Characterization of virus-specific cytotoxic T cell clones from allogeneic bone marrow chimeras

ER-associated degradation adapter Sel1L is required for CD8+ T cell function and memory formation following acute viral infection

The maintenance of antigen-specific CD8+ T cells underlies the efficacy of vaccines and immunotherapies. Pathways contributing to CD8+ T cell loss are not completely understood. Uncovering the pathways underlying the limited persistence of CD8+ T cells would be of significant benefit for developing novel strategies of promoting T cell persistence. Here, we demonstrate that murine CD8+ T cells experience endoplasmic reticulum (ER) stress following activation and that the ER-associated degradation (ERAD) adapter Sel1L is induced in activated CD8+ T cells. Sel1L loss limits CD8+ T cell function and memory formation following acute viral infection. Mechanistically, Sel1L is required for optimal bioenergetics and c-Myc expression. Finally, we demonstrate that human CD8+ T cells experience ER stress upon activation and that ER stress is negatively associated with improved T cell functionality in T cell-redirecting therapies. Together, these results demonstrate that ER stress and ERAD are important regulators of T cell function and persistence.

Gene editing of hematopoietic stem cells restores T-cell response in familial hemophagocytic lymphohistiocytosis

BACKGROUND

Hemophagocytic lymphohistiocytosis (HLH) is a hyperinflammatory disorder characterized by a life-threatening cytokine storm and immunopathology. Familial HLH type 3 (FHL3) accounts for approximately 30% of all inborn HLH cases worldwide. It is caused by mutations in the UNC13D gene that result in impaired degranulation of cytotoxic vesicles and hence compromised T-cell- and natural killer-cell-mediated killing. Current treatment protocols, including allogeneic hematopoietic stem cell (HSC) transplantation, still show high mortality.

OBJECTIVE

We sought to develop and evaluate a curative genome editing strategy in the preclinical FHL3 Jinx mouse model. Jinx mice harbor a cryptic splice donor site in Unc13d intron 26 and develop clinical symptoms of human FHL3 upon infection with lymphocytic choriomeningitis virus (LCMV).

METHODS

We employed clustered regularly interspaced short palindromic repeats (CRISPR)-Cas technology to delete the disease-causing mutation in HSCs and transplanted Unc13d-edited stem cells into busulfan-conditioned Jinx recipient mice. Safety studies included extensive genotyping and chromosomal aberrations analysis by single targeted linker-mediated PCR sequencing (CAST-Seq)-based off-target analyses. Cure from HLH predisposition was assessed by LCMV infection.

RESULTS

Hematopoietic cells isolated from transplanted mice revealed efficient gene editing (>95%), polyclonality of the T-cell receptor repertoire, and neither signs of off-target effects nor leukemogenesis. Unc13d transcription levels of edited and wild-type cells were comparable. While LCMV challenge resulted in acute HLH in Jinx mice transplanted with mock-edited HSCs, Jinx mice grafted with Unc13d-edited cells showed rapid virus clearance and protection from HLH.

CONCLUSIONS

Our study demonstrates that transplantation of CRISPR-Cas edited HSCs supports the development of a functional polyclonal T-cell response in the absence of genotoxicity-associated clonal outgrowth.

The SWI/SNF chromatin remodeling complexes BAF and PBAF differentially regulate epigenetic transitions in exhausted CD8+ T cells

CD8⁺ T cell exhaustion (Tex) limits disease control during chronic viral infections and cancer. Here, we investigated the epigenetic factors mediating major chromatin-remodeling events in Tex-cell development. A protein-domain-focused in vivo CRISPR screen identified distinct functions for two versions of the SWI/SNF chromatin-remodeling complex in Tex-cell differentiation. Depletion of the canonical SWI/SNF form, BAF, impaired initial CD8⁺ T cell responses in acute and chronic infection. In contrast, disruption of PBAF enhanced Tex-cell proliferation and survival. Mechanistically, PBAF regulated the epigenetic and transcriptional transition from TCF-1⁺ progenitor Tex cells to more differentiated TCF-1^- Tex subsets. Whereas PBAF acted to preserve Tex progenitor biology, BAF was required to generate effector-like Tex cells, suggesting that the balance of these factors coordinates Tex-cell subset differentiation. Targeting PBAF improved tumor control both alone and in combination with anti-PD-L1 immunotherapy. Thus, PBAF may present a therapeutic target in cancer immunotherapy.

Immunoproteasome Upregulation Is Not Required to Control Protein Homeostasis during Viral Infection

The prime function of proteasomes is the control of protein homeostasis in cells (i.e., the removal of proteins that are not properly folded, damaged by stress conditions like reactive oxygen species formation, or degraded on the basis of regular protein turnover). During viral infection, the standard proteasome is replaced by the so-called immunoproteasome (IP) in an IFN-γ-dependent manner. It has been proposed that the IP is required to protect cell viability under conditions of IFN-induced oxidative stress. In this study, we investigated the requirement for IP to cope with the enhanced need for protein degradation during lymphocytic choriomeningitis virus (LCMV) infection in mice lacking the IP subunit LMP7. We found that IP are upregulated in the liver but not in the spleen during LCMV infection, although the total proteasome content was not altered. The expression of standard proteasome subunits is not induced in LMP7-deficient mice, indicating that enhanced proteasomal activity is not required during viral infection. Furthermore, ubiquitin accumulation, apoptosis induction, and viral titers were similar in LCMV-infected mice lacking LMP7 compared with wild-type mice. Taken together, these data indicate that the IP is not required to regulate protein homeostasis during LCMV infection.

TCR Transgenic Mice: A Valuable Tool for Studying Viral Immunopathogenesis Mechanisms

Viral infectious diseases are a significant burden on public health and the global economy, and new viral threats emerge continuously. Since CD4⁺ and CD8⁺ T cell responses are essential to eliminating viruses, it is important to understand the underlying mechanisms of anti-viral T cell-mediated immunopathogenesis during viral infections. Remarkable progress in transgenic (Tg) techniques has enabled scientists to more readily understand the mechanisms of viral pathogenesis. T cell receptor (TCR) Tg mice are extremely useful in studying T cell-mediated immune responses because the majority of T cells in these mice express specific TCRs for partner antigens. In this review, we discuss the important studies utilizing TCR Tg mice to unveil underlying mechanisms of T cell-mediated immunopathogenesis during viral infections.

Cyclophilin D Regulates Antiviral CD8+ T Cell Survival in a Cell-Extrinsic Manner

CD8+ T cell-mediated immunity is critical for host defense against viruses and requires mitochondria-mediated type I IFN (IFN-I) signaling for optimal protection. Cyclophilin D (CypD) is a mitochondrial matrix protein that modulates the mitochondrial permeability transition pore, but its role in IFN-I signaling and CD8+ T cell responses to viral infection has not been previously explored. In this study, we demonstrate that CypD plays a critical extrinsic role in the survival of Ag-specific CD8+ T cell following acute viral infection with lymphocytic choriomeningitis virus in mice. CypD deficiency resulted in reduced IFN-I and increased CD8+ T cell death, resulting in a reduced antiviral CD8+ T cell response. In addition, CypD deficiency was associated with an increase in pathogen burden at an early time-point following infection. Furthermore, our data demonstrate that transfer of wild-type macrophages (expressing CypD) to CypD-deficient mice can partially restore CD8+ T cell responses. These results establish that CypD plays an extrinsic role in regulating optimal effector CD8+ T cell responses to viral infection. Furthermore, this suggests that, under certain circumstances, inhibition of CypD function may have a detrimental impact on the host's ability to respond to viral infection.

Analysis of the T Cell Response to Zika Virus and Identification of a Novel CD8+ T Cell Epitope in Immunocompetent Mice

Zika virus (ZIKV) is an emerging arbovirus of the Flaviviridae family. Although ZIKV infection is typically mild and self-limiting in healthy adults, infection has been associated with neurological symptoms such as Guillain-Barré syndrome, and a causal link has been established between fetal microcephaly and ZIKV infection during pregnancy. These risks, and the magnitude of the ongoing ZIKV pandemic, have created an urgent need for the development of animal models to study the immune response to ZIKV infection. Previous animal models have primarily focused on pathogenesis in immunocompromised mice. In this study, we provide a model of ZIKV infection in wild-type immunocompetent C57BL/6 mice, and have provided an analysis of the immune response to infection. We evaluated the activation of several innate immune cell types, and studied the kinetics, phenotype, and functionality of T cell responses to ZIKV infection. Our results demonstrate that ZIKV infection is mild in wild-type immunocompetent C57BL/6 mice, resulting in minimal morbidity. Our data establish that at the peak of the adaptive response, antigen-experienced CD4⁺ T cells polarize to a Th1 phenotype, and antigen-experienced CD8⁺ T cells exhibit an activated effector phenotype, producing both effector cytokines and cytolytic molecules. Furthermore, we have identified a novel ZIKV CD8⁺ T cell epitope in the envelope protein that is recognized by the majority of responding cells. Our model provides an important reference point that will help dissect the impact of polymorphisms in the circulating ZIKV strains on the immune response and ZIKV pathogenesis. In addition, the identification of a ZIKV epitope will allow for the design of tetramers to study epitope-specific T cell responses, and will have important implications for the design and development of ZIKV vaccine strategies.

Zika virus (ZIKV) is a mosquito-borne human pathogen of the Flaviviridae family. Most notably, it is responsible for an ongoing epidemic in the Americas, and has been causally linked to birth defects such as fetal microcephaly. These factors have led to an urgent need for small animal models, which may be used to study ZIKV infection, pathogenesis, and the antiviral immune response. To date, the majority of mouse models developed have used mice that lack a competent immune system. These models are excellent for characterizing infection and the pathogenesis of the virus, but are unable to provide a comprehensive analysis of the immune response to ZIKV infection, which will be useful for the design of effective vaccine strategies. Herein, we demonstrate that ZIKV is able to infect wild-type immunocompetent C57BL/6 mice, resulting in activation of the innate immune response and induction of an antiviral T cell response. We have also identified a novel epitope recognized by CD8⁺ T cells within the ZIKV envelope protein. Our model provides an important reference point regarding the T cell response to ZIKV infection, which will be useful in comparative analyses and will have implications in the design and development of effective vaccines.

Inflammatory IL-15 is required for optimal memory T cell responses

Due to their ability to rapidly proliferate and produce effector cytokines, memory CD8+ T cells increase protection following reexposure to a pathogen. However, low inflammatory immunizations do not provide memory CD8+ T cells with a proliferation advantage over naive CD8+ T cells, suggesting that cell-extrinsic factors enhance memory CD8+ T cell proliferation in vivo. Herein, we demonstrate that inflammatory signals are critical for the rapid proliferation of memory CD8+ T cells following infection. Using murine models of viral infection and antigen exposure, we found that type I IFN-driven expression of IL-15 in response to viral infection prepares memory CD8+ T cells for rapid division independently of antigen reexposure by transiently inducing cell-cycle progression via a pathway dependent on mTOR complex-1 (mTORC1). Moreover, exposure to IL-15 allowed more rapid division of memory CD8+ T cells following antigen encounter and enhanced their protective capacity against viral infection. Together, these data reveal that inflammatory IL-15 promotes optimal responses by memory CD8+ T cells.

Polymicrobial Sepsis Increases Susceptibility to Chronic Viral Infection and Exacerbates CD8+ T Cell Exhaustion

Patients who survive sepsis display suppressed immune functions, often manifested as an increased susceptibility to secondary infections. Recently, using a cecal-ligation and puncture (CLP) model of sepsis, we showed that sepsis induces substantial and long-lasting changes in the available naive CD8(+) T cell repertoire affecting the capacity of the host to respond to newly encountered acute infections. However, the extent to which sepsis changes the host susceptibility to chronic infection and affects CD8(+) T cell responses is currently unknown. In this study, we demonstrate that inbred and outbred mice recovering from a septic event are more susceptible to lymphocytic choriomeningitis virus (LCMV) clone-13 infection exhibited by mortality and viral burden. Primary virus-specific CD8(+) T cells in LCMV clone-13-infected septic mice displayed exacerbated CD8(+) T cell exhaustion illustrated by increased inhibitory molecule expression (e.g., programmed cell death 1, lymphocyte-activation gene 3, and 2B4) and diminished Ag-driven cytokine production (e.g., IFN-γ, TNF-α) compared with similarly infected sham-treated mice. Importantly, therapeutic inhibitory molecule dual blockade (anti-PD-L1 and anti-lymphocyte-activation gene 3) increased the number of circulating LCMV-specific CD8(+) T cells, and improved CD8(+) T cell function and pathogen control in chronically infected septic mice. Together, these results illustrate that polymicrobial sepsis compromises the overall health of the host leading to increased vulnerability to chronic infection and exacerbated CD8(+) T cell exhaustion. Collectively, our findings suggest that septic survivors may be more susceptible and at greater risk for developing exhaustible CD8(+) T cells upon encountering a subsequent chronic infection.

Cutting edge: Expression of FcγRIIB tempers memory CD8 T cell function in vivo

During reinfection, high-affinity IgG Abs form complexes with both soluble Ag and Ag displayed on the surface of infected cells. These interactions regulate cellular activation of both innate cells and B cells, which express specific combinations of activating FcγRs (FcγRI, FcγRIII, FcγRIV) and/or the inhibitory FcγR (FcγRIIB). Direct proof for functional expression of FcγR by Ag-specific CD8 T cells is lacking. In this article, we show that the majority of memory CD8 T cells generated by bacterial or viral infection express only FcγRIIB, and that FcγRIIB could be detected on previously activated human CD8 T cells. Of note, FcγR stimulation during in vivo Ag challenge not only inhibited the cytotoxicity of memory CD8 T cells against peptide-loaded or virus-infected targets, but FcγRIIB blockade during homologous virus challenge enhanced the secondary CD8 T cell response. Thus, memory CD8 T cells intrinsically express a functional FcγRIIB, permitting Ag-Ab complexes to regulate secondary CD8 T cell responses.

Sustained and incomplete recovery of naive CD8+ T cell precursors after sepsis contributes to impaired CD8+ T cell responses to infection

Patients who survive severe sepsis often display compromised immune function with impairment in innate and adaptive immune responses. These septic patients are highly susceptible to "secondary" infections with intracellular pathogens that are usually controlled by CD8(+) T cells. It is not known when and if this observed immunoparalysis of CD8(+) T cell immunity recovers, and the long-term consequences of sepsis on the ability of naive CD8(+) T cells to respond to subsequent infections are poorly understood. In this study, using the cecal-ligation and puncture mouse model of sepsis, we show that sepsis induces a rapid loss of naive CD8(+) T cells. However, IL-15-dependent numerical recovery is observed a month after initial septic insult. Numerical recovery is accompanied by IL-15-dependent phenotypic changes where a substantial proportion of naive (Ag-inexperienced) CD8(+) T cells display a "memory-like" phenotype (CD44(hi)/CD11a(hi)). Importantly, the impairment of naive CD8(+) T cells to respond to viral and bacterial infection was sustained for month(s) after sepsis induction. Incomplete recovery of naive CD8(+) T cell precursors was observed in septic mice, suggesting that the availability of naive precursors contributes to the sustained impairment in primary CD8(+) T cell responses. Thus, sepsis can result in substantial and long-lasting changes in the available CD8(+) T cell repertoire affecting the capacity of the host to respond to new infections.

Pathogen-specific inflammatory milieux tune the antigen sensitivity of CD8(+) T cells by enhancing T cell receptor signaling

CD8(+) T cells confer host protection through T-cell-receptor (TCR)-mediated recognition of foreign antigens presented by infected cells. Thus, generation of CD8(+) T cell populations with high antigen sensitivity is critical for efficient pathogen clearance. Besides selection of high-affinity TCRs, the molecular mechanisms regulating the antigen sensitivity of CD8(+) T cells remain poorly defined. Herein, we have demonstrated that the antigen sensitivity of effector and memory CD8(+) T cells is dynamically regulated and can be tuned by pathogen-induced inflammatory milieux independently of the selection of cells with higher TCR affinity. Mechanistically, we have demonstrated that the signal-transduction capacity of key TCR proximal molecules is enhanced by inflammatory cytokines, which reduced the antigen density required to trigger antimicrobial functions. Dynamic tuning of CD8(+) T cell antigen sensitivity by inflammatory cytokines most likely optimizes immunity to specific pathogens while minimizing the risk of immunopathology at steady state.

Probing CD8 T cell responses with Listeria monocytogenes infection

CD8 T cells play a critical role in the control and eradication of intracellular pathogens. Increased understanding of CD8 T cell biology provides insight that can be translated into improved vaccination strategies. The intracellular bacterium, Listeria monocytogenes, has been used as a model organism to study every phase of the CD8 T cell response to intracellular bacterial infection. Infection of laboratory mice with L. monocytogenes has provided insight into the factors that are involved in primary T cell responses, memory CD8 T cell generation, maintenance, functionality, and diversification following repeated pathogenic challenges. In this review, we will focus on work from our laboratories utilizing the murine model of L. monocytogenes to investigate the characteristics of CD8 T cell responses to infection. This model has profoundly advanced our understanding of the CD8 T cell response to infection and is likely to continue to provide invaluable basic insights that can be translated into the development of effective vaccination strategies to protect against pathogens.

B7-H4 mediates inhibition of T cell responses by activated murine hepatic stellate cells

Liver fibrosis is mediated by the transformation of hepatic stellate cells (HSC) from a quiescent to an activated state. To understand the role of HSC in liver immunity, we investigated the effect of this transition on T cell stimulation in vitro. Unlike quiescent HSC, activated HSC did not induce proliferation of antigen-specific T cells. Phenotypic analysis of quiescent and activated HSC revealed that activated HSC expressed the coinhibitory molecule B7-H4. Silencing B7-H4 by small interfering RNA (siRNA) in activated HSC restored the ability of T cells to proliferate, differentiate, and regain effector recall responses. Furthermore, expression of B7-H4 on HSC inhibits early T cell activation and addition of exogenous interleukin (IL)-2 reversed the T cell anergy induced by activated HSC.

CONCLUSION

These studies reveal a novel role for activated HSC in the attenuation of intrahepatic T cell responses by way of expression of the coinhibitory molecule B7-H4, and may provide fundamental insight into intrahepatic immunity during liver fibrogenesis.

Designer T cells by T cell receptor replacement

T cell receptor (TCR) gene transfer is a convenient method to produce antigen-specific T cells for adoptive therapy. However, the expression of two TCR in T cells could impair their function or cause unwanted effects by mixed TCR heterodimers. With five different TCR and four different T cells, either mouse or human, we show that some TCR are strong--in terms of cell surface expression--and replace weak TCR on the cell surface, resulting in exchange of antigen specificity. Two strong TCR are co-expressed. A mouse TCR replaces human TCR on human T cells. Even though it is still poorly understood why some TCRalpha/beta combinations are preferentially expressed on T cells, our data suggest that, in the future, designer T cells with exclusive tumor reactivity can be generated by T cell engineering.

Rapid peptide turnover and inefficient presentation of exogenous antigen critically limit the activation of self-reactive CTL by dendritic cells

This study evaluated to what extent presentation of exogenously acquired self-Ags via MHC class I molecules on DC might contribute to the activation of self-reactive CTL and subsequent development of autoimmune disease. We show here by using the rat insulin promotor lymphocytic choriomeningitis virus glycoprotein model of autoimmune diabetes that the activation of self-reactive CTL by DC after uptake of exogenous Ag is very limited, first by the short half-life of MHC class I-associated peptides on DC in vitro and in vivo, and second by the rather inefficient MHC class I presentation of cell-associated self-Ags by DC. These two mechanisms are probably crucial in establishing high thresholds for the induction of self-reactive CTL that prevent autoimmune sequelae after release of sequestered and previously immunologically ignored tissue Ags.

Break of T cell ignorance to a viral antigen in the liver induces hepatitis

To study peripheral tolerance of CD8 T cells to a classically MHC-restricted peptide Ag expressed in hepatocytes, ALB1 transgenic (tg) mice expressing the CTL epitope GP33 of the lymphocytic choriomeningitis virus glycoprotein under control of the mouse albumin promoter were generated. ALB1 mice exclusively expressed the GP33 transgene in the liver and, at a 100- to 1000-fold lower level, in the thymus. TCR-tg mice specific for the GP33 epitope were used to directly follow GP33-specific T cells in vivo. These experiments revealed that 1) thymic expression of the GP33 transgene led to incomplete central deletion of TCR-tg cells; and 2) peripheral TCR-tg cells in ALB1 mice ignored the GP33 transgene expressed in hepatocytes. Ignorance of adoptively transferred TCR-tg cells in ALB1 mice was broken by infection with lymphocytic choriomeningitis virus, leading to induction of hepatitis in ALB1, but not in control, mice. Taken together, we have established a novel model of virus-induced CD8 T cell-mediated autoimmune hepatitis in mice and demonstrate that naive CD8 T cells may ignore Ags expressed in the liver.

The standard model of T-cell receptor function: a critical reassessment

The Standard Model of T-cell receptor (TCR) function is the distillation of many views. Here we provide a summary that is intended to capture the flavour of the whole, without assigning particular blame, or credit, to any one part. The Standard Model is based on the notion of a single TCR-combining site that sums the binding contributions of MHC and peptide to produce a single signal to the T cell. How this signal is interpreted can vary with the state of the T cell. A growing number of creaks in the tweaks needed to maintain the Standard Model suggest that it may be timely to make a critical reassessment of the facts and their interpretation. The result of this effort has been to uncover a long-overlooked fact that T cells do not recognize hybrid class II major histocompatibility complex alleles; they recognize only those haplotypes directly associated with each alpha- or beta- subunit of class II. Our attempts to tweak the Standard Model to deal with lack of recognition of hybrid class II alleles led us, by surprise, to a quite different framework with which to view TCR function.

An inhibitor of HIV-1 protease modulates proteasome activity, antigen presentation, and T cell responses

Inhibitors of the protease of HIV-1 have been used successfully for the treatment of HIV-1-infected patients and AIDS disease. We tested whether these protease inhibitory drugs exerted effects in addition to their antiviral activity. Here, we show in mice infected with lymphocytic choriomeningitis virus and treated with the HIV-1 protease inhibitor ritonavir a marked inhibition of antiviral cytotoxic T lymphocyte (CTL) activity and impaired major histocompatibility complex class I-restricted epitope presentation in the absence of direct effects on lymphocytic choriomeningitis virus replication. A potential molecular target was found: ritonavir selectively inhibited the chymotrypsin-like activity of the 20S proteasome. In view of the possible role of T cell-mediated immunopathology in AIDS pathogenesis, the two mechanisms of action (i.e., reduction of HIV replication and impairment of CTL responses) may complement each other beneficially. Thus, the surprising ability of ritonavir to block the presentation of antigen to CTLs may possibly contribute to therapy of HIV infections but potentially also to the therapy of virally induced immunopathology, autoimmune diseases, and transplantation reactions.

What can we learn about human immunodeficiency virus infection from a study of lymphocytic choriomeningitis virus?

The role of cytotoxic T lymphocytes (CTL) in human immunodeficiency virus (HIV) infection remains elusive. Since the discovery 10 years ago of high levels of specific CTL in this disease, some have argued that they play an important role in virus control, others that they drive disease progression through destruction of T helper cells, and others still that they play no obvious role at all. By contrast, the central role of CTL in murine lymphocytic choriomeningitis virus (LCMV) infection has been very clearly worked out through the use of in vivo depletion and adoptive transfer experiments, as well as knockout and transgenic mice. To interpret the possible roles for CTL in HIV, we have therefore made a comparison between what is known about CTL and their interaction with virus-infected cells in these two infections. This illustrates a potential critical role for these cells in both control of HIV replication and immune-mediated pathology, but one that is highly dependent on virus dose, distribution and dynamics.

Functional management of an antiviral cytotoxic T-cell response

Lymphocytic choriomeningitis virus (LCMV) is known to induce strong, polyclonal cytotoxic T-lymphocyte (CTL) responses. Using a set of variant peptides derived from the major CTL epitope of LCMV, we analyzed the functional fine specificity of the LCMV-specific CTL response. During the primary response, almost all the tested peptides were recognized. In contrast, the secondary response was purged of all minor cross-reactivities and very few peptides were significantly recognized. This study is the first demonstration of the functional maturation of a T-cell response and has important clinical and biological implications.

Specific recognition of thymic self-peptides induces the positive selection of cytotoxic T lymphocytes

To understand how thymic selection gives rise to T cells that are capable of major histocompatibility complex (MHC)-restricted recognition of antigen but are tolerant of self, we directly examined how peptide/MHC ligands expressed on thymic epithelial cells trigger the positive selection of immature thymocytes. We demonstrate that abundant self-peptides, purified from the H-2D(b) molecules of thymic epithelial cells, are specifically recognized during the positive selection of CD8+ T cells, implying that positive selection generates a repertoire of T cells that is weakly self-reactive. We also found that this recognition is somewhat cross-reactive, thereby providing an explanation for how the specific recognition of a limited repertoire of thymic self-peptides can select a diverse repertoire of T cells.

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

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

Immunobiology of cytotoxic T-cell escape mutants of lymphocytic choriomeningitis virus

Infection with virus variants exhibiting changes in the peptide sequences defining immunodominant determinants that abolish recognition by antiviral cytotoxic T cells (CTL) presents a considerable challenge to the antiviral T-cell immune system and may enable some viruses to persist in hosts. The potential importance of such variants with respect to mechanisms of viral persistence and disease pathogenesis was assessed by infecting adult mice with variants of lymphocytic choriomeningitis virus (LCMV) strain WE. These variants were selected in vivo or in vitro for resistance to lysis by CD8+ H-2b-restricted antiviral CTL. The majority of anti-LCMV CTL in infected H-2b mice recognize epitopes defined by residues 32 to 42 and 275 to 289 (epitopes 32-42 and 275-289) of the LCMV glycoprotein or 397 to 407 of the viral nucleoprotein. The 8.7 variant exhibits a change in the epitope 32-42 (Val-35-->Leu), and variant CL1.2 exhibits a change in the epitope 275-289 (Asn-280-->Asp) of the wild-type LCMV-WE. The double-mutated 8.7-B23 variant had the variation of 8.7 and an additional change located in the epitope 275-289 (Asn-280-->Ser). The 8.7 variant peptide with unchanged anchor positions bound efficiently to H-2Db and H-2Kb molecules but induced only a very weak CTL response. CTL epitope 275-289 of CL1.2 and 8.7-B23 altered at predicted anchor residues were unable to bind Db molecules and were also not recognized by antiviral CTL. Infection of C57BL/6 mice (H-2b) with the variants exhibiting mutations of one of the CTL epitopes, i.e., 8.7 or CL1.2, induced CTL responses specific for the unmutated epitopes comparable with those induced by infection with WE, and these responses were sufficient to eliminate virus from the host. In contrast, infection with the double-mutated variant 8.7-B23 induced CTL activity that was reduced by a factor of about 50-fold compared with wild-type LCMV. Consequently, high doses (10(7) PFU intravenously) of this virus were eliminated slowly and only by about day 100 after infection. 8.7-B23 failed to cause lethal lymphocytic choriomeningitis after intracerebral infection with a dose of > 10(4) PFU in C57BL/6 mice (but not in mice of nonselecting H-2d haplotype); with the other variants or wild-type LCMV, doses greater than 10(6) to 10(7) PFU were necessary to avoid lethal choriomeningitis.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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

Mice expressing both B7-1 and viral glycoprotein on pancreatic beta cells along with glycoprotein-specific transgenic T cells develop diabetes due to a breakdown of T-lymphocyte unresponsiveness

T lymphocytes have been implicated in the onset of many autoimmune diseases; however, the mechanisms underlying T-cell activation toward self antigens are poorly understood. To study whether T-lymphocyte costimulation can overcome the immunologic unresponsiveness observed in an in vivo model, we have created transgenic mice expressing the costimulatory mouse molecule B7-1, a ligand for the CD28 receptor, on pancreatic beta cells. We now report that triple-transgenic mice expressing both B7-1 and a viral glycoprotein on their beta cells, along with T cells expressing the viral-glycoprotein-specific transgenic T-cell receptor, all develop insulitis (lymphocytic infiltration of the pancreatic islets) and diabetes. In striking contrast, the T cells in double-transgenic mice expressing the same viral glycoprotein (but no B7) on their pancreatic beta cells and the transgenic T-cell receptor on their T cells, reported earlier, remain indifferent to the glycoprotein-expressing beta cells. In fact, all three transgenes are required to initiate immune-mediated destruction of the beta cells. Mice expressing any of the transgenes alone, or any two in combination, maintain normal islet architecture and never spontaneously develop insulitis or diabetes. Our results show that aberrant B7 expression on peripheral tissues may play an important role in the activation of self-reactive T cells and further suggest that abnormal expression of costimulatory receptors may be involved in various T-cell-mediated autoimmune diseases.

Peripheral clonal deletion of antiviral memory CD8+ T cells

Antiviral cytotoxic memory CD8+ T cells adoptively transferred to mice which are persistently infected with lymphocytic choriomeningitis virus WE or DOCILE initially proliferated extensively; they either caused the death of the recipient or, alternatively, disappeared within a few days. Apparently, the complete and coordinated induction and stimulation by widely distributed viral antigen caused these memory T cells to die before virus had been eliminated from the host. Thus memory T cells are as susceptible to peripheral exhaustion/deletion as unprimed T cells. These results indicate possible limitations of exclusively CD8+ T cell-mediated adoptive immunotherapy against viral infections or tumors.

T cell immunity after a viral infection versus T cell tolerance induced by soluble viral peptides

The fate of in vivo activated CD8+ cytotoxic T cells was studied in transgenic mice expressing a T cell receptor (TCR) specific for the lymphocytic choriomeningitis virus (LCMV) glycoprotein peptide 33-41 presented by major histocompatibility complex (MHC) class I molecules. LCMV infection of TCR transgenic mice induced LCMV-specific effector and memory T cells whereas injection of soluble LCMV glycoprotein peptide 33-41 resulted in tolerance by peripheral deletion and anergy of LCMV-specific T cells after an initial expansion phase. Similarly, LCMV peptide 33-41-specific tolerance could be achieved in normal C57BL/6 mice and was not abrogated by an LCMV infection. These results obtained with a classically MHC-restricted peptide antigen parallel previous findings with retroviral or bacterial superantigens and indicate a possibility to modulate specifically mature peripheral cytotoxic T lymphocytes in vivo.

B cells turn off virgin but not memory T cells

There are three possible outcomes when a T cell recognizes a cell bearing a self or foreign antigen. (i) The T cell is not sufficiently signaled and is unaffected. (ii) The T cell is activated. (iii) The T cell is turned off. The differentiation state of the T cell is critical to the outcome. Although both virgin and memory T cells can be activated by antigens presented by "professional" antigen-presenting cells such as dendritic cells, they differ in their responses to B cells. Experienced T cells respond to antigen presented by B cells, whereas virgin T cells are rendered tolerant. These findings may relate to the phenomena of low- and high-zone tolerance, neonatal tolerance, and the beneficial effect of blood transfusions on allograft survival.

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

A monoclonal antibody, B20.1, was generated by fusing spleen cells from a Lou rat immunized with a soluble alpha/beta T cell receptor (TcR; V alpha 2/V beta 2) to mouse myeloma cells. Analysis of a panel of V alpha 2 mRNA-expressing T cell lines, hybridomas and transfectants revealed that the B20.1 antibody was specific for murine TcR V alpha 2 chains. The V alpha 2+ T cell population was examined in various inbred strains by two-color immunofluorescence using B20.1 and CD4- and CD8-specific antibodies with the following results: (a) the B20.1 antibody detected most members of the TcR V alpha 2 subfamily in the four TcR V alpha haplotypes tested; (b) in most strains examined, TcR V alpha 2 expression was biased to the CD4 subset (7.4%-17.4% V alpha 2+ T cells) as compared to the CD8 compartment (3.8%-13.3%); (c) TcR V alpha 2 expression was not influenced by Mls gene products and (d) increased positive selection of V alpha 2+ CD8+ T cells by H-2k major histocompatibility complex molecules occurred in all murine strains tested of the TcR V alpha a, but not in those bearing the TcR V alpha b haplotype.

In vitro selection of lymphocytic choriomeningitis virus escape mutants by cytotoxic T lymphocytes

Cytotoxic T lymphocyte (CTL)-mediated cytolysis is induced via the interaction of the specific T-cell antigen receptor and the peptidic viral antigen associated with the major histocompatibility complex class I antigen. Here we demonstrate in vitro that lymphocytic choriomeningitis virus (LCMV) can escape the cytotoxic activity of LCMV-specific cloned CTLs by single amino acid changes within the recognized T-cell epitope defined by residues 275-289 of the LCMV glycoprotein [LCMV-GP-(275-289)]. LCMV-infected fibroblasts at a multiplicity of infection of 10(-3) exposed to virus-specific CTL at an effector-to-target cell ratio of 4:1 4 hr after infection was optimal for virus mutant selection. The selections were carried out with three LCMV-GP-(275-289)-specific CTL clones expressing T-cell antigen receptors containing the identical variable gene segments V alpha 4 and V beta 10 but different junctional regions; selection was also possible with LCMV-GP-(275-289)-specific cytotoxic polyclonal T cells. The most common escape mutation was an amino acid change of asparagine (AAT) to aspartic acid (GAT) at position 280; an additional mutation was glycine (GGT) to aspartic acid (GAT) at position 282. The results presented show that relevant point mutations within the T-cell epitope of LCMV-GP-(275-289) occur frequently and that they are selectable in vitro by CTLs.

Involvement of both T cell receptor V alpha and V beta variable region domains and alpha chain junctional region in viral antigen recognition

We have studied the lymphocytic choriomeningitis virus (LCMV)-specific cytotoxic T cell response in transgenic mice expressing either the T cell receptor (TcR) alpha (V alpha 2/J alpha TA31) or the corresponding TcR beta (V beta 8.1/D beta/J beta 2.4) chain originally isolated from the LCMV glycoprotein specific (residues 32-42), H-2Db-restricted T cell clone P14. The expression of single transgenic TcR chains did not influence the corresponding endogenous TcR V gene usage in unstimulated T cells indicating that one particular TcR alpha or beta chain can randomly pair with different V beta or V alpha chains without any obvious bias. However, upon infection with LCMV, reactive cytotoxic T lymphocytes (CTL) from P14 beta-transgenic mice were predominantly V alpha 2+ whereas CTL from P14 alpha-transgenic mice preferentially expressed V beta 8.1 and unexpectedly also V beta 8.3 (but not V beta 8.2). Correspondingly, the LCMV-specific CTL response in both alpha and beta TcR-transgenic mice was strongly biased to the original P14 T cell epitope (LCMV glycoprotein residues 32-42). Sequence analysis of a large panel of LCMV-reactive "half-transgenic" TcR from P14 single receptor chain-transgenic mice revealed a highly conserved VJ alpha and a more diverse VDJ beta junctional region. This report demonstrates that the antigen specificity of the studied TcR depends on the specific combination of both TcR alpha and beta chains which implies that amino acids located in the TcR V alpha and V beta segments as well as in the junctional region are involved in binding of the viral antigenic fragment.

T and B cell tolerance and responses to viral antigens in transgenic mice: implications for the pathogenesis of autoimmune versus immunopathological disease

Experiments with transgenic mice illustrate clonal elimination of T cells specific for antigens expressed appropriately in the thymus, but presence of inducible T cells when the antigen presented on class I MHC antigens is expressed exclusively on non-lymphohemopoietic cells in the periphery (pancreatic beta islet cells). TCR-transgenic LCMV-carrier mice expressing LCMV in the thymus exhibit clonal elimination at the early CD4+8+ thymocyte stage, causing CTL unresponsiveness in these mice. In contrast, studies with RIP LCMV-GP-transgenic mice (expressing GP in pancreatic beta cells) and with TCR-RIP LCMV-GP double-transgenic mice show that CTL reactivity is normal. These experiments argue against so-called peripheral anergy of class I MHC antigen-restricted cytotoxic T cells as a general mechanism of peripheral immunological tolerance to self. They reveal that self epitopes that are genetically self and presented by class I antigens may not be considered immunologically self if expressed solely extrathymically, despite the fact that they are antigenic and can be recognized by induced effector T cells. Genetic self that is presented on cells which can induce neither tolerance nor an immune response is immunologically dealt with as foreign and therefore may be called nonimmunological self. Appropriate presentation of the same epitope on antigen-presenting cells promptly induces effector T cells and causes disease; such disease should not be called autoimmune because it is an immunopathological T-cell mediated disease, comparable to an unfavorably balanced immunopathological T-cell response to a virus. Mechanisms that control autoantibody responses were studied in mice expressing a viral transgene. Such mice generate neutralizing antiviral autoantibody responses only when the transgenic viral antigen is linked to a foreign T-helper determinant. These findings, therefore, document differences in levels of T- vs B-cell tolerance (so-called split tolerance) under a given expression level of a "self" antigen. They illustrate how unresponsiveness of B cells to produce T-independent IgM is dose-dependent and that IgG autoantibodies are triggered by introducing foreign T-helper determinants that can be recognised in a linked fashion. This model suggests that, while T-cell tolerance to tolerogenic self in the thymus is solid, B-cell tolerance in general is not. From the point of view of autoantibody responses these T-helper cells may also be called immunopathological; i.e., these T-helper cells are specific for foreign epitopes that, via linked recognition, trigger truly autoimmune B cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Lower receptor avidity required for thymic clonal deletion than for effector T-cell function

Clonal deletion in the thymus plays a major part in T-cell tolerance to self antigens. But the mechanism of negative selection, its fine specificity and the threshold of affinity and avidity remains unknown. We have now examined these aspects of negative selection with mice expressing a transgenic T-cell receptor with specificity for lymphocytic choriomeningitis virus (LCMV) glycoprotein in association with the class I H-2Db molecule. These mice were rendered tolerant to LCMV by neonatal infection with mutant LCMVs bearing point mutations in the T-cell epitope recognized by the transgenic T-cell receptor. Variant LCMVs were also tested for their ability to elicit antiviral responses in transgenic mice in vivo and in vitro. Comparison in vivo revealed that a low-avidity receptor interaction, which was unable to induce effector T cells in the periphery, was still sufficient for clonal deletion in the thymus.

Ablation of "tolerance" and induction of diabetes by virus infection in viral antigen transgenic mice

To address the mechanisms of tolerance to extrathymic proteins, we have generated transgenic mice expressing the lymphocytic choriomeningitis viral (LCMV) glycoprotein (GP) in the beta islet cells of the pancreas. The fate of LCMV GP-specific T cells was followed by breeding the GP transgenic mice with T cell receptor transgenic mice, specific for LCMV and H-2Db. These studies suggest that "peripheral tolerance" of self-reactive T cells does not involve clonal deletion, clonal anergy, or a decrease in the density of T cell receptors or accessory molecules. Instead, this model indicates that self-reactive cytotoxic T cells may remain functionally unresponsive, owing to a lack of appropriate T cell activation. Infection of transgenic mice with LCMV readily abolishes peripheral unresponsiveness to the self LCMV GP antigen, resulting in a CD8+ T cell-mediated diabetes. These data suggest that similar mechanisms may operate in several so-called "T cell-mediated autoimmune diseases."

Viral escape by selection of cytotoxic T cell-resistant virus variants in vivo

Viruses persist in an immune population, as in the case of influenza, or in an individual, as postulated for human immunodeficiency virus, when they are able to escape existent neutralizing antibody responses by changing their antigens. It is now shown that viruses can in principle escape the immunosurveillance of virus-specific cytotoxic T cells by mutations that alter the relevant T-cell epitope.

Specific deletion of the J-C delta locus in murine alpha/beta T cell clones and studies using transgenic mice

A deletion event in the T cell receptor (TcR) delta locus has been characterized in a panel of mouse alpha/beta cytotoxic T lymphocyte (CTL) clones. Data presented here shows that J delta 1, J delta 2 and C delta are absent from functional CTL clones while a germ-line D delta 1 fragment is retained, thus suggesting a specific deletion of this region. We have investigated the possible significance of the J-C delta deletion by generating T cell lines from TcR alpha/beta transgenic mice. Unlike control T cell lines which included a T cell line derived from a beta transgenic mouse, the lines expressing the transgenic alpha/beta heterodimer have not deleted the C delta region. This strongly suggests that the J-C delta deletion event is not responsible for directing T cells to the alpha/beta lineage, but rather is involved in the rearrangement or transcriptional activity of the alpha locus. In addition, to ensure that the alpha/beta transgene does not have any inhibitory affects on the rearrangement of the delta loci in general, the gamma/delta expressing dendritic epithelial T cell (DETC) population was examined in TcR alpha/beta transgenic mice and alterations in this T cell subset were not found. This finding that normal gamma/delta DETC cells are present in alpha/beta transgenic mice, together with the data showing that the D delta 1 region remains in an unrearranged germ-line configuration in functional alpha/beta CTL, suggests that commitment to the alpha/beta or gamma/delta lineage is predetermined at a particular stage in early T cell ontogeny.

Preferential usage of V alpha 4 and V beta 10 T cell receptor genes by lymphocytic choriomeningitis virus glycoprotein-specific H-2Db-restricted cytotoxic T cells

Correlations between the T cell receptor (TcR) V gene usage and the specificity of T cells have been primarily described for major histocompatibility complex (MHC) class II-restricted helper T cell responses. In the present study the TcR genes expressed by MHC class I-restricted murine cytotoxic T cells (CTL) specific for a major epitope of the lymphocytic choriomeningitis virus (LCMV), LCMV-GP2(275-289), were investigated. The TcR primary structure of an LCMV-GP2(275-289) specific H-2Db-restricted CTL clone has been determined. It uses a member of the V alpha 4 family joined to J alpha AN14.4 for the alpha chain and V beta 10 rearranged to D beta 2.1 and J beta 2.4 for its beta chain. Four other independent LCMV-GP2(275-289) specific H-2Db-restricted CTL clones also expressed V alpha 4 and V beta 10 gene elements. Furthermore, V alpha 4 and V beta 10 were preferentially expressed by polyclonal CTL of C57BL/6 origin specific for LCMV. These results suggest that both TcR V alpha and V beta regions are important for the recognition of the LCMV-GP2(275-289) epitope on H-2Db molecules.

T cell receptor (TcR) beta chain transgenic mice: studies on allelic exclusion and on the TcR+ gamma/delta population

To study allelic exclusion of TcR genes we analyzed two types (I and II) of TcR beta transgenic mice. T cells derived from both types of mice contained similar amounts of transgenic RNA transcripts; however, surface expression of the transgenic beta chain was drastically reduced in type II compared to type I. In type I transgenic mice, productive rearrangements and expression of endogenous TcR beta genes were suppressed whereas on T cells of type II mice, both transgenic and endogenous TcR beta chains were expressed on the surface of the same cell. These findings suggest that allelic exclusion of TcR genes in beta transgenic mice depends on amount and/or onset of transgene expression during thymic development. Furthermore, TcR gamma rearrangements and the population of TcR gamma/delta-bearing double-negative CD4-CD8- thymocytes were reduced fivefold in type I transgenic animals. However, the V gamma usage and the gamma/delta+ dendritic epidermal cell populations appeared normal. RNase protection analysis further revealed low levels of transgenic TcR beta chain transcripts in TcR+ gamma/delta CD4-CD8- thymocytes. These results suggest that the beta transgene only quantitatively influences the gamma/delta T cell compartment, and supports the independence of the gamma/delta population.

Tolerance induction in double specific T-cell receptor transgenic mice varies with antigen

The crucial role of the thymus in immunological tolerance has been demonstrated by establishing that T cells are positively selected to express a specificity for self major histocompatibility complex (MHC), and that those T cells bearing receptors potentially reactive to self antigen fragments, presumably presented by thymic MHC, are selected against. The precise mechanism by which tolerance is induced and the stage of T-cell development at which it occurs are not known. We have now studied T-cell tolerance in transgenic mice expressing a T-cell receptor with double specificities for lymphocytic choriomeningitis virus (LCMV)-H-2Db and for the mixed-lymphocyte stimulatory (MIsa) antigen. We report that alpha beta TCR transgenic mice tolerant to LCMV have drastically reduced numbers of CD4+CD8+ thymocytes and of peripheral T cells carrying the CD8 antigen. By contrast, tolerance to MIsa antigen in the same alpha beta TCR transgenic MIsa mice leads to deletion of only mature thymocytes and peripheral T cells and does not affect CD4+CD8+ thymocytes. Thus the same transgenic TCR-expressing T cells may be tolerized at different stages of their maturation and at different locations in the thymus depending on the antigen involved.

Distinct patterns of virus-specific T cell-mediated cytolysis of transformed versus primary target cells

Ideas about the mechanism(s) by which cytotoxic T lymphocytes (CTL) lyse appropriate target cells are still controversial. We studied the action of established murine CTL clones as well as of freshly prepared primarily induced CTL against two types of target cells. Transformed tumour cell lines (MC57G and L929) and untransformed cells such as peritoneal exudate cells (PEC) or fibroblasts were examined as target cells by scanning and transmission electron microscopy and with the 51Cr-release assay. We found independent of which CTL used, that in transformed cells the cell membrane seemed to be the first target of CTL attack, whereas in untransformed cells the first noticeable events appeared to take place in the nucleus of the target cells; the membrane of attacked untransformed PEC or fibroblasts was found to be intact at a time when the cellular organelles already were disintegrated. The morphological observations were paralleled by differences in the kinetics of 51Cr-release; untransformed target cells released their label only after a 2 h long period, whereas transformed cells released 51Cr considerably earlier.

Molecular analysis of the antigen receptor of virus-specific cytotoxic T cells and identification of a new V alpha family

We have determined the primary structure of the alpha/beta-antigen receptor of a lymphocytic choriomeningitis virus (LCMV)-specific cytotoxic T cell clone (P14). The beta chain of the T cell receptor is composed of V beta 8.1, D beta and J beta 2.4 gene segments while the alpha chain uses a member of the V alpha 2 family and J alpha TA31. Southern blot analysis of a panel of six other independent LCMV-specific cytotoxic T cell clones did not reveal any preferential usage of the V alpha 2 or V beta 8 gene families in the cytotoxic LCMV response. Additionally, we have isolated and sequenced cDNA clones derived from the non-functionally rearranged alpha allele of P14. This transcript is composed of a unique V alpha gene segment which belongs to a new V alpha gene family.