Resting and activated T cells display different requirements for CD8 molecules

Antigen-specific clonal expansion and cytolytic effector function of CD8+ T lymphocytes depend on the transcription factor Bcl11b

CD8(+) T lymphocytes mediate the immune response to viruses, intracellular bacteria, protozoan parasites, and tumors. We provide evidence that the transcription factor Bcl11b/Ctip2 controls hallmark features of CD8(+) T cell immunity, specifically antigen (Ag)-dependent clonal expansion and cytolytic activity. The reduced clonal expansion in the absence of Bcl11b was caused by altered proliferation during the expansion phase, with survival remaining unaffected. Two genes with critical roles in TCR signaling were deregulated in Bcl11b-deficient CD8(+) T cells, CD8 coreceptor and Plcgamma1, both of which may contribute to the impaired responsiveness. Bcl11b was found to bind the E8I, E8IV, and E8V, but not E8II or E8III, enhancers. Thus, Bcl11b is one of the transcription factors implicated in the maintenance of optimal CD8 coreceptor expression in peripheral CD8(+) T cells through association with specific enhancers. Short-lived Klrg1(hi)CD127(lo) effector CD8(+) T cells were formed during the course of infection in the absence of Bcl11b, albeit in smaller numbers, and their Ag-specific cytolytic activity on a per-cell basis was altered, which was associated with reduced granzyme B and perforin.

T cell receptor-dependent regulation of lipid rafts controls naive CD8+ T cell homeostasis

T cell receptor (TCR) contact with self ligands keeps T cells alive and is shown here to cause naive CD8(+), but not CD4(+), T cells to be hypersensitive to certain gamma(c) cytokines, notably interleukin (IL)-2, IL-15, and IL-7. Hypersensitivity of CD8(+) T cells to IL-2 was dependent on a low-level TCR signal, associated with high expression of CD5 and GM1, a marker for lipid rafts, and was abolished by disruption of lipid rafts. By contrast, CD4(+) T cells expressed low amounts of GM1 and were unresponsive to IL-2. Physiologically, sensitivity to IL-7 and IL-15 maintains survival of resting CD8(+) T cells, whereas sensitivity to IL-2 may be irrelevant for normal homeostasis but crucial for the immune response. Thus, TCR contact with antigen upregulates GM1 and amplifies responsiveness of naive CD8(+) T cells to IL-2, thereby making the cells highly sensitive to exogenous IL-2 from CD4(+) T helper cells.

Primed CD8(+) T-cell responses to allogeneic endothelial cells are controlled by local complement activation

CD8 T cells primed by transplantation recognize allogeneic class I MHC molecules expressed on graft vascular endothelium and contribute to allograft injury. We previously showed that immune cell-derived complement activation fragments are integral to T cell activation/expansion. Herein we tested the impact of local complement production/activation on T cell/endothelial cell (EC) interactions. We found that proinflammatory cytokines upregulated alternative pathway complement production by ECs, yielding C5a. We further found that ECs deficient in the cell surface C3/C5 convertase regulator decay accelerating factor (DAF, CD55) induced greater CD8 T-cell proliferation and more IFNgamma(+) and perforin(+) effector cells than wild-type (WT) ECs. Allogeneic C3(-/-) EC induced little or no CD8 responses. Abrogation of responses following C5a receptor (C5aR) blockade, or augmentation following addition of recombinant C5a demonstrated that the effects were mediated through T-cell-expressed-C5aR interactions. Analyses of in vivo CD8 cell responses to transplanted heart grafts deficient in EC DAF showed similar augmentation. The findings reveal that EC-derived complement triggers secondary CD8 T-cell differentiation and expansion and argue that targeting complement and/or C5aR could limit T-cell-mediated graft injury.

Characterization of MHC class-I restricted TCRalphabeta+ CD4- CD8- double negative T cells recognizing the gp100 antigen from a melanoma patient after gp100 vaccination

The immune attack against malignant tumors require the concerted action of CD8+ cytotoxic T lymphocytes (CTL) as well as CD4+ T helper cells. The contribution of T cell receptor (TCR) alphabeta+ CD4- CD8- double-negative (DN) T cells to anti-tumor immune responses is widely unknown. In previous studies, we have demonstrated that DN T cells with a broad TCR repertoire are present in humans in the peripheral blood and the lymph nodes of healthy individuals. Here, we characterize a human DN T cell clone (T4H2) recognizing an HLA-A2-restricted melanoma-associated antigenic gp100-peptide isolated from the peripheral blood of a melanoma patient. Antigen recognition by the T4H2 DN clone resulted in specific secretion of IFN-gamma and TNF. Although lacking the CD8 molecule the gp100-specific DN T cell clone was able to confer antigen-specific cytotoxicity against gp100-loaded target cells as well as HLA-A2+ gp100 expressing melanoma cells. The cytotoxic capacity was found to be perforin/granzymeB-dependent. Together, these data indicate that functionally active antigen-specific DN T cells recognizing MHC class I-restricted tumor-associated antigen (TAA) may contribute to anti-tumor immunity in vivo.

Role of alpha3 domain of class I MHC molecules in the activation of high- and low-avidity CD8+ CTLs

CD8 can serve as a co-receptor or accessory molecule on the surface of CTL. As a co-receptor, CD8 can bind to the alpha3 domain of the same MHC class I molecules as the TCR to facilitate TCR signaling. To evaluate the role of the MHC class I molecule alpha3 domain in the activation of CD8(+) CTL, we have produced a soluble 227 mutant of H-2D(d), with a point mutation in the alpha3 domain (Glu227 --> Lys). 227 mutant class I-peptide complexes were not able to effectively activate H-2D(d)-restricted CD8 T cells in vitro, as measured by IFN-gamma production by an epitope-specific CD8(+) CTL line. However, the 227 mutant class I-peptide complexes in the presence of another MHC class I molecule (H-2K(b)) (that cannot present the peptide) with a normal alpha3 domain can induce the activation of CD8(+) CTL. Therefore, in order to activate CD8(+) CTL, the alpha3 domain of MHC class I does not have to be located on the same molecule with the alpha1 and alpha2 domains of MHC class I. A low-avidity CD8(+) CTL line was significantly less sensitive to stimulation by the 227 mutant class I-peptide complexes in the presence of the H-2K(b) molecule. Thus, low-avidity CTL may not be able to take advantage of the interaction between CD8 and the alpha3 domain of non-presenting class I MHC molecules, perhaps because of a shorter dwell time for the TCR-MHC interaction.

Regulation of T lymphocyte subsets

Patterns of cytokine secretion and functional differences distinguish T lymphocyte subsets. T lymphocyte subsets are also regulated differentially. Most established CD8+ lymphocyte clones secrete gamma-interferon (IFN-gamma) but not interleukin 2 (IL-2) or IL-4. Using murine T cells which express a transgenic, antigen-specific alpha/beta T cell receptor (TCR) specific for L(d) class I major histocompatibility complex antigen, we have found that CD8+ lymphocytes can be divided into functional subsets. Freshly isolated CD8+ T cells are not cytolytic, do not proliferate and do not proliferate and do not secrete cytokines. Stimulation of TCR alone does not induce cytokine secretion, but cells become responsive to exogenous IL-2 or IL-4. Stimulation of CD28 together with TCR induces secretion of IL-2 and IFN-gamma, and cells proliferate without exogenous cytokines. Proliferation is necessary for the development of cytolytic activity. If IL-4 is present during initial stimulation, IL-4 is secreted following restimulation. Upon stimulation, some IL-4-producing murine CD8+ T cell clones express CD40 ligand (CD40L), and they potentiate proliferation and immunoglobulin secretion by small resting B cells. Thus, the CD8+ T cell subsets T cytotoxic 1 (Tc1) and Tc2 are analogous to CD4+ T helper 1 (Th1) and Th2. IL-2 production by naive CD8+ cells requires co-stimulation. IL-4 production by CD8+ T cells requires the presence of IL-4 during initial stimulation. Some IL-4-producing CD8+ T cells express CD40L following TCR stimulation and provide help for B cells.

Differential requirement for Lck during primary and memory CD8+ T cell responses

T cell receptor (TCR) signaling mediates cell fate decisions throughout the life of a T cell. The earliest biochemical events during antigen-stimulated TCR signaling include activation of the Src-family protein tyrosine kinase, p56(Lck) (Lck), which is an integral component of the TCR signaling complex by its association with the cytoplasmic tails of CD8 or CD4. CD8 and Lck are obligatory during thymic selection of CD8+ T cells. What remain unknown are when and with what stringency Lck is required for effective TCR-mediated activation and function throughout the life of a mature CD8+ T cell. Using mice that express an inducible Lck transgene in T cells, we have investigated the temporal importance of Lck-mediated TCR signaling in antigen-specific CD8+ T cell responses during acute viral infections. We show that Lck deficiency induced in naive mice abrogated the antigen-specific activation and clonal expansion of CD8+ T cells during a primary response to acute viral infections. Moreover, the magnitude of primary CD8 T cell expansion depended on the duration of Lck-dependent TCR signaling. Quite unexpectedly, however, Lck was dispensable for enhanced functional avidity, maintenance, and reactivation of memory CD8+ T cells in vitro and in vivo. These observations suggest that the TCR signaling apparatus is rewired from an Lck-dependent state in naive CD8+ T cells to an Lck-independent state in memory CD8+ T cells. Less stringent requirements for antigen-specific TCR signaling to activate memory CD8+ T cells could, in part, account for their unique hyperreactivity to antigen, which contributes to accelerated immune control during secondary infections.

Activated mouse T cells downregulate, process and present their surface TCR to cognate anti-idiotypic CD4+ T cells

The ability of activated T cells to present foreign antigens through the MHC class II pathway has been shown in the case of human, rat and mouse T cells. In the present study, the ability of activated T cells to present their endogenous TCR in association with MHC class II molecules to CD4+ T cells was shown. Upon activation mouse T cells downregulate their surface TCR, which are degraded into peptides in endosomal/lysosomal compartments. The idiopeptides (peptides derived from the variable region of the TCR) are presented to cognate anti-idiotypic CD4+ T cells, resulting in activation and proliferation of these cells. Interaction of idiotypic and anti-idiotypic T cells brought about by presentation of TCR idiopeptide may have important implications for T-cell vaccination and perpetuation of T-cell memory not requiring persisting antigen or long-lived memory cells.

Idiotypic T cells specific for Morbillivirus nucleocapsid protein process and present their TCR to cognate anti-idiotypic CD8+ T cells

CD8(+) T cells are activated by the presentation of antigenic peptide through MHC class I molecules. Newly synthesized proteins formed as defective ribosomal products (DRiPs) can act as a major source of antigenic peptides for MHC class I presentation pathway. Majority of these peptides are generated from the intracellular degradation of self antigens. In the present study, we have shown that newly synthesized T cell receptor (TCR) beta chains formed as DRiPs in T cells are ubiquitinated and degraded by the proteasomes. These TCR-DRiPs are processed and presented by activated T cells to cognate anti-idiotypic CD8(+) T cells. Presentation of TCR idiopeptide (peptide derived from the variable region of idiotypic TCR) by activated T cells leads to Bcl-2 expression and cytokine secretion by anti-idiotypic CD8(+) T cells. Presentation of intracellular antigen by T cells may have important implications in immunoregulation, control of lymphotropic virus infection and autoimmune diseases.

Memory CD8 T cells require CD8 coreceptor engagement for calcium mobilization and proliferation, but not cytokine production

Memory T-cell responses are faster and more robust than those of their naive counterparts. The mechanisms by which memory T cells respond better to subsequent antigenic exposure remain unresolved. A portion of the more rapid response is undoubtedly the result of the increased frequency of antigen-specific cells. In addition, there are also differences in the cells themselves with respect to their requirements for costimulation and the apparent avidity of the T cells. We used major histocompatibility complex (MHC) class I tetramers to stimulate T cells to focus on the interaction of T-cell receptor (TCR)/MHC and CD8 in the absence of other molecules that are present on cell surfaces and so contribute to the activation of T cells by undefined mechanisms. Mutated MHC class I tetramers that are unable to engage CD8 were used to investigate the role of CD8 engagement in memory cell activation. Either wild-type tetramers or tetramers carrying the mutation were used to stimulate both memory and naive TCR transgenic T cells in vitro. Surprisingly, like naive cells, memory CD8(+) T cells required CD8 engagement for calcium mobilization and optimum proliferation. In contrast, the requirements for cytokine production differed. Unlike naive cells, memory cells were able to produce cytokine in the absence of CD8 engagement. This suggests both a CD8-dependent pathway for early events and a CD8-independent pathway for cytokine production in memory cells.

Alteration of cell surface sialylation regulates antigen-induced naive CD8+ T cell responses

The strength of interactions with APC instructs naive T cells to undergo programmed expansion and differentiation, which is largely determined by the peptide affinity and dose as well as the duration of TCR ligation. Although, most ligands mediating these interactions are terminally sialylated, the impact of the T cell sialylation status on Ag-dependent response remains poorly understood. In this study, by monitoring TCR transgenic CD8+ T cells, OT-I, we show that biochemical desialylation of naive OT-I T cells increases their sensitivity for agonist as well as partial agonist peptides. Desialylation enhances early activation and shortens the duration of TCR stimulation required for proliferation and differentiation, without increasing apoptosis. Moreover, desialylation of naive OT-I T cells augments their response to tumor-presented Ag. These results provide direct evidence for a regulatory role for sialylation in Ag-dependent CD8+ T cell responses and offer a new approach to sensitize or dampen Ag-specific CD8+ T cell responses.

Interplay between TCR affinity and necessity of coreceptor ligation: high-affinity peptide-MHC/TCR interaction overcomes lack of CD8 engagement

CD8 engagement is believed to be a critical event in the activation of naive T cells. In this communication, we address the effects of peptide-MHC (pMHC)/TCR affinity on the necessity of CD8 engagement in T cell activation of primary naive cells. Using two peptides with different measured avidities for the same pMHC-TCR complex, we compared biochemical affinity of pMHC/TCR and the cell surface binding avidity of pMHC/TCR with and without CD8 engagement. We compared early signaling events and later functional activity of naive T cells in the same manner. Although early signaling events are altered, we find that high-affinity pMHC/TCR interactions can overcome the need for CD8 engagement for proliferation and CTL function. An integrated signal over time allows T cell activation with a high-affinity ligand in the absence of CD8 engagement.

B cells activated by lipopolysaccharide, but not by anti-Ig and anti-CD40 antibody, induce anergy in CD8+ T cells: role of TGF-beta 1

B cells recognize Ag through their surface IgRs and present it in the context of MHC class II molecules to CD4(+) T cells. Recent evidence indicates that B cells also present exogenous Ags in the context of MHC class I to CD8(+) T cells and thus may play an important role in the modulation of CTL responses. However, in this regard, conflicting reports are available. One group of studies suggests that the interaction between B cells and CD8(+) T cells leads to the activation of the T cells, whereas other studies propose that it induces T cell tolerance. For discerning this dichotomy, we used B cells that were activated with either LPS or anti-Ig plus anti-CD40 Ab, which mimic the T-independent and T-dependent modes of B cell activation, respectively, to provide accessory signals to resting CD8(+) T cells. Our results show that, in comparison with anti-Ig plus anti-CD40 Ab-activated B cells, the LPS-activated B cells (LPS-B) failed to induce significant levels of proliferation, cytokine secretion, and cytotoxic ability of CD8(+) T cells. This hyporesponsiveness of CD8(+) T cells activated with LPS-B was significantly rescued by anti-TGF-beta1 Ab. Moreover, it was found that such hyporesponsive CD8(+) T cells activated with LPS-B had entered a state of anergy. Furthermore, LPS-B expresses a significantly higher level of TGF-beta1 on the surface, which caused the observed hyporesponsiveness of CD8(+) T cells. Therefore, this study, for the first time, provides a novel mechanism of B cell surface TGF-beta1-mediated hyporesponsiveness leading to anergy of CD8(+) T cells.

p53 deficiency and defective mitotic checkpoint in proliferating T lymphocytes increase chromosomal instability through aberrant exit from mitotic arrest

During the proliferation of T cells for successful immune responses against pathogens, the fine regulation of cell cycle is important to the maintenance of T cell homeostasis and the prevention of lymphoproliferative disorders. However, it remains to be elucidated how the cell cycle is controlled at the mitotic phase in proliferating T cells. Here, we show that during the proliferation of primary T cells, the disruption of the mitotic spindle leads to cell-cycle arrest at mitosis and that prolonged mitotic arrest results in not only apoptosis but also the form of chromosomal instability observed in human cancers. It is interesting that in response to spindle damage, the phosphorylation of BubR1, a mitotic checkpoint kinase, was significantly induced in proliferating T cells, and the expression of the dominant-negative mutant of BubR1 compromised mitotic arrest and subsequent apoptosis and thus led to the augmentation of polyploidy formation. We also show that in response to prolonged spindle damage, the expression of p53 but not of p73 was significantly induced. In addition, following sustained mitotic arrest, p53-deficient T cells were found to be more susceptible to polyploidy formation than the wild type. These results suggest that during flourishing immune response, mitotic checkpoint and p53 play important roles in the prevention of chromosomal instability and in the maintenance of the genomic integrity of proliferating T cells.

Quantitative analysis of the contribution of TCR/pepMHC affinity and CD8 to T cell activation

The relative roles of CD8, TCR:pepMHC affinity, and TCR:pepMHC dissociation rate in T cell activation have remained controversial. To determine the relationships among these factors, we used T cells transfected with normal and in vitro engineered alphabeta TCRs, in the presence or absence of CD8. The TCRs exhibited a wide range of affinities (K(D) values of 80 microM to 5 nM). T cells with the highest affinity TCRs were efficiently stimulated by peptide, with or without CD8. In contrast, CD8 was required for T cells that expressed TCRs with affinities typical of syngeneic reactions (K(D) values above approximately 3 microM). The results suggest that virtually all normal syngeneic interactions require CD8, which enhances peptide sensitivity by one million-fold or more.

Role of the actin cytoskeleton in T cell absorption and internalization of ligands from APC

A feature of T-APC interaction is that, via either TCR or CD28, T cells can absorb molecules from APC on to the cell surface and then internalize these molecules. Here, using both normal and TCR-transgenic T cells, we investigated the mechanism of T cell absorption of molecules from APC and the role of the cytoskeleton. The results show that although activated T cells could absorb APC molecules in the form of cell fragments, uptake of molecules by resting T cells required direct T-APC interaction. Based on studies with latrunculin B, surface absorption of molecules by resting T cells was crucially dependent upon the actin cytoskeleton for both CD28- and TCR-mediated absorption. Significantly, however, TCR-mediated absorption became strongly resistant to latrunculin B when the concentration of MHC-bound peptide on APC was raised to a high level, implying that the actin cytoskeleton is only important for absorption when the density of receptor/ligand interaction is low. By contrast, in all situations tested, the actin cytoskeleton played a decisive role in controlling T cell internalization of ligands from the cell surface.

Dependence of lymphopenia-induced T cell proliferation on the abundance of peptide/ MHC epitopes and strength of their interaction with T cell receptors

Factors that affect naive T cell proliferation in syngeneic lymphopenic hosts were investigated. 2C T cell receptor (TCR) transgenic T cells lacking both CD8 and CD4 survived but hardly proliferated. Proliferation of CD8(+) 2C cells was proportional to the abundance of cognate peptide/MHC complexes and was severely inhibited by injection of anti-CD8 antibody. Weakly reactive self-peptides slightly enhanced CD8(+) 2C cell proliferation whereas a potent agonist peptide promoted much more rapid proliferation, but inflammation-stimulating adjuvant had only a small effect on the rate of cell proliferation. The findings suggest that under uniform lymphopenic conditions, the widely different rates of proliferation of T cells expressing various TCR, or the same TCR in the presence or absence of CD8, reflect the strength of interaction between TCR and MHC associated with particular self-peptides.

Anti-Tumor CC49-zeta CD4 T cells possess both cytolytic and helper functions

The authors report that the nature of the T-cell-receptor--derived signal in normal CD4+ T cells can induce interleukin-2 (IL-2) secretion or perforin-mediated cytolytic activity. Normal human T cells were genetically modified to express the tumor antigen specific chimeric immune receptor, CC49-zeta. The CC49-zeta chimeric immune receptor is comprised of the intracellular signaling domains of the TCR CD3zeta protein fused to the single chain scFv of the humanized CC49 antibody, which binds the pan-adenocarcinoma tumor antigen TAG-72. Patient-specific T cells genetically modified to express the CC49-zeta receptor have been used in patients with colon cancer. The authors report that both CD4 and CD8 T cells expressing the CC49-zeta receptor mediated the major histocompatibility complex-unrestricted lysis of TAG-72--expressing tumor cells with comparable efficiency. However, although the CC49-zeta receptor mediated target cell lysis, it did not support the production of IL-2, even in the presence of CD28 stimulation. Robust IL-2 secretion and T-cell proliferation were observed when the same CD4 CC49-zeta T cells were stimulated through the CD28 receptor and endogenous T-cell receptor. These results indicate that CD4 T lymphocytes possess the capacity to act as both cytolytic and helper T cells and that this difference in effector function is controlled by the nature of the T-Cell receptor--derived signals.

Cutting edge: In situ tetramer staining of antigen-specific T cells in tissues

Staining Ag-specific T cells with fluorescently labeled tetrameric MHC/peptide complexes has provided a powerful experimental approach to characterizing the immune response. In this report, we describe an extension of this method to directly visualize Ag-specific T cells in tissues. We successfully stained transgenic T cells with MHC tetramers in spleen sections from both 2C and OT-1 TCR transgenic mice. In addition, with the in situ tetramer staining technique, we detected a very small population of Ag-specific T cells in tissue after adoptive transfer of transgenic TCR T cells to a syngeneic nontransgenic mouse. We also show that the in situ tetramer technique can be applied to lightly fixed as well as frozen tissue, thus extending the method to archived tissue collections. This in situ tetramer staining technique offers a general approach to tracking the Ag-specific T cells in tissues.

Identification of a previously unknown antigen-specific regulatory T cell and its mechanism of suppression

Despite increasing evidence for the existence of antigen-specific regulatory T cells, the mechanisms underlying suppression remain unclear. In this study we have identified and cloned a novel subset of antigen-specific regulatory T cells and demonstrated that these T cells possess a unique combination of cell surface markers and array of cytokines. The regulatory T cells are able to inhibit the function of T cells carrying the same T-cell receptor specificity and prevent skin allograft rejection in an antigen-specific, dose-dependent manner. The regulatory T cells are able to acquire alloantigen from antigen-presenting cells, present the alloantigen to activated syngeneic CD8+ T cells and then send death signals to CD8+ T cells. These findings provide a novel mechanism of regulatory T-cell-mediated, antigen-specific suppression.

T cells can use either T cell receptor or CD28 receptors to absorb and internalize cell surface molecules derived from antigen-presenting cells

At the site of contact between T cells and antigen-presenting cells (APCs), T cell receptor (TCR)-peptide-major histocompatibility complex (MHC) interaction is intensified by interactions between other molecules, notably by CD28 and lymphocyte function-associated antigen 1 (LFA-1) on T cells interacting with B7 (B7-1 and B7-2), and intracellular adhesion molecule 1 (ICAM-1), respectively, on APCs. Here, we show that during T cell-APC interaction, T cells rapidly absorb various molecules from APCs onto the cell membrane and then internalize these molecules. This process is dictated by at least two receptors on T cells, namely CD28 and TCR molecules. The biological significance of T cell uptake of molecules from APCs is unclear. One possibility is that this process may allow activated T cells to move freely from one APC to another and eventually gain entry into the circulation.

Critical role for CD8 in T cell receptor binding and activation by peptide/major histocompatibility complex multimers

Recent data using MHC/peptide tetramers and dimers suggests that the T cell coreceptors, CD4 and CD8, although important for T cell activation, do not play a direct role in facilitating T cell receptor (TCR) binding to multivalent MHC/peptide ligands. Instead, a current model proposes that coreceptors are recruited only after a stable TCR-MHC/peptide complex has already formed and signaled. In contrast, we show using multimeric class I MHC/peptide ligands that CD8 plays a critical (in some cases obligatory) role in antigen-specific TCR binding. T cell activation, measured by calcium mobilization, was induced by multimeric but not monomeric ligands and also showed CD8 dependency. Our analysis using anti-CD8 antibodies revealed that binding to different epitopes of CD8 can either block or augment TCR-MHC/peptide interaction. These effects on TCR binding to high-affinity agonist ligands were even more pronounced when binding to multimeric low-affinity ligands, including TCR antagonists, was studied. Our data have important implications for the role of CD8 in TCR binding to MHC/peptide ligands and in T cell activation. In addition, our results argue against the view that multimeric MHC/peptide ligands bind directly and solely to the TCR; rather, our data highlight a pivotal contribution of CD8 for this association.

Visualization, Fate, and Pathogenicity of Antigen-Specific CD8+ T Cells in the Graft-Versus-Host Reaction

To follow the fate of alloreactive T cell effectors in graft-vs-host disease, Ld-specific CD8+ T cells from C57BL/6 2C TCR-transgenic donors were transplanted into sublethally irradiated (750 cGy) Ld+ or Ld− recipients. In Ld− C57BL/6 or (BALB/c-dm2 × C57BL/6)F1 recipients, naive 2C T cells engrafted and survived long term, but did not acquire effector function. In Ld+ (BALB/c × C57BL/6)F1 recipients, 2C T cells engrafted, expanded, became cytolytic, destroyed host B cells and double-positive thymocytes, and later disappeared. Despite marked damage to lymphoid and hemopoietic cells by 2C T cells, no significant pathology was detected in other organs, and recipients survived. Ld+ (BALB/c × C57BL/6)F1 recipients died when LPS/endotoxin was administered on day 7 after cell transfer, while Ld− (BALB/c-dm2 × C57BL/6)F1 recipients survived. Our findings show that under certain conditions, a CD8+ T cell population recognizing an extremely limited repertoire of Ags can initiate graft-vs-host disease.

Defective CD8+ T Cell Activation and Cytolytic Function in the Absence of LFA-1 Cannot Be Restored by Increased TCR Signaling

Signaling through the TCR as well as engagement of costimulatory molecules are required for efficient T cell activation and progression into differentiated effector cells. The β2 integrin LFA-1 (CD11a/CD18) has been implicated in TCR costimulation as well as in cell-cell adhesion function, but its exact role is still ambiguous. The present study focuses on the requirement for LFA-1 in CD8+ T cell activation and effector function using LFA-1-deficient cells expressing the 2C transgenic TCR as a model system. The lack of LFA-1 expression in 2C T cells resulted in severely diminished proliferative response toward allogeneic BALB/c splenocytes. Increase in TCR signaling alone by pulsing stimulators with high affinity peptides, p2Ca or QL9, had minimal effects in restoring proliferation. Addition of exogenous IL-2, however, enhanced the effect of peptide pulsing on proliferation of LFA-1-deficient 2C T cells. LFA-1-deficient 2C CTLs generated from alloantigen stimulation exhibited a defective cytotoxic activity when tested on a variety of target cells. Cytolysis could be improved, but not fully rectified by peptide pulsing of target cells. Thus, in the 2C TCR model, LFA-1 has a requisite role for optimal CD8+ T cell activation and effector function, which cannot be overcome by increasing peptide/MHC density on either the APCs or target cells, respectively.

CD8+TCR+ and CD8+TCR- cells in whole bone marrow facilitate the engraftment of hematopoietic stem cells across allogeneic barriers

Although purified hematopoietic stem cells (HSC) are sufficient to engraft irradiated allogeneic recipients, bone marrow (BM) contains other cells that facilitate engraftment. Here, several candidate facilitators were tested by cotransplantation with HSC. Both TCR+ and TCR- CD8alpha+ BM subpopulations have facilitative potential. CD8+TCR+ cells are typical T lymphocytes. CD8+TCR- facilitators are CD3 , not CD3+, have a granular morphology, and are CD8beta- and CD11c+; they share phenotypic characteristics with CD8(alpha)alpha lymphoid dendritic cells and veto cells. We also demonstrate that lytic function is nqt necessary for facilitation and that the CD8alpha molecule is either important for facilitation or in the development of facilitators.

Requirements for Stimulating Naive CD8+ T Cells via Signal 1 Alone

In the absence of costimulation, TCR recognition of peptide/MHC complexes is generally considered to be nonimmunogenic. In agreement with this view, naive TCR transgenic CD8+ cells failed to respond to specific peptides presented by MHC class I (Ld) molecules bound to mouse RBC. However, peptide/Ld complexes presented by cell-sized beads or bound to plastic led to overt proliferative responses in the absence of added cytokines. Significantly, equivalent strong proliferative responses occurred when mouse RBC were fixed with glutaraldehyde before Ld coupling. The implication therefore is that the intensity of signaling via the TCR is a reflection of the mobility of the ligand being recognized; TCR signaling is weak when the ligand can move laterally on the cell membrane but strong when the ligand is immobilized.

Secondary but not primary T cell responses are enhanced in CTLA-4-deficient CD8+ T cells

Negative as well as positive co-stimulation appears to play an important role in controlling T cell activation. CTLA-4 has been proposed to negatively regulate T cell responses. CTLA-4-deficient mice develop a lymphoproliferative disorder, initiated by the activation and expansion of CD4+ T cells. To assess the function of CTLA-4 on CD8+ T cells, CTLA-4(-/-) animals were crossed to an MHC class I-restricted 2C TCR transgenic mouse line. We demonstrate that although the primary T cell responses were similar, the CTLA-4-deficient 2C TCR+ CD8+ T cells displayed a greater proliferative response upon secondary stimulation than the 2C TCR+ CD8+ T cells from CTLA-4 wild-type mice. These results suggest that CTLA-4 regulates antigen-specific memory CD8+ T cell responses.

Probing the activation requirements for naive CD8+ T cells with Drosophila cell transfectants as antigen presenting cells

Activation of T cells involves multiple receptor-ligand interactions between T cells and antigen presenting cells (APC). At least two signals are required for T-cell activation: Signal 1 results from recognition of MHC/peptide complexes on the APC by cell surface T-cell receptors (TCR), whereas Signal 2 is induced by the interactions of co-stimulatory molecules on APC with their complementary receptors on T cells. This review focuses on our attempts to understand these various signals in a model system involving the 2C TCR. The structural basis of Signal 1 was investigated by determining the crystal structure of 2C TCR alone and in complex with MHC/peptide. Analysis of these structures has provided some basic rules for how TCR and MHC/peptide interact; however, the critical question of how this interaction transduces Signal 1 to T cells remains unclear. The effects of Signal 1 and Signal 2 on T-cell activation were examined with naive T cells from the 2C TCR transgenic mice, defined peptides as antigen and transfected Drosophila cells as APC. The results suggest that, except under extreme conditions, Signal 1 alone is unable to activate naive CD8 T cells despite the induction of marked TCR downregulation. Either B7 or intercellular adhesion molecule (ICAM)-1 can provide the second signal for CD8 T-cell activation. However, especially at low MHC/peptide densities, optimal activation and differentiation of CD8 T cells required interaction with both B7 and ICAM-1 on the same APC. Thus, the data suggest that at least two qualitatively different co-stimulation signals are required for full activation of CD8 T cells under physiological conditions.

Kinetics of circulating cytotoxic T lymphocyte precursors that have a high avidity for donor antigens: correlation with the rejection status of the human cardiac allograft

Studies on graft infiltrating cells demonstrated that accumulation of cytotoxic T lymphocytes (CTL) with high avidity for donor antigens (Ag) coincided with acute cardiac rejection. In the present study, we analyse whether such high-avidity CTL are present within the peripheral blood of cardiac transplant recipients and whether their kinetics correspond with the rejection status of the allograft. Using limiting dilution analysis (LDA), donor-specific CTL were enumerated in serial blood samples of seven patients. From each patient, 7-11 samples were obtained during the first year after transplantation and up to three samples were obtained at a later date. Enumerated donor-specific CTL were divided into CTL with high or low avidity for donor Ag, depending on their sensitivity to CD8-blocking. In contrast to the situation in the graft, the donor-specific CTL present within the peripheral blood were CTL precursors (pCTL) and not fully mature CTL (cCTL). The number of donor-specific pCTL among peripheral blood cells fluctuated irrespective of the rejection grade of the allograft, indicating that the frequency of circulating donor-specific CTL does not reflect the immunological status of the allograft. During acute cardiac rejection, 66% (median) of the circulating donor-specific pCTL had a high avidity for donor Ag. This percentage significantly exceeded pre- and postrejection values obtained during the first year post-transplantation (median, 39% and 37%, respectively). The disparity in avidity increased even further more than 1 year after transplantation, when stable engraftment was achieved. Among donor-specific pCTL in peripheral blood, those with a high avidity were absent (median, 0%). Hence the avidity of circulating donor-specific CTL might inform us about the immune status of the cardiac allograft.

Prevention of Marrow Graft Rejection Without Induction of Graft-Versus-Host Disease by a Cytotoxic T-Cell Clone That Recognizes Recipient Alloantigens

In allogeneic marrow transplantation, donor T cells that recognize recipient alloantigens prevent rejection but also cause graft-versus-host disease (GVHD). To evaluate whether the ability to prevent marrow graft rejection could be dissociated from the ability to cause GVHD, we generated a panel of four different CD8 cytotoxic T-lymphocyte clones specific for H2d alloantigens. Three of the clones caused no overt toxicity when as many as 20 × 106 cells were infused intravenously into irradiated H2d-positive recipients, and one clone caused acute lethal toxicity within 1 to 3 days after transferring 10 × 106cells into H2d-positive recipients. One clone that did not cause toxicity was able to prevent rejection of (C57BL/6J × C3H/HeJ)F1 marrow in 800 cGy-irradiated (BALB/cJ × C57BL/6J)F1 recipients without causing GVHD. Large numbers of cells and exogenously administered interleukin-2 were required to prevent rejection. These results with different CD8 clones suggest that GVHD and prevention of rejection could be separable effects mediated by distinct populations of donor T cells that recognize recipient alloantigens.

Prevention of Marrow Graft Rejection Without Induction of Graft-Versus-Host Disease by a Cytotoxic T-Cell Clone That Recognizes Recipient Alloantigens

In allogeneic marrow transplantation, donor T cells that recognize recipient alloantigens prevent rejection but also cause graft-versus-host disease (GVHD). To evaluate whether the ability to prevent marrow graft rejection could be dissociated from the ability to cause GVHD, we generated a panel of four different CD8 cytotoxic T-lymphocyte clones specific for H2d alloantigens. Three of the clones caused no overt toxicity when as many as 20 × 106 cells were infused intravenously into irradiated H2d-positive recipients, and one clone caused acute lethal toxicity within 1 to 3 days after transferring 10 × 106cells into H2d-positive recipients. One clone that did not cause toxicity was able to prevent rejection of (C57BL/6J × C3H/HeJ)F1 marrow in 800 cGy-irradiated (BALB/cJ × C57BL/6J)F1 recipients without causing GVHD. Large numbers of cells and exogenously administered interleukin-2 were required to prevent rejection. These results with different CD8 clones suggest that GVHD and prevention of rejection could be separable effects mediated by distinct populations of donor T cells that recognize recipient alloantigens.

Alloreactive T Cells That Do Not Require TCR and CD8 Coengagement Are Present in Naive Mice and Contribute to Graft Rejection

Class I alloreactive CTL populations have been defined as either CD8 dependent or CD8 independent, based upon their ability to kill target cells in the presence of Ab to CD8. The CD8-dependent population uses CD8 in a coreceptor role with the TCR, and mutations in the class I molecule that destroy the CD8 binding site abrogate CTL killing, even if the target cell expresses other allelic forms of class I molecules with an intact binding site for CD8. The CD8-independent population apparently does not require CD8, as Ab to CD8 has no effect on the ability of these cells to kill appropriate target cells. We have isolated a third population of CTL that is inhibited by the addition of CD8 Ab yet can kill target cells that express the alloantigenic molecule incapable of binding CD8, provided that the target cells also express non antigenic class I molecules that contain an intact binding site for CD8. We refer to these cells as CD8 bystander-dependent CTL. Many (10 of 12) of these CTL were able to kill H-2Kb-expressing transfectants of T2 cells, consistent with the idea that they recognize a peptide-independent determinant that may be expressed at a high density on the cell surface. These CD8 bystander-dependent CTL are only readily detectable in vitro when spleen cells from mice primed in vivo with a skin graft are used.

Consequences of Intrathymic TCR Engagement by Partial Agonist on Selection Events and Peripheral T Cell Activation Program

Functions elicited from mature T cells depend on the nature of the Ag. Thus, an agonist induces a larger set of cytokine responses than a partial agonist. Additionally, Ags present in the thymus influence both the selection of TCRs generated by gene rearrangement and the potential functional program of developing thymocytes. This can be approached by analysing the development of T cells in mice expressing the same transgenic TCR (tgTCR) under different conditions of intrathymic selection. H-2Kbm8 was found to act as a partial agonist for CD8+ T cells expressing a tgTCR specific for the H-2Kb alloantigen. Intrathymic exposure to full or to partial agonist affected the development of thymocytes at different stages, consistent with the respective CD8-independent and -dependent characteristic of the tgTCR/Ag interaction. The presence of the partial agonist led to the accumulation of a major population of thymocytes (tgTCRhighCD4−CD8low) originating from TCR engagement at the immature single-positive CD8low stage as evidenced by: 1) results from reaggregated thymic organ culture in the presence of H-2k/bm8 thymic stromal cells; 2) the absence of CD4+ thymocytes, the development of which depends on rearrangements of endogenous TCRα genes; and 3) the identification of the CD8low thymocytes as cycling cells. Peripheral CD8low T cells selected in an H-2k/bm8 thymus expressed a partial functional program in response to H-2Kb, akin to the response of CD8high T cells to a partial agonist. The analysis of the molecular bases for partial reactivity revealed a correlation with inefficient AP-1, but efficient NF-κB transactivation.

A structure-based approach to designing synthetic CD8alpha peptides that can inhibit cytotoxic T-lymphocyte responses

CD8 molecules function as co-receptors on cytotoxic T lymphocytes (CTLs), interacting with a nonpolymorphic region of the major histocompatibility complex (MHC) class I a3 domain on antigen-presenting cells. Analogues were designed from a structural model of the mouse CD8a molecule to identify surfaces involved in CD8 function. Peptides were screened for in vitro biological activity on alloreactive CTLs, and analogue SC4 (p54-59) was found to be inhibitory during both the generation and effector stages. SC4 was also able to significantly prolong skin allograft survival across a MHC class I barrier. Thus, such CD8 analogues may have therapeutic potential as immunoregulators of CTL immune responses.

Positive Selection of Thymocytes Expressing the Same TCR by Different MHC Ligands Results in the Production of Functionally Distinct Thymocytes Distinguished by Differential Expression of the Heat Stable Antigen

The issue of whether the signaling process during positive selection can affect the efficiency by which the positively selected T cells respond to antigenic stimulation has not been addressed. We approached this question by determining the consequences of positive selection of a particular transgenic TCR (2C TCR) in the H-2b and the H-2k thymus. The H-2b thymus provides a strong positive-selecting environment for the 2C TCR, whereas the H-2k thymus selects weakly for the 2C TCR. Although the positively selected CD8 thymocytes from the H-2b or H-2k thymus expressed similar levels of the CD8 coreceptor molecule, those for the H-2k thymus expressed a slightly lower level of the 2C TCR. This lower level of 2C TCR expression by H-2k CD8 thymocytes was not a result of coexpression of endogenous TCRs. Interestingly, CD8 thymocytes from H-2k mice were hyporesponsive to Ag stimulation compared with those from the H-2b mice. The functional maturity of positively selected CD8 thymocytes from the H-2b or H-2k thymus was inversely correlated with the level of heat stable Ag expressed by these cells. Furthermore, TCR-derived signals appear to be more efficiently coupled to downstream pathways leading to proliferation and cytokine production in CD8 thymocytes from H-2b 2C mice than those derived from H-2k 2C mice. These results provide the first demonstration that the intensity of the signaling process during positive selection affects the efficiency by which TCR-derived signals in positively selected thymocytes are coupled to downstream effector pathways.

Evidence for clonal deletion and clonal anergy after intrathymic antigen injection in a transplantation model

Intrathymic (IT) antigen injection has been shown to induce antigen-specific systemic tolerance in the rodent. To delineate the mechanisms responsible for the induction of tolerance, we used the 2C line of T cell receptor transgenic mice. The majority of T cells in 2C mice express an antigen receptor specific for the major histocompatibility complex class I alloantigen Ld and can be identified with the clonotypic monoclonal antibody 1B2. IT injection of lymphoid cells expressing Ld was found to induce a significant prolongation in BALB/c skin allograft survival. The allograft prolongation was associated with a marked reduction in the number of developing 1B2+ thymocytes (clonal deletion), which occurred primarily at the CD4+ CD8+ stage of thymocyte development, as well as a reduction in the number of mature CD8+ 1B2+ 2C T cells in peripheral lymphoid tissue. In addition, CD8+ 1B2+ 2C T cells that survive deletion have decreased CD8 expression levels and a significantly reduced in vitro proliferative response to specific alloantigen (clonal anergy). Exogenous recombinant interleukin 2 restores the capacity of 2C T cells to respond in vitro to alloantigen. Experiments involving separation of cells by fluorescence-activated cell sorter indicate that there is a precise correlation between the reduction in CD8 expression and anergy induction. Collectively, these data indicate that IT antigen injection can induce antigen-specific systemic tolerance by both clonal deletion and clonal anergy.

Naive alloreactive CD8 T cells are activated by purified major histocompatibility complex class I and antigenic peptide

In this report, we demonstrate stimulation of T cell receptor (TCR) transgenic CD8 T cells by isolated major histocompatibility complex (MHC) class I H-2Ld complexes and antigenic peptide. This is the first demonstration of CD8 T cells activated by MHC and antigenic peptide in the absence of antigen priming. Furthermore, isolated MHC and a potent peptide antigen can stimulate phenotypically naive CD44- T cells to become CTL effectors and to produce interleukin-2 in nanogram per milliliter amounts. These results demonstrate that particular TCR antigen pairs may overcome the need for specialized antigen-presenting cells and have implications for mechanisms of autoimmunity and tolerance induction.

Requirements for peptide-induced T cell receptor downregulation on naive CD8+ T cells

The requirements for inducing downregulation of alpha/beta T cell receptor (TCR) molecules on naive major histocompatibility complex class I-restricted T cells was investigated with 2C TCR transgenic mice and defined peptides as antigen. Confirming previous results, activation of 2C T cells in response to specific peptides required CD8 expression on the responder cells and was heavily dependent upon costimulation provided by either B7-1 or ICAM-1 on antigen-presenting cells (APC). These stringent requirements did not apply to TCR downregulation. Thus, TCR downregulation seemed to depend solely on TCR/peptide/interaction and did not require either CD8 or B7-1 expression; ICAM-1 potentiated TCR downregulation, but only with limiting doses of peptides. TCR downregulation was most prominent with high affinity peptides and appeared to be neither obligatory nor sufficient for T cell activation. In marked contrast to T cell activation, TCR downregulation was resistant to various metabolic inhibitors. The biological significance of TCR downregulation is unclear, but could be a device for protecting T cells against excessive signaling.

Transfected Drosophila cells as a probe for defining the minimal requirements for stimulating unprimed CD8+ T cells

Stimulation of naive T cells by antigen-presenting cells (APC) is thought to involve two qualitatively different signals: signal one results from T-cell receptor (TCR) recognition of antigenic peptides bound to major histocompatibility complex (MHC) molecules, whereas signal two reflects contact with one or more costimulatory molecules. The requirements for stimulating naive T cells were studied with MHC class I-restricted CD8+ T cells from a T-cell receptor transgenic line, with defined peptides as antigen and transfected Drosophila cells as APC. Three main findings are reported. First, stimulation of naive T cells via signal one alone (MHC plus peptide) was essentially nonimmunogenic; thus T cells cultured with peptides presented by MHC class I-transfected Drosophila APC lacking costimulatory molecules showed little or no change in their surface phenotype. Second, cotransfection of two costimulatory molecules, B7-1 and intercellular adhesion molecule 1 (ICAM-1), converted class I+ Drosophila cells to potent APC capable of inducing strong T-proliferative responses and cytokine (interleukin 2) production. Third, B7-1 and ICAM-1 acted synergistically, indicating that signal two is complex; synergy between B7-1 and ICAM-1 varied from moderate to extreme and was influenced by both the dose and affinity of the peptide used and the parameter of T-cell activation studied. Transfected Drosophila cells are thus a useful tool for examining the minimal APC requirements for naive T cells.

Resting memory CD8+ T cells are hyperreactive to antigenic challenge in vitro

The characteristics of CD8+ T cells responsible for memory responses are still largely unknown. Particularly, it has not been determined whether different activation thresholds distinguish naive from memory CD8+ T cell populations. In most experimental systems, heterogeneous populations of primed CD8+ T cells can be identified in vivo after immunization. These cells differ in terms of cell cycle status, surface phenotype, and/or effector function. This heterogeneity has made it difficult to assess the activation threshold and the relative role of these subpopulations in memory responses. In this study we have used F5 T cell receptor transgenic mice to generate a homogeneous population of primed CD8+ T cells. In the F5 transgenic mice, peptide injection in vivo leads to activation of most peripheral CD8+ T cells. In vivo BrdU labeling has been used to follow primed T cells over time periods spanning several weeks after peptide immunization. Our results show that the majority of primed CD8+ T cells generated in this system are not cycling and express increased levels of CD44 and Ly6C. These cells remain responsive to secondary peptide challenge in vivo as evidenced by short term upregulation of activation markers such as CD69 and CD44. The activation thresholds of naive and primed CD8+ T cells were compared in vitro. We found that CD8+ T cells from primed mice are activated by peptide concentrations 10-50-fold lower than naive mice. In addition, the kinetics of interleukin 2R alpha chain upregulation by primed CD8+ T cells differ from naive CD8+ T cells. These primed hyperresponsive CD8+ T cells might play an important role in the memory response.

Alloreactive anti-HLA-B7 cytolytic T cell clones use restricted T cell receptor genes

Most alloreactive T cells specifically recognize peptides bound to donor MHC molecules. In addition to peptides, solvent accessible MHC residues also may stimulate alloreactive T cells. We studied T cell receptor (TCR) usage by 16 independent anti-HLA-B7 alloreactive cytolytic T lymphocyte (CTL) clones. Most or all of these CTL clones recognized unique peptides bound to HLA-B7. Despite the diversity of peptides recognized, 11 out of 15 CTL clones analyzed expressed TCR V(alpha) gene segment (AV) subgroups 1 and 3. Within AV subgroup 1, four of six clones expressed AV2; within AV subgroup 3, three clones used AV6. Ten of 14 CTL clones analyzed expressed V(beta) gene segment (BV) subgroups 4 and 1. In subgroup 4, BV14 was expressed by four of five alloreactive CTL clones. Similar AV and BV usage restriction was not found in mitogen-stimulated peripheral blood T cells from the major donor of the CTL clones. TCR A and TCR B junctional region sequences were quite diverse in length and sequence, although two CTL clones expressed nearly identical TCR B chains. We found no correlation between TCR AV or TCR BV usage and CTL recognition of 81 HLA-B7 variants. These results are consistent with models of TCR structure, in which very diverse TCR CDR3 regions contact very diverse peptides, and moderately diverse TCR CDR1 and CDR2 regions contact moderately diverse MHC alpha-helices.

A single T cell receptor recognizes structurally distinct MHC/peptide complexes with high specificity

The 2C T cell is a CD8+, alloreactive T cell, which recognizes cells bearing Ld and Kbm3 class I major histocompatability complex molecules. Here, we characterize an allopeptide, designated dEV-8, that is a ligand in the Kbm3 molecule for the 2C TCR but is not a ligand in the Ld molecule. By biochemical and immunological properties, dEV-8 is distinct from P2Ca, the Ld allopeptide that is also recognized by the 2C TCR. Using the deduced amino acid sequence of dEV-8, we isolate a candidate endogenous source of the peptide. The endogenous protein, MLRQ, contains a peptide sequence identical to dEV-8. This degenerate recognition of two distinct peptide/MHC complexes by a single TCR has important implications for understanding allorecognition.

Progressive accumulation of CTL with high avidity for donor antigens during the development of acute cardiac rejection

To study the importance of cytotoxic T lymphocytes (CTL) with high avidity for donor antigens (Ag) in the development of acute cardiac allograft rejection, their appearance within the graft in relation to rejection was analyzed. For this study, donor directed CTL propagated from sequentially taken endomyocardial biopsies (EMB) were enumerated by limiting dilution analysis (LDA). Subsequentially, the fraction of these CTL having high avidity for donor Ag was determined by addition of a CD8 monoclonal antibody (mAb) to the cytotoxic phase of the LDA. Analysis of 37 EMB cultures obtained from 11 heart transplant (HTx) patients before, during, or after they experienced rejection, revealed the kinetics of donor specific CTL in relation to rejection for HTx patients in general. For 5 individual recipients, a more detailed analysis was performed. The kinetics found for individual patients confirmed the pattern found for the total group of HTx recipients tested. Frequencies of donor specific precursor CTL (pCTL) as well as of in vivo primed donor reactive CTL (committed CTL or cCTL) increased towards rejection and decreased after successful rejection therapy. More than 2 weeks before rejection was diagnosed, only a small fraction of the graft infiltrating donor specific pCTL and cCTL had high avidity for donor Ag (median = 35% and 11%, respectively). Within 2 weeks preceding rejection, this fraction increased gradually (median = 52% and 55%, respectively) and became dominant during rejection (median = 87% and 78%, respectively). After successful rejection therapy, a decrease to basal levels (median = 18% and 24%, respectively) was observed. Conclusively, intragraft accumulation of high avidity, donor specific pCTL and cCTL may cause transplant rejection.

Immune deviation of 2C transgenic intraepithelial lymphocytes in antigen-bearing hosts

The present study examined self-tolerance for T cell receptor (TCR) alpha beta intestinal intraepithelial lymphocytes (iIELs) using the 2C transgenic (Tg) mouse model specific for a peptide antigen (Ag) presented by the class I major histocompatibility complex H-2Ld. Although Tg+ T cells were largely deleted from the periphery of Ag+ mice, equivalent numbers of Tg iIELs were present in Ag+ compared to Ag- mice. Tg iIELs in Ag- mice contained CD8 alpha beta, CD8 alpha alpha, and CD4-CD8- subsets, whereas only CD8 alpha alpha and CD4-CD8- Tg iIEL subsets were detected in Ag+ mice. Analysis of surface markers revealed that Tg iIELs in Ag+ mice expressed decreased levels of Thy-1 and increased CD45R/B220 as compared to Ag- Tg iIELs. In response to activation with exogenous peptide or immobilized anti-TCR mAB, iIELs from Ag- mice proliferated at high levels and produced interleukin (IL)-2 and interferon (IFN)-gamma, while Tg+ iIELs from Ag+ mice proliferated at low levels and failed to produce detectable IL-2 or IFN-gamma. Activation of sorted iIEL subsets from Ag- mice revealed that CD8 alpha alpha and CD4-CD8- subsets produced low levels of IL-2 and IFN-gamma in response to activation with antigen-presenting cells and added peptide or immobilized anti-TCR mAb, while CD8 alpha beta + iIELs responded to endogenous levels of peptide. In response to APC and exogenous peptide, sorted iIEL subsets from Ag+ mice produced IL-2 and IFN-gamma, and proliferated at greatly reduced levels compared to corresponding subsets from Ag- mice. Analysis of cytokine mRNA levels revealed that activation in vitro induced IL-2 mRNA only in Ag-, but not Ag+ iIELs, whereas a high level of IL-4 mRNA induction was detected in Tg+ iIELs from Ag+ mice, and to a lesser degree, from Ag- mice. These data suggest that tolerance for Tg+ iIELs resulted in the deletion of CD8 alpha beta + subsets and the persistence of Tg+ iIEL subsets with decreased sensitivity to endogenous levels of self-peptide. A comparison of the cytokine profiles expressed by Tg+ iIEL subsets in Ag- and Ag+ mice suggested that tolerance induction had involved the functional deviation of cells from TC1 (T helper-1-like) to a less inflammatory TC2 (T helper-2-like) phenotype capable of mediating humoral immune responses in the mucosa.

Dual function of Drosophila cells as APCs for naive CD8+ T cells: implications for tumor immunotherapy

With unseparated mouse spleen cells as responders, Drosophila cells expressing MHC class I (L(d)) molecules alone lead to peptide-specific responses of CD8+ cells in the absence of exogenous cytokines. Under these conditions, DNA released from dying cells stimulates the B cells in spleen to up-regulate costimulatory molecules; these activated B cells then provide bystander costimulation for CD8+ cells responding to class I-peptide complexes on the Drosophila APCs. By stimulating B cells and presenting antigen to T cells, Drosophila cells thus serve two different functions in promoting primary responses of CD8+ cells in vitro. With this system, we show that Ld-transfected Drosophila cells are able to induce autologous spleen cells to respond to a tumor-specific peptide in vitro and, after transfer, cause tumor rejection in vivo.

Influence of antigen dose and costimulation on the primary response of CD8+ T cells in vitro

The influence of costimulation on the primary response of CD8+ T cells to class I alloantigens was studied with the aid of a T cell receptor transgenic model and defined peptides as antigen. With small doses of antigen, the proliferative response of CD8+ cells was high early in culture but was of brief duration and declined to low levels by day 4; this abbreviated response was associated with limited production of interleukin 2 (IL-2) and was strongly dependent upon costimulation via CD8-major histocompatibility complex class I and CD28-B7 interactions. The response to large doses of antigen was quite different in two respects. First, large doses of antigen inhibited the early (day 3) proliferative response but caused a marked elevation of the response late in culture (day 5); these altered kinetics were associated with increased production of IL-2. Second, the initial proliferative response to large doses of antigen did not require costimulation: indeed, blocking costimulation with CTLA4lg or anti-CD8 monoclonal antibody enhanced the early proliferative response. However, blocking costimulation impaired IL-2 production and prevented the late proliferative response. These findings indicate that the requirement for costimulation of T cells can be partly overcome by increasing the dose of antigen to a high level. However, costimulation plays a key role in prolonging the response, presumably by triggering strong and sustained production of IL-2.