Analysis of V beta 17a expression in new mouse strains bearing the V beta a haplotype

ATM deficiency disrupts Tcra locus integrity and the maturation of CD4+CD8+ thymocytes

Mutations in ATM (ataxia-telangiectasia mutated) cause ataxia-telangiectasia (AT), a disease characterized by neurodegeneration, sterility, immunodeficiency, and T-cell leukemia. Defective ATM-mediated DNA damage responses underlie many aspects of the AT syndrome, but the basis for the immune deficiency has not been defined. ATM associates with DNA double-strand breaks (DSBs), and some evidence suggests that ATM may regulate V(D)J recombination. However, it remains unclear how ATM loss compromises lymphocyte development in vivo. Here, we show that T-cell receptor β (TCRβ)–dependent proliferation and production of TCRβlow CD4+CD8+ (DP) thymocytes occurred normally in Atm−/− mice. In striking contrast, the postmitotic maturation of TCRβlow DP precursors into TCRβint DP cells and TCRβhi mature thymocytes was profoundly impaired. Furthermore, Atm−/− thymocytes expressed abnormally low amounts of TCRα mRNA and protein. These defects were not attributable to the induction of a BCL-2–sensitive apoptotic pathway. Rather, they were associated with frequent biallelic loss of distal Va gene segments in DP thymocytes, revealing that ATM maintains Tcra locus integrity as it undergoes V(D)J recombination. Collectively, our data demonstrate that ATM loss increases the frequency of aberrant Tcra deletion events, which compromise DP thymocyte maturation and likely promote the generation of oncogenic TCR translocations.

Biased V beta usage in immature thymocytes is independent of DJ beta proximity and pT alpha pairing

During thymus development, the TCR beta locus rearranges before the TCR alpha locus. Pairing of productively rearranged TCR beta-chains with an invariant pT alpha chain leads to the formation of a pre-TCR and subsequent expansion of immature pre-T cells. Essentially nothing is known about the TCR V beta repertoire in pre-T cells before or after the expression of a pre-TCR. Using intracellular staining, we show here that the TCR V beta repertoire is significantly biased at the earliest developmental stage in which VDJ beta rearrangement has occurred. Moreover (and in contrast to the V(H) repertoire in immature B cells), V beta repertoire biases in immature T cells do not reflect proximity of V beta gene segments to the DJ beta cluster, nor do they depend upon preferential V beta pairing with the pT alpha chain. We conclude that V gene repertoires in developing T and B cells are controlled by partially distinct mechanisms.

Regulation of interleukin (IL)-12 receptor beta2 subunit expression by endogenous IL-12: a critical step in the differentiation of pathogenic autoreactive T cells

The interleukin (IL)-12 receptor (R)beta2 subunit is the critical molecule involved in maintaining IL-12 responsiveness and controlling T helper cell type 1 lineage commitment. We demonstrate that IL-12 and interferon (IFN)-gamma play separate, but complementary, roles in regulating IL-12Rbeta2 expression on antigen-specific CD4(+) T cells. These results are consistent with our previous observation that IL-12 can promote autoimmune disease through IFN-gamma-independent as well as -dependent pathways. Therefore, we compared the induction of IL-12 by, and the expression of the IL-12Rbeta2 subunit on, myelin basic protein (MBP)-specific T cells from experimental allergic encephalomyelitis (EAE)-susceptible SJL (H-2(s)) mice and from EAE- resistant B10.S mice (H-2(s)). B10.S mice had an antigen-specific defect in their capacity to upregulate the IL-12Rbeta2 subunit. Defective expression was not secondary to the production of suppressive cytokines, but to a failure of B10.S MBP-specific T cells to upregulate CD40 ligand expression and to induce the production of IL-12. IL-12Rbeta2 expression as well as encephalitogenicity of these cells could be restored by the addition of IL-12. These results suggest that the development of immunotherapies that target the IL-12Rbeta2 subunit may be useful for the treatment of autoimmune diseases.

An alternate pathway for T cell development supported by the bone marrow microenvironment: recapitulation of thymic maturation

In the principal pathway of alpha/beta T cell maturation, T cell precursors from the bone marrow migrate to the thymus and proceed through several well-characterized developmental stages into mature CD4+ and CD8+ T cells. This study demonstrates an alternative pathway in which the bone marrow microenvironment also supports the differentiation of T cell precursors into CD4+ and CD8+ T cells. The marrow pathway recapitulates developmental stages of thymic maturation including a CD4+CD8+ intermediary cell and positive and negative selection, and is strongly inhibited by the presence of mature T cells. The contribution of the marrow pathway in vivo requires further study in mice with normal and deficient thymic or immune function.

CD8+ T-cell clones in old mice

Most old mice and human beings contain large clones of CD8+ alpha beta TCR+ T cells. In mice, clones bearing V beta 7 appear more frequently in animals infected with mouse hepatitis virus than in uninfected animals. This property is controlled by some non-MHC gene in the animals. The frequency of old mice containing such clones is affected by the origin of the animals. Although the clones are relatively anergic to acute stimuli in vitro, they can divide in vivo since in old animals they divide and turnover with about the same kinetics as other, non-clonally expanded CD8+ T cells. Moreover the clones expand slowly but inexorably after transfer into recipient animals. These data suggest that the CD8+ alpha beta TCR clones arise because they are specific for some exogenous or auto antigen to which the cells are continuously exposed in vivo.

Characteristics and maintenance of CD8+ T-cell clones found in old mice

Old mice, like old human beings, contain large clones of CD8+ T-cells. These cells grow poorly in tissue culture, therefore it is difficult to maintain the cells in vitro. The cells can be grown after transfer to sublethally irradiated mice. This technique will be useful in further studies on the properties of the cells. Based on observations from such transfer experiments, we conclude that: (1) expansion of the T-cell clones in recipients is dramatic but slow; (2) chance events caused by endogenous antigens or gene mutations rather than exogenous antigens may account for the expansion of these clones; and (3) the expanded T-cell clones are benign and do not cause malignancies.

The c-kit+ maturation pathway in mouse thymic T cell development: lineages and selection

Positive selection of T cells begins with TCR alpha beta lo thymic progenitors. Here, we show that the most efficient TCRlo progenitors are c-kit+ with intermediate levels of CD4 and CD8 (DPint). Positive selection of DPint TCRlo c-kit+ cells results in TCRmed CD69+ c-kit+ transitional intermediates that show increased TCRV beta frequencies to selecting superantigen (SAg) that are committed to the CD4 or CD8 pathway. The cells on the c-kit+ maturation pathway maintain Bcl-2 expression. Most DPint c-kit+ progenitors fail positive selection, and become DPhi c-kit- cells that lose Bcl-2 expression. Some DPhi c-kit blast cells can be salvaged to produce mature single-positive (SP) cells. DPint c-kit+ maturation to SP cells can occur in <12 hr in vitro on thymic stromal monolayers.

Monoclonal antibodies defining functional sites on the toxin superantigen staphylococcal enterotoxin B

Four monoclonal antibodies (mAbs) were produced binding to four nonoverlapping epitopes on the superantigen staphylococcal enterotoxin B (SEB). The mAbs were tested for their ability to detect SEB bound to major histocompatibility complex (MHC) class II, to inhibit SEB binding to MHC class II, to inhibit SEB stimulation of T cell hybridomas, to bind to various nonfunctional mutants of SEB, and to capture and present SEB and its mutants to T cells in the absence of MHC class II. We concluded that two mAbs, B344 and B327, bound to epitopes not required for superantigen function, one mAb, 2B33, blocked an MHC interaction site on SEB, and the fourth mAb, B87, blocked the T cell recognition site on SEB. Moreover, two mAbs (B344 and 2B33) were capable of presenting SEB, although much less efficiently than APC, to CD4- but not CD4+ T cell hybridomas. The results confirm the functional domains on SEB originally defined by mutation and show that MHC class II is not always an essential component of the superantigen ligand.

(SWR x SJL)F1 mice: a new model of lupus-like disease

During the study of autoimmune models we found that (SWR x SJL)F1 mice (both parental strains with the V beta a phenotype) spontaneously produced immunoglobulin G (IgG) antibodies directed against Sm/U1 small nuclear ribonucleoproteins (snRNPs). In some of these females, the presence of these autoantibodies was found as early as 10 wk of age. Their frequency increased with age i.e., 70% at 40 wk. At that time, only 10% of males developed anti-Sm/U1snRNP antibodies. Anti-Sm/U1snRNP antibodies from positive mice generally recognized the peptides BB', D, 70 kD, and A from RNPs. These polypeptides are known to bear the autoantigenic epitopes that are recognized by human sera containing anti-Sm and anti-U1snRNP antibodies. Reactivity of IgG antibodies with the octapeptide sequence PPPGMRPP was also found in 30% of anti-Sm/U1snRNP positive (SWR x SJL)F1 mice that precipitated BB' peptides. This octapeptide has been described as the most immunoreactive linear epitope in systemic lupus erythematosus (SLE) patients with anti-Sm and anti-U1snRNP antibodies. Approximately 30% of anti-Sn/U1snRNP positive females, later produced anti-dsDNA antibodies. This fact was accompanied by the development of proteinuria due to glomerulonephritis mediated by immunocomplexes. In addition to the specific autoimmune response, (SWR x SJL)F1 females also showed other immunologic abnormalities such as hypergammaglobulinemia, and an approximately twofold increase in spleen cell number compared with control mice. These results indicate that (SWR x SJL)F1 females develop clinical and serological abnormalities similar to those observed in human SLE and constitute a novel model for the study of the genetic mechanisms that result in autoimmunity.

Influence of viral superantigens on V beta- and V alpha-specific positive and negative selection

In mice, V beta-specific negative selection is mediated by a number of superantigens encoded by various mouse mammary tumor viruses. We have identified Mtv-3, Mtv-27, Mtv-44, Mtv-8, Mtv-9, Mtv-11, and MMTV(D2.GD), and have confirmed Mtv-1. Although specificities of superantigens correlate well with sequences of their carboxy terminal regions, Mtv-44 appears to be an exception: the product is specific for V beta 3, V beta 6, V beta 8.1, and V beta 9. It remains to be determined whether Mtv-44 produces one or two different superantigens to exhibit this specificity. V beta 5+ T-cell deletion is induced by two groups of superantigens: V beta 3-specific superantigens encoded by Mtv-1, Mtv-3, Mtv-6, Mtv-13, Mtv-27, and Mtv-44, and V beta 11-specific superantigens encoded by Mtv-8, Mtv-9, and Mtv-11. Furthermore, these V beta 3-specific superantigens are also specific for V beta 17a(cz). In contrast, V beta-specific positive selection and V alpha-specific positive and negative selection do not seem to involve non-H-2 (super)antigens, although their involvement can not be excluded. In the near future, superantigens, powerful modulators of T-cell functions, will be exploited for clinical applications.

The bacterial and mouse mammary tumor virus superantigens; two different families of proteins with the same functions

In conclusion, the bacterial toxins are completely unlike the MTV superantigens in primary sequence and structure. The former are soluble globular proteins which do not have to be proteolytically cleaved before they act. The latter are synthesized as type II membrane proteins and may be clipped before they reach the cell surface and act to stimulate T cells. Table III summarizes the similarities and differences between the two sets of superantigens. The most notable quality of these molecules is that both sets of families have developed strategies whereby they bind to Class II and engage V beta. As far as the microorganisms which produce them are concerned, these two properties appear to be essential since they are absolutely conserved over proteins of a number of different structures. Several questions can now be addressed as follows. a. Why do all known superantigens bind to Class II? For the microorganism which produces them, the function of superantigens appears to be T-cell and perhaps directly or indirectly B-cell and macrophage stimulation. Activation of virgin T cells requires engagement with antigen plus MHC on professional antigen-presenting cells. Unlike other cell surface proteins, for example Class I, most Class II in animals is expressed on such cells. Therefore it is likely that superantigens have evolved to engage Class II because presentation to T cells by Class II-bearing cells offers the superantigen the best chance of activating its target T cells. b. Why do superantigens engage TCR V beta and not V alpha or CD3? It is possible that superantigens bind to the V beta portion of the TCR rather than V alpha because the latter does not have a consistently well exposed face for engagement. The fact that it is perhaps relatively easier to produce anti-V beta rather than anti-V alpha antibodies supports this idea. We have shown that N-glycosylation of V beta can interfere with recognition by vSAGs (Pullen et al. 1991), perhaps glycosylation of V alpha tends to conceal otherwise available sites. As far as C beta, C alpha or CD3 engagement is concerned, this may be just too dangerous for MTVs. The role of MTVs SAgs in the life history of the virus seems to be to stimulate T cells in the suckling recipient and thereby create a pool of activated lymphocytes in which the virus may survive until the mouse gives birth and transmits the virus to her own progeny (Hainaut et al. 1990, Golovkina et al. 1992).(ABSTRACT TRUNCATED AT 400 WORDS)

Superantigens and their potential role in human disease

In the past few years, there has been a virtual explosion of information on the viral and bacterial molecules now known as superantigens. Some structures have been defined and the mechanism by which they interact with MHC class II and the V beta region of the T cell receptor is being clarified. Data are accumulating regarding the importance of virally encoded superantigens in infectivity, viral replication, and the life cycle of the virus. In the case of MMTV, evidence also suggests that superantigens encoded by a provirus may be maintained by the host to protect against future exogenous MMTV infection. Experiments in animals have also begun to elucidate the dramatic and variable effects of superantigens on responding T cells and other immune processes. Finally, the role of superantigens in certain human diseases such as toxic shock syndrome, some autoimmune diseases like Kawasaki syndrome, and perhaps some immunodeficiency disease such as that secondary to HIV infection is being addressed and mechanisms are being defined. Still, numerous important questions remain. For example, it is not clear how superantigens with such different structures, for example, SEB, TSST-1, and MMTV vSAG, can interact with MHC and a similar region of the TCR in such basically similar ways. It remains to be determined whether there are human equivalents of the endogenous murine MMTV superantigens. The functional role of bacterial superantigens also remains to be explained. Serious infection and serious consequences from toxin-producing bacteria are relatively rare events, and it is questionable whether such events are involved in the selection pressure to maintain production of a functional superantigen. Hypotheses to explain these molecules, which can differ greatly in structure, include T cell stimulation-mediated suppression of host responses or enhancement of environments for bacterial growth and replication, but substantiating data for these ideas are mostly absent. It also seems likely that only the tip of the iceberg has been uncovered in terms of the role of superantigens in human disease. Unlike toxic shock syndrome, other associations, especially with viral superantigens, may be quite subtle and defined only after considerable effort. The definition of these molecules and mechanisms of disease may result in new therapeutic strategies. Finally, it is apparent that superantigens have dramatic effects on the immune system. One wonders whether these molecules or modifications of them can be used as specific modulators of the immune system to treat disease.

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.

Metabolically active antigen presenting cells are required for human T cell proliferation in response to the superantigen streptococcal M protein

M protein from type 5 group A streptococci has been identified as a member of the family of polyclonal T cell activators termed superantigens because it preferentially stimulates T cells bearing specific V beta elements of the T cell receptor (TCR). In this study the molecular and cellular requirements for presentation of this protein to T cells were investigated. Only accessory cells (AC) expressing class II major histocompatibility complex (MHC) molecules were capable of supporting T cell activation in response to a 22 kDa fragment of M protein (pep M). Despite the need for class II elements, processing of pep M5 by the antigen-presenting cells (APC) was not required for T cell proliferation induced by pep M5. Fixation of APC by paraformaldehyde (PF) treatment impaired their ability to induce optimal T cell proliferation in response to pep M5 without significantly affecting interleukin (IL-2) production. In contrast, PF-fixation of cells from the B cell lymphoma line, Raji, did not affect their ability to present pep M5 to human T cells. Addition of rIL-1 and IL-6 to PF-treated APC restored pep M5-induced blastogenesis. Our data suggest that pep M5 directly associates with HLA class II molecules forming a complex that can induce IL-2 production but not optimal proliferation by T cells. Additional signals provided by the AC are required to trigger optimal T cell proliferation in response to this superantigen.

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.

Limitations in plasticity of the T-cell receptor repertoire

How constrained is T-cell recognition? Is a truncated T-cell receptor (TCR) repertoire, missing half of its V beta components (where V indicates variable), still broad enough to produce an antigen-specific T-cell response to all determinants? These questions can be answered for certain T-cell antigenic determinants whose response in the wild type is limited to specific gene segments. Our results show that mice with such a deletion in their TCR V beta genes (V beta truncated haplotype, Va beta) are unable to respond to two antigen determinants (sperm whale myoglobin 111-121/I-Ed and myelin basic protein 1-11/I-Au) whose response in the wild type is restricted to the missing V beta (V beta 8.2 in the case of 111-121/I-Ed and V beta 8.2 and V beta 13 in the case of 1-11/I-Au) gene segments. Fundamentally, this restriction could have been attributed to another aspect of immunodominance--that a favored TCR with high affinity would dominate the response, but in its absence, a hierarchy of T cells with lesser efficiency and expressing alternate TCR V genes could take over. However, from our experiments it has become evident that there is an absolute limit to the flexibility inherent in the TCR repertoire. Since it is clear that mouse populations have many ambient deletion ligands (such as self-superantigens) that can result in the loss of multiple V beta gene segments during normal T-cell development, these deletions can have serious consequences, such as unresponsiveness to the antigen as a whole--a hole in the repertoire--if a dominant determinant of that antigen normally shows restricted TCR V beta gene usage.

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.

Chronic experimental autoimmune encephalomyelitis induced by the 89-101 myelin basic protein peptide in B10RIII (H-2r) mice

Development of experimental allergic encephalomyelitis (EAE) in the SJL (H-2s) mice is associated with a T cell-dependent autoimmune response to the C-terminal part of the myelin basic protein (MBP). In this study the influence of both H-2 and non-H-2 genetic background on EAE induced with the MBP89-101 peptide is described. Analysis of different H-2q haplotype strains, B10G, B10Q, SWR and NFR/N, showed that the B10 background is relatively resistant to disease induction. Both SWR and NFR/N were susceptible to EAE showing that the H-2q haplotype is permissive for EAE development induced with MBP89-101 and that the T cell receptor (TcR) haplotype or complement C5 deficiency exert no significant influence on disease susceptibility. In a series of H-2-congenic strains on the B10 background only B10RIII (H-2r) mice were susceptible to EAE. The B10RIII mice developed a severe EAE with early onset and chronic progressive or relapsing course of disease. In addition, B10RIII mice treated with Freund's complete adjuvant and pertussis toxin alone showed an early monophasic disease. The clinical observations were confirmed by immunohistopathologic analysis of the central nervous system. In these studies, we also applied antibodies to different TcR V beta elements which showed no specific limitation of the used TcR among infiltrating T cells in the target tissue in any of the strains. It is concluded that an MBP peptide-specific disease can be induced in three different haplotypes and it is possible that shared structures between the As, Aq and Ar molecules are of importance for the trigger of encephalitogenic T cells with different TcR V elements. The presently described chronic EAE model induced in the B10RIII mice will be of value as a model for multiple sclerosis.

A maternally inherited superantigen encoded by a mammary tumour virus

A collection of superantigens, molecules which in combination with class II major histocompatibility complex (MHC) engage T cells bearing particular V beta chains as part of their alpha beta receptors, have recently been described. The mouse self superantigen, Mls-1a, for example, in conjunction with many MHC class II proteins, engages mouse T cells bearing V beta 6, 7, 8.1 and 9, almost regardless of the sequences of the other variable components of the receptors on the T cells. Two types of superantigen have been identified so far: first, superantigens encoded in the mouse genome, such as Mls-1a; second, superantigens produced by bacteria, such as the staphylococcal enterotoxins. Although the latter type of superantigens are in many cases known to be proteins of about 220 amino acids, nothing is known about the structures of any of the superantigens encoded in mouse. Here we describe the properties of a new mouse superantigen. The antigen is maternally transmitted in milk and is probably encoded by a mammary tumour virus (MTV). Given the known genetic linkage between at least one of the mouse genomic superantigens and endogenous MTV integration sites, it is tempting to speculate that the superantigen described here and some of the endogenous mouse superantigens are encoded by MTVs.

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

Of 41 wild-derived mouse strains analyzed, 14 contained T cells bearing V beta 17 receptors in spite of the concomitant expression of I-E antigens. Reciprocal F1 and F2 hybrids of one of these strains, PWK, with laboratory strains revealed different patterns of V beta 17 T cell deletions from those observed with V beta 17 T cells from SJL, implying that the two V beta 17 regions are associated with recognition of distinct superantigens. The structures of the V beta 17 alleles differ by two amino acid substitutions, which lie together in an area distant from the predicted site of T cell receptor interaction with peptide-MHC complexes but overlapping with that implicated in V beta 8.2 recognition of Mls-1 superantigen. This demonstrates that the self-superantigen leading to V beta 17 T cell deletion varies with the allele of the receptor gene and confirms that T cell deletions by such ligands involve interactions with a region of the V beta domain that is distinct from the conventional combining site.

Thymic selection defines multiple T cell receptor V beta 'repertoire phenotypes' at the CD4/CD8 subset level

We describe here the use of a sensitive and accurate multiprobe V beta RNase protection assay in characterizing the expression levels of 17 V beta genes in separated CD4+ and CD8+ subsets of selected mouse strains. The IE-reactive V beta genes (V beta s 11, 12, 5.1 and 16) showed various patterns of skewed subset expression in different strains, suggesting additional influences of IA, class I, and non-MHC genes in the selection process. Clonal deletion of V beta 11- and V beta 12-bearing T cells, among others, was skewed strongly towards the CD4+ subset in many IE+ mouse strains, supporting the notion that negative selection can cause incomplete, subset biased, V beta clonal deletions. Broad analysis in separated CD4+ and CD8+ subsets gave improved resolution of V beta repertoire selection, and revealed significant strain and/or subset specific skewing for additional V beta genes; with consistent bias towards higher expression of V beta 7 and V beta 13 in the CD8+ subset, and V beta 15 in the CD4+ subset of most mouse strains. The influence of diverse non-MHC ligands in V beta repertoire selection was further illustrated by the identification of unique V beta repertoires for six different MHC-identical (H2k) strains. Such polymorphisms in TCR repertoire expression may help to define better disease susceptibility phenotypes.

T cell receptor-mediated negative selection of autoreactive T lymphocyte precursors occurs after commitment to the CD4 or CD8 lineages

To identify the maturational stage(s) during which T cell receptor (TCR)-mediated positive and negative selection occurs, we followed the development of CD4+8- and CD4-8+ T cells from TCRlo CD4+8+ thymic blasts in the presence of different positive and negative selecting (major histocompatibility complex or Mls) elements. We describe novel lineage-committed transitional intermediates that are TCRmed CD4+8lo or TCRmed CD4lo8+, and that show evidence of having been positively selected. Furthermore, negative selection is not evident until after cells have attained one of the TCRmed transitional phenotypes. Accordingly, we propose that negative selection in normal mice occurs only after TCRlo CD4+8+ precursors have been positively selected into either the CD4 or CD8 lineage.

The role of the T cell receptor in positive and negative selection of developing T cells

Although many combinations of alpha beta T cell receptors are available to the T cells in any given organism, far fewer are actually used by mature T cells. The combinations used are limited by two selective processes, positive selection of T cells bearing receptors that will be useful to the host, and clonal elimination or inactivation of T cells bearing receptors that will be damaging to the host. The ways in which these two apparently contradictory processes occur, and the hypotheses that have been suggested to reconcile them, are discussed.

The toxicity of staphylococcal enterotoxin B in mice is mediated by T cells

Staphylococcal enterotoxin B (SEB) has been shown in the past to be a potent T cell stimulant in mouse or man. The toxin acts as a superantigen that is, it binds to class II MHC proteins and, as such a complex, stimulates T cells bearing particular V beta s as part of their receptors. The toxin also has several pathological effects, causing, in mice, rapid weight loss, thymus atrophy, immunosuppression, and, at high doses, death. The data in this paper show that at least one of these effects, weight loss, is T cell mediated. Staphylococcal enterotoxin-mediated weight loss is MHC dependent, and is almost absent in animals expressing MHC class II molecules, which, complexed with SEB, are poor T cell stimulants. Also, mice that lack T cell function, genetically or because of cyclosporin A treatment, lose no or less weight than controls in response to SEB. Finally, animals bred such that they express few T cells bearing V beta s with which SEB can interact lose much less weight in response to the toxin than littermate controls that have higher numbers of reactive T cells. It is therefore suggested that the pathological effects of the staphylococcal, T cell-stimulating toxins in mouse and man may be partially or wholly the consequence of massive T cell stimulation.

Immunological alterations inducible by mercury compounds. III. H-2A acts as an immune response and H-2E as an immune "suppression" locus for HgCl2-induced antinucleolar autoantibodies

In responder mouse strains repeated injections of subtoxic doses of HgCl2 induce formation of antinuclear autoantibodies (ANA) and antinucleolar autoantibodies (ANolA). Others have shown that responsiveness to HgCl2-induced formation of ANA and ANolA is linked to H-2. Here, we extend these studies to a variety of mouse strains not tested previously. After confirming that strain B10.S (H-2s) is a high responder we have shown that strains B10.D2 (H-2d) and B10.BR (H-2k) are nonresponders. By comparing a panel of strains carrying appropriate intra-H-2 recombinant haplotypes derived from d, k and s, we were able to map responsiveness to As. Interestingly, among four strains all of which were As, and thus responsive, only the two H-2E- ones, B10.S and B10.RSD2, were high responders whereas the two H-2E+ ones, B10.HTT and B10.S(9R), were significantly less responsive. Thus, the genetics of HgCl2-induced autoantibody formation follow the rules established for immune responses to a variety of different antigens in that expression of H-2E "suppressed" the response.

Reactivity of V beta 17a+ CD8+ T cell hybrids. Analysis using a new CD8+ T cell fusion partner

Tolerance to IE molecules leads to deletion of V beta 17a-bearing T cells. Both, the CD4+ as well as the CD8+ T cell subsets are affected. A large percentage of CD4+ V beta 17a+ T cell hybrids recognize IE molecules. We now have investigated the reactivity for IE antigens of CD8+ V beta 17a+ T cell hybrids. Using a transfection approach, we have introduced the murine CD8 molecule into different V beta 17a+ T cell hybrids. Furthermore, the CD8 cDNA was transfected into the BW5147 alpha-beta- fusion partner. This allowed us to generate a large number of V beta 17a+ T cell hybrids by fusion with the appropriate T cells. Only 6% of T cell hybrids were stimulated to produce IL-2 upon incubation with IE+ cells. However, in those, the CD8 molecule seemed not to contribute to the IE reactivity of the hybrid, since mAbs against the CD8 molecule failed to inhibit their reactivity. This low percentage of V beta 17a+ CD8+ IE-reactive T cell hybrids contrasts with the strong reduction of CD8+ V beta 17a+ T cells in IE+ mice, strongly suggesting that elimination of such cells in the thymus occurs when they are coexpressing CD4 and CD8. This view was confirmed by the occasional expression of CD4 in some hybrids in which case IE reactivity was detected. Furthermore, we demonstrated the functional integrity of the introduced CD8 molecule by: (a) reconstitution of the IL-2 response in a class I-restricted TNP-specific T cell hybrid; and (b) by generation of alloreactive class I-restricted T cell hybrids using the new CD8+ fusion cell line. This CD8+ fusion partner, BWLyt2-4, should prove useful to study antigen processing and antigen presentation requirements of class I-restricted T cells.

Positive selection of CD4+ T cells mediated by MHC class II-bearing stromal cell in the thymic cortex

T lymphocytes differentiate in the thymus, where functionally immature, CD4+CD8+ (double positive) thymocytes develop into functionally mature CD4+ helper cells and CD8+ cytotoxic (single positive) T cells. The thymus is the site where self-reactive T cells are negatively selected (clonally deleted) and where T cells with the capacity to recognize foreign antigens in association with self-proteins encoded by the major histocompatibility complex (MHC) are positively selected. The net result of these developmental pathways is a T-cell repertoire that is both self-tolerant and self-restricted. One unresolved issue is the identity of the thymic stromal cells that mediate the negative and positive selection of the T-cell repertoire. Previous work has pointed to a bone-marrow-derived macrophage or dendritic cell as the inducer of tolerance, whereas a radiation-resistant, deoxyguanosine-resistant thymic cell seems to mediate the positive selection of self-MHC restricted T cells. Thymic stromal cells in the cortex interact with the T-cell antigen receptor on thymocytes. Using several strains of transgenic mice that express the class II MHC molecule I-E in specific regions of the thymus, we show directly that the positive selection of T cells is mediated by an I-E-bearing cell in the thymic cortex.

Role of MHC, Mls and TCR in immune tolerance

MHC class II molecules and self antigens, such as Mls, influence T-cell selection by clonal deletion of potentially self-reactive T cells. In order to examine the role of various class II molecules in the T-cell receptor-self antigen interaction, class II transgenic and recombinant mice were analysed for TCR expression. Our studies indicate that the A alpha and E alpha chains can present Mls gene products for the clonal deletion of V beta 6-bearing T cells, and that the A alpha q chain is defective in this process. We have also shown that E alpha A beta heterodimer in transgenic and recombinant mice is expressed and functions to delete I-E reactive V beta llT cells, demonstrating again the role of the E alpha molecule.

Influence of the major histocompatibility complex on positive thymic selection of V beta 17a+ T cells

A monoclonal antibody was used to show directly positive thymic selection of the T cell repertoire in mouse strains expressing the 17a beta-chain variable domain (V beta 17a) of the T cell receptor. In the absence of the potent tolerizing class II major histocompatibility complex (MHC) molecule, I-E, peripheral expression of V beta 17a+ T cell receptors varied with the MHC haplotype of the mouse strain. In the most extreme case, H-2q mice expressed high peripheral levels of CD4+ V beta 17a+ T cells (14 to 19 percent), whereas H-2b mice expressed low levels (3 to 4 percent). Analysis of (b x q)F1 mice and chimeric mice showed that these differences were determined by positive thymic selection and implicated the thymic epithelium as the controlling cell type.