Endothelial and Leukocyte Adhesion Molecules in Inflammation

Introduction

1. Margination and rolling
2. Activation (integrin activation by chemokines)
3. Firm (stable) adhesion
4. Transmigration (diapedesis)

Families of Adhesion Molecules

1. SELECTINS

Structure

Members

P-Selectin (CD62P)

• Location: Endothelial cells and platelets
• Storage: Stored in cytoplasmic granules of endothelial cells called Weibel-Palade bodies (not expressed on resting endothelium)
• Induction: Rapidly mobilized to cell surface within minutes of exposure to histamine (from mast cells) or thrombin (from coagulation cascade)
• Ligands on leukocytes: Sialyl-Lewis X, particularly on P-selectin glycoprotein ligand 1 (PSGL-1) - present on neutrophils, monocytes, and some T cells
• Function: Mediates initial rolling of leukocytes on endothelium

E-Selectin (CD62E)

• Location: Activated endothelial cells only (not on resting endothelium)
• Induction: Synthesized and expressed within 1-2 hours in response to IL-1, TNF (cytokines from tissue macrophages and DCs), and bacterial LPS
• Ligands on leukocytes: Sialyl-Lewis X on PSGL-1, E-selectin ligand-1, and certain glycolipids on neutrophils, monocytes, and activated T lymphocytes
• Function: Mediates rolling and weak adhesion

L-Selectin (CD62L)

• Location: Expressed on leukocytes (not on endothelial cells) - constitutively expressed on naive T cells and neutrophils
• Ligands on endothelium: Sialomucins on endothelial cells whose expression is upregulated by IL-1, TNF; the major determinant is sialyl 6-sulfo Lewis X
• Function: Promotes adhesion of neutrophils to endothelium at sites of inflammation; also mediates homing of naive lymphocytes to lymph nodes via interaction with HEV (high endothelial venules)

Selectin Ligands Summary

• PSGL-1 (P-selectin glycoprotein ligand 1) - primary P-selectin ligand
• PSGL-1 and E-selectin ligand-1 - for E-selectin
• Sialomucins on endothelium - for L-selectin

2. INTEGRINS

Structure

Key Feature: Inside-Out Signaling (Integrin Activation)

• Low-affinity (bent) state - on resting leukocytes; do not bind their ligands effectively
• High-affinity (extended) state - after inside-out activation by chemokines

Key Members

3. IMMUNOGLOBULIN (Ig) SUPERFAMILY MOLECULES ON ENDOTHELIUM

ICAM-1 (Intercellular Adhesion Molecule-1, CD54)

• Structure: Membrane glycoprotein with immunoglobulin (Ig) domains
• Expression: Constitutively low; markedly upregulated on endothelium, lymphocytes, DCs, macrophages, fibroblasts, and epithelial cells by IL-1 and TNF
• Binds to: LFA-1 and MAC-1 on leukocytes
• Role: Mediates firm (stable) adhesion of leukocytes to endothelium; also critical for T cell interaction with APCs

ICAM-2 (CD102)

• Expression: Constitutively expressed on endothelial cells
• Binds to: LFA-1
• Role: Contributes to leukocyte adhesion and transmigration

VCAM-1 (Vascular Cell Adhesion Molecule-1, CD106)

• Expression: Induced on endothelium by IL-1, TNF
• Binds to: VLA-4 (α4β1) integrin on lymphocytes, monocytes, and eosinophils
• Clinical relevance: Important in lymphocyte and eosinophil recruitment; target of natalizumab (anti-α4 integrin antibody used in MS and Crohn's disease)

PECAM-1 (Platelet-Endothelial Cell Adhesion Molecule-1, CD31)

• Expression: Expressed on endothelial cells (particularly at intercellular junctions) and on leukocytes
• Homophilic binding: PECAM-1 on leukocytes binds PECAM-1 on endothelial junctions
• Role: Mediates transmigration (diapedesis) - leukocytes squeeze through intercellular junctions using PECAM-1 interactions. CD31 blockade inhibits leukocyte extravasation.

MAdCAM-1 (Mucosal Addressin Cell Adhesion Molecule-1)

• Expression: Expressed on endothelial cells of gut-associated lymphoid tissue
• Binds to: α4β7 integrin on gut-homing lymphocytes
• Role: Directs lymphocyte homing to intestinal Peyer's patches and lamina propria

4. THE MULTISTEP PROCESS OF LEUKOCYTE RECRUITMENT (Step-by-Step)

Step 1: Margination

• Under normal laminar flow, red cells occupy the vessel center, pushing leukocytes peripherally
• In early inflammation, as blood flow slows (stasis), leukocytes accumulate along the endothelial surface
• Called margination

Step 2: Rolling

• Molecules involved: Selectins (E-selectin and P-selectin on endothelium; L-selectin on leukocytes) binding sialyl-Lewis X carbohydrates
• P-selectin rapidly redistributes from Weibel-Palade bodies to the surface in response to histamine and thrombin (within minutes)
• E-selectin is synthesized within 1-2 hours in response to TNF and IL-1
• These weak, transient, reversible interactions cause leukocytes to slow down and roll along the endothelial surface
• Rolling allows leukocytes to survey the endothelial surface for further activation signals

Step 3: Integrin Activation by Chemokines

• Chemokines (e.g., CXCL8/IL-8 for neutrophils; CCL2 for monocytes) are produced at the inflammatory site and displayed on endothelial surface proteoglycans
• Rolling leukocytes encounter surface-bound chemokines
• Chemokine binding to GPCR-type chemokine receptors on leukocytes triggers intracellular signaling (via RAP GTPases) that converts integrins from low-affinity to high-affinity conformation
• Integrins also cluster on the leukocyte surface, increasing avidity

Step 4: Firm (Stable) Adhesion

• Molecules involved: Integrins on leukocytes (LFA-1, MAC-1, VLA-4) binding Ig-superfamily ligands on endothelium (ICAM-1, VCAM-1)
• The high-affinity integrins bind firmly to ICAM-1 and VCAM-1 (upregulated on endothelium by TNF/IL-1)
• Results in strong, stable adhesion - leukocytes stop rolling and flatten on the endothelium

Step 5: Transmigration (Diapedesis)

• Molecules involved: PECAM-1 (CD31) on both leukocytes and endothelial junctions
• Leukocytes squeeze between endothelial cells at intercellular junctions (paracellular route - most common) or rarely through the endothelial cell body (transcellular route)
• PECAM-1 homophilic binding drives migration through the junction
• After crossing the endothelium, leukocytes pierce the basement membrane using matrix metalloproteinases (collagenases)

Step 6: Chemotaxis in Tissues

• Once in the extracellular space, leukocytes migrate along a concentration gradient of chemokines and other chemoattractants toward the site of injury/infection
• Chemoattractants include: CXCL8 (IL-8), C5a, leukotriene B4 (LTB4), fMet-Leu-Phe (fMLF) bacterial peptides

5. SUMMARY TABLE: Robbins Table 2.3 (Endothelial and Leukocyte Adhesion Molecules)

6. REGULATION OF ADHESION MOLECULE EXPRESSION

7. CLINICAL RELEVANCE AND DISEASE CORRELATES

Leukocyte Adhesion Deficiency (LAD)

• Type I: Autosomal recessive deficiency of CD18 (β2 integrin subunit) → absent LFA-1 and MAC-1 → failure of leukocyte firm adhesion and transmigration → recurrent bacterial infections, impaired pus formation, delayed umbilical cord separation, leukocytosis without inflammation at sites of infection
• Type II: Deficiency of fucosyl transferase → absent sialyl-Lewis X → failure of selectin-mediated rolling (leukocytes never slow down enough to adhere)
• Type III: Deficiency in kindlin-3 (activates integrin inside-out signaling)

Therapeutic Targeting

• Anti-TNF antibodies (infliximab, etanercept): reduce upregulation of E-selectin, ICAM-1, VCAM-1 - used in rheumatoid arthritis, IBD, psoriasis
• Natalizumab (anti-α4 integrin): blocks VLA-4/VCAM-1 and α4β7/MAdCAM-1 interactions - used in multiple sclerosis and Crohn's disease
• Vedolizumab (anti-α4β7): selectively blocks gut lymphocyte trafficking - used in IBD

8. SEQUENCE OF CELLULAR INFILTRATION

• First 6-24 hours: Neutrophil predominance (respond rapidly, more numerous in blood, attach firmly to early-induced P- and E-selectin)
• After 24-48 hours: Monocyte/macrophage predominance (neutrophils undergo apoptosis; macrophages survive longer and proliferate)
• Exceptions: Pseudomonas infections - prolonged neutrophil dominance; viral infections - lymphocytes first; allergic reactions/parasitic infections - eosinophils predominant

Key Points to Remember for Exam

1. Selectins mediate rolling - weak, reversible, calcium-dependent, carbohydrate-binding
2. P-selectin is from Weibel-Palade bodies (rapid, minutes); E-selectin is newly synthesized (slow, 1-2 hours)
3. Integrins mediate firm adhesion after activation by chemokines (inside-out signaling, conformational change)
4. PECAM-1 (CD31) mediates transmigration at intercellular junctions
5. ICAM-1 and VCAM-1 are the endothelial Ig-superfamily ligands upregulated by TNF/IL-1
6. LAD type I = β2-integrin deficiency (absent CD18); LAD type II = absent sialyl-Lewis X
7. Chemokines are displayed on endothelial surface proteoglycans - they activate integrins during rolling, not before