Role of Phagocytic Cells in Periodontal Health
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Transcript of 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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