility

Which chemokine stabilizes directional migration of DCs into lymphatics?

CCL21, secreted by lymphatic endothelial cells, binds CCR7 on DCs

What was the key finding from Sixt et al., Science (2013)?

Provided first direct evidence that chemokine gradients (e.g., CCL21) regulate DC migration into lymphatic vessels

How do T cells “scan” for antigen in lymph nodes?

T cells rapidly contact multiple DCs (“speed dating”) until they encounter cognate pMHC, triggering stable contact and TCR signaling

How long is the clonal expansion cycle for activated CD4⁺ T cells in the LN?

~1 division every 12 hours
→ After multiple divisions, T cells gain effector function

What is the role of ACKR4 in the lymph node chemokine environment?

ACKR4 sharpens CCL21 gradients, ensuring accurate DC entry positioning into LN cortex

What evidence showed that CD69 inhibits lymphocyte exit?

Swan et al., PLOS One, 2012:

• CD69 upregulated early after T cell activation

• Blocks S1P₁R, preventing egress

• CD69 is diluted with each cell division, allowing eventual re-expression of S1P₁R and LN exit

How was CD69 expression experimentally linked to cell division?

• CFSE labeling used to track T cell divisions

• Flow cytometry correlated division count with decreasing CD69 expression

What is the role of S1P₁R in lymphocyte trafficking?

S1P₁R detects high S1P in blood/lymph → promotes T cell egress from LNs
Blocked by CD69 during early activation

Which molecule is the main integrin that mediates gut homing?

α4β7 integrin, which binds to MadCAM-1 on intestinal endothelium

What is the source of retinoic acid (RA) that induces gut-homing receptors?

Gut dendritic cells, which convert vitamin A to RA via retinal dehydrogenase

What is the role of RA in gut T cell priming?

RA induces expression of CCR9 and α4β7 on T cells during priming in mesenteric lymph nodes

How was RA-induced gut-homing signature validated experimentally?

Newcastle PhD project:
In vitro RA exposure led to CCR9 upregulation on activated T cells, confirmed by flow cytometry

What completes the multi-step gut-homing process after T cells leave circulation?

CCL25, secreted by gut epithelium, forms a gradient that attracts CCR9⁺ T cells into gut tissue

What is the therapeutic rationale behind blocking immune cell trafficking?

Reduces tissue-specific inflammation (e.g., in IBD) without causing global immune suppression

Which three drugs target immune cell trafficking, and what are their mechanisms?

• Fingolimod: internalizes S1P₁R → lymphocyte retention

• Natalizumab: blocks α4β1 integrin

• Vedolizumab: blocks α4β7–MadCAM-1 interaction in gut