role of phagocytic cells in periodontal health

7/30/2019 Role of Phagocytic Cells in Periodontal Health

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Presented by:

Dr. Rajendra Kumar

Role of phagocytic cells in periodontal

health


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The phagocytic cells of the innate or nonspecific

immune system, especially neutrophils, monocytes and

macrophages, maintain health by preventing and

controlling infection of the host by bacteria.The nonspecific defense systems constitute the first line

of defense, or acute reaction to insult, by invading

bacteria or other foreign material.


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This contrasts with acquired immunity: the development

of immune recognition by lymphoid cells and specific

reaction to defined antigens.

Phagocytic cells are able to detect invaders through avariety of mechanisms. The predominant mechanism is

opsonization, which is mediated by complement proteins

in the acute phase and antibody after the development of

acquired immunity.

The neutrophil plays a pivotal role in host defense

against infectious periodontal disease.


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Understanding the role of phagocytic cells in protecting the host

from periodontal disease requires and awareness of normal

neutrophil function.

Phagocytic cells are derived from the lymphoid and myeloid arms

of hemopoietic system. In the bone marrow, the myeloid arm giverise to phagocytes.

Phagocytes

Mononuclear macrophages Polymorphonuclear Microphages

( monocytes) (Neutrophils Eosinophils Basophils)


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Neutrophils and Monocytes/Macrophages

Neutrophils and monocytes are closely related phagocytic

leukocytes.

The fundamental difference between these two cells is

that neutrophils differentiate almost completely within thebone marrow (14 days), whereas monocytes exit the bone

marrow after 2 days in a relatively immature state and

may differentiate in the tissues.

Neutrophils and monocytes are the same size (9-10m

diameter) in the blood.


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They possess many lysosomes within their cytoplasm. Because

neutrophils do not need to differentiate substantially to function,

they are suited for rapid responses.

When neutrophils leave the blood, they always retain their small

size and hence were once called microphages.

Neutrophils possess receptors for metabolites of the complement

molecule C3, designated complement receptor 1, 3, and 4 (CR1,

CR3, CR4); and C5aR.

They also possess receptors for IgG antibody (FcyR). Thesereceptors enable neutrophils to participate in the inflammatory

response and to ingest foreign molecules and cells in the process of

phagocytosis.


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Monocytes are referred to as macrophages when they leave the

blood.

They complete their differentiation in the local tissues and may

become greater than 22 m diameter.

Because macrophages differentiate and live in the local tissues,

they are suited for communicating with lymphocytes and other

surrounding cells.Together, macrophages and lymphocytes coordinate the chronic

immune response.

Monocytes/macrophages possess CRl, CR3, CR4, C5aR receptors,

several classes of Fcy receptors, and molecules important in antigen

presentation (MHC Class II receptor, CD1).


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Role of Neutrophils in Phagocytosis

Neutrophils are the primary phagocytic cells in the

acute response.

The normal function of neutrophils can be broken

down into a defined series of molecular events usefulin describing how neutrophils respond to bacterial

invasion.

Normal neutrophil function can be discussed asquantifiable events:


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1.Stimulation of the acute phase (generation of the

signal)

2. Recognition of the signal

3.Migration to the source of the signal

4. Recognition of the invader

5. Phagocytosis

6. Microbicidal activity


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Stimulation of the acute phase (generation of the

signal) or complement pathway

The complement cascade comprises 30 heat-labile

plasma proteins that autoassemble after initiation of

inflammation, forming a series of enzymes that

catalyze each subsequent step. The net effect of complement activation is to augment

opsonization of bacteria by antibodies, to allow some

antibodies to kill bacteria, to recruit phagocytes to the

site of complement activation and to attack the

membrane of pathogens forming pores in the cell and

lysis.


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Complement is activated through three differentpathways:

1. The classical pathway is activated by an antigen-antibody complex,

requiring an acquired humoral immune response.

2. The lectin pathway is initiated by binding of a serum lectin, the

mannose binding protein, to mannose-containing proteins or to

carbohydrates on bacteria.

3. The alternative pathway is initiated by lipopolysaccharide or other

bacterial products, resulting in the direct cleavage of the third

component of complement, C3, which initiates activation of the

terminal proteins of the cascade.


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Although multiple pathways exist for the initial

activation of complement, all pathways ultimately result

in the production of a protease called C3 convertase

that is covalently bound to the surface of the pathogen.


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Biological effects of complement components

C3a, C5a and C5b are mediators of inflammation that

recruit fluid, cells and proteins to the site of infection.

In all three pro-inflammatory complement components,

C5a is the most stable and has the highest specificbiological activity.

All three mediators induce smooth muscle contraction

and increase vascular permeability; C3a and C5a canactivate mast cells to release mediators that cause

similar effects.


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The increased vascular permeability ensures that

antibody, complement and phagocytic cells are readily

exposed to the site of the infection, accelerating

clearance of the pathogen through opsonization andphagocytosis or clearance to the local lymph nodes.

Importantly, C5a is a potent chemotactic factor, or

chemical attractant, for neutrophils and monocytes and

macrophages.


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Recognition of the signal

Chemotactic factor receptors are a distinct group of

molecules with a unique structure found on the surface

of neutrophils and other cells.

The neutrophil is a fully differentiated, dedicatedphagocyte constituting the primary cellular defense

against bacterial insult.

Neutrophils are selectively recruited into distressed

tissues by a sequence of pro-inflammatory events,

promoted at the site of insult by secretion of soluble

mediators by both indigenous host cells and the

bacteria themselves


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Neutrophils are initially the predominant host defense

cell found in bacterial infections, including periodontal

lesions.

Neutrophils have several selective mechanisms forcontrolling bacteria, including both intracellular and

extracellular oxidative and nonoxidative killing

mechanisms , which can be triggered by endogenous

signals, such as antibody and complement component

binding (C3b), or exogenous bacterial factors.


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These signals profoundly affect chemotaxis,

anaphylaxis and phagocytosis.

In the neutrophil, C5a and neutrophil granule enzyme

breakdown of C5 serves as a positive feedback loop forneutrophil chemotaxis, phagocytosis and granule

release .

Both C5a and C3a have been reported to bechemotactic for monocytes and macrophages.


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Migration to the site of infection

Neutrophils and monocytes appear to move into the

tissues through similar mechanisms.

The initial adherence of cells to the endothelium in the

area of an infection is mediated through interactionbetween surface adhesion glycoproteins on the

neutrophil and adhesion molecules expressed by the

endothelial cells.

Cells in the peripheral blood may be moving, and the

initial binding between the cells and the endothelium is

not absolute but slows the cells, causing them to roll

across the endothelium.


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ICAM- intercellular cell adhesion molecule, LFA- leukocyte function associated

antigen, VLA 4-very late antigen, VCAM

vascular cell adhesion molecule


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C


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Chemotaxis

After traversing the endothelium, neutrophils enters the

connective tissue to the site of bacterial challenge in

response to chemoattractants expressed in a gradient.

Chemoattractant agents liberated at the site includeactivated complement fragment C5a, an arachidonic

acid metabolite leukotriene B, and formylated peptides

(formylation is a unique characteristic of bacterial

peptides).

In addition, IL-8 can function as a chemoattractant

specific for neutrophils.


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3a


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Fig. Leukocytes can "hone in" on various irritants, such as a microorganism, by

chemotaxis. This requires a chemotaxin receptor (A). The chemotaxin receptors aremembers of the G-protein coupled family. Seven transmembranous domains, three

extracellular loops (ELl-3), and three cytosolic loops (CL1-3) characterize this

family of molecules. B, Neutrophils polarize, forming an anterior lamellipod and a

posterior uropod. Cytoplasm appears to squirt through a contractile ring.

Neutrophils exhibit strikingly sensitive chemotaxis and can detect a 1 % gradient

over the length of its cell body at nanomolar concentrations.


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PHAGOCYTOSIS

Phagocytosis is the process by which cells ingest particles of a size

visible to light microscope.

Phagocytic cells: Neutrophils and monocytes / macrophages to be

considered professional phagocytes

Phagocytosis result in the evantual contaminent of a pathogen

within a membrane-delimited structre the phagosome.

The process of phagocytosis can be divided into the following steps:

1. Recognition and attachment ( Opsonisation)2. Engulfment stage

3. Secretion stage

4. Digestion or degradation


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Opsonisation

The immune system has evolved mechanisms of coating

the pathogen with a few recognizable ligands or opsonin,

which enable the phagocyte to bind and ingest the

pathogen. This is referred to as opsonization


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Fig. Routes of opsonization and phagocytosis. Bacterial control in the periodontal environment is

achieved largely by opsonization, phagocytosis, and killing of the bacteria by neutrophils.

Opsonization refers to the coating of the bacterial cell with host-derived proteins such as LPS

binding protein (LBP), specific antibody, or the complement component iC3b. Opsonization with

specific antibody of the Ig G subclass is required for phagocytosis of certain bacteria, such as

Actinobacillus actinomycetemcomitans.


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Engulfment stage

Opsonized particle bound to the surface of phagocyte

Formation of cytoplasmic pseudopodes around it due toactivation of actin filament beneath cell wall

Phagocytic vacuole

Phagolysosome

+ lysosome of cell cytoplasm


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Preformed granules / stored products of PMNs are discharged or

secreted into phagosome ( or extracellular environment).

Neutrophils contain three types of granules:1. Specific or secondary granules:

Both extracellulur and intra cellular secretion.

Eg: lactoferrin, lysozyme, alkaline phosphatase and collagenase

2. Azurophil granules:Intra phagolysosomal secretion.

Eg: myeloperoxidase, acid hydrolases and neutral proteases such

as elastase, collagenase and proteinase

3. Tertiary granules: gelatinase , cathepsin

Degranulation stage


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Killing and degradation stage

Once microbe is ingested, it is killed by two broad

categories of killing mechanism:

a) Non-oxidative mechanism: Neutrophild do not requireoxygen for energy can fusion under anaerobic condition.

b) Oxidative mechanism: Based on the reduction of

oxygen

NON OXIDATIVE KILLING


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NON- OXIDATIVE KILLING

Phagosome + lysosome

phagolysosome

Secretion of lysosomal components into the phagolysosome

In less than 30 seconds neutrophil secretes specific granules(

lysosomes, lactoferrin, etc..)

After secretion of specific granule it secrete azurophil granule

among the microbiciadal compounds are small antimicrobial peptides

compounds are:

Defensins : Defensins are small cationic amphipathic, arginine- and

cysteine-rich peptides composed of 29 to 38 amino acids. They make

up 5-7% of the total protein and 30-50% of the azurophil granule

content of human neutrophils.


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Defensins are membrane-permeabilizing molecules that can kill a

variety of gram negative bacteria, gram-positive bacteria, fungi and

eukaryotic cellscathepsin G: Cathepsin G is a neutral serine protease found in the

granules of neutrophils. Cathepsin G has proteolytic and esterolytic

activities resembling that of chymotrypsin. have found that

cathepsin G is the most potent nonoxidative

antimicrobial agent against periodontal pathogens found in the

human neutrophil.

Calprotectin: Calprotectin exerts microbiostatic activity against

fungi, including Candida albicans and bacteria, including

Capnocytophuga sputigenaserprocidins ( elastase, proteinase 3, azurocidin)


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OXIDATIVE KILLING:

Based on reduction of oxygen.

Requires: 1) presence of oxygen

2) an oxidation-reduction potential In particular neutrophil exert intense microbicidal activity by

forming toxic, reduced oxygen metabolites such as superoxide

anion using NADPH oxidase system.

Superoxide anion also contribute to formation of H2O2, which

is capable diffusing inside the membrane.

A phase of increased oxygen consumption(respiratory burst)

by activated neutrophil requires the essential presence of

NADPH oxidase.


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Oxygen molecule

Superoxide ion

NADPH NADP + hydrogen ion

NADPH oxidase

Bactericidal properties carried out by:

1) Myeloperoxidase(MPO) dependent

pathway

2) Myeloperoxidase independent pathway


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1) MPO Dependent killing

2) MPO Independent killingHydroxyl

radical

Hydroxyl radical

NADPH oxidase deficiency

Chronic granulomatous disease

Inconsistently associated with

aggressive periodontal (AP)

disease

Oxidative microbicidal mechanisms

are of some importance in PDL

infection

Hypochlorous acid


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Role of Macrophages in Phagocytosis

The role of mononuclear phagocytes (monocytes in blood,

macrophages in tissues) are closely related to neutrophils.

Role of macrophages vary within and between the different tissue

compartments. In general, macrophages follow two divergent

pathways of functioning.

Some macrophages function primarily as phagocytes, engulfing

and destroying foreign substances including microorganisms and

particulate insoluble agents

Examples of these macrophages include alveolar macrophages in

the lungs and peritoneal macrophages in the abdomen.


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Other macrophages reside in the tissues, where they

ingest foreign substances (antigens) and then present

the antigens on the macrophage cell surface after

processing. The combination of the antigen with appropriate

histocompatibility molecules enables T-lymphocyte

recognition, stimulating the release of cytokines that

amplify the specific immune response.


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In addition to processing antigens for activation of the

specific immune system, macrophages within the

tissues secrete a number of cytokines in response to

antigens and other agents associated withmicroorganisms.

These cytokines have several functions, including

amplifying the specific immune system, inducing and

amplifymg inflammation and stimulating tissue

breakdown.


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Tissue breakdown can occur either directly by

macrophage-secreted enzymes or indirectly by

stimulating enzymes in cells such as fibroblasts

(collagenase). In addition, the macrophage produces cytokines such

as interleukin (IL-1) and prostaglandin E2 that can

promote breakdown of bone by stimulating

osteoclasts. Hence, although macrophages have a

critical role in the overall immune response, they play

a somewhat secondary role in the acute response.


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Fig. Mononuclear phagocytes (monocytes in blood, macrophages in tissues) are closely related to

neutrophils. Their activities are similar, and they exhibit chemotaxis (A), phagocytosis (B), and killing

(C), similar to neutrophils. Mononuclear phagocytes are particularly adept at processing and presenting

antigen to T-cells (D), a process that may require more than 20 hours. That antigen is presented in

association with MHC Class II molecules along with a costimulatory signal. Mononuclear phagocytes

also release cytokines (E) that direct lymphocyte differentiation.


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Major histocompatibility complex (MHC)/ human leukocyte

antigen complex (HLA)

All cells of the body possess some form of glycoproteins

unique to an individual. These glycoproteins are known as major

histocompatibility complex (MHC) Proteins.

The major histocompatibility complex (MHC) is a locus on

the short arm of chromosome of that encodes a number of

molecules, including MHC classes I, II and III molecules, whichare involved with antigen uptake, processing and presentation.

There are two major classes of MHC: MHC class I, MHC

class II, MHC class I glycoproteins are found on the surfaces of

all nucleated somatic cells. except (RBCs). MHC class IIglycoproteins are found only on certain cells including B-

lymphocytes, T- lymphocytes, macrophages and macrophage like

cells that present antigens to T-cells.


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The processing and presenting of antigen by the macrophage to T

cell require that both the cells possess surface determinants coded

for the same major histocompatibility complex (MHC) genes. The

T cell can accept the processed antigen only if it is presented by a

macrophage carrying on its surface the self -MHC antigens. Cytotoxic T lymphocytes are able to kill and lysis virus infected

target cells only when the T cells and target cells are of the MHC

type, so that the T-cells can recognize the class I MHC antigens

on the target cells. Helper T cells can accept antigen presented bymacrophages only when the macrophages bear the same class II

MHC molecules on the surface.


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role of phagocytic cells in periodontal health

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