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Phagocytosis: An Evolutionarily Conserved Mechanism to Remove Apoptotic Bodies

and Microbial Pathogens

Phagocytosis of IgG-coated Targets by Macrophages

3 min 10 min

Mast Cells Can Phagocytose Too!

Extension of an F-actin-rich“Phagocytic Cup” Around Phagocytic Targets Motor Proteins and Exocytosis Power Phagocytosis

From: Chavrier, Nature Cell Biol. 4:E169, 2002

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Elie Metchnikoff, 1845-1916

Phagosome-Lysosome Fusion?

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Post-phagocytic Events:Phagosome-Lysosome Fusion

Lysosomes

Pathogen Macrophage

Phagolysosomes

0-3 min 1-5 min 30 min-hrs

Phagocytosis of Bacteriais Followed by Phagosome-Lysosome Fusion

From: Allen et al., J. Exp. Med. 191:115, 2000

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Granulomatous inflammationconsists of epithelioid macro-phages, giant cells, lymphocytes,plasma cells, and fibroblasts.

Epithelioid cells accumulate around the center of a granuloma. They get their name from the factthat they have pink cytoplasmsimilar to squamous epithelia.

Langhans-type giant cells represent fused macrophages.The nuclei are lined up around the peripheryof the cell.

The Granuloma: a Delayed Response toIndigestible Pathogens and Particles in Macrophages

Epithelioid CellsGranulomasLanghans-type

Giant Cells

Oxidant-dependent Killing ofBacteria and Fungi

From: Lekstrom-Himes and Gallin, N Engl J Med, 343:1703, 2000

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Post-phagocytic Events:“Phagosome-Oxidase Fusion”

NADPH oxidase

Pathogen Macrophage

2O2 ⎯⎯⎯⎯⎯⎯⎯⎯→ 2O2- + H+

Post-phagocytic Events:Generation of H2O2

NADPH oxidase

Pathogen Macrophage

2O2 ⎯⎯⎯⎯⎯⎯⎯⎯→ 2O2- + H+

O2•- + O2•- + 2H+ ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H2O2 + O2Superoxide dismutase

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Post-phagocytic Events:Myeloperoxidase Activity

NADPH oxidase

Pathogen Macrophage

2O2 ⎯⎯⎯⎯⎯⎯⎯⎯→ 2O2- + H+

O2•- + O2•- + 2H+ ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H2O2 + O2Superoxide dismutase

H2O2 + Cl- ⎯⎯⎯⎯⎯⎯⎯⎯→ HOCl + OH•Myeloperoxidase

Post-phagocytic Events:Peroxynitrite Production

NADPH oxidase

Pathogen Macrophage

2O2 ⎯⎯⎯⎯⎯⎯⎯⎯→ 2O2- + H+

O2•- + O2•- + 2H+ ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H2O2 + O2Superoxide dismutase

H2O2 + Cl- ⎯⎯⎯⎯⎯⎯⎯⎯→ HOCl + OH•Myeloperoxidase

2O2- + NO• ⎯⎯→ ONOO-

Peroxynitrite

Bacterial Virulence Factors Subvert Host Defenses

Phagosomematuration stalled

(M. tuberculosis; Legionella)

Ingestion phaseimpaired(Yersinia)

Resistance tolysosomal degradation

(Salmonella)

Modification ofphagocytic receptors

(P. aeruginosa)

Escape from phagosome into cytosol (Listeria, Shigella)

Immunological Consequencesof Phagocytosis

Death of pathogenic microbe Persistence of pathogenic microbeResolution of infection Failure of resolution of infection

Clearance of pathogens

Clearance of apoptotic corpses

Suppression of inflammation Inappropriate inflammationTolerance Break in tolerance

Dendritic Cells Engulf Influenza-infected Monocytesand Cross-present Antigen

From: Albert et al., Nature 392:86, 1998

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Biology of Fcγ Receptors

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C1q binding site

Glycosylation site

FcγR binding site

Functional Sites on the IgG Molecule

VH

VL

Y

Y

YYPP

YYPP

Y

Y

FcγRIIIA

γ γ subunitsubunit

Src familyTK

Y Y Y Y Y Y Y Y Y Y Y

Opsonized Bacterium

Fcγ Receptor Signaling: ITAM Phophorylation

YPP

PTPase

YPP YPPYPP

YPP

Y Y

Y

TK substrates

FcγRIIA ligand-binding domain

Y Y Y Y Y Y Y Y Y Y Y

Opsonized Bacterium

PPY YPP YPP

Syk

Fcγ Receptor Signaling: Syk Activation

Syk SHIP

+ −

Phagocytosis

Activating FcγR Inhibitory FcγR

γ γ

ITAMITAM ITIMITIM

IgβIgαIgβ Igα

ITAMITAM

Ag

Clustering of the BCR by Antigen Initiates Signaling

IgβIgαIgβ Igα

ITAMITAM

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Ig-α

Activating BCR Inhibitory FcγRIIB

Syk

Ca2+, Proliferation

Ig-β

ITAM

ITIM

PLC-γ

++−−

P

PIP3

BTKSHIP

PI3PI3--kinasekinase

IgβIgαIgβ Igα

CD21 (CR2)CD21 (CR2)

ITIMITIM

CD19CD19

AgC3d

PI 3-kinase

-- ++SHP-1

CD22CD22

Positive and Negative Regulation of the BCR

Ag

SHIP

FcFcγγRIIBRIIB

ITAMITAM

Y

IgG

SHPSHP--1: 1: A protein tyrosine phosphatase PI 3PI 3--kinase:kinase: Generates PIP3SHIP:SHIP: A phosphoinositide phosphatase

The “Dark Side” of Fc Receptors:Immune Complex-mediated Injury

Hypersensitivity Diseases

The Arthus Reaction: A Model of Type IIIHypersensitivity

1-2 hrAbsence of the γ subunit of Fc receptors leads to enhanced survival in the F1 generation of NZB/NZW (lupus-prone) mice, a model for autoimmune, immune complex-mediated glomerulonephritis.

Requirement of Activating FcγRs in Immune Complex-mediated Glomerulonephritis

From: Clynes et al., Science 279:1052, 1998.

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Glomerulonephritis is blocked in γ chain-deficient NZB/NZW (lupus-prone)mice. Pathological features include mesangial thickening and hypercellularityevolving into end-stage sclerotic and crescentic changes.

Requirement of Activating FcγRs in Immune Complex-mediated Glomerulonephritis

From: Clynes et al., Science 279:1052, 1998.

Strain: C57Bl/6 NZB/NZW NZB/NZW γ chain: -/- -/- +/-

Summary1. Phagocytosis is a component of innate and aquired immunity. It is the principalmeans of destroying pathogenic bacteria and fungi. Phagocytosis initiates theprocess of antigen presentation.

2. Many phagocytic receptors recognize a diverse array of microbial pathogens. Some pathogens (e.g., S. pneumoniae) require opsonization for their clearance.

3. Bugs fight back.

4. Phagocytosis is an essential component of development and tissueremodeling. Ingestion of apoptotic bodies is immunologically “silent” and isnormally accompanied by a suppression of inflammation.

5. Failure of this mechanism may result in autoimmunity.

6. Fc receptors come in two basic types: activating (ITAM-associated) and inhibitory (ITIM-associated).

7. The relative expression of activating and inhibitory Fc receptors determinesthe outcome of a given engagement of Fc receptors.

8. Fc receptor-driven pathology includes formation and deposition of immune complexes, which play a major role in autoimmunity.