Preferential positive selection of V alpha 2+ CD8+ T cells in mouse strains expressing both H-2k and T cell receptor V alpha a haplotypes: determination with a V alpha 2-specific monoclonal antibody

On integrity in immunity during ontogeny or how thymic regulatory T cells work

The Standard model of T cell recognition asserts that T cell receptor (TCR) specificities are positively and negatively selected during ontogeny in the thymus and that peripheral T cell repertoire has mild self-major histocompatibility complex (MHC) reactivity, known as MHC restriction of foreign antigen. Thus, the TCR must bind both a restrictive molecule (MHC allele) and a peptide reclining in its groove (pMHC ligand) in order to transmit signal into a T cell. The Standard and Cohn's Tritope models suggest contradictory roles for complementarity-determining regions (CDRs) of the TCRs. Here, I discuss both concepts and propose a different solution to ontogenetic mechanism for TCR-MHC-conserved interaction. I suggest that double (CD4+ CD8+ )-positive (DP) developing thymocytes compete with their αβTCRs for binding to self-pMHC on cortical thymic epithelial cells (cTECs) that present a selected set of tissue-restricted antigens. The competition between DPs involves TCR editing and secondary rearrangements, similar to germinal-centre B cell somatic hypermutation. These processes would generate cells with higher TCR affinity for self-pMHC, facilitating sufficiently long binding to cTECs to become thymic T regulatory cells (tTregs). Furthermore, CD4+ Foxp3+ tTregs can be generated by mTECs via Aire-dependent and Aire-independent pathways, and additionally on thymic bone marrow-derived APCs including thymic Aire-expressing B cells. Thymic Tregs differ from the induced peripheral Tregs, which comprise the negative feedback loop to restrain immune responses. The implication of thymocytes' competition for the highest binding to self-pMHC is the co-evolution of species-specific αβTCR V regions with MHC alleles.

Characterization of Mouse γδ T Cell Subsets in the Setting of Type-2 Immunity

Accumulating evidence indicates that γδ T cells are a critical component of type-2 immunity. However, the role of these cells in type-2 immune responses seems to be divergent. γδ T cells are heterogeneous lymphocytes that can be further divided into TCR-Vγ/δ definable subsets. Different subsets have distinct and sometimes opposite function during immune responses. In this chapter, we describe the detailed protocol for characterization of γδ T cell subsets in a mouse model of ovalbumin (OVA)/alum-induced type-2 immunity. Our protocol includes identifying γδ T cell subsets by flow cytometry, functionally inactivating individual subsets in vivo, purifying γδ T cell subsets, and using adoptive cell transfer to explore the role of individual subsets in OVA/alum-induced IgE responses.

The somatically generated portion of T cell receptor CDR3α contributes to the MHC allele specificity of the T cell receptor

Mature T cells bearing αβ T cell receptors react with foreign antigens bound to alleles of major histocompatibility complex proteins (MHC) that they were exposed to during their development in the thymus, a phenomenon known as positive selection. The structural basis for positive selection has long been debated. Here, using mice expressing one of two different T cell receptor β chains and various MHC alleles, we show that positive selection-induced MHC bias of T cell receptors is affected both by the germline encoded elements of the T cell receptor α and β chain and, surprisingly, dramatically affected by the non germ line encoded portions of CDR3 of the T cell receptor α chain. Thus, in addition to determining specificity for antigen, the non germline encoded elements of T cell receptors may help the proteins cope with the extremely polymorphic nature of major histocompatibility complex products within the species.

Antigen Processing in the Endoplasmic Reticulum Is Monitored by Semi-Invariant αβ TCRs Specific for a Conserved Peptide-Qa-1b MHC Class Ib Ligand

Ag processing in the endoplasmic reticulum (ER) by the ER aminopeptidase associated with Ag processing (ERAAP) is central to presentation of a normal peptide-MHC class I (MHC I) repertoire. Alternations in ERAAP function cause dramatic changes in the MHC I-presented peptides, which elicit potent immune responses. An unusual subset of CD8⁺ T cells monitor normal Ag processing by responding to a highly conserved FL9 peptide that is presented by Qa-1^b, a nonclassical MHC Ib molecule (QFL) in ERAAP-deficient cells. To understand the structural basis for recognition of the conserved ligand, we analyzed the αβ TCRs of QFL-specific T cells. Individual cells in normal wild-type and TCRβ-transgenic mice were assessed for QFL-specific TCR α- and β-chains. The QFL-specific cells expressed a predominant semi-invariant TCR generated by DNA rearrangement of TRAV9d-3-TRAJ21 α-chain and TRBV5-TRBD1-TRBJ2-7 β-chain gene segments. Furthermore, the CDR3 regions of the α- as well as β-chains were required for QFL ligand recognition. Thus, the αβ TCRs used to recognize the peptide-Qa-1 ligand presented by ERAAP-deficient cells are semi-invariant and likely reflect a conserved mechanism for monitoring the fidelity of Ag processing in the ER.

Allelic exclusion of TCR α-chains upon severe restriction of Vα repertoire

Development of thymocytes through the positive selection checkpoint requires the rearrangement and expression of a suitable T cell receptor (TCR) α-chain that can pair with the already-expressed β-chain to make a TCR that is selectable. That is, it must have sufficient affinity for self MHC-peptide to induce the signals required for differentiation, but not too strong so as to induce cell death. Because both alleles of the α-chain continue to rearrange until a positively-selectable heterodimer is formed, thymocytes and T cells can in principle express dual α-chains. However, cell-surface expression of two TCRs is comparatively rare in mature T cells because of post-transcriptional regulatory mechanisms termed “phenotypic allelic exclusion”. We produced mice transgenic for a rearranged β-chain and for two unrearranged α-chains on a genetic background where endogenous α-chains could not be rearranged. Both Vα3.2 and Vα2 containing α-chains were efficiently positively selected, to the extent that a population of dual α-chain-bearing cells was not distinguishable from single α-chain-expressors. Surprisingly, Vα3.2-expressing cells were much more frequent than the Vα2 transgene-expressing cells, even though this Vα3.2-Vβ5 combination can reconstitute a known selectable TCR. In accord with previous work on the Vα3 repertoire, T cells bearing Vα3.2 expressed from the rearranged minilocus were predominantly selected into the CD8⁺ T cell subpopulation. Because of the dominance of Vα3.2 expression over Vα2 expressed from the miniloci, the peripheral T cell population was predominantly CD8⁺ cells.

Pervasive and stochastic changes in the TCR repertoire of regulatory T-cell-deficient mice

We hypothesize that regulatory T-cell (Treg)-deficient strains have an altered TCR repertoire in part due to the expansion of autoimmune repertoire by self-antigen. We compared the Vbeta family expression profile between B6 and Treg-lacking B6.Cg-Foxp3(sf)(/Y) (B6.sf) mice using fluorescent anti-Vbeta mAbs and observed no changes. However, while the spectratypes of 20 Vbeta families among B6 mice were highly similar, the Vbeta family spectratypes of B6.sf mice were remarkably different from B6 mice and from each other. Significant spectratype changes in many Vbeta families were also observed in Treg-deficient IL-2 knockout (KO) and IL-2Ralpha KO mice. Such changes were not observed with anti-CD3 mAb-treated B6 mice or B6 CD4+CD25- T cells. TCR transgenic (OT-II.sf) mice displayed dramatic reduction of clonotypic TCR with concomitant increase in T cells bearing non-transgenic Vbeta and Valpha families, including T cells with dual receptors expressing reduced levels of transgenic Valpha and endogenous Valpha. Collectively, the data demonstrate that Treg deficiency allows polyclonal expansion of T cells in a stochastic manner, resulting in widespread changes in the TCR repertoire.

The Yersinia enterocolitica invasin protein promotes major histocompatibility complex class I- and class II-restricted T-cell responses

Yersinia enterocolitica invasin (Inv) protein confers internalization into and expression of proinflammatory cytokines by host cells. Both events require binding of Inv to beta1 integrins, which initiates signaling cascades including activation of focal adhesion complexes, Rac1, mitogen-activated protein kinase, and NF-kappaB. Here we tested whether Inv might be suitable as a delivery molecule and adjuvant if used as a component of a vaccine. For this purpose, hybrid proteins composed of Inv and ovalbumin (OVA) were prepared, applied as a coating to microparticles, and used for vaccination. Fusion of OVA to Inv did not significantly disturb the ability of Inv to promote host cell binding, internalization, and interleukin-8 (IL-8) secretion when applied as a coating to microparticles. The microparticles were used for vaccination of mice adoptively transferred with OVA-specific T cells from OT-1 or DO11.10 mice. Administration of OVA-Inv-coated microparticles induced OVA-specific T-cell responses. OVA-specific CD4 T cells produced both gamma interferon (IFN-gamma) and IL-4 as determined by enzyme-linked immunosorbent assay. Likewise, pronounced OVA-specific CD8 T-cell responses associated with IFN-gamma production were observed. Together, these results suggest that Inv might be an attractive tool in vaccination as it confers both host cell uptake and adjuvant activity by engagement of beta1 integrins of host cells, which leads to CD4 as well as CD8 T-cell responses.

Induction of CD8+ T cell responses by Yersinia vaccine carrier strains

Yersinia enterocolitica employs a type III secretion system (TTSS) to target virulence factors (e.g. YopE) into the cytosol of the host cells. We utilized the TTSS to introduce a recombinant antigen directly into the cytosol of host cells and to investigate the potential of Y. enterocolitica and Y. pseudotuberculosis as live carrier for vaccines. The model antigen ovalbumin (Ova) was fused to defined secretion or translocation domains of the Yersinia effector protein YopE and introduced into attenuated mutant strains of Y. enterocolitica and Y. pseudotuberculosis. In vitro experiments showed secretion and translocation of YopE-Ova hybrid proteins into host cells. To investigate the resulting immune responses, mice expressing transgenic Ova-specific T cell receptors were used. Both Y. enterocolitica and Y. pseudotuberculosis mutants induced efficaciously Ova-specific CD8+ T cell responses. The translocation domain of YopE was required for induction of CD8+ T cell responses in vivo, but not for T cell responses induced in vitro. The in vivo frequency of Ova-specific splenic T cells was up to six-fold higher in mice immunized with YopE-Ova-translocating Y. enterocolitica/Y. pseudotuberculosis mutants than in control mice. The Ova-specific T cells were shown to produce high amounts of IFN-gamma. We did not observe significant Ova-specific CD4+ T cell or antibody responses upon vaccination with either of the strains. In conclusion, Yersinia live carrier vaccine strains are suitable to target antigens into the MHC class I pathway and stimulate CD8+ T cell responses and thus, might be useful in vaccine approaches against intracellular pathogens.

CD1d-Independent Developmental Acquisition of Prompt IL-4 Gene Inducibility in Thymus CD161(NK1)−CD44lowCD4+CD8− T Cells Is Associated with Complementarity Determining Region 3-Diverse and Biased Vβ2/Vβ7/Vβ8/Vα3.2 T Cell Receptor Usage

Among Ag-inexperienced naive T cells, the CD1d-restricted NKT cell that uses invariant TCR-alpha-chain is the most widely studied cell capable of prompt IL-4 inducibility. We show in this study that thymus CD161-CD44lowCD4+CD8- T cells promptly produce IL-4 upon TCR stimulation, a response that displays biased Vbeta(2/7/8) and Valpha3.2 TCR usage. The association of Vbeta family bias and IL-4 inducibility in thymus CD161-CD44lowCD4+CD8- T cells is found for B6, B10, BALB/c, CBA, B10.A(4R), and ICR mouse strains. Despite reduced IL-4 inducibility, there is a similarly biased Vbeta(2/7/8) TCR usage by IL-4 inducibility+ spleen CD161-CD44lowCD4+CD8- T cells. Removal of alpha-galacotosylceramide/CD1d-binding cells from CD161-CD44lowCD4+CD8- thymocytes does not significantly affect their IL-4 inducibility. The development of thymus CD161-CD44lowCD4+CD8- T cells endowed with IL-4 inducibility and their associated use of Vbeta(2/7/8) are beta2-microglobulin-, CD1d-, and p59fyn-independent. Thymus CD161-CD44lowCD4+CD8- T cells produce low and no IFN-gamma inducibility in response to TCR stimulation and to IL-12 + IL-18, respectively, and they express diverse complementarity determining region 3 sequences for both TCR-alpha- and -beta-chains. Taken together, these results demonstrate the existence of a NKT cell distinct, TCR-repertoire diverse naive CD4+ T cell subset capable of prompt IL-4 inducibility. This subset has the potential to participate in immune response to a relatively large number of Ags. The more prevalent nature of this unique T cell subset in the thymus than the periphery implies roles it might play in intrathymic T cell development and may provide a framework upon which mechanisms of developmentally regulated IL-4 gene inducibility can be studied.

Mismatched antigen prepares gamma delta T cells for suppression of airway hyperresponsiveness

Gammadelta T cells suppress airway hyperresponsiveness (AHR) induced in allergen-challenged mice but it is not clear whether the suppression is allergen specific. The AHR-suppressive cells express TCR-Vgamma4. To test whether the suppressive function must be induced, we adoptively transferred purified Vgamma4(+) cells into gammadelta T cell-deficient and OVA-sensitized and -challenged recipients (B6.TCR-Vgamma4(-/-)/6(-/-)) and measured the effect on AHR. Vgamma4(+) gammadelta T cells isolated from naive donors were not AHR-suppressive, but Vgamma4(+) cells from OVA-stimulated donors suppressed AHR. Suppressive Vgamma4(+) cells could be isolated from lung and spleen. Their induction in the spleen required sensitization and challenge. In the lung, their function was induced by airway challenge alone. Induction of the suppressors was associated with their activation but it did not alter their ability to accumulate in the lung. Vgamma4(+) gammadelta T cells preferentially express Vdelta4 and -5 but their AHR-suppressive function was not dependent on these Vdeltas. Donor sensitization and challenge not only with OVA but also with two unrelated allergens (ragweed and BSA) induced Vgamma4(+) cells capable of suppressing AHR in the OVA-hyperresponsive recipients, but the process of sensitization and challenge alone (adjuvant and saline only) was not sufficient to induce suppressor function, and LPS as a component of the allergen was not essential. We conclude that AHR-suppressive Vgamma4(+) gammadelta T cells require induction. They are induced by allergen stimulation, but AHR suppression by these cells does not require their restimulation with the same allergen.

Cutting edge: precursor frequency affects the helper dependence of cytotoxic T cells

Generation of CTL immunity often depends on the availability of CD4 T cell help. In this report, we show that CTL responses induced by cross-priming can be converted from CD4-dependent to CD4-independent by increasing the frequency of CTL precursors. In the absence of CD4 T cells, high numbers of CTL precursors were able to expand in number and become effector CTL. The ability of high frequencies of CD8 T cells to override help was not due to their ability to signal CD40 via expression of CD154. These findings suggest that when precursor frequencies are high, priming of CD8 T cell responses may not require CD4 T cell help.

Characterizing the functionality of recombinant T-cell receptors in vitro: a pMHC tetramer based approach

The very low affinity of the T-cell receptor (TCR) for the peptide-major histocompatibility complex (pMHC) has made it very challenging to design assays for testing the functionality of these molecules on small scales, which in turn has severely hampered the progress in developing expression and refolding methodologies for the TCR. We have now developed an ELISA assay for detecting pMHC binding to functional recombinant TCRs. It uses tetramers of biotinylated pMHCs bound to a neutravidin-horseradish peroxidase conjugate and detects the presence of functional TCR, bound in a productive orientation to an immobilized anti-Cbeta antibody. Specificity can be stringently demonstrated by inhibition with monomeric pMHCs. The assay is very sensitive and specific, and requires only very small amounts of protein. It has allowed us to study the unstable recombinant TCR P14, which we expressed and refolded from Escherichia coli. The TCR P14 is directed against the most abundant epitope of LCMV. We have confirmed the specificity of the interaction by BIAcore, and were able to determine the dissociation constant of the interaction of the P14 TCR and of the gp33-pMHC as 6 microM. This affinity ranks it among the tighter ones of TCR-pMHC interactions, and unusually low affinity thus does not seem to be the cause of the modest protective power of these T-cells, compared to others elicited in the anti-LCMV response. This strategy of multimerizing one partner and immobilizing the other in both a native form and productive orientation should be generally useful for characterizing the weak interactions of cell-surface molecules.

Amino Acids Specifying MHC Class Preference in TCR Vα2 Regions

Some TCR variable regions are preferentially expressed in CD4+ or CD8+ T cells, reflecting a predilection for interacting with MHC class II or class I molecules. The molecular basis for MHC class bias has been studied previously, in particular for Vα3 family members, pointing to a dominant role for two amino acid positions in complementary-determining regions (CDRs) 1 and 2. We have evaluated the generality of these findings by examining the MHC class bias of Vα2 family members, an attractive system because it shows more variability within the CDR1 and -2, exhibits variation in the framework regions, and includes a member for which the crystal structure has been determined. We find that preferential recognition of MHC class I or II molecules does not always depend on residues at the same positions of CDR1 and -2; rules for one family may be reversed in another. Instead, there are multiple influences exerted by various CDR1/2 positions as well as the CDR3s of both the TCR α- and TCR β-chains.

Reciprocal Expression in CD4 or CD8 Subsets of Different Members of the Vα11 Gene Family Correlates with Sequence Polymorphism

Previous staining studies with TCR Vα11-specific mAbs showed that Vα11.1/11.2 (AV11S1 and S2) expression was selectively favored in the CD4+ peripheral T cell population. As this phenomenon was essentially independent of the MHC haplotype, it was suggested that AV11S1 and S2 TCRs exert a preference for recognition of class II MHC molecules. The Vα segment of the TCR α-chain is suggested to have a primary role in shaping the T cell repertoire due to selection for class I or II molecules acting through the complementarity determining regions (CDR) 1α and CDR2α residues. We have analyzed the repertoire of Vα11 family members expressed in C57BL/6 mice and have identified a new member of this family; AV11S8. We show that, whereas AV11S1 and S2 are more frequent in CD4+ cells, AV11S3 and S8 are more frequent in CD8+ cells. The sequences in the CDR1α and CDR2α correlate with differential expression in CD4+ or CD8+ cells, a phenomenon that is also observed in BALB/c mice. With no apparent restriction in TCR Jα usage or CDR3α length in C57BL/6, these findings support the idea of Vα-dependent T cell repertoire selection through preferential recognition of MHC class I or class II molecules.

Inhibition of thymocyte positive selection by natural MHC: peptide ligands

The 3A9 transgenic mouse line carries the rearranged TCR genes from a T cell hybridoma that recognizes hen egg lysozyme peptide 46-61 in the context of MHC class II Ak molecules. As expected, positive selection of immature 3A9 thymocytes to become mature CD4+ 8- T cells was efficient on the "selecting" CBA (H-2k) genetic background but not on the "non-selecting" C57BL/6 (H-2b) background. Surprisingly, positive selection was also inefficient on the CBA x C57BL/6 F1 background (H-2kb). We present evidence that expression of A(beta)b molecules on thymus epithelium (in conjunction with A(alpha)b or A(alpha)k molecules) inhibits the positive selection of 3A9 thymocytes mediated by A(alpha)k:A(beta)k complexes, in a process evocative of peptide antagonism of mature T cells.

Initiation of autoimmune diabetes by developmentally regulated presentation of islet cell antigens in the pancreatic lymph nodes

Little is known about the events triggering lymphocyte invasion of the pancreatic islets in prelude to autoimmune diabetes. For example, where islet-reactive T cells first encounter antigen has not been identified. We addressed this issue using BDC2.5 T cell receptor transgenic mice, which express a receptor recognizing a natural islet beta cell antigen. In BDC2.5 animals, activated T cells were found only in the islets and the lymph nodes draining them, and there was a close temporal correlation between lymph node T cell activation and islet infiltration. When naive BDC2.5 T cells were transferred into nontransgenic recipients, proliferating cells were observed only in pancreatic lymph nodes, and this occurred significantly before insulitis was detectable. Surprisingly, proliferation was not seen in 10-day-old recipients. This age-dependent dichotomy was reproduced in a second transfer system based on an unrelated antigen artificially expressed on beta cells. We conclude that beta cell antigens are transported specifically to pancreatic lymph nodes, where they trigger reactive T cells to invade the islets. Systemic or extrapancreatic T cell priming, indicative of activation via molecular mimicry or superantigens, was not seen. Compromised presentation of beta cell antigens in the pancreatic lymph nodes of juvenile animals may be the root of a first "checkpoint" in diabetes progression.

T cell receptor (TCR) antagonism without a negative signal: evidence from T cell hybridomas expressing two independent TCRs

Antagonist peptides inhibit T cell responses by an unknown mechanism. By coexpressing two independent T cell receptors (TCRs) on a single T cell hybridoma, we addressed the question of whether antagonist ligands induce a dominant-negative signal that inhibits the function of a second, independent TCR. The two receptors, Valpha2Vbeta5 and Valpha2Vbeta10, restricted by H-2Kb and specific for the octameric peptides SIINFEKL and SSIEFARL, respectively, were coexpressed on the same cell. Agonist stimulation demonstrated that the two receptors behaved independently with regard to antigen-induced TCR downregulation and intracellular biochemical signaling. The exposure of one TCR (Valpha2Vbeta5) to antagonist peptides could not inhibit a second independent TCR (Valpha2Vbeta10) from responding to its antigen. Thus, our data clearly demonstrate that these antagonist ligands do not generate a dominant-negative signal which affects the responsiveness of the entire cell. In addition, a kinetic analysis showed that even 12 h after engagement with their cognate antigen and 10 h after reaching a steady-state of TCR internalization, T cells were fully inhibited by the addition of antagonist peptides. The window of susceptibility to antagonist ligands correlated exactly with the time required for the responding T cells to commit to interleukin 2 production. The data support a model where antagonist ligands can competitively inhibit antigenic peptides from productively engaging the TCR. This competitive inhibition is effective during the entire commitment period, where sustained TCR engagement is essential for full T cell activation.

Visualization of CD4/CD8 T cell commitment

A system to innocuously visualize T cell lineage commitment is described. Using a "knock-in" approach, we have generated mice expressing a beta-galactosidase reporter in place of CD4; expression of beta-galactosidase in these animals appears to be an accurate and early indicator of CD4 gene transcription. We have exploited this knock-in line to trace CD4/CD8 lineage commitment in the thymus, avoiding important pitfalls of past experimental approaches. Our results argue in favor of a selective model of thymocyte commitment, demonstrating a fundamentally symmetrical process: engagement of either class of major histocompatibility complex (MHC) molecule by a differentiating CD4(+)CD8(+) cell can give rise to T cell antigen receptor (TCR)hi thymocytes of either lineage. Key findings include (a) direct demonstration of a substantial number of CD4-committed, receptor/coreceptor-mismatched cells in MHC class II- deficient mice, a critical prediction of the selective model; (b) highly efficient rescue of such "mismatched" intermediates by forced expression of CD8 in a TCR transgenic line, and an explanation of why previous experiments of this nature were less successful-a major past criticism of the selective model; (c) direct demonstration of an analogous, though smaller, population of CD8-committed mismatched intermediates in class I-deficient animals. Finally, we found no evidence of a CD4 default pathway.

Highly restricted T cell repertoire shaped by a single major histocompatibility complex-peptide ligand in the presence of a single rearranged T cell receptor beta chain

The T cell repertoire is shaped by positive and negative selection of thymocytes through the interaction of alpha/beta-T cell receptors (TCR) with self-peptides bound to self-major histocompatibility complex (MHC) molecules. However, the involvement of specific TCR-peptide contacts in positive selection remains unclear. By fixing TCR-beta chains with a single rearranged TCR-beta irrelevant to the selecting ligand, we show here that T cells selected to mature on a single MHC-peptide complex express highly restricted TCR-alpha chains in terms of Valpha usage and amino acid residue of their CDR3 loops, whereas such restriction was not observed with those selected by the same MHC with diverse sets of self-peptides including this peptide. Thus, we visualized the TCR structure required to survive positive selection directed by this single ligand. Our findings provide definitive evidence that specific recognition of self-peptides by TCR could be involved in positive selection of thymocytes.

Epitope glycosylation plays a critical role for T cell recognition of type II collagen in collagen-induced arthritis

Immunization of mice with type II collagen (CII) leads to collagen-induced arthritis (CIA), a model for rheumatoid arthritis. T cell recognition of CII is believed to be a critical step in CIA development. We have analyzed the T cell determinants on CII and the TCR used for their recognition, using twenty-nine T cell hybridomas derived from C3H.Q and DBA/1 mice immunized with rat CII. All hybridomas were specific for the CII(256-270) segment. However, posttranslational modifications (hydroxylation and variable O-linked glycosylation) of the lysine at position 264 generated five T cell determinants that were specifically recognized by different T cell hybridoma subsets. TCR sequencing indicated that each of the five T cell epitopes selected its own TCR repertoire. The physiological relevance of this observation was shown by in vivo antibody-driven depletion of TCR Valpha2-positive T cells, which resulted in an inhibition of the T cell proliferative response in vitro towards the non-modified CII(256-270), but not towards the glycosylated epitope. Most hybridomas (20/29) specifically recognized CII(256-270) glycosylated with a monosaccharide (beta-D-galactopyranose). We conclude that this glycopeptide is immunodominant in CIA and that posttranslational modifications of CII create new T cell determinants that generate a diverse TCR repertoire.

Thymic skewing of the CD4/CD8 ratio maps with the T-cell receptor alpha-chain locus

The thymic preference for CD4+ T cells over CD8+ T cells is often attributed to a default pathway favouring CD4+ T cells or to homeostatic mechanisms. It is also clear, however, that T-cell receptor (TCR) preferences for major histocompatibility complex (MHC) class I versus class II binding will strongly influence an individual clone's skewing to the CD4 or CD8 subset. The variable region of each TCR alpha chain (V alpha) studied to date is found to be overrepresented in either CD4+ or CD8+ cells, suggesting that each V alpha element can interact more favourably with either MHC class I or class II molecules. Indeed, TCRs appear to have an intrinsic ability to interact with MHC molecules, and single amino acid residues present in germline-encoded complementarity determining region 1 (CDR1) and CDR2 of the V alpha element can be responsible for determining MHC specificity. Interestingly, the degree of CD4/CD8 skewing is variable among different mouse strains and in human populations. Here, we have shown that polymorphism in CD4/CD8 skewing between B6 and BALB/c mice is determined by the stem cell genotype and not by environmental effects, and that it maps in or near the TCR alpha-chain complex, Tcra. This was confirmed by comparing Tcra(b) with Tcra(a) or Tcra(c) haplotypes in congenic mice. We propose that the array of V alpha genes in various Tcra haplotypes exerts influence over the proportion of CD4 and CD8 subsets generated and may account in part for the observed thymic skewing. Thus, while it has been suggested that the TCR genes have been selected by evolution for MHC binding, our results further indicate selection for class II MHC preference.

Posttranslational Regulation of TCR Vα Allelic Exclusion During T Cell Differentiation

We have previously shown that phenotypic allelic exclusion of TCR α-chain is functional only in mature thymocytes. A significant proportion of immature thymocytes (TCRlow) express more than one cell surface α-chain, but mature thymocytes (TCRhigh) show phenotypic allelic exclusion and express only a single α-chain. We have analyzed thymocytes for both surface and intracellular α-chain expression and find that the majority of mature thymocytes express a second α-chain intracellularly. This result is predicted by a model in which the developmentally regulated allelic exclusion of the TCR α-chain is caused by competition between α-chains for the β-chain rather than by models in which one α-chain is down-regulated or in which selection favors cells with only a single α-chain species. Changes in the relative amounts of α- and β-chains available for pairing may therefore allow competition between the two α-chains for the β-chain. Peripheral T cells also frequently express second α-chains in the cytoplasm (18–27%), despite a rather low frequency of dual α-chain expression on the cell surface (2–4%). The frequency of nonsurface expressed α-chains is reduced somewhat compared with thymocytes, indicating that an additional level of control of allelic exclusion operates during the maturation of peripheral T cells.

Polymorphism Within a TCRAV Family Influences the Repertoire Through Class I/II Restriction

Antibody-staining experiments have shown that closely related members of the TCRAV3 family are reciprocally selected into the CD4 or CD8 peripheral T cell subsets. This has been attributed to the individual AV3 members interacting preferentially with either MHC class I or MHC class II molecules. Single amino acid residues present in the complementarity-determining regions (CDR) CDR1α and CDR2α are important in determining MHC class specificity. We have now extended these observations to survey the expressed repertoire of the AV3 family in C57BL/6 mice. Three of the four expressed AV3 members are preferentially selected into the CD4+ subset of T cells. These share the same amino acid residue in both CDR1α and CDR2α that differ from the only CD8-skewed member. Preferential expression of an individual AV3 is not caused by other endogenous α- or β-chains, by any conserved CDR3 sequence, or by the usage of TCRAJ regions. This study shows that residues in the CDR1 and CDR2 regions are primary determinants for MHC class discrimination and suggests that polymorphism found within a TCRAV family has an important effect on the overall shaping of the T cell repertoire.

The Level of CD4 Surface Protein Influences T Cell Selection in the Thymus

During T cell development thymocytes are subjected to positive and negative selection criteria to ensure that the mature T cell repertoire is MHC restricted, yet self tolerant at the same time. The CD4 and CD8 coreceptors are thought to play a crucial role in this developmental process. To elucidate the role of CD4 in T cell selection, we have produced a mouse strain that expresses CD4 at a reduced level. We used homologous recombination in embryonic stem cells to insert neo into the 3′ untranslated region of CD4. The resulting mice have a reduction in the percentage of CD4+ cells in the thymus and a concomitant increase in CD8+ cells. In addition, breeding two individual class II-restricted TCR transgenic mice onto the CD4low (low level of CD4) mutant background affects the selection of each TCR differentially. In one case (AND TCR transgenic), significantly fewer CD4+ cells with the transgenic TCR develop on the CD4low mutant background, whereas in the other (5C.C7 TCR transgenic), selection to the CD4 lineage is only slightly reduced. These data support the differential avidity model of positive and negative selection. With little or no avidity, the cell succumbs to programmed cell death, low to moderate avidity leads to positive selection, and an avidity above a certain threshold, presumably above one that would lead to autoreactivity in the periphery, results in clonal deletion. These data also support the idea that a minimum avidity threshold for selection exists and that CD4 plays a crucial role in determining this avidity.

Conformational integrity and ligand binding properties of a single chain T-cell receptor expressed in Escherichia coli

We recently showed that a soluble, heterodimeric murine D10 T-cell receptor (TCR) (Valpha2Calpha, Vbeta8.2Cbeta) expressed in insect cells binds both Vbeta8.2-specific bacterial superantigen staphylococcal enterotoxin C2 (SEC2) and a soluble, heterodimeric major histocompatibility complex class II I-Ak.conalbumin peptide complex with a low micromolar affinity. To define further the structural requirements for the TCR/ligand interactions, we have produced in Escherichia coli a soluble, functional D10 single chain (sc) TCR molecule in which the Valpha and Vbeta domains are connected by a flexible peptide linker. Purified and refolded D10 scTCR bound to SEC2 and murine major histocompatibility complex class II I-Ak.conalbumin peptide complex with thermodynamic and kinetic binding constants similar to those measured for the baculovirus-derived heterodimeric D10 TCR suggesting that neither the TCR constant domains nor potential N- or O-linked carbohydrate moieties are necessary for ligand recognition and for expression and proper folding of the D10 scTCR. Purified D10 scTCR remained soluble at concentrations up to 1 mM. Circular dichroism and NMR spectroscopy indicated that D10 scTCR is stabilized predominantly by beta-sheet secondary structure, consistent with its native-like conformation. Because of its limited size, high solubility, and structural integrity, purified D10 scTCR appears to be suitable for structural studies by multidimensional NMR spectroscopy.

Function of the TCR alpha enhancer in alphabeta and gammadelta T cells

We have used gene targeted mutational approaches to assess the role of the T cell receptor alpha (TCR alpha) enhancer (E alpha) in the control of TCR alpha and TCR delta gene rearrangement and expression. We show that E alpha functions in cis to promote V alpha to J alpha rearrangement across the entire J alpha locus, a distance of greater than 70 kb. We also show that E alpha is required for normal alphabeta T cell development; in this lineage, E alpha is required for germline J alpha expression, for normal expression levels of rearranged V alpha J alpha genes, and for expression of a diverse V alpha repertoire. In gamma delta T cells, E alpha is not required for VdeltaDJdelta rearrangement, but, surprisingly, is required for normal expression levels of mature VdeltaDJdelta transcripts and for expression of germline J alpha transcripts. Our findings imply that E alpha function is not limited to the TCR alpha components of the TCRalpha/delta locus or to the alpha beta lineage; rather, E alpha function is important in both alphabeta and gammadelta lineage T cells.

Two separable T cell receptor signals reconstitute positive selection of CD4 lineage T cells in vivo

Positive selection is an obligatory step during intrathymic T cell differentiation. It is associated with rescue of short-lived, self major histocompatibility complex (MHC)-restricted thymocytes from programmed cell death, CD4/CD8 T cell lineage commitment, and induction of lineage-specific differentiation programs. T cell receptor (TCR) signaling during positive selection can be closely mimicked by targeting TCR on immature thymocytes to cortical epithelial cells in situ via hybrid antibodies. We show that selection of CD4 T cell lineage cells in mice deficient for MHC class I and MHC class II expression can be reconstituted in vivo by two separable T cell receptor signaling steps, whereas a single TCR signal leads only to induction of short-lived CD4+CD8lo intermediates. These intermediates remain susceptible to a second TCR signal for 12-48 h providing an estimate for the duration of positive selection in situ. While both TCR signals induce differentiation steps, only the second one confers long-term survival on immature thymocytes. In further support of the two-step model of positive selection we provide evidence that CD4 T cell lineage cells rescued by a single hybrid antibody pulse in MHC class II-deficient mice are pre-selected by MHC class I.

CD5- CD8 alpha beta intestinal intraepithelial lymphocytes (IEL) are induced to express CD5 upon antigen-specific activation: CD5- and CD5+ CD8 alpha beta IEL do not represent separate T cell lineages

We followed alpha beta T cell receptor (TCR) usage in subsets of gut intraepithelial lymphocytes (IEL) in major histocompatibility complex class I-restricted alpha beta TCR-transgenic (tg) mice. The proportion of tg alpha beta TCR+ CD8 alpha beta IEL is reduced compared with CD8+ splenocytes of the same animal, particularly under conventional conditions of maintenance. Further fractionation of CD8 alpha beta IEL according to the expression level of surface CD5 revealed that in conventionally housed animals tg TCR+ CD5- CD8 alpha beta IEL are as frequent as in specific pathogen-free (SPF) mice, whereas tg TCR+ CD5int or, even more pronounced, tg TCR+ CD5hi CD8 alpha beta IEL are greatly diminished when compared with mice kept under SPF conditions. Upon antigen-specific stimulation of CD5- CD8 alpha beta IEL in vitro, CD5 surface expression is up-regulated on a large fraction of cells within 48 h. Up-regulation of CD5 surface expression is further enhanced by the presence of the anti-alpha IEL monoclonal antibody 2E7. This clearly demonstrates that CD5-, and CD5+ CD8 alpha beta IEL cannot be considered as separate T cell lineages.

Affinity and kinetics of the interactions between an alphabeta T-cell receptor and its superantigen and class II-MHC/peptide ligands

Immune activation is mediated by a specific interaction between the T-cell receptor (TCR) and an antigenic peptide bound to the major histocompatibility complex (MHC). T-cell activation can also be stimulated by superantigens which bind to germline-encoded variable domain sequences of certain TCR beta-chains. We have used a surface plasmon resonance biosensor to characterize the molecular interactions between a class II-restricted alphabeta TCR and its superantigen and MHC/peptide ligands. The extracellular domains of the murine D10 TCR (Valpha2, Vbeta8.2) were expressed in insect cells and secreted as a disulfide-linked heterodimer. In the absence of MHC class II, purified soluble D10 TCR bound to Staphylococcus aureus enterotoxin C2 with an association rate of 1.69+/-0.12 x 10(4)M(-1) sec(-1) and a dissociation rate of 1.9+/-0.47 x 10(-2) sec(-1), giving a dissociation constant of 1.1 microM. Binding of the TCR to S. aureus enterotoxin B was barely detectable and could not be measured accurately due to the rapid dissociation rate. Soluble D10 TCR also bound to a soluble murine MHC class II I-A(k) molecule containing a fused antigenic conalbumin peptide and complementary leucine zipper sequences to facilitate efficient chain pairing. The purified I A(k) chimera specifically stimulated proliferation of the D10 T-cell clone, and bound to immobilized soluble D10 TCR with an association rate of 1.07+/-0.19 x 10(4)M(-1)sec(-1) and a dissociation rate of 2.2+/-0.65 x 10(-2) sec(-1), giving a dissociation constant of 2.1 microM.

A T cell receptor V alpha domain expressed in bacteria: does it dimerize in solution?

To evaluate the potential for dimerization through a particular T cell receptor (TCR) domain, we have cloned the cDNA encoding a TCR V alpha from a hybridoma with specificity for the human immunodeficiency virus (HIV) envelope glycoprotein 120-derived peptide P18-110 (RGPGRAFVTI) bound to the murine major histocompatibility complex (MHC) class I molecule, H-2Dd. This cDNA was then expressed in a bacterial vector, and protein, as inclusion bodies, was solubilized, refolded, and purified to homogeneity. Yield of the refolded material was from 10 to 50 mg per liter of bacterial culture, the protein was soluble at concentrations as high as 25 mg/ml, and it retained a high level of reactivity with an anti-V alpha 2 monoclonal antibody. This domain was monomeric both by size exclusion gel chromatography and by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Circular dichroism spectra indicated that the folded V alpha domain had secondary structure similar to that of single immunoglobulin or TCR domains, consisting largely of beta sheet. Conditions for crystallization were established, and at least two crystal geometries were observed: hexagonal bipyramids that failed to diffract beyond approximately 6 A, and orthorhombic crystals that diffracted to 2.5 A. The dimerization of the V alpha domain was investigated further by solution nuclear magnetic resonance spectroscopy, which indicated that dimeric and monomeric forms of the protein were about equally populated at a concentration of 1 mM. Thus, models of TCR-mediated T cell activation that invoke TCR dimerization must consider that some V alpha domains have little tendency to form homodimers or multimers.

MHC class II molecules are not required for survival of newly generated CD4+ T cells, but affect their long-term life span

We grafted fetal thymi from wild-type mice into immunodeficient RAG-2-/- or class II-/-RAG-2-/- (class II MHC-) recipients and followed the fate of naive CD4+ T cells derived from the grafts. In both types of recipients, newly generated CD4+ T cells proliferated to the same extent in the periphery and rapidly filled the empty T cell compartment. However, CD4+ T cells in class II- recipients gradually decreased in number over 6 months. These results show that interactions between the TCR and class II molecules are not required for newly generated CD4+ T cells to survive and proliferate, but are necessary to maintain the size of the peripheral T cell pool for extended periods.

Constitutive class I-restricted exogenous presentation of self antigens in vivo

Ovalbumin (OVA)-specific CD8+ T cells from the T cell receptor-transgenic line OT-I (OT-I cells) were injected into unirradiated transgenic RIP-mOVA mice, which express a membrane-bound form of OVA (mOVA) in the pancreatic islet beta cells and the renal proximal tubular cells. OT-I cells accumulated in the draining lymph nodes (LN) of the kidneys and pancreas and in no other LN. They displayed an activated phenotype and a proportion entered cell cycle. Unilateral nephrectomy 7-13 d before inoculation of OT-I cells into RIP-mOVA mice allowed the injected T cells to home only to the regional LN of the remaining kidney (and pancreas), but when the operation was performed 4 h before injecting the T cells, homing to the LN of the excised kidney was evident. When the bone marrow of RIP-mOVA mice was replaced with one of a major histocompatibility haplotype incapable of presenting OVA to OT-I cells, no homing or activation was detectable. Therefore, OT-I cells were activated by OVA presented by short-lived antigen-presenting cells of bone marrow origin present in the draining LN of OVA-expressing tissue. These results provide the first evidence that tissue-associated "self" antigens can be presented in the context of class I via an exogenous processing pathway. This offers a constitutive mechanism whereby T cells can be primed to antigens that are present in nonlymphoid tissues, which are not normally surveyed by recirculating naive T cells.

Expression of a second receptor rescues self-specific T cells from thymic deletion and allows activation of autoreactive effector function

Allelic exclusion at the T-cell receptor alpha chain locus is incomplete resulting in the generation of T cells that express two T-cell receptors. The potential involvement of such T cells in autoimmunity has been suggested [Padovan, E., Casorati, G., Dellabona, P., Meyer, S., Brockhaus, M. & Lanzavecchia, A. (1993) Science 262, 422-424; Heath, W. R. & Miller, J. F. A. P. (1993) J. Exp. Med. 178, 1807-1811]. Here we show that expression of a second T-cell receptor can rescue T cells with autospecific receptors from thymic deletion and allow their exit into the periphery. Dual receptor T cells, created by constitutive expression of two transgenic T-cell receptors on a Rag1-/- background, are tolerant to self by maintaining low levels of autospecific receptor, but selfreactive effector function (killing) can be induced through activation via the second receptor. This opens the possibility that T cells carrying two receptors in the periphery of normal individuals contain putatively autoreactive cells that could engage in autoimmune effector functions after recognition of an unrelated environmental antigen.

Control of MHC restriction by TCR Valpha CDR1 and CDR2

Individual T cell receptor (TCR) Valpha elements are expressed preferentially in CD4 or CD8 peripheral T cell subsets. The closely related Valpha3.1 and Valpha3.2 elements show reciprocal selection into CD4 and CD8 subsets, respectively. Transgenic mice expressing site-directed mutants of a Valpha3.1 gene were used to show that individual residues in either the complementarity-determining region 1 (CDR1) or CDR2 were sufficient to change selection from the CD4 subset to the CD8 subset. Thus, the germline-encoded Valpha elements are a major influence on major histocompatibility class complex (MHC) restriction, most likely by a preferential interaction with one or the other class of MHC molecule.

Role of the T cell receptor alpha-chain in superantigen recognition

Superantigens bind to antigen-presenting cells on the outside of the major histocompatibility complex (MHC) class II molecule and to T cells via the external face of the T cell receptor (TCR) V beta element. As a consequence, superantigens stimulate populations of T cells in a V beta-specific, non-MHC-restricted manner. However, accumulating evidence has shown an additional contribution of the TCR alpha-chain and polymorphic residues of the MHC molecule to superantigen recognition by some T cells. These data suggest that the TCR and MHC come into contact during superantigen engagement and indirectly modulate the superantigen reactivity. Thus, additional interactions between non-V beta elements of the TCR and MHC play a role in the overall stability of the superantigen/MHC/TCR complex, explaining the influence of the TCR alpha-chain. It is likely that this additional interaction is of greater consequence for weakly reactive T cells. This modulation of superantigen reactivity in individual T cells may have physiological consequences, for example, in the induction of autoimmunity.

Derivation of T-cell receptor alpha-chain double expresser lines from normal murine mature T cells

Because the T-cell receptor (TCR) alpha-chain locus is known to lack allelic exclusion of rearrangements, and as a recent report revealed the existence of alpha-chain double expressers among normal human peripheral blood lymphocytes (PBL), the possible existence of TCR alpha-chain double expressers among mature murine T cells was examined. Although two-colour staining analysis of normal T-cell populations did not immediately reveal recognizable clusters of V alpha double expressers, alternative in vitro stimulations of normal murine T cells with antibodies to two different TCR V alpha chains reproducibly induced TCR alpha-chain double-expresser lines. TCR complexes with different alpha-chains on such T cells were both shown to be functional. The cell lines were heterogeneous with respect to V beta usage and the ratio of the expressed amounts of the two alpha-chains on the surface. The ratio of the two expressed alpha-chains was found to be very stable over a long period of time. These results are consistent with the earlier report on alpha-chain double expressers among human T cells and also show normal occurrence of TCR alpha-chain double expressers in murine T-cell populations.

The roles of perforin- and Fas-dependent cytotoxicity in protection against cytopathic and noncytopathic viruses

In vitro, T cell-dependent cytotoxicity is mediated by two distinct mechanisms, one being perforin-, the other Fas-dependent. The contribution of both of these mechanisms to clearance of viral infections was investigated in mice for the non-cytopathic lymphocytic choriomeningitis virus (LCMV) and the cytopathic vaccinia, vesicular stomatitis (VSV) and Semliki forest (SFV) viruses. Clearance of an acute LCMV infection was mediated by the perforin-dependent mechanism without measurable involvement of the Fas-dependent pathway. For the resolution of vaccinia virus infection and for resistance against VSV and SFV, however, neither of the two pathways was required. These data suggest that perforin-dependent cytotoxicity mediated by T cells is crucial for protection against non-cytopathic viruses, whereas infections with cytopathic viruses are controlled by nonlytic T cell-dependent soluble mediators such as cytokines (IFN-gamma against vaccinia virus) and neutralizing antibodies (against VSV and SFV).

Preferential positive selection of T lymphocytes which express two different TCR alpha chains, an endogenous and a transgenic

A hallmark of positive selection in T-cell receptor (TCR)-transgenic mice is a strong skewing towards the CD4+ or the CD8+ subset, depending on the class II or I restriction of the TCR, respectively. However, previous experiments in TCR transgenic mice specific for an Ig light chain (lambda 2(315)/I-Ed class II molecule did not fit into this scheme because the authors observed an anomalous skewing towards CD8. In this paper the authors show that endogenous TCR alpha chains are expressed on > 90% of CD4+ and CD8+ cells in this particular transgenic strain, even on a selecting H-2d haplotype. Endogenous TCR alpha chains are first detected when double-positive thymocytes down-regulate either CD4 or CD8. Endogenous V alpha seems to influence generation of T-cell subsets because CD4+ and CD8+ cells express different frequencies of endogenous V alpha 2 and V alpha 8. In the absence of endogenous TCR alpha chains in recombination-deficient TCR-transgenic severe combined immunodeficiency (SCID) mice, a strong skewing towards CD4+ T cells is seen, but such mice are severely T-cell deficient. As an explanation for these results, the authors suggest that the transgenic TCR has a too low affinity for efficient positive selection, therefore, TCR alpha gene rearrangements proceed. Endogenous TCR alpha paired with transgenic TCR beta could bind to class I or class II molecules, enhance positive selection and thereby production of CD4+ or CD8+ cells. Most of the 'mismatched' CD8+ cells are lambda 2(315)-specific and I-Ed class II restricted, and may function as idiotype-specific suppressors of B cells. These results may help explain the origin of dual TCR alpha T cells. Furthermore, the authors suggest that T cells 'mismatched' for co-receptor/TCR MHC-specificity may be enriched among dual TCR alpha T cells.

Allelic exclusion of mouse T cell receptor alpha chains occurs at the time of thymocyte TCR up-regulation

We report a detailed analysis of TCR V alpha and V beta chain expression on immature versus mature thymocytes of normal, TCR beta-transgenic, and TCR alpha-hemizygous mice. Chain pairing between TCR V alpha and V beta chains is random on immature thymocytes, but individual chain pairs are selected in mature thymocytes. This indicates that V alpha-V beta chain pairing preferences are determined during thymic selection, and not as a result of structural constraints. Dual V alpha chain expression is found frequently on immature, but not mature thymocytes. It is not found in TCR alpha-hemizygous mice, showing that cell surface expression of dual alpha chains is caused by lack of allelic exclusion in immature thymocytes. Down-regulation of one of the alpha chains occurs concurrently with differentiation from TCRlo, CD69- to TCRhi, CD69+ phenotype, suggesting that it is associated with positive selection of the functional TCR.

T cell development and repertoire of mice expressing a single T cell receptor alpha chain

We examined T cell development and T cell repertoire in transgenic mice expressing a single T cell receptor (TCR) alpha chain derived from the H-2Db-lymphocytic choriomeningitis virus (LCMV)-specific cytolytic T lymphocyte (CTL) clone P14. To generate these alpha P14 mice, mice transgenic for the P14 TCR alpha chain were backcrossed to TCR alpha-deficient mice. Thymi from alpha P14 mice exhibited a marked decrease of mature CD4+8- and CD8+4- single-positive thymocytes comparable to thymi from TCR alpha-deficient mice. Correspondingly, the number of peripheral T cells was reduced in the CD4 (tenfold) and in the CD8 (twofold) subsets when compared to normal mice. T cells from alpha P14 mice generated a primary anti-LCMV CTL response when stimulated in vitro with LCMV in contrast to normal mice which require priming in vivo; elimination of LCMV in vivo was, however, not improved. Flow cytometric analysis of T cells with V beta-specific antibodies showed a diverse endogenous TCR V beta repertoire. Functional analysis of the T cell repertoire, however, revealed a strongly reduced (30-fold) allogeneic and the absence of a vesicular stomatitis virus-specific CTL response and an impaired ability to provide T cell help for antibody isotype switching. Thus, T cell selection in the thymus was impaired and the T cell repertoire was limited in mice expressing only one type of TCR alpha chain.

Peripheral deletion of autoreactive CD8+ T cells in transgenic mice expressing H-2Kb in the liver

The response of T cells specific for liver antigens was examined in transgenic mice expressing the allogeneic major histocompatibility complex class I molecule H-2Kb (Kb) under the control of the sheep metallothionein promoter (Met-Kb mice). To follow the fate of Kb-specific T cells, and to prevent any aberrant thymic expression of the Kb transgene, the mice were thymectomized, lethally irradiated, protected with bone marrow cells from transgenic mice expressing in their T cells a Kb-specific T cell receptor identifiable by a clonotypic antibody, and given syngeneic non-transgenic thymus grafts. Although Kb-specific CD8+ T cells were produced in the thymus grafts of these manipulated Met-Kb mice, only small numbers of such cells could be detected in the spleen and lymph nodes. The livers, however, showed signs of damage and were heavily infiltrated by actively dividing CD8+ T cells. We provide strong evidence that the hepatocytes, not generally regarded as antigen-presenting cells, activated the Kb-specific CD8+ T cells and that these disappeared after a vigorous autoimmune response that resulted in deletion.

Expression of two T cell receptor alpha chains on the surface of normal murine T cells

We have previously reported that a subset of T cells in T cell receptor (TCR)-transgenic mice may express two different alpha chains on their surface. The expression of two functional alpha chains has also been demonstrated for human peripheral blood T cells. In this report, we show that a proportion of normal murine lymph node T cells express two functional alpha chains on their surface. The extrapolated frequency of these cells present in the normal repertoire ranges from 7-21%, with an average of 15%. Our analysis of a small number of antigen-specific T cell clones suggests that the frequency of antigen-responsive cells expressing two surface alpha chains is relatively low. This raises the possibility that dual alpha chain T cells may have a selective disadvantage in responding to specific antigen.

Immunological function of a defined T-cell population tolerized to low-affinity self antigens

In the thymus there are two major mechanisms of T-lymphocyte tolerance: clonal deletion and clonal inactivation. One important problem underlying the mechanism of clonal inactivation is why unresponsive cells are maintained in the mature peripheral T-cell repertoire. Here we report that transgenic alpha beta T-cells may be tolerized to a self antigen Mls-1a, but still retain proliferative responses for alternative peptide antigens and superantigens. These self-tolerant T cells can also provide immunopathological and memory cytotoxic function in vivo. We propose that high-affinity/avidity self-reactive T cells are deleted in the thymus, whereas lower-affinity/avidity interactions lead to unresponsiveness and define the 'resting threshold' for a given T cell. These low-affinity self-tolerant T cells remain functionally competent for high-affinity foreign antigens, and efficiently eliminate natural pathogens in vivo.

Modulation of T cell development by an endogenous altered peptide ligand

T cells potentially encounter numerous endogenous peptides during selection in the thymus and in the periphery. We examined the impact of an endogenous peptide on in vivo T cell development, using a TCR transgenic mouse model based on a hemoglobin-specific T cell clone. In these mice, the transgenic beta chains paired with endogenous alpha chains. This led to a serendipitous primary reactivity to Ser69 peptide, an altered peptide ligand of the Hbd (64-76) epitope of the parent clone. Two Ser69-reactive T cell populations were identified. A smaller population of the Ser69-reactive T cells responded both to Ser69 and Hbd (64-76). A majority reacted only to Ser69, and not to Hbd(64-76); in fact, Hbd(64-76) was a specific TCR antagonist for these Ser69-only-reactive T cells. Thus, in this unique experimental system, Ser69 became an agonist, and Hbd (64-76) was an antagonist. Endogenous presentation of the antagonist ligand in the thymus selectively eliminated the high-avidity cells, while sparing low-avidity cells in the Ser69-reactive T cell repertoire. These results highlight how specificity guides developing T cells through a network of ligands and indicate that the endogenous peptide pool has a profound effect on T cell development and repertoire.

Mechanisms of tolerance induction in major histocompatibility complex class II-restricted T cells specific for a blood-borne self-antigen

Transgenic mice expressing a major histocompatibility complex class II-restricted T cell receptor with specificity for a natural self-antigen, the fifth component of complement, were generated to analyze the mechanism of tolerance induction to a blood-borne self-protein. In the absence of C5 protein thymocytes from T cell receptor transgenic mice develop into mature CD4 single positive cells which emigrate into the periphery and mount C5-specific T cell responses upon immunization with C5. In the presence of circulating C5 protein, CD4 single positive thymocytes do not develop. Negative selection occurs late in thymic ontogeny leaving the bulk of CD4+8+ thymocytes unaffected. This phenotype may be due to a delay in contact with self-antigen presentation which, under physiological conditions, is inefficient in the cortex of C5+ mice, and therefore does not affect most immature double positive thymocytes. In contrast, in vitro exposure to C5(-)-presenting dendritic cells or in vivo injection of C5 peptide results in deletion of double positive thymocytes. C5+ transgenic mice are tolerant in vivo, but contain T cells in spleen and lymph nodes that secrete interleukin 2 and interferon gamma in response to C5 activation in vitro. When crossed onto a Rag1-/- background to prevent endogenous T cell receptor rearrangements, these peripheral potentially autoreactive cells do not appear. This indicates that endogenous T cell receptor rearrangements possibly leading to the expression of two receptors might be a prerequisite for their survival and export into the periphery.

Positive and negative thymocyte selection induced by different concentrations of a single peptide

T lymphocyte maturation is dependent on interactions between the T cell receptor (TCR) expressed on the developing thymocyte and intrathymic major histocompatibility complex (MHC)-peptide ligands. The relation between the peptide-MHC complex that results in negative or positive selection has not been identified. Here, the requirements for the maturation of thymocytes expressing a defined transgenic TCR specific for a viral peptide are studied in fetal thymic organ culture. Low concentrations of the viral peptide antigen recognized by this transgenic TCR can mediate positive selection, whereas high concentrations result in thymocyte tolerance. These findings support the affinity-avidity model of thymocyte selection.

Regulation of RAG-1 and CD69 expression in the thymus during positive and negative selection

Successful interaction of the T cell receptor (TCR) with major histocompatibility complex (MHC) molecules during thymic selection down-regulates the expression of the recombination activating genes (RAG)-1 and -2 in cortical thymocytes and thereby prevents further endogenous TCR alpha-chain gene rearrangements (Borgulya, P., Kishi, H., Uematsu, Y. and von Boehmer, H., Cell. 1992. 69: 529-537; Brändle, D., Müller, C., Rülicke, T., Hengartner, H. and Pircher, H., Proc. Natl. Acad. Sci. USA 1992. 89: 9529-9533). To address the question whether down-regulation of RAG-1 activity represents an irreversible process we have blocked TCR-MHC interactions of thymocytes with thymic stromal cells. Firstly, transgenic (Tg) mice expressing a virus-specific MHC class I (H-2Db)-restricted TCR were injected with anti-Db or anti-CD8 monoclonal antibodies and RAG-1 expression was examined by in situ hybridization on thymus sections. The results show that cortical thymocytes up-regulated RAG-1 expression within 24 h after antibody administration. Secondly, immature thymocytes from TCR Tg mice were released from the thymic microenvironment and cultured in vitro for 14 h in single-cell suspension. The amount of RAG-1 mRNA was increased sixfold in cultured cells when compared to freshly isolated thymocytes. In addition, we show that immature thymocytes from TCR transgenic mice bearing non-selective MHC molecules (H-2d) down-regulated RAG-1 expression after antigen-induced TCR engagement. Cytofluorometric analysis further revealed that surface expression of CD69 on immature thymocytes inversely correlated with RAG-1 expression during positive and negative selection processes.