Superantigens

Superantigens Talal Chatila and Raif S. Geha Harvard

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Introduction The term ‘superantigens’ has been coined in reference to a group of molecules which, when bound to MHC class II molecules, stimulate T cells bearing particular T-cell receptor (TcR) VP or, in some case, Vy elements. Superantigens differ in several important aspects from peptide antigens in their interaction with the TCR. In the case of peptide antigens, all the variable elements of both TCR ct- and p-chains (Vcl, Ju, VP, Dp, Jp) contribute to the interaction of the TCR with peptid&viIK complexes. In contrast, superantigen-MHC class II complexes interact exclusively with the VP element of the TCR at a region on the lateral face of the VP domain not usually involved in antigen recognition. Because of the large number of po tential combinations of all variable elements of the TCR u- and P-chains, the frequency of a T-cell response to a particular eptide antigen is usually vety low (1 in 104 to 1 in 1og>. The capacity of superantigens to recognize one or more of a restricted number of VP gene products (approximately 20 in mice and 50 in humans) and to stimulate all T cells bearing these VP products allows for the stimulation of large numbers of T cells. For some superantigens this can reach up to 30% of the entire T-cell population. Another major difference between antigens and superantigens is that whereas the recognition of peptide antigens by the TCR is restricted by the allele of the presenting MIX molecule, recognition of superantigens is for the most part only marginally a&ted by the identity of the presenting MHC allele. Finally, unlike peptide antigens, superantigens do not require intracellular processing prior to presentation by MHC class II molecules, and they bind to MHC class II molecules at sites distinct from the peptide antigen groove. Both endogenous (self) and foreign superantigens have been described. The former are exemplified by the murine minor lymphocyte stimulating determinants, or h4ls. Foreign superantigens are typified by the staphylococcal enterotoxins, causative agents of food poisoning

1992, 4:74--78

and of toxic shock syndrome. Both types of superantigen induce intense in vitro proliferation of T cells bearing target VP products but continuous exposure in vivo to either type induces the elimination or inactivation of such T cells. The past year has witnessed a major growth in our knowledge of both types of superantigen. Perhaps the most exciting development was the identification of some superantigens, including the Mls determinants and other self superantigens, as retrovifal gene products. More bacterial toxins have been identified as superantigens, and superantigens have been implicated in the pathogenesis of autoimmune diseases and irnmunodeficiency. New information has become available concerning the interaction of superantigens with both TCR Vpa and MIX class II molecules, and the ability of superantigens to transduce biologic signals via MHC class II molecules. This review will highlight and discuss some of these new findings.

Retrovirus-encoded

superantigens

Several groups have now published reports documenting tight linkage of self superantigens to endogenous retrotiral integrants. Frankel et al. [ 1.1 demonstrated perfect concordance between the presence of endogenous mouse mammary tumor virus (MMTV) integrants and the expression of Mls gene products. MMTV-7 cosegregated with Mlsla on chromosome 1, MM’I’V-6 with Mls-3a on chromosome 16, h&s-2a with W-13 and an Mls-2-like product cosegregated with MMTV1 on chromosome 7. Other self superantigens also cosegregate with MMTV integrants, Woodland et al. [2*] demonstrated that the self superantigen DVbll-2 (also known as Etc-I), cosegregated with W-9. Dyson et al. [3*] demonstrated concordance between DvBll-2 and MMTV-9, and additionally demonstrated that two related self superantigens, Dvbll-1 and Dvbll-3, similarly cosegregate with MMTV integrants (W-8 and MM’IV-11, respectively).

Abbreviations MAIDsmurine

74

HIV-human immunodeficiency virus; LTR ORbpen reading frame of the 3’ long terminal repeat; acquired immunodeficiency syndrome; MMTV-mouse mammary tumor virus; SEA-staphylococcal SE&staphylococcal exotoxin E; TCR-T-cell receptor; TSST-toxic shock syndrome toxin. @ Current Biology Ltd ISSN0952-7915

exotoxin A;

SuperantigensChatila and Geha

Exogenous MMTV isolates also encode a superantigen that can be vertically transmitted in the milk of mice infected with the virus and which deletes V@14+ T cells [4*]. Two groups have now demonstrated that this superantigen is encoded by a gene in the open reading frame of the 3’ long terminal repeat (LTR ORF) of MMTV. Choi et al. [5**] reported that a B-cell lymphoma line transfected with an expression vector containing the LTR ORF was effective in stimulating Vg14+ T-cell clones. Acha-Orbea et UC [G*] demonstrated that in mice transgenic for the whole MMTV provirus or for LTR ORF, V~l4bearing T cells are deleted. Analysis of LTR ORF sequences of different MMTV associated with the deletion of Vpllbearing T cells reveals a high degree of homology in the carboxy-terminal region but marked differences in the amino-terminal region, suggesting that the VP specificity is determined by the carboxy-terminal region. This is supported by the observation that an MMTV variant that differs from the wild-type virus in the carboxy-terminal 32 amino acids of LTR ORF was incapable of inducing the deletion of Vfl14+ T cells. An important development in the link between retroviruses and superantigens has been the finding by Hugin et al. [7**] that a defective murine leukemia virus that induces an acquired immunodeficiencylike disease in mice (MAIDS) also codes for a VPS-specific superantigen. B-cell lymphomas expressing the viral gagp30 fusion protein stimulated T cells in a VPS-specific manner, and antigagp30 antibodies inhibited this stimulation. The implications of these findings for human diseases induced by retroviruses, especially the acquired immunodeficiency syndrome, are obvious and profound. For example, a superantigen encoded by the human immunodeficiency virus (HIV) may initiate an expansion of T cells bearing the appropriate VP element followed by induction of tolerance or deletion of these same T cells. Such a sequence of events may play a critical role in the T-cell dysfunction and attrition associated with HIV infection. No self superantigens have been documented so far in humans. However, the identification of self superantigens as products of retroviral gene integrants has heightened the interest in identifying potential self superantigens in humans. A recent study by Paliard et al. [8*] addressing the role of superantigens in rheumatoid arthritis (discussed below) documented important differences in the usage of VP elements amongst different individuals, consistent with molding of the VP repertoire by self superantigens.

Bacterial superantigens The superantigenicity of many bacterial products con-

tinues to be a focus of intense study by researchers. Recently identified bacterial superantigens include streptococcal erythrogenic toxins A and B [9,10] and streptococcal M protein [ 11,121. The latter is the major virulence factor for rheumatogenic strains of Strepte COCCUS @genes, pathogens that induce multiple autoimmune diseases in humans including acute rheumatic fever, glomerulonephritis, arthritis and carditis (see below).

Interaction of superantigens with TCR and MHC class II molecules The details of the interaction between superantigens, TCR, and MHC class II molecules are now beginning to emerge as a result of a flurry of recent studies. Gascoigne et al. [13] have demonstrated that a secreted, truncated TCR p-chain product interacts directly with superantigens presented on MHC class II molecule-bearing cells. This interaction was dependent on MHC class II expression by the presenting cell, was specifically inhibited by antibodies to MI-K class II molecules and to superantigen, and proceeded in the absence of TCR u-chain. The accumulated evidence suggests that detemrinants on both superantigens and MHC class II molecules interact with TCR VP elements. The dual capacity of superantigens to interact with TCR and MHC class II molecules was explored by Grossman et al. [ 141, who demonstrated that, in the case of staphylococcal enterotoxins, these two binding activities could be dissociated by reduction and alkylation of a disullide loop conserved amongst these toxins. Toxins treated in this manner lost their mitogenic activity but retained their capacity to bind to and signal through MHC class II molecules, indicating that disruption of the disulfide loop interfered with toxin-TCR interaction. Evidence for a direct interaction between superantigens and TCR molecules was also provided by Fleischer et al. [9], who demonstrated that bacterial superantigens can in duce the proliferation of resting T cells in the absence of MHC class II molecules when presented co-crosslinked on beads together with CD2 or CD8 antibodies. Fleischer et al. also provided evidence for a direct interaction between TCR and MHC class II molecules. This was suggested by the observation that a given T cell may respond to a superantigen presented on one accessory cell but fails to respond when the same superantigen is presented on an accessory cell that bears a different set of MHC class II alleles. In previous studies by Kappler, Marrack and their colleagues [ 15,161, the Interaction site for both self and foreign superantigens on TCR VP elements was mapped to a P-pleated sheet region lying away from the antigen and MHC recognition site. Cazenave et d. [17] reached a similar conclusion studying variant forms of VP17a that have lost the ability to react with self superantigen plus I-E. Examination of one mutant VP17a region revealed two amino acid substitutions that lie in an area that overlaps the area implicated by the Denver group in superantigen recognition. More recently, Pullen et ul. [ 181 have further analyzed the interaction of VP elements with superantigen. They ConfrmXd that the recognition site for the superantigen Mls-la lies on the side of VP, and described the masking of this site by amino-linked carbohydrates. They also described mutations in VP that interfere with both antigen and superantigen recognition. These mutations, which map to the first complementarity determining region of VP, apparently disrupted TCR-MHC interaction. The latter iinding confirms the requirement for direct TCR-MHC interaction during superantigen presentation. Unlike the case of MHC class II-presented peptide antigens, several lines of evidence argue against a significant

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Antigen recognition

role for CD4 in the response of T cells to bacterial superantigens. Both CD4 + and CD8+ as well as CD4-, CD8- cells proliferate well in response to bacterial superantigens [ 193,and in most cases transfection of CD4 murlne T-cell hybridomas with CD4 molecules does not affect the response of these hybridomas to bacterial superantigens [20]. That T-cell stimulation by bacterial superantigens is CD4 independent may be a reflection of the high affinity of interaction between the MHC-class II bacterial toxin complex and the TCR This is suggested by the finding that CD4 functions to bolster low ai%ity but not high affinity nominal antigen-TCR interactions [21]. In the case of self superantigens, it was originally reported that only CD4 + T cells respond to Mls determinants [ 221. Recent studies indicate that CD8+ T cells do respond to Mls determinants presented in the context of MHC class II molecules, but that this response is negatively inIluenced by CD8 expression [ 231. This suggests that the interaction between Mls-MHC and TCR is of lower affinity compared with microbial toxin-MHC-TCR interaction, and that self superantigen recognition is enhanced by an additional interaction between CD4 and MHC class II molecules.

transcriptional activation of monokine genes in monocytes [30], and to upregulate mononuclear-cell adhesiveness. Perhaps the most interesting iinding was the demonstration that, in the presence of accessory cells, superantigens can bypass the VP restriction rule to induce the activation and proliferation of VP-mismatched, MHC class II positive, but not MHC class II negative T-cell clones [ 31~:. This has important implications for diseases mediated by superantigens, as mediators such as interferon-y released by T cells responding to toxins in a Vj%restricted manner can upregulate MHC class II expression on bystander VP-mismatched T cells resulting in a polyclonal T-cell activation and greater immune activation. The same report has also documented induction of proliferation of CD56+, CD3 - natural-killer-cell clones by bacterial toxins. This raises the interesting possibility that natural killer cells may play a role in immune responses triggered by superantigens. Superantigens

in autoimmune

diseases

Recent studies have also focused on the interaction of superantigens with MHC class II molecules. It was previously appreciated that microbial superantigens bind outside the antigen-binding groove [24], and that different staphylococcal exotoxins bind to distinct sites on MHC class II molecules [25]. The al domain of HLA-DR a-chain was found to be critical for binding of the staphylococcal toxic shock syndrome toxin (TSST)1 to Ia molecules [26]. More recently, Pontzer et al [27] have used the synthetic peptide approach to map the binding sites for other staphylococcal exotoxins on MHC class II molecules. Using this approach, a peptide corresponding to residues 65-85 of I- b inhibited the binding of staphylococcal exotoxin A ($ EA), but not of TSST-1, to MHC class II molecules. A critical role for the P-chain for the binding of SEA and staphylococcal exotoxin E (SEE) to MIX class II molecules was also UIICOVered by studies by Karp and Long and their colleagues [28]. These authors identified the SEA- and SEE-binding sites on HLA-DR molecules by examining the binding of these toxins to hybrid HLA-DR molecules containing wild-type a-chain and mutant P-chains containing sequences from DRlp, which supports SEA and SEE binding, and DRw538, which does not support toxin binding. The binding site of both toxins was localized by this method to residues 59-94 of the p-chain, in the a-helical portion of the 81 domain.

The capacity of superantigens to break the barriers of MIX restriction to activate large numbers of T and B lymphocytes has led to the hypothesis that superantigens may activate autoreactive T- and B-cell clones to lnitiate and/or accentuate autoimmune diseases. In favour of this hypothesis is the frequent occurrence of synovitis and arthritis in patients with toxic shock syndrome [32], the superantigenicity of bacterial products implicated in the pathogenesis of autoimmune diseases, such as Myc@mrzu artbriditis mitogen and streptococcal M protein, and the Induction of joint Wlammation upon injection of superantigens lntra-articularly [ 331. A useful approach in establishing a role for superantigens in an otherwise idiopathic autoimmune disease is to search for evidence of restricted VP expression by inliltrating lymphocytes. Using this approach, Paliard et al [8*] have described a preferential expression of Vj314 by lymphocytes isolated from the synovial fluids of patients with rheumatoid arthritis. However, the Iinding that the majority of Vj314-bearing cells demonstrated identical VDJ recombination argues against a superantigen-mediated stimulation, as the latter would be expected to result in the expansion of cells bearing the same (or a small set of) VP element but distinct VDJ recombinations. On the other hand, as Paliard et al argue, a superantigen may initially expand all VP14 T cells but only the subset of Vj314 cells that recognize autoantigen may continue to expand, resulting in chronic disease. More studies are required to verify this possibility.

Signal transduction

Future

by superantigens

through

directions

MHC class II molecules In addition to their capacity to signal via TCR, superantigens can also transmit signals via MHC class II molecules that can profoundly affect the functioning of the various limbs of the immune system. Signals delivered by superantigens via MHC class II molecules have been demonstrated to synergize with signals delivered via antigen receptors in inducing lymphocyte proliferation [ 291. Additionally, superantigens have been demonstrated to induce

A number of important issues remain to be resolved. Do endogeneous self superantigens exist in man; what is their nature and what is their role in shaping the human T-cell repertoire? What is the real role of superantigens in autoimmune disw what is the mechanism by which superantigens trigger autoreactive T and B cells and what is the influence of MHC polymorphism on superantigen triggered autoimmunity? Iastly, do superantigens play a role in graft versus host disease, a condition aggravated

Smerantirrens Chatila and Geha

by microbial recontamination; immunodeficiencies?

and in acquired

References and recommended

human

reading

Papers of particular interest, published within the annual period of review, have been highlighted as: . of special interest .. of outstanding interest FRANKEL WN, RUDYC, COFFINJM, HUBERBT: Linkage of Mls Genes to Endogenous Mammary Tumour Vises of Inbred Mice. Nature 1991, 349:526528. This paper documents the linkage of MIS-la gene with m-7, Mls-2a gene with m-13, a Mls-2-likegene with MMTV1 and Mls-3a gene tith m-6.

12.

TOM.UMA, AELIONJA, DOCK??ZR ME, M.%JUMDAR G, SPINEUA DG, KOTB M: T Cell Receptor V Gene Usage by Human T Cells Stimulated with the Superantigen Streptococcal M Protein. J E.@ Med 1991, 174:285-288.

13.

GAXOIGNE NR, AMES KT: Direct Binding of Secreted Tcell Receptor Beta Chain to Superantigen Associated with Class II Major Histocompatibiity Complex Protein. Pnx Natf Acud Sci US A 1991, 88:613Xi16.

14.

GROSSMAN D, COOK RG, SPARROW JT, MOLLICK JA, RICH RR: Dissociation of the Stimulatory Activity of Staphylococcal Enterotoxins for T Cells and Monocytes. J Exp Med 1990,

15.

CHOI YW, HERMANG DIGIUSTOD, WADE T, MARRACK P, KAPPIER J: Residues of the Variable Region of the T-cell Receptor Beta-chain that Interact with S. aureus Toxin Superantigens. Nature 1990, 346:471-473.

16.

PUUEN AM, WADE T, MARRACK P, KAPPLER JW: Identification of the Region of T Cell Receptor Beta Chain that Interacts with the Self-superantigen Mls-la. Cell 1990, 61:13651374.

17.

CAZENAVE PA, MARCHE PN, JOLMN ME, VOEG~ D, BONHOMME F, BANDEIRA A, COUTINHO A: V Beta 17 Gene Polymorphism in Wild-derived Mouse Strains: Two Amino Acid Substitutions in the V Beta 17 Region Greatly Alter T Cell Receptor Specificity. Cell 1990, 63:717-728.

18.

PKJUENAi%,BIlLJ,KUEIORT, bfARR.kCKP, bPPL!ZRJW: zbdJ'SiS of the Interaction Site for the Self Superantigen Mls-la on T Cell Receptor V Beta. J Ezp Med 1991, 173:118?%1192.

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FLEISCHER B, SCHREZENMEIER H: T Cell Stimulation by Staphylococcal Enterotoxins. J Elcp Med 1988: 167:1697-1707.

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SEKALYR-P, CROTF.AU G, BOWMANM, SCHOLLP, BURAKOFFS, GEHA RS: The CD4 Molecule is not Always Required for the T Cell Response to Bacterial Enterotoxins. J Exp Med

1. .

2. .

WOODLAND DL, WP MP, Gomi KJ, PALMERE: An Endogenous Retrovirus Mediating Deletion of Alpha Beta T Cells? Nature 1991, 349:529-530. Documents the linkage of another self superantigen gene, DVbll-2,

with m-9. 3.

172:1831-1841.

DYSON PJ, KNIGHTAM, FAIRCHILD S, SU~PSON E, TOMONAR~ K:

.

Genes Encoding Ligands for Deletion of V Beta 11 T Cells Cosegregate with Mammary Tumour Virus Genomes. Nature 1991, 349531-532. Demonstrates the linkage of genes coding for the DVb family of superantigenswith MMTVintergrants. 4. .

MARRACK P, KUSHN~R E, KAPPIERJ: A Maternally Inherited Superantigen Encoded by a Mammary Tumour Viis. Nature 1991, 349:524-526.

Documents the vertical tmnsmission in the milk of infected mice of a superantigen encoded by infectious MMTV isolates. CHOIY, KAPP~ER JW, MARRACK P: A Superantigen Encoded in the Open Reading Frame of the 3’ Long Terminal Repeat of Mouse Mammary Tumour Virus. Nature 1991, 350:20>207. This paper demonstrates that an open reading frame in the 3’ LTR of exogenous MMTVcodes for a superantigen which, when expressed in a B-cell lymphoma line, specifically stimulates V@4+ T cells. 5. ..

6. ..

ACHA-ORBEA H, SHAKHOV AN, SCAWEILINO L, KOLB E, MULLER V, VESSAS-SHAW A, FUCHSR, BIOCHLINGER K, ROLLINI P, BILIQTE J, ET AL: Cloti Deletion of Vgl4-bearing T Cells in Mice Transgenic for Mammary Tumor Virus. Nature 1991, 350:207-211.

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R, ZLOTN~A, DLALYNAS D, MARRACK P, ENDER~R, SHIMONKEVITZ FITCHF, KAPPLER J: The Major Histocompatibility Complexrestricted Antigen Receptor on T Cells. II. Role of the L3T4 Product. J EM Med 1983, 158:1077-1091.

22.

JANEWAYCA JR, IERNEREA, JASONJM, JONES B: T Lymphocytes Responding to Ml.+locus Antigens are Lytl+-2 and I-A Restricted. Immunogenetics 1980, 10:481-497.

23.

CHVATCHKO Y, MACDONALD HR: CD8+ T Cell Responses to Mls-la Determinan ts Involves Major Histocompatibility Complex Class II Molecules. J Exp Med 1991, 173:77%782.

24.

DEUABONAP, PECCOUDJ, KAPPLERJ, MARRACK P, BENOIST C, MATHS D: Superantigens Interact with MHC Class II Molecules Outside of the Antigen Groove. Cell 1990, 62:111~1121.

25.

SCHOIJ. PR, DIEZ A, GEHA RS: Staphylococcal Enterotoxin B and Toxic Shock Syndrome Toxin-l Bind to Distinct Sites on HLA-DR and HLA-DQ Molecules. J Immund 1989,

26.

KARF DR, TELETXI CL, SCHOLL P, GEHA R, LONG EO: The

Thispaper reaches similar conclusions to [5**] using the transgenic approach and provides evidence for the localization of the supcrantigenic determinant in the carboxy-terminal 32.amino-acid peptide of LTR ORF. 7. ..

HUGINAW, VACCHIOMS, MORSEHC: A Virus-encoded ‘Superantigen’ in a Retrovirus-induced Immunodeficiency Syndrome of Mice. Science 1991, 252~424427. This paper makes the case for a superantigen encoded by a Merent retrotirus, mutine leukemia virus, and derived from another retroviral gene, gag. PAL~ARD X, WEST SG, LAFFERTY JA, CIEMENTSJR, KAPPLER JW, MARRACK P, KO’IZNBL: Evidence for the EtTects of a Superantigen in Rheumatoid Arthritis. Science 1991, 253:325329. Proposes a superantipbased model for the induction of one of the most widespread of the human autoimmune diseases. Provides for the first time a detailed insight into the human TCR VP repertoire. 8. .

9.

FW.CHER B, GERARDY R, ME’IZROTH B, CARRELS, GEUCH D, KOHLER W: An Evolutionary Conserved Mechanism of T Cell Activation by Microbial toxins. J lmmunoll991, 146:11-17.

10.

ABEJ, FORRESTER J, NAKAHARA T, IAFFERTY JA, KO-IZINBL, LEUNG DY: Selective Stimulation of Human T Cells with Streptococcal Erythrogenic Toxins A and B. J Immunol 1991, 146:3747-3750.

11.

TOMAIM, Ko’m M, MAJUMDAR G, BEACHEYEH: Superantigenicity of Streptococcal M Protein. J Exp Med 1990, 172:35+362.

143:2583.

Alpha 1 Domain of the HLA-DR Molecule is Essential for High-afllnity Binding of the Toxic Shock Syndrome Toxin-l. Nature 1990, 346~474-476. 27.

PONTZERCH, RUSSELL JK, JOHNSONHM: Structural Basis for Differential Biding of Staphylococcal Enterotoxin A and Toxic Shack Syndrome Toxin 1 to Class II Major Histocompatibility Molecules. Proc Nat1 Acud Sci U S A 1991, g&125-128.

28.

KARPDR, TELETSKICL, LONGEO: Identilication of HLA-DR Beta Chain Residues Responsible for Binding of Staphylococcal Toxins. FtiEB J 1991, 5~727.

29.

FULEIHAN R, MOURAD W, GEHARS, CHATILA T: Engagement of MHC-class II Molecules by Staphylococcal Rxotoxins Dc-

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N, GEHARS, CHATM T: Transcriptional Activation of Monokine Genes by MHC Class II Ligands. J Immunol19!X, 146:231@2315.

30.

TREDE

31. .

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F, SPITS

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RW, Ww

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DN: Clinical and Laboratory ManifesShock Syndrome. Ann Intern Med 1982,

CANNON GW, COLE BC, WAROJR, .ShU?IiJI, EICHWALO EJ: Arthritogenic Effects of MycopIasma Arthriditis T Ceil Mi-

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Activation Signals to Human T-cells Via Major HistocompatibiIity Complex Class II Molecules. Proc N&l Acad Sci U S A 1991, 88:753%7537. Presents evidence for the activationby superantigensof T cells and naturalkiller cells via MHC class II molecules in a VP unrestricted manner.

T Chatila and RS Geha, Division of Immunology, The Children’s Hospital and the Department of Pediatrics, Harvard Medical School, Boston, Massachusetts 02115, USA.