Primate testing of TGN1412: right target, wrong cell

The failure of toxicity studies in non-human primates to predict the cytokine release syndrome during a first-in-man study of the CD28-specific monoclonal antibody TGN1412 has remained unexplained so far. In this issue of the BJP, work from the NIBSC first identifies the effector memory subset of human T-lymphocytes as the most likely source of the pro-inflammatory cytokines released during the study, and goes on to show that in cynomolgus monkeys, this subset lacks CD28, the target molecule of TGN1412. We discuss the implications for the TGN1412 catastrophe and for preclinical evaluation of biologicals in animal models in general.

Superagonistic CD28 antibody induces donor-specific tolerance in rat renal allografts

The ultimate goal of organ transplantation is to establish graft tolerance where CD4+CD25+FOXP3+ regulatory T (Treg) cells play an important role. We examined whether a superagonistic monoclonal antibody specific for CD28 (CD28 SA), which expands Treg cells in vivo, would prevent acute rejection and induce tolerance using our established rat acute renal allograft model (Wistar to Lewis). In the untreated or mouse IgG-treated recipients, graft function significantly deteriorated with marked destruction of renal tissue, and all rats died by 13 days with severe azotemia. In contrast, 90% of recipients treated with CD28 SA survived over 100 days, and 70% survived with well-preserved graft function until graft recovery at 180 days. Analysis by flow cytometry and immunohistochemistry demonstrated that CD28 SA induced marked infiltration of FOXP3+ Treg cells into the allografts. Furthermore, these long-surviving recipients showed donor-specific tolerance, accepting secondary (donor-matched) Wistar cardiac allografts, but acutely rejecting third-party BN allografts. We further demonstrated that adoptive transfer of CD4+CD25+ Treg cells, purified from CD28 SA-treated Lewis rats, significantly prolonged allograft survival and succeeded in inducing donor-specific tolerance. In conclusion, CD28 SA treatment successfully induces donor-specific tolerance with the involvement of Treg cells, and thus the therapeutic value of this approach warrants further investigation and preclinical studies.

Combination of donor-specific blood transfusion with anti-CD28 antibody synergizes to prolong graft survival in rat liver transplantation

Donor-specific blood transfusion (DST) has been shown to effectively induce tolerance to certain allografts. In addition, it is well known that blockade of costimulatory signals reduces the ability of T cells to respond to alloantigens, prolonging allograft survival in some transplant models. We assessed the effects of single or multiple DSTs in the absence or presence of anti-CD28 monoclonal antibodies (mAbs) on graft function and host survival in rat liver transplantation (LTx). Fully MHC-mismatched adult male Dark Agouti (DA) and Lewis (LEW) rats were used as donors and recipients, respectively. Heparinized DA blood was administered to naïve LEW rats 7 days before LTx [DST(-7d)], 14 and 7 days before LTx [DST(1 x 2)], twice a week for 2 weeks prior to LTx [DST(2 x 2)] and once a week for 4 weeks prior to LTx [DST(1 x 4)]. For some experiments, two different monoclonal antibodies (mAb) to rat CD28 (JJ316 and JJ319) were administered in combination with some DST treatments. We found that DST administration induced a time- and dose-dependent increase in host survival. Treatment of LEW rats with JJ316 or JJ319 mAb alone failed to prolong graft survival over untreated rats; however, the combination of DST(1 x 2) with JJ316 or JJ319 mAb induced indefinite survival at 100 days following surgery. We found that this protective effect was associated with increased numbers of splenic CD4+ CD45RC- but not CD4+ CD25+ foxp3+ T-cells in long-term survivors. Our data suggest that the combination of suboptimal DST with CD28 mAb induces donor-specific tolerance that correlates with enhanced numbers of regulatory T-cells.

Superagonistic anti-CD28 antibodies: potent activators of regulatory T cells for the therapy of autoimmune diseases

This paper reviews the existing evidence regarding the use of superagonistic anti-CD28 antibodies (CD28 superagonists) for therapeutic manipulation of regulatory T cells (T(reg) cells). The molecular properties of superagonistic anti-CD28 antibodies allow the generation of a strong activating signal in mature T cells, including T(reg) cells, without additional stimulation of the T cell receptor complex. CD28 superagonist administration in vivo leads to the preferential expansion and strong activation of naturally occurring CD4+CD25+CTLA-4+FoxP3+ T(reg) cells over conventional T cells. In animal models, both prophylactic and therapeutic administration of a CD28 superagonist prevented or at least greatly mitigated clinical symptoms and induced remission. Adoptive transfer experiments have further shown that CD28 superagonists mediate protection by expansion and activation of CD4+CD25+ T(reg) cells. Therefore, superagonistic anti-CD28 antibodies offer a promising novel treatment option for human autoimmune diseases and the first clinical trials are eagerly awaited.

Molecular and cellular mechanisms of T cell development

The thymus is central to the establishment of a functioning immune system. Here is the place where T cells mature from hematopoietic progenitors, driven by mutual interactions of stromal cells and the developing thymocytes. As a result, different types of T cells are generated, all of which have been carefully selected for the ability to act in host defense towards non-self and against the potential to mount pathogenic self-reactive autoimmune responses. In this review we summarize our present knowledge on the lineage decisions taking place during this development, the selection processes responsible for shaping the T cell antigen-receptor repertoire, the interactions with the stromal components and the signal transduction pathways which transform the interactions with the thymic microenvironment into cellular responses of survival, proliferation, differentiation and, importantly, also of cell death.

HIV type 1 abrogates TAP-mediated transport of antigenic peptides presented by MHC class I. Transporter associated with antigen presentation

Downregulation of MHC class I expression following human immunodeficiency virus 1 (HIV-1) infection is thought to play an important role in viral escape from immune recognition by cytotoxic T-lymphocytes (CTLs). Since exogenous addition of HIV-1-derived peptides restores susceptibility of HIV-1-infected cells to CTL-mediated lysis, we tested whether endogenous peptide loading is impaired in these cells. Our results show that in HIV-1-infected cells the ability of the transporter associated with antigen presentation (TAP) to translocate antigenic peptides from the cytosol to the lumen of the ER for presentation on MHC class I molecules is abolished. These data suggest that interference with the supply of antigenic peptides to the MHC class I pathway provides an additional mechanism by which HIV-1 evades the CTL-mediated immune response.

Thymic development and repertoire selection: the rat perspective

This review summarizes our current knowledge of T-cell maturation and repertoire selection in the rat thymus. Some unique features of early thymocyte development and of CD4/CD8 lineage decision are described. A detailed analysis of lineage progression through the CD4, CD8 "double positive" compartment and T-cell receptor-induced CD8 T-cell maturation in cell culture is provided. A second emphasis is placed on interactions between germline-encoded T-cell receptor elements with MHC molecules in thymic repertoire selection and alloreactivity

Control of T cell hyperactivation in IL-2-deficient mice by CD4(+)CD25(-) and CD4(+)CD25(+) T cells: evidence for two distinct regulatory mechanisms

In IL-2-deficient mice, antigen-activated CD4 T cells accumulate and cause lethal immune pathology. Wild-type cells of hematopoietic origin present in the same animal are able to prevent this hyperactivation of T cells, but the mechanisms and cells controlling the IL-2-deficient cells are unknown. Here we show that IL-2(-) CD4 cells with an ovalbumin-specific transgenic TCR (IL-2(-) OVAtg) undergo both clonal expansion and clonal contraction when transferred to euthymic recipients and challenged with antigen, but continuously expand in athymic hosts. Cotransfer of wild-type CD4 T cells prevents the accumulation of IL-2-deficient cells. On the residual IL-2(-) TCRtg cells CD69 and CD25 are up-regulated, suggesting that activation per se is not suppressed and that the cells had received an IL-2 signal. Since IL-2 is able to restore the defective antigen-induced cell death (AICD) of IL-2-deficient T cells in vitro, paracrine IL-2 provided by the wild-type CD4 cells may thus be able to allow clonal contraction of IL-2-deficient cells also in vivo. Interestingly however, regulatory CD4(+)CD25(+) cells also efficiently contain the clone size of antigen-stimulated IL-2-deficient T cells. Since CD4(+)CD25(+) cells do not produce IL-2, this suggests a mechanism of suppression distinct from paracrine IL-2 delivery. In keeping with this, the residual IL-2(-) TCRtg cells recovered after cotransfer of regulatory CD4(+)CD25(+) cells do not show increased CD25 or CD69 expression, suggesting that they had not received paracrine IL-2 and that clonal containment occurred at the level of initial activation rather than clonal contraction by AICD. IL-2 deficiency therefore may upset T cell homeostasis by two distinct mechanisms: the failure to program expanding T cells for apoptosis, and the failure to generate functional CD4(+)CD25(+) regulatory cells.

Genetic control of peripheral TCRAV usage by representation in the preselection repertoire and MHC allele-specific overselection

TCRAV segments contribute significantly to MHC restriction as illustrated by their general preference for either the CD4 or CD8 T cell subset and additional, MHC allele-specific overselection during T cell differentiation. The 10-fold over-representation of the TCRAV8S2 (VA8S2) segment in CD8 over CD4 T cells by the RT1(f) haplotype of LEW.1F rats provides the most striking example of MHC allele-specific overselection of a VA segment reported so far. Also in alloreactivity, VA8S2(+) CD8 cells from RT1(f-) rats are preferentially expanded by RT1(f+) stimulators. We have identified the class I molecule, A(f), mediating VA8S2 overselection and report that it differs only in four amino acids at the MHC-TCR interface from the class I molecule A(a), which is neutral with regard to selection of VA8S2. We also provide an extensive survey of the TCRAV8 family and show that among 14 functional VA8 segments in LEW rats, the dramatic A(f)-dependent overselection is unique for VA8S2. Surprisingly, VA8S2 expression in CD8 T cells of RT1(f+) rats derived from a Sprague-Dawley stock was only 3% as compared to the 12% observed in LEW.1F. The VA8S2 segment of Sprague-Dawley (VA8S2(SD)) differs from VA8S2 of the LEW background (VA8S2(l)) in only two amino acids, one of which is located in CDR2 and could thus participate in allele-specific recognition of A(f). However, analysis of the pre- and postselection thymic repertoires of Sprague-Dawley and LEW.1F rats and of the repertoire of CD8 cells from both strains expanded in the alloreactive response to RT1(f) revealed that the difference in VA8S2 representation between the two backgrounds is explained by differential availability in the preselection repertoires and not by a difference in overselection. Sequence comparisons of A(f) and A(a) and of both VA8S2 segments suggest a predominant role of CDR1 in hyper-reactivity to A(f). Thus, the VA composition of the mature TCR repertoire is influenced by TCRA: locus polymorphisms at two levels: the regulation of VA usage in the preselection repertoire and the composition of structural elements which contribute to specific VA-MHC interactions during thymic selection.

MEK activity regulates negative selection of immature CD4+CD8+ thymocytes

CD4+CD8+ thymocytes are either positively selected and subsequently mature to CD4 single positive (SP) or CD8 SP T cells, or they die by apoptosis due to neglect or negative selection. This clonal selection is essential for establishing a functional self-restricted T cell repertoire. Intracellular signals through the three known mitogen-activated protein (MAP) kinase pathways have been shown to selectively guide positive or negative selection. Whereas the c-Jun N-terminal kinase and p38 MAP kinase regulate negative selection of thymocytes, the extracellular signal-regulated kinase (ERK) pathway is required for positive selection and T cell lineage commitment. In this paper, we show that the MAP/ERK kinase (MEK)-ERK pathway is also involved in negative selection. Thymocytes from newborn TCR transgenic mice were cultured with TCR/CD3epsilon-specific Abs or TCR-specific agonist peptides to induce negative selection. In the presence of the MEK-specific pharmacological inhibitors PD98059 or UO126, cell recovery was enhanced and deletion of DP thymocytes was drastically reduced. Furthermore, development of CD4 SP T cells was blocked, but differentiation of mature CD8 SP T cells proceeded in the presence of agonist peptides when MEK activity was blocked. Thus, our data indicate that the outcome between positively and negatively selecting signals is critically dependent on MEK activity.

Autonomous induction of proliferation, JNK and NF-alphaB activation in primary resting T cells by mobilized CD28

Induction of proliferation in primary resting T cells requires engagement of both the antigen-specific TCR and the co-stimulatory receptor CD28. Here we report that CD28 functions as an autonomous mitogenic receptor which is mobilized by TCR signaling through cytoskeletal rearrangement. Shortcutting of TCR-dependent CD28 recruitment by stimulation with monoclonal antibodies specific for mobilized CD28 results in maximum proliferation and IL-2 secretion in primary resting T cells without activation of ZAP-70, a central component of the TCR's signal transduction machinery. Engagement of mobilized CD28 fully activates the c-Jun N-terminal kinase cascade and translocation of NF-alphaB, two key targets of signal integration in co-stimulation. We propose a two-step activation model for co-stimulation in primary resting T cells in which antigen recognition recruits co-stimulatory receptors which then autonomously transduce signals promoting T cell proliferation.

Triggering of T cell proliferation through CD28 induces GATA-3 and promotes T helper type 2 differentiation in vitro and in vivo

The relative contribution of T cell receptor-versus CD28-mediated signals in co-stimulation of resting CD4 T cells is thought to influence their functional differentiation towards T helper (Th) 1 versus Th2 subsets. We have used a conventional and a mitogenic CD28-specific monoclonal antibody to assess the effect of polyclonal T cell activation through CD28 alone on CD4 subset differentiation. In vivo, mitogenic but not conventional anti-CD28 induces massive lymphocytosis, the Th2 cytokines interleukin (IL)-4 and IL-10, and Th2-dependent immunoglobulin isotypes, most notably IgE. In vitro, it is shown that mitogenic anti-CD28 primes for IL-4-dependent induction of IL-4 expression much more efficiently than conventional co-stimulation. At the molecular level, we show for the first time that the activation of the "Th2 promoting" transcription factor GATA-3 requires co-stimulation by CD28 and is also induced by mitogenic anti-CD28 alone. We suggest that CD28-dependent induction of GATA-3 in concert with other transcription factors, which are preferentially induced by strong CD28-signals, primes CD4 T cells for IL-4-dependent Th2 differentiaton.

Impaired thymopoietic potential of immature CD3(-)CD4(+)CD8(-) T cell precursors from SIV-infected rhesus monkeys

Immature thymocyte subpopulations were examined for their capacity to differentiate in a newly developed xenogeneic monkey-mouse fetal thymus organ culture (FTOC) system. We provide evidence for impaired precursor function of CD3(-)CD4(+)CD8(-) thymocytes after in vivo infection with SIVmac251 as indicated by a reduced cell number per FTOC and a lower percentage of thymocytes with more mature phenotypes. Addition of recombinant SIV glycoprotein 120 (rgp120) also resulted in a dose-dependent impairment of T cell maturation in FTOC. The data suggest that in patients infected with HIV, T cell maturation and thus replenishment of peripheral pools may be compromised as a result of intrathymic infection or circulating viral gp120.

Ligation of the CD4 receptor induces activation-independent down-regulation of L-selectin

Lymphocyte circulation plays an important role in the generation of a specific immune response. Mature lymphocytes continuously circulate between blood and lymph, entering the lymphoid tissue via high endothelial venules. Trafficking across high endothelial venules of peripheral lymph nodes (PLN) depends on the expression of L-selectin. It has been shown that L-selectin is rapidly cleaved from the surface by a metalloproteinase after in vitro activation. Here, we show that ligation of CD4, without ligation of the T cell receptor for antigen, causes down-regulation of L-selectin on T helper cells. This down-regulation is caused by proteolytic cleavage by a metalloproteinase and is reversible by the addition of hydroxamic acid-based metalloproteinase inhibitors. We show that in vivo down-regulation of L-selectin in huCD4tg mice by mAb reduces the homing of lymphocytes to PLN in adoptive transfer experiments. Because CD4 is a coreceptor for HIV-1, the down-regulation of L-selectin induced by CD4 ligation could play a role in the pathogenesis of AIDS. We provide evidence that CD4 ligation by HIV-1 induces metalloproteinase-dependent L-selectin down-regulation. Reduced levels of L-selectin expression might contribute to immune deficiency in individuals infected with HIV by inhibiting T cell redistribution and decreasing the probability of an encounter between specific lymphocytes and viral antigens in PLN.

Differential CD4/CD8 subset-specific expression of highly homologous rat Tcrb-V8 family members suggests a role of CDR2 and/or CDR4 (HV4) in MHC class-specific thymic selection

Different rat Tcrb haplotypes express either TCR beta variable segment (Tcrb-V) 8.2l or 8.4a. Both V segments bind the mAb R78 but differ by one conservative substitution (L14V) and clusters of two and four substitutions in the complementarity-determining region (CDR) 2 and CDR4 [hypervariable loop 4 (HV4)]. Independently of MHC alleles numbers of R78+ CD4+ cells are lower in Tcrb-V8.2l-expressing than in Tcrb-V8.4a-expressing strains. Expression of R78+ TCR during T cell development, analysis of backcross populations and generation of a Tcrb congenic strain [LEW.TCRB(AS)] define two mechanisms how Tcrb haplotypes affect the frequency of R78+ cells, one acting prior to thymic selection leading to up to 2-fold higher frequency of Tcrb-V8.4a versus Tcrb-V8.2l in unselected thymocytes and another occurring between the TCRlow and the CD4/CD8 single-positive stage. The latter leads to a 50% reduction of frequency of Tcrb-V8.4a CD8+ cells but not CD4+ cells and does not affect either subset of Tcrb-V8.2l cells. A comparison of rat classical class I MHC (RT1.A) sequences and current models of TCR-MHC-peptide interaction suggests that this reduction in frequency of Tcrb-V8.4a CD8 cells may be a consequence of differential selection of Tcrb-V8.2l versus Tcrb-V8.4a TCR by differential binding of CDR2beta to highly conserved areas of C-terminal parts of the alpha helices of class I MHC molecules.

Prolonged allograft survival but no tolerance induction by modulating CD28 antibody JJ319 after high-responder rat heart transplantation

BACKGROUND

Allograft rejection depends on T cell immune responses requiring antigen recognition and costimulatory signals through accessory T cell receptors, including CD28. Inhibition of CD28 signaling with a CTLA-4-immunoglobulin (Ig) fusion protein has resulted in immunosuppression and occasional T cell anergy in mouse transplant models, but not in rats. Because this approach also inhibits a potentially tolerizing signal through CTLA-4, selective blockade of CD28 ligation might induce more profound immunosuppression and transplant tolerance.

METHODS

The effects of escalating doses of the rat CD28 monoclonal antibody JJ319 on allograft survival were studied after vascularized heterotopic heart transplantation in a high responder strain combination (DA to Lewis). CD28 antigen modulation and circulating antibody levels were monitored by flow cytometry.

RESULTS

CD28 antibody JJ319 markedly prolonged cardiac graft survival compared with untreated controls (7 days, range: 6-8). A strictly dose-dependent increase in median graft survival time was demonstrated with a maximum of 36 days (range: 30-40; p <0.001) after the administration of 8 x 1 mg JJ319 i.p. (days -1 to +6 before/after transplantation). However, indefinite graft survival and tolerance could not be induced by JJ319 treatment. At the maximal dose, flow cytometry showed complete down modulation of the CD28 receptor for 10-14 days without T cell depletion in close temporal relation to antibody presence in serum. In vitro, CD28-modulated T cells showed significantly reduced responses to activation.

CONCLUSIONS

CD28 antibody JJ319 induces profound immunosuppression after rat heart transplantation, however without development of transplant tolerance. The underlying mechanism seems to be receptor modulation during primary alloantigen recognition. While still potentially applicable clinically, there are no qualitative or quantitative differences to the treatment with CTLA-4/lg or the blockade of CD2 or LFA-1, as reported elsewhere. Thus, a CD28-modulating approach seems not to allow therapeutic exploitation of a tolerizing signal delivered by CTLA-4 but may still be clinically applicable, especially in combined immune interventions.

In vitro skewing of TCR transgenic CD4+ T cells from interleukin-2 deficient mice towards Th1 and Th2 in the absence of exogenous interleukin-2

Previous experiments from several groups have indicated that in vitro priming for Th2 cells rigorously requires IL-4 but also depends on IL-2 [1-3]. On the other hand, IL-2 deficient mice characteristically have highly increased serum levels of the Th2-dependent isotypes IgG1 and IgE [4]. The overproduction of these isotypes is lost in IL-2 x IL-4 double deficient animals [5]. To readdress the question of a need for IL-2 for Th2 skewing in vitro we used T cells from IL-2-/- mice also transgenic for the DO11.10 TCR which is specific for OVA + IAd [6]. CD4+ cells from these mice were primed in vitro on IL-2-/- dendritic cells in the presence of OVA peptide and IL-4, IL-12 and IL-15, respectively. Following restimulation, cytokine production was analysed by intracellular staining with anti IL-4 and anti IFNgamma antibodies and flow cytometry. The data show that IL-4 primes IL-2-/- T cells for IL-4 production even in the absence of exogenous IL-2, while IL-12, as expected, polarises towards IFNgamma production. The ability to be primed for IL-4 production in the absence of IL-2 was also exhibited by naive CD4+CD62LlowTCR transgenic IL-2-/- cells and thus was not restricted to the CD44high CD62Llow cells which make up a high proportion of CD4+ cells in IL-2 deficient mice. We conclude that IL-2 is not absolutely required for in vitro skewing of naive T cells towards Th2.

Abnormal thymocyte development and generation of autoreactive T cells in mixed and cortical thymomas

To gain insight into the pathogenesis of thymoma-associated myasthenia gravis, thymocyte maturation was investigated in mixed and cortical thymomas by three-color flow cytometry. Although we detected cells at all recognizable stages, we noted an unusual increased percentage of early CD4+/CD3- thymocytes--especially in mixed thymoma--and a pronounced decreased percentage of mature CD4+/CD3+ cells in cortical thymomas as well. The percentage of CD3+/CD69+ cells that arose after positive selection was reduced in both thymoma subtypes compared with control thymuses, which suggests differences in the rate or efficiency of positive selection particularly in mixed thymomas. Mature T cells in 10 of 11 thymomas were not activated in situ as shown by the absence of CD25 expression. After stimulation with recombinant human acetylcholine receptor alpha-subunit fragments, thymocytes from 8 of 11 thymomas of both subtypes proliferated more strongly than those from controls, regardless of whether the donors were myasthenic. Responses of residual thymus cells to tetanus toxoid correlated well with those of autologous blood T cells, whereas those from the thymomas clearly did not--implying minimal colonization of thymomas by mature recirculating T cells. In conclusion, our results show that cortical and mixed thymomas exhibited differences in thymocyte maturation. Nevertheless, both thymoma subtypes seem to contribute to the pathogenesis of paraneoplastic myasthenia gravis by generating naive but potentially autoaggressive T cells; in some thymomas, these cells may then be actively immunized inside the tumor.

Co-evolution of rat TAP transporters and MHC class I RT1-A molecules

The genes for rat major histocompatibility complex (MHC) class I molecules are associated either with those for the A allele of the transporter associated with antigen processing (TAP-A), which can transport peptides with basic carboxy-terminal residues, or with those for TAP-B, which cannot [1-5]. To explore whether these associations have a functional basis, we compared the sequences of 13 rat MHC class la RT1-A cDNAs from nine MHC haplotypes. Of seven TAP-A- linked RT1-A molecules, six possess strongly acidic F pockets, and these bind a high proportion of peptides with basic carboxy-terminal residues. The F pockets of TAP-B-linked molecules, by contrast, were more basic. Furthermore, we identified six positions at the 'righthand end' of the peptide-binding groove, at which a majority of TAP-B-linked molecules diverge from the consensus sequence for class la molecules whereas, at these positions, all the TAP-A-linked molecules reflect the consensus sequence. Our results suggest that the linked rat class la and TAP genes have co-evolved to maximize the supply of appropriate peptides to the presenting molecules.

Opposite CD4/CD8 lineage decisions of CD4+8+ mouse and rat thymocytes to equivalent triggering signals: correlation with thymic expression of a truncated CD8 alpha chain in mice but not rats

Unselected CD4+8+ rat thymocytes, generated in vitro from their direct precursors, are readily converted to functional TCRhigh T cells by stimulation with immobilized TCR-specific mAb plus IL-2. Lineage decision invariably occurs toward CD4-8+, regardless of the timing of TCR stimulation after entry into the CD4+8+ compartment or the concentration of TCR-specific mAb used for stimulation. CD4-specific mAb synergizes with suboptimal TCR-specific mAb in inducing T cell maturation, but lineage decision remains exclusively CD4-8+. These results contrast with those obtained in mice, in which Abs to the TCR complex were shown to promote CD4+8- T cell maturation from CD4+8+ thymocytes. Surprisingly, when rat and mouse CD4+8+ thymocytes were stimulated with PMA/ionomycin under identical conditions, the opposite lineage commitment was observed, i.e., mouse thymocytes responded with the generation of CD4+8- and rat thymocytes with the generation of CD4-8+ cells. It thus seems that CD4+8+ thymocytes of the two species respond with opposite lineage decisions to strong activating signals such as given by TCR-specific mAb or PMA/ionomycin. A possible key to this difference lies in the availability of p56lck for coreceptor. supported signaling. We show that in contrast to mouse CD4+8+ thymocytes, which express both a complete and a truncated CD8 alpha-chain (CD8 alpha') unable to bind p56lck, rat thymocytes only express full-length CD8 alpha molecules. Mice, but not rats, therefore may use CD8 alpha' as a "dominant negative" coreceptor chain to attenuate the CD8 signal, thereby facilitating MHC class II recognition through the higher amount of p56lck delivered, and rats may use a different mechanism for MHC class distinction during positive selection.

Systemic autoimmune disease as a consequence of defective lymphocyte death

A major group of systemic autoimmune diseases is associated with abnormal lymphoproliferation, as a result of defects in the termination of lymphocyte activation and growth. Recent progress has been made in understanding the causes and consequences of these abnormalities. At the molecular level, the defects in CD95 and its ligand are only the most obvious reasons for the breakdown of 'clonal contraction' which in fact requires the participation of multiple gene products, including the IL-2-IL-2-receptor system, to set up a functional apoptotic machinery.

Control of TCR V alpha-mediated positive repertoire selection and alloreactivity by differential J alpha usage and CDR3 alpha composition

In rats expressing the f allele of the rat MHC (RT1f), CD8 T cells utilizing the V alpha 8.2 segment are 10-fold overselected during thymic development, resulting in V alpha 8.2 expression by 14% of mature CD8 T cells as compared to 1-2% in MHC congenic strains. In the alloreactive responses of CD8 T cells from RT1f-negative rats against RT1f, V alpha 8.2+ CD8 T cells are also preferentially expanded. Neither overselection nor alloreactivity of V alpha 8.2+ TCR require selective V beta pairing. However, RT1f alloreactive V alpha 8.2+ TCR preferentially use a related set of J alpha segments which contribute short homogeneous CDR3 alpha loops, with features suggesting peptide promiscuity, and little N additions. In contrast, only few overselected V alpha 8.2+ CD8 T cells showed an imprint of positive selection on J usage or CDR3 composition. The results demonstrate that a single V alpha segment can promote both MHC allele-specific positive selection and alloreactivity, and that the latter is more dependent on an additional contribution of CDR3 alpha, possibly by promoting reactivity with a diverse set of MHC-bound peptides or by providing additional MHC contacts.

CD28-mediated induction of proliferation in resting T cells in vitro and in vivo without engagement of the T cell receptor: evidence for functionally distinct forms of CD28

JJ316 and JJ319 are rat CD28-specific monoclonal antibodies (mAb) of the gamma1 kappa isotype with identical co-stimulatory potency. At a concentration 100-1000-fold higher than that required for co-stimulation, JJ316, but not JJ319 induces massive proliferation of all T cell subsets in vitro without T cell receptor (TCR) triggering. "Direct" stimulation by JJ316 is fully blocked by JJ319, indicating that it is not due to cross-reactivity of JJ316 with the TCR complex or other activating receptors. JJ316 binds much more slowly to primary T cells than JJ319, whereas both antibodies bind with similar kinetics to CD28-transfected L-929 cells, suggesting that JJ316 binding to T cells requires redistribution or a conformational change of CD28. In vivo, JJ316 but not JJ319 induces rapid and transient proliferation of most CD4 T cells and, indirectly, of B cells. These data show that TCR engagement is not an absolute prerequisite either in vitro or in vivo for the induction of T cell proliferation through CD28 and suggest that mAb JJ316 is able to stimulate resting T cells directly by recruiting CD28 molecules from an inactive to an active form.

The canonical T cell receptor of dendritic epidermal gamma delta T cells is highly conserved between rats and mice

Two monoclonal antibodies with specificity for rat gammadelta T cell receptor (TCR) were generated. One, called V65, reacts with all CD3+ alphabeta TCR- rat Tcells and thus recognizes a constant determinant of the rat gammadelta TCR (Kühnlein et al., Journal of Immunology 1994, 153: 979). The other, called V45, reacts with approximately 80% of gammadelta T cells in peripheral lymphoid organs. In rat epidermis, V65 but not V45 detects a dense network of the dendritic epidermal Tcells (DETC). Analysis of epidermal RNA by polymerase chain reaction (PCR) indicated that Vgamma3 and Vdelta1 are the predominant, if not exclusive TCR V transcripts present at this site. Sequence analysis of cDNA clones obtained by reverse transcription-PCR with Vgamma3- and Vdelta1-specific primers revealed that the variable domains of rat DETC gamma and delta chains are very homologous to those described in mice (92% and 95% identity at the protein level). The complete conservation between the two species of the amino acid sequences at the V-(D)-J transitions of this monomorphic receptor indicates that the interaction of the DETC TCR with its as yet unknown ligand must be of central importance for DETC function.

Alleles of highly homologous rat T cell receptor beta-chain variable segments 8.2 and 8.4: strain-specific expression, reactivity to superantigens, and binding of the mAb R78

This study addresses the molecular basis of a Tcrb-V polymorphism in the reactivity to the superantigens staphylococcus enterotoxin B (SEB) and the mtv-7 sag (MIs1a) of T cells recognized by the mAb R78, which reacts with the T cell receptor beta-chain variable segment 8.2 (Tcrb-V8.2) of Lewis (LEW) rats. Tcrb-V8.2-like sequences were isolated from liver DNA of the responder strain LEW (I) and the nonresponder strain DA (a) and alleles of the Tcrb-V8.2 and the highly homologous Tcrb-V8.4 were identified. Their expression was analyzed by RNase protection studies and cDNA clones were characterized. A comparison of thymocytes, activated R78+ cells, Con A-stimulated and SEB-stimulated cells allows the following conclusions: the newly identified Lewis allele of Tcrb-V8.4 (Trcb-V8.4I) is nonfunctional due to a frame shift induced by deletion of one nucleotide. The R78 epitope is expressed by Tcrb-V8.2I and Tcrb-V8.4a but not by Tcrb-V8.2a. The implication of this finding for mapping of the R78 epitope and the study of V region usage in experimental autoimmune encephalitis are discussed. Finally, the expression of both Tcrb-V8.2 alleles but not of Tcrb-V8.4a in SEB-stimulated cells defines a polymorphism of the CDR2 and/or CDR4 as the molecular basis of the differential superantigen reactivity.

Alleles of highly homologous rat T cell receptor beta-chain variable segments 8.2 and 8.4: strain-specific expression, reactivity to superantigens, and binding of the mAb R78

This study addresses the molecular basis of a Tcrb-V polymorphism in the reactivity to the superantigens staphylococcus enterotoxin B (SEB) and the mtv-7 sag (MIs1a) of T cells recognized by the mAb R78, which reacts with the T cell receptor beta-chain variable segment 8.2 (Tcrb-V8.2) of Lewis (LEW) rats. Tcrb-V8.2-like sequences were isolated from liver DNA of the responder strain LEW (I) and the nonresponder strain DA (a) and alleles of the Tcrb-V8.2 and the highly homologous Tcrb-V8.4 were identified. Their expression was analyzed by RNase protection studies and cDNA clones were characterized. A comparison of thymocytes, activated R78+ cells, Con A-stimulated and SEB-stimulated cells allows the following conclusions: the newly identified Lewis allele of Tcrb-V8.4 (Trcb-V8.4I) is nonfunctional due to a frame shift induced by deletion of one nucleotide. The R78 epitope is expressed by Tcrb-V8.2I and Tcrb-V8.4a but not by Tcrb-V8.2a. The implication of this finding for mapping of the R78 epitope and the study of V region usage in experimental autoimmune encephalitis are discussed. Finally, the expression of both Tcrb-V8.2 alleles but not of Tcrb-V8.4a in SEB-stimulated cells defines a polymorphism of the CDR2 and/or CDR4 as the molecular basis of the differential superantigen reactivity.

Negative regulation of rat natural killer cell activity by major histocompatibility complex class I recognition

The cytolytic activity of human and mouse natural killer (NK) cells is negatively regulated by self major histocompatibility complex (MHC) class I molecules on potential target cells. In the rat, protection by RT1 class I gene products has so far not been formally shown although the complex effects of foreign and self RT1 genes on polyclonal NK cell activity suggest that MHC recognition can have both stimulatory and inhibitory effects. Here we report that the expression of self-MHC class I molecules on target cells strongly inhibits lysis by a long term NK cell line derived from LEW (RT1l) rats and by LEW NK cells activated by short-term culture in the presence of interleukin-2. This was demonstrated with mouse-rat hybridoma target cells expressing different rat MHC alleles and with mouse tumor target cells transfected with classical (RT1.Al) and nonclassical (RT1.Cl) rat MHC class I genes. With hybridoma target cells, the strongest reduction in lysis as compared to the parental mouse myeloma line was observed when "self" (LEW) MHC was expressed, while hybridomas expressing other MHC alleles showed less and variable reduction. Transfection of RT1.Al protected both L-929 fibroblasts and P815 mastocytoma cells from lysis by the NK cell line, while RT1.Cl only protected P815 cells, indicating that additional target cell properties regulate rat NK cell activity.

Identification and cellular distribution of the rat interleukin-2 receptor beta chain: induction of the IL-2R alpha- beta+ phenotype by major histocompatibility complex class I recognition during T cell development in vivo and by T cell receptor stimulation of CD4+8+ immature thymocytes in vitro

A mouse monoclonal antibody (mAb) to the rat interleukin-2 receptor beta (IL-2R beta) chain was generated using IL-2R beta cDNA-transfected mouse L929 cells for immunization and differential screening. This antibody, called L316, detects a cell surface protein with an apparent molecular mass of about 80 kDa. In peripheral lymphoid organs of young adult rats, IL-2R beta expression is restricted to T and natural killer (NK) cells, and less than 10% of IL-2R beta+ cells co-express the IL-2R alpha chain. IL-2R beta was detected on all NKRP-1hi (NK-) and NKRP-1lo cells (T-lineage cells of unknown function), most peripheral gama delta T cells and on 30-40 % of CD8 and 10 % of CD4 alpha beta T cells. In the adult rat thymus, mAb L316 detects a small subset (about 1%) of predominantly IL-2R alpha- cells which express cell surface markers characteristic of mature T lymphocytes and contain a high proportion of CD4-8- and CD4-8+ alpha beta T cell receptor (TCR)+ thymocytes. TCR-V usage suggests that major histocompatibility complex (MHC) class I plays a more important role than MHC class II in the selection of these cells. On immature CD4+8+ rat thymocytes, IL-2R beta cell surface expression is readily induced by TCR stimulation in vitro, supporting the idea that in vivo, the IL-2R beta+ phenotype is the the result of TCR engagement during thymic selection.

Orally induced, peptide-specific gamma/delta TCR+ cells suppress experimental autoimmune uveitis

We investigated the role of gamma/delta TCR+ T cells in induction and suppression of the T cell-mediated disease experimental autoimmune uveitis. Disease induction was studied in Lewis rats perinatally depleted of alpha/beta or gamma/delta TCR+ subpopulations. Depletion of alpha/beta TCR+ cells completely abrogated disease, whereas treatment with anti-gamma/delta antibodies had no influence on onset or intensity of uveitis. However, adoptively transferred gamma/delta+ cells from orally tolerized rats could mediate suppression of uveitis in an antigen-specific fashion. Uveitis induced by a peptide derived from the uveitogenic retinal soluble antigen (S-Ag) was suppressed by gamma/delta+ cells from rats orally tolerized with the same peptide as well as HLA peptide B27PD. This disease ameliorating effect could also be observed when rats were fed with the HLA peptide before immunization with S-Ag peptide. Transfer of alpha/beta+ T cells from the same donors as well as gamma/delta+ or alpha/beta+ cells from animals fed with control peptide had no ameliorating effect.

Depletion of gamma/delta T cells does not prevent or ameliorate, but rather aggravates, rat adjuvant arthritis

OBJECTIVE

To investigate the role of gamma/delta T cells in Mycobacterium tuberculosis-induced rat adjuvant arthritis.

METHODS

Rats with adjuvant arthritis were injected with anti-T cell receptor gamma/delta (anti-TCRgamma/delta) monoclonal antibody V65 according to a preventive protocol, a pre-arthritis peak protocol, and a late therapeutic protocol. Arthritis severity and joint destruction were monitored, and depletion of target cells was analyzed by flow cytometry.

RESULTS

Although all protocols led to successful depletion of TCRgamma/delta(bright) cells in peripheral blood and lymph nodes, none of the regimens influenced clinical parameters of adjuvant arthritis. If rats were treated before the clinical peak of adjuvant arthritis, however, joint destruction was significantly more severe than in vehicle-treated rats.

CONCLUSION

Rat adjuvant arthritis is not promoted or perpetuated by gamma/delta T cells. Aggravation of joint destruction with pre-arthritis peak anti-gamma/delta treatment suggests a stage-dependent protective role of gamma/delta T cells in adjuvant arthritis.

Immunopathology of interleukin (IL) 2-deficient mice: thymus dependence and suppression by thymus-dependent cells with an intact IL-2 gene

Interleukin (IL) 2-deficient mice develop a fatal immunopathology characterized by lymphoadenopathy, splenomegaly, T cell infiltration of the bone marrow, loss of B cells, anemia, and inflammation of the gut. The thymus dependence of these disease symptoms was tested by introducing the IL-2 mutation into athymic mice. With the exception of an increase in CD8+ intrahepatic alpha/beta T cells, IL-2 deficiency had no detectable effect on leukocyte composition or health of athymic mice, indicating a key role for thymus-derived T cells in the initiation of disease and demonstrating that B cell development and survival are independent of IL-2. In adoptive transfer studies, lymph node and spleen cells from euthymic IL-2-deficient mice induced disease in athymic mice with an intact IL-2 gene, suggesting that thymus-independent IL-2-expressing cells are unable to control the development of immune pathology. Both IL-2+ and IL-2-/- bone marrow cells repopulated the thymus and the peripheral T cell compartment of the recombination activator gene 2-deficient recipients, and chimeras that had received IL-2-deficient bone marrow developed immune pathology. Disease development was, however, fully or at least partially prevented when 30% of the bone marrow inoculum was derived from mice able to express IL-2. These results demonstrate that the IL-2 deficiency syndrome depends on the intrathymic differentiation of T cells carrying the IL-2 mutation, and that the abnormal activation of IL-2-deficient lymphocytes can be controlled by thymus-derived but not thymus-independent lymphocytes.

Characterization of mouse CD53: epitope mapping, cellular distribution and induction by T cell receptor engagement during repertoire selection

The pan-leukocyte antigen CD53 is a member of the poorly understood transmembrane 4 superfamily (TM4SF) of cell membrane glycoproteins. CD53 is proposed to play a role in thymopoiesis, since rat CD53 is expressed on immature CD4-8-thymocytes and the functionally mature single-positive subset, but is largely absent from the intermediate CD4+8+ cells. We have characterized CD53 in the mouse through the production of two new monoclonal antibodies, MRC OX-79 and OX-80, which were raised against the RAW 264 cell line and screened on recombinant CD53 fusion proteins. The epitopes recognized by both antibodies are dependent on disulfide bonding and map to the major extracellular region of CD53, requiring the presence of a single threonine residue at position 154. Mouse CD53 has a molecular mass of 35-45 kDa and is expressed on virtually all peripheral leukocytes, but not on cells outside the lymphoid or myeloid lineages. CD53 expression distinguishes subpopulations of thymocytes in the mouse and resembles the expression pattern of rat CD53. Amongst the immature CD4-8-thymocytes, mouse CD53 is clearly detectable on the earliest CD44high25- subset, but down-regulated on the later CD44high25+, CD44low25+ and CD44low25- stages. Also, the subsequent transient TcR-/low CD4-8+ cells and most CD4+8+ thymocytes express little or no CD53. This is consistent with the idea that cells which are committed to enter the selectable CD4+8+ compartment switch off CD53. The effect of T cell receptor (TcR) engagement on the re-expression of CD53 on CD4+8+ thymocytes was studied both ex vivo and in vitro using F5 mice, transgenic for the H-2b/influenza nucleoprotein-peptide-specific TcR, back-crossed onto an H-2q or H-2b background of RAG-2-deficient mice. CD4+8+ thymocytes from non-selecting H-2q F5 mice are CD53 negative, but in vitro stimulation through the TcR dramatically induces CD53 expression. In contrast, a fraction of CD4+8+ thymocytes from positively selecting H-2b F5 transgenic mice express CD53. Therefore TcR engagement by selecting major histocompatibility complex peptide complexes, or surrogate ligands, induces CD53 expression on otherwise CD53-negative, non-selected CD4+8+ thymocytes. Whether CD53 itself participates as a signaling molecule in further stages of thymic selection is still a matter of speculation.

Prevention and treatment of Lewis rat experimental allergic encephalomyelitis with a monoclonal antibody to the T cell receptor V beta 8.2 segment

The predominance of T cell receptor (TCR) V beta 8.2 utilization by encephalitogenic T cells induced in Lewis rats by immunization with myelin basic protein (MBP) is controversial. Thus, both an almost exclusive usage of V beta 8.2 [Burns, F. R., Li, X., Shen, N., Offner, H., Chou, Y. K., Vandenbark, A. A. and Heber-Katz, E., J. Exp. Med. 1989, 169: 27; Chluba, J., Steeg, C., Becker, A., Wekerle, H. and Epplen, J. T., Eur. J. Immunol. 1989. 19: 279] and a quite diverse V beta composition of CD4 T cells causing experimental autoimmune encephalomyelitis (EAE) [Sun, D., Gold, P. D., Smith, L., Brostoff, S. and Coleclough, C., Eur. J. Immunol, 1992. 22: 591; Sun, D., Le, J. and Coleclough, C., Eur. J. Immunol. 1993. 23: 494] have been reported. Using a recently developed monoclonal antibody (mAb) specific for TCR V beta 8.2, we show that postnatal treatment effectively eliminates V beta 8.2-bearing cells and prevents MBP-induced EAE in the majority of Lewis rats. Moreover, treatment of adult Lewis rats with V beta 8.2-specific mAb as late as on day 12 after MBP immunization suppressed the development of neurological symptoms. Thus, V beta 8.2-bearing cells do play a decisive role in Lewis rat EAE, and suppression of the small (5%) V beta 8.2-expressing T cell subset provides an effective therapeutic strategy.

Cellular distribution and costimulatory function of rat CD28. Regulated expression during thymocyte maturation and induction of cyclosporin A sensitivity of costimulated T cell responses by phorbol ester

CD28 has been identified in man and mouse as a potent costimulatory receptor on T cells. We have generated a mAb, called JJ319, to rat CD28 and show that it is expressed on virtually all peripheral rat alpha beta and on most gamma delta T cells, and on about half of NK cells. In contrast to the mouse but as in humans, most immature CD4+8+TCRlow thymocytes express little or no CD28, whereas CD28 expression is high on TCRintermediate and TCRhigh cells. mAb JJ319 very effectively costimulates T cell proliferation and IL-2 secretion by resting rat T cells. In contrast to results obtained in mice and humans, phorbol ester did not synergize in T cell activation with CD28-specific mAb but even induced sensitivity to cyclosporin A in T cell cultures that were optimally costimulated by mAbs to the TCR and to CD28. This result points to a novel effect of protein kinase activation by phorbol ester on signal transduction by TCR plus CD28 costimulation which only becomes apparent if, as in the rat, the TCR-mediated signal cannot be replaced by phorbol ester.

Cellular distribution and costimulatory function of rat CD28. Regulated expression during thymocyte maturation and induction of cyclosporin A sensitivity of costimulated T cell responses by phorbol ester

CD28 has been identified in man and mouse as a potent costimulatory receptor on T cells. We have generated a mAb, called JJ319, to rat CD28 and show that it is expressed on virtually all peripheral rat alpha beta and on most gamma delta T cells, and on about half of NK cells. In contrast to the mouse but as in humans, most immature CD4+8+TCRlow thymocytes express little or no CD28, whereas CD28 expression is high on TCRintermediate and TCRhigh cells. mAb JJ319 very effectively costimulates T cell proliferation and IL-2 secretion by resting rat T cells. In contrast to results obtained in mice and humans, phorbol ester did not synergize in T cell activation with CD28-specific mAb but even induced sensitivity to cyclosporin A in T cell cultures that were optimally costimulated by mAbs to the TCR and to CD28. This result points to a novel effect of protein kinase activation by phorbol ester on signal transduction by TCR plus CD28 costimulation which only becomes apparent if, as in the rat, the TCR-mediated signal cannot be replaced by phorbol ester.

Gamma delta T cells down-regulate primary IgE responses in rats to inhaled soluble protein antigens

The biologic role and repertoire of cells bearing the gamma delta T cell receptor has not been fully defined. However, their tropism for epithelial microenvironments is recognized and suggests an important role for these cells in immune defense at mucosal tissue surfaces. The study presented below utilizes an experimental model in which repeated exposure of Brown Norway rats to OVA by inhalation induces a state of Ag-specific, IgE isotype-specific "tolerance" via immune deviation. This process seems similar to oral tolerance in the gut. This form of tolerance was adoptively transferred to naive syngeneic recipients by i.p. injection of as few as 10(3) positively selected TCR-gamma delta+ cells from OVA-exposed rats. These TCR-gamma delta+ T-cells are demonstrated to produce high levels of INF-gamma in response to OVA stimulation, and this provides a potential mechanism for the inhibition of Th2 cell proliferation, resulting in suppression of IgE production. The unique potency of these cells in selective suppression of IgE Ab production in response to natural "mucosal" Ag exposure suggests a potentially important role in protection against primary allergic sensitization in vivo.

Gamma delta T cells down-regulate primary IgE responses in rats to inhaled soluble protein antigens

The biologic role and repertoire of cells bearing the gamma delta T cell receptor has not been fully defined. However, their tropism for epithelial microenvironments is recognized and suggests an important role for these cells in immune defense at mucosal tissue surfaces. The study presented below utilizes an experimental model in which repeated exposure of Brown Norway rats to OVA by inhalation induces a state of Ag-specific, IgE isotype-specific "tolerance" via immune deviation. This process seems similar to oral tolerance in the gut. This form of tolerance was adoptively transferred to naive syngeneic recipients by i.p. injection of as few as 10(3) positively selected TCR-gamma delta+ cells from OVA-exposed rats. These TCR-gamma delta+ T-cells are demonstrated to produce high levels of INF-gamma in response to OVA stimulation, and this provides a potential mechanism for the inhibition of Th2 cell proliferation, resulting in suppression of IgE production. The unique potency of these cells in selective suppression of IgE Ab production in response to natural "mucosal" Ag exposure suggests a potentially important role in protection against primary allergic sensitization in vivo.

Impaired survival of T cell receptor V gamma 3+ cells in interleukin-4 transgenic mice

The mouse epidermis contains a network of Thy-1+ dendritic T cells. Most of these cells express a homogeneous T cell receptor (TCR) configuration (V gamma 3/V delta 1) with only negligible junctional diversity. Because fetal thymocytes are precursors of these dendritic epidermal T cells (DETC) and the addition of interleukin (IL)-4 to fetal thymic organ cultures causes an early arrest in thymopoiesis, we examined DETC development in transgenic (tg) mice expressing IL-4 under the control of major histocompatibility complex class I regulatory sequences. Immunohistologic examination of epidermal sheets and polymerase chain reaction analysis of total skin RNA from IL-4 tg mice failed to reveal TCR V gamma 3+ DETC and V gamma 3 mRNA, respectively. In contrast, the sizes of TCR gamma delta subpopulations in lymphoid organs were unchanged in these mice. Although the numbers and staining intensities of TCR V gamma 3+ thymocytes in early fetal (days 14-17) IL-4 tg mice were similar to those of littermate controls, we observed a preferential death of these cells in thymic organ cultures from IL-4 tg mice. We observed further that epidermal sheets prepared from 9-day-old mice whose mothers had been treated with an IL-4-neutralizing antibody from day 12 to day 18 of pregnancy contained DETC numbers similar to those of controls. However, upon termination of the anti-IL-4 treatment, DETC ceased to expand. We conclude that IL-4 impairs the survival of TCR V gamma 3+ cells.

Expression of cell interaction molecules by immature rat thymocytes during passage through the CD4+8+ compartment: developmental regulation and induction by T cell receptor engagement of CD2, CD5, CD28, CD11a, CD44 and CD53

Rat thymocytes of the T cell receptorlow (TcRlow) CD4+8+ subset which is the target of repertoire selection are heterogeneous with respect to expression of the cell interaction (CI) molecules CD2, CD5, CD11a/CD18 (LFA-1), CD28 and CD44. We show that this heterogeneity is due to the developmental regulation of these CI molecules during passage through the CD4+8+ compartment, and to up-regulation by TcR engagement. Thus, cohorts of CD4+8+ cells differentiating synchronously in vitro from their direct precursors, the immature CD4-8+ cells, were homogeneous with regard to CI molecule expression. Upon entry into the CD4+8+ compartment, they expressed relatively high levels of CD2 and CD44, and moderate levels of CD5, CD28 and CD11a. CD2, CD28 and CD44 were slightly down-regulated during the following 2 days, whereas CD5 slightly increased and CD11a remained constant. TcR stimulation using immobilized monoclonal antibodies resulted in rapid and dramatic up-regulation of CD2, CD5 and CD28 and, to a lesser extent, of CD11a and CD44. Finally CD53, a triggering structure absent from unstimulated CD4+8+ thymocytes was also rapidly induced by TcR stimulation. Inclusion of interleukin (IL)-2, IL-4, or IL-7 in this in vitro differentiation system did not affect the levels of CI molecules studied. Since the high levels of CI molecules induced by TcR-stimulation correspond to those found in vivo on TcRintermediate thymocytes known to be undergoing repertoire selection, these results suggest that upregulation of CI molecules by TcR engagement provides a mechanism by which thymocytes that have entered the selection process gain preferential access to further interactions with stromal and lymphoid cells in the thymus.

Encephalitogenic, myelin basic protein-specific T cells from naive rat thymus: preferential use of the T cell receptor gene V beta 8.2 and expression of the CD4-CD8- phenotype

Using a primary limiting dilution approach to generate T cell lines, we compared myelin basic protein (MBP)-specific T cell clones from naive unprimed Lewis rat thymuses with the corresponding T cell repertoire of primed rats. We found that in the native thymus repertoire MBP-specific, encephalitogenic T cell clones preferentially use T cell receptor V beta 8.2 genes, along with CDR3 sequences typical for the primed Lewis anti-MBP response. In contrast to T cells from primed immune organs, which all display the CD4+CD8- phenotype, the majority of naive thymus-derived T cell clones expressed reduced levels of the CD4 co-receptor. Some clones were completely CD4-CD8-, while others included CD4-CD8- subpopulations along with CD4+CD8- T cells. In the one mixed population examined in detail, the CD4-CD8- and CD4+CD8-T cell subpopulations used a T cell receptor with identical beta chain sequence. The data suggest that in the Lewis rat the biased T cell receptor gene usage by encephalitogenic T cells is a property of the natural thymic T cell repertoire, possibly as a consequence of positive selection. The unusually low expression of CD4 in the major histocompatibility complex class II-restricted autoreactive T cells could be related to their escape from negative selection within the thymus.

Structural analysis of the rat T-cell receptor Tcra V4 gene family

The rat Tcra V gene locus is only poorly characterized, although rats are widely used in a variety of T-cell-mediated experimental animal models. Recently, we described the first monoclonal antibody, G99, directed against a rat Tcra V4 segment. We examined cDNA transcripts of G99-positively sorted T cells and show that the monoclonal antibody G99 most likely recognizes at least two members of the Tcra V4 family. Moreover, we analyzed the genomic repertoire of this VA family and report 15 novel Tcra V4 DNA sequences. Based on sequence and Southern blot analysis, the Tcra V4 family could be divided into four subgroups, which were also detected in mice. These findings corroborate previous findings of a similar genetic organization of the Tcra V loci in both species.

Gamma/delta T cells in fetal, neonatal, and adult rat lymphoid organs

In the present study, we have analyzed the appearance and maturation of gamma/delta T cells, recognized with a new mAb V65, in the central and peripheral lymphoid organs of fetal, neonatal, and adult Wistar rats. Cytofluorometrical analysis demonstrated the first V65+ gamma/delta T cells in the thymus of 16-17-day embryonic rats, although by immunohistology, they were identified only in 19-day rat embryos in both the cortico-medullary border and thymic medulla. Phenotypically, gamma/delta thymocytes from fetal and neonatal thymus expressed CD3, CD2, and CD5, but only 60-80% were CD8+ and approximately 40-50% expressed the alpha chain (p55) of the IL-2R. In the periphery, the immunohistological study identified for the first time gamma/delta T cells in the splenic white pulp and the gut of 21-day fetal rats, where they occurred within the epithelium as well as in the lamina propria. After birth, gamma/delta lymphocytes appeared in the skin, where they were present as dendritic epidermal T cells in increasing numbers during postnatal life. Whereas these gamma/delta T cells formed the predominant T-cell population in the rat skin, gamma/delta T cells in peripheral lymphoid organs, BALT, or the gut only represented a minor T-cell population. These results are discussed in comparison to gamma/delta T cells of other vertebrate species.

Induction of interleukin 2 receptor beta chain expression by self-recognition in the thymus

1-2% of adult mouse thymocytes express the T cell receptor alpha/beta (TCR-alpha/beta) together with the interleukin (IL) 2R beta (p70), but not the alpha (p 55) chain. We show that the previously described alpha/beta-TCR +CD4-8- and the partially overlapping Ly6C+ thymocytes are contained within this subset. Most IL-2R beta+ alpha/beta-TCR+ cells have a mature and activated (heat stable antigen [HSA]-, thymic shared antigen 1 [TSA-1]-, CD44high, CD69+) phenotype. Overrepresentation of V beta 8.2 in both CD4-8- and CD4 and/or CD8+ IL-2R beta+ thymocytes suggests that IL-2R beta expression is induced by a TCR-mediated activation event. In mice transgenic for an H-2Kb-specific TCR, IL-2R beta+ cells were abundant under conditions of mainstream negative selection, i.e., in the presence of Kb, but absent under conditions of mainstream positive selection or in a nonselecting environment. Together, these results show that in addition to clonal deletion, self-recognition by immature thymocytes leads to phenotypic maturation of a small subset of thymocytes expressing IL-2R beta. IL-2-deficient mice contain normal numbers of IL-2R beta+ alpha/beta-TCR+ thymocytes, indicating that like mainstream T cell development, this minor pathway of positive selection does not depend on IL-2. However, in the absence of IL-2, the CD4/CD8 subset composition of IL-2R beta+ thymocytes is skewed towards CD4-8+, mostly at the expense of CD4-8-. A possible relevance of this finding for the development of the immune pathology of IL-2-deficient mice is discussed.