Pathology, Lecture 9 (Lecture Notes)

8/8/2019 Pathology, Lecture 9 (Lecture Notes)

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Phagocytosis & Chemical Mediators

Ismaeel Matalqa

Mohammad Abu-Assi

17/10/2010


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Sunday, Oct be

17, 2010Path

l

ylecturebyDr.IsmaeelMatalqaChapter tw ; Inflammati n

Doneby:Mohammad Abu-Assi

***************************************************

Phagocytosis & ChemicalMediators

Some notes aboutthis lecture:1. Keyconcepts arewritten BOLD.2.Additional notes fromthe doctor and myown

clarifications arewritten in italics.3. Sometimes the doctorwas showing a picture or a figure onthe slide, and hewas just readingwhatiswritten on them,so these figures areincluded in this summary.

The doctor declared two announcementsbeforehe startedthelecture:

1. Regarding studentswho did not register forpathologylab,their names are on the pathologydepartment,and these students should registerbeforetheyexceed theallowed number of absence, otherwisetheywillbeconsidered failed in thetheoretical subject.

2. Regarding the firstexam,itwillbe on Saturday,October 30, 2010, and allthelecturesbeforethat date areincluded in the firstexam,which are 6 lectures for cellinjury,and other 6 for inflammation. The doctor ensured thateachstudent shouldbecommittedwiththeinformation senttohim

PHAGOCYTOSIS

Phagocytosisis the process ofingestion and digestionbycells of solid substances (other cells,bacteria,microbes, necrotictissue or foreign material), so necrotictissue or foreign materialwillbeengulfed (phagocytoted),then itwillbe degraded ordigestedbe phagocyticcellsbyseveralmechanisms.


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There are steps of phagocytosis 1. Recognition of the foreign material, necrotic tissue,

microbes, etc. by attachment and binding to cellularreceptors.There are cellular receptors at the surface of theleukocytes

hich

ill bind to these foreign bodies

hich are

usually coated by circulating proteins, they are calledOpsonins such as IgG (immunoglobulin G), C3b (acomplement protein) and collectins.The process of proteincoating to foreign bodies is called Opsonin

iz

ion.

Circul

ting proteins (opsonins)coat the foreign materialtodefine it as foreign and make it recognizable for phagocyticcells, which then identify it, bind to it by itscellular receptorsandstart the process of phagocytosis.

2.Engulfment3. Fusion of phagocytic acuoles ith lysosomes4. Killing or degradation of ingested material (iNO

and RO )

Recognition and attachment by receptors Mannose receptors: bind to terminal mannose residues onmicrobes cell

alls.

They are present at the surface ofleukocytes, and they are anadditionalmechanism to recognize microbes. (They do not bind tocoating proteins but to specific molecule,i.e. mannose, which is

present only issome microbes and absent in mammalian cells)Mammalian cells are not recognized by mannose receptors

because they contain terminal sialic acid and N-acetylgalactosamine. So they are not recognized by these types ofreceptors

hich are present on the leukocytes.

Other receptors are:


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Scavengerreceptors: oxidi

ed LDL, and microbes. Opsoninreceptors (high affinity): IgG, C3b,

collectins

Summaryfor the steps of phagocytosis:

Themicrobes are firstcoatedwith opsonins (whichhave a highaffinityforbinding to microbes),then theleu

ocytes recogniethe

coated microbes andbind to itbyopsonin receptors on theirsurface,engulfmentthen occurs,the plasma membrane ofthephagocyte zips up around themicrobe. After invagination,thephagosome (theendocytoticvacuolecontaining themicrobe)willfusewithlysosomes to formthephagolysosome(whichcontainsbothmicrobe and lysosomalenzymes). Oncethemicrobe and theenzymes aretogether,theystimulate several degradation andbacteriocidal (bacterial-

illing)mechanisms. Mostimportantmechanisms aretheiNOS and ROS. Stimulation ofiNOS(inducibleNitric Oxide Synthase)willlead to the activation ofNO(Nitric Oxide). Stimulation of phagocyte oxidasewilllead to theactivation of ROS (Reactive Oxygen Species). Thesetwomechanisms participatein the

illing ofmicrobes in addition tolysosomalenzymes. In manycases thesemechanisms are not reallyadequateto

illthebacteria. So thesemechanisms are usuallyassociatedwith other mechanisms (wewillcome on themlater)tocompletely

ill and degradethemicrobe.

Bacteriocidal agents: agentsthatkillthe bacteria.

Baceriostatic agents: agentsthatsto

thegro

tho

thebacteria.

This figure (next page) summarizes the steps:Note differenttypes of receptors.


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The generation of the o ygen metabolites is the mostimportant step in the degradation of microbes.This process isintiated by a trigger,

!

hich is usually an infection, thats!

hy!

esee increase in o

ygen metabolites during infections especially

microbial. So during infection, o

ygen consumption is increasedbecause of the increased acti" ity ofNAD

# $

oxidase,%

hichconverts O2 to O2-(superoxide ion) and H+ &

Under the effect ofDismutase, O2- reacts % ith H+ to formhydrogen pero ' ide H2O2; % hich is one of the bacteriocidal agents:

If halides are available, like Cl-, % hich is present % ithin thecells and in theirvicinity, hydrogen pero ' ide

%ill react

%ith them

forming hydrochlorous radical HO(

l-


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This reaction is catalyzed be the enzymeMyeloperoxidase(MPO), this enzyme is the cause of the color of eosinophericgranules present inneutrophils. Hydrochlorous radical HOCl- isthe most efficient bacteriocidal agent in the cell. HOCl- is used alsoin industry, in manufacturing the chemical compounds of cleaning

and sterilization(Hype)

, Flash.)

A summary of the generation of o)ygen metabolites:

Ma 3alena,,

Howdo leukocytes kill infectious agents??1. Oxygen burst products, 0 hich is 0 hat 0 e havejust discussed

(oxygen metabolites)2. Bacteriocidal permeability increasing protein, 0 hich are

produced by the neutrophils.3. 1 ysozyme.4. Major basic protein, 0 hich is a specific protein produced by

the granules 0 ithin the eosinophils.These kinds of proteinsspecifically attach to parasites, and thats 0 hy 0 hen you have

a parasitic infection, you

0

ill have a larger thannormalnumber of eosinophils.5. 2 efensins.6. 1 actoferrin.

*** In alltypes of microbialinfections we see an increasednumber of WBCs generally, but each type of infection ischaracterized by a large increase in one of the WBCs, forexample:


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Bacterialin3

ection -> largenumbero3

neutro4 hilsParasiticin

3


eosino4 hilsViralin

3


lymphocytes

Anydefect or abnormalityin anyoftheseenzymes, receptorproteins or steps of phagocytosis,mightlead to serious diseases.For example:

1. PatientswithLeukocyteA5 hesion Deficiency16

LAD1

7

disease, usually, suffer from recurrentinfections in thelungs, s

8

in, uterine,etc. Thecause ofthis diseaseis adefectin CD18 subunit ofintegrin.

2. In theLAD 2 disease,whichhas the presentationbutmore or less in a milder form,the deficiencyis in Sialyl-Lewis X,whichis a receptor found on the surface oftheleu

9ocytes.

3. In neutrophil-specificgranule deficiencythe defectis absence of specific granules, andwehave justmentioned theimportance of neutrophilic granules andtheir rolein secretion somecriticalcompounds like theenzymeMyeloperoxidase.

4. ChronicGranulomatous Disease @ CGDA has twotypes; X-liked CGD and autosomal recessive CGD.Themain cause ofbothtypes is a defectin NADPHoxidase,butin X-liked CGD the defectis in themembranous NADPH oxidase,whilein autosomal

recessive CGD the defectis in thecytoplasmicNADPHoxidase. The defectin NADPH oxidaseleads to decreasednumber of oxygen metabolites,butthebodypartiallyovercomes this problembyrecruiting morehistocytes(macrophages), so the problemis partiallysolved. Suchcondition ofthehistocyteswilllead to the formation ofgranulomas (collection of activated macrophages),and thatswhywecallitchronic granulomatous disease.

***Xchromosomeisthesexualchromosomethatcarries

genes(Ychromosomealsocarriessomegenes butmuchlessthan Xsoitisconsideredto beemptyo

B

genes)***Autosomalchromosomesarethe 22 somaticchromosomes

5.MyeloperoC idaseDMPO

E

deficiencyleads to theabsence ofMPO-H2O2 system.

6. Chediak-Higashi diseaseis causedbecause of adeficiencyin organelletrafficking. You sawthat one ofthe


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steps of phagocytosis is the fusion oflysosomes andphagosomes, suchmovementis called organelletrafficking. So ifthereis a deficiencyin organelletrafficking,the fusionwill not occur.

These defects areeither inherited orbecause of a mutation ina specific gene.

Q) Whatis the differencebetween histocytes and macrophages??A)Monocytes,histocytes and macrophages are allthe same,butwegivethem different names according to the site of action,iftheyarein thebloodwecallthemmonocytes,iftheyarein thetissueswecallthemmacrophages or histocytes.

In addition to the previous diseases,wehave also someacquired diseases. These diseases are not related to genes,butweacquirethem during our life.

Acquired defects in leukocyte functions:Chemotaxis defects

Burns, diabetes, sepsis,etc.Adhesion

Hemodialysis, diabetesPhgocytosis and microbicidal activity

Leukemia, sepsis, diabetes,malnutrition,etc

CHEMICAL MEDIATORS

There aretwo sources for chemicalmediators;circulatingplasma proteins and cells. There are differenttypes ofcirculatingplasma proteinswhich formtogether different systems,mostimportant systems are:

x Coagulation / fibrinolytic system (i.e. coagulation cascade)x Complement systemx KininsCell derived mediators are allelements normallysequestered

in granules, so theyare alreadyformed and storedwithin thesegranuleswhich are presentin thecytoplasm. Theclassicalexampleofthesemediators is Vasoactiveamines. The other type ofcellderived mediators is the newlysynthesized mediators in responseto stimulation, such as PGs (prostaglandins), LT (leukotrienes), O2species,NO (nitrous oxide), Cytokines and PAF (platelet-activating


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factor).( So allthesemediators are not formed unless thereisstimulation for their synthesis ). Ifwehavethesemediatorscontinuouslysynthesized,thiswilllead to harmfuleffects on thetissue. After thesemediators finishtheir function, some antibodieswilleliminate or inactivatethem; otherwise, serious effects to the

tissuemighthappen.

The following figureis averyimportant figurewhichthedoctor ensured the students to studyitwell and understand it;otherwiseitwillbe difficultto understand at anystagelater.

There are generalcharacteristics ofchemicalmediators ofinflammation;you need to understand themwelltobe abletounderstand the steps ofinflammation later in this chapter. Thesegeneralcharacteristics are:

Theybind to specificcellular receptors, or haveenzymaticactivitywhichcanbe used to stimulate other cells torespond. So this is theirwayof action.

Theymaystimulatetargetcells to release secondarymediatorswith similar or opposing functions, for example,ifthereis a mediator,e.g. cytokine A, releasedbymacrophages,itwillinduce neutrophils to producecytokineBwhichincrease and exaggerate anti-inflammatoryreactions, so ithas the same function as cytokine A.Cytokine A also induces neutrophils to producecytokine C,whichhas exactlythe opposing function ofcytokine A.

Mayhavelimited targets, orwide spread activities Shortlived function,because of:


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x Shorthalf-life (Arachedonic acid metabolites)x Inactivatedbyenzymes (kininases inactivate

bradykinins)x Eliminated (antioxidants eliminate O2 species

which are a type ofcell derived mediators)x

Inhibited (complementinhibitoryproteins) Ifthesechemicalmediators remain unchecked, unopposedor activated,theywillcauseharmfuleffects. So chemicalmediators havebeneficial and harmfuleffects on tissues,and thebodys taskis to maintain thebalancebetweenbeneficial and harmfuleffects.

VasoactiveAmines, aswe said, arethemostimportantclass ofcell derived chemicalmediators. Themain two types ofvasoactive amines areserotonin and histamine. Release ofhistaminecanbeinitiated in responseto manystimuli,like:

PhysicalinjuryBinding of IgEto Fc receptors. In allergy(hypersensitivity),

IgEbinding to Fc receptors is stimulated. This,in turn,induces mastcells to secretelarge amounts ofhistamine. Intreatment of allergy, patients are administered anti-histamine drugs,which areeither histamine receptorblockers, mastcell stabilizerswhich are sprays (nasalsprays)that stabilizemastcells, or steroids.

Anaphylatoxins (C3a, C5a)bindingHistamine releasing proteins derived from PMNs***PMNsstands for Polymorphonuclear Neutrophils,anothername forneutrophils.

Neuropeptides (substance P)Cytokines (IL-1, IL-8)

The release of serotonin is mainlyactivatedby:Platelets aggregationPAF

THATS ALL FOLKS

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Pathology, Lecture 9 (Lecture Notes)