Leukocyte Recruitment to Sites of Inflammation

Slide 1 · Leukocyte Recruitment

to Sites of Inflammation

From endothelial activation to leukocyte emigration

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

Explain how vascular stasis favors leukocyte margination.

Distinguish selectin-dependent rolling from integrin-mediated arrest.

Describe endothelial transmigration, CD31, and basement-membrane passage.

Relate cellular recruitment patterns to inflammatory stimuli and clinical consequences.

Slide 3 · Leukocyte recruitment is a multistep response

Circulating phagocytes must leave the vascular lumen to reach microbes and damaged tissue.

Adhesion molecules and cytokines coordinate sequential, anatomically distinct steps.

Leukocyte emigration links the vascular phase of acute inflammation to the tissue response.

Margination / rolling

Firm adhesion

Transmigration

Tissue migration

Slide 4 · Why recruited leukocytes matter

Neutrophils and macrophages ingest and destroy microbes.

These phagocytes clear necrotic material and foreign substances.

Macrophages also release growth factors that support tissue repair.

Activated leukocytes may injure otherwise viable host tissue.

Slide 5 · Leukocyte defense carries a cost

Host protection

Potential host injury

Phagocytosis of microbes

Removal of necrotic tissue

Initiation of repair

Toxic microbicidal products

Damage to normal cells

Excessive inflammation

Slide 6 · Recruitment crosses distinct compartments

First: Contact with activated endothelium in the vessel lumen.

Second: Migration across endothelium and vessel wall.

Third: Directed interstitial movement toward inflammatory signals.

Blood lumen

Endothelial junction

Basement membrane

Inflamed tissue

Slide 7 · Resting endothelium limits leukocyte traffic

Normal vascular endothelium generally resists adhesion of circulating leukocytes.

A leukocyte cannot simply pass through the intact vessel wall without recruitment signals.

Inflammation induces endothelial changes that permit attachment and emigration.

Vessel lumen

Endothelium

Extravascular tissue

WBC near the peripheral stream

Slide 8 · Activated endothelium becomes adhesive

Microbes and tissue damage activate local sentinel cells.

Cytokine-driven endothelial activation increases adhesion-molecule expression.

Chemokines displayed at the endothelial surface activate passing leukocytes.

Local injury

TNF / IL-1

Endothelial activation

Adhesive surface

Slide 9 · Postcapillary venules are a principal exit site

Leukocyte transmigration occurs mainly across postcapillary venules.

The same microvascular segment is important in increased permeability.

Leukocyte exit is a regulated junctional process, not simply unrestricted leakage.

Vessel lumen

Endothelium

Extravascular tissue

Passage at endothelial junctions

Slide 10 · The major participating leukocytes

Neutrophils

Monocytes / macrophages

Numerous circulating phagocytes

Rapidly recruited in most acute responses

Short-lived after tissue entry

Become prominent later in typical responses

Persist longer within tissue

Participate in clearance and repair

Slide 11 · Margination: Movement toward the vessel wall

Red cells are concentrated in the axial stream in flowing venous blood.

Leukocytes are displaced toward the vascular periphery.

Slowed flow increases leukocyte contact with the endothelial surface.

Vessel lumen

Endothelium

Extravascular tissue

Slide 12 · Vascular stasis favors leukocyte margination

Vasodilation and fluid loss slow microvascular blood flow.

More leukocytes occupy the peripheral stream as flow becomes sluggish.

Margination increases opportunities for molecular adhesion.

Vasodilation / leakage

Sluggish flow

Peripheral WBCs

Contact with endothelium

Slide 13 · Margination is a positional event

Margination redistributes circulating leukocytes; It does not itself establish firm attachment.

Rolling is the next molecular interaction with endothelial adhesion molecules.

Separating these events prevents conflating altered hemodynamics with receptor–ligand binding.

Vessel lumen

Endothelium

Extravascular tissue

Slide 14 · Rolling consists of repeated weak contacts

Leukocytes briefly attach to, detach from, and reattach to activated endothelium.

Rapid bond dissociation allows motion along the endothelial surface.

The dominant adhesion-molecule family at this stage is the selectins.

Vessel lumen

Endothelium

Extravascular tissue

Transient contacts: Bind → release → bind

Slide 15 · Selectins form a three-member family

L-selectin is found on leukocytes.

E-selectin is expressed by activated endothelial cells.

P-selectin is found in platelets and endothelial cells.

Selectins interact with carbohydrate-bearing ligands.

L-selectin

Leukocyte

E-selectin

Activated endothelium

P-selectin

Platelets / endothelium

Slide 16 · L-selectin is leukocyte-associated

L-selectin (CD62L) is present on several circulating leukocyte populations.

Leukocyte microvilli display L-selectin to increase access to endothelial ligands.

It participates in the rolling family of interactions.

L-selectin

(leukocyte)

Endothelial

Selectin ligand

Rolling-associated contact

Slide 17 · E-selectin is induced by cytokines

E-selectin (CD62E) is expressed by endothelial cells exposed to TNF and IL-1.

Its ligands include sialylated glycans on leukocyte glycoproteins.

Induced E-selectin supports leukocyte rolling at an inflamed focus.

Leukocyte

Endothelial surface

Sialylated glycan

On glycoprotein

E- / P-selectin

Weak, reversible tether

Schematic — proteins not drawn to molecular scale

Slide 18 · P-selectin can appear rapidly

P-selectin (CD62P) is stored in endothelial Weibel–Palade bodies.

Histamine and thrombin stimulate its movement to the cell surface.

Surface P-selectin binds carbohydrate-bearing leukocyte ligands.

Stored P-selectin

Histamine / thrombin

Surface P-selectin

Leukocyte rolling

Slide 19 · Selectin ligands carry sialylated sugars

Selectins recognize carbohydrate motifs on mucin-like glycoproteins.

Sialyl-Lewis X is a representative selectin-binding carbohydrate motif.

Adhesion depends on matched endothelial and leukocyte ligands.

Leukocyte

Endothelial surface

Sialylated glycan

On glycoprotein

E- / P-selectin

Weak, reversible tether

Schematic — proteins not drawn to molecular scale

Slide 20 · Selectin interactions are easily broken

Rolling contacts are relatively low-affinity and rapidly reversible.

Blood-flow forces continually break and reform transient bonds.

The result is progressive movement, not a fixed stop.

Vessel lumen

Endothelium

Extravascular tissue

Transient contacts: Bind → release → bind

Slide 21 · Sentinel cells initiate local recruitment

Tissue macrophages, mast cells, and endothelium respond to microbes and injury.

They generate cytokines, including TNF, IL-1, and chemokines.

Local production helps direct leukocyte recruitment toward the affected tissue.

Microbes / dead cells

Sentinel cells

Cytokines

Nearby venule

Slide 22 · TNF and IL-1 activate nearby venules

TNF and IL-1 induce a coordinated endothelial adhesion program.

The response includes molecules involved in rolling and firm adhesion.

Endothelial induction localizes inflammatory recruitment to stimulated tissue.

TNF / IL-1

Endothelium

E-selectin + ICAM-1 + VCAM-1

Slide 23 · Endothelial responses have different kinetics

Rapid redistribution

Cytokine-driven induction

Histamine or thrombin signal

Preformed P-selectin reaches the surface

No de novo synthesis is required for the stored pool

TNF and IL-1 act on endothelial cells

E-selectin becomes detectable over ~1–2 hours

Integrin ligands are also upregulated

Slide 24 · Induced E-selectin appears within 1–2 hours

TNF and IL-1 produce time-dependent endothelial activation.

The source describes endothelial E-selectin expression within 1–2 hours.

This timing is distinct from mobilization of preformed P-selectin.

Local cytokines

Transcriptional response

E-selectin display

Rolling

Slide 25 · Endothelial activation coordinates two steps

Selectin expression favors initial rolling contacts.

ICAM-1 and VCAM-1 provide ligands for firm integrin-mediated adhesion.

Chemokine presentation raises integrin affinity on leukocytes.

Selectins: Rolling

Chemokines: Activate integrins

ICAM / VCAM: Arrest

Slide 26 · Rolling and firm adhesion are mechanistically distinct

Rolling

Firm adhesion

Selectin-dominated interactions

Transient, repeatedly disrupted bonds

Leukocyte continues moving

Activated leukocyte integrins

Stable binding to ICAM-1 or VCAM-1

Leukocyte arrests and spreads

Slide 27 · Chemokines are displayed on the endothelial surface

Locally produced chemokines associate with endothelial proteoglycans.

Surface presentation exposes rolling leukocytes to concentrated signals.

Chemokine receptors transmit the activation signals needed for firm adhesion.

Chemokine

(on proteoglycan)

Rolling leukocyte

Receptor

Local signaling at the vessel wall

Slide 28 · Integrins start in a low-affinity state

Leukocytes normally carry integrins that do not bind strongly.

Low integrin affinity permits continued circulation rather than universal adhesion.

Inflammation must first convert these receptors into a stronger binding state.

Leukocyte

Endothelial surface

Low-affinity

Integrin

Endothelial ligand

Weak interaction

Schematic — proteins not drawn to molecular scale

Slide 29 · Chemokines increase leukocyte integrin affinity

Rolling leukocytes encounter chemokines attached to endothelial surfaces.

Chemokine receptor signaling switches integrins to high-affinity conformations.

Activated integrins now bind their endothelial ligands firmly.

Chemokine signal

Leukocyte receptor

Integrin activation

Stable binding

Slide 30 · Firm adhesion requires coordinated changes

The leukocyte increases its integrin-binding capacity.

The inflamed endothelium increases integrin-ligand expression.

Together, receptor activation and ligand availability produce firm arrest.

Activated leukocyte integrin

Endothelial ICAM-1 / VCAM-1

Firm leukocyte adhesion

Slide 31 · β2 integrins recognize endothelial ICAMs

LFA-1 (CD11a/CD18) binds ICAM-1 and ICAM-2.

MAC-1 (CD11b/CD18) also interacts with ICAM proteins.

ICAM-1 is increased on activated endothelium.

Leukocyte

Endothelial surface

High-affinity

LFA-1 / MAC-1

ICAM-1

Firm leukocyte arrest

Schematic — proteins not drawn to molecular scale

Slide 32 · VLA-4 recognizes VCAM-1

VLA-4 (α4β1; CD49d/CD29) is a leukocyte β1 integrin.

Activated endothelial cells express VCAM-1.

VLA-4–VCAM-1 supports firm leukocyte attachment.

Leukocyte

Endothelial surface

VLA-4

(β1 integrin)

VCAM-1

Firm leukocyte adhesion

Schematic — proteins not drawn to molecular scale

Slide 33 · Principal adhesion pairs are specific

Leukocyte molecule

Endothelial partner

Functional stage

Selectin ligands

E-selectin / P-selectin

Rolling

LFA-1 / MAC-1

ICAM-1 / ICAM-2

Firm adhesion

VLA-4

VCAM-1

Firm adhesion

PECAM-1 (CD31)

PECAM-1 (CD31)

Transmigration

Slide 34 · Firmly adherent leukocytes spread out

Integrin-dependent contacts arrest a rolling leukocyte.

The cytoskeleton reorganizes as the cell spreads along endothelium.

This stabilizes the cell before transmigration.

Vessel lumen

Endothelium

Extravascular tissue

Firm integrin-mediated adhesion

Slide 35 · Rolling progresses to stable arrest

Selectins slow leukocytes with serial reversible contacts.

Chemokines induce a high-affinity integrin state.

ICAM-1 / VCAM-1 then bind activated leukocyte integrins.

The arrested cell is positioned to cross the endothelium.

Rolling

Chemokine signal

Integrin activation

Arrest

Slide 36 · Match each recruitment stage to its molecule

Event

Dominant molecular mediator

Effect

Rolling

Selectins + glycan ligands

Transient tethering

Integrin activation

Chemokines on proteoglycans

Higher integrin affinity

Stable adhesion

Integrins + ICAM / VCAM

Arrest and spreading

Transmigration

PECAM-1 among junction proteins

Cross endothelial barrier

Slide 37 · Transmigration is passage through endothelium

Transmigration (diapedesis) follows stable leukocyte adhesion.

Leukocytes pass through interendothelial gaps while the endothelium remains intact overall.

Migration is guided by signals originating from inflamed tissues.

Vessel lumen

Endothelium

Extravascular tissue

Passage at endothelial junctions

Slide 38 · Diapedesis is concentrated in venules

Postcapillary venules are the main location of leukocyte transmigration.

Their endothelial junctions are involved in both permeability and cellular exit.

Adhesion must precede efficient passage across the vessel wall.

Vessel lumen

Endothelium

Extravascular tissue

Passage at endothelial junctions

Slide 39 · Interendothelial junctions provide a route

The leukocyte approaches a cell–cell junction after arrest.

Junction-associated adhesion molecules mediate regulated cellular passage.

The cell then enters the subendothelial compartment.

Firm attachment

Junctional entry

Across endothelium

Subendothelial space

Slide 40 · PECAM-1 (CD31) participates in transmigration

PECAM-1 is an immunoglobulin-superfamily adhesion molecule.

It is expressed on leukocytes and endothelial cells.

Homotypic CD31 interactions contribute to the transmigration step.

Leukocyte

Endothelial surface

PECAM-1

(CD31)

PECAM-1

(CD31)

Junction-associated migration

Schematic — proteins not drawn to molecular scale

Slide 41 · The basement membrane is crossed after diapedesis

Following endothelial passage, leukocytes must traverse the basement membrane.

Collagenases are implicated in permitting matrix penetration.

The cell then enters the extravascular inflammatory focus.

Endothelial junction

Basement membrane

Interstitial tissue

Slide 42 · Vascular structure is restored after exit

Leukocyte passage does not imply permanent destruction of the vessel wall.

The basement membrane becomes continuous again after emigration.

Localized remodeling allows defense while preserving vascular integrity.

Slide 43 · Exit from blood is not the end of recruitment

Leukocytes accumulate in tissue after they have crossed the vessel wall.

Directed migration is needed to reach the highest concentration of inflammatory signals.

The chemotactic stimulus originates from injury or infection.

Vessel lumen

Endothelium

Extravascular tissue

Chemotactic cue

Slide 44 · Chemotaxis follows a chemical gradient

Chemotaxis is locomotion toward a higher concentration of chemoattractant.

The stimulus can originate from microbial products or host mediators.

Cell movement is directional rather than random.

Extravascular tissue

Increasing chemoattractant

Source of tissue injury

Schematic — concentration trend, not measured data

Slide 45 · Chemoattractants may be exogenous or endogenous

Microbe-derived

Host-derived

Bacterial N-formylated peptides

Other microbial lipids and products

Signals associated with infection

Chemokines such as IL-8

Complement fragment C5a

Leukotriene B4 (LTB4)

Slide 46 · Chemoattractant receptors reorganize the cell

Chemoattractants activate seven-transmembrane G protein–coupled receptors.

Second messengers trigger actin polymerization at the leading edge.

Myosin localization toward the rear supports directed cell movement.

GPCR signaling

Actin polymerization

At the leading edge

Myosin localization

At the rear

Directed motility

Slide 47 · Inflamed tissue determines the direction of travel

Chemokines and other chemoattractants originate near damaged tissue or microbes.

Migrating leukocytes follow the spatial gradient toward its source.

This mechanism concentrates recruited cells where effector function is required.

Extravascular tissue

Increasing chemoattractant

Source of tissue injury

Schematic — concentration trend, not measured data

Slide 48 · Typical early infiltrates are neutrophil-rich

Neutrophils usually predominate during the first 6–24 hours of acute inflammation.

They enter quickly in response to chemokines and early endothelial adhesion signals.

This is a typical pattern, not a universal clock for all inflammatory stimuli.

Slide 49 · Monocytes usually follow in the next phase

In many acute inflammatory reactions, monocytes become prominent after 24–48 hours.

Recruited monocytes differentiate into tissue macrophages.

The shift reflects differences in recruitment and survival.

Slide 50 · Why neutrophils usually arrive first

Neutrophils are abundant in circulating blood.

They respond rapidly to chemokines.

They interact efficiently with rapidly induced endothelial adhesion molecules.

Abundance

Rapid chemokine response

Prompt adhesion

Early predominance

Slide 51 · Most extravasated neutrophils are short-lived

Tissue neutrophils undergo apoptosis after a relatively brief lifespan.

Neutrophil disappearance helps make room for later mononuclear infiltrates.

The cellular profile evolves even when the initial insult is resolving.

Slide 52 · Macrophages persist and may proliferate

Monocyte-derived macrophages survive longer than recruited neutrophils.

Macrophages may multiply within tissues.

They become prominent in sustained inflammatory responses and participate in repair.

Slide 53 · The inflammatory stimulus changes the infiltrate

The conventional neutrophil-to-monocyte sequence has important exceptions.

Different pathogens preferentially evoke distinct leukocyte populations.

Interpret the cellular composition in relation to cause and timing.

Slide 54 · Some bacterial infections remain neutrophil-rich

In some infections, continued neutrophil recruitment persists for days.

Pseudomonas infections as an example.

A neutrophil-rich lesion therefore need not be very early.

Slide 55 · Viral infections may recruit lymphocytes early

Lymphocytes may be among the earliest recruited cells in some viral infections.

This differs from the conventional early neutrophilic pattern.

Cell type alone is insufficient to date an inflammatory lesion.

Slide 56 · Allergy and helminths favor eosinophils

Eosinophils may predominate in allergic reactions.

Helminthic infections are another classic eosinophil-associated stimulus.

The leukocyte pattern reflects the nature of the immune response.

Slide 57 · Some hypersensitivity reactions are mononuclear

Activated lymphocytes and macrophages may dominate immune-mediated reactions.

Plasma cells can also form part of the infiltrate.

Recruitment is shaped by the underlying adaptive immune response.

Slide 58 · Cell composition is a clue, not a diagnosis

Setting

Potential prominent cells

Interpretation

Typical early acute inflammation

Neutrophils

Often 6–24 h

Typical later phase

Monocytes / macrophages

Often 24–48 h

Certain bacterial infections

Persistent neutrophils

Continued recruitment

Some viral infections

Lymphocytes

Stimulus-dependent exception

Allergy / helminths

Eosinophils

Type of inflammatory trigger

Slide 59 · Recruitment supports both killing and repair

Neutrophils and macrophages reach the tissue to remove microbes and debris.

Macrophages contribute to repair-promoting growth-factor production.

Microbicidal activity can also cause collateral tissue injury.

Recruitment

Phagocytosis / killing

Debris removal

Repair ± injury

Slide 60 · Adhesion defects predispose to infection

Genetic defects of leukocyte adhesion molecules impair movement from blood into tissues.

The clinical consequence can be increased susceptibility to bacterial infections.

This demonstrates that adhesion is essential for protective inflammation.

Slide 61 · Therapeutic suppression has an immune cost

Blocking TNF can attenuate cytokine-dependent inflammatory recruitment.

Integrin antagonists can diminish leukocyte adhesion.

Interfering with host defense increases vulnerability to microbial infection.

Reduce inflammatory signal

Less recruitment

Less tissue inflammation

Defense tradeoff

Slide 62 · Clinical application: A fresh skin wound

Clinical vignette (illustrative)

A patient develops focal erythema and swelling around a contaminated cut. Circulating neutrophils must reach the affected tissue.

Which molecular event converts rolling into firm arrest?

A. Selectin binding alone

B. Chemokine-induced activation of leukocyte integrins

C. Permanent endothelial rupture

Slide 63 · Case analysis: Firm arrest needs integrin activation

Chemokines → high-affinity integrins → adhesion

Correct: B — surface chemokines activate leukocyte integrins.

Activated integrins bind ICAM-1 or VCAM-1 and stop the rolling cell.

Selectins alone generally mediate transient rolling, not stable arrest.

Slide 64 · Clinical application: Recurrent bacterial infections

Clinical vignette (illustrative)

An individual has recurrent bacterial infections and a suspected inherited defect affecting leukocyte attachment to endothelium.

Which phase of the inflammatory response may fail?

A. Leukocyte adhesion and subsequent tissue entry

B. Production of circulating erythrocytes

C. Formation of plasma proteins in the liver

Slide 65 · Case analysis: Failure of leukocyte recruitment

Impaired adhesion → impaired tissue defense

Correct: A — adhesion and emigration can fail despite circulating leukocytes.

Adhesion molecules are required to establish and maintain contacts with endothelium.

Increased bacterial infection susceptibility in genetic adhesion defects.

Slide 66 · Clinical application: Persistent neutrophils

Clinical vignette (illustrative)

A tissue lesion remains neutrophil-rich several days after infection began. The suspected organism is Pseudomonas.

Does neutrophil predominance prove the lesion is less than 24 hours old?

A. Yes — neutrophils only occur during the first day

B. No — ongoing recruitment can sustain neutrophil predominance

Slide 67 · Case analysis: Interpret kinetics cautiously

The stimulus modifies leukocyte kinetics

Correct: B — stimulus can override the typical recruitment sequence.

Some bacterial infections sustain neutrophil influx for several days.

Cell type must be interpreted alongside stimulus, clinical course, and tissue findings.

Slide 68 · Compare the main molecular checkpoints

Checkpoint

Key molecules

What changes?

Rolling

E-/P-selectin; Selectin ligands

Reversible slowing

Activation

Chemokines; Leukocyte receptors

Integrins gain affinity

Firm adhesion

LFA-1, MAC-1, VLA-4; ICAM / VCAM

Stable cell arrest

Diapedesis

PECAM-1 (CD31) and junction proteins

Exit across endothelium

Chemotaxis

IL-8, C5a, LTB4; GPCR

Migration in tissue

Slide 69 · Leukocyte recruitment: Integrated mechanism

Inflammation produces stasis and activates local endothelium.

Selectin interactions promote rolling; Chemokines activate integrins.

Firm adhesion precedes PECAM-associated transmigration.

Chemotaxis delivers emigrated leukocytes to the site of injury.

Stasis + rolling

Integrin-mediated arrest

Diapedesis

Chemotaxis

Slide 70 · Interpretation principles for pathology

Identify the stage being described before assigning a molecular mediator.

Distinguish the time course of a typical infiltrate from pathogen-specific exceptions.

Explain both host-defense benefits and possible tissue injury.

Remember that an intact endothelium is actively regulated during leukocyte transit.

Slide 71 · Primary source and terminology

Robbins, Cotran & Kumar: Pathologic Basis of Disease, 11th edition (2026).

Chapter 3, Inflammation and Repair, pages 69–72.

Sections: Leukocyte Recruitment to Sites of Inflammation; Leukocyte Adhesion to Endothelium; Leukocyte Migration Through Endothelium; Chemotaxis of Leukocytes.

Source text was provided as Markdown extraction; the original textbook figures were not reproduced.