Antigen Processing and Presentation, MHC Restriction and Role of Thymus Antigen Processing and Presentation, MHC Restriction and Role of Thymus
II.. CCoommppaarriissoon n oof f BBCCR R aannd d TTCCRR
B cells and T cells recognize different substances as antigens and in a different form. The B cell B cells and T cells recognize different substances as antigens and in a different form. The B cell uses cell surface-bound immunoglobulin as a receptor and the specificity of that receptor is the uses cell surface-bound immunoglobulin as a receptor and the specificity of that receptor is the same as the immunoglobulin that it is able to secrete after activation. B cells recognize the same as the immunoglobulin that it is able to secrete after activation. B cells recognize the fo
follllowowining g anantitigegens ns in in sosolulublble e foformrm: : 1) 1) prprototeieins ns (b(bototh h coconfnforormamatitiononal al dedetetermrmininanants ts anandd determinants exposed by denaturation or proteolysis); 2) nucleic acids; 3) polysaccharides; 4) determinants exposed by denaturation or proteolysis); 2) nucleic acids; 3) polysaccharides; 4) some lipids; 5) small chemicals (haptens).
some lipids; 5) small chemicals (haptens).
In contrast, the overwhelming majority of antigens for T cells are proteins, and these must be In contrast, the overwhelming majority of antigens for T cells are proteins, and these must be fr
fragmagmentented ed and and recrecognognizeized d in in assassociociatiation on witwith h MHMHC C proproducducts ts expexpresressed sed on on the the sursurfacface e of of nucleated cells, not in soluble form. T cells are grouped functionally according to the class of nucleated cells, not in soluble form. T cells are grouped functionally according to the class of MHC molecules that associate with the peptide fragments of protein: helper T cells recognize MHC molecules that associate with the peptide fragments of protein: helper T cells recognize only those peptides associated with class II MHC molecules, and cytotoxic T cells recognize only only those peptides associated with class II MHC molecules, and cytotoxic T cells recognize only those peptides associated with class I
those peptides associated with class I MHC molecules.MHC molecules. II
II.. AnAntitiggeen Pn Prrococesesssining ag and nd PPrreseseenntatatitionon An
Antigtigen en proprocescessinsing g rereferfers s to to the abilithe ability ty of of and APCs and APCs to to brebreak ak dowdown n a a proproteitein n antantigeigen n inintoto pept
peptides and to assocides and to associate thosiate those e peptpeptides with MHC molides with MHC moleculeecules. s. AntiAntigen presegen presentatntation is theion is the process of displaying peptide antigens associated with MHC molecules to a T cell. The path process of displaying peptide antigens associated with MHC molecules to a T cell. The path leading to the association of
leading to the association of protein fragments with MHC molecules differs for class I and protein fragments with MHC molecules differs for class I and class IIclass II MH
MHC. C. MHMHC C clclasass s I I momolelecuculeles s prpresesenent t dedegrgradadatatioion n prprododucucts ts dedeririveved d frfrom om inintrtracacelellululalarr (endogenous) proteins in the cytosol. MHC class II molecules present fragments derived from (endogenous) proteins in the cytosol. MHC class II molecules present fragments derived from extracellular (exogeno
extracellular (exogenous) proteins that us) proteins that are located are located in an in an intracellulaintracellular compartment.r compartment. A.
A. ClasClass I MHC Ps I MHC Pathwathway - Aay - All nucll nucleateleated cells d cells exprexpress cess class I Mlass I MHC. As sHC. As shown hown in Figin Figure 1, ure 1, protproteinseins are fragmented in the cytosol by proteosomes (a complex of proteins having proteolytic are fragmented in the cytosol by proteosomes (a complex of proteins having proteolytic activity) or by other proteases. The
activity) or by other proteases. The fragments are then transported across the membrane of fragments are then transported across the membrane of the endoplasmic reticulum by transporter
the endoplasmic reticulum by transporter
Figure 1.
Antige
Antigen n ProcessiProcessing ng and and PresentaPresentationtion- Class I MHC Pathway- Class I MHC Pathway proteins. (The transporter proteins andproteins. (The transporter proteins and some components of the proteosome are encoded by genes in
some components of the proteosome are encoded by genes in the MHC complex). Synthesis andthe MHC complex). Synthesis and assembly of class I heavy chain and β2- microglobulin occurs in the endoplasmic reticulum. assembly of class I heavy chain and β2- microglobulin occurs in the endoplasmic reticulum. Within the endoplasmic reticulum, the MHC class I heavy chain, β2-microglobulin and peptide Within the endoplasmic reticulum, the MHC class I heavy chain, β2-microglobulin and peptide form a stable complex that is transported to the
form a stable complex that is transported to the cell surface.cell surface. B.
B. Class II MHC pathwayClass II MHC pathway - Whereas all nucleated cells express class I MHC, only a limited- Whereas all nucleated cells express class I MHC, only a limited group of cells express class II MHC, which includes the antigen presenting cells (APC). The group of cells express class II MHC, which includes the antigen presenting cells (APC). The principal APC are macrophages, dendritic cells (Langerhans cells), and B cells, and the principal APC are macrophages, dendritic cells (Langerhans cells), and B cells, and the exp
expresressiosion n of of claclass ss II II MHMHC C momolecleculeules s is is eiteither her conconstistituttutive ive or or indinducuciblible, e, espespecieciallally y byby interferon-g
interferon-gamma in the case of macamma in the case of macrophages. rophages. As shown in FiguAs shown in Figure 2, exogenous prore 2, exogenous proteinsteins taken in by endocytosis are fragmented by proteases in an endosome. The alpha and beta taken in by endocytosis are fragmented by proteases in an endosome. The alpha and beta chains of MHC class II, along with an invariant chain, are synthesized and assembled in the chains of MHC class II, along with an invariant chain, are synthesized and assembled in the endoplasm
endoplasmic reticulum. ic reticulum. The invariant chain prevThe invariant chain prevents endogenous peptients endogenous peptides from the cytosoldes from the cytosol fr
from assoom associaciatinting g witwith h claclass II ss II MHMHC C momolecleculeules. s. The claThe class II ss II MHMHC C molmolecuecules witles with h thethe associated invariant chain are transported through the Golgi and trans-Golgi apparatus to associated invariant chain are transported through the Golgi and trans-Golgi apparatus to reach the endosome, where the
reach the endosome, where the invariant chain is digested, and the invariant chain is digested, and the peptide fragments frompeptide fragments from the exogenous protein are able to associate with the class II MHC molecules, which are the exogenous protein are able to associate with the class II MHC molecules, which are finally transport
finally transported to ed to the cell surface.the cell surface.
Figure 2.
Figure 2. Antigen Processing and Antigen Processing and Presentation - Class Presentation - Class II MHC II MHC PathwayPathway C.
C. OthOther poier points cnts conconcernerning aning antigtigen proen procecessssing aning and presd presententatiationon 1.
1. One way of ratioOne way of rationaliznalizing why two diffing why two differenerent antigen prot antigen processcessing pathwing pathways deveays developed is to sayloped is to say that each ultimately stimulates the population of T cells that is most effective in
that each ultimately stimulates the population of T cells that is most effective in eliminating aeliminating a particular type of antigen. Viruses replicate within nucleated cells in the cytosol and produce particular type of antigen. Viruses replicate within nucleated cells in the cytosol and produce
endogenous antigens that can associate with class I MHC. By killing these infected cells, endogenous antigens that can associate with class I MHC. By killing these infected cells, cytotoxic T cells help to control the spread of the virus. Bacteria mainly reside and replicate cytotoxic T cells help to control the spread of the virus. Bacteria mainly reside and replicate extracellularly. By being taken up and fragmented inside cells as exogenous antigens that extracellularly. By being taken up and fragmented inside cells as exogenous antigens that can associate with class II MHC molecules, helper Th2 T cells can be
can associate with class II MHC molecules, helper Th2 T cells can be activated to assist B cellsactivated to assist B cells to
to mamake ke antantiboibody dy agaagainsinst t bacbacterteria, whicia, which h limlimits the its the grogrowth of wth of thethese se ororganganismisms. s. SomSomee bacteria grow intracellularly inside the vesicles of cells
bacteria grow intracellularly inside the vesicles of cells like macrophageslike macrophages. Inflammatory Th1 T. Inflammatory Th1 T cells help to
cells help to activate macrophages to kill the activate macrophages to kill the intracellular bacteria.intracellular bacteria. 2.
2. FragFragmentments of s of self, as well as non-sself, as well as non-self, proteelf, proteins assoins associatciate e with MHC molwith MHC moleculecules of both classeses of both classes and are expr
and are expressessed at ed at ththe e celcell l sursurfacface. e. T T celcells capals capable of ble of recrecognogniziizing self peptng self peptideides s areare eliminated during development (Clonal Selection Postulate)
eliminated during development (Clonal Selection Postulate) 3.
3. WhWhich protich protein fraein fragmegments bind is a nts bind is a fufunctnction of ion of the chemthe chemicaical l natnature of ure of the groothe groove for ve for thathatt specific MHC molecule
specific MHC molecule
II.. SSeellf f MMHHC C RReessttrriiccttiioonn
In order for a T
In order for a T cell to recognize and respond tocell to recognize and respond to a foreign protein antigen, it must recognize the a foreign protein antigen, it must recognize the MHC on
MHC on the presentithe presenting cell as ng cell as self MHC. self MHC. This isThis is te
termrmed ed seself lf MHMHC C rereststririctctioion. n. HeHelplper er T T cecellllss re
recocogngnizize e anantitigegen n in in cocontntexext t of of clclasass s II II seself lf MH
MHC. C. CytCytotootoxic xic T T celcells ls recrecognognize ize antantigeigen n inin co
contntexext t of clof clasass s I I seself MHlf MHC. C. ThThe e prprococesesss w
whheerreebby y T T cceelllls s bebeccoomme e rreessttrriicctteed d ttoo recognizing self MHC molecules occurs in the recognizing self MHC molecules occurs in the thym
thymus. us. The exThe experiperimentmental basal basis of seis of self MHClf MHC restriction for APC-helper T cell interactions and restriction for APC-helper T cell interactions and for class I MHC-cytotoxic T cell interactions are for class I MHC-cytotoxic T cell interactions are shown in Figures 3 and 4,
shown in Figures 3 and 4, respectivelyrespectively..
Fi
Figugure 3. re 3. SeSelf MHlf MHC ResC Restrtricictition of Th/on of Th/APAPCC Interactions.
Figure 4. Self MHC Restriction of Tc/ target cell
Figure 4. Self MHC Restriction of Tc/ target cell InteractionsInteractions
IIII.. AAnnttiiggeen Pn Prreesseennttiinng Cg Ceellllss
The three main types of antigen presenting cells are dendritic cells, macrophages and B cells, The three main types of antigen presenting cells are dendritic cells, macrophages and B cells,
although other cells, that express class II
although other cells, that express class II MHC molecules, (e.g., thymic epithelial cells) can act MHC molecules, (e.g., thymic epithelial cells) can act asas an
antigtigen presen presententining g celcells in ls in somsome e cacasesses. . DenDendridritic celltic cells, whics, which h arare e foufound in nd in skskin in anand d othotherer tissues, ingest antigens by pinocytosis and transport antigens to the
tissues, ingest antigens by pinocytosis and transport antigens to the lymph no
lymph nodes and spdes and spleen. leen. In the lympIn the lymph nodes and h nodes and spleen they spleen they are found are found predominantly predominantly in the Tin the T cells are
cells areas. as. DendDendritic celritic cells are the ls are the mosmost effectivt effective antigen prese antigen presentinenting cells and can g cells and can prespresentent an
antigtigenens s to to naïnaïve ve (v(virgirgin) in) T T celcells. ls. FurFurthethermrmorore, e, thethey y cacan n prepresensent t intinternernalialized zed antantigeigens ns inin ass
associaociation tion with either class with either class I I or or clasclass s II II MHC moleculMHC molecules es (cro(cross ss prespresentatentation), althouion), although gh thethe pr
predoedominminanant t papathwthway ay for intefor internarnalizlized ed antantigigen en is is the clasthe class s II II patpathwahway. y. ThThe e secseconond d typtype e of of an
antigtigen en prepresesentinting ng celcell l is is ththe e mamacrocrophaphage. ge. ThThese cells ingeese cells ingest st antantigeigen n by phagoby phagocycytostosis is of of pinocytos
pinocytosis. is. MacrophaMacrophages are not as effective in presenting antigen to naïve T cells but they areges are not as effective in presenting antigen to naïve T cells but they are very good in a
very good in activating memctivating memory T cells. ory T cells. The third typThe third type of antigen presente of antigen presenting cell is the B cell.ing cell is the B cell. T
Thehese se cecelllls s bibind nd anantitigegen n vivia a ththeieir r susurfrfacace e Ig Ig anand d iningegest st anantitigegens ns by by pipinonocycytotosisis. s. LiLikeke macrophages these cells are not as effective as dendrite cells in presenting antigen to naïve T macrophages these cells are not as effective as dendrite cells in presenting antigen to naïve T cells
cells. . B cells are very effectB cells are very effective in presentive in presenting antiging antigen to en to memomemory T ry T cellcells, especis, especially when theally when the antigen concentra
antigen concentration is low because tion is low because surface Ig on the B surface Ig on the B cells binds antigen with a high affinity.cells binds antigen with a high affinity. III.
III. Presentation of Presentation of SuperantigeSuperantigensns
Superantigens are antigens that can polyclonally activate T cells (see lecture on antigens) to Superantigens are antigens that can polyclonally activate T cells (see lecture on antigens) to produce large quantit
produce large quantities of cytokines that can have pathologicaies of cytokines that can have pathological effects. l effects. These antigens must beThese antigens must be presented to T cells in association with class II MHC molecules but the antigen does not need to presented to T cells in association with class II MHC molecules but the antigen does not need to be processed.
be processed. Figure 5 compaFigure 5 compares how conventiores how conventional antigens and supnal antigens and superantigens are preserantigens are presentedented to T cells.
to T cells. In the case of a superantigeIn the case of a superantigen the intact protein binds to cln the intact protein binds to class II MHC molecules aass II MHC molecules and tond to one or mor
one or more Vβ e Vβ regions of thregions of the TCR. e TCR. The antigen iThe antigen is not bound s not bound to the peptide to the peptide binding grobinding groove of ove of the MH
the MHC moleC moleculcule or e or to the antito the antigen bingen bindiding site of the TCR. ng site of the TCR. ThuThus, any T cell that uses, any T cell that uses as a particular Vβ
particular Vβ in its TCR will be activain its TCR will be activated by a superantigented by a superantigen, resulting in the activ, resulting in the activation of a largeation of a large numbers of T cells
IIVV.. RRoolle e oof f TThhyymmuuss
Both Th and Tc cells are self-MHC restricted. In Both Th and Tc cells are self-MHC restricted. In addition, T cells do not normally recognize addition, T cells do not normally recognize self-anti
antigensgens. . How arHow are self MHC re self MHC restrestricted T ceicted T cellslls generated and why are self-reacting T cells not generated and why are self-reacting T cells not pr
produoducedced? ? RaRandondom m VDJ VDJ rearearrrrangangemeements nts in in TT cells would be expected to generate some T cells would be expected to generate some T cells that can recognize non-self MHC and some cells that can recognize non-self MHC and some T cel
T cells that cls that can recoan recognize sgnize self-aelf-antigentigens. ns. It isIt is the role of the thymus to ensure that the
the role of the thymus to ensure that the only Tonly T cel
cells ls thathat t get get to to the the perperipipherhery y arare e selself-Mf-MHCHC restricted and unable to react with self-antigen. restricted and unable to react with self-antigen. Fu
Funcnctitiononal al T T cecelllls s in in ththe e pepeririphpherery y hahave ve toto recognize foreign antigens associated with self recognize foreign antigens associated with self MH
MHC, C, bebecacaususe e APAPC C or or tatargrget et cecelllls s prpresesenentt fo
forereigign n anantitigegen n asassosociciatated ed wiwith th seself lf MHMHC.C. H
Hoowweveverer, , aan n iinnddiivviidduuaal l ddooees s nnoot t nneeeedd ffuunnccttiioonnaal l T T ccelellls s iin n tthhe e ppeerriipphheerry y tthhaatt reco
recognize gnize antiantigen gen (sel(self f or or foreforeign) ign) assassociaociatedted with foreign
with foreign MHC. MHC. An individAn individual especially ual especially doesdoes not want functional T cells in the periphery that not want functional T cells in the periphery that can recognize self-antigens associated with can recognize self-antigens associated with self self MH
MHC C becbecausause e thethey y coucould ld lealead d to to damdamage of age of h
heeaalltthhyy, , nnoorrmmaal l ttiissssuueess..
Figure 5.
Figure 5. Presentation of Presentation of SuperantigensSuperantigens
As
As a a reresusult lt of of rarandndom om VDVDJ J rerecocombmbininatatioionn events occurring in immature T cells within the events occurring in immature T cells within the thymus, TCRs of all specificities are produced. thymus, TCRs of all specificities are produced. Processes in the thymus determine which TCR Processes in the thymus determine which TCR sp
spececifificicititieies s arare e reretatainineded. . ThTherere e arare e twtwoo sequ
sequentiential stepal steps shos shown in Fiwn in Figure 6gure 6. . FirsFirst, Tt, T ce
celllls s wiwith th ththe e ababililitity y to to bibind nd to to seself lf MMHCHC m
mololececulules es exexprpresessesed d by by cocortrticical al ththymymicic epithelial
epithelial cells arcells are retained. e retained. This is This is known aknown ass po
positsitive seive seleclectiotion. n. ThoThose that do nose that do not bind,t bind, undergo a
undergo apoptosis. poptosis. Thus, T Thus, T cells havcells having a TCRing a TCR that recognizes self MHC survive. Next, T cells that recognizes self MHC survive. Next, T cells with the ability to bind to self MHC molecules with the ability to bind to self MHC molecules ass
associaociated ted with with selfself-mo-molecullecules es exprexpresseessed d byby th
thymymic ic epepitithehelilial al cecelllls, s, dedendndrirititic c cecelllls s anandd ma
macrcropophahageges s arare e kikilllleded. . ThThis is is is knknowown n asas negative selection. Those that do not bind are negative selection. Those that do not bind are ret
retainained. ed. As a resAs a resulult of thest of these two ste two stepseps, T, T cells having a TCR that recognizes self MHC cells having a TCR that recognizes self MHC and for
and foreign aneign antigen stigen surviurvive. ve. Each T celEach T cell thatl that survives positive and negative selection in the survives positive and negative selection in the th
thymymus us anand d is is rereleleasased ed ininto to ththe e pepeririphphereryy retains its specific T cell receptor (TCR). While retains its specific T cell receptor (TCR). While positive and negative selection is occurring in positive and negative selection is occurring in th
the e ththymymus us ththe e imimmamatuture re T T cecelllls s arare e alalsoso ex
exprpresessising ng CDCD4 4 or or CDCD8 8 anantitigegens ns on on ththeieirr
surface.
surface. Initially thInitially the pre T cell e pre T cell that enters ththat enters thee tthyhymmuus s iis s CDCD4-4-CCD8D8--. . IIn n ththe e tthhyymmus itus it be
becocomemes s CDCD4+4+CDCD8+ 8+ anand d as as poposisititive ve anandd neg
negatiative ve selselectection ion proproceeceeds ds a a celcell l becbecomeomess either a C
either a CD4+ or CDD4+ or CD8+ cell. 8+ cell. The comThe commitmentmitment tto o bbececomome e eeitithher er a a CDCD44+ + oor r CCDD8+ 8+ ccelellsls depends on which class of MHC molecule the depends on which class of MHC molecule the ce
cell enll encocoununtetersrs. . If a CDIf a CD4+4+CDCD8+ ce8+ cell isll is presented with a class I molecule it will down presented with a class I molecule it will down regulate CD4 and become a CD8+ cell. If
regulate CD4 and become a CD8+ cell. If a cella cell is presented with a class II MHC molecule it is presented with a class II MHC molecule it will down regulate CD8 and become a CD4+ will down regulate CD8 and become a CD4+
cell
cell FiguFigure re 6. 6. PosPositivitive e and and neganegative tive seleselectioction n inin the thymus.
Figure 7.
Figure 7. Generation of CD4+ or Generation of CD4+ or CD8+ cells in CD8+ cells in the thymus.the thymus. V.
V. NNegegaatitivve se seellecectition on in in ththe pe peeririppheheryry Positive and negativ
Positive and negative selection in the thymus is not a 100% efficient procese selection in the thymus is not a 100% efficient process. s. In addition, not allIn addition, not all self-antige
self-antigens may be expresns may be expressed in the thymsed in the thymus. us. Thus somThus some self-reactive T cele self-reactive T cells may get to thels may get to the periphery.
periphery. Thus, there are additional mecThus, there are additional mechanisms that are designed to eliminhanisms that are designed to eliminate self-reactive Tate self-reactive T cells in the perip
cells in the peripheryhery. . ThesThese will be e will be discdiscusseussed in d in the tolerthe tolerance lectuance lecture. Sincre. Since B e B cellcells are nots are not MHC-rest
MHC-restricted there is no ricted there is no need for positive selection of B need for positive selection of B cells. However, negative selection (i.e.,cells. However, negative selection (i.e., elimination of sel
elimination of self-reactive clones) of-reactive clones) of B cells is required. f B cells is required. This occurs durThis occurs during B cell developmenting B cell development in the bone marrow.
in the bone marrow. However, negativHowever, negative selection of B cells is not a critical as for T cells since, ine selection of B cells is not a critical as for T cells since, in most instanc
most instances, B cells require T cell help in ores, B cells require T cell help in order to become activated. der to become activated. Thus, if a self-reactiThus, if a self-reactive Bve B cell does get to the periphery it will not be activated due to lack of T cell help.
