Superantigens: mechanism of T-cell stimulation and role in immune responses

Acquired allo-tolerance to major or minor histocompatibility antigens indifferently contributes to preventing diabetes development in non-obese diabetic (NOD) mice

Diabetes in NOD mice represents the end stage of a genetically-programmed autoimmune process mediated by T lymphocytes and directed against insulin-producing beta cells. We have shown in a previous study that the course of the disease is significantly inhibited in NOD mice which have been made tolerant at birth to foreign histocompatibility antigens. This early T cell manipulation results in a significant delay of disease onset, reduced overall incidence and less severe alterations of islet cells. In order to characterize better the nature of the foreign tolerogenic determinants responsible for this protection, we have now examined separately the contribution of MHC and non-MHC antigens. Two lines of congenic mice were used as donors of tolerogenic cells, NOD.H-2b, which differ from NOD by the MHC-encoded antigens only, and B10.H-2g7, which differ by all the minor histocompatibility antigens encoded by the B10 background, but which share with NOD mice the same MHC haplotype. Our results show that NOD recipients of F1 semi-compatible cells become specifically tolerant to the set of alloantigens to which they were neonatally exposed. Unresponsiveness, assessed by lack of CTL generation, is profound and specific. Yet, despite the fact that distinct sets of alloreactive T cell precursors are silenced, mice made tolerant indifferently to major or minor histocompatibility antigens are significantly protected against overt diabetes. These results could mean that each set of MHC and non-MHC encoded determinants can independently cross-tolerize a sufficient proportion of the autoreactive repertoire to slow the natural course of the disease. Alternatively, neonatally-acquired tolerance might induce polyclonal activation of the immune system resulting in the suppression or the immunodeviation of potentially harmful, autoreactive T cell clones.

Molecular detection and in vivo analysis of the specific T cell response to a protein antigen

We have analyzed in detail the repertoire of transcripts encoding the V beta chains of the T cell receptor and investigated the T cell response of B10.A mice to pigeon cytochrome c. We were thus able to follow the specific T cell response in vivo after immunization with this protein antigen. The response is first detectable in the draining lymph nodes, then in the spleen and in the blood. It is qualitatively similar in individual animals. It is dominated by a major category of specific T cells harboring a V beta 3-J beta 1.2 rearrangement, and a limited and well-defined set of nucleotide sequences, previously found in several specific T cell hybridomas and clones. This predominance is observed from the onset of the immune response strongly suggesting the notion that there is no variation and, therefore, no maturation of the T cell response in the course of immunization.

pp59fyn mutant mice display differential signaling in thymocytes and peripheral T cells

We have generated mutant mice that do not express pp59fyn, a nonreceptor protein tyrosine kinase related to pp60src, by homologous recombination in embryonic stem cells. fyn- mice did not display an overt phenotype. Because fyn is associated with the T cell receptor (TCR), thymocyte and T cell signaling was analyzed in the mutant background. Cross-linking of TCR-CD3 in thymocytes led to markedly reduced calcium fluxes and abrogated proliferation, whereas mature splenic T cells retained largely normal proliferation despite depressed calcium movements and IL-2 production. Similarly, proliferation induced by Thy-1 cross-linking was reduced in thymocytes but not in splenic T cells. fyn- thymocytes were impaired at a late stage of maturation and showed limited clonal deletion to the Mls-1a self-super-antigen but not to staphylococcal enterotoxin A. These results implicate fyn as a critical component in TCR signaling in thymocytes and, potentially, in the process that determines T cell repertoire in the adult mouse.

Group A streptococcal M proteins: virulence factors and protective antigens

Rebecca Lancefield described group A streptococcal M proteins over 50 years ago, and they have remained at the forefront of investigations into streptococcal pathogenicity to the present day. As described in this review, they form cell surface fibrils with several functions, ranging from resisting phagocytosis and inducing host-crossreactive antibodies, to presenting the host immune system with an accessible protective antigen.

Memory T cells are anergic to the superantigen staphylococcal enterotoxin B

We have used staphylococcal enterotoxin B (SEB) to study the role of naive and memory T cells in the induction of peripheral tolerance. After administration of SEB to mice, the numbers of naive and memory T cells increase, as does the proportion of memory T cells, which are unresponsive to further stimulation with SEB in vitro. In addition, memory T cells generated in response to conventional antigen, which proliferate and provide help to B cells in the presence of the conventional antigen, fail to respond to superantigen. Hence, memory T cells, in general, are anergized by SEB. These results suggest that SEB-induced activation and anergy reflect the combined responses of naive and memory T cells. The differential activation vs. anergy of naive and memory T cells by superantigen may be related to cytokine production and may play an important role in the etiology of autoimmune diseases or immunodeficiency diseases such as acquired immune deficiency syndrome.

Superantigens

The endogenous superantigens (the enigmatic minor lymphocyte-stimulating antigens) have been identified; they are encoded by integrated mouse mammary tumor viruses. The retroviral superantigens appear to be transmembrane glycoproteins, and their highly variable extracellular carboxyl terminus is responsible for V beta interaction. In spite of intensive efforts the precise structure-function relationship for the superantigens is not yet clear. The most important consequences of the introduction of the superantigens in vivo are shock and T-cell depletion and anergy. The search for novel superantigens related to human diseases has started.

Expression of a transgenic T cell receptor beta chain enhances collagen-induced arthritis

SWR/J transgenic (tg) mice were generated expressing the TCR beta chain derived from an anticollagen type II (CII) arthritogenic T cell clone. The SWR/J strain was selected because it is resistant to collagen-induced arthritis (CIA) and lacks the V beta gene segment used by the T cell clone. Expression of the tg beta chain on all thymocytes and peripheral lymph node T cells led to a more efficient anti-CII immune response, but did not confer CIA susceptibility to SWR/J mice. Nevertheless, this tg beta chain enhanced predisposition to CIA as (DBA/1 x SWR) F1 beta tg mice were more susceptible than normal F1 littermates. Our results demonstrate that the expression of the tg beta chain contributes to CIA susceptibility, but by itself it is not sufficient to overcome CIA resistance in the SWR/J strain.

Analysis of clonality of tumour infiltrating lymphocytes in breast cancer and uveal melanoma

Fresh tumour infiltrating lymphocytes (TILs) from 6 uveal melanomas and 4 breast cancers were analysed by flow cytometry with a panel of 6 monoclonal antibodies to V beta regions of the T cell receptor (V betas 5a, 5b, 5c, 6, 8a and 12a). With a single exception where one TIL sample lacked V beta 12a, lymphocytes from both tumour and blood contained cells reactive with all 6 probes, and no probe was highly dominant or missing. The proportions of reactive cells differed between tumour and blood within each patient. The data indicate that while tumour infiltrating lymphocytes have a capacity to locate selectively within the tumour they nonetheless comprise a population expressing a diversity of TCR V beta genes.

Superantigen implicated in dependence of HIV-1 replication in T cells on TCR V beta expression

In the pathogenesis of AIDS it is not yet understood whether the small fraction of CD4+ T cells (approximately 1%) infected with the human immunodeficiency virus (HIV) are randomly targeted or not. Here we present evidence that human CD4 T-cell lines expressing selected T-cell antigen receptor V beta gene products can all be infected in vitro with HIV-1, but give markedly different titres of HIV-1 virion production. For example, V beta 12 T-cell lines from several unrelated donors reproducibly yielded up to 100-fold more gag gene product (p24gag antigen) than V beta 6.7a lines. This is consistent with a superantigen effect, because the V beta selectivity was observed with several divergent HIV-1 isolates, was dependent on antigen-presenting cells and on major histocompatibility complex (MHC) class II but was not MHC class II-restricted. The in vivo significance of these findings is supported by the preferential stimulation of V beta 12+ T cells by freshly obtained irradiated antigen-presenting cells from some HIV-1-seropositive but not HIV-1-negative donors. Moreover, cells from patients positive for viral antigen (gp120) were enriched in the V beta 12 subpopulation. V beta 12+ T cells were not deleted in AIDS patients, however, raising the possibility that a variety of mechanisms contribute to T-cell depletion. Our results indicate that a superantigen targets a subpopulation of CD4+ cells for viral replication.

Specificity of rat T cell receptor V beta chain usage in proliferative responses to staphylococcal enterotoxin B

The staphylococcal enterotoxins (SE) are potent stimulators of T cell proliferative responses in humans and in mice. In these systems, the toxins function as superantigens and stimulate T cells bearing particular V beta. Although homology between the V beta of mice and humans is limited, related V beta families may respond to certain SE in a similar fashion. In this report, we have characterized the rat T cell response to staphylococcal enterotoxin B (SEB). Rat T cells from several lymphoid organs proliferated strongly in response to both commercially available and recombinant SEB. Using a polymerase chain reaction assay, we identified the predominant V beta families stimulated by this enterotoxin. The T cell receptor V beta elements used by rat T cells were similar to but not completely identical with those used by mice. The V beta profile stimulated depended on the purity of the SEB preparation used.

Effects of staphylococcal enterotoxin B on rodent mast cells

Staphylococcal enterotoxin B (SEB) was tested in rodent mast cell cultures for the release of serotonin. Both rat RBL-2H3 mast cells and murine peritoneal cells released serotonin after SEB stimulation in culture. Release of serotonin in RBL-2H3 cells depended on the concentration of SEB; an appreciable release was seen at 50 micrograms/ml. The release of serotonin was not due to cell death. Serotonin release could be enhanced by bradykinin but not by vasoactive intestinal peptide, substance P, lipopolysaccharide from Salmonella typhimurium, the calcium ionophore A23187, acetylcholine, adenosine, 5-hydroxyeicosatetraenoic acid, indomethacin, or phorbol myristate acetate. SEB bound directly to the membrane of RBL-2H3 mast cells, and the SEB-binding site, the presumptive receptor, appeared to be a protein. The SEB receptor could not be capped under membrane-capping conditions, and serotonin release could not be enhanced by attempts to cross-link the receptor. These results suggest that mast cells may be an important cell type involved in SEB toxicosis and that release of serotonin may be enhanced by activation of the kinin-kallikrein system.

Multiple, autoreactive TCR V beta genes utilized in response to a small pathogenic peptide of an autoantigen in EAU

The restricted usage of particular T cell receptor beta chain genes in autoimmune disease was studied in LEW rats using T cell hybridomas specific for an immunodominant sequence of bovine retinal S-Ag, which induces experimental autoimmune uveoretinitis. T cell hybridomas from a pathogenic T cell line, R858, specific for residues 273-289 of bovine retinal S-Ag were analyzed in order to determine the contribution of their TCR V beta to self specificity as determined by recognition of the pathogenic epitope represented in the autologous rat S-Ag sequence. Six different, functional TCR rearrangements were expressed by the panel of hybridomas, including two distinct V beta 8.2 rearrangements and functional V beta 10, V beta 14, V beta 19 rearrangements, and an unidentified V beta gene. All hybridomas were Ag specific and reacted both to nonself-peptide derivatives as well as to self-peptide homologues. No unique pattern of peptide reactivity distinguished V beta 8.2+ hybridomas from V beta 8.2- hybridomas; all of the hybridomas were most reactive to the nonself sequences and reacted to self peptide with one to three orders of magnitude less sensitivity. However, all V beta 8.2+ hybridomas were much better responders overall and were activated by lower concentrations of all peptides than were V beta 8.2- hybridomas. Although V beta 8.2 gene usage is strongly associated with autoimmune pathology, these data show that in LEW rats several different TCR V beta genes are utilized in response to a short pathogenic sequence of this autoantigen and show that V beta 8.2 receptors are not uniquely self-reactive. However, the enhanced reactivity to Ag of V beta 8.2+ hybridomas relative to V beta 8.2- hybridomas specific for the same peptide may help explain the close association of V beta 8.2 TCR gene usage with pathogenicity found in autoimmune disease models.

Silent allelic variants of a T-cell receptor V beta 12 gene are present in diverse human populations

Amino acid substitutions in variable regions of the T-cell receptor (TCR) can alter T-cell reactivity; however, relatively little is known about the extent of allelic variation in human TCR coding sequences. In the present studies, coding region variation in the human TCR V beta 12.2 gene was examined in detail. Virtually the entire V beta 12.2 coding region was screened for nucleotide substitutions by single-stranded conformational polymorphism analysis. Four alleles were identified in a sample population of 90 unrelated people from diverse genetic backgrounds. Three of the alleles were common, with estimated frequencies of 0.32, 0.47, and 0.20. Sequence analyses revealed that variation between the alleles was confined to three single-base differences in codons 24, 31, and 45; none of these changes altered the amino acid sequence. No evidence for other coding region differences in this gene were found. This analysis suggests that coding region variation in V beta 12.2 is limited, and amino acid sequence is highly conserved.

Major histocompatibility complex-restricted recognition of retroviral superantigens by V beta 17+ T cells

It has been established that at least some V beta 17+ T cells interact with an endogenous superantigen encoded by the murine retrovirus, Mtv-9. To analyze the role of major histocompatibility complex (MHC) class II molecules in presenting the Mtv-9 encoded superantigen, vSAG-9 to V beta 17+ hybridomas, a panel of nine hybridomas was tested for their ability to respond to A20/2J (H-2d) and LBK (H-2a) cells which had been transfected with the vSAG-9 gene. Whereas some of the hybridomas recognized vSAG-9 exclusively in the context of H-2a, other hybridomas recognized vSAG-9 exclusively in the context of H-2d or in the context of both H-2d and H-2a. These results suggest that: (a) the class II MHC molecule plays a direct role in the recognition of retroviral superantigen by T cells, rather than serving simply as a platform for presentation; and, (b) it is likely that components of the TCR other than V beta are involved in the vSAG-9/TCR/class II interaction.

Selective manipulation of the human T-cell receptor repertoire expressed by thymocytes in organ culture

A recently described organ culture system for human thymocytes is shown to support the generation of a diverse T-cell receptor repertoire in vitro: thymocytes of the alpha beta lineage, including representatives of the V beta families 5.2/5.3, 6.7, and 8, accounted for the majority of T-cell receptor-positive cells throughout a 3-week culture period. Thymocytes bearing gamma delta receptors were also identified, particularly among the CD4 CD8 double-negative subset. The T-cell receptor repertoire expressed in organ culture responded to experimental manipulation with staphylococcal enterotoxins. Staphylococcal enterotoxin D (a powerful activator of human peripheral T cells expressing V beta 5.2/5.3 receptors) caused a marked reduction of V beta 5.2/5.3 expression, as determined with the V beta-specific antibody 42/1C1. Evidence is presented that this loss of V beta 5.2/5.3 expression resulted from the selective deletion of activated thymocytes by apoptosis, in concert with T-cell receptor modulation. These effects of staphylococcal enterotoxin D were specific (since staphylococcal enterotoxin E did not influence V beta 5.2/5.3 expression) and V beta-selective (since expression of V beta 6.7 remained unaffected by staphylococcal enterotoxin D). On the basis of these observations, we suggest that thymic organ culture provides a powerful approach to study the generation of the human T-cell repertoire.

Heat shock protein Hsp60-reactive gamma delta cells: a large, diversified T-lymphocyte subset with highly focused specificity

Previously, we detected a subset of gamma delta T cells in the newborn mouse thymus that responded to the mycobacterial heat shock protein Hsp60, as well as with what seemed to be a self-antigen. All of these cells expressed V gamma 1, most often in association with V delta 6+. It was not clear, however, whether similar, mature gamma delta cells with Hsp60 reactivity are common outside of the thymus, or rather, whether they are largely eliminated during development. From the data presented here, we estimate that gamma delta cells responding to Hsp60 comprise 10-20% of normal splenic and lymph node gamma delta T cells. Such cells, derived from adult spleen, always express a V gamma 1-J gamma 4-C gamma 4 gamma chain, although not all cells with this gamma chain show Hsp60 reactivity. Many of these V gamma 1+ cells also express V delta 6-J delta 1-C delta, though fewer than in V gamma 1+ cells from the newborn thymus. Extensive diversity is evident in both the gamma and delta chain junctional amino acids of the receptors of these cells, indicating that they may largely develop in the thymus of older animals or undergo peripheral expansion. Finally, we found that all such cells responding to both a putative self-antigen and to mycobacterial Hsp60 respond to a 17-amino acid synthetic peptide representing amino acids 180-196 of the Mycobacterium leprae Hsp60 sequence. This report demonstrates that a large subset of Hsp60-reactive peripheral lymphoid gamma delta T cells preexists in normal adult mice, all members of which respond to a single segment of this common heat shock protein.

Superantigens: biology, immunology, and potential role in disease

Superantigens are unique products of bacteria and viruses which, in combination with class II major histocompatibility complex molecules, are capable of stimulating a large fraction of T cells in an affected individual. This stimulation primarily involves the variable region of the T cell receptor beta chain (V beta). The discovery of superantigens and the elucidation of their immunologic properties have provided valuable tools for the investigation of the immune system in both normal and diseased animals. Most importantly, recent work suggests that superantigens play a role in a number of diverse pathological conditions, including toxic shock syndrome and autoimmune diseases such as rheumatoid arthritis.

Human hematopoietic cells and thymic epithelial cells induce tolerance via different mechanisms in the SCID-hu mouse thymus

To study the role of thymic education on the development of the human T cell repertoire, SCID-hu mice were constructed with fetal liver and fetal thymus obtained from the same or two different donors. These animals were studied between 7 and 12 mo after transplantation, at which times all thymocytes and peripheral T cells were derived from stem cells of the fetal liver graft. Immunohistology of the thymus grafts demonstrated that thymic epithelial cells were of fetal thymus donor (FTD) origin. Dendritic cells and macrophages of fetal liver donor (FLD) origin were abundantly present in the medullary and cortico-medullary areas. Thymocytes of SCID-hu mice transplanted with liver and thymus of two different donors (FLDA/FTDB animals) were nonresponsive to Epstein-Barr virus-transformed B cell lines (B-LCL) established from both the FLDA and FTDB, but proliferated vigorously when stimulated with third-party allogeneic B-LCL. Mixing experiments showed that the nonresponsiveness to FTDB was not due to suppression. Limiting dilution analysis revealed that T cells reacting with the human histocompatibility leukocyte antigens (HLA) of the FLD were undetectable in the CD8+ T cell population and barely measurable in the CD4+ subset. On the other hand, CD4+ and CD8+ T cells reactive to the HLA antigens of the FTD were readily detectable. These results indicate that FLD-reactive cells were clonally deleted, whereas FTD-reactive cells were not. However, the frequencies of FTD-reactive T cells were consistently twofold lower than those of T cells specific for any third-party B-LCL. In addition, the cytotoxic activity and interleukin 2 production by FTD-specific T cells were lower compared with that of third-party-reactive T cell clones, suggesting that FTD-specific cells are anergic. These data demonstrate that T cells become tolerant to autologous and allogeneic HLA antigens expressed in the thymus via two different mechanisms: hematopoietic cells present in the thymus induce tolerance to "self"-antigens by clonal deletion, whereas thymic epithelial cells induce tolerance by clonal energy and possibly deletion of high affinity clones.

Induction of nitric oxide synthase activity by toxic shock syndrome toxin 1 in a macrophage-monocyte cell line

Toxic shock syndrome toxin 1 (TSST-1) is a Mr 22,000 protein produced by Staphylococcus aureus. It is thought to be the cause of toxic shock syndrome. We investigated the hypothesis that TSST-1 induces nitric oxide (NO) synthase and that the NO formed may be involved in the pathogenesis of toxic shock syndrome. We used the murine monocyte-macrophage cell line J744.2 that responds to TSST-1 and also expresses NO synthase activity upon immunological stimulation. J774.2 macrophages stimulated with TSST-1 (10-100 nM) generated nitrite, a breakdown product of NO, and induced concentration-dependent elevations of cGMP in the pig kidney epithelial cell line (LLC-PK1). This latter effect was due to the generation of L-arginine-derived NO for it was (i) abolished by oxyhemoglobin (10 microM), a scavenger of NO, or by methylene blue (10 microM), an inhibitor of NO-activated guanylate cyclase; (ii) potentiated by superoxide dismutase (100 units/ml), which prolongs the life of NO; (iii) inhibited by NG-monomethyl-L-arginine (0.3 mM), an inhibitor of NO synthase; (iv) significantly decreased when L-arginine (0.4 mM) in the medium was replaced by D-arginine (0.4 mM). Moreover, TSST-1 (100 nM) enhanced the activity of cytosolic NO synthase in J774.2 cells. Hydrocortisone (1 microM) but not indomethacin (5 micrograms/ml) or salicylic acid (5 micrograms/ml) prevented the generation of NO2- and the increases in cGMP levels in LLC-PK1 cells induced by J774.2 cells stimulated with TSST-1. The effects of hydrocortisone were partially reversed by coincubation with RU 486 (1 microM), an antagonist of glucocorticoid receptors. Thus, TSST-1 and perhaps other exotoxins produced by Gram-positive bacteria induce NO synthase and the increased NO formation may contribute to toxic shock syndrome and possibly to changes in the immune responses that accompany infection.

V beta 17 T-cell deletion by endogenous mammary tumor virus in wild-type-derived mouse strain

The wild-type-derived mouse strain PWK possesses a beta-chain variable region V beta 17a2 allele, which is expressed on mature T cells as part of the T-cell receptor of most mice expressing I-E, whereas V beta 17 T cells are deleted in all I-E+ laboratory mice bearing a V beta 17a1 allele. However, (PWK x CBA/J)F1 progeny and the wild-type-derived mouse strain MAI, which possesses the V beta 17a2 allele, display deletion of V beta 17 T cells. Analysis of (PWK x CBA/J) x PWK and of (PWK x MAI) x PWK backcrosses demonstrates that endogenous mouse mammary tumor virus MTV-6 from CBA/J and a MTV from strain MAI control the clonal deletion of V beta 17a2 as well as V beta 3 T cells. Furthermore, among I-E- progeny of a (MAI x C57BL/6) x C57BL/6 backcross, we observed that mice inheriting MTV of MAI have a reduced level of V beta 17 T cells, suggesting that the clonal deletion of V beta 17a2 T cells can be mediated in the absence of the I-E molecule. The 3' long terminal repeat of MTV MAI was cloned and translation of the open reading frame was compared to those of MTV known to encode superantigens. Comparisons indicate that MTV MAI has significantly diverged from the other MTVs. However, MTV MAI and MTV-6 share a stretch of 11 identical amino acids at the C terminus, which is divergent in MTV reacting with other V beta s. This suggests that this region is involved in determining the specificity toward V beta s and has been selectively conserved through evolution of the Mus species.

Mycoplasma arthritidis mitogen. V beta engaged in mice, rats, and humans, and requirement of HLA-DR alpha for presentation

OBJECTIVE

Mycoplasma arthritidis mitogen (MAM) is mitogenic for mouse, rat, and human T cells, and behaves as a superantigen in mice through its capacity to bind to the alpha chain of I-E molecules and engage entire sets of T cells expressing specific V beta. Here, we have attempted to fully characterize the V beta-engaging activities of MAM in mice, and define similar activities in rats and humans.

METHODS

Multiprobe RNase-protection assays and mice transgenic for human DR alpha, DR beta, and DR alpha beta were utilized for this purpose.

RESULTS

MAM-reactive V beta in the mouse included not only the previously reported V beta 6, V beta 8.1, V beta 8.2, and V beta 8.3, but also V beta 5.1. In the rat, engagement of V beta 5.1, V beta 6, V beta 8.1, and V beta 8.2, but not V beta 8.3, was documented, whereas in humans, the engaged V beta included primarily V beta 19.1 (alternatively termed V beta 17.1) and, to a lesser extent, V beta 3.1, V beta 11.1, V beta 12.1, and V beta 13.1. In DR transgenic E alpha- E beta- mice, presentation of MAM and engagement of specific V beta was effected by DR alpha.

CONCLUSIONS

Homologous V beta are engaged by MAM in mice, rats, and humans, presumably through a binding site similar to that proposed previously for other superantigens. MAM presentation primarily via the nonpolymorphic DR alpha makes it unlikely that there is involvement of such a superantigen in the pathogenesis of autoimmune diseases known to be associated with certain DR haplotypes. The possibility cannot be excluded, however, that superantigen-activated T cells may lead to disease by cross-reactions with self-antigens presented by particular DR haplotypes.

Revisiting and revising suppressor T cells

A great deal of experimental evidence supports the phenomenon of immunological suppression. The molecular mechanisms to explain the phenomenology have, however, remained controversial. In this review, the data are reinterpreted in light of the recent advances in the understanding of T-cell subsets, the cross-regulatory properties of lymphokines and the differential presentation capacities of different antigen-presenting cell types.

Macaque CD4+ T-cell subsets: influence of activation on infection by simian immunodeficiency viruses (SIV)

Simian immunodeficiency virus (SIV) infects a small number of CD4+ T cells including "memory" T cells. The following describes the cell surface markers which may delineate subsets of CD4+ memory T cells and reviews how memory CD4+ T cells are activated and regulated through the T-cell receptor and such accessory receptors as CD28. The factors which may influence initial expression and infection of T cells by CD4 are discussed. Unlike activated and infected T cells, unstimulated CD4+ T cells have little or no SIV DNA detectable in the genomic fraction, but key activation signals may promote integration of viral DNA in memory T cells. Bacterial superantigens (SuperAg) can promote increased levels of SIV viral DNA in mature and immature T cells. Immunodeficiency virus products such as gp120, Nef, and Tat can affect CD4+ T-cell function. Whereas Nef can reduce expression of CD4, Tat reduces the expression of CD28. We hypothesize that the lack of expression of key accessory molecules on CD4 lineage T cells infected with immunodeficiency viruses may make infected T cells more susceptible to recall-antigen-induced programmed cell death.

V beta repertoire in rats and implications for endogenous superantigens

Endogenous superantigens of mice, encoded by mammary tumor virus proviral integrants, induce intrathymic deletion of entire T cell populations that express specific V beta gene products, a phenomenon proposed to be important in self-tolerance and prevention of toxic responses to exogenous microbial superantigens. Evidence for the presence of V beta selecting/deleting endogenous superantigens in other species is lacking. We report here that rats do not exhibit endogenous superantigen-induced V beta clonal deletions despite their strong response to bacterial superantigens. These findings indicate that endogenous superantigens are not obligatory in V beta repertoire shaping.

Mutations defining functional regions of the superantigen staphylococcal enterotoxin B

Staphylococcal enterotoxin B (SEB) is both a superantigen and toxin. As a superantigen, SEB can bind to major histocompatibility complex (MHC) class II molecules to form a ligand for alpha/beta T cell receptors bearing particular V beta elements. As a toxin, SEB causes rapid weight loss in mice sometimes leading to death. We show here that both of these functions map to the NH2-terminal portion of the toxin. Three regions were identified: one important in MHC class II binding, one in T cell recognition, and one in both functions. These results support the conclusion that the toxicity of SEB is related to massive T cell stimulation and release of cytokine mediators and show that the residues interacting with MHC and the T cell receptor are intertwined.

Dual antigenic recognition by cloned human gamma delta T cells

The function of gamma delta T cells is still elusive. The nature of the antigens that they recognize and the mode of presentation of these antigens are largely unknown. The majority of human peripheral gamma delta T cells bear a V gamma 9/V delta 2 T cell receptor, and display nonclonal reactivity to mycobacteria, without restriction by MHC. It is unknown whether these cells have clonal antigenic specificity as well. Here we describe rheumatoid arthritis-derived V gamma 9/V delta 2 T cell clones, displaying dual antigenic recognition: a nonclonal, MHC-unrestricted recognition of mycobacteria, and a clonal recognition of a short tetanus toxin peptide presented by HLA-DRw53, a nonpolymorphic class II MHC molecule associated with susceptibility to rheumatoid arthritis. This is the first evidence that V gamma 9/V delta 2 T cells can recognize nominal antigenic peptides presented by class II MHC molecules. These results suggest that much like alpha beta T cells, V gamma 9/V delta 2 cells may contribute to the immune response against foreign antigens in an antigen-specific and MHC-restricted manner. The reactivity of these gamma delta T cells to mycobacteria may represent a superantigen-like phenomenon.

Recombinant epidermolytic (exfoliative) toxin A of Staphylococcus aureus is not a superantigen

The epidermolytic (exfoliative) toxins produced by Staphylococcus aureus cause epidermolysis and skin blistering. In addition, they have been implicated to belong to the group of T lymphocyte stimulating molecules known as "superantigens". Here we show that recombinant epidermolytic toxin A produced in S. aureus is not mitogenic for human and murine T lymphocytes. We discuss the possibility that minute contaminations of highly mitogenic exoproteins may cause the mitogenicity in several proteins that are reported to be superantigens.

Induction of proliferative responses of T cells from Babesia bovis-immune cattle with a recombinant 77-kilodalton merozoite protein (Bb-1)

A major portion of a Babesia bovis-specific gene encoding a 77-kDa merozoite protein (Bb-1) produced during natural infection in cattle and in microaerophilous culture was subcloned into the pGEX1N expression vector. Recombinant Bb-1 protein fused to glutathione S-transferase (Bb-1-GST) was used to examine cellular immune responses in B. bovis-immune cattle. Sera from rabbits immunized with Bb-1-GST reacted with fusion protein and with the native antigen present in crude B. bovis but not with B. bigemina merozoites. Bb-1-GST but not GST induced strong proliferation of T lymphocytes from these immune cattle, and Bb-1-reactive T-cell lines which consisted of a mixed population of either CD4+ and CD8+ cells or CD4+, CD8+, and "null" (gamma delta T) cells were established by in vitro stimulation of peripheral blood mononuclear cells with the recombinant fusion protein. Three CD4+ CD8- and three CD4- CD8+ Bb-1-specific T-cell clones were identified after limiting-dilution cloning of the cell lines. The studies described here demonstrate that the 77-kDa protein of B. bovis contains T-cell epitopes capable of eliciting proliferation of two types of T cells in immune cattle, an important consideration for the design of a recombinant subunit vaccine.

Variable gene usage of T cell receptor gamma- and delta-chain transcripts expressed in synovia and peripheral blood of patients with rheumatoid arthritis

The synovial tissue and fluid of patients with rheumatoid arthritis (RA) contain activated T cells that probably have a central role in the disease process which leads to joint destruction. A subset of T cells, gamma delta T cells detected at the site of inflammation, may be important in the pathogenesis of the disease. This study investigated variable (V) gene usage of gamma delta T cell receptors (TcRs) expressed in synovia and peripheral blood of patients with RA by using the polymerase chain reaction (PCR) to amplify TcR gamma- and delta-chain transcripts. Most patients showed no restriction in V gamma gene usage since synovial mononuclear cells (SMC) expressed TcR gamma-chain transcripts which used the same set of V gamma genes as peripheral blood mononuclear cells (PBMC). In contrast, the majority of patients expressed a restricted SMC V delta-chain repertoire biased towards V delta 1, but V delta 2 mRNA transcripts were also detected, albeit at low levels in some patients. The TcR delta-chain repertoires of PBMC from healthy control subjects were also characterized. There was variation in the TcR delta-chain repertoires of PBMC from patients when compared with controls, particularly with respect to expression of V delta 4. V delta 4 mRNA transcripts were expressed in PBMC of only two of seven RA patients in contrast with eight of the nine controls (P = 0.03). These findings are compatible with reports that gamma delta T cells in the rheumatoid synovium are reactive to Mycobacterium tuberculosis and that response to M. tuberculosis is restricted to V gamma 9/V delta 2-bearing T cells, if a superantigen is involved in the pathogenesis of RA.

Animal models for the study of human immunodeficiency virus infections

Recent studies in animal models have advanced our understanding of determinants of acquired immune deficiency syndrome pathogenesis. They have also indicated the potential importance of both the envelope glycoprotein of the virus that causes acquired immune deficiency syndrome and cell surface molecules in vaccine-elicited protective immunity.

The development of functionally responsive T cells

The work reviewed in this article separates T cell development into four phases. First is an expansion phase prior to TCR rearrangement, which appears to be correlated with programming of at least some response genes for inducibility. This phase can occur to some extent outside of the thymus. However, the profound T cell deficit of nude mice indicates that the thymus is by far the most potent site for inducing the expansion per se, even if other sites can induce some response acquisition. Second is a controlled phase of TCR gene rearrangement. The details of the regulatory mechanism that selects particular loci for rearrangement are still not known. It seems that the rearrangement of the TCR gamma loci in the gamma delta lineage may not always take place at a developmental stage strictly equivalent to the rearrangement of TCR beta in the alpha beta lineage, and it is not clear just how early the two lineages diverge. In the TCR alpha beta lineage, however, the final gene rearrangement events are accompanied by rapid proliferation and an interruption in cellular response gene inducibility. The loss of conventional responsiveness is probably caused by alterations at the level of signaling, and may be a manifestation of the physiological state that is a precondition for selection. Third is the complex process of selection. Whereas peripheral T cells can undergo forms of positive selection (by antigen-driven clonal expansion) and negative selection (by abortive stimulation leading to anergy or death), neither is exactly the same phenomenon that occurs in the thymic cortex. Negative selection in the cortex appears to be a suicidal inversion of antigen responsiveness: instead of turning on IL-2 expression, the activated cell destroys its own chromatin. The genes that need to be induced for this response are not yet identified, but it is unquestionably a form of activation. It is interesting that in humans and rats, cortical thymocytes undergoing negative selection can still induce IL-2R alpha expression and even be rescued in vitro, if exogenous IL-2 is provided. Perhaps murine thymocytes are denied this form of rescue because they shut off IL-2R beta chain expression at an earlier stage or because they may be uncommonly Bcl-2 deficient (cf. Sentman et al., 1991; Strasser et al., 1991). Even so, medullary thymocytes remain at least partially susceptible to negative selection even as they continue to mature.(ABSTRACT TRUNCATED AT 400 WORDS)

Unresponsiveness to self-peptides of S-antigen in EAU: an overview of recent results

Several observations in the characterization of EAU are examined. First, sequences of heterologous S-Ag (bovine S-Ag in LEW rats) which induce strong in vitro T cell proliferative responses are dissociated from sequences which induce EAU. Strong in vitro responses were detected only to nonself peptide homologues. Second, T cells specific for self-sequences of S-Ag are unresponsive in vitro. Third, TCR V beta 8 gene usage is associated with pathogenic T cells. V beta 8.2 bearing hybridomas from a pathogenic line exhibited enhanced reactivity to pathogenic self-peptides, but were unresponsive unless presented Ag on nonirradiated, splenic APC. We propose that these findings reflect self, nonself discrimination of the epitopes on heterologous autoantigen, and examine the hypothesis that TCR containing V beta 8 have enhanced avidity for MHC complexed with autologous sequences, but that these V beta 8 autoreactive T cells appear unresponsive in vitro due to mechanisms of self-tolerance involving superantigen/coligand participation.

Retroviral super-antigens and T cells

For many years immunologists have been intrigued by a series of potent antigens encoded in the murine genome. These antigens, originally termed minor lymphocyte stimulating (Mls) antigens, are capable of inducing extremely strong T cell proliferative responses when presented in the context of MHC class II molecules. Recently, Mls antigens have been shown to stimulate T cells bearing particular T cell receptor V beta elements, leading to the designation of super-antigens. The endogenous expression of these super-antigens in mice results in the clonal elimination of large numbers of T cells in order to maintain self-tolerance. In this review we discuss the recent identification of endogenous super-antigens as retroviral gene products. In addition, we analyze the role of class II MHC molecules in the presentation of endogenous super-antigens to T cells. Finally, we discuss the dramatic effect of retroviral super-antigens on the T cell repertoire.

T cells and human autoimmune thyroid disease: emerging data show lack of need to invoke suppressor T cell problems

Human T cells recognize self and foreign antigens when such antigens are processed into small peptides and bound to molecules coded for by genes of the HLA region on chromosome 6. The part of the T-cell surface which is responsible for such recognition is a set of molecules coded for by a variety of genes and known as the T-cell-receptor complex. In animal models, T cells are able to transfer autoimmune thyroiditis and T cells have, therefore, long been implicated in the etiology of human autoimmune thyroid disease (AITD). Information gained from the study of intrathyroidal T cells and thyroid antigen-specific T-cell clones has shown that in patients with Graves' disease, mainly helper T-cell clones have been obtained, whereas in autoimmune (Hashimoto's) thyroiditis cytolytic T-cell clones may be predominant. Such thyroid antigen-specific T cells have now been shown to recognize one or other of the three major thyroid-specific antigens; thyroglobulin, thyroid peroxidase, or the TSH receptor and efforts are currently in progress to characterize the T-cell epitopes of these major thyroid autoantigens. Recent findings of restricted T-cell receptor V gene use amongst intrathyroidal T cells confirm the primary role of T cells in human thyroid autoimmune processes leading to AITD. However, the mechanisms whereby such autoreactive T cells escape deletion and anergy, and how they become activated, remain uncertain. There is compelling evidence that the thyroid cell itself, by expressing HLA molecules, and presenting antigen directly to the T cells, may initiate disease, perhaps after an external insult.

Identification of the staphylococcal enterotoxin A superantigen binding site in the beta 1 domain of the human histocompatibility antigen HLA-DR

The staphylococcal enterotoxin A (SEA) is a superantigen that must bind to class II molecules of the major histocompatibility complex to be recognized by T cells. In humans, most HLA-DR class II allelic and isotypic forms, such as DR1, bind SEA well. DRw53 is an exception, binding SEA very poorly. We have localized this difference to a single residue (amino acid 81) in the beta 1 domain. A highly conserved histidine at residue 81 allows SEA binding, but a tyrosine does not. Residue 81 is predicted to lie in an alpha-helix on the surface of the molecule, with its side chain pointing up out of the pocket associated with binding of conventional peptide antigens. This finding supports the hypothesis that superantigens and conventional antigens bind to different sites on the class II molecule.

Identification of a T cell receptor beta chain variable region, V beta 20, that is differentially expressed in various strains of mice

A cDNA library of TCR beta chain transcripts from BALB/c thymocytes was constructed using anchored polymerase chain reaction (PCR). Screening of this library led to the identification of a V beta gene segment, V beta 20, structurally related to V beta 3 and V beta 17. Genomic analysis of mice displaying deletions in their V beta loci, together with mapping of cosmid clones, situated V beta 20 2.5 kb beside V beta 17. The expression of V beta 20 was estimated by PCR in mice of different H-2 and Mls types. Peripheral T cells from H-2k and H-2d mice did not express V beta 20, whereas in I-E-negative mice (C57Bl/6 and SJL), V beta 20 transcripts were detected. The lack of V beta 20 transcripts in (C57Bl/6 x CBA/J)F1, (C57Bl/6 x BALB/c)F1, and in congenic B6.H-2k mice suggests that the differential use of V beta 20 is due to an I-E-mediated clonal deletion process. The involvement of the Mls super antigens was excluded by analysis of all Mls type combinations. The nature of the V beta 20-deleting element(s) is discussed in the context of the I-E/superantigen systems controlling the expression of V beta 11 and V beta 17.

Endogenous superantigen expression is controlled by mouse mammary tumor proviral loci

Superantigens are defined by their ability to stimulate T cells based predominantly on their V beta expression and ability to delete T cells in the thymus when expressed endogenously. We show here that the expression of one endogenous superantigen, Etc-1, is controlled by the expression of the open reading frame region of the 3' long terminal repeat of the mouse mammary tumor proviral gene, Mtv-9. We show that Mtv-8 controls a superantigen with similar specificity, and that both Mtv-8 and Mtv-9 stimulate some V beta 17+ T cells. A third provirus, Mtv-6, controls a superantigen with specificity for V beta 3. Data presented raise the possibility that endogenous superantigens may compete for class II molecules in a single B cell.

Mls--a retrovirus exploits the immune system

The identity of minor lymphocytes stimulating (Mls) antigens, endogenous superantigens that can activate, or induce the deletion of, large portions of the T-cell repertoire, has recently been revealed: they are encoded by mouse mammary tumor viruses (MMTV) that have integrated into the germ line as DNA proviruses. As Hans Acha-Orbea and Ed Palmer point out, Mls-mediated modulation may be only the tip of the retrovirus iceberg; already murine leukemia virus (MuLV), with similar superantigen properties, has been discovered.

A method for production of antibodies to human T-cell receptor beta-chain variable regions

Mouse T-cell hybridomas bearing human V beta elements were produced by transfection of human/mouse hybrid T-cell receptor beta-chain genes into a mouse T-cell hybridoma lacking an endogenous beta-chain gene. These hybridomas were entirely mouse in origin except for the human V beta region. These cells were used to immunize mice against human V beta elements. Mouse monoclonal antibodies have thus been generated against human V beta 13.1 and -13.2. We expect that the method outlined in this paper will be useful in the production of monoclonal antibodies specific for other human V beta or V alpha elements.

Altered antigen receptor signaling in anergic T cells from self-tolerant T-cell receptor beta-chain transgenic mice

T-cell tolerance to the minor lymphocyte-stimulating antigen Mls-1a in a T-cell receptor (TcR) V beta 8.1 transgenic line of mice is maintained by both clonal deletion and clonal anergy. Approximately 20-50% of peripheral CD4+ (but not CD8+) T cells isolated from these mice are anergic and fail to proliferate following TcR ligation. We have examined key events in T-cell signaling in peripheral T cells isolated from these mice. In this report, we show that the anergic CD4+ T cells did not mobilize calcium or express receptors for interleukin 2 (IL-2) following TcR ligation. However, the cells retained viability and functional potential because stimulation with phorbol 12-myristate 13-acetate and ionomycin bypassed the block in receptor-mediated signaling and induced IL-2 receptor expression and proliferation of the anergic cells.