Thymic selection and adaptability of cytotoxic T lymphocyte responses in transgenic mice expressing a viral protein in the thymus

Molecular mimicry and immune-mediated diseases

Molecular mimicry has been proposed as a pathogenetic mechanism for autoimmune disease, as well as a probe useful in uncovering its etiologic agents. The hypothesis is based in part on the abundant epidemiological, clinical, and experimental evidence of an association of infectious agents with autoimmune disease and observed cross-reactivity of immune reagents with host 'self' antigens and microbial determinants. For our purpose, molecular mimicry is defined as similar structures shared by molecules from dissimilar genes or by their protein products. Either the molecules' linear amino acid sequences or their conformational fits may be shared, even though their origins are as separate as, for example, a virus and a normal host self determinant. An immune response against the determinant shared by the host and virus can evoke a tissue-specific immune response that is presumably capable of eliciting cell and tissue destruction. The probable mechanism is generation of cytotoxic cross-reactive effector lymphocytes or antibodies that recognize specific determinants on target cells. The induction of cross-reactivity does not require a replicating agent, and immune-mediated injury can occur after the immunogen has been removed a hit-and-run event. Hence, the viral or microbial infection that initiates the autoimmune phenomenon may not be present by the time overt disease develops. By a complementary mechanism, the microbe can induce cellular injury and release self antigens, which generate immune responses that cross-react with additional but genetically distinct self antigens. In both scenarios, analysis of the T cells or antibodies specifically engaged in the autoimmune response and disease provides a fingerprint for uncovering the initiating infectious agent.

Mice expressing the E7 oncogene of HPV16 in epithelium show central tolerance, and evidence of peripheral anergising tolerance, to E7-encoded cytotoxic T-lymphocyte epitopes

In order to derive mice which expressed both the E7 open reading frame transgene of human papillomavirus type 16 in skin and MHC class 1 restriction elements for several E7-encoded cytotoxic T-lymphocyte (CTL) epitopes, K14.HPV16E7 mice which express E7 in basal keratinocytes were crossed to the F1 generation with A2.1 Kb transgenic mice which express the MHC binding cleft domains of human HLA A*0201, and murine H-2b. F1 mice (denoted K14E7 x A2.1) expressed E7 in the thymus at least as early as 2-5 days before birth. Immunisation of FVB x A2.1 control mice (transgenic for HLA A*0201 and H-2b but not for E7), with two HLA A*0201-restricted epitopes of E7 and one H-2b-restricted CTL epitope of E7, gave strong primary CTL responses recognising epitope-pulsed or constitutively E7-expressing syngeneic target cells. In contrast, in immunised K14E7 x A2.1 mice, the CTL responses to the H-2b epitope and one of the HLA A*0201 CTL epitopes were strongly down-regulated, and to the other HLA A*0201 epitope, completely abolished, as demonstrated by percentage specific killing by bulk splenocyte cultures in cytotoxicity assays, and by CTL precursor frequency analysis. In thymus-transplanted bone marrow radiation chimeras in which the immune system of K14E7 x A2.1 mice was replaced by a FVB x A2.1 immune system, specific immunisation did not result in reemergence of strong E7-directed CTL responses. In agreement with these in vitro findings, specific immunisation failed to significantly alter the course of E7-associated tumour development in K14E7 x A2.1 mice. These data are consistent with a model of central deletional CTL tolerance to E7-encoded epitopes recognised in the context of two distinct MHC class 1 restriction elements, and with the possibility of peripheral T-cell anergy maintained by expression of E7 in the skin.

Relationship Among Immunodominance of Single CD8+ T Cell Epitopes, Virus Load, and Kinetics of Primary Antiviral CTL Response

The primary CTL response of BALB/c mice infected with the lymphocytic choriomeningitis (LCM) virus strain WE is directed exclusively against one major epitope, n118, whereas a viral variant, ESC, that does not express n118 induces CTL against minor epitopes. We identified one minor epitope, g283, that induces primary lytic activity in ESC-infected mice. Infections of mice with WE and ESC were used to study the hierarchical control of a T cell response. Presentation of minor epitopes is not reduced in WE-infected cells. Generation of CTL against n118 does not suppress the generation of minor epitope-specific CTL systemically, as mice coinfected with WE and ESC developed CTL against n118 and g283. However, elimination of ESC and development of minor epitope-specific CTL in ESC infection were slower than elimination of WE and development of CTL against n118. CD8+ T cells against the minor epitope were activated in ESC and WE infection, but did not expand in the latter to show lytic activity in a primary response. We explain the absence of minor epitope-specific lytic activity in WE infection by the fast reduction of virus load due to the early developing n118-specific CTL. Immunodominance of CTL epitopes in primary virus infections thus can be explained as a kinetic phenomenon composed of 1) expansion of CD8+ T cells specific for individual epitopes, 2) stimulatory effect of virus load, and 3) negative feedback control on virus load by the fastest CTL population.

Viral pathogenesis and immunity within the thymus

Replication of viruses within the thymic microenvironment may have a unique impact on viral persistence and pathology. The author's laboratory has studied thymic infection by both human and murine retroviruses. For human lentiviruses, such as HIV-1, the consequences of persistent thymic replication are frequently a severe disruption of the normal processes of thymopoiesis and potentially of progression to AIDS. Murine retroviruses, such as Gross murine leukemia virus, establish persistent infection with less cytopathic, but no less devastating effects. These include the alteration of immune recognition to retroviral antigens by the peripheral immune response, the thymic persistence of virus, and the establishment of viral-induced thymic leukemia. This article summarizes the analysis of both the common and distinctive means of pathology induced by these two retroviral families with particular attention on the influence and impact of the thymus as a unique site of virus replication.

Expression of adenoviral E3 transgenes in beta cells prevents autoimmune diabetes

The adenovirus (Ad) genome contains immunoregulatory and cytokine inhibitory genes that are presumed to function in facilitating acute infection or in establishing persistence in vivo. Some of these genes are clustered in early region 3 (E3), which contains a 19-kDa glycoprotein (gp19) that inhibits the transport of selected class I major histocompatibility complex (MHC) molecules out of the endoplasmic reticulum. In addition, the E3 region contains three protein inhibitors of the cytolytic function of tumor necrosis factor alpha (TNF-alpha). Because type I autoimmune diabetes destroys islets by mechanisms that involve class I MHC and TNF-alpha, we investigated whether the entire cassette of Ad E3 genes might prevent the onset of diabetes in a well studied lymphocytic choriomeningitis viral (LCMV) murine model of virus-induced autoimmune diabetes. In this model, a LCMV polypeptide (either glycoprotein or nucleoprotein) expressed as a transgene in the islets is a target for autoimmune destruction of beta cells after LCMV infection. In this scenario the LCMV-induced immune response is directed not only against the virus but also against the LCMV transgenes expressed in the beta cells. Our experiments demonstrated a very efficient prevention of this LCMV-triggered diabetes by the Ad E3 genes. This resulted from the inhibition of target cell recognition by a fully competent and LCMV-primed immune system. Unlike the results from the beta-2 microglobulin gene deletion experiments, our approach shows that selective regulation at the level of the target cell is sufficient to prevent autoimmune diabetes without disrupting the function of the systemic immune response. Although the Ad genes in these experiments were provided as transgenes, recent experiments may permit the introduction of such genes through the use of viral vectors. Although the decrease in class I MHC in islets by Ad genes was demonstrated in these in vivo studies, the relative importance of this process and the control of TNF-alpha cytolysis must await further genetic dissection of the introduced Ad genes.

Interferon gamma (IFN-gamma) is necessary for the genesis of acetylcholine receptor-induced clinical experimental autoimmune myasthenia gravis in mice

Experimental autoimmune myasthenia gravis (EAMG) is an animal model of human myasthenia gravis (MG). In mice, EAMG is induced by immunization with Torpedo californica acetylcholine receptor (AChR) in complete Freund's adjuvant (CFA). However, the role of cytokines in the pathogenesis of EAMG is not clear. Because EAMG is an antibody-mediated disease, it is of the prevailing notion that Th2 but not Th1 cytokines play a role in the pathogenesis of this disease. To test the hypothesis that the Th1 cytokine, interferon (IFN)-gamma, plays a role in the development of EAMG, we immunized IFN-gamma knockout (IFN-gko) (-/-) mice and wild-type (WT) (+/+) mice of H-2(b) haplotype with AChR in CFA. We observed that AChR-primed lymph node cells from IFN-gko mice proliferated normally to AChR and to its dominant pathogenic alpha146-162 sequence when compared with these cells from the WT mice. However, the IFN-gko mice had no signs of muscle weakness and remained resistant to clinical EAMG at a time when the WT mice exhibited severe muscle weakness and some died. The resistance of IFN-gko mice was associated with greatly reduced levels of circulating anti-AChR antibody levels compared with those in the WT mice. Comparatively, immune sera from IFN-gko mice showed a dramatic reduction in mouse AChR-specific IgG1 and IgG2a antibodies. However, keyhole limpet hemocyanin (KLH)-priming of IFN-gko mice readily elicited both T cell and antibody responses, suggesting that IFN-gamma regulates the humoral immune response distinctly to self (AChR) versus foreign (KLH) antigens. We conclude that IFN-gamma is required for the generation of a pathogenic anti-AChR humoral immune response and for conferring susceptibility of mice to clinical EAMG.

Uncovering subdominant cytotoxic T-lymphocyte responses in lymphocytic choriomeningitis virus-infected BALB/c mice

The cytotoxic T-lymphocyte response against lymphocytic choriomeningitis virus (LCMV) in BALB/c mice is predominantly directed against a single, Ld-restricted epitope in the viral nucleoprotein (residues 118 to 126). To investigate whether any Kd/Dd-restricted responses were activated but did not expand during the primary response, we used a BALB/c mutant, BALB/c-H-2dm2, which does not express the Ld molecule. Splenocytes from LCMV-infected BALB/c mice were transferred into irradiated BALB/c-H-2dm2 mice and rechallenged with LCMV. Thus, they were exposed to an antigenic stimulus without the involvement of the immunodominant Ld-restricted epitope. In this adoptive transfer model, the donor splenocytes protected the recipient mice against chronic LCMV infection by mounting a potent Kd- and/or Dd-restricted secondary antiviral response. Analysis of a panel of Kd binding LCMV peptides revealed that residues 283 to 291 from the viral glycoprotein (GP(283-291)) comprise a major new epitope in the adoptive transfer model. Because the donor splenocytes were first activated during the primary infection in BALB/c mice, the GP(283-291) epitope is a subdominant epitope in BALB/c mice that becomes dominant after rechallenge in BALB/c-H-2dm2 mice. This study makes two points. First, it shows that subdominant CTL responses can be protective, and second, it provides a general experimental approach for uncovering subdominant CTL responses in vivo. This strategy can be used to identify subdominant T-cell responses in other systems.

Differential immune recognition of LCMV nucleoprotein and glycoprotein in transgenic mice expressing LCMV cDNA genes

We have generated doubly transgenic (DT) mice that independently express cDNA genes for the nucleocapsid protein (NP) and the surface glycoproteins (GP) of lymphocytic choriomeningitis virus (LCMV). By RT-PCR, transcription of both transgenes was detected at low levels in brain and kidney but was not observed in the thymus. Additionally, transcription of the GP transgene was observed in the spleen. Following challenge with exogenous LCMV, an anti-NP CTL response was induced in LCMV-infected DT mice, suggesting that nonresponsiveness to NP had not been established. In contrast, LCMV- infected DT mice were nonresponsive to GP and failed to mount any CTL response against GP, either at Day 7 or Day 30 postinfection or following expansion of splenocyte populations in vitro. A significant number (33%) of adult DT mice survived intracerebral infection with LCMV, suggesting that virus-induced immunopathology in the central nervous system can be diminished by combined expression of the transgenes whereas no protective effect was conferred on singly transgenic mice, expressing NP or GP alone. The DT mice therefore create a novel host genetic background for comparative studies of the anti-LCMV immune responses relative to parental C57Bl/6 mice.

Peptide-induced positive selection of TCR transgenic thymocytes in a coreceptor-independent manner

T cell receptor (TCR) transgenic thymocytes specific for the LCMV gp peptide are normally positively selected to the CD8 lineage. Transgenic thymocyte development was substantially reduced in the absence of these CD8 coreceptors. However, efficient positive selection was restored when TCR transgenic CD8-/- fetal thymic lobes were cultured with a peptide variant of the wild-type ligand. These mature thymocytes were functional, as shown by their ability to respond against strong peptide agonists. Additional experiments demonstrated that transgenic positive selection was peptide-specific. These results prove that CD8 does not possess essential signaling properties that are necessary for T cell development. In addition, the unilateral commitment of transgenic thymocytes to mature CD4-TCR(hi) T cells expressing intracellular perforin suggests that there must be some instructive component to CD4 down-regulation and lineage commitment during thymocyte selection.

Low-affinity cytotoxic T-lymphocytes require IFN-gamma to clear an acute viral infection

The majority of the response of cytotoxic T-lymphocytes (CTL) to lymphocytic choriomeningitis virus (LCMV) in H-2d mice is directed toward one epitope located on the nucleoprotein (NP, aa 118-126), and usually no primary responses to other epitopes are detectable. Previous studies have shown that thymic expression of lymphocytic choriomeningitis virus-nucleoprotein (LCMV-NP) in H-2d transgenic mice (Thy-NP mice) leads to deletion of high-affinity anti-LCMV-NP CTL by negative selection. Selection is incomplete, so that low-affinity NP-specific CTL pass through the thymus and are detectable in the periphery. To analyze the importance of interferon-gamma (IFN-gamma) in the ability of low-affinity antiviral CTL to clear an acute viral infection, double transgenic mice were generated that are IFN-gamma deficient and express the NP of LCMV in the thymus (Thy-NP x IFN-gamma -/- mice). When infected with LCMV, these bigenic mice were unable to clear the infection despite generating low-affinity primary antiviral CTL, and they became persistently infected. In contrast, IFN-gamma competent Thy-NP mice cleared LCMV within 7-8 days and IFN-gamma deficient mice that did not express NP in their thymus generated high-affinity CTL that terminated an acute LCMV infection within 10-12 days post-viral challenge. Persistently infected IFN-gamma deficient mice selectively depleted LCMV-specific CTL and displayed reduced levels of antigen-presenting cells in the spleen, and 60% of these mice died at 2-3 months postinfection. Thus, IFN-gamma is required for clearing an acute viral infection in the absence of a high-affinity CTL response. In the absence of IFN-gamma persistent viral infection results despite the presence of low-affinity CTL.

Tolerance to p53 by A2.1-restricted cytotoxic T lymphocytes

Elevated levels of the p53 protein occur in approximately 50% of human malignancies, which makes it an excellent target for a broad-spectrum T cell immunotherapy of cancer. A major barrier to the design of p53-specific immunotherapeutics and vaccines, however, is the possibility that T cells may be tolerant of antigens derived from wild-type p53 due to its low level of expression in normal thymus and lymphohemopoetic cells. The combination of p53 deficient (p53-/-) and p53+/+ HLA-A2.1/Kb transgenic mice was used as a model to explore the possibility that A2.1-restricted cytotoxic T lymphocytes (CTL) are functionally tolerant of self peptides derived from the wild-type p53 tumor suppressor protein. A2.1-restricted CTL specific for a naturally processed p53 self-epitope spanning residues 187-197 were completely aborted in p53+/+ as opposed to p53-/- transgenic mice. In contrast, CTL specific for a second self-epitope spanning residues 261-269 of the murine p53 sequence were detected in both p53-/- and p53+/+ A2.1/Kb transgenic mice. However, the avidity of the CTL effectors obtained from p53+/+ mice was 10-fold lower than that obtained from p53-/- mice, again suggesting elimination of CTL with high avidity for the A2.1-peptide complex. The circumvention of functional tolerance of high avidity CTL may therefore be a necessary prerequisite for optimizing immunotherapy against A2.1-restricted wild-type p53 epitopes in humans.

Insulin expression in human thymus is modulated by INS VNTR alleles at the IDDM2 locus

Type 1 diabetes or insulin-dependent diabetes mellitus (IDDM) is due to autoimmune destruction of pancreatic beta-cells. Genetic susceptibility to IDDM is encoded by several loci, one of which (IDDM2) maps to a variable number of tandem repeats (VNTR) minisatellite, upstream of the insulin gene (INS). The short class I VNTR alleles (26-63 repeats) predispose to IDDM, while class III alleles (140-210 repeats) have a dominant protective effect. We have reported that, in human adult and fetal pancreas in vivo, class III alleles are associated with marginally lower INS mRNA levels than class I, suggesting transcriptional effects of the VNTR. These may be related to type 1 diabetes pathogenesis, as insulin is the only known beta-cell specific IDDM autoantigen. In search of a more plausible mechanism for the dominant effect of class III alleles, we analysed expression of insulin in human fetal thymus, a critical site for tolerance induction to self proteins. Insulin was detected in all thymus tissues examined and class III VNTR alleles were associated with 2- to 3-fold higher INS mRNA levels than class I. We therefore propose higher levels of thymic INS expression, facilitating immune tolerance induction, as a mechanism for the dominant protective effect of class III alleles.

Interferon-gamma is essential for destruction of beta cells and development of insulin-dependent diabetes mellitus

Autoimmune mediated destruction of beta cells of the islets of Langerhans leads to insulin-dependent diabetes mellitus (IDDM). Rat insulin promoter (RIP) lymphocytic choriomeningitis virus (LCMV) transgenic mice that express the nucleoprotein (NP) or glycoprotein (GP) of LCMV under control of the RIP in their beta cells develop IDDM after infection with LCMV and serve as a model for virus-induced IDDM. Recently, Kagi et al. (Kagi, D., B. Odermatt, P. Ohashi, R.M. Zinkernagel, and H. Hengartner, 1996, J. Exp. Med. 183:2143-2149) showed, using RIP LCMV perforin-deficient mice, that IDDM does not occur in the absence of perforin. They concluded that perforin-mediated killing by cytotoxic T lymphocytes (CTLs) is the main factor needed for beta cell injury and destruction. Here we provide evidence that killing of beta cells is more complex and multifactorial. By the use of our RIP LCMV model, we show that in perforin competent but interferon-gamma (IFN-gamma)-deficient mice, beta cell injury is limited and IDDM does not occur. For these studies, double transgenic mice were generated that were genetically deficient in the production of IFN-gamma and express LCMV NP or GP in their beta cells. In such mice, IDDM was aborted despite the generation of LCMV-specific antiself CTLs that displayed normal cytolytic activity in vitro and in vivo and entered the pancreas. However, mononuclear infiltration into the islets did not occur, and upregulation of class I and II molecules usually found in islets of RIP LCMV single transgenic mice after LCMV infection preceding the onset of clinical IDDM was not present in these bigenic mice. Our findings indicate that in addition to perforin, beta cell destruction, development of insulitis, and IDDM also depend on the cytokine INF-gamma, presumably through enhancement of major histocompatibility complex expression and antigen presentation.

Virus-induced autoimmune disease

The breaking of tolerance or unresponsiveness to self-antigens, involving the activation of autoreactive lymphocytes, is a critical event leading to autoimmune diseases. The precise mechanisms by which this can occur are mostly unknown. Viruses have been implicated in this process, among other etiological factors, such as genetic predisposition and cytokine activity. Several ways have been proposed by which a viral infection might break tolerance to self and trigger an autoreactive cascade that ultimately leads to the destruction of a specific cell type or an entire organ. The process termed "molecular mimicry' and the use of transgenic models in which viral and host genes can be manipulated to analyze their effects in causing autoimmunity have been particular focuses for research. For example, there is a transgenic murine model of virus-induced autoimmune disease, in which a known viral gene is selectively expressed as a self-antigen in beta cells of the pancreas. In these mice, insulin-dependent diabetes develops after either a viral infection, the release of a cytokine such as IFN-gamma, or the expression of the costimulatory molecule B7.1 in the islets of Langerhans. Recent studies using this model have contributed to the understanding of the pathogenesis of virus-induced autoimmune disease and have furthered the design and testing of novel immunotherapeutic approaches.

Virus-induced autoimmune disease: transgenic approach to mimic insulin-dependent diabetes mellitus and other autoimmune diseases

The technology of cloning viral genes and expressing them in vivo under cell-specific promoters allows to dissect the role of viruses, host self proteins, host genetics and immune responses in the complex etiology of autoimmune disease. Expression of a viral transgene, that is really a marker for a host "self" protein per se in beta cells of the islets of Langerhans, need not cause disease. In our model, expression of a viral gene was not associated with disease over the lifetime of the animal. However, when the host becomes infected with a virus encoding the same gene as the transgene or one closely related to it, a resultant immune response directed against the virus also recognizes the transgene leading to progressive T-cell-mediated response and destruction of the tissue expressing the viral ("self") gene, leading to autoimmune disease. This multifactorial process is influenced by whether the viral transgene is expressed in the thymus as well as in the disease-related cell or target tissue. Thymic expression influences negative selection of responder lymphocytes and thus delays the onset of the autoimmune disorder. Further, the MHC haplotype or other background genes of an individual undergoing autoimmune dysfunction play a role in the affinity of binding of the transgene products to the MHC molecule and influence the degree of negative selection that occurs, thereby influencing the vigor of the resulting immune response. The current ability to express host or viral genes in unique cell populations, and to make double- or triple-tg mice in which various cytokine genes or lymphocyte activation genes can be expressed along with the viral gene, offers a unique possibility for molecular dissection of autoimmunity. With the information on hand, approaches to the prevention and treatment of human autoimmune disease are likely to be uncovered. Finally, animal models are available in which the onset, progression and control of molecular mimicry can be evaluated. Future studies should define roles played by cytokines, bystander and immune-specific cross-reactivity to viruses and other microbes in several autoimmune diseases.

Oral insulin treatment suppresses virus-induced antigen-specific destruction of beta cells and prevents autoimmune diabetes in transgenic mice

Oral administration of self-antigens has been proposed as a therapy to prevent and treat autoimmune diseases. Here we report that oral treatment with insulin prevents virus-induced insulin-dependent diabetes mellitus (IDDM) in a transgenic (tg) mouse model. Such mice express the viral nucleoprotein (NP) of lymphocytic choriomeningitis virus (LCMV) under control of the rat insulin promoter in their pancreatic beta cells and < 2% spontaneously develop diabetes. However, 2 mo after challenge with LCMV, IDDM occurs in > 95% of tg mice but not in controls. Oral treatment with 1 mg of insulin twice per week for 2 mo starting either 1 wk before or 10 d after initiating LCMV infection prevents IDDM in > 50% of the tg mice (observation time 8 mo). Thus, insulin therapy is effective in preventing progression to overt IDDM in prediabetic tg mice with ongoing islet infiltration. Oral administration of insulin does not affect the generation of LCMV-NP-specific anti-self cytotoxic T lymphocytes nor the infiltration of lymphocytes into the pancreas. However, less beta cells are destroyed in insulin-treated mice, upregulation of MHC class I and II molecules does not occur, and antiviral (self) cytotoxic T lymphocytes are not found in the islets, events present in tg mice developing IDDM. The majority of lymphocytes in the islets of insulin-treated tg mice without IDDM produces IL-4, IL-10, and TGF-beta. In contrast, lymphocytes from islets of tg mice developing IDDM mainly make gamma-IFN.

Using transgenic mouse models to dissect the pathogenesis of virus-induced autoimmune disorders of the islets of Langerhans and the central nervous system

Viruses have often been associated with autoimmune diseases. One mechanism by which self-destruction can be triggered is molecular mimicry. Many examples of cross-reactive immune responses between pathogens and self-antigens have been described. This review presents two transgenic models of autoimmune disease induced by a virus through activation of anti-self lymphocytes. Viral antigens are expressed as transgenes either in beta-cells of the pancreas or in the oligodendrocytes of the CNS. Infection by a virus encoding the same gene activated autoreactive T cells that cleared the viral infection, and as a consequence of transgene expression resulted in organ-specific autoimmune disease. In both transgenic mouse models, autoreactive lymphocytes that escaped thymic negative selection were present in the periphery. Several factors are described that play a role in the regulation of the self-reactive process precipitated by a viral infection. These include the quantity of activated autoreactive T cells, the affinity of these T cells, the number of memory T cells generated following primary infection, costimulation by accessory molecules, and the types and locations of cytokines produced. In addition, unique barriers exist in target tissues that prevent or suppress autoreactive responses and define to a large extent the outcome of disease. Restimulation of autoreactive memory lymphocytes may be required to bypass these barriers and enhance autoimmune disease. Therapy directed at modifying these factors can reduce and even prevent autoimmune disease after it has been initiated.

Binding of viral antigens to major histocompatibility complex class I H-2Db molecules is controlled by dominant negative elements at peptide non-anchor residues. Implications for peptide selection and presentation

Binding of viral antigens to major histocompatibility complex (MHC) class I molecules is a critical step in the activation process of CD8(+) cytotoxic T lymphocytes. In this study, we investigated the impact of structural factors at non-anchor residues in peptide-MHC interaction using the model of lymphocytic choriomeningitis virus (LCMV) infection of its natural host, the mouse. Altering viral genes by making reassortants, recombinants, and using synthetic peptides, CD8(+) cytotoxic T lymphocytes were shown to recognize only three H-2Db-restricted epitopes, GP amino acids 33-41/43, GP 276-286, and NP 396-404. However, LCMV NP and GP proteins contain 31 other peptides bearing the H-2Db motif. These 34 LCMV peptides and 11 other known H2-Db-restricted peptides were synthesized and examined for MHC binding properties. Despite the presence of the H-2Db binding motif, the majority of LCMV peptides showed weak or no affinity for H-2Db. We observed that dominant negative structural elements located at non-anchor positions played a crucial role in peptide-MHC interaction. By comparative sequence analysis of strong versus non-binders and using molecular modeling, we delineated these negative elements and evaluated their impact on peptide-MHC interaction. Our findings were validated by showing that a single mutation of a favorable non-anchor residue in the sequence of known viral epitopes for a negative element resulted in dramatic reduction of antigen presentation properties, while conversely, substitution of one negative for a positive element in the sequence of a non-binder conferred to the peptide an ability to now bind to MHC molecules.

On T cell memory: arguments for antigen dependence

Memory is a hallmark of the immune system. Considerable progress has been made towards understanding B cell memory, but T cell memory remains poorly understood and its nature is controversial. There is good evidence that B cell memory is driven by antigen, but the antigen dependence of T cell memory is still being debated. For several years we have investigated the nature, duration and antigen dependence of different aspects of CD8+ T cell memory and this review will discuss our findings as well as how and why they differ from some other results. As others, we find that antigen, due to proliferation of antigen-specific T cell clones, induces a shift in the T cell repertoire which remains detectable for years as an elevated cytotoxic T cell precursor frequency (CTLp) in lymphoid organs. Also in the absence of antigen, in vitro assays for T cell memory which invariably isolate memory T cells from these lymphoid organs therefore remain positive. In contrast, immunity against reinfection with a pathogen requires more than just elevated numbers of CTLp in lymphoid organs. Since reinfection usually takes place via peripheral nonlymphoid tissue, these CTLp have to a) efficiently extravasate and patrol through such tissues, and b) be immediately able to exert effector function in case of reinfection. Both functions, require a certain level of activation which critically depends on T cell stimulation by persisting antigen.

Balancing immunity and tolerance: deleting and tuning lymphocyte repertoires

Immunological self-tolerance is ensured by eliminating or inhibiting self-reactive lymphocyte clones, creating physical or functional holes in the B- and T-lymphocyte antigen receptor repertoires. The nature and size of these gaps in our immune defenses must be balanced against the necessity of mounting rapid immune responses to an everchanging array of foreign pathogens. To achieve this balance, only a fraction of particularly hazardous self-reactive clones appears to be physically eliminated from the repertoire in a manner that fully prevents their recruitment into an antimicrobial immune response. Many self-reactive cells are retained with a variety of conditional and potentially flexible restraints: (i) their ability to be triggered by antigen is diminished by mechanisms that tune down signaling by their antigen receptors, (ii) their ability to carry out inflammatory effector functions can be inhibited, and (iii) their capacity to migrate and persist is constrained. This balance between tolerance and immunity can be shifted, altering susceptibility to autoimmune disease and to infection by genetic or environmental differences either in the way antigens are presented, in the tuning molecules that adjust triggering set points for lymphocyte responses to antigen, or in the effector molecules that eliminate, retain, or expand particular clones.

Mature T cell reactivity altered by peptide agonist that induces positive selection

Recent studies have investigated how defined peptides influence T cell development. Using a T cell receptor-transgenic beta2-microglobulin-deficient model, we have examined T cell maturation in fetal thymic organ cultures in the presence of various peptides containing single-alanine substitutions of the strong peptide agonist, p33. Cocultivation with the peptide A4Y, which contains an altered T cell contact residue, resulted in efficient positive selection. Several in vitro assays demonstrated that A4Y was a moderate agonist relative to p33. Although A4Y promoted positive selection over a wide concentration range, high doses of this peptide could not induce clonal deletion. Thymocytes maturing in the presence of A4Y were no longer able to respond to A4Y, but could proliferate against p33. These studies demonstrate that (a) peptides that induce efficient positive selection at high concentrations are not exclusively antagonists; (b) some agonists do not promote clonal deletion; (c) positive selection requires a unique T cell receptor-peptide-major histocompatibility complex interaction; and (d) interactions with selecting peptides during T cell ontogeny may define the functional reactivity of mature T cells.

Prevention of murine lupus by an I-E alpha chain transgene: protective role of I-E alpha chain-derived peptides with a high affinity to I-Ab molecules

The expression of a transgene encoding the I-E alpha chain prevents a lupus-like autoimmune syndrome in BXSB mice. However, it had not been elucidated whether the E alpha d transgene-mediated protective effect results from I-E expression or from the generation of I-E alpha chain-derived peptides (E alpha peptide) displaying high affinity for the I-Ab molecule. To address this question, two different BXSB lines expressing the transgene at low or high levels were crossed with lupus-prone MRL mice; this resulted in three types of (MRL x BXSB)F1 mice, differing in the expression levels of I-E molecules and of E alpha peptides presented by I-Ab molecules. Comparative analysis of these three (MRL x BXSB)F1 mice as well as several BXSB transgenic lines showed that the E alpha d transgene-mediated protection paralleled the expression levels of E alpha peptide presented by I-Ab molecules, but not of I-E molecules on B cells. In addition, use of transgenic and nontransgenic double bone marrow chimeras showed a selective activation of nontransgenic B cells during I-Ab-restricted T cell-dependent immune responses, while both transgenic and nontransgenic B cells were comparably activated during T cell-independent responses. These results favor a model of autoimmunity prevention based on competition for antigen presentation, in which excessive generation of E alpha peptides prevents, because of their high affinity to the I-A molecules, activation of potential autoreactive T and B cells.

The Yaa gene-mediated acceleration of murine lupus: Yaa- T cells from non-autoimmune mice collaborate with Yaa+ B cells to produce lupus autoantibodies in vivo

The BXSB Y chromosome-linked mutant gene, Yaa, promotes autoimmune responses in mice predisposed to a lupus-like autoimmune disease. We have previously shown that a cognate interaction of T cells with B cells expressing the Yaa gene appears to be responsible for the accelerated production of autoantibodies. To investigate whether T cells that provide help for autoantibody production by Yaa+ B cells need to express the Yaa gene, we have made radiation bone marrow chimeras containing two sets of T and B cells from mice with or without the Yaa gene and differing by the Thy-1 and Igh allotypes. We then determined autoantibody production following the selective elimination of T cells of Yaa+ origin by treating mice with allele-specific anti-Thy-1 monoclonal antibody. Our results demonstrated that the selective production of autoantibodies by Yaa+ B cells in Yaa(+)-Yaa- double bone marrow chimeras can be mediated as efficiently by T cells from non-autoimmune mice lacking the Yaa gene as by T cells from autoimmune mice bearing the Yaa gene. This indicates that T cells from non-autoimmune Yaa- mice are capable of providing help for autoimmune responses by collaborating with Yaa+ B cells. These data thus strongly suggest that the Yaa gene defect is not functionally expressed in T cells, but only in B cells, and contrast with parallel experiments in the lpr model, in which defects of the Fas antigen in both T and B cells are crucial for the lpr gene-mediated promotion of autoantibody production.

Coexpression of B7-1 and viral ("self") transgenes in pancreatic beta cells can break peripheral ignorance and lead to spontaneous autoimmune diabetes

We evaluated the role of the costimulatory molecule B7-1 in overcoming peripheral ignorance in transgenic mice, which expressed the glycoprotein (GP) or nucleoprotein (NP) of lymphocytic choriomeningitis virus (LCMV) as the self-antigen in pancreatic beta cells. The viral transgenes or B7-1 alone did not induce autoimmune diabetes (IDDM). However, in bigenic mice expressing B7-1 and LCMV-GP, anti-self (viral) cytotoxic T lymphocytes (CTL) were activated without viral infection and spontaneous IDDM occurred. In contrast, bigenic RIP-B7-1 x RIP-NP mice with thymic expression of the self (viral-NP) antigen deleted the majority of their autoreactive CTL and did not develop spontaneous IDDM. However, these mice developed fast-onset IDDM 14 days after LCMV infection, whereas single-transgenic RIP-NP littermates developed IDDM only within 4-5 months. Rapid IDDM was associated with increased numbers of anti-self CTL and a predominance of IFN gamma produced by islet-infiltrating lymphocytes, whereas single transgenic RIP-NP littermates with slow-onset IDDM displayed less anti-self CTL and more IL-4- and IL-10-producing T lymphocytes in pancreatic infiltrates.

Emergence of virus escape mutants after immunization with epitope vaccine

BALB/c and C57BL/6J mice were immunized with recombinant vaccines consisting of lymphocytic choriomeningitis virus CD8+ T-lymphocyte epitopes and a carrier protein. During challenge infection with WE strain lymphocytic choriomeningitis virus, mutants with alterations in distinct amino acid residues of the epitopic nonapeptides appeared and multiplied. Splenocytes from WE-infected BALB/c mice lysed cells coated with the WE-type epitope; lysis was considerably less effective when the epitopic nonapeptide with which the syngeneic cells had been sensitized was the mutated form. Neither target was lysed by splenocytes from BALB/c mice infected with the variant virus. Mutants were not detected in F1 hybrid mice immunized with two viral epitopes that were restricted by class I molecules of both parents.

Focal expression of interleukin-2 does not break unresponsiveness to "self" (viral) antigen expressed in beta cells but enhances development of autoimmune disease (diabetes) after initiation of an anti-self immune response

The participation of IL-2 in insulin-dependent (type 1) diabetes (IDDM) was analyzed in transgenic (tg) mice expressing the nucleoprotein (NP) of lymphocytic choriomeningitis virus and IL-2 under control of the rat insulin promoter focally in beta cells of the islets of Langerhans. Insertion and expression of the viral (self) gene or of the IL-2 gene alone did not lead to IDDM. Infiltration primarily of CD4 and B lymphocytes and increased expression of MHC class I and II molecules occurred in islets where IL-2 was expressed. By contrast, neither cellular infiltrates nor expression of MHC class I or II glycoproteins above base levels was noted in tgs expressing the viral protein alone. Double tg mice expressing both the viral protein and IL-2 in their islets displayed a modest increase in incidence of spontaneous diabetes compared with that of single transgenic mice expressing IL-2 alone. Breaking of immunological unresponsiveness or sensitization to self antigens did not occur. Neither cytotoxic T lymphocytes (CTL) nor antibodies directed against the viral tg (NP) were generated. However, after challenge with lymphocytic choriomeningitis virus, double tg mice developed anti-self (viral) CTL and IDDM (incidence > 95%) within 2 mo. The generation of virus ("self")-specific MHC-restricted CTL was dependent on CD4+ help. In contrast, viral inoculum to single tg mice expressing either the viral protein or IL-2 failed to enhance the incidence of IDDM over 30% for viral protein or 10% for IL-2 after an 8-mo observation period. Hence, in this autoimmune model in situ expression of IL-2 did not break unresponsiveness but markedly enhanced ongoing disease.