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ARTICLE IN PRESS+ModelIMM-2443; No. of Pages 16

Molecular Immunology xxx (2007) xxx–xxx

Review

T cell allorecognition and MHC restriction—A case of Jekyll and Hyde?

Julia K. Archbold a, Lauren K. Ely a, Lars Kjer-Nielsen b, Scott R. Burrows c,Jamie Rossjohn a, James McCluskey b,∗, Whitney A. Macdonald a,∗

a The Protein Crystallography Unit, Department of Biochemistry and Molecular Biology, School of Biomedical Sciences,Monash University, Clayton, Victoria 3800, Australia

b Department of Microbiology & Immunology, University of Melbourne, Parkville, Victoria 3010, Australiac Queensland Institute of Medical Research, 300 Herston Road, Brisbane, 4029 Qld, Australia

Received 16 March 2006; accepted 19 May 2006

bstract

A great paradox in cellular immunology is how T cell allorecognition exists at high frequencies (up to 10%) despite the stringent requirementsf discriminating ‘self’ from ‘non-self’ imposed by MHC restriction. Thus, in tissue transplantation, a substantial proportion of the recipient’scells will have the ability to recognize the graft and instigate an immune response against the transplanted tissue, ultimately resulting in graft

ejection—a manifestation of T cell alloreactivity. Transplantation of human organs and lymphoid cells as treatment for otherwise life-threateningiseases has become a more routine medical procedure making this problem of great importance. Immunologists have gained important insightsnto the mechanisms of T cell alloreactivity from cytotoxic T cell assays, affinity-avidity studies, and crystal structures of peptide-MHC (pMHC)

olecules and T cell receptors (TCRs) both alone and in complex. Despite the clinical significance of alloreactivity, the crystal structure of an
lloreactive human TCR in complex with both cognate pMHC and an allogeneic pMHC complex has yet to be determined. This review highlightsome of the important findings from studies characterizing the way in which alloreactive T cell receptors and pMHC molecules interact in anttempt to resolve this great irony of the cellular immune response.
2007 Elsevier Ltd. All rights reserved.

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eywords: T cell allorecognition; Major histocompatibility complex; T cell rec

. Introduction

Incompatibility between donor and recipient cells is therincipal barrier to the long term success of solid organransplantation. T cell alloreactivity – that is, recognition of allo-eneic major histocompatibility complex (MHC) molecules byHC-restricted T cells – has been a long-standing paradox in

ellular immunity.MHC molecules (HLA in humans; H-2 in mice) were discov-

red for their role in transplantation and responses to tumors,efining them as genetically important in regulating immuneesponses (Benacerraf et al., 1972; Snell, 1976). The primary

Please cite this article in press as: Archbold, J.K., et al., T cell allorecognitio(2007), doi:10.1016/j.molimm.2006.05.018

unction of MHC molecules is to initiate an immune responsey presenting antigenic peptides on the cell surface for recogni-ion by TCRs. This interaction is generally restricted, occurring

∗ Corresponding authors.E-mail addresses: [email protected] (J. McCluskey),

[email protected] (W.A. Macdonald).

datAdr(

161-5890/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.oi:10.1016/j.molimm.2006.05.018

nly between T cells and antigen presenting cells (APC) origi-ating from a syngeneic background, giving rise to the conceptf ‘MHC restriction’ (Fig. 1) (Zinkernagel and Doherty, 1974).espite this restriction between self-MHC and TCR, as many as0% of a donor or host’s T cell population can specifically rec-gnize allogeneic MHC molecules (Lindahl and Wilson, 1977;uchin et al., 2001; Wilson and Blyth, 1968), resulting in thepparent paradox of T cell allorecognition. This paradox is clin-cally significant in tissue transplantation involving donor/hostairs that are not perfectly matched at each HLA locus (with theossibility of variation of all 12 HLA alleles). A substantial pro-ortion of host T cells will have the ability to recognize disparateonor MHC molecules present in the graft. Such recognition canctivate an immune response directed against the transplantedissue, ultimately resulting in graft rejection (Terasaki, 1990).

n and MHC restriction—A case of Jekyll and Hyde?, Mol. Immunol.

dvances in immunosuppression and improved ability to matchonor and host at the genetic level have translated to a survivalate of >80% of most solid organ grafts, 5 years post-transplantOPTN/S 2005 Annual, 2005).

dx.doi.org/10.1016/j.molimm.2006.05.018

mailto:[email protected]

mailto:[email protected]


ARTICLE IN PRESS+ModelMIMM-2443; No. of Pages 16

2 J.K. Archbold et al. / Molecular Immunology xxx (2007) xxx–xxx

Fig. 1. MHC Restriction of T cells. T cells recognize antigen presented by MHC molecules in a highly restricted manner. T cells (e.g. from a HLA-B8+ individual)t l) wil( ptide

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hat recognize a particular pMHC combination (e.g. peptide/HLA-B8) (left panee.g. HLA-B44) (middle panel), nor will they be able to recognize a different pe

Alternatively, following hemopoetic stem cell transplantationcells derived from the donor have the potential to recognize

ifferences in the host’s pMHC molecules. This can lead tovigorous immune response – marked by a cytokine stormas T cells in the grafted tissue ‘reject’ the donor’s cells, a

ondition called ‘graft-versus-host-disease’ (GVHD) (Barnest al., 1956; Simonsen, 1957). GVHD is the major cause oforbidity and mortality in allogeneic bone marrow transplan-

ation, with increasing incidence of GVHD corresponding tohe level of HLA mismatch between donor and host (Beattyt al., 1985; Clift et al., 1979). However, cases of graft rejec-ion (Fleischhauer et al., 1990) and acute GVHD (Keever et al.,994) can also occur as the result of single amino acid mis-atches between donor and host. Chronic GVHD occurs in

pproximately 50% of patients who receive an HLA matchedtem cell transplant from an unrelated donor (Vogelsang et al.,003).

Even when donor and host are matched for each MHC allele,ifferences in other expressed proteins – known as minor his-ocompatibility antigens (mHags) – can lead to GVHD or graftejection. These antigens have been extensively studied in miceMalarkannan et al., 1998, 2000; Mendoza et al., 1997, 2001;

ommaas et al., 2002; Ostrov et al., 2002; Sahara and Shastri,003; Yadav et al., 2003) and in humans (den Haan et al., 1998;olstra et al., 1999; Goulmy, 1996; Goulmy et al., 1976, 1983,996; Maruya et al., 1998; Thomas et al., 1975a,b; Tiercy et al.,995). Several epitopes derived from mHags are also potential


arget antigens in alloresponses (see Table 1 for details). Themportance of mHags in human bone marrow transplantationas recognized 30 years ago after a female host received anLA-matched bone marrow transplant from her brother. Fol-

(

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l not be able to recognize the same peptide bound by a different MHC moleculebound to the same MHC (e.g. HLA-B8).

owing the transplant, in vitro analysis of the donor–host cellsemonstrated the ability of female derived CTL to recognizeifferences in the male derived target cells, characterizing theHag as ‘H-Y’ (Goulmy et al., 1976, 1977). Though often

lower and less severe, morbidity due to disparities in mHagsollowing HLA-matched transplants is still considerable enougho warrant further characterization of these antigens. In addition

Hags that are restricted to expression in hemopoetic cells areseful targets of graft versus leukaemia responses in the absencef GHVD reactivity.

. Molecules encoded by the major histocompatibilityomplex—a leading role in T cell allorecognition

The molecules encoded by the MHC are divided into twoain subsets, class I molecules and class II molecules. Classmolecules consist of a 45 kDa heavy chain comprised of

hree domains: �1, �2, and �3, and the non-covalently asso-iated light chain �2-microglobulin (�2m)—encoded by aon-polymorphic gene separate from the MHC (Little andarham, 1999; Strominger et al., 1976; York and Rock, 1996).

Within the class I loci, there are three loci that belong tohe class Ia group, denoted in humans as HLA-A, B, and C. Theroducts of these loci are referred to as classical class I moleculesnd are co-dominantly expressed in all nucleated cells. Thesehree loci are among the most polymorphic in the genome,esulting in almost 1500 alleles of classical class I molecules


Robinson et al., 2003).Class I molecules bind intracellular peptides derived from

he cytosol for presentation to CD8+ T cells. These peptidesre created by the proteasome (Rock et al., 1994) prior to


Please cite this article in press as: Archbold, J.K., et al., T cell allorecognition and MHC restriction—A case of Jekyll and Hyde?, Mol. Immunol.(2007), doi:10.1016/j.molimm.2006.05.018


J.K. Archbold et al. / Molecular Immunology xxx (2007) xxx–xxx 3

Table 1Summary of structural data available for both cognate and alloreactive T cell/pMHC complexes

T cell clone Cognate ligand Allo-MHCrecognized

Allo-peptiderecognized

Peptide source Structural data References

LC13 HLA-B*0801FLRGRAYGL

HLA-B*4402 Unknown Unknown HLA-B*0801/FLRGRAYGL(cognate)

Kjer-Nielsen et al.(2003), Macdonald etal. (2002), Macdonaldet al. (2003)

HLA-B*4405HLA-B*4407 HLA-B44/non-stimulating

peptide (allo)LC13LC13/HLA-B*0801/FLRGRAYGL(cognate)

JL12 HLA-B*0801FLRGRAYGL

HLA-B*3501 KPIVVLHGY Cytochrome P450 HLA-B*0801/FLRGRAYGL(cognate)

Burrows et al.(1997a,b)

JL12 HLA-B*0801FLRGRAYGL

HLA-B*3502 KPSPPYFGL CTP (cytidinetriphosphate)Cynthase

HLA-B*0801/FLRGRAYGL(cognate)

Burrows et al.(1997a,b)

MHN24 Unknown HLA-A*0201 YLDPAQQNL Zinc finger ProteinFamily

HLA-A*0201 bound to avariety of peptides but notwith the allopeptide

Garboczi et al. (1996),Wang et al. (1997)

Crystal structures:B7/HLA-A*0201/LLFGYPVYVA6/HLA-A2/TaxA6/HLA-A2/TaxP6AA6/HLA-A2/TaxV7RA6/HLA-A2/TaxY8A

DBS clone 1.5 Unknown HLA-A*03 GPPGVTIVK Kidney-specificprotein

No structural data available Poindexter et al.(1995)

27S69 HLA-B*2705RRFFPYYV

HLA-B*2702 RRFFPYYV Proteasome C5subunit

Model ofHLA-B*2705/RRFFPYYV(cognate)

Madden et al. (1992),Paradela et al. (1998)

HLA-B*2703HLA-B*2705 bound to avariety of peptides, but notwith the cognate peptide

121 HLA-B*3503FPSDSWCYF

HLA-B*3501 HIPDVITY Squalene Synthase HLA-A*3501 bound to avariety of peptides but notwith the allopeptide

Mandruzzato et al.(2000)

121 HLA-B*3503FPSDSWCYF

HLA-B*3501 QFADVIVLF 2-Hydroxyphytanoylcoenzyme A lyase

HLA-A*3501 bound to avariety of peptides but notwith allopeptide

Mandruzzato et al.(2000)

2C H-2Kb (SIYRYYGL) H-2Ld QLSPFPFDL 2-oxoglutaratedehydrogenase

H-2Ld bound to a variety ofpeptides but not withallopeptide

Sykulev et al. (1994)

(SIYR = superagonist) Speir et al. (1998)2C/H-2Kb/SIYRYYGL(cognate)Model of the allogeneic2C/H-2Ld/QL9 (allo)

2C H-2Kb (SIYRYYGL) H-2Ld LSPFPFDL 2-oxoglutaratedehydrogenase

Crystal structures of H-2Ld

bound to a variety of peptidesbut not with allopeptide

Udaka et al. (1992),Udaka et al. (1993),Speir et al. (1998)

2C/H-2Kb/EQYKFYSV2C/H-2Kb/SIYRYYGL(cognate)Model allogeneic2C/H-2Ld/QL9 (allo)


Please cite this article in press as: Archbold, J.K., et al., T cell allorecognition and MHC restriction—A case of Jekyll and Hyde?, Mol. Immunol.(2007), doi:10.1016/j.molimm.2006.05.018



Table 1 (Continued )


Allo-peptide recognized Peptide source Structural data References

2C H-2Kb (SIYRYYGL) H-2Kbm3 EQYKFYSV(dEV8 = weak agonist)

MLRQ(nuclear-encodedmitrochondrialprotein - acomponent

2C/H-2Kb/SIYRYYGL(cognate)

Tallquist et al. (1996),Speir et al. (1998)

of themitochondrialNADHubiquinonecomplex)

H-2Kbm3/EQYKFYSV2C/H-2Kbm3/EQYKFYSV2C/H-2Kb/EQYKFYSV

BM3.3 Unknown H-2Kb INFDFNTI Protein kinase H-2Kb/INFDFNTI Reiser et al. (2000),Guimezanes et al.(2001), Reiser et al.(2003)

BM3.3/H-2Kb/INFDNTI

BM3.3 Unknown H-2Kb RGYVYQGL Vesicularstomatitis virusNuclear Protein

H-2Kb/RGYVYQGL Guimezanes et al.(1992), Kern et al.(1998), Reiser et al.(2003)

BM3.3/H-2Kb/RGYVYQGL

C20 Unknown H-2Kb KVITFIDL GTP-bindingprotein AGP-1

H-2Kb/KVITFIDL Housset et al. (1997),Guimezanes et al.(2001), Reiser et al.(2002)

C20 TCRC20/H-2Kb/KVITFIDL

C20 Unknown H-2Kb KVLHYNV Match toexpressedsequence tag

C20 TCR Housset et al. (1997),Guimezanes et al.(2001), Reiser et al.(2002)

C20/H-2Kb/KVITFIDL

C20 Unknown H-2Kb KIITYRNL Match to anexpressedsequence tag

C20 TCR Housset et al. (1997),Guimezanes et al.(2001), Reiser et al.(2002)

C20/H-2Kb/KVITFIDL

bm1BZ19.4 Unknown H-2Kb SVVEFSSL Cryptic translationproduct

H-2Kb bound to a variety ofpeptides but not withallopeptide

Malarkannan et al.(1995)

bm1BZ19.4 Unknown H-2Kb SLVELTSL Adenosinephosphoribosyltransferase

Crystal structures of H-2Kb

bound to a variety of peptidesbut not with allopeptide

Malarkannan et al.(1995)

Qa-1b AMAPRTLL H-2d/Ld leadersequence

No structural data available. Aldrich et al. (1994)

HLA-DR3 VTTLNSDLKYNALDLTN ApolipoproteinB-100

HLA-DR3/CLIP* (fragment87-101 of invariant chain) butnot with the allopeptide

de Koste et al. (1998)

HLA-DR3 KVHGSLARAGKVRGQTPKV Fau-1 HLA-DR3/CLIP* (fragment87-101 of the invatiant chain)but not with the allopeptide

de Koste et al. (1998)

HLA-DR11 IPTLVEVSRNLGK Serum albumin No structural data available Panina-Bordignon etal. (1991)

H-2I-Ab ASFEAQGALANIAVDKA I-E� 52–68 I-Ab(murine)/human CLIP* Weber et al. (1995),Zhu et al. (2003)

2.102 Hemoglobin(64–76)/I-Ek

H-2I-Ep YFLLVTFTWNGLSAT G protein-coupledreceptor 128(531–545)

No structural data available Felix et al. (2006)




Table 1 (Continued )


Allo-peptiderecognized

Peptide source Structural data References

Minor antigensH-2Db SSVVGVWYL

(H13)Novel Protein(unknown function)

No structural data available Mendoza et al. (1997)

H-2Ab HAFVEAIPELQ(H46)

IL-4–inducedIL4i1gene (unknownfunction)

No structural data available Sahara and Shastri(2003)

C57BI/6 H-2Kb LTFNYRNL(H60)

Novel Type ITransmembraneProtein

No structural data available Malarkannan et al.(1998)

C57BI/6 H-2Kb ILENFPRL (H28) Novel CytoplasmicProtein

No structural data available Malarkannan et al.(2000)

*

t(lts(rnapctepHl

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CLIP = class II invariant chain-associated peptide.

rimming in the endoplasmic reticulum by aminopeptidasesShastri et al., 2002) (Hammer et al., 2006) to the optimalength of 8–11 residues. A substantial proportion of the pep-ides assembled with MHC molecules are derived from newlyynthesized peptides, ensuring efficient immune surveillanceReits et al., 2000). Recently, unusually long peptides (13–14esidues) have emerged as important targets in antiviral immu-ity (Probst-Kepper et al., 2001; Miles et al., 2005; Tynan etl., 2005a,b,c). However, it is not known whether or not longeptides also play a role in T cell allorecognition. The CTLlone SB27 is restricted against a 13mer EBV peptide boundo HLA-B*3508, yet is also alloreactive on HLA-B44 (Tynant al., 2005b). Although HLA-B44 has been found to bind longeptides (Macdonald et al., 2003), it is not known whether theLA-B44 bound allopeptide seen by SB27 is also unusually

ong.Pamela Bjorkman and colleagues solved the first crystal

tructure of an MHC molecule, HLA-A2, revealing that the mostolymorphic region of an MHC molecule is the peptide bindingleft, a groove bordered by helices from the �1 and �2 domains,ith the floor consisting of an eight stranded anti-parallel betaleated sheet (Bjorkman et al., 1987a,b). The peptide is boundn an extended conformation with the N- and C-termini boundn the groove while the central region of the peptide poten-ially bulged out (Guo et al., 1992; Madden et al., 1991, 1992).his renders two or three residues bound in the peptide-bindingroove as ‘anchor residues’ with the majority of the remain-ng peptide residues either forming minor anchor sites or beingrientated away from the MHC forming potential TCR contactoints.

Class II molecules are comprised of a � chain and a � chain,hich non-covalently associate to form heterodimeric glycopro-

eins. Class II molecules present peptides to CD4+ T cells forhe initiation of cytokine production or antibody production via

helper functions. The length of these peptides is less con-trained and typically between 12 and 19 amino acids long,


erived from exogenous sources, endosomes, and lysosomesEngelhard, 1994). Class II molecules are found on the surfacef dendritic cells and other such specialized antigen presentingells (APC).

tai1

There are also three pairs of MHC class II genes in humans:LA-DR, HLA-DP and HLA-DQ (there are only two mouse

quivalents, H-2A and H-2E). The MHC class II genes are veryolymorphic, however, unlike the class Ia genes where the poly-orphic sites are distributed throughout the �1 and �2 domains,

olymorphism in the MHC class II is more predominant in the-chain, particularly for HLA-DR (Marsh et al., 2005). Therst structure of an MHC class II molecule, solved by Wileynd colleagues, illustrated that they also adopt a similar con-ormation to that of the class I molecules (Brown et al., 1993).nlike the MHC class I molecules, the peptide-binding cleftf a class II structure is open-ended, accommodating the typi-ally longer class II ligands. The class II peptides do not formcentral bulge but rather extend beyond the groove in a more

longated conformation. While the mode of peptide binding dif-ers between classes I and II molecules, the final peptide surfacerea available for TCR recognition is quite comparable (Sternt al., 1994).

. The T cell receptor—the other leading role inllorecognition

Each � and � chain of the TCR consists of a variableegion, a constant region containing a disulfide bond linkinghe two chains, a transmembrane region, and a short cytoplas-

ic tail. Individual TCRs in a T cell repertoire are extremelyiverse, enabling them to recognize the vast amounts of peptidesresented by MHC molecules. This extraordinary amount ofiversity is generated through somatic rearrangement of variableV), diversity (D), and joining (J) gene segments during T cellevelopment (Davis and Bjorkman, 1988; Garcia et al., 1999a;ilson et al., 1988). Additional diversity is seen within the V-

D)-J junctional regions by the addition and deletion of randomucleotides (called N regions). Much like antibodies, TCRs pos-ess complementary determining regions (CDRs). These areocated in the variable region, which is the distal portion of


he TCR. The hypervariability corresponding to the V–D–Jnd V–J junctions is located in the complementarity determin-ng region (CDR3) (Davis and Bjorkman, 1988; Garcia et al.,999b). The first crystal structure of a TCR (murine derived


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ARTICLEIMM-2443; No. of Pages 16

J.K. Archbold et al. / Molecula

C) was published in 1996 (Garcia et al., 1996), and now therere 24 structures of TCRs in complex with pMHC that haveegun to elucidate how TCRs recognize antigen in the con-ext of MHC molecules (for a recent review see (Rudolph etl., 2006). Analysis of the initial pMHC-TCR complexes sug-ested a common diagonal docking mode in which CDR1 andDR2 interact predominantly with the �1 and �2 helices of theHC molecule, while the CDR3 region interacts with the pep-

ide (Garboczi et al., 1996; Garcia et al., 1996, 1999b; Reisert al., 2003; Rudolph and Wilson, 2002). However, we nownow that TCRs can dock onto pMHC surfaces in a varietyf ways from orthogonal to diagonal. Despite the vast potentialCR diversity discussed above, immunodominant CTLs (selec-

ion of the same TCR variable chains in unrelated individuals)ave been identified with specificity for EBV and InfluenzaKjer-Nielsen et al., 2003; Stewart-Jones et al., 2003). The crys-al structures of these TCRs in complex with their respectiveestricted pMHC molecules have elucidated the structural prop-rties of such TCRs that confer pMHC specificity, resulting inmmunodominance.

. Positive and negative selection of thymocytes—wherelloreactivity all starts

In order for mature T cells to be able to survey and respondo all of the diverse pathogens that they face, it is a necessity forCRs to cross-react onto different antigenic surfaces, as there areore potential antigens than there are possible TCR combina-

ions. Additionally, in order for developing T cells to progress toature lymphocytes, they must exhibit a degree of self-reactivity

rom early in their development. In order to understand the rela-ionship between T cell alloreactivity and MHC restriction, it iselpful to look at the events that lead to positive and negativeelection.

T cells are educated during thymic development to be ableo distinguish between self and non-self (Bevan, 1997; Sebzdat al., 1999) (Figure 1.3). Self-peptides are presented by MHColecules on the surface of thymic epithelial cells, to imma-

ure thymocytes that express both the CD4 and CD8 coreceptorsViret and Janeway, 1999). Thymocytes that react weakly withhese self pMHC complexes are chosen for further maturationo single positive (either CD4+ or CD8+) lymphocytes in a pro-ess called positive selection (Fink and Bevan, 1978; Starr et al.,003; Zinkernagel et al., 1978). Those that do not react at all withelf-pMHC complexes die by neglect. Alternatively, immaturehymocytes that react too strongly with these complexes willndergo clonal deletion by apoptosis, a process known as neg-tive selection (Matzinger et al., 1984; Rammensee and Bevan,984; Viret and Janeway, 1999). Positive and negative selectionre balancing interactions, with the purpose of creating a matureopulation of lymphocytes that exhibit specificity for self-MHCnd foreign antigen whilst lacking autoreactive properties.

The path that leads to the fork in the road of thymic


evelopment, delineating positive and negative selection, isot unanimously agreed upon. Two prominent schools ofhought exist, the ‘quantitative/avidity’ theory and the ‘qualita-ive/peptide’ hypothesis (Sebzda et al., 1999). The avidity theory

ptmM

PRESSunology xxx (2007) xxx–xxx

roposes that the same ligand can result in either positive or neg-tive selection, depending on the affinity of the interaction andhe density of selecting ligand (Ashton-Rickardt et al., 1994;ook et al., 1997; Delaney et al., 1998; Jameson et al., 1995;ebzda et al., 1994). Support for the avidity theory came fromxperiments using mice transgenic for the alloreactive 2C TCRCook et al., 1997), which is known to be negatively selected inhe presence of H-2Ld and positively selected on H-2Kb (Sha etl., 1988a,b, 1990). The 2C TCR is also alloreactive against the2Ca peptide bound to Ld (Udaka et al., 1992). In this study, aignificant decrease in surface expression of Ld (2% of wild typexpression levels, generated by �2m deficient mice possessingnly one copy of the Ld gene) led to positive selection of the 2CCR (Cook et al., 1997).

The qualitative differential signaling theory states that posi-ive and negative selection come about by qualitatively differentignals to the TCR based on the individual nature of the selectingMHC complex (Janeway, 1995; Janeway et al., 1989; Mannie,991). Under this theory, one set of ligands would result ineak or incomplete signaling through interactions with the TCR

esulting in the positive selection of the T cell, while stimulatorynteractions would promote negative selection thereby prevent-ng autoreactivity in the periphery. According to this theory,ntagonists are thought to be examples of peptides that induceositive selection while interactions with agonist peptides wouldead to negative selection (Viret and Janeway, 1999).

.1. Theories on allorecognition

Transplantation of human organs and lymphoid cells asreatment for otherwise life-threatening diseases has become

routine medical procedure since the first successful trans-lant was executed between identical twins almost 50 years agoHarrison et al., 1956; Merrill et al., 1956). Since before theransplant success of Merrill and Murray, scientists have beenrying to understand the mechanism of T cell alloreactivity.

Graft rejection through T cell allorecognition can either beirect or indirect (Lafferty et al., 1983) (Fig. 2). Direct allorecog-ition involves interaction with donor APC that have beenransplanted into the host via the donor organ, travelling tohe lymph nodes and activating alloreactive T cells. Indirectllorecognition occurs when the host’s APCs take up allogeneicHC proteins from the graft tissue. The host’s T cells then

espond to the resultant foreign peptides presented by the self-HC molecules. The indirect pathway predominantly involvesD4+ T cells recognizing syngeneic MHC class II moleculesresenting allopeptides usually derived from allogeneic MHColecules (Fangmann et al., 1992a,b; Liu et al., 1992). Although

he role of direct allorecognition was traditionally thought to beore significant in transplant rejection, evidence is emerging

hat implicates indirect allorecognition as being a key mecha-ism of graft rejection (Gould and Auchincloss, 1999).

The structural basis of T cell allorecognition has led to two


rominent historical views that differ on whether the alloreac-ive TCR directly recognizes polymorphisms in the allo-MHC

olecule or whether it recognizes similarities of the foreignHC molecule and focuses on the presented peptide as foreign.




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ig. 2. Direct and indirect allorecognition. Direct allorecognition is the recognif foreign cells by self-CD4+ and CD8+ T cells. Indirect allorecognition is they self-MHC class II molecules to CD4+ T cells.

he first theory, explored by Bevan, proposes an affinity-basedesponse, termed the “antigen density model” (Fig. 3a). It pro-oses that engrafted cells will contain a high concentrationf allo-MHC molecules, such that an alloreactive T cell willecognize the differences occurring within the MHC moleculeindependent of the bound peptide) as antigenic, thus initiat-ng a cellular immune response against the graft (Bevan, 1984).


his peptide independent theory would account for the higherumbers of alloreactive T cells compared to the lower frequen-ies of most antigen-specific T cells that focus their recognitionpon the peptide. The theory suggests that all of the allo-MHC

lafp

ig. 3. Allorecognition Theories. The multiple binary complexes theory proposes thaty the allogeneic MHC molecule. The antigen density theory suggests that alloreactigh density of these molecules on the antigen presenting cell would induce a strong

f intact allogeneic class I and class II MHC molecules expressed on the surfacenition of peptides derived from allogeneic MHC molecules that are presented

xpressing cells simultaneously present thousands of antigeniceterminants per cell in the form of the polymorphisms withinhe MHC molecule itself, thus activating lower affinity T cellsBevan, 1984). Examples of peptide-independent alloreactive Tell clones have been documented in the literature (Elliott andisen, 1990; Smith et al., 1997; Villadangos et al., 1994).

The second theory, proposed by Matzinger and Bevan, out-


ines alloreactivity as being peptide-dependent. It assumes thello-MHC molecule is similar enough to the self-MHC moleculeor the alloreactive TCR to bind to it and recognize the presentedeptide as foreign (Matzinger and Bevan, 1977). Under this the-

allorecognition is the result of recognition of many different peptides presentedive T cells recognize the differences in the allogeneic MHC molecule and thealloresponse.


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ry, also referred to as “determinant frequency”, the peptides the key element controlling the specificity of the responseFig. 3b). This could result from the alloreactive TCR recog-izing the deceivingly similar allo-MHC molecule bound tone or more endogenous peptides, that appear foreign becausehey have not been privy to the negative selection processAuchincloss and Sachs, 1993; Heath et al., 1991). Alterna-ively, recognition could be due to a peptide that is commono the graft and host, but looks foreign to the alloreactive TCRue to adoption of a distinct conformation induced by the poly-orphic residues from within the allo-MHC molecule (Housset

nd Malissen, 2003; Lechler et al., 1990; Rudolph and Wilson,002) (Bluestone et al., 1993; Chattopadhyay et al., 1994; Luzt al., 2002a; Sherman, 1982). A report from Obst and col-eagues detailed how the spectrum of peptide-dependent (andpecific) alloresponses varied according to the level of simi-arities between the self-MHC and allo-MHC molecules, withncreased peptide-specific responses observed with increasedimilarities (Obst et al., 2000). Attempts to reconcile the oppos-ng theories of allorecognition have turned to analysis of crystaltructures of alloreactive TCRs in complex with allogeneicMHC molecules. The majority of observations have supportedpeptide-dependent theory, as the bound peptide participates

n a significant number of interactions with the alloreactiveCR (Housset and Malissen, 2003; Reiser et al., 2000, 2002,003).

. Identifying alloligands

One of the major bottlenecks in understanding T cellllorecognition is the limited knowledge of allopeptideequences. While there are currently over 20 peptides derivedrom natural sources known to stimulate alloreactive T cellsTable 1), the ‘allopeptide database’ is relatively minute givenhat up to 10% of T cells are able to recognize an allogeneicMHC target. Unfortunately, identifying the peptide target ofT cell response is not a trivial exercise and often is the rate-

imiting step in studying the T cell responses to viruses, cancer,utoimmunity, and organ transplantation. Many groups havepent years developing and optimising experimental techniqueso fish out one or several peptide ligands from a sea of tens ofhousands of presented peptides on the cell surface (Rammenseet al., 1993). Add to this the possibility that the peptide mighte in low abundance—occurring at as low as three copies perell (Purbhoo et al., 2004). Alternatively, the presented peptideould constitute a low affinity ligand for the TCR, or perhaps becryptic translation product (Malarkannan et al., 1995, 1999;chwab et al., 2003), and the experimental task becomes expo-entially more difficult. However, despite this difficulty, someechniques have emerged that have proved successful for iden-ifying allogeneic T cell epitopes.

One such method that has been exploited by Shastri and col-eagues to identify several peptides derived from minor antigens


n mice is the use of cDNA expression cloning (Malarkannan etl., 2000; Mendoza et al., 1997; Sahara and Shastri, 2003). Thisenetic approach involves making a cDNA library using RNAsolated from a tissue source that contains the antigenic peptide.

ke

a


PC that expressed the appropriate MHC molecule (either nat-rally or from transfection) are then transfected with the cDNAibrary and screened using a T cell readout assay to fish out indi-idual cDNA that stimulate T cells. The individual cDNA clonesre sequenced, and the APC are then transfected with truncatedersions of the stimulating construct and screened again. Even-ually the cDNA clone can be narrowed down to the sequenceoding for the activating peptide ligand. One important elementhat has aided in Shastri and co-workers’ success with cDNAxpression library screening is their development of a robust

cell readout assay. They have developed a system that usesacZ inducible mouse T cell hybrids to detect specific T cellctivation at the single cell level (Karttunen and Shastri, 1991;anderson and Shastri, 1994). The robust nature of this sys-

em enables detection of even rare antigens (Karttunen et al.,992).

An alterative method to the expression cloning approach fordentifying ligands of alloreactive T cells is to screen a syntheticeptide library. This technique often makes use of a known pep-ide binding motif of the particular allo-MHC molecule to limithe size and biochemical properties of the library. Rammenseend collegues successfully screened an H-2Kb peptide libraryontaining more than 2000 unique sequences to identify allopep-ides that demonstrated a correlation between increasing peptideependence and increasing similarities amongst the allogeneicnd syngeneic MHC molecules (Obst et al., 2000). In addition,hey were able to generate high avidity alloreactive CTL againstndividual peptides isolated from the synthetic peptide libraryObst et al., 1998).

Purification of naturally presented peptides from MHColecules is another successful method that has also resulted

n the identification and characterization of MHC-bound pep-ides (Aldrich et al., 1994; Paradela et al., 2000; Paradela et al.,998; Reiser et al., 2000). Peptide purification (or ‘peptide elu-ion’ to which it is also referred) involves stripping the boundeptides from the cleft of HLA molecules that have been isolatedrom a large number of antigenic cells. The eluted peptides arehen fractionated using most commonly high performance liq-id chromatography (HPLC). Fractions are tested for presencef allo-antigenic stimuli using an alloreactive T cell read-outssay. Once an active fraction is detected, the antigenic peptideequence is identified using Mass Spectrometry (MS) and/ordman degradation techniques (Henderson et al., 1992; Hunt etl., 1992; Udaka et al., 1992).

A fourth technique for identifying allogeneic antigens is tose a ‘bioinformatics’ approach in which protein data bases areearched for corresponding peptides containing a binding motifelevant to the particular MHC molecule, candidate sequencesre synthesized and tested for antigenicity in a T cell assayBurrows et al., 1997a,b; Felix et al., 2006). This techniqueequires that the peptide binding characteristics of the present-ng MHC molecule are already known, and thus peptide elutions sometimes used prior to bioinformatics searches if little is


nown about the peptide repertoire of the MHC molecule (Felixt al., 2006).

Kappler and Marrack and colleagues have recently developednovel system for identifying T cell epitopes (and mimotopes)


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Crawford et al., 2004; Wang et al., 2005), which they havesed to identify the alloligand for an alloreactive mouse TCRCrawford et al., 2004). This technique uses a baculovirus, pep-ide display library—that is, a genetically produced peptideibrary as opposed to a synthetic peptide library. Using thisechnique has enabled them to identify both class I and classI bound antigenic peptides. Their method utilizes a baculovirusystem to infect cells such that MHC molecules and a library ofeptides are displayed on the surface of the cell. Using recom-inant, fluorescently labeled TCRs, they were able to identifyT cell mimotope of the allopeptide by sequential enrichmentf cells expressing the antigenic peptide bound to the MHColecule.Although all of these methods have worked to identify a

ariety of T cell epitopes, each technique has significant limita-ions, which can greatly reduce the ability to identify the desiredpitope. Factors such as peptide abundance, chemical character-stics of the peptide, availability of an APC that does not giveigh levels of background in the T cell readout assay, levels ofxpression of the ligand on the surface of the APC, and robust-ess and sensitivity of the T cells used in the read out assay canmpact the overall success of each method.

Kappler’s novel technique avoids many of the disadvantagesssociated with the other common methods of T cell epitopedentification. Firstly, the peptide display library is constructeduch that it is covalently linked to the MHC molecule, thus elim-nating problems associated with exogenous peptide loading, orndogenous peptide processing. Factors such as peptide abun-ance are no longer an obstacle. Secondly, using a baculovirusystem ensures high level of antigen expression, which is mostmportant for optimal detection in the read out assay. Lastly, these of recombinant soluble T cell receptors enables a robust sys-em as working with recombinant soluble protein can often be

ore consistently reliable than using live T cells. Often living Tells in culture can be adversely affected by factors beyond thexperimenter’s control, rendering them less robust. To overcomehis problem by using recombinant soluble TCR technology ismajor advantage.

. Mountains from Molehills?

Although our knowledge about the nature of the T celllloresponse has greatly increased in the last 10 years, muchtill remains to be discovered. Immunologists have traditionallyhought that the severity of the alloresponse (manifested as graftejection or GvHD) is directly correlated to the extent to whichhe donor and recipient are mismatched at the HLA loci, whileewer mismatches at the sequence level are more likely to beell tolerated (Auchincloss and Sachs, 1993). There has beenebate regarding the relevant importance of matching particularlements of the MHC with distinctions made between classesand II antigens right through to individual alleles. There arenumber of reports describing alloreactive CTLs that are able


o discriminate between closely related alleles, differing some-imes by only a single amino acid (Barouch et al., 1995; Burrowst al., 1994; Fleischhauer et al., 1994, 1990; Keever et al., 1994;oelle et al., 2002; Nakayama et al., 1994; Paradela et al., 1998;

B1Fc

PRESSunology xxx (2007) xxx–xxx 9

mith et al., 1996; Steinle et al., 1993), such as in the HLA-B44imorphism.

HLA-B44 alleles are common in all ethnic populations atfrequency averaging around 15 per cent, and constitute one

f the nine major supertypes (Sette and Sidney, 1999). Both*4402 and B*4403 substantially contribute to that frequencynd have long been known as a ‘taboo mismatch’ in tissueransplantation, followed by B*4405 (though B*4405 is muchess common). These three allotypes differ from each other bynly 1–2 amino acids; yet this difference is enough to invoke aubstantial difference in T cell recognition.

HLA-B*4405 differs from B*4402 and B*4403 by the sub-titution of a tyrosine at position 116 in place of an aspartic acid.LA-B*4402 and B*4405 share an aspartic acid at residue 156,hile the only distinguishing factor of B*4403 from B*4402 is

he presence of a leucine at this position (Burrows et al., 1994;leischhauer et al., 1991; Petersdorf et al., 1994). The singlemino acid disparity between the B*4402 and B*4403 resultsn graft rejection and GvHD when the donor and recipient are

ismatched for these alleles (Fleischhauer et al., 1990; Keevert al., 1994), defining them as a ‘taboo mismatch’ in tissueransplantation. Residue 156 is located on the �2 helix, with itside chain pointing into the peptide-binding groove. Elucidationf the three-dimensional structures of HLA-B*4402, B*4403,nd B*4405 all bound to a dominant self-peptide demonstratedhat the polymorphic residue 156 was indeed buried withinhe antigen binding cleft (Macdonald et al., 2003; Zernich etl., 2004). This observation ruled out the earlier hypothesishat differential alloresponses generated by this polymorphicesidue were due to the amino acid being solvent exposednd thus directly detectable by alloreactive TCRs (Hermant al., 1999; Parham et al., 1988). In addition, the polymor-hism at position 156 induces conformational changes evenhen the same peptide is bound by these allotypes, thereby

ngendering structurally different epitopes. Thus the 156 poly-orphism alters the heavy chain conformation of these allotypes

n the presence of the same ligand, which could affect recog-ition by CTL (Ajitkumar et al., 1988; Macdonald et al.,003).

The presentation of unique ligands by these allotypesMacdonald et al., 2003) most certainly contributes to thetrong alloresponse observed between B*4402 and B*4403Fleischhauer et al., 1990; Herman et al., 1999; Keever et al.,994; Macdonald et al., 2003), with the crystal structures of thesewo molecules demonstrating how the polymorphic residue atosition 156 structurally controls for the selection of differ-nt peptides, providing a structural explanation for how theselleles bind different peptides and present functionally differentpitopes.

Previous studies have shown that the HLA-B8 restricted CTLlone LC13, which uses an immunodominant TCR in responseo the Epstein–Barr viral epitope FLRGRAYGL (FLR), detectshis polymorphism at position 156 as it is alloreactive against


*4402 and B*4405, but not B*4403 (Fig. 4) (Burrows et al.,994, 1997a; Herman et al., 1999) (Macdonald et al., 2003).urthermore, in heterozygous HLA-B8/B*4402 individuals, Tells that express this particular �� TCR are absent from the




Fig. 4. Polymorphic residues of HLA-B8 and HLA-B44. The LC13 T cell is able to recognize HLA-B8 (coloured in blue) presenting the EBV epitope, FLR (colouredi ferenp interpt

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n yellow). The LC13 T cell is also able to bind to HLA-B44 presenting a difolymorphic residues (that would be seen by LC13) are coloured in aqua. (Foro the web version of the article.)

epertoire thus avoiding autoreactivity, and instead alternativeCRs are selected producing a comparable CTL response to theLR epitope (Burrows et al., 1995). Interestingly, although theC13 CTL clones are not alloreactive towards B*4403+ cells,

he LC13-like public TCR is also absent from the repertoiref mature anti-EBV CTL from HLA-B8/B*4403 heterozygousndividuals, suggesting that recognition of B*4403 occurs dur-ng the process of thymic development purging the repertoire ofhe LC13 specificity (Burrows et al., 1997a).

Another alloreactive CTL clone (called HV23) that recog-ises HLA-B*4402, HLA-B*4403 and HLA-B*4407 but nothe closely related HLA-B*4404 and HLA-B*4405 has alsoeen isolated in the CTL response to Herpes Simplex VirusKoelle et al., 2001). The HLA-B*4407 molecule differs fromLA-B*4402 by a single polymorphic residue at position 41

Thr → Ala), the HLA-B*4404 molecule has two polymorphicesidues at positions 156 (Asp → Arg) and 163 (Leu → Thr)


hile the HLA-B*4405 molecule has just one residue differ-nce from B*4402 at position 116 (Asp → Tyr). This clonotypeas isolated from herpes simplex virus type 2 (HSV-2) lesions

nd is restricted against HLA-A*0201 (HLA-A2) presenting a

idMt

t peptide. HLA-B44 differs from HLA-B8 by 24 amino acids. Some of theseretation of the references to colour in this figure legend, the reader is referred

eptide epitope from the HSV-2 viral protein VP13/14 (residues89–297, encoded by the gene UL47) (Koelle et al., 2002). Thellopeptide involved in the HV23 TCR–HLA-B44 response islso unknown.

. Structural basis of allorecognition

Structural studies have attempted to elucidate the molecularasis of TCR allorecognition. To date there have been no humanCR–pMHC complex structures solved that reveal how a singleCR can recognise the disparate surfaces of a cognate pMHCnd allogeneic pMHC complexes. Nevertheless, there has beenome insight provided from the structure of the alloreactiveurine BM3.3 TCR bound to allo-pMHC molecules (Reiser et

l., 2000) and the 2C TCR bound to both H-2Kb and H-2Kbm3

llotypes (Luz et al., 2002b). The BM3.3 TCR ternary com-lexes illustrate that allo-pMHC molecules can be recognised


n much the same way as cognate pMHC, with the BM3.3 TCRocking in a diagonal orientation and interacting with both theHC heavy chain and the peptide side chains (Fig. 5). Unfor-

unately, the cognate pMHC for this TCR is unknown and thus


ARTICLE IN+ModelMIMM-2443; No. of Pages 16

J.K. Archbold et al. / Molecular Imm

Fig. 5. TCR footprint on allogeneic pMHCs. Surface representation of allo-geneic pBM1-H-2Kb (A) and VSV8-H-2Kb (B) with the footprint of thealloreactive mouse TCR BM3.3 shown. The peptide is coloured yellow, and thefoot print is as follows: CDR1� red, CDR2� magenta, CDR3� blue, CDR1�

orange, CDR2� green and CDR3� cyan. (For interpretation of the referencesta

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o colour in this figure legend, the reader is referred to the web version of therticle.)

he mode of binding for this alloresponse could not be comparedo that of the cognate ligand. Nevertheless, the BM3.3 TCR haseen solved in complex with two distinct allopeptides, pBM1nd VSV8, bound to the H-2Kb molecule. These ternary com-lexes illustrate how a single TCR can accommodate uniquetructural features whilst still maintaining a similar diagonalocking angle and positioning over the peptide (Fig. 5). Theost significant conformational change between these struc-

ures is observed in the BM3.3 CDR3� loop. In the BM3.3CR–pBM1-H-2Kb structure the CDR3� loop minimally con-

acts the �1-helix (Fig. 5A) whilst the recognition of the alternateSV8 peptide sequence results in this loop forming a significant


nterface with the VSV8-H-2Kb complex (Fig. 5B).The second structural study of TCR allorecognition focused

n the 2C TCR system. Wilson and colleagues have solved the

cQe


omplexes of the 2C TCR bound to both the cognate H-2Kb

nd the closely related H-2Kbm3 both bound to the dev8 ligand.he H-2Kb and H-2Kbm3 molecules differ by Asp77Ser andla89Lys substitutions on the MHC heavy chain; this allelicariation results in the 2C TCR being negatively selected in H-Kbm3+ mice (Sha et al., 1990). The Asp77Ser substitution ismplicated in the alloresponse, however position 77 in the H-Kbm3 structure is occluded by the peptide and thus does notirectly contact the TCR but rather influences the peptide con-ormation (Luz et al., 2002b). The 2C TCR recognises both ofhese pMHC complexes in a very similar manner however TCRigation elicits subtle changes in both buried surface area andurface complementarity of the TCR-pMHC interface that mayonfer the alloresponse. Although this study successfully char-cterised an alloreactive system, the information is somewhatimited due to the structural similarity of the “cognate” H2Kb

nd H2Kbm3 alloligands.The TCR from the CTL clone SB27 is also involved in

n alloresponse that differentiates between prevalent HLA-B44llotypes. This TCR is generated in the HLA-B35 restrictedesponse to the unusually long Epstein–Barr viral epitope LPE-LPQGQLTAY (LPEP). The crystal structure of the SB27 TCR

n its liganded state with HLA-B35LPEP has recently been deter-ined (Tynan et al., 2005b). The structure of this complex shows

hat the rigid-bulged nature of the 13-mer peptide results in annteraction between the TCR and only six residues from the

HC-I � helices. The SB27/HLA-B*3508LPEP complex hasuggested the minimal MHC-I contribution to the binding inter-ace needed for MHC restriction of this TCR. This requirementor MHC restriction is exemplified by the non-recognition ofhe same peptide bound to a closely related HLA-B35 subtypeHLA-B*3501), that differs by a single amino acid also at posi-ion 156 (Tynan et al., 2005a). The crystal structures of the twoLA-B35 allotypes bound to the 13-mer peptide revealed that

lthough the peptide was bound in virtually the same conforma-ion, the MHC �2-helix polymorphism resulted in a wideningf this helix in HLA-B*3508 in the region where the SB27 TCRakes contacts.Despite this stringent requirement for MHC restriction exhib-

ted by the SB27 TCR, it elicits a strong alloresponse againstLA-B*4402, B*4403, and B*4405, allotypes that differ fromLA-B*3508 by up to 21 residues. Interestingly, the four

esidues that are contacted by the SB27 TCR on the B*35082 helix are conserved in these B44 allotypes. Moreover, there

s strong similarity between the structures of the �2 helicesrom B*3508 and B*4402 (Tynan et al., 2005a,b; Macdonaldt al., 2003), suggesting that the binding mode of the SB27 TCRnables it to override the requirements of MHC restriction andlicit an alloresponse.

The 2C TCR also interacts with the alloligands p2Ca-H-2Ld

nd QL9-H-2Ld. Although there is no ternary crystal structurevailable for this interaction, Wilson and colleagues have mod-lled the 2C TCR–QL9-H-2Ld complex based on the structures


omplex (Speir et al., 1998). Efforts to crystallise the binaryL9-H-2Ld were not fruitful, thus the QL9 peptide was mod-

lled based on the conformation of several other ligands. It


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s predicted that the peptide in the allogeneic H-2Ld complexulges out of the binding cleft at the C-terminal end; assum-ng that the 2C TCR has a similar footprint on both pMHC

olecules, this peptide bulge will allow for increased interac-ions with the �-chain of the 2C TCR. It is hypothesised thathe greater contribution of electrostatic interactions between theL9 peptide and the 2C TCR contribute to the increased affinityf the 2C TCR for the alloligands over the cognate dev8-H-2Kb.hese electrostatic interactions made with the QL9 peptide are

hought to mimic those that would be formed between the H-2Kb

HC heavy chain in the self-pMHC complex, thus this modelllustrates how both polymorphic and peptide differences canontribute to TCR alloreactivity. The resolution of this ternarytructure, and/or other similar alloreactive systems, will be nec-ssary to confirm these findings.

. Concluding remarks

Recognition of pMHC by a TCR is a highly specific interac-ion, such that substitutions in key contact residues of the peptidean significantly alter the functional outcome of a pMHC-TCRnteraction (Burrows et al., 1995; Degano et al., 2000; Ding etl., 1999; Kjer-Nielsen et al., 2003). Burrows and colleaguesemonstrated the fine specificity of the alloreactive TCR JL12,emonstrating the requirement for a glycine at position eightn the allopeptide in order to induce cell lysis (Burrows et al.,997b). In addition, an altered peptide ligand (APL) consist-ng of a single substitution from the EBV cognate ligand forhe alloreactive LC13 CTL clone is a strong antagonist peptideor this CTL (Ely et al., 2005), also inhibiting allorecognitionf HLA-B44 by LC13. Additionally, an antagonist peptide wasdentified recently in the mouse 2C system that also abolishedllorecognition (Rudolph et al., 2004).

Success of tissue transplantion is limited by the high degreef polymorphism at the HLA loci, which makes finding HLAatched donor–host pairs for transplantation a difficult task.lthough immunosuppressive theraputics have been used with

ome success to overcome this immunological barrier, there isoom for improved therapeutics to minimize transplant relatedorbidity and mortality. As such, for decades there has beensubstantial amount of effort devoted towards elucidating theechanism of T cell alloreactivity.Exploiting the fine specificity exhibited by alloreactive TCRs

o design APLs with antagonist activity could be useful in creat-ng therapeutic agents for reducing graft rejection and GVHD.nsight into multiple TCR-pMHC allogeneic interactions willrovide invaluable insight into any general mechanisms byhich the TCR accommodates alternate ligands and allogeneicHC allotypes.

ote in proof

The recently solved structure of a TCR in complex with an


llogeneic MHC molecule by Garcia and colleagues (Colf etl., 2007) has provided important insights into the structuralechanism of T cell allorecognition. The authors compare the

llogeneic MHC-TCR complex to the previously solved struc-

B


ure of the TCR in complex with its positively selecting ligandnd show that that particular alloresponse is not a result ofmolecular mimicry’ – that is, that the TCR binds in distinctlyifferent orientations on its foreign and self ligands. Interest-ngly, in a recently published study of a known pair of foreignnd self ligands for a human TCR, Burrows, Rossjohn, and col-eagues solved and compared the structures of these ligandsArchbold et al, 2006). While the structures of this TCR in com-lex with its foreign and self ligands have not yet been solved,he authors provide data from the binary structures, together withunctional data, to suggests that this alloresponse is the result ofolecular mimicry. It will important to obtain more paired struc-

ures of TCRs in complex with both self and foreign ligands toully understand the molecular basis for T cell allorecognition.

Archbold, J.K., Macdonald, W.A., Miles, J.J., Brennan, R.M.,jer-Nielsen, L., McCluskey, J., Burrows, S.R., Rossjohn J.,006. Alloreactivity between disparate cognate and allogeneicMHC-I complexes is the result of highly focused, peptide-ependent structural mimicry. J. Biol. Chem. 281, 34324–34332.

Colf, L.A., Bankovich, A.J., Hanick, N.A., Bowerman, N.A.,ones, L.L., Kranz, D.M., Garcia, K.C., 2007. How a single Tell receptor recognizes both self and foreign MHC. Cell 129,35–146.

cknowledgements

This work was supported by the National Health and Medi-al Research Council, the Australian Research Council and theoche Organ Transplantation Research. J.R. is supported by anustralian Research Council Professorial Fellowship; L.K.E. is

upported by a National Health and Medical Research Coun-il CJ Martin Fellowship; W.A.M. is supported by a NHMRCeter Doherty Fellowship; J.K.A. is supported by a NHMRCora Lush Scholarship.

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ldrich, C.J., DeCloux, A., Woods, A.S., Cotter, R.J., Soloski, M.J., Forman,J., 1994. Identification of a tap-dependent leader peptide recognized byalloreactive T cells specific for a class Ib antigen. Cell 79, 649–658.

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B

B

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B

B

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B

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C

C

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D

D

D

d

D

D

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