Lecture on Antigen processing and presentation pathways

Describes the pathways for antigen processing and presentation - The cytosolic and Endocytic pathways

1. ANTIGEN PROCESSING AND PRESENTATION PATHWAYS
Dr.Manikandan Kathirvel M.Sc., Ph.D., (NET)
Assistant Professor,
Department of Life Sciences,
Kristu Jayanti College (Autonomous),
(Reaccredited with "A" Grade by NAAC)
Affiliated to Bengaluru North University,
K. Narayanapura, Kothanur(PO)
Bengaluru
Email: [email protected]
ORCID ID: 0000000270066334

2. ANTIGEN PROCESSING AND PRESENTATION:
a.TherecognitionofforeignantigenbyaTcellrequiresthatpeptidesderivedfromthe
antigenbedisplayedwithinthecleftofanMHCmoleculeonthemembraneofacell.
b.Theformationofthesepeptide-MHCcomplexesrequiresthataproteinantigenbe
degradedintopeptidesbyasequenceofeventscalledantigenprocessing.
c.ThedegradedpeptidesthenassociatewithMHCmoleculeswithinthecellinterior,and
thepeptideMHCcomplexesaretransportedtothemembrane,wheretheyare
displayedtoTcells(antigenpresentation)forTcellactivationanddifferentiation.

3. •ClassIandclassIIMHCmoleculesassociatewithpeptidesthathavebeenprocessedin
differentintracellularcompartments.
•ClassIMHCmoleculesbindpeptidesderivedfromendogenousantigensthathavebeen
processedwithinthecytoplasmofthecell–CytosolicPathway(e.g.,normalcellular
proteins,tumorproteins,orviralandbacterialproteinsproducedwithininfectedcells).
•ClassIIMHCmoleculesbindpeptidesderivedfromexogenousantigensthatare
internalizedbyphagocytosisorendocytosisandprocessedwithintheendocyticpathway.
“AllnucleatedCellscanpresentantigenwithClassIMHC;
PresentationwithClassIIMHCisrestrictedonlytoantigenpresentingcells(APCs)”
•CellsexpressingeitherclassIorclassIIMHCmoleculescanpresentantigenicpeptidestoT
C
andT
Hcells.
•CellsthatdisplaypeptidesassociatedwithclassIMHCmoleculestoCD8
+
T
Ccellsand
•CellsthatdisplaypeptidesassociatedwithclassIIMHCmoleculestoCD4
+
T
Hcellsare
calledantigen-presentingcells(APCs).

4. Threecelltypesareclassifiedasprofessionalantigen-presentingcells:dendriticcells,
macrophages,andBlymphocytes.Thesecellsdifferfromeachotherintheirmechanisms
ofantigenuptake,inwhethertheyconstitutivelyexpressclassIIMHCmolecules,andin
theirco-stimulatoryactivity.

5. ANTIGENPROCESSINGANDPRESENTATIONPATHWAYS:
Theimmunesystemusestwodifferentpathwaystoeliminateintracellularandextracellular
antigens.
Endogenousantigens(thosegeneratedwithinthecell)areprocessedinthecytosolic
pathwayandpresentedonthemembranewithclassIMHCmolecules;
Exogenousantigens(thosetakenupbyendocytosis)areprocessedintheendocyticpathway
andpresentedonthemembranewithclassIIMHCmolecules.

6. A.Endogenous Antigens:The
CytosolicPathway
B.Exogenous Antigens:The
EndocyticPathway

7. A. Endogenous Antigens:The
CytosolicPathway

8. A.Endogenous Antigens:TheCytosolic
Pathway
Thepathwaybywhichendogenousantigensare
degradedforpresentationwithclassIMHC
moleculesutilizesthesamepathwaysinvolvedin
thenormalturnoverofintracellularproteins.
I)Peptidesforpresentationaregeneratedby
ProteaseComplexesCalledProteasomes:
1.Intracellularproteinsaredegradedintoshort
peptidesbyacytosolicproteolyticsystem
presentinallcells.Proteinstobedegraded
(proteinstargetedforproteolysis)areoften
covalentlylinkedtoasmallproteincalled
ubiquitin,attachedtothem.
2.Ubiquitin-proteinconjugatescanbedegraded
byamultifunctionalproteasecomplexcalleda
proteasome.Eachproteasomeisalarge
(26S),cylindricalparticleconsistingoffour
ringsofproteinsubunitswithacentralchannel
ofdiameter10–50Å.
3.Aproteasomecancleavepeptidebonds
between2or3differentaminoacid
combinationsinanATP-dependentprocess.
4.Degradationofubiquitin-proteincomplexes
occurswithinthecentralhollowofthe
proteasome.

9. II). Peptides Are Transported from the Cytosolto
the Rough Endoplasmic Reticulum:
1.Thetransporterprotein,designatedTAP(for
transporterassociatedwithantigenprocessing)is
amembrane-spanningheterodimerconsistingof
twoproteins:TAP1andTAP2.
2.TheTAP1andTAP2proteins,eachhaveadomain
projectingintothelumenoftherough
endoplasmicreticulum(RER)andanATP-binding
domainthatprojectsintothecytosol.
3.(BothTAP1andTAP2belongtothefamilyofATP-
bindingcassetteproteinsfoundinthemembranesof
manycells,includingbacteria;theseproteinsmediate
ATP-dependenttransportofaminoacids,sugars,ions,
andpeptides).
4.Peptidesgeneratedinthecytosolbythe
proteasomearetranslocatedbyTAPintotheRER
byaprocessthatrequiresthehydrolysisofATP.
5.TAPhasthehighestaffinityforpeptides
containing8–10aminoacids,whichisthe
optimalpeptidelengthforclassIMHCbinding.
6.Thus,TAPisoptimizedtotransportpeptidesthat
willinteractwithclassIMHCmolecules.
Generation of antigenic peptide–class I MHC complexes in the
cytosolicpathway

10. III).PeptidesAssemblewithClassIMHCAidedby
ChaperoneMolecules
1.Theαchainandβ2-microglobulincomponentsofthe
classIMHCmoleculearesynthesizedonpolysomes
alongtheroughendoplasmicreticulum.
2.AssemblyofthesecomponentsintoastableclassIMHC
molecularcomplexinvolvesseveralstepswhichincludes
theparticipationofmolecularchaperones,which
facilitatethefoldingofpolypeptides.
3.ThefirstmolecularchaperoneinvolvedinclassIMHC
assemblyiscalnexin,aresidentmembraneproteinof
theendoplasmicreticulum.
4.CalnexinassociateswiththefreeclassIαchainand
promotesitsfolding.Whenβ2-microglobulinbindsto
theαchain,calnexinisreleasedandtheclassImolecule
associateswiththechaperonecalreticulinandwith
tapasin.
5.Tapasin(TAP-associatedprotein)bringstheTAP
transporterintoproximitywiththeclassImoleculeand
allowsittoacquireanantigenicpeptide.Thephysical
associationoftheαchainandβ2-microglobulin
heterodimerwiththeTAPproteinpromotespeptide
capturebytheclassImolecule.Peptidesthatarenot
boundbyclassImoleculesarerapidlydegraded.
6.Onceapeptidehasbound,thepeptide–classIcomplex
istransportedtotheplasmamembraneandpresentsit
toT
Ccells(cytotoxicTcells).

11. B.ExogenousAntigens:TheEndocytic
Pathway:

12. B.ExogenousAntigens:TheEndocyticPathway:
1.Antigen-presentingcellscaninternalizeantigenby
phagocytosis,endocytosis,orboth.
2.Macrophagesinternalizeantigenbybothprocesses,
whereasmostotherAPCsarenotphagocyticorare
poorlyphagocyticandthereforeinternalize
exogenousantigenonlybyendocytosis(either
receptormediatedendocytosisorpinocytosis).
3.Bcells,forexample,internalizeantigenvery
effectivelybyreceptor-mediatedendocytosisusing
antigen-specificmembraneantibodyasthe
receptor.

13. I).PeptidesAreGeneratedfromInternalized
MoleculesinEndocyticVesicles
1.Onceanantigenisinternalized,itis
degradedintopeptideswithin
compartmentsoftheendocyticprocessing
pathway.
2.Theendocyticpathwayappearstoinvolve
threeincreasinglyacidiccompartments:
earlyendosomes(pH6.0–6.5);late
endosomes,orendolysosomes(pH5.0–
6.0)andlysosomes(pH4.5–5.0).
Internalizedantigenmovesfromearlyto
lateendosomesandfinallytolysosomes,
encounteringhydrolyticenzymesanda
lowerpHineachcompartment.
3.(Lysosomescontainauniquecollectionof
morethan40acid-dependenthydrolases,
includingproteases,nucleases,
glycosidases,lipases,phospholipases,and
phosphatases).
4.Withinthecompartmentsoftheendocytic
pathway,antigenisdegradedinto
oligopeptidesofabout13-18residues,
whichbindtoclassIIMHCmolecules.
Generation of antigenic peptides in the
endocyticprocessing pathway

14. II).TheInvariantChainGuides
TransportofClassIIMHCMolecules
toEndocyticVesicles
1.WhenclassIIMHCmoleculeare
synthesizedwithintheRER,three
pairsofclassIIαβchainsassociate
withapreassembledtrimerofa
proteincalledinvariantchain(Ii,
CD74).
2.Thistrimericproteininteractswith
thepeptide-bindingcleftoftheclass
IImolecules,preventingany
endogenouslyderivedpeptidesfrom
bindingtothecleftwhiletheclassII
moleculeiswithintheRER.

15. III).PeptidesAssemblewithClassIIMHC
MoleculesbyDisplacingCLIP
1.ClassIIMHC–invariantchaincomplexesare
transportedfromtheRER,throughtheGolgi
complexandtrans-Golginetwork,andthen
throughtheendocyticpathway,movingfrom
earlyendosomestolateendosomes,andfinally
tolysosomes.Astheproteolyticactivity
increasesineachsuccessivecompartment,the
invariantchainisgraduallydegraded.
2.However,ashortfragmentoftheinvariant
chaintermedCLIP(forclassII–associated
invariantchainpeptide)remainsboundtothe
classIImoleculeaftertheinvariantchainhas
beencleavedwithintheendosomal
compartment.
3.CLIPphysicallyoccupiesthepeptide-binding
grooveoftheclassIIMHCmolecule,
presumablypreventinganyprematurebinding
ofantigenicpeptide.
4.ClassIIMHCmoleculecalledHLA-DMis
requiredtocatalyzetheexchangeofCLIPwith
antigenicpeptides,inthepresenceofHLA-DO.
5.Onceapeptidehasbound,thepeptide–classII
complexistransportedtotheplasma
membraneandpresentsittoT
Hcells(T-helper
cells).

16. Antigen-presentingpathways
utilizedforendogenous
(green)andexogenous(red)
antigens.
Themodeofantigenentry
intocellsandthesiteof
antigenprocessingdetermine
whetherantigenicpeptides
associatewithclassIMHC
moleculesintherough
endoplasmicreticulumor
withclassIImoleculesin
endocyticcompartments.