Immunoprecipitation of cell surface structures of cloned cytotoxic T lymphocytes by clone-specific antisera

Mouse Models of Antigen Presentation in Hematopoietic Stem Cell Transplantation

Allogeneic stem cell transplantation (alloSCT) is a curative therapy for hematopoietic malignancies. The therapeutic effect relies on donor T cells and NK cells to recognize and eliminate malignant cells, known as the graft-versus-leukemia (GVL) effect. However, off target immune pathology, known as graft-versus-host disease (GVHD) remains a major complication of alloSCT that limits the broad application of this therapy. The presentation of recipient-origin alloantigen to donor T cells is the primary process initiating GVHD and GVL. Therefore, the understanding of spatial and temporal characteristics of alloantigen presentation is pivotal to attempts to separate beneficial GVL effects from detrimental GVHD. In this review, we discuss mouse models and the tools therein, that permit the quantification of alloantigen presentation after alloSCT.

Identification of the transgenic integration site in 2C T cell receptor transgenic mice

2C T cell receptor (TCR) transgenic mice have been long used to study the molecular basis of TCR binding to peptide/major compatibility complexes and the cytotoxicity mechanism of cytotoxic T lymphocytes (CTLs). To study the role of variable gene promoters in allelic exclusion, we previously constructed mutant mice in which the Vβ13 promoter was deleted (P13 mice). Introduction of 2C transgene into P13 mice accelerated the onset of systemic CD8 T cell lymphoma between 14 and 27 weeks of age, although parental P13 mice appeared to be normal. This observation suggests that the lymphoma development may be linked to features of 2C transgene. To identify the integration site of 2C transgene, Southern blotting identified a 2C-specific DNA fragment by 3' region probe of 2C TCR α transgene, and digestion-circularization-polymerase chain reaction (DC-PCR) amplified the 2C-specific DNA fragment with inverse primers specific to the southern probe. Sequence analysis revealed that DC-PCR product contained the probe sequences and the junction sequences of integration site, indicating that 2C TCR α transgene is integrated into chromosome 1. Further genomic analysis revealed cytosolic phospholipase A2 group IVA (cPLA2) as the nearest gene to the integration site. cPLA2 expression was upregulated in the normal thymi and T cell lymphomas from 2C transgenic mice, although it was not altered in the lymph nodes of 2C transgenic mice. The result is the first report demonstrating the integration site of 2C TCR transgene, and will facilitate the proper use of 2C transgenic mice in studies of CTLs.

Role of T cell receptor affinity in the efficacy and specificity of adoptive T cell therapies

Over the last several years, there has been considerable progress in the treatment of cancer using gene modified adoptive T cell therapies. Two approaches have been used, one involving the introduction of a conventional αβ T cell receptor (TCR) against a pepMHC cancer antigen, and the second involving introduction of a chimeric antigen receptor (CAR) consisting of a single-chain antibody as an Fv fragment linked to transmembrane and signaling domains. In this review, we focus on one aspect of TCR-mediated adoptive T cell therapies, the impact of the affinity of the αβ TCR for the pepMHC cancer antigen on both efficacy and specificity. We discuss the advantages of higher-affinity TCRs in mediating potent activity of CD4 T cells. This is balanced with the potential disadvantage of higher-affinity TCRs in mediating greater self-reactivity against a wider range of structurally similar antigenic peptides, especially in synergy with the CD8 co-receptor. Both TCR affinity and target selection will influence potential safety issues. We suggest pre-clinical strategies that might be used to examine each TCR for possible on-target and off-target side effects due to self-reactivities, and to adjust TCR affinities accordingly.

TCR-ligand koff rate correlates with the protective capacity of antigen-specific CD8+ T cells for adoptive transfer

Adoptive immunotherapy is a promising therapeutic approach for the treatment of chronic infections and cancer. T cells within a certain range of high avidity for their cognate ligand are believed to be most effective. T cell receptor (TCR) transfer experiments indicate that a major part of avidity is hardwired within the structure of the TCR. Unfortunately, rapid measurement of structural avidity of TCRs is difficult on living T cells. We developed a technology where dissociation (koff rate) of truly monomeric peptide-major histocompatibility complex (pMHC) molecules bound to surface-expressed TCRs can be monitored by real-time microscopy in a highly reliable manner. A first evaluation of this method on distinct human cytomegalovirus (CMV)-specific T cell populations revealed unexpected differences in the koff rates. CMV-specific T cells are currently being evaluated in clinical trials for efficacy in adoptive immunotherapy; therefore, determination of koff rates could guide selection of the most effective donor cells. Indeed, in two different murine infection models, we demonstrate that T cell populations with lower koff rates confer significantly better protection than populations with fast koff rates. These data indicate that koff rate measurements can improve the predictability of adoptive immunotherapy and provide diagnostic information on the in vivo quality of T cells.

Alloreactivity

The alloimmune response between individuals genetically disparate for antigens encoded within the major histocompatibility complex (MHC) remains a substantial barrier to transplantation of solid organs, tissues, and hematopoietic stem cells. Alloreactivity has been an immunological paradox because of its apparent contradiction to the requirement of MHC restriction for the induction of normal T lymphocyte mediated immune responses. Through crystallographic analyses and experimental systems utilizing murine CD8(+) cytolytic T cell clones, major advances have been achieved in understanding the molecular and structural basis of T cell receptor recognition of MHC-peptide complexes and the basis of T cell mediated alloreactivity. These studies have further provided an explanation for the relatively high frequencies of alloreactive T cells compared to the frequencies of T cells for microbial derived antigens.

Long-term persistence of CD4(+) but rapid disappearance of CD8(+) T cells expressing an MHC class I-restricted TCR of nanomolar affinity

Most T cells have T cell receptors (TCR) of micromolar affinity for peptide-major histocompatibility complex (MHC) ligands, but genetic engineering can generate TCRs of nanomolar affinity. The affinity of the TCR used, m33, for its cognate non-self peptide-MHC-I complex (SIYRYYGL-K(b)) is 1,000-fold higher than of the wild-type TCR 2C. The affinity of m33 for the self-peptide dEV-8 on K(b) is only twofold higher. Mouse CD8(+) T cells transduced with an m33-encoding retrovirus showed binding of SIY-K(b) and potent function in vitro, but in vivo these T cells disappeared within hours after transfer into syngeneic hosts without causing graft-versus-host disease (GVHD). Accordingly, in cases where such CD8-dependent self-reactivity might occur in human adoptive T cell therapies, our results show that a peripheral T-cell deletion mechanism could operate to avoid reactions with the host. In contrast to CD8(+) T cells, we show that CD4(+) T cells expressing m33 survived for months in vivo. Furthermore, the m33-transduced CD4(+) T cells were able to mediate antigen-specific rejection of 6-day-old tumors. Together, we show that CD8(+) T cell expressing a MHC class I-restricted high-affinity TCR were rapidly deleted whereas CD4(+) T cells expressing the same TCR survived and provided function while being directed against a class I-restricted antigen.

The effect of mutations on the alloreactive T cell receptor/peptide-MHC interface structure: a molecular dynamics study

T cells orchestrate adaptive, pathogen-specific immune responses. T cells have a surface receptor (called TCR) whose ligands are complexes (pMHCs) of peptides (derived from pathogens or host proteins) and major histocompatibility complex proteins (MHCs). MHC proteins vary between hosts. During organ transplants, host TCRs interact with peptides present in complex with genetically different MHCs. This usually causes a vigorous immune response: alloreactivity. Studies of alloreactive protein interactions have yielded results that present a puzzle. Some crystallographic studies concluded that the alloreactive TCR/MHC interface is essentially unaffected by changing the TCR peptide-binding region, suggesting that the peptide does not influence the interface. Another biochemical study concluded from mutation data that different peptides can alter the binding interface with the same TCR. To explore the origin of this puzzle, we used molecular dynamics simulations to study the dependence of the TCR/pMHC interface on changes in both the peptide and the TCR. Our simulations show that the footprint of the TCR on the pMHC is insensitive to mutations of the TCR peptide-binding loops, but peptide mutations can make multiple local changes to TCR/pMHC contacts. Therefore, our results demonstrate that the structural and mutation data do not conflict and reveal how subtle, but important, characteristics of the alloreactive TCR/pMHC interface are influenced by the TCR and the peptide.

Opposite effects of endogenous peptide-MHC class I on T cell activity in the presence and absence of CD8

Nonstimulatory or endogenous peptide-MHC (pepMHC) presented on the surfaces of APCs, either alone or alongside agonist pepMHC, plays various roles in T cell selection and activation. To examine these properties in more detail, we explored several model systems of TCR and pepMHC ligands with sufficient affinity to be activated in the absence of CD8. The TCRs had a range of affinities for agonist and nonstimulatory ligands and were restricted by MHC class I alleles with different properties. We observed CD8-independent antagonism from TCR-pepMHC interactions with very low affinities (e.g., K(D) = 300 μM). In addition, endogenous peptide-L(d) complexes on APCs antagonized activation of coreceptor (CD8)-negative 2C T cells even by the strong agonist QL9-L(d). In contrast, TCRs m33 and 3D-PYY, restricted by K(b) and D(b), respectively, did not show signs of antagonism by endogenous pepMHC in the absence of CD8. This did not appear to be an inherent difference in the ability of the TCRs to be antagonized, as altered peptide ligands could antagonize each TCR. In the presence of CD8, endogenous pepMHC ligands acted in some cases as coagonists. These results show that endogenous pepMHC molecules exhibit complex behavior in T cells, leading to either reduced activity (e.g., in cases of low coreceptor levels) or enhanced activity (e.g., in presence of coreceptor). The behavior may be influenced by the ability of different TCRs to recognize endogenous pepMHC but also perhaps by the inherent properties of the presenting MHC allele.

Evolving concepts of specificity in immune reactions

Our goal is to provide a perspective on current understanding of the origins of specificity in immune reactions, a topic that has intrigued scientists for over a century. A fundamental property of adaptive immune responses is the ability to discriminate among an immense variety of substances by means of antibodies (Abs) and Ab-like receptors on T lymphocytes [T-cell receptors (TCRs)], each able to bind a particular chemical structure [the antigen (Ag)] and not, or only weakly, similar alternatives. Evidence has long existed, however, and has grown, especially recently, that while exhibiting remarkable specificity, many individual Abs and TCRs can also bind a variety of very different ligands. How can Ag recognition by these receptors exercise the great specificity for which they are renowned and yet react with a variety of different ligands (degeneracy)? We critically consider the mechanistic bases for this specificity/degeneracy enigma and also compare and contrast Ag recognition by Abs and TCRs.

Effector function-deficient memory CD8+ T cells clonally expand in the liver and give rise to peripheral memory CD8+ T cells

Upon adoptive transfer into histocompatible mice, naive CD8(+) T cells stimulated ex vivo by TCR+IL-4 turn into long-lived functional memory cells. The liver contains a large number of so formed memory CD8(+) T cells, referred to as liver memory T cells (T(lm)) in the form of cell clusters. The CD62L(low) expression and nonlymphoid tissue distribution of T(lm) cells are similar to effector memory (T(em)) cells, yet their deficient cytotoxicity and IFN-γ inducibility are unlike T(em) cells. Adoptive transfer of admixtures of TCR+IL-4-activated Vβ8(+) and Vβ5(+) CD8(+) T cells into congenic hosts reveals T(lm) clusters that are composed of all Vβ5(+) or Vβ8(+), not mixed Vβ5(+)/Vβ8(+) cells, indicating that T(lm) clusters are formed by clonal expansion. Clonally expanded CD8(+) T cell clusters are also seen in the liver of Listeria monocytogenes-immune mice. T(lm) clusters closely associate with hepatic stellate cells and their formation is IL-15/IL-15R-dependent. CD62L(low) T(LM) cells can home to the liver and secondary lymphoid tissues, remain CD62L(low), or acquire central memory (T(cm))-characteristic CD62L(hi) expression. Our findings show the liver as a major site of CD8(+) memory T cell growth and that T(lm) cells contribute to the pool of peripheral memory cells. These previously unappreciated T(lm) characteristics indicate the inadequacy of the current T(em)/T(cm) classification scheme and help ongoing efforts aimed at establishing a unifying memory T cell development pathway. Lastly, our finding of T(lm) clusters suggests caution against interpreting focal lymphocyte infiltration in clinical settings as pathology and not normal physiology.

Transnuclear mice with predefined T cell receptor specificities against Toxoplasma gondii obtained via SCNT

Mice that are transgenic for rearranged antigen-specific T cell receptors (TCRs) are essential tools to study T cell development and function. Such TCRs are usually isolated from the relevant T cells after long-term culture, often after repeated antigen stimulation, which unavoidably skews the T cell population used. Random genomic integration of the TCR alpha and beta chain and expression from nonendogenous promoters represent additional drawbacks of transgenics. Using epigenetic reprogramming via somatic cell nuclear transfer, we demonstrated that T cells with predefined specificities against Toxoplasma gondii can be used to generate mouse models that express the TCR from their endogenous loci, without experimentally introduced genetic modification. The relative ease and speed with which such transnuclear models can be obtained holds promise for the construction of other disease models.

Engineering the binding properties of the T cell receptor:peptide:MHC ternary complex that governs T cell activity

The potency of a T cell is determined in large part by two interactions, binding of a cognate peptide to the MHC, and binding of the T cell receptor (TCR) to this pepMHC. Various studies have attempted to assess the relative importance of these interactions, and to correlate the corresponding binding parameters with the level of T cell activity mediated by the peptide. To further examine the properties that govern optimal T cell activity, here we engineered both the peptide:MHC interaction and the TCR:pepMHC interaction to generate improved T cell activity. Using a system involving the 2C TCR and its allogeneic pepMHC ligand, QL9-L(d), we show that a peptide substitution of QL9 (F5R), increased the affinity and stability of the pep-L(d) complex (e.g. cell surface t(1/2)-values of 13 min for QL9-L(d) versus 87 min for F5R-L(d)). However, activity of peptide F5R for 2C T cells was not enhanced because the 2C TCR bound with very low affinity to F5R-L(d) compared to QL9-L(d) (K(D)=300 microM and K(D)=1.6 microM, respectively). To improve the affinity, yeast display of the 2C TCR was used to engineer two mutant TCRs that exhibited higher affinity for F5R-L(d) (K(D)=1.2 and 6.3 microM). T cells that expressed these higher affinity TCRs were stimulated by F5R-L(d) in the absence of CD8, and the highest affinity TCR exhibited enhanced activity for F5R compared to QL9. The results provide a guide to designing the explicit binding parameters that govern optimal T cell activities.

Different strategies adopted by K(b) and L(d) to generate T cell specificity directed against their respective bound peptides

Mouse T cell clone 2C recognizes two different major histocompatibility (MHC) ligands, the self MHC K(b) and the allogeneic MHC L(d). Two distinct peptides, SIY (SIYRYYGL) and QL9 (QLSPFPFDL), act as strong and specific agonists when bound to K(b) and L(d), respectively. To explore further the mechanisms involved in peptide potency and specificity, here we examined a collection of single amino acid peptide variants of SIY and QL9 for 1) T cell activity, 2) binding to their respective MHC, and 3) binding to the 2C T cell receptor (TCR) and high affinity TCR mutants. Characterization of SIY binding to MHC K(b) revealed significant effects of three SIY residues that were clearly embedded within the K(b) molecule. In contrast, QL9 binding to MHC L(d) was influenced by the majority of peptide side chains, distributed across the entire length of the peptide. Binding of the SIY-K(b) complex to the TCR involved three SIY residues that were pointed toward the TCR, whereas again the majority of QL9 residues influenced binding of TCRs, and thus the QL9 residues had impacts on both L(d) and TCR binding. In general, the magnitude of T cell activity mediated by a peptide variant was influenced more by peptide binding to MHC than by binding the TCR, especially for higher affinity TCRs. Findings with both systems, but QL9-L(d) in particular, suggest that many single-residue substitutions, introduced into peptides to improve their binding to MHC and thus their vaccine potential, could impair T cell reactivity due to their dual impact on TCR binding.

Different thermodynamic binding mechanisms and peptide fine specificities associated with a panel of structurally similar high-affinity T cell receptors

To understand the mechanisms that govern T cell receptor (TCR)-peptide MHC (pMHC) binding and the role that different regions of the TCR play in affinity and antigen specificity, we have studied the TCR from T cell clone 2C. High-affinity mutants of the 2C TCR that bind QL9-L(d) as a strong agonist were generated previously by site-directed mutagenesis of complementarity determining regions (CDRs) 1beta, 2alpha, 3alpha, or 3beta. We performed isothermal titration calorimetry to assess whether they use similar thermodynamic mechanisms to achieve high affinity for QL9-L(d). Four of the five TCRs examined bound to QL9-L(d) in an enthalpically driven, entropically unfavorable manner. In contrast, the high-affinity CDR1beta mutant resembled the wild-type 2C TCR interaction, with favorable entropy. To assess fine specificity, we measured the binding and kinetics of these mutants for both QL9-L(d) and a single amino acid peptide variant of QL9, called QL9-Y5-L(d). While 2C and most of the mutants had equal or higher affinity for the Y5 variant than for QL9, mutant CDR1beta exhibited 8-fold lower affinity for Y5 compared to QL9. To examine possible structural correlates of the thermodynamic and fine specificity signatures of the TCRs, the structure of unliganded QL9-L(d) was solved and compared to structures of the 2C TCR/QL9-L(d) complex and three high-affinity TCR/QL9-L(d) complexes. Our findings show that the QL9-L(d) complex does not undergo major conformational changes upon binding. Thus, subtle changes in individual CDRs account for the diverse thermodynamic and kinetic binding mechanisms and for the different peptide fine specificities.

Distinct CDR3 conformations in TCRs determine the level of cross-reactivity for diverse antigens, but not the docking orientation

T cells are known to cross-react with diverse peptide MHC Ags through their alphabeta TCR. To explore the basis of such cross-reactivity, we examined the 2C TCR that recognizes two structurally distinct ligands, SIY-K(b) and alloantigen QL9-L(d). In this study we characterized the cross-reactivity of several high-affinity 2C TCR variants that contained mutations only in the CDR3alpha loop. Two of the TCR lost their ability to cross-react with the reciprocal ligand (SIY-K(b)), whereas another TCR (m67) maintained reactivity with both ligands. Crystal structures of four of the TCRs in complex with QL9-L(d) showed that CDR1, CDR2, and CDR3beta conformations and docking orientations were remarkably similar. Although the CDR3alpha loop of TCR m67 conferred a 2000-fold higher affinity for SIY-K(b), the TCR maintained the same docking angle on QL9-L(d) as the 2C TCR. Thus, CDR3alpha dictated the affinity and level of cross-reactivity, yet it did so without affecting the conserved docking orientation.

Regulated expression of a tumor-associated antigen reveals multiple levels of T-cell tolerance in a mouse model of lung cancer

Maximizing the potential of cancer immunotherapy requires model systems that closely recapitulate human disease to study T-cell responses to tumor antigens and to test immunotherapeutic strategies. We have created a new system that is compatible with Cre-LoxP-regulatable mouse cancer models in which the SIY antigen is specifically overexpressed in tumors, mimicking clinically relevant TAAs. To show the utility of this system, we have characterized SIY-reactive T cells in the context of lung adenocarcinoma, revealing multiple levels of antigen-specific T-cell tolerance that serve to limit an effective antitumor response. Thymic deletion reduced the number of SIY-reactive T cells present in the animals. When potentially self-reactive T cells in the periphery were activated, they were efficiently eliminated. Inhibition of apoptosis resulted in more persistent self-reactive T cells, but these cells became anergic to antigen stimulation. Finally, in the presence of tumors overexpressing SIY, SIY-specific T cells required a higher level of costimulation to achieve functional activation. This system represents a valuable tool in which to explore sources contributing to T-cell tolerance of cancer and to test therapies aimed at overcoming this tolerance.

Structural features of T cell receptor variable regions that enhance domain stability and enable expression as single-chain ValphaVbeta fragments

The variable (V) domains of antibodies and T cell receptors (TCRs) share sequence homology and striking structural similarity. Single-chain antibody V domain constructs (scFv) are routinely expressed in a variety of heterologous systems, both for production of soluble protein as well as for in vitro engineering. In contrast, single-chain T cell receptor V domain constructs (scTCR) are prone to aggregation and misfolding and are refractory to display on phage or yeast in their wild-type form. However, through random mutagenesis and yeast display engineering, it has been possible to isolate scTCR mutants that are properly folded and displayed on the yeast surface. These displayed mutants can serve not only as a scaffold for further engineering but also as scTCR variants that exhibit favorable biophysical properties in Escherichia coli expression. Thus, a more comprehensive understanding of the V domain mutations that allowed display would be beneficial. Our goal here was to identify generalizable patterns of important mutations that can be applied to different TCRs. We compared five different scTCRs, four from mice and one from a human, for yeast surface display. Analysis of a collection of mutants revealed four distinct regions of TCR V domains that were most important for enabling surface expression: the Valpha-Vbeta interface, the HV4 of Vbeta, and the region of the Valpha and Vbeta domains normally apposed against the constant (C) domains. Consistent with the role of the V-C interface in surface display, reconstitution of this interface, by including the constant domains of each chain, allowed V domain display and alphabeta chain association on the yeast surface, thus providing an alternative TCR scaffold. However, the surface levels of TCR achieved with engineered scTCR mutants were superior to that of the ValphaCalpha/VbetaCbeta constructs. Therefore, we describe further optimization of the current strategy for surface display of the single-chain format in order to facilitate yeast display engineering of a broader range of scTCRs.

How a single T cell receptor recognizes both self and foreign MHC

alphabeta T cell receptors (TCRs) can crossreact with both self- and foreign- major histocompatibility complex (MHC) proteins in an enigmatic phenomenon termed alloreactivity. Here we present the 2.35 A structure of the 2C TCR complexed with its foreign ligand H-2L(d)-QL9. Surprisingly, we find that this TCR utilizes a different strategy to engage the foreign pMHC in comparison to the manner in which it recognizes a self ligand H-2K(b)-dEV8. 2C engages both shared and polymorphic residues on L(d) and K(b), as well as the unrelated QL9 and dEV8 peptide antigens, in unique pair-wise contacts, resulting in greater structural complementarity with the L(d)-QL9 complex. In the structure of an engineered, high-affinity 2C TCR variant bound to H-2L(d)-QL9, the "wild-type" TCR-MHC binding orientation persists despite modified TCR-CDR3alpha interactions with peptide. Thus, a single TCR recognizes two globally similar, but distinct ligands by divergent mechanisms, indicating that receptor-ligand crossreactivity can occur in the absence of molecular mimicry.

Stage-dependent reactivity of thymocytes to self-peptide--MHC complexes

In mice that express a transgene for the 2C T cell antigen-receptor (TCR) and lack a recombinase-activating gene (2C(+)RAG(-/-) mice) most of the peripheral T cells are CD8(+), a few are CD4(+), and a significant fraction are CD4(-)CD8(-) [double negative (DN)]. The DN 2C cells, like DN T cells that are abundant in various other alphabeta TCR-transgenic mice, appear to be derived directly from DN thymocytes that prematurely express the TCR transgene. The DN 2C cells are virtually absent in mice deficient in major histocompatibility complex class II (MHC-II) but more abundant in mice deficient in MHC-I, suggesting that the DN 2C thymocytes are positively selected by self-peptide-MHC-II (pMHC-II) complexes and negatively selected by self-pMHC-I complexes. The pMHC-I complexes, however, positively select CD8(+) 2C T cells in the same mice. The different effects of thymic pMHC-I on DN and CD8(+) thymocytes are consistent with the finding that DN 2C thymocytes are more sensitive than more mature CD4(+)CD8(+) [double positive (DP)] thymocytes to a weak pMHC-I agonist for the 2C TCR. Together with previous evidence that DP thymocytes respond more sensitively than T cells in the periphery to weak pMHC agonists, the findings suggest progressive decreases in responsiveness to self-pMHC-I complexes as thymocytes develop from DN to DP thymocytes and then to mature naïve T cells in the periphery.

An essential role for IFN-gamma in regulation of alloreactive CD8 T cells following allogeneic hematopoietic cell transplantation

We previously found that CD8 T cells from IFN-gamma gene knockout (GKO) donors induce more severe lethal GVHD compared with CD8 T cells from wild-type (WT) donors in fully MHC-mismatched strain combinations. In this study, we investigated the mechanisms by which IFN-gamma inhibits GVHD in a parent --> F1 (B6 --> B6D2F1) allogeneic HCT (allo-HCT) model. IFN-gamma was strongly protective against GVHD in this parent --> F1 haplotype-mismatched allo-HCT model. Irradiated B6D2F1 mice that received GKO B6 CD4-depleted splenocytes developed lethal GVHD with severe lung and liver injury, whereas those receiving a similar cell population from WT B6 donors survived long term. Donor CD8 cells showed rapid activation, accelerated cell division, and reduced/delayed activation-induced cell death in allogeneic recipients in which donor cells were incapable of producing IFN-gamma. In consequence, the numbers of activated/effector (ie, CD25+, CD62L-, and CD44(high)) donor CD8 T cells in the recipients of GKO allo-HCT significantly exceeded those in mice receiving WT allo-HCT. These data show that IFN-gamma negatively regulates the CD8 T cell response by inhibiting cell division and promoting cell death and suggest that blockade of IFN-gamma could augment the severity of GVHD in patients undergoing allo-HCT.

Development of CD4+ T cells expressing a nominally MHC class I-restricted T cell receptor by two different mechanisms

Differences in T cell receptor (TCR) signaling initiated by interactions among TCRs, coreceptors, and self-peptide-MHC complexes determine the outcome of CD4 versus CD8 lineage of T cell differentiation. The H-2Ld and Kbm3 alloreactive 2C TCR is positively selected by MHC class I Kb and a yet-to-be identified nonclassical class I molecule to differentiate into CD8+ T cells. Here we describe two mechanisms by which CD4+ 2C T cells can be generated in 2C TCR-transgenic mice. In the RAG-/- background, development of CD4+ 2C T cells requires the expression of both I-Ab and the TAP genes, indicating that both MHC class I and II molecules are required for positive selection of these T cells. Notably, only some of the 2C+ RAG-/- mice (approximately 30%) develop CD4+ 2C T cells, with frequencies in individual mice varying from 0.5% to as high as approximately 50%. In the RAG+ background, where endogenous TCRalpha genes are rearranged and expressed, CD4+ 2C T cells are generated because these cells express the 2C TCR as well as additional TCRs, consisting of the 2C TCRbeta and endogenous TCRalpha chains. Similarly, T cells expressing the OT-1 TCR, which is nominally MHC class I-restricted, can also develop into CD4+ T cells through the same two mechanisms. Thus, expression of two TCRs by a single thymocyte, TCR recognition of multiple MHC molecules, and heterogeneity of TCR, coreceptors, and peptide-MHC interactions in the thymus all contribute to the outcome of CD4 versus CD8 lineage development.

Mechanistic insights into achievement of cardiac allograft long-term survival by treatment with immature dendritic cells and sub-dose sirolimus

BACKGROUND

Administration of immature dendritic cells (DC) prolongs but does not result in indefinite allograft survival. We attempted to achieve this goal by adding a sub-therapeutic dose of immunosuppression.

METHODS

DC propagated from B10 (H-2(b)) mouse bone marrow (BM) were transfected with nuclear factor-kappaB (NF-kappaB)-binding-site-specific oligodeoxyribonucleotide (ODN). The allostimulatory activity of transfected and normal DC were examined in mixed-lymphocyte reaction (MLR) and cytotoxic T-lymphocyte (CTL) assays in vitro, and their influence on allograft survival by systemic administration of DC in vivo.

RESULTS

Transfection of DC with NF-kappaB ODN resulted in complete abrogation of NF-kappaB activity and inhibition of co-stimulation. Allogeneic (C3H, H-2(k)) T cells stimulated by ODN DC demonstrated impairment in MLR and CTL activity. Administration of ODN DC significantly prolonged B10 allograft survival. In contrast to cyclosporine, which failed to enhance the effect of ODN DC, a combination of ODN DC with sirolimus at 6 mg/kg/day for 6 days achieved long-term survival in all allografts. This was associated with low CTL activity of either graft-infiltrating cells or splenic T cells and increased TUNEL-positive cells in T-cell areas of recipient mesenteric lymph nodes. Analysis of transcription factor nuclear translocation with Cellomics indicated that stimulation with ODN DC showed inhibited T-cell nuclear translocation of signal transducer and activator of transcription (Stat)1 and Stat3, extracellular signal-related kinase (ERK) and activating transcription factor (ATF)-2, but not NF-kappaB and P38, compared with mature DC. The selective inhibition was enhanced by sirolimus, but not cyclosporine.

CONCLUSIONS

Sirolimus enhances immature DC tolerogenicity by induction of T-cell apoptosis, and promotes immature DC-induced inhibition of Stat1, ERK and ATF-2 activation.

Rapamycin Prolongs Susceptibility of Responding T Cells to Tolerance Induction by CD8+ Veto Cells

BACKGROUND

CD8 veto cells are an antigen-specific immunoregulatory cell type that induces tolerance by causing apoptosis in T cells that recognize antigens on the veto cell. However, responding T cells are only susceptible to veto based deletion for a 48-hour window, which represents a severe limitation to the use of veto cells as a cellular therapeutic. Several immunosuppresant drugs inhibit T cell differentiation at distinct points. We hypothesized that immunosuppresants, which arrest T cell differentiation, would maintain responding T cells in a vetoable state.

METHODS

CD8 veto cells were generated from BALB/c (H-2) mice. The 2C transgenic mouse, which expresses a T cell receptor specific for L was used to visualize responding cells. Veto activity was measured by both visualizing apoptosis of the responding 2C T cells and by measuring a decrease in lysis of H-2 targets. Several immunosuppressant drugs were tested for their effect on veto cell activity.

RESULTS

BALB/c veto cells exhibited a potent and antigen-specific deletion of 2C responder cells, but not mature 2C effectors. The addition of rapamycin significantly extended the window of opportunity to veto based deletion by preventing the responding 2C T cells from differentiating out of a vetoable state.

CONCLUSION

These findings demonstrate that by utilizing rapamycin, the window of opportunity for veto-based induction of tolerance to transplantation antigens is significantly extended. This approach circumvents a serious obstacle in the development of veto cells as an efficacious cellular therapy to induce allospecific tolerance to transplantation antigens.

Signal 3 tolerant CD8 T cells degranulate in response to antigen but lack granzyme B to mediate cytolysis

Naive CD8 T cells that respond in vivo to Ag and costimulation in the absence of a third signal, such as IL-12, fail to develop cytolytic function and become tolerized. We show in this study that CD8 T cells purified from TCR transgenic mice and stimulated in vitro in the presence or absence of IL-12 form conjugates with specific target cells, increase intracellular Ca2+, and undergo degranulation to comparable extents. Perforin is also expressed at comparable levels in the absence or presence of a third signal, but expression of granzyme B depends upon IL-12. Levels of granzyme B also correlate strongly with the cytolytic activity of cells responding in vivo. In contrast, an increase in CD107a (lysosomal-associated membrane protein 1) expression resulting from degranulation cannot distinguish in vivo generated lytic effector cells from tolerized, noncytolytic cells. Thus, it appears that cells rendered tolerant as a result of stimulation in the absence of a third signal fail to lyse target cells because they are "shooting blanks."

Critical Role for CD103+CD8+ Effectors in Promoting Tubular Injury following Allogeneic Renal Transplantation

Immune destruction of the graft renal tubules is an important barrier to the long-term function of clinical renal allografts, but the underlying mechanisms remain obscure. CD103-an integrin conferring specificity for the epithelial cell-restricted ligand, E-cadherin-defines a subset of CD8 effectors that infiltrate the graft tubular epithelium during clinical rejection episodes, predicting a causal role for CD103+CD8+ effectors in tubular injury. In the present study, we used rodent transplant models to directly test this hypothesis. Surprisingly, CD8 cells infiltrating renal allografts undergoing unmodified acute rejection did not express significant levels of CD103. However, we demonstrate that a brief course of cyclosporine A to rat renal allograft recipients promotes progressive accumulation of CD103+CD8+ cells within the graft, concomitant with the development of tubular atrophy and interstitial fibrosis. As in the known clinical scenario, graft-associated CD103+CD8+ cells exhibited a T effector phenotype and were intimately associated with the renal tubular epithelium. Treatment with anti-CD103 mAb dramatically attenuated CD8 infiltration into the renal tubules and tubular injury. Mouse studies documented that CD103 expression is required for efficient destruction of the graft renal tubules by CD8 effectors directed to donor MHC I alloantigens. Taken together, these data document a causal role for CD103+CD8+ effectors in promoting tubular injury following allogeneic renal transplantation and identify novel targets for therapeutic intervention in this important clinical problem.

TGF-{beta}-dependent CD103 expression by CD8(+) T cells promotes selective destruction of the host intestinal epithelium during graft-versus-host disease

Destruction of the host intestinal epithelium by donor effector T cell populations is a hallmark of graft-versus-host disease (GVHD), but the underlying mechanisms remain obscure. We demonstrate that CD8⁺ T cells expressing CD103, an integrin conferring specificity for the epithelial ligand E-cadherin, play a critical role in this process. A TCR transgenic GVHD model was used to demonstrate that CD103 is selectively expressed by host-specific CD8⁺ T cell effector populations (CD8 effectors) that accumulate in the host intestinal epithelium during GVHD. Although host-specific CD8 effectors infiltrated a wide range of host compartments, only those infiltrating the intestinal epithelium expressed CD103. Host-specific CD8 effectors expressing a TGF-β dominant negative type II receptor were defective in CD103 expression on entry into the intestinal epithelium, which indicates local TGF-β activity as a critical regulating factor. Host-specific CD8 effectors deficient in CD103 expression successfully migrated into the host intestinal epithelium but were retained at this site much less efficiently than wild-type host-specific CD8 effectors. The relevance of these events to GVHD pathogenesis is supported by the finding that CD103-deficient CD8⁺ T cells were strikingly defective in transferring intestinal GVHD pathology and mortality. Collectively, these data document a pivotal role for TGF-β–dependent CD103 expression in dictating the gut tropism, and hence the destructive potential, of CD8⁺ T cells during GVHD pathogenesis.

A novel model of allograft rejection: immune reconstitution of Rag-1 recipients with 2C transgenic T-cell receptor lymphocytes

The complexity of allorejection (cell activation, homing, and effector function) makes detailed studies difficult. We have developed a model of allograft rejection using purified monoclonal alloreactive effector cells. Immunodeficient C57Bl/6-Rag-1 (H-2(b)) recipients of Balb/c (H-2(d)) islet or skin grafts were reconstituted via adoptive transfer of splenocytes from 2C transgenic mice containing CD8+ cytotoxic effector cells directed against L(d). Recipients were assessed for engraftment, activation and homing of effector cells, and ability to reject grafts. Both unpurified 2C splenocytes and purified 2C/CD8+ cells durably reconstitute immunodeficient mice. Naive 2C effector cells reject skin grafts, but not islet allografts. However, when effector cells are primed prior to reconstitution, islet allografts are rejected. Using this model, blockade of adhesion molecules LFA-1 and alpha4-integrin delayed infiltration of islet allografts and prolonged allograft survival. This model of allorejection may be useful to study the activation and homing of allospecific cells in vivo.

Intrinsic versus environmental influences on T-cell responses in aging

A decline in T-cell responses and a switch to memory T-cell predominance occur with aging. We have used the T-cell receptor (TCR) transgenic mouse model to study age-associated changes in T-cell responses that are a consequence of shifts in subset representation versus changes intrinsic to T cells versus changes in the 'aged' microenvironment. We found that naive transgene-expressing (Tg(+)) CD4(+) T cells from aged mice respond to antigen with reduced interleukin-2 (IL-2) production, decreased cell expansion, and limited differentiation to effectors. Comparable to the characteristic accumulation of memory phenotype T cells in aged humans and conventional rodents, Tg(+) CD4(+) T cells from old OTII and 6.5 TCR transgenic mice acquire a memory phenotype without immunization and become hyporesponsive. The naive Tg(+) CD8(+) T cells from aged 2C mice expressed activation markers, produced IL-2, proliferated, and differentiated into cytotoxic T lymphocytes as efficiently as their young counterparts. Responses by adoptive transferred Tg(+) cells from young mice, immunized in young and old conventional hosts, indicated that the host age influences the onset of cell division, level of cell expansion, and number of cytokine-producing cells. Co-transfer of dendritic cells (DCs) from young and less so from aged conventional mice partially restored responses. Furthermore, DCs and T-cell migration to draining lymphoid organs was reduced due to deficiencies intrinsic to aged cells and the aged environment. Thus, alterations in T-cell responses in aging are attributable to intrinsic and environmental influences.

Regulation of CD103 expression by CD8+ T cells responding to renal allografts

CD103 is an integrin with specificity for the epithelial cell-specific ligand, E-cadherin. Recent studies indicate that CD103 expression endows peripheral CD8 cells with a unique capacity to access the epithelial compartments of organ allografts. In the present study we used a nonvascularized mouse renal allograft model to 1) define the mechanisms regulating CD103 expression by graft-infiltrating CD8 effector populations, and 2) identify the cellular compartments in which this occurs. We report that CD8 cells responding to donor alloantigens in host lymphoid compartments do not initially express CD103, but dramatically up-regulate CD103 expression to high levels subsequent to migration to the graft site. CD103+CD8+ cells that infiltrated renal allografts exhibited a classic effector phenotype and were selectively localized to the graft site. CD8 cells expressing low levels of CD103 were also present in lymphoid compartments, but three-color analyses revealed that these are almost exclusively of naive phenotype. Adoptive transfer studies using TCR-transgenic CD8 cells demonstrated that donor-specific CD8 cells rapidly and uniformly up-regulate CD103 expression following entry into the graft site. Donor-specific CD8 cells expressing a dominant negative TGF-beta receptor were highly deficient in CD103 expression following migration to the graft, thereby implicating TGF-beta activity as a dominant controlling factor. The relevance of these data to conventional (vascularized) renal transplantation is confirmed. These data support a model in which TGF-beta activity present locally at the graft site plays a critical role in regulating CD103 expression, and hence the epitheliotropism, of CD8 effector populations that infiltrate renal allografts.

The Ig-Like Domain of Tapasin Influences Intermolecular Interactions

Presentation of antigenic peptides to T lymphocytes by MHC class I molecules is regulated by events involving multiple endoplasmic reticulum proteins, including tapasin. By studying the effects of substitutions in the tapasin Ig-like domain, we demonstrated that H-2L(d)/tapasin association can be segregated from reconstitution of folded L(d) surface expression. This finding suggests that peptide acquisition by L(d) is influenced by tapasin functions that are independent of L(d) binding. We also found that the presence of a nine-amino acid region in the Ig-like domain of mouse or human tapasin is required for association with L(d), and certain point substitutions in this sequence abrogate human, but not mouse, tapasin association with L(d). These data are consistent with a higher overall affinity between L(d) and mouse tapasin compared with human tapasin. In addition, we found that other point mutations in the same region of the tapasin Ig-like domain affect MHC class I surface expression and Ag presentation. Finally, we showed that the cysteine residues in the Ig-like domain of tapasin influence tapasin's stability, its interaction with the MHC class I H chain, and its stabilization of TAP. Mutagenesis of these cysteines decreases tapasin's electrophoretic mobility, suggesting that these residues form an intramolecular disulfide bond. Taken together, these results reveal a critical role for the tapasin Ig-like domain in tapasin function.

A Peptide That Antagonizes TCR-Mediated Reactions with Both Syngeneic and Allogeneic Agonists: Functional and Structural Aspects

We identify and consider some characteristics of a peptide antagonist for the Ag-specific receptor on 2C cells (the 2C TCR). The peptide, GNYSFYAL (called GNY), binds to H-2K(b), and a very high-resolution crystal structure of the GNY-K(b) complex at 1.35 A is described. Although the GNY peptide does not bind to L(d), the potency of GNY-K(b) as an antagonist is evident from its ability to specifically inhibit 2C TCR-mediated reactions to an allogenic agonist complex (QLSPFPFDL-L(d)), as well as to a syngeneic agonist complex (SIYRYYGL-K(b)). The crystal structure and the activities of alanine-substituted peptide variants point to the properties of the peptide P4 side chain and the conformation of the Tyr-P6 side chain as the structural determinants of GNYSFYAL antagonist activity.

Peripheral tolerance in transgenic mice expressing class I MHC L(d) only on cardiac cells

BACKGROUND

The fate of autoreactive T cells exposed to extrathymic self-antigen is examined in a double transgenic (DTG) mouse [(L(d+) cardiacx2C)F1], where cardiac myocytes alone express L(d) and T cells express an antigen receptor (2C TCR) against L(d).

METHODS

Naïve cardiac L(d+) single transgenic (STG) mice (before breeding with 2C) and DTG mice were examined for evidence of autoimmunity. The L(d+) STG hearts were then transplanted to syngeneic L(d-) wild type C57BL/6 to evaluate the heart's immunogenicity. L(d+) skin grafts were transplanted to non-transgenic B6, transgenic 2C, STG, and DTG mice. Phenotype analysis of peripheral 1B2+(identifies 2C T cells), CD4+, and CD8+ T cells was performed by FACS. In vitro MLC and CTL, with and without the addition of IL-2 and suppressor cell assays, were evaluated.

RESULTS

Neither STG nor DTG hearts developed any evidence of autoimmunity by histology. In contrast, B6 mice rejected the L(d+) STG heart in 17+/-9.7 days (P<0.01), while a syngeneic B6 heart transplant was accepted indefinitely. Survival of L(d+) skin grafts was prolonged in both STG and DTG mice. FACS quantitation revealed that while there was no deletion of peripheral 2C cells in the DTG, these 2C T cells did have a significantly reduced proliferative and cytotoxic response to H-2L(d). Restoration of the proliferative and cytotoxic response of the DTG cells by the addition of IL-2 was consistent with a state of anergy.

CONCLUSIONS

These findings suggest that the expression of extrathymic class I MHC expression alone did not trigger autoimmune reactions but that the T cells can be rendered anergic to the specific 'self' antigen.

Direct stimulation of naive T cells by membrane vesicles from antigen-presenting cells: distinct roles for CD54 and B7 molecules

T cell stimulation usually requires direct contact with viable antigen-presenting cells (APCs). However, we show here that small exosome-like membrane vesicles shed from APCs can be recognized by naïve CD8+ T cells in the absence of viable APCs. T cell antigen receptor-dependent binding of vesicles by CD8+ cells is MHC class I/peptide-specific and requires that the vesicles coexpress intercellular adhesion molecule 1 (ICAM-1, CD54), although not B7 (B7-1). In the absence of B7, T cell binding of vesicles is nonimmunogenic. By contrast, vesicles expressing both ICAM-1 and B7 are strongly immunogenic and cause purified APC-depleted CD8+ cells to mount peptide-specific proliferative responses and differentiate into effector cells.

Homology between an alloantigen and a self MHC allele calibrates the avidity of the alloreactive T cell repertoire independent of TCR affinity

The self-restricted T cell repertoire exhibits a high frequency of alloreactivity. Because these alloreactive T cells are derived from the pool of cells selected on several different self MHC alleles, it is unknown how development of the alloantigenic repertoire is influenced by homology between a self MHC allele and an alloantigen. To address this, we used the 2C transgenic TCR that is selected by K(b), is alloreactive for L(d), and cross-reacts with L(q). L(q) is highly homologous to L(d) and binds several of the same peptide ligands, including p2Ca, the peptide recognized by 2C. We find that L(d)/p2Ca is a high avidity agonist ligand, whereas L(q)/p2Ca is a low avidity agonist ligand for 2C T cells. When mice transgenic for the 2C TCR are bred to L(q)-expressing mice, 2C(+) T cells develop; however, they express lower levels of either the 2C TCR or CD8 and require a higher L(d)/p2Ca ligand density to be activated than 2C(+) T cells selected by K(b). Furthermore, the 2C T cells selected in the presence of L(q) fail to detect L(q)/p2Ca complexes even at high ligand density. Thus, despite possessing the identical TCR, there is a functional avidity difference between 2C(+) T cells selected in the presence of L(q) vs K(b). These data provide evidence that homology between the selecting ligand and an alloantigen can influence the avidity of the T cell repertoire for the alloantigen, and suggest that thymic selection can fine tune T cell avidity independent of intrinsic TCR affinity.

The paradox of immune molecular recognition of alpha-galactosylceramide: low affinity, low specificity for CD1d, high affinity for alpha beta TCRs

CD1 resembles both class I and class II MHC but differs by the important aspect of presenting lipid/glycolipids, instead of peptides, to T cells. Biophysical studies of lipid/CD1 interactions have been limited, and kinetics of binding are in contradiction with functional studies. We have revisited this issue by designing new assays to examine the loading of CD1 with lipids. As expected for hydrophobic interactions, binding affinity was not high and had limited specificity. Lipid critical micelle concentration set the limitation to these studies. Once loaded onto CD1d, the recognition of glycolipids by alphabeta T cell receptor was studied by surface plasmon resonance using soluble Valpha14-Vbeta8.2 T cell receptors. The Valpha14 Jalpha18 chain could be paired with NK1.1 cell-derived Vbeta chain, or any Vbeta8 chain, to achieve high affinity recognition of alpha-galactosylceramide. Biophysical analysis indicated little effect of temperature or ionic strength on the binding interaction, in contrast to what has been seen in peptide/MHC-TCR studies. This suggests that there is less accommodation made by this TCR in recognizing alpha-galactosylceramide, and it can be assumed that the most rigid part of the Ag, the sugar moiety, is critical in the interaction.

A model T-cell receptor system for studying memory T-cell development

When T-cell clones were first grown in long-term cell culture, each clone was considered to be capable of displaying a limited range of functional activities, constrained by the clones' coreceptor, CD4 or CD8, and the specificity of its antigen-specific receptor (TCR) for one or a few peptides in association with a class I or class II MHC molecule. Subsequent studies, especially with transgenic mice, have shown, however, that T cells expressing the same receptor can be obtained in a variety of differentiated states, including naïve cells, activated effector cells, memory cells, and anergic or tolerant cells, as well as cells with or without a coreceptor. In each of these states T cells can display distinctly different responses to the peptide-MHC (pepMHC) complexes the TCR recognizes. Recently, memory T cells have received considerable attention, in part because of the likelihood that they confer long-term protective immunity against diverse infectious agents and possibly against some forms of cancer. Here we review some recent studies that our colleagues and we have carried out on memory CD8(+) T cells. These studies have made extensive use of cells that express the TCR called 2C. The diverse set of cells expressing the 2C TCR, in mice and in cultured clones and cell lines, are referred to as the 2C system. Before reviewing the studies of memory cells, we summarize the 2C system's main features, including evidence that a large and diverse array of pepMHC complexes, involving at least four class I MHC proteins, can stimulate TCR-mediated responses of 2C cells.

A yeast display system for engineering functional peptide-MHC complexes

In a cellular immune response, antigenic peptides derived by intracellular processing of foreign pathogens are bound to the class I major histocompatability complex (MHC I) and presented to CD8(+) cytotoxic T cells. Although the crystal structures of several different MHC products have been solved, many MHC molecules, including some associated with diseases, have not been amenable to biochemical and structural studies. The variability in this success is based largely on the fact that peptide-MHC complexes vary extensively in their stability. These properties also are intimately tied to the biological activity of the complexes. The ability to apply the techniques of directed evolution to this system in order to engineer stable complexes has been complicated by the trimeric structure of peptide-MHC complexes, requiring association of three polypeptides: the heavy chain, beta2-microglubulin (beta2m), and a short peptide. We show here that single-chain forms of peptide-MHC complexes can be expressed as Aga-2 fusions on the surface of yeast. Three different complexes, SIYRYYGL-K(b)-beta2m (SIYR-K(b)), EQYKFYSV-K(b)-beta2m (dEV8-K(b)), and SIINFEKL-K(b)-beta2m (OVA-K(b)), were expressed on yeast and detected by flow cytometry with a conformation-specific anti-K(b) antibody (B.8.24.3). In addition, yeast displaying K(b) loaded with exogenous SIYR and OVA peptides were recognized by a high-affinity T cell receptor that is specific for SIYR-K(b) and by an antibody (25.D1-16) that is specific for OVA-K(b), respectively. Finally, yeast that display the SIYRYYGL-K(b) also directly stimulated CD69 up-regulation on naive 2C T cells. Hence, yeast display represents a technology that can be used for directed evolution of any of the components of the trimeric pep-MHC complex.

TCRs with high affinity for foreign pMHC show self-reactivity

T cells with high-affinity T cell receptors (TCRs) for a foreign peptide-major histocompatibility complex (pMHC) appear to be negatively selected, even though they have never seen the foreign antigen. To examine how this process operates, we used in vitro yeast display to isolate high-affinity TCRs from the T cell clone 2C. The TCRs showed fast on-rates, which were consistent with reduced CDR (complementarity determining region) flexibility, and cross-reactivity with other cognate pMHCs. T cell hybridomas transfected with a high-affinity TCR were stimulated by endogenous self-pMHC, which suggested that T cells bearing the TCR would be negatively selected. The immune system appears to maintain a repertoire of flexible, low-affinity TCRs at the expense of more effective high-affinity TCRs.

Allogeneic and syngeneic class I MHC complexes drive the association of CD8 and TCR on 2C T cells

In most cases, cytotoxic T cell activation is dependent on the interaction of the T cell receptor (TCR) and CD8 with MHC class I molecules. In the CD8(+) T cell system based on the mouse cytotoxic T cell clone 2C, recognition of the allogeneic MHC L(d) exhibits a less significant role for CD8 than recognition of the syngeneic MHC K(b). Here, we examined whether this difference is related to the relative abilities of the two pepMHC complexes to drive the association of CD8 and TCR on the T cell surface. We show that both the syngeneic and allogeneic pepMHC induced association of CD8 and TCR, as revealed by fluorescence resonance energy transfer (FRET). Thus, the orientation of the syngeneic and allogeneic ligands when bound to the same TCR both allow CD8 to be recruited to the TCR complex. The conserved diagonal orientation of TCRs on different pepMHC ligands may facilitate such associations. The FRET results are consistent with the known binding properties and the CD8 involvement of the two different TCR:pepMHC interactions.

The roles of IL-12 in providing a third signal for clonal expansion of naive CD8 T cells

Stimulation of an effective in vitro or in vivo response by naive CD8 T cells requires three signals: TCR engagement, costimulation/IL-2, and a third signal that can be provided by IL-12. In addition to being required for acquisition of cytolytic function, IL-12 is required for optimal IL-2-dependent proliferation and clonal expansion. In experiments examining in vitro stimulation of naive CD8 T cells, IL-12 is shown to stimulate expression of the IL-2R alpha-chain (CD25) to much higher levels than are reached in response to just TCR and costimulation and/or IL-2. In addition, high CD25 expression is substantially prolonged in the presence of IL-12. As a consequence, the cells proliferate more effectively in response to low levels of IL-2. Examination of adoptively transferred TCR transgenic CD8 T cells responding to peptide Ag confirmed that IL-12 up-regulates CD25 in vivo, even when B7-mediated costimulation is largely blocked. TCR- and IL-2-dependent proliferation of CD8 T cells from mice deficient in CD25 was also found to increase in the presence of IL-12, indicating that CD25 up-regulation is not the only mechanism by which IL-12 increases clonal expansion of the cells. IL-2 and IL-12 both act to increase expression of both CD25 and the IL-12R, thus providing positive cross-regulation of receptor expression. These results suggest that when cross-priming dendritic cells present class I/Ag and costimulatory ligands, and produce IL-12, naive CD8 T cells will begin to produce IL-2 and both receptors will be optimally up-regulated to insure that an effective response is generated.

CD4-dependent generation of dominant transplantation tolerance induced by simultaneous perturbation of CD154 and LFA-1 pathways

CD154 and LFA-1 (CD11a) represent conceptually distinct pathways of receptor/ligand interactions (costimulation and adhesion/homing, respectively) that have been effectively targeted to induce long-term allograft acceptance and tolerance. In the current study, we determined the relative efficacy and nature of tolerance induced by mAbs specific for these pathways. In vitro analysis indicated that simultaneous targeting of CD154 and LFA-1 resulted in profound inhibition of alloreactivity, suggesting that combined anti-CD154/anti-LFA-1 therapy could be highly effective in vivo. Thus, we evaluated combining mAb therapies targeting CD154 and LFA-1 for inducing transplantation tolerance to pancreatic islet allografts. Monotherapy with either anti-CD154 or anti-LFA-1 was partially effective for inducing long-term allograft survival, whereas the combination resulted in uniform allograft acceptance in high-responder C57BL/6 recipients. This combined therapy was not lymphocyte depleting and did not require the long-term deletion of donor-reactive T lymphocytes to maintain allograft survival. Importantly, combined anti-CD154/anti-LFA therapy uniquely resulted in "dominant" transplantation tolerance. Therefore, simultaneous perturbation of CD154 and LFA-1 molecules can result in profound tolerance induction not accomplished through individual monotherapy approaches. Furthermore, results show that such regulatory tolerance can coexist with the presence of robust anti-donor reactivity, suggesting that active tolerance does not require a corresponding deletion of donor-reactive T cells. Interestingly, although the induction of this regulatory state was highly CD4 dependent, the adoptive transfer of tolerance was less CD4 dependent in vivo.

Localization of Fas ligand in cytoplasmic granules of CD8+ cytotoxic T lymphocytes and natural killer cells: participation of Fas ligand in granule exocytosis model of cytotoxicity

Fas ligand (FasL) has been implicated in cytotoxic T lymphocyte (CTL)- and natural killer (NK) cell-mediated cytotoxicity. In the present study, we investigated the localization of FasL in murine CTL and NK cells. Immunocytochemical staining showed that FasL was stored in cytoplasmic granules of CD8+ CTL clones and in vivo activated CTL and NK cells, where perforin and granzyme A also resided. Immunoelectron microscopy revealed that FasL was localized on outer membrane of the cytoplasmic granules, while perforin was localized in internal vesicles. Western blot analysis showed that the membrane-type FasL of 40 kDa was stored in CD8+ CTL clones but not in CD4+ CTL clones. By utilizing a granule exocytosis inhibitor (TN16), we demonstrated that FasL translocated onto cell surface upon degranulation of anti-CD3-stimulated CD8+ CTL clones. Moreover, TN16 markedly inhibited the FasL-mediated cytotoxicity by CD8+ T cell clones and NK cells. These results suggested a substantial contribution of FasL to granule exocytosis-mediated target cell lysis by CD8+ CTL and NK cells.

Early antigen-specific response by naive CD8 T cells is not altered with aging

Both a dramatic decline in CD8 responses and a switch to memory T cell predominance occur with aging. The extent to which the loss of responsiveness is the consequence of the accumulation of more differentiated vs intrinsically defective T cells (or both) has been unclear. Using similar conditions of Ag stimulation, we have examined the responses generated by CD8(+) cells isolated from aged TCR transgenic mice. We found that the naive transgene(+) CD8(+) cells from aged 2C mice expressed activation markers, produced IL-2, proliferated, and differentiated into cytotoxic T cells as efficiently as their young counterparts. The extent of responsiveness and the level of the responses were comparable in both age groups regardless of the stimulatory conditions used, i.e., partial costimulation/adhesion molecule expression on APCs, or presentation of lower affinity peptide or diminished peptide concentrations. By day 4 after Ag stimulation, no significant age-related differences were observed in the number of effector cells generated nor in the levels of secreted IL-2 or IFN-gamma. Upon restimulation of effector cells, IL-2 secretion and to a lesser extent TNF-alpha expression, but not IFN-gamma secretion, were diminished with age. These findings suggest that age-associated alterations in naive CD8 cell function are not found after primary stimulation, but may become apparent upon restimulation.

Peptide antigen priming of naive, but not memory, CD8 T cells requires a third signal that can be provided by IL-12

Challenge with peptide Ag in the absence of adjuvant results in tolerance of CD8 T cells specific for the Ag. In contrast, administration of IL-12 along with peptide results in massive clonal expansion, development of effector function, and establishment of a long-lived memory population. Using adoptive transfer of TCR-transgenic CD8 T cells, this effect of IL-12 is shown to be independent of CD4 T cells and to require costimulation provided by CD28 and possibly LFA-1. IL-12 supports responses when IL-12Rbeta1-deficient mice are used as recipients for the adoptively transferred CD8 T cells, demonstrating that the IL-12 is acting directly on the T cells rather than on host APC. These results provide strong support for a three-signal model for in vivo activation of naive CD8 T cells by peptide Ag, in which the presence or absence of the third signal determines whether tolerance or activation occurs. In contrast, memory CD8 T cells are effectively activated by peptide Ag in the absence of IL-12 or adjuvant.

Donor-derived interferon gamma separates graft-versus-leukemia effects and graft-versus-host disease induced by donor CD8 T cells

The graft-versus-leukemia (GVL) effects and graft-versus-host disease (GVHD)-inducing activity of CD8 T cells was compared in murine recipients of wild-type (WT) or interferon gamma (IFN-gamma)-deficient (GKO) allogeneic donor cells. CD8 T cells (or CD4-depleted splenocytes) from GKO donor mice induced more severe GVHD in lethally irradiated allogeneic recipients compared to the same cell populations from WT donors. Consistent with GVHD severity, donor CD8 T-cell expansion in allogeneic recipients was augmented in the absence of IFN-gamma. These results demonstrate that IFN-gamma does not stimulate but instead down-modulates GVHD induced by donor CD8 T cells. Remarkably, antihost lymphohematopoietic reactions, including GVL effects against host leukemia/lymphoma cells, of CD8 T cells correlated inversely with their GVHD-inducing activity, and those of GKO donors were markedly weaker than those mediated by WT donor CD8 T cells. These data show for the first time that GVHD-inducing activity and GVL effects of allogeneic CD8 T cells can be separated by a single cytokine, IFN-gamma. (Blood. 2002;99:4207-4215)

Different contributions of thymopoiesis and homeostasis-driven proliferation to the reconstitution of naive and memory T cell compartments

Following transfer into lymphopenic hosts, naive CD8 T cells proliferate and acquire memory phenotype. Although the acquired phenotype is stable in recombination activating gene-1-deficient (RAG-/-) recipients, in sublethally irradiated mice naive CD8 T cells of donor origin gradually accumulate. The naive cells have been attributed to phenotypic reversion of homeostatic memory cells, implying instability of memory phenotype and restoration of the naive T cell compartment by homeostasis-driven proliferation. We show here that (i) the accumulation of naive CD8 T cells of donor origin only occurs in recipients that have been irradiated and have an intact thymus; (ii) the apparent reversion of memory to naive cells actually results from de novo T cell development of hematopoietic stem cells, present in the donor spleen or lymph node cell populations, in the thymus of irradiated recipients; and (iii) the number of homeostatic memory cells generated in both RAG-/- and irradiated hosts reaches a plateau value and their phenotype is stably maintained even after retransfer into nonirradiated normal mice for 30 days. These findings demonstrate that homeostatic memory T cells do not revert to naive cells. After severe T cell depletion homeostasis-driven proliferation restores only the memory T cell compartment, whereas thymopoiesis is required for the reconstitution of the naive T cell compartment.

How do cultured CD8(+) murine T cell clones survive repeated ligation of the TCR?

Many murine T cell clones grow continuously in culture despite weekly ligation of their TCR by antigen. To learn how the cultured cells avoid or minimize antigen-induced cell death (AICD), we compared Fas and tumor necrosis factor (TNF) receptors (TNFR) on several long-term cultured CD8(+) T cell clones with those on naive and activated naive cells expressing the same TCR (2C). In contrast to the naive cells, Fas was absent on the cultured clones and the TNFR-II receptor, present initially at high levels on the cultured cells, was rapidly down-modulated in response to TCR ligation and had virtually disappeared by 2 h, when only approximately 10% of the cloned cells had been induced to express TNF-alpha. The extent of AICD of the cultured clones in response to cognate peptide-MHC on the presenting cells used for routine stimulation of the cultures was also considerably less than the massive cell death of the clones following exposure to anti-CD3 antibody plate-bound at high density.

Developmental and functional analyses of CD8(+) NK1.1(+) T cells in class-I-restricted TCR transgenic mice

Using a class-I-restricted T cell receptor (TCR) transgenic mice (Tgm), 2C (Valpha3.1/Vbeta 8.2, specific for L(d) + LSPFPFDL), the development and cytokine production of tg-TCR(+) NKT cells were analyzed. We found that CD8(+) or double negative (DN) NKT cells constituted a major population of NKT cells in the H-2(b/b) 2C Tgm (positive selecting background) or the H-2(b/d) 2C Tgm (negative selecting background), respectively. Virtually no NKT cells were generated in the H-2(k/k) 2C Tgm (neutral selecting background). CD8(+) NKT cells in the H-2(b/b) 2C Tgm expressed CD8alphabeta heterodimers, whereas those in the H-2(b/d) 2C Tgm expressed CD8alphaalpha homodimers. These findings suggest that development of a subpopulation of NKT cells is influenced by the H-2 molecules. Upon stimulation with anti-CD3 mAb, tg-TCR(+) NKT cells generated in the H-2(b/b) and H-2(b/d) backgrounds produced IFN-gamma, but not IL-4.

Immunogenicity of L(d+) transgenic mouse hearts

BACKGROUND

C57BL/6 mice transfected with the L(d) gene coupled to the alpha-myosin heavy chain promoter result in transgenic mice with L(d) antigen expressed only on cardiac tissue. These transgenic animals allow the examination of immune reactivity against cardiac L(d) by "self" or by adoptively transferred L(d) specific 2C cells, and the response of nontransgenic C57BL/6 mice to the transplanted L(d+) heart.

METHODS

Naïve cardiac L(d+) transgenic mice were examined for evidence of L(d) "autoimmunity." Forty million fresh 2C cells or 2C cells sensitized in vitro for 7 days against Balb/c (L(d+)) + interleukin-2 were also given intravenously to L(d+) transgenic mice. At 5 and 12 days after injection, heart-infiltrating lymphocytes were analyzed by fluorescence-activated cell sorter. The L(d+) transgenic hearts were also transplanted to syngeneic L(d-) nontransgenic C57BL/6 to evaluate the heart's immunogenicity.

RESULTS

Naïve L(d+) transgenic mice did not exhibit any evidence of lymphocytic infiltration on histologic examination. Adoptive transfer of either fresh or in vitro sensitized 2C cells was also unable to reject the native L(d+) heart in transgenic mice (100% of the mice survived long term [more than 60 days]). Sensitization of the L(d+) transgenic mice with a Balb/c skin graft and interleukin-2 pump infusion (7 days) beginning 1 day before 2C cell injection also did not promote rejection of the native L(d+) heart. However, fluorescence-activated cell sorter analysis did reveal that a significantly greater number of in vitro sensitized 2C cells homed to the L(d+), but not L(d-), heart after both 5 and 12 days (P <.01, P <.001). In contrast, C57BL/6 mice rejected the L(d+) (C57BL/6 background) transgenic heart in a mean survival time of 17 +/- 9.7 days (P <.01), whereas a syngeneic C57BL/6 heart transplant was accepted indefinitely. Lymphocytic infiltration consistent with rejection was present in all animals receiving an Ld+ transgenic heart transplant, whereas no infiltrate was present in those receiving a syngeneic C57BL/6 heart transplant.

CONCLUSIONS

Although the class I L(d) transgene is not recognized in its native host, its immunogenicity is shown by the homing of anti-L(d) 2C cells to the heart in situ and rejection of L(d+) heart grafts when transplanted into syngeneic C57BL/6 mice.

Potent cytolytic response by a CD8+ CTL clone to multiple peptides from the same protein in association with an allogeneic class I MHC molecule

CTL clone 2C recognizes the allogeneic class I MHC molecule L(d) in association with peptides derived from alpha-ketoglutarate dehydrogenase (oxoglutarate dehydrogenase (OGDH)), a ubiquitous intracellular protein. One of these peptides, QLSPFPFDL (QL9), elicits more vigorous cytolytic responses than two previously identified naturally processed peptides with overlapping sequences, LSPFPFDL (p2Ca) and VAITRIEQLSPFPFDL (p2Cb), from OGDH. In this study, we show that QL9 forms a more stable complex with cell surface L(d) than does p2Ca or p2Cb and is processed from the longer, naturally occurring peptide p2Cb by 20S proteosomes in vitro. The N-terminal cyclized pyroglutaminyl QL9 (pyroQL9), a form of QL9 to which it is converted at the low pH used for peptide isolation from tissue extracts, is even more active than QL9 in cytotoxicity assays with 2C CTL. Overall, the results indicate that along with the abundant natural peptides p2Ca and p2Cb, the QL9 and other OGDH peptides of various lengths, sharing a conserved C-terminal sequence, are also processed and presented with L(d) as allogeneic ligands for T cells expressing 2C TCR. All these peptides, each available in a low amount, could act in concert at the cell surface, resulting in a high density of cognate ligands that accounts for the exceptionally potent cytolytic response by 2C CTL.