Intracellular Destruction of Microbes and Debris

Slide 1 · Intracellular Destruction

of Microbes and Debris

Leukocyte microbicidal mechanisms in acute inflammation

From recognition and engulfment to intracellular killing and collateral injury

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Relate phagocytosis to the intracellular killing of microbes and dead-cell debris.

Explain the NADPH oxidase–ROS and iNOS–reactive nitrogen pathways.

Distinguish the functions of neutrophil granules, antimicrobial proteins, and antiproteases.

Predict how defective killing or extracellular release causes disease.

Focus: Mechanism → cellular compartment → consequence

Slide 3 · Intracellular destruction completes phagocytosis

Recognize and attach

Engulf into phagosome

Fuse with lysosome

Kill and degrade

The final step removes infectious agents and necrotic debris.

Most efficient killing follows leukocyte activation.

Slide 4 · Two professional phagocytes, complementary responses

Property

Neutrophils

Macrophages

Speed

Rapid; short-lived

Slower; more prolonged

Key oxidant response

Prominent respiratory burst

Reactive nitrogen species prominent

Granule release

Major response

Less prominent

Cytokine production

Limited

Major functional response

Slide 5 · Leukocyte activation makes intracellular killing effective

Microbial signals / cytokines

Receptor-initiated signals

Cytoskeletal and enzymatic activation

Microbial destruction

Activation coordinates ingestion with the microbicidal apparatus.

Phagocytosis and killing are distinct but linked outputs.

Slide 6 · Phagocytosis has three linked phases

Recognition

Microbe or debris engages receptors on the leukocyte.

Engulfment

Actin-driven membrane extension creates a phagosome.

Destruction

Lysosomal fusion brings toxic molecules to the cargo.

Slide 7 · Direct recognition of microbial surfaces

Pattern-binding receptors

Mannose receptor binds terminal mannose/fucose.

Scavenger receptors bind microbes and modified lipoproteins.

Additional binding route

Mac-1 integrin (CD11b/CD18) may bind microbes.

Different receptor classes support particle attachment.

Slide 8 · Why the mannose receptor discriminates microbes

Microbial surfaces

Terminal mannose and fucose residues are common.

Macrophage lectin receptor recognizes these sugars.

Mammalian surfaces

Many terminal sugars are modified.

Sialic acid or N-acetylgalactosamine helps distinguish host cells.

Slide 9 · Opsonins amplify phagocytic efficiency

Microbe coated with opsonin

High-affinity leukocyte receptor

Stable attachment

Efficient uptake

Main opsonins: IgG; Complement-derived C3b; Plasma lectins.

This provides an indirect recognition pathway alongside direct binding.

Slide 10 · Recognition pathways converge on engulfment

Microbial particle

Direct receptor binding

Opsonin-dependent binding

Phagocytosis

Slide 11 · Engulfment is an actin-dependent event

Receptor binds particle

Actin polymerizes

Membrane surrounds cargo

Phagosome closes

Actin-driven cytoskeletal remodeling explains the membrane movement.

The microbe remains enclosed in a membrane-bound compartment.

Slide 12 · Fusion forms the microbicidal compartment

Cargo

Lysosomal

Enzymes

Phagolysosome

Lysosomal granules fuse with the phagosome.

Hydrolases and microbicidal substances enter the phagolysosome.

Delivery into this compartment concentrates destructive activity.

Slide 13 · Compartmentalization protects the phagocyte

Contained killing

Toxic products act near ingested cargo.

Cytoplasm and nucleus are spatially shielded.

Extracellular escape

Oxidants and proteases can reach healthy tissue.

The same effectors now cause collateral injury.

Slide 14 · Three major intracellular effector systems

Reactive oxygen species

Produced rapidly after phagocyte oxidase assembly.

Reactive nitrogen species

Generated via inducible nitric oxide synthase.

Lysosomal contents

Hydrolases, proteases and antimicrobial proteins.

Slide 15 · Reactive oxygen species: Pathway overview

NADPH oxidase

Superoxide (O₂•⁻)

Hydrogen peroxide (H₂O₂)

Microbicidal oxidants

MPO can use hydrogen peroxide to make hypochlorite.

Hydroxyl radicals and related oxidants also damage cell components.

Slide 16 · Phagocyte oxidase is inactive as a dispersed complex

Resting neutrophil

Enzyme components reside in different locations.

Some components are membrane associated.

Others are distributed in the cytoplasm.

Functional requirement

Stimulus-dependent assembly at a membrane.

Assembly brings the oxidase components together.

Slide 17 · Activation assembles NADPH oxidase on the phagosome

Microbial / inflammatory stimulus

Cytosolic components translocate

Membrane oxidase assembles

Local superoxide generation

ROS synthesis is concentrated at the site of ingested cargo.

The complex comprises multiple protein components.

Slide 18 · Respiratory burst is rapid oxygen consumption

Trigger

A stimulus activates the phagocyte oxidase system.

Biochemical event

Oxygen is used to form superoxide anion.

Functional purpose

Generate antimicrobial oxidants in the phagosome.

Slide 19 · NADPH oxidase initiates oxygen reduction

NADPH + O₂

NADPH oxidase

O₂•⁻

Reduced electron donor

Electron transfer

Superoxide anion

The important first product is superoxide, not hypochlorite.

Slide 20 · Superoxide produces hydrogen peroxide

O₂•⁻

Dismutation

H₂O₂

Primary oxidant

Spontaneous conversion

Substrate for further oxidants

Hydrogen peroxide is an intermediate in the stronger neutrophil killing system.

Slide 21 · Myeloperoxidase is a primary-granule enzyme

Cargo

Lysosomal

Enzymes

Phagolysosome

Azurophilic (primary) granules are rich in MPO.

Granule fusion supplies MPO to the phagolysosome.

MPO uses H₂O₂ and a halide to generate a stronger antimicrobial oxidant.

Slide 22 · MPO generates hypochlorite

H₂O₂ + Cl⁻

Myeloperoxidase

ClO⁻

Hydrogen peroxide + chloride

MPO–halide system

Hypochlorite

The H₂O₂–MPO–halide pathway is the most potent bactericidal system of neutrophils.

Slide 23 · Hypochlorite injures microbial molecules

Hypochlorite (ClO⁻)

Halogenation of cell constituents

Oxidation of proteins and lipids

Microbial damage

Slide 24 · The neutrophil ROS cascade, integrated

O₂ + NADPH

O₂•⁻

H₂O₂

ClO⁻ via MPO + Cl⁻

Several oxidants and enzymes act together rather than as one isolated reaction.

Main site: Phagolysosome after membrane oxidase assembly.

Slide 25 · Hydroxyl radical provides another toxic route

Hydrogen peroxide (H₂O₂)

MPO + halide → hypochlorite

Conversion → hydroxyl radical (•OH)

Oxidant-mediated damage

Slide 26 · Oxidants target multiple microbial macromolecules

Lipids

Oxidative disruption of cellular membranes.

Proteins

Chemical modification impairs structure and function.

Nucleic acids

Reactive species chemically damage genetic material.

Slide 27 · ROS are not restricted to the phagolysosome

Microbe / immune complex / chemokine

Leukocyte activation

Extracellular ROS release

Bystander tissue injury

Extracellular release converts defense into a source of inflammation-associated injury.

Tissue damage depends on whether ROS reach host structures.

Slide 28 · Antioxidants buffer oxidant injury

Cellular enzymes

Superoxide dismutase

Catalase

Glutathione peroxidase

Tissue and plasma protection

Ceruloplasmin

Iron-free transferrin fraction

Multiple protective mechanisms operate together.

Slide 29 · Superoxide dismutase limits the superoxide burden

Substrate

Superoxide anion is a reactive first product.

Antioxidant

Superoxide dismutase participates in oxidant handling.

Balance

Defense systems reduce exposure to harmful radicals.

Slide 30 · Catalase and glutathione peroxidase neutralize H₂O₂

Catalase

Detoxifies hydrogen peroxide.

Protects cells from oxidant accumulation.

Glutathione peroxidase

Also removes hydrogen peroxide.

Provides an additional protective route.

Slide 31 · Plasma antioxidants add extracellular protection

Ceruloplasmin

Copper-containing plasma protein.

Included among protective antioxidant mechanisms.

Transferrin

Iron-free transferrin fraction is protective.

Represents another defense in tissue fluids.

Slide 32 · Chronic granulomatous disease: Oxidase deficiency

Inherited oxidase defect

Impaired ROS production

Defective oxygen-dependent killing

Infection susceptibility

This as a phagocyte oxidase immunodeficiency.

The topic is developed further in the immunodeficiency chapter.

Slide 33 · MPO deficiency demonstrates microbicidal redundancy

MPO deficiency

One strong oxidant-generating route is deficient.

Minimal increase in infection susceptibility.

NADPH oxidase deficiency

Upstream ROS generation is impaired.

Associated with chronic granulomatous disease.

Slide 34 · Redundant killing explains a clinical paradox

Observation

MPO–halide chemistry is highly bactericidal.

Observation

Inherited MPO deficiency often has little infection impact.

Inference from source

Other leukocyte effectors compensate for an isolated loss.

Slide 35 · Reactive nitrogen species begin with nitric oxide

L-arginine

Nitric oxide synthase (NOS)

Nitric oxide (NO)

Substrate

Enzymatic production

Soluble reactive signaling molecule

The inducible NOS isoform participates in microbial killing.

Slide 36 · Three nitric oxide synthases have distinct roles

Isoform

Expression / function

eNOS

Constitutive, low-level NO; vascular tone

nNOS

Constitutive, low-level NO; neurotransmission

iNOS

Induced in activated phagocytes; microbial killing

Slide 37 · Inducible NOS responds to macrophage activation

Microbial products / cytokines

Macrophage activation

INOS transcription / expression

Nitric oxide generation

Interferon-γ is one cited induction signal.

Neutrophils can contribute, but macrophage iNOS is emphasized.

Slide 38 · Macrophages: Nitric oxide as an antimicrobial effector

Activation input

Microbial products

Cytokines including interferon-γ

Effector output

Induced nitric oxide synthesis

Reactive nitrogen-derived antimicrobial damage

Slide 39 · NO and superoxide form peroxynitrite

NO + O₂•⁻

Chemical reaction

ONOO⁻

Nitric oxide + superoxide

Reactive species combine

Peroxynitrite

Peroxynitrite provides a powerful nitrogen-derived microbicidal oxidant.

Slide 40 · ROS and RNS share major molecular targets

Oxygen-derived species

Oxidize lipids, proteins and nucleic acids.

Include superoxide-derived oxidants.

Nitrogen-derived species

Damage the same classes of targets.

Include NO-derived peroxynitrite.

Slide 41 · Experimental evidence illustrates overlap

Single pathway missing

Mice lacking oxidase OR iNOS have only mild infection susceptibility.

Both pathways missing

Double-knockout mice develop severe disseminated infection.

Interpretation

Cooperating ROS and RNS pathways provide functional backup.

Slide 42 · NO has an additional vascular action

Microbicidal setting

INOS-derived NO acts in activated macrophages.

Can yield peroxynitrite with superoxide.

Vascular setting

NO relaxes vascular smooth muscle.

Its precise contribution to acute inflammatory vascular changes is uncertain.

Slide 43 · Granules organize nonoxidative killing

Cargo

Lysosomal

Enzymes

Phagolysosome

Lysosomal granules store enzymes and antimicrobial proteins.

Fusion delivers these effectors into the ingested cargo compartment.

Discharge outside the cell can damage tissue.

Slide 44 · Neutrophils carry two major granule populations

Primary / azurophilic

Larger granules.

MPO; Defensins; Lysozyme; Acid hydrolases.

Neutral proteases including elastase.

Secondary / specific

Smaller granules.

Lysozyme; Collagenase; Gelatinase.

Lactoferrin and additional enzymes.

Slide 45 · Primary granules combine oxidants and proteases

Primary granule constituent

Principal relevance

Myeloperoxidase

H₂O₂–halide oxidant generation

Defensins and lysozyme

Direct antimicrobial effects

Acid hydrolases

Digestion of phagocytosed material

Elastase / cathepsin G / proteinase 3

Proteolytic activity

Slide 46 · Secondary granules broaden enzymatic activity

Secondary granule constituent

Functional class

Lysozyme

Antibacterial enzyme

Collagenase and gelatinase

Matrix-degrading enzymes

Lactoferrin

Iron-binding antimicrobial protein

Histaminase / alkaline phosphatase

Other granule enzymes

Slide 47 · Granules may fuse internally or release externally

Activated neutrophil granules

Fusion with phagocytic vacuole

Extracellular degranulation

Microbial control ± host injury

Slide 48 · Acidified phagolysosomes favor digestion

Phagosome–lysosome fusion

Proton pump acidification

Acid proteases / hydrolases

Digestion of microbes and debris

Compartment acidification creates an enzymatic environment for digestion.

This route is mechanistically distinct from oxidant-mediated killing.

Slide 49 · Neutral proteases have broad extracellular targets

Extracellular matrix

Collagen; Basement membrane; Elastin; Cartilage.

Protein deposits

Fibrin is susceptible to degradation.

Inflammatory mediators

Proteases can process complement and kinin pathways.

Slide 50 · Elastase can protect and injure

Host defense

Neutrophil elastase degrades bacterial virulence factors.

Contributes to antimicrobial activity.

Tissue damage

Uncontrolled elastase degrades elastic support structures.

Damage depends on antiprotease protection.

Slide 51 · Proteases can amplify mediator cascades

Released neutrophil proteases

C3 / C5 cleavage and kininogen processing

Complement / kinin-derived products

Further inflammation

Protein degradation is not limited to structural damage.

The source describes mediator-generating effects of neutral proteases.

Slide 52 · Macrophages also contain degradative enzymes

Macrophage enzymes

Acid hydrolases and phospholipase

Collagenase and elastase

Plasminogen activator

Functional implication

Digestion of engulfed debris

Potential connective-tissue remodeling

Potential tissue injury if released

Slide 53 · Antimicrobial granule peptides act directly

Defensins

Cationic, arginine-rich granule peptides.

Direct toxicity to microorganisms.

Cathelicidins

Antimicrobial proteins of neutrophils and other cells.

Additional antimicrobial defense.

Slide 54 · Lysozyme and lactoferrin act differently

Lysozyme

Hydrolyzes the muramic acid–N-acetylglucosamine bond.

Acts on bacterial cell-wall components.

Lactoferrin

Iron-binding protein in specific granules.

Contributes to antimicrobial granule functions.

Slide 55 · Eosinophil major basic protein: A distinct example

Cell context

Major basic protein is a cationic eosinophil protein.

Antibacterial action

Its bactericidal activity is limited.

Parasite defense

It is cytotoxic to many helminthic parasites.

Slide 56 · Antiproteases restrain leukocyte-mediated injury

Protease release

Serum / tissue antiproteases

Restriction of excessive proteolysis

Preservation of host tissue

Antiprotease control is essential because enzymes cannot intrinsically select host versus microbe.

α₁-Antitrypsin and α₂-macroglobulin are named protective proteins.

Slide 57 · α₁-Antitrypsin restrains neutrophil elastase

α₁-Antitrypsin deficiency

Relative excess elastase activity

Loss of lung elastic support

Risk of emphysema

This is a mechanism linking inflammatory proteolysis to organ pathology.

α₁-antitrypsin as the major neutrophil elastase inhibitor.

Slide 58 · α₂-Macroglobulin provides another antiprotease layer

Protease challenge

Activated leukocytes release multiple proteases.

Serum and tissue fluids require counter-regulation.

Protective response

α₂-Macroglobulin is a circulating antiprotease.

It supplements other inhibitory proteins.

Slide 59 · Frustrated phagocytosis releases toxic contents

Material too large / firmly deposited

Particle cannot be fully engulfed

Persistent leukocyte activation

Extracellular granule release

Example in source: Immune complexes deposited on a broad surface.

Extracellular enzymes can injure host tissue nearby.

Slide 60 · Ingested crystals can breach the compartment

Urate crystals are phagocytosed

Phagolysosomal membrane damage

Leakage of granule contents

Inflammatory tissue injury

The protective phagolysosomal barrier may fail after particular challenges.

This mechanism connects particle uptake to collateral damage.

Slide 61 · Leukocytes injure tissues through their defense machinery

Oxidants

Extracellular ROS modify host molecules.

Reactive nitrogen species

RNS may contribute to cellular injury.

Proteases

Matrix and membrane constituents are degraded.

Slide 62 · Three contexts for collateral leukocyte injury

Persistent infection

Prolonged responses damage adjacent tissues.

Autoimmunity

Inflammatory effectors are directed toward self tissues.

Allergy

Excess responses target usually innocuous stimuli.

Slide 63 · Damage may arise without a defect in phagocytosis

Effective containment

Microbes are engulfed and lysed intracellularly.

Microbicidal substances remain largely compartmentalized.

Mislocalized effectors

ROS and granule contents reach extracellular structures.

Inflammation itself becomes injurious.

Slide 64 · Mechanism comparison: Deficiency versus excess

Disturbance

Mechanism

Pathologic implication

Oxidase deficiency

Reduced antimicrobial ROS

Susceptibility to infection (CGD)

MPO deficiency

Less hypochlorite production

Often mild impact because of redundancy

Antiprotease deficiency

Unopposed elastase

Lung elastic-fiber destruction

Extracellular release

Oxidants / enzymes hit host tissue

Inflammation-associated injury

Slide 65 · Clinical reasoning: Defective upstream oxidant formation

A patient is known to have an inherited defect in the phagocyte oxidase complex.

Which part of microbial killing is directly compromised?

Interpretation

Production of superoxide during the respiratory burst is impaired, reducing downstream ROS generation.

Slide 66 · Clinical reasoning: Strong MPO biochemistry, mild deficiency

A patient has isolated inherited myeloperoxidase deficiency with little infection susceptibility.

How can this occur despite the potency of the MPO–halide system?

Interpretation

Leukocyte killing is redundant; Other ROS, RNS and lysosomal mechanisms can partly compensate.

Slide 67 · Clinical reasoning: Emphysema and protease balance

A patient has α₁-antitrypsin deficiency and loss of lung elastic support.

Which leukocyte effector explains this connection?

Interpretation

Neutrophil elastase is insufficiently inhibited, increasing elastin and matrix destruction.

Slide 68 · Clinical reasoning: Broad extracellular immune deposits

Immune complexes are deposited across a broad tissue surface that a neutrophil cannot surround.

What produces injury even without complete particle ingestion?

Interpretation

Frustrated phagocytosis leads to granule release and extracellular proteolytic injury.

Slide 69 · Integrated sequence: Defense and collateral damage

Activated leukocyte

Sequestered phagolysosomal attack

Extracellular effector release

Clearance or tissue injury

Slide 70 · Five mechanistic conclusions

NADPH oxidase produces superoxide; MPO transforms H₂O₂ into hypochlorite.

Induced nitric oxide can generate peroxynitrite with superoxide.

Acidified phagolysosomes deliver proteolytic and antimicrobial proteins.

Antioxidants and antiproteases limit self-inflicted damage.

Disrupted containment or defective killing explains important disease links.

Slide 71 · Primary textbook reference

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

Chapter 3: Inflammation and Repair — phagocytosis and intracellular destruction of microbes and debris.

Relevant textbook material: printed pp. 72–76; diagrams discussed conceptually without reproducing textbook figures.

All original diagrams in this presentation are editable teaching schematics; they are not clinical or histologic specimens.