T cell tolerance by clonal elimination in the thymus

A third T-cell receptor beta-chain variable region gene encodes reactivity to Mls-1a gene products

Using a quantitative RNA hybridization method, we have identified a third murine T-cell receptor beta-chain variable (V) region gene, V beta 9, that engenders recognition of Mls-1a gene products. V beta 9-expressing T cells are clonally deleted from the periphery of mice that carry the Mls-1a allele, and the fine specificity of V beta 9+ hybridomas suggests that V beta 6-, V beta 8.1-, and V beta 9-encoded T-cell receptors may recognize a similar antigenic epitope. Comparison of these three protein sequences identifies two residues that may be important in determining this recognition specificity.

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.

Structure of the gene of tum- transplantation antigen P91A: the mutated exon encodes a peptide recognized with Ld by cytolytic T cells

Mutagen treatment of mouse P815 tumor cells produces immunogenic mutants that express new transplantation antigens (tum- antigens) recognized by cytolytic T cells. We found that the gene conferring expression of tum- antigen P91A contains 12 exons, encoding a 60 kd protein lacking a typical N-terminal signal sequence. The sequence shows no significant similarity with sequences in current data bases. A mutation that causes expression of the antigen is located in exon 4; it is the only apparent difference between the normal and the antigenic alleles. A short synthetic peptide corresponding to a region of exon 4 located around this mutation makes P815 cells sensitive to lysis by anti-P91A cytolytic T cells. The mutation creates a strong aggretope enabling the peptide to bind the H-2 Ld molecule. Several secondary tumor cell variants that no longer express tum- antigen P91A were found to carry deletions in the gene.

Immunologic tolerance: collaboration between antigen and lymphokines

Immunologic tolerance is the process whereby limits are placed on the degree to which lymphocytes respond to an animal's inherent antigens. It is a quantitative rather than an absolute term, as some autoantibody formation is common. Contrary to early hopes, it is not due to some single, simple causative mechanism confined to early developmental stages of the fetal immune system. Rather, self-tolerance results from a variety of complementary mechanisms and feedback loops in the immune system and is thus best seen as part of the general process of immunoregulation.

Positive selection of a T-cell subpopulation in the thymus in which it develops

In SWR mice the expression with high-density V beta 17a (high V beta 17a) of the T-cell antigen receptors correlates with the CD4+8- subpopulation of thymocytes. By contrast, in thymocytes of SJL mice the expression of high V beta 17a is observed on the CD4+8- or CD4-8+ subpopulation. However, when the thymocytes from SWR mice have been developed in the SJL or B10 thymus but not in the H-2 compatible DBA/1 thymus, a greater proportion of thymocytes that express high V beta 17a was found to be CD4-8+. By contrast, only a small proportion of KJ23a+ thymocytes from SJL mice that had differentiated in the thymus of SWR or DBA/1 mice was CD4-8+, whereas a high proportion of CD4+8- cells expressed V beta 17a. Further, an intermediate proportion of KJ23a+ thymocytes that had derived from SJL donor mice was present on CD4-8+ thymocytes that had developed in B10.A(4R) thymus. These findings demonstrate that the appearance of a particular subpopulation of thymocytes (CD4-8+ with a beta chain of T-cell antigen receptor identified as V beta 17a) is determined by the histocompatibility complex products that are expressed in the thymic microenvironment in which the T cells develop.

Monoclonal IgMs with anti-Gal(beta 1-3) GalNAc activity in lower motor neuron disease; identification of glycoprotein antigens in neural tissue and cross-reactivity with serum immunoglobulins

IgM monoclonal antibodies (M-proteins) with anti-Gal(beta 1-3)GalNAc and anti-Gal(beta 1-3)GlcNAc activity that bind to gangliosides GD1b and GM1, from two patients with lower motor neuron disease were tested for binding to neural glycoproteins. The M-proteins bound to several glycoproteins in the central and peripheral nervous system including to some in the non-myelin or axonal fraction only. Peanut agglutinin (PNA) which is specific for Gal(beta 1-3)GalNAc, bound to the same protein bands. Since serological studies revealed that the M-proteins were complexed to IgG, serum immunoglobulins were tested for presence of Gal(beta 1-3)GalNAc epitopes. Both PNA and the M-proteins bound to immunoglobulin heavy and light chains, suggesting that the circulating M-proteins bind to Gal(beta 1-3)GalNAc on other immunoglobulins. These studies indicate that in addition to gangliosides the M-proteins might bind to Gal(beta 1-3)GalNAc bearing glycoproteins in vivo and that carbohydrate epitopes on immunoglobulins might have a role in the development and regulation of autoantibodies which cross-react with neural antigens and may cause neurological disease.

Positive selection determines T cell receptor V beta 14 gene usage by CD8+ T cells

We report here a mAb, 14-2, reactive with TCRs that include V beta 14. The frequency of V beta 14+ T cells varies with CD4 and CD8 subset and is controlled by the H-2 genes. Thus CD8+ T cells from H-2b mice include approximately 2.3% V beta 14+ T cells while CD8+ T cells from mice expressing K kappa include greater than 8% V beta 14+ T cells. In all strains examined, 7-8% of CD4+ T cells express V beta 14. The frequent usage of V beta 14 in CD8+ T cells of K kappa-expressing mice is a result of preferential positive selection of V beta 14+ CD8+ T cells as demonstrated by analysis of radiation chimeras. These studies demonstrate that H-2-dependent positive selection occurs in unmanipulated mice. Furthermore, the results imply that positive selection, and possibly H-2 restriction, can be strongly influenced by a V beta domain, with some independence from the beta-junctional sequence and alpha chain.

Tolerance in transgenic mice expressing class II major histocompatibility complex on pancreatic acinar cells

To study the nature of tolerance to antigens not expressed by cells of the lymphoid system, expression of class II MHC I-E was targeted to the acinar cells of the exocrine pancreas in transgenic mice (elastase [EL]-I-E). Despite the absence of detectable I-E in the thymus of EL-I-E transgenic mice, both thymocytes and peripheral T lymphocytes were tolerant to I-E, and the pancreas was free of autoimmune infiltrates. Nontolerant T cells adoptively transferred into irradiated or T-depleted transgenic mice rapidly destroy the I-E+ components of the pancreas; however, adoptive transfer of nontolerant T lymphocytes into nonirradiated transgenic mice do not. These results suggest that tolerance in transgenic mice is maintained by some peripheral tolerance mechanism. However, further studies indicate that tolerance in transgenic mice is not maintained by specific Ts cells. For example, cell mixing experiments both in vitro and in vivo fail to reveal dominant unresponsiveness. Furthermore, nontolerant T cells injected into otherwise unmanipulated EL-I-E mice can be primed in situ (by injections of I-E+ spleen cells) to destroy the I-E+ acinar cells.

Thymocyte development: an analysis of T cell receptor gene expression in 519 newborn thymocyte hybridomas

We have examined the frequency of expression of individual alpha and beta chain V gene families in a population of immature T cells that has not been selected or tolerized. To accomplish this, we generated 519 T cell hybridomas from freshly isolated thymocytes of newborn C57BL/10 mice and subjected RNA from these hybrids to hybridization analysis with 11 V alpha, 16 V beta, C gamma and C delta probes. Comparison of the expressed repertoire of V beta gene segments in this newborn thymocyte population with similar data previously generated from adult peripheral T cells (Bill et al., Proc. Natl. Acad. Sci. USA 1988. 85: 9184.) revealed two V beta genes, V beta 12 and V beta 15, whose expression is decreased in the periphery possibly due to the effects of tolerance. An additional V beta gene segment (V beta 10) and a V beta gene family (V beta 5.1, V beta 5.2) were expressed more frequently in the mature, peripheral population than in the newborn thymus. These findings may represent two instances of positive selection of T cells. Furthermore, unlike VH gene segments, D beta-proximal V beta genes are not overrepresented on this collection of immature thymocyte hybridomas. A similar analysis of V alpha gene family expression was hampered by the fact that V alpha gene segments were expressed in only 15% of newborn thymocyte hybridomas (compared to 58% of adult hybridomas). An unexpectedly large fraction (57%) of those newborn hybrids expressing a V alpha gene segment was also found to express C delta mRNA and further examination revealed that several V alpha gene probes were actually detecting delta chain mRNA. The most notable of these was the V alpha 7 gene family, which accounted for approximately one-third of the expressed V alpha genes but was expressed exclusively as part of a delta chain mRNA. We found no examples of hybridomas co-expressing both full-length alpha and delta chain mRNA, despite significant numbers of hybridomas co-expressing full-length beta and gamma chain transcripts. This observation suggests that a large number of mature alpha/beta T cells may have proceeded through development without having gone through a stage where a delta chain message is expressed, thereby precluding their development into gamma/delta cells.

Differential pattern of T cell recognition of the 65-kDa mycobacterial antigen following immunization with the whole protein or peptides

The 65-kDa stress protein from Mycobacterium bovis (Bacillus Calmette Guérin) elicited T cell proliferation and antibody responses in seven B10 congenic mouse strains with different H-2 haplotypes. To analyze T cell determinants on this antigen, seven peptides corresponding to six predicted T cell epitopes, and one defined B cell epitope were synthesized. Mice were either immunized with the whole antigen and the specificity of the response was ascertained in respect of the six peptides, or mice were immunized with seven of the peptides and tested for proliferative responses to the whole molecule. The results showed that three peptides carried epitopes to which mice responded following injection of the whole molecule and that immunization with two additional peptides could prime for in vitro stimulation with the native antigen. The latter result indicates the feasibility of generating T cell responses to "cryptic" epitopes on proteins by immunizing with peptides. The peptide-specific T cell responses were distinctly influenced by the H-2 haplotype of mouse strains. However, two peptides were recognized by several H-2-disparate mouse strains, and one peptide could be presented by both I-A and I-E molecules. Immunization with several peptides induced a cross-reactive T cell proliferative response to the homologous GroEL protein isolated from E. coli. The amount of cross-reactivity was influenced by the extent of sequence homology between mycobacterial and E. coli proteins and the major histocompatibility complex class II molecule used to present the peptide.

Tolerance of class I histocompatibility antigens expressed extrathymically

Although convincing evidence has been obtained for the imposition of self-tolerance by the intrathymic deletion of self-reactive T cells, the development of tolerance to antigens which are expressed only in the periphery is not so well understood. We have approached this question by creating transgenic mice which carry a class I major histocompatibility complex (MHC) gene (H-2Kb) linked to the rat insulin promoter. Mice expressing the transgene develop diabetes, but do not appear to mount an immune response against the transgene-expressing pancreatic beta-cells, even when the transgene is allogeneic with respect to the endogenous host H-2 antigens. We have now explored the mechanism of this tolerance further. We find that spleen cells from pre-diabetic transgenic (RIP-Kb) mice do not kill targets bearing H-2Kb, whereas thymus cells from the same mice do. The unresponsiveness of these spleen cells can be reversed in vitro by providing recombinant interleukin-2 (rIL-2). In older, diabetic mice, responsiveness develops as the pancreatic beta-cells are lost. Our results point to an extrathymic mechanism of tolerance induction, dependent on the continuous presence of antigen and the lack of IL-2 in the local environment of potentially reactive T cells.

Clonal anergy induced in mature V beta 6+ T lymphocytes on immunizing Mls-1b mice with Mls-1a expressing cells

Tolerance to self-antigens has been shown to develop during ontogeny as a result of the clonal deletion of self-reactive T cells. Tolerance, or better 'nonresponsiveness', to specific antigens can also be induced in adult animals but the mechanism(s) involved are not well understood. Most murine T-helper cells that express the V beta 6 T-cell receptor gene segment are specific for Mls-1a antigens. We have therefore been able to use an anti-V beta 6 monoclonal antibody to follow the fate of Mls-1a specific T cells in adult Mls-1b mice made specifically unresponsive to Mls-1a. We show that the induced unresponsiveness is not due to clonal deletion, but rather to clonal anergy. The anergic V beta 6 T-helper cells express IL-2 receptors and undergo limited blastogenesis in vitro upon stimulation, but do not produce IL-2, in marked contrast to V beta 6 cells from naive mice. Our data appear to represent an in vivo correlate for the induction of anergy that has been observed in T-cell lines in vitro.

A combination of a particular HLA-DP beta allele and an HLA-DQ heterodimer confers susceptibility to coeliac disease

Coeliac disease is an autoimmune disease of the intestinal mucosa, elicited by ingestion of wheat gluten in genetically susceptible individuals. Susceptibility to coeliac disease has been associated with the serologically defined variants DR3 and DR7 of the class II antigens encoded by the HLA-D region. Three related class II antigens, each consisting of an alpha and a beta glycoprotein chain, have been identified and are designated HLA-DR, HLA-DQ, and HLA-DP. These highly polymorphic transmembrane proteins bind peptides derived from the processing of foreign antigens and present them to T lymphocytes; they also influence the specificity of the mature T-cell repertoire. The role of HLA-DP polymorphism in susceptibility has not been as fully explored as that of the other class II antigens because of the complexity of the primed lymphocyte typing (PLT) method for determining DPw specificities. Here we use a new DNA-based method of HLA-DP typing to analyse the distribution of DP beta alleles in a group of coeliac disease patients and healthy controls. Two specific DP beta alleles (DPB4.2 and DPB3) are increased in the patient population. Comparison of the DP beta sequences suggests that the polymorphic residues at position 69 and at 56 and 57 may be critical in conferring susceptibility. Further, the contribution of the susceptible DP beta alleles appears to be independent of linkage to the previously reported DR3 and DR7 markers for coeliac disease. The distribution of DQ alpha and beta alleles in patients suggests that a specific DQ heterodimer may be responsible for the observed DR associations. Individuals with both this DQ antigen and a specific DP beta allele are at increased risk for coeliac disease.

Deletion of Mls-reactive T cells in H-2-compatible but Mls-incompatible bone marrow chimeras

The cellular basis of tolerance induction to Mls-encoded antigens in radiation bone marrow (BM) chimeras has been investigated in two H-2-compatible strain combinations of AKR/J (H-2k, Thy-1.1, Mls-Ia) and C3H/He (H-2k, Thy-1.2, Mls-IIa). Sequential appearance of host- and donor-derived T cell subsets and T cell receptor gene messages occurred in the peripheral lymphoid organs of both irradiated AKR/J mice reconstituted with C3H/He BM cells (C3H/He-AKR/J chimera) and irradiated C3H/He mice reconstituted with AKR/J BM cells (AKR/J-C3H/He chimera). A large number of cells expressing T cell receptor gamma genes were detected in spleen on day 21 after reconstitution, while the normal level of alloreactivity was first detected in the spleen on day 56 after reconstitution in correlation with the appearance of appreciable levels of Thy-1 high cells and T cell receptor alpha and beta gene transcripts. T cells bearing V beta 6, that is strongly correlated with reactivity to antigens encoded by the Mls-Ia genetic locus, were virtually abolished in spleen on day 56 in both C3H/He-AKR/J chimera and AKR/J-C3H/He chimera. Furthermore, expression of V beta 3 gene transcripts, that are important for recognizing Mls-IIa, was undetected either in the peripheral lymphoid cells of AKR/J-C3H/He chimera or in those of C3H/He-AKR/J chimera. These results suggested that clonal elimination of self-reactive T cells bearing V beta 3 or V beta 6 was induced by both host-derived radioresistant cells and donor-derived repopulating cells in the thymus of radiation BM chimeras.

Role of CD4 in thymocyte selection and maturation

We examined the possible role of CD4 molecules during in vivo and in vitro fetal thymic development. Our results show that fetal thymi treated with intact anti-CD4 mAbs fail to generate CD4 single-positive T cells, while the generation of the other phenotypes remains unchanged. Most importantly, the use of F(ab')2 and Fab anti-CD4 mAb gave identical results, i.e., failure to generate CD4+/CD8- T cells, with no effect on the generation of CD4+/CD8+ T cells. Since F(ab')2 and Fab anti-CD4 fail to deplete CD4+/CD8- in adult mice, these results strongly argue that the absence of CD4+/CD8- T cells is not due to depletion, but rather, is caused by a lack of positive selection, attributable to an obstructed CD4-MHC class II interaction. Furthermore, we also observed an increase in TCR/CD3 expression after anti-CD4 (divalent or monovalent) mAb treatment. The TCR/CD3 upregulation occurs in the double-positive population, and may result from CD4 signaling after mAb engagement, or may be a consequence of the blocked CD4-class II interactions. One proposed model argues that the CD3 upregulation occurs in an effort to compensate for the reduction in avidity or signaling that is normally provided by the interaction of the CD4 accessory molecule and its ligand. As a whole, our findings advocate that CD4 molecules play a decisive role in the differentiation of thymocytes.

Postnatal disappearance of self-reactive (V beta 6+) cells from the thymus of Mlsa mice. Implications for T cell development and autoimmunity

The postnatal ontogeny of potentially autoreactive T cells has been studied in a model system where a particular TCR beta chain variable domain (V beta 6) is correlated with reactivity to a minor antigen encoded by the Mlsa locus. Although absent among mature (CD4+ or CD8+) T cells in adult mice expressing Mlsa, brightly staining V beta 6+ cells were readily detectable in the thymus of neonatal animals, reaching a maximum after 4 d and decreasing rapidly thereafter. These V beta 6+ thymocytes were predominantly of the CD4+ phenotype and were localized in the medulla of the developing thymus. Furthermore, the intensity of TCR expression by these CD4+ cells was significantly (twofold) reduced as compared with age-matched Mlsb controls. A rapid disappearance of CD4+V beta 6+ cells (and corresponding decrease in TCR density) could also be observed in the thymus of Mlsb mice that had been injected neonatally with Mlsa spleen cells. Taken together, these results raise the possibility that some autoreactive T cells may persist after birth and that TCR downregulation may occur as a physiological response to tolerogenic signals in vivo.

Donor and recipient specific tolerance in cells from semi-allogeneic, H-2 subregion compatible or fully allogeneic bone marrow chimeras attributable to clonal deletion

Specificities of tolerance induced in allogeneic bone marrow (BM) chimeras which had been established by injecting allogeneic BM cells pretreated with anti-Thy-1 mAb alone (without complement (C)) were analyzed using Simonsen's splenomegaly assay. Lymphocytes from fully allogeneic, semi-allogeneic and H-2 subregion compatible BM chimeras were specifically unresponsive to donor and recipient antigens (Ag). However, cells from H-2 subregion compatible chimeras initiated as vigorously a GVHR in F1 recipient mice, which were disparate at H-2K and I-A regions, as did spleen cells of donor mice, which were incompatible at the entire H-2 and minor histocompatibility regions of the recipients. The donor cells from such chimeras that initiated these considerable GVHR were either CD4+ or CD8+ T cells. Furthermore, synergistic effects by the CD4+ and CD8+ T lymphocytes were also observed. We found no evidence for a suppressive mechanism(s) in maintenance of the specific tolerance in allogeneic chimeras. Further, when lymphoid cells from these chimeras were adoptively transferred to irradiated mice of the donor strain and maintained for 5 days in the absence of recipient Ag (tolerogen), the adoptively transferred cells were shown to retain their unresponsiveness to the recipient Ag. These results reveal that T lymphocytes from allogeneic BM chimeras prepared by our method had been specifically induced to a tolerant state to both donor and recipient Ag and that the major mechanism of induction and maintenance of long-lasting tolerance is attributable to clonal deletion of both CD4+ and CD8+ T cell subsets rather than to the development of a population of suppressor cells of any sort.

Insulin B chain functions as an effective competitor of antigen presentation via peptide homologies present in the thymus

The B chain of mammalian insulins contains appropriately spaced amino acids that predict recognition by T cells. However, all T cell clones from an HLA-DR1, Dw6 diabetic donor recognize epitopes associated with the A chain, and the B chain was found to inhibit these responses. Effective intramolecular competition at the level of the APC, not a direct effect on the T cell, is responsible for the inhibition. Insulin B chain contains two clusters of amino acid homology with the TCR beta chain and B chain peptides lacking these clusters do not compete for antigen presentation. A hole in the repertoire for T cells that recognize this portion of the insulin molecule may arise in the thymus by deletion of T cells that recognize similar peptides.

RIII S/J (H-2r). An inbred mouse strain with a massive deletion of T cell receptor V beta genes

We have identified an inbred strain of mouse, RIII S/J (H-2r), that has the largest known deletion of the TCR V beta genes by screening with mAb and TCR V beta specific probes. Upon screening of PBL with mAb F23.1, which is specific for V beta 8 TCR, RIII S/J was found to be negative. On further screening with mAb KJ 23a, which is specific for V beta 17a TCR, RIII S/J was completely negative. We next tested RIII S/J with mAb 44-22-1, which is specific for V beta 6 TCR, and found it also to be negative. The (B10 X RIII)F1 mice showed a 50% expression of V beta 6 gene, indicating a genomic rather than a clonal deletion. mAb KJ25, detecting V beta 3, was positive in RIII S/J, denoting the downstream boundary for the deletion. Southern blot analysis of liver DNA using TCR V beta-specific probes confirmed the deletion of V beta 8 gene subfamily and V beta 5 gene subfamily, along with V beta 9, V beta 11, V beta 12, and V beta 13 genes similar to the known TCR V beta deletion mutants (SWR, SJL, C57L, and C57Br). In addition, RIII S/J is missing V beta 6, V beta 15, and V beta 17 genes. Our mapping of the deletion indicates that RIII S/J has lost approximately 130 kb of V beta chromosome and with it 13 V beta genes out of the known 21 V beta genes of the TCR. The deletion is marked by the presence of V beta 10 gene upstream and V beta 3 gene downstream.

Two monoclonal antibodies specific for the T cell receptor V alpha 8

Two monoclonal antibodies, KT50 and KT65, specific for V alpha 8 have been established. This was determined as follows: (a) 4 T cell clones, C6, R1, G22 and I9, out of 43 T cell clones with various antigen specificities, major histocompatibility complex restrictions and V beta usages not only bound KT50 and KT65 but also expressed V alpha 8 mRNA, (b) KT50 and KT65 precipitated molecules from the clone C6 similar to the T cell receptor molecules precipitated in C6 cells by KT11 (anti-V beta 11) or KTL2 (anti-Ti) and (c) KT50 and KT65 were mitogenic and induced cytotoxicity. All strains of mice so far examined have populations of KT50+ and KT65+ T cells of 1.4%-3.6% and 0.9%-2.6%, respectively. Different H-2 haplotypes were not observed to affect the number of cells expressing KT50 or KT65. In addition KT15 (anti-CD8), without cross-linking to KT50 or KT65, augmented proliferation triggered by KT50 or KT65.

Molecular aspects of autoimmunity

Sustained humoral and/or cellular autoimmune responses are currently thought to be the primary causes of a wide spectrum of systemic and organ-specific human and animal diseases. Although a very good picture of the immunopathological characteristics of these diseases has emerged, their etiologies remain unknown. Studies initiated to define these diseases at the molecular genetic level were the subject of a recent meeting in Ville D'Esterel, Canada. Because these diseases are diverse and enormously complex, several avenues of investigation have been pursued. The primary focus of this meeting was the tripartite system of immunoglobulin (Ig), T-cell antigen receptor (TCR) and major histocompatibility complex (MHC) genes.

Developmental biology of T cell receptors

T cell receptors are the antigen-recognizing elements found on the effector cells of the immune system. Two isotypes have been discovered, TCR-gamma delta and TCR-alpha beta, which appear in that order during ontogeny. The maturation of prothymocytes that colonize the thymic rudiment at defined gestational stages occurs principally within the thymus, although some evidence for extrathymic maturation also exists. The maturation process includes the rearrangement and expression of the T cell receptor genes. Determination of these mechanisms, the lineages of the cells, and the subsequent thymic selection that results in self-tolerance is the central problem in developmental immunology and is important for the understanding of autoimmune diseases.

V beta-specific stimulation of human T cells by staphylococcal toxins

The staphylococcal toxins are responsible for a number of diseases in man and other animals. Many of them have also long been known to be powerful T cell stimulants. They do not, however, stimulate all T cells. On the contrary, each toxin reacts with human T cells bearing particular V beta sequences as part of their receptors for major histocompatibility complex protein-associated antigen. The specificity of these toxins for V beta s puts them in the recently described class of superantigens and may account for the differential sensitivity of different individuals to the toxic effects of these proteins.

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

A set of new mouse strains were produced that carry the V beta a haplotype of the TCR-alpha/beta and any of a number of different H-2 haplotypes on backgrounds derived from related C57BL, C57L, and C57BR mice. Study of V beta 17a expression in these mice confirms the association between the presence of IE and the deletion of V beta 17a+ T cells. A second H-2 gene causing deletion of V beta 17a+ T cells was mapped in these mice to the K end of H-2k, and H-2 influences on the level of selection of CD4+ V beta 17a+ T cells were indicated.

Recognition of idiotypic structures in the thymus

During the passage through the thymus, T cells are selected which recognize self major histocompatibility complex (MHC) antigens with low avidity. Whether the T-cell repertoire for recognition of altered self is also built up intrathymically or in the periphery, and whether it is determined exclusively by external antigens or is shaped by the internal environment is still a matter of debate. This question was addressed by analysing the responsiveness of thymocytes during post-natal development towards a nominal antigen [trinitrophenyl (TNP)] and an anti-TNP monoclonal antibody (Sp6), which carries a recurrent idiotype. During the first weeks of life, in vitro cultures of thymocytes proliferated strongly in the absence of nominal antigen. Proliferation rates were not increased by the addition of nominal antigen [TNP-ovalbumin (OA)], but a significant increase was noted in the presence of Sp6, thymocytes recognizing the processed immunoglobulin. After in vivo stimulation with TNP conjugates, 'antigen-specific' clones could also be detected in the thymus, the frequency of clones proliferating in response to Sp6 being further augmented. With increasing age, the proliferative capacity of thymocytes from unstimulated and antigenically stimulated mice decreased significantly. Responsiveness of spleen cells (SC) differed in some respects. The response towards Sp6 decreased with age, while antigen-specific clones were detected at increasing frequencies during post-natal development. Furthermore, after antigenic stimulation, the frequency solely of antigen-specific, but not of Sp6-specific clones was increased. Thus, it appears that the T-cell repertoire is shaped already during the intrathymic passage, being influenced primarily by the B-cell repertoire and modulated further by external antigen.

Relative V beta transcript levels in thymus and peripheral lymphoid tissues from various mouse strains. Inverse correlation of I-E and Mls expression with relative abundance of several V beta transcripts in peripheral lymphoid tissues

We have measured the relative levels of transcripts for 15 of the 22 known V beta gene segments. The level of transcripts for the highest and lowest expressed V beta gene segment differed by greater than 20-fold in the thymus and an even larger difference was observed in the periphery. The levels of expressions were unrelated to the order of the V beta genes on the chromosome. For most of the V beta gene segments, the relative transcript levels were the same in the thymus and periphery, suggesting that thymic selection in general does not act solely upon the V beta gene segment. One V beta gene segment in the BALB and B10 mice strains was an exception to this rule. V beta 5.2 expression in the periphery of BALB and B10 mice inversely correlated with the expression of the MHC class II molecule I-E. Five V beta gene segments had reduced transcript levels in the periphery of Mls-1a mice compared with their thymic levels or to the levels found in Mls-1b mice. The peripheral level of V beta 3 transcripts vary with MHC and Mls-2 haplotypes. The observation that certain V beta transcript levels are reduced in the periphery when compared with the thymus favors the hypothesis that self tolerance at the T cell level results in the elimination of self-reactive T cells, rather than paralysis by a block at some post-transcriptional step. Finally, the wide variability of V beta gene segment expression in the thymus suggests mechanisms exist to import an early bias to the repertoire. Whether this bias results from differential V beta segment rearrangement rates, differential V beta expression rates, or events occurring after TCR-alpha/beta expression on immature/nonmature thymocyte cell surfaces is yet to be determined.

Expression in transgenic mice of class I histocompatibility antigens controlled by the metallothionein promoter

To study the effects of increased expression of major histocompatibility complex class I molecules on the development of self-tolerance, transgenic mice were produced that expressed the H-2Kb gene under the control of the metallothionein promoter. Administration of zinc enhanced transgene expression in liver, kidney and exocrine pancreas. No evidence suggestive of an autoimmune response was found in transgene-expressing tissues in mice otherwise allogeneic to H-2Kb. Despite this lack of responsiveness in vivo, T cells could be stimulated in vitro to lyse H-2Kb-bearing target cells. No infiltration was detected in transgenic mice after irradiation and reconstitution with bone marrow cells. When spleen cells were used for reconstitution, however, dense lymphocytic infiltration was seen, particularly in the portal tracts of the liver, and this was accompanied by piecemeal necrosis and apoptosis of periportal hepatocytes. This aggressive response progressively diminished with time, and by 12 weeks after reconstitution many of the portal tracts were free of infiltration while the others showed no accompanying necrosis. The picture at this stage was similar to that seen in chronic persistent hepatitis. These results suggest that, in addition to negative selection in the thymus, peripheral mechanisms not involving clonal deletion or permanent clonal anergy can prevent immune responses to self molecules.

Disease resistance and immune response genes in cattle: strategies for their detection and evidence of their existence

The possibility of breeding or genetically engineering cattle for resistance to disease has tremendous potential for increasing the efficiency of milk and meat production. In cattle and other species, major genes that control humoral and cellular immune responses to a variety of antigens have been mapped to a chromosomal region known as the major histocompatibility complex. However, resistance or susceptibility to viral, bacterial, and parasitic diseases in noninbred species is often a complex phenotype, with age, stress, and physiologic status all being important factors in the outcome of infection. This paper reviews the function of major histocompatibility complex gene products and the relationship between polymorphism of these genes and infectious diseases. A discussion of strategies for detecting immune response genes and disease associations is presented, with particular reference to the problems and advantages of working with cattle. The present knowledge of the bovine major histocompatibility complex and its relationship to immune responsiveness and disease resistance are also reviewed, with special consideration given to enzootic bovine leukosis because of the significant relationship between alleles of the bovine lymphocyte antigen system and resistance or susceptibility to subclinical progression of bovine leukemia virus infection. Finally, potential applications of this research to genetic improvement and animal health are considered.

T-cell antigen receptor beta-chain variable region families: a study of their distribution in normal and reactive tissues

The beta chain of the T-cell antigen receptor is constructed using a variable region gene from one of approximately 20 variable gene families. Antibodies that react with the products of these gene families have been used to demonstrate clonal proliferation of T cells. Here, we describe the expression of two beta-chain variable region families in normal and reactive lymphoid tissues. In all the tissues studied, expression was scattered throughout the T-cell areas, without any clustering of expression.

Model for clonal elimination in the thymus

A thymic stromal cell clone, MRL104.8a, expresses class I as well as class II H-2k antigens after exposure to gamma-interferon. This clone also produces thymic stroma-derived T-cell growth factor (TSTGF), which is distinct from other known interleukins and is capable of promoting the growth of various antigen-specific helper T cell (Th) clones without requiring a specific antigen or interleukin 2. When the keyhole limpet hemocyanin (KLH)-specific, I-Ek-restricted Th clone 9-16 was cultured on an Ia (I-Ak and I-Ek)-expressing MRL104.8a monolayer, potent proliferation of the 9-16 cells was induced by TSTGF produced by the monolayer. In contrast, the addition of KLH resulted in lethal growth inhibition of Th clone 9-16 cells. Another Th clone that is KLH-specific but I-Ab-restricted was capable of proliferating on the Iak-expressing MRL104.8a monolayer whether or not KLH was present. More importantly, death of Th clone 9-16 cells cultured on a MRL104.8a monolayer in the presence of KLH was almost completely prevented by the addition of anti-I-Ek or anti-CD3 monoclonal antibodies, which are capable of blocking antigen recognition by the T-cell receptor. However, when Th clone 9-16 cells were cultured in the presence of KLH but on a monolayer of MRL28.8a cells, another thymic stromal clone that expresses a comparable amount of I-Ek antigen but produces a marginal amount of TSTGF, cells did not die; a lethal effect was induced by adding TSTGF. These results indicate that the TSTGF-producing and Ia-expressing thymic stromal cells induce the continuous proliferation or selective elimination of each T-cell clone, depending on whether the T-cell receptor is stimulated by the relevant antigen associated with Ia molecules expressed on the stromal cell surface.

Antibodies against the T cell receptor/CD3 complex interfere with distinct intra-thymic cell-cell interactions in vivo: correlation with arrest of T cell differentiation

Postnatal treatment of mice with antibodies against the T cell receptor complex (TcR) prevents the differentiation of mature T cells in the thymic medulla without affecting the generation of most immature cortical thymocytes, thus interfering with a discrete stage of intra-thymic T cell differentiation at the cortex/medulla transition. This result has been interpreted as indicating a direct role of the TcR in the differentiation of immature to mature T cells, possibly via TcR-ligand interactions during direct cell-cell contact. Here we analyze the effect of anti-TcR (V beta 8 family) and anti-CD3 (epsilon chain) antibodies on distinct intra-thymic cell-cell interactions in vivo. We find that the maturation arrest of thymocytes correlates with a nearly complete abrogation of interactions of corresponding immature thymocyte with I-A/E+ cortical epithelial cells and I-A/E+ medullary dendritic cells, while preserving interactions with adherent I-A/E- macrophages. It is proposed that the blockade of thymocyte-epithelial cell recognition in the cortex by anti-TcR antibodies prevents the translocation of thymocytes into the medulla and their subsequent differentiation and selection, including interactions with dendritic cells. Interestingly, the anti-CD3 mAb treatment seems to spare the intra-thymic development of the CD3+, CD4-/CD8- T cell lineage.

Preferential differentiation of T cell receptor specificities based on the MHC glycoproteins encountered during development. Evidence for positive selection

T cells recognize foreign antigens together with those MHC glycoproteins they have encountered during their development in the thymus. How the repertoire of antigen-specific TCRs is selected has not yet been fully defined. We have investigated the T cell repertoire specificities of CD4-CD8+ cytotoxic T cells developing under conditions where one of the class I MHC-encoded molecules is blocked, while other class I-MHC glycoproteins are still expressed. We show that antigen-specific T cells restricted to the blocked class I fail to develop, while generation of other class I-specific T cell proceeds undisturbed. This highly selective perturbation of the T cell receptor repertoire demonstrates that development of CD4-CD8+ T cells with a certain TCR specificity requires expression of particular alleles of class I MHC. Thus, TCR-MHC interactions provide signals essential to the differentiation of precursor T cells.

Peripheral T lymphocytes: expansion potential and homeostatic regulation of pool sizes and CD4/CD8 ratios in vivo

Peripheral T lymphocytes are self-renewable cell populations since, when transferred into syngeneic T cell-deficient athymic mice, they expand in the absence of exogenous antigen stimulation. Quantification of the expansion potential of CD4+ cells by transfer of the same population into successive host mice shows that these cells are able to divide up to 56 times in vivo. Therefore, as a population, CD4+ cells can increase in size 8 x 10(5)-fold, an expansion potential of similar magnitude to that previously reported for colony-forming units. Injection of different numbers of T cells at different CD4/CD8 ratios is followed by T cell accumulation to a similar plateau in recipient nude mice. This indicates that peripheral T lymphocytes are tightly regulated by homeostatic mechanisms that control pool sizes and CD4/CD8 ratios, in a manner independent of the cell input into the peripheral compartment. This kinetic behavior of mature T cells permits the maintenance at the periphery of any T cell specificity previously selected in the thymus. The expansion capacity of peripheral T cells may also allow extensive modulation of peripheral T cell specificities, which would confer a major role to post-thymic selection of mature peripheral T cell repertoires.

Peptide-specific prevention of experimental allergic encephalomyelitis. Neonatal tolerance induced to the dominant T cell determinant of myelin basic protein

Experimental allergic encephalomyelitis (EAE) is a model of antigen-specific T cell-mediated autoimmune disease. The alpha-acetylated, NH2-terminal nine amino acids (1-9NAc) of myelin basic protein (MBP) represents the dominant T cell epitope for the induction of EAE in the B10.PL (H-2u) strain. We tolerized neonatal B10.PL mice to 1-9NAc and studied the proliferative responses to this peptide and to whole MBP. Mice exposed to 1-9NAc in the neonatal period were tolerant to subsequent challenge at the proliferative T cell level. Similarly, in the 1-9NAc-tolerant group, both the incidence and severity of 1-9NAc induced EAE were greatly reduced. The fact that we were able to tolerize mice normally responsive to MBP suggests that this self antigen is sequestered (within the central nervous system) and hence tolerance to it is not normally induced. No significant difference in disease incidence was seen in response to rat MBP between control animals and 1-9NAc-tolerized mice (50% in both groups), demonstrating the presence of at least one additional encephalitogenic determinant elsewhere on the molecule. We have successfully prevented disease induction by peptide-induced tolerization. Tolerance induction by peptides provides a new and specific strategy in the prevention of autoimmunity. However, it will be clearly necessary to fully define all epitopes potentially capable of inducing pathogenic T cells to ensure complete and effective therapy of T cell-mediated autoimmune disease.

Veto function in vitro and in vivo

A CTLp recognizing another cell, called veto cell, is suppressed by that veto cell. The veto cell can itself be a CTL. For the veto function, the TCR of the veto cell is not required, hence the veto function is a backward action of CTL. Since, from the point of view of the veto cell, only self-reactive CTLp are suppressed, the veto function could be a mechanism for maintaining self tolerance of CTL. The characteristics of veto function in vitro and in vivo are discussed, as well as the potential physiological relevance.

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.

Presentation of antigen, foreign major histocompatibility complex proteins and self by thymus cortical epithelium

In mouse and man most peripheral T cells bear clonally variable receptors made up of alpha- and beta-chains which bind ligands on target cells consisting of peptide fragments of foreign antigens, complexed with cell surface proteins encoded by the major histocompatibility complex (MHC) of the individual. In the thymus, developing T cells are selected to mature only if their receptors will be able to participate in self-MHC plus antigen recognition in the periphery. This positive selection occurs in the presence of self-MHC, but in the apparent absence of antigen, leading to the paradoxical conclusion that developing thymocytes must be positively selected by engagement of their receptors and self-MHC alone, although thymocytes that react too well with self-MHC are eliminated. To account for this, it has been suggested that MHC molecules in the thymus are not identical to those found elsewhere. To test this and other hypotheses, we have examined the ability of the presumed selecting cells, those of the thymus cortical epithelium, to present various MHC complexes to T cells. Our results indicate that MHC molecules on thymus epithelium are not always the same as those found elsewhere.

The MHC molecule I-E is necessary but not sufficient for the clonal deletion of V beta 11-bearing T cells

We have generated an mAb, RR3-15, that recognizes murine TCRs containing the V beta 11 domain. Using this antibody to stain peripheral T cells, we have demonstrated that V beta 11-bearing T cells are largely absent from strains of mice that express the class II MHC molecule, I-E. Studies with F1 mice demonstrate that this effect is dominant, consistent with tolerance. The clonal deletion of V beta 11-bearing T cells appears to occur intrathymically, as immature but not mature V beta 11+ T cells are present in the thymus of I-E-bearing mice. Examination of B6 x DBA/2 recombinant inbred strains demonstrates that the expression of I-E molecules is necessary for the clonal deletion of V beta 11-bearing T cells, but that other non-MHC genes control the clonal deletion process, as well. Paradoxically, only a small fraction of V beta 11+ T cell hybridomas are I-E reactive.

Masking of veto function in vivo by activated CD4+ T lymphocytes

Donor CD8+ T lymphocytes injected into recipient mice incompatible at major histocompatibility complex (MHC) class I genes induce donor-specific CTL nonresponsiveness, attributed to the veto function of donor cells. Here we show that conditions leading to strong activation of CD4+ T cells, namely the presence in the recipient of foreign MHC class II determinants, lead to the apparent loss of veto function of donor cells. This "masking" of veto function is dependent on the dose of foreign MHC class II present. Veto function can be partially restored by treatment of recipients in vivo with CD4-specific antibody, a measure which has been shown to eliminate the function of CD4+ T cells in vivo. We conclude that CD4+ T cells activated by contact with antigen can interfere with the veto function of CD8+ T cells. Consequences of this finding are: (a) veto function of a sample cell population can be overlooked when activation of CD4+ T cells occurs simultaneously. (b) The balance between veto function of recipient cells and its abrogation might be responsible for the kind of graft-vs.-host reaction generated (CD8+ T cell-mediated and frequently lethal or CD4+ T cell-mediated and not lethal) when parental T cells are injected into recipients incompatible at MHC class I and class II genes. (c) A possible contribution of veto cells should be considered in several protocols in which donor hemopoetic cells were used in conjunction with CD4-specific antibodies to induce transplantation tolerance. (d) Veto function in vivo does not require a contribution of CD4+ T cells.

The long road to functional maturity for developing T cells

The past two years have clarified the role of the thymus as a filter for T cells with appropriate recognition specificities. What remains a knottier problem is the role of the thymus as an inductive microenvironment for changes in the regulation of genes involved in effector function--that is, making T cells into competent elements in immune defenses. It is still uncertain to what extent the functional role of a T cell--as an interleukin 2 (IL-2)-producing helper, an IL-4-producing helper, or a killer expressing granzymes and perforin--is irreversibly determined by intrathymic events. Another, related question is whether the stimuli required by a cell to induce expression of any of its 'response' genes may also change in development, through maturation of specialized signal transduction pathways. If so, some differentiation events that profoundly affect the immunological role of T cells might be subtle and difficult to score.(ABSTRACT TRUNCATED AT 250 WORDS)

Medullary epithelial cell lines from murine thymus constitutively secrete IL-1 and hematopoietic growth factors and express class II antigens in response to recombinant interferon-gamma

In this report, we describe the generation of two cloned epithelial cell lines, TE-71 and TE-75, from murine thymus. These cell lines resemble medullary thymic epithelium by a number of criteria, including reactivity with the monoclonal antibodies A2B5 and ER-TR5, the fucose-specific lectin derived from Ulex europeus, and the expression of keratins normally expressed by medullary thymic epithelial cells in situ. Constitutive Class II antigen expression by these cells is not detectable at the light or electron microscopic level or with flow cytometry. Following exposure to recombinant interferon-gamma or supernatants from mitogen-stimulated spleen cells, expression of Class II antigens by these thymic epithelial cell lines is increased, although less than the levels expressed by spleen cells. Medium conditioned by TE-71 and TE-75 cells exhibited colony-stimulating activity for bone marrow cells. In addition, TE-71-conditioned medium exhibited IL-1-like activity which could be neutralized with anti-IL-1 antibodies.