Molecular mimicry in T cell-mediated autoimmunity: viral peptides activate human T cell clones specific for myelin basic protein

Failure to induce organ-specific autoimmunity by breaking of tolerance: importance of the microenvironment

Peripheral tolerance is considered to be a safeguard against autoimmunity. Using a TCR-transgenic mouse system displaying peripheral tolerance against a liver-specific MHC class I Kb antigen, we investigated whether the breaking of tolerance would result in autoimmunity. Reversal of tolerance was achieved by simultaneous challenge with cells expressing the Kb autoantigen and IL-2. Tolerance could not be broken with IL-2 alone or when Kb- and IL-2-expressing cells were applied to different sites of the mice. However, despite the presence of activated autoreactive T cells that were able to reject Kb-positive grafts no autoaggression against the Kb-positive liver was observed. These results indicate that breaking of tolerance per se is not sufficient to cause liver-specific autoimmunity. However, when in addition to breaking tolerance the mice were infected with a liver-specific pathogen, autoaggression occurred. Thus, in this system at least two independent steps seem to be required for organ-specific autoimmunity: reversal of peripheral tolerance resulting in functional activation of autoreactive T cells and conditioning of the liver microenvironment which enables the activated T cells to cause tissue damage.

Serum amyloid A protein is elevated in relapsing-remitting multiple sclerosis

In multiple sclerosis (MS), the signs of inflammation that can be detected in the central nervous system are not mirrored by unequivocal markers of activation of the immune system in the periphery. We performed a serial monitoring of serum amyloid A protein (SAA), a major acute phase reactant, in peripheral blood of patients with relapsing-remitting MS over a 3-month period. Patients were monitored in parallel with gadolinium-enhanced magnetic resonance imaging (Gd-MRI) of the brain. The results show that signs of ongoing peripheral inflammation, reflected by elevations of SAA levels, can be detected in MS patients.

Persisting viruses and chronic inflammation: understanding their relation to autoimmunity

Viral infections may induce and sustain autoimmune processes via several and overlapping mechanisms. We outline how chronic inflammation, sustained by persisting viruses, may be "the prerequisite" for initiation and maintenance of the multistep process leading to autoimmunity. Chronic inflammation may favour priming of autoreactive T cells which have escaped thymic tolerance and are able to mount a cross-reactive response to self-mimicking antigens carried by viruses in the periphery. Moreover, chronic inflammation and persisting viruses can synergistically support autoimmunity through other relevant mechanisms: unveiling of cryptic self-epitopes, determinant spreading, activation of dendritic cells, constant priming of new autoreactive T cells, and efficient generation and restimulation of memory cells. Therefore, viruses seem to play a key role among the many environmental factors which, together with the genetic background, have been implicated in the pathogenesis of autoimmune diseases. We will also discuss some hypotheses explaining why autoimmunity is a rare event.

T Cell Vaccination in Experimental Autoimmune Encephalomyelitis: A Mathematical Model

T cell vaccination (TCV) is a method to induce resistance to autoimmune diseases by priming the immune system with autoreactive T cells. This priming evokes an anti-idiotypic regulatory T cell response to the receptors on the autoreactive T cells. Hence resistance is induced. To prevent the inoculated autoreactive cells from inducing autoimmunity, cells are given in a subpathogenic dose or in an attenuated form. We developed a mathematical model to study how the interactions between autoreactive T cells, self epitopes, and regulatory cells can explain TCV. The model is based on detailed data on experimental autoimmune encephalomyelitis, but can be generalized to other autoimmune diseases. We show that all of the phenomena collectively described as TCV occur quite naturally in systems where autoreactive T cells can be controlled by anti-idiotypic regulatory T cells. The essential assumption that we make is that TCV generally involves self epitopes for which T cell tolerance is incomplete. The model predicts a qualitative difference between the two vaccination methods: vaccination with normal autoreactive cells should give rise to a steady state of long lasting protection, whereas vaccination with attenuated cells should only confer transient resistance. Moreover, the model shows how autoimmune relapses can occur naturally without the involvement of T cells arising due to determinant spreading.

Epitope spreading: the role of self peptides and autoantigen processing by B lymphocytes

A complex interplay of cells, soluble macromolecules, and antigen lead to a productive immune response that evolved for the survival of species. While the immune system is intended to protect from foreign agents, such as bacterial and viral infection, the presence of autoimmune diseases indicates that the system is not perfect in differentiating antigen that may cause harm from benign self constituents. The concept of epitope spreading, where many determinants on an offending antigen are the focus of immune attack, is an efficient means of clearing an infectious agent. However, the same mechanisms that lead to a diverse immune response may be harmful when the targets of attack are self tissues or self macromolecules. This review will examine the forms of self antigens that may initiate autoimmunity and the potential role of B lymphocytes, as autoantigen-presenting cells, as one mechanism by which diversification of autoimmunity may occur.

CD4+CD25+ immunoregulatory T cells suppress polyclonal T cell activation in vitro by inhibiting interleukin 2 production

Peripheral tolerance may be maintained by a population of regulatory/suppressor T cells that prevent the activation of autoreactive T cells recognizing tissue-specific antigens. We have previously shown that CD4+CD25+ T cells represent a unique population of suppressor T cells that can prevent both the initiation of organ-specific autoimmune disease after day 3 thymectomy and the effector function of cloned autoantigen-specific CD4+ T cells. To analyze the mechanism of action of these cells, we established an in vitro model system that mimics the function of these cells in vivo. Purified CD4+CD25+ cells failed to proliferate after stimulation with interleukin (IL)-2 alone or stimulation through the T cell receptor (TCR). When cocultured with CD4+CD25- cells, the CD4+CD25+ cells markedly suppressed proliferation by specifically inhibiting the production of IL-2. The inhibition was not cytokine mediated, was dependent on cell contact between the regulatory cells and the responders, and required activation of the suppressors via the TCR. Inhibition could be overcome by the addition to the cultures of IL-2 or anti-CD28, suggesting that the CD4+CD25+ cells may function by blocking the delivery of a costimulatory signal. Induction of CD25 expression on CD25- T cells in vitro or in vivo did not result in the generation of suppressor activity. Collectively, these data support the concept that the CD4+CD25+ T cells in normal mice may represent a distinct lineage of "professional" suppressor cells.

Future perspectives in specific immunotherapy of melanoma

With the discovery of T-cell recognised tumour-associated antigens (TAAs), interest in specific immunotherapy for treatment of malignancies has increased substantially. The majority of studies investigating TAAs have focused on melanoma-associated antigens because of evidence that the immune system influences the pathogenesis of melanoma. This paper reviews the different types of melanoma antigens, their in vitro and in vivo immunogenicity and clinical data regarding the use of specific immunotherapy in patients with stage I-IV melanoma. Results of clinical studies are highly variable but encourage further research in these patients. Developing and perfecting laboratory and clinical correlates of response to these specific immunotherapies are vital to determining their role in clinical practice.

A Viral Peptide with Limited Homology to a Self Peptide Can Induce Clinical Signs of Experimental Autoimmune Encephalomyelitis

Molecular mimicry has been suggested as a mode of autoreactive T cell stimulation in autoimmune diseases. Myelin basic protein (MBP) peptide 1–11 induces experimental autoimmune encephalomyelitis (EAE) in susceptible strains of mice. Here we show that a herpesvirus Saimiri (HVS) peptide, AAQRRPSRPFA, with a limited homology to MBP1–11 peptide, ASQKRPSQRHG (underlined letters showing homology), can stimulate a panel of MBP1–11-specific T cell hybridomas and more importantly cause EAE in mice. We demonstrate that this is due to cross-recognition of these two peptides by TCRs. Results presented in this communication are the first demonstration that a viral peptide with homology at just 5 amino acids with a self peptide can induce clinical signs of EAE in mice. These findings have important implications in understanding the breakdown of T cell tolerance to self Ags in autoimmune diseases by means of cross-reactivity with unrelated peptides.

Gammadelta T cell responses to activated T cells in multiple sclerosis patients induced by T cell vaccination

To explore the hypothesis that gammadelta T cells may regulate activated alphabeta T cells, we studied gammadelta T cell responses to alphabeta T cell clones in Multiple Sclerosis (MS) patients who received attenuated autologous autoreactive T cells. We recently conducted a pilot study of T cell vaccination with myelin basic protein reactive T cells in MS. Since T cell vaccination upregulates the anti-vaccine T cell responses, we evaluated gammadelta T cell reactivity towards the vaccine in the vaccinated patients. Lymphocytes were stimulated in vitro with irradiated vaccine cells and the responding lines were checked for the presence of gammadelta T cells. Our data demonstrate that in the majority of vaccinated MS patients gammadelta T cells expand upon stimulation with the vaccine cells. The responding gammadelta T cells were predominantly Vdelta1+/Vgamma1+, and represented diverse clonal origins. The gammadelta T cells could not inhibit in vitro proliferation of the vaccine T cells and displayed low cytotoxic reactivity towards the vaccine clones. However, they produced high levels of IL2, TNFalpha and IL10. These results indicate that gammadelta T cells can be stimulated by activated alphabeta T cells, and that these gammadelta T cell responses are upregulated after T cell vaccination. These findings suggest that gammadelta T cells are involved in peripheral mechanisms to control activated autoreactive T cells.

Mycobacterial Cpn10 promotes recognition of the mammalian homologue by a mycobacterium-specific antiserum

Self-tolerance, a key feature of the immune system, is still a matter of intense debate. We give here evidence for a peculiar behavior of an antiserum against Mycobacterium tuberculosis chaperonin 10 (m-Cpn10), which could have implications for the mechanism of self-recognition by antibodies against non-self. We show that this antiserum can interact in terms of both inhibition of biological activity and physical association (immunoprecipitation), with the mammalian homologue of m-Cpn10, but only if the bacterial protein is present. Several lines of evidence led us to exclude that the two proteins physically associate to form heterocomplexes: (1) the behavior of the antiserum was not shared by a monoclonal antibody against m-Cpn10; (2) a matrix selective for human Cpn10 (h-Cpn10) did not co-purify m-Cpn10; (3) the distribution pattern in non-denaturing isoelectric focusing of labeled m-Cpn10 was not altered by the presence of the unlabeled h-Cpn10. We conclude therefore that the antiserum against M. tuberculosis Cpn10 also recognizes mammalian Cpn10, with an affinity/avidity regulated by the mycobacterial protein, or by the promotion of hetero-oligomerization. This emergence of self-recognition in the presence of M. tuberculosis Cpn10 could imply a breaking of self-tolerance in situations of infection or vaccination.

Idiotypic immunization induces immunity to mutated p53 and tumor rejection

The p53 molecule might serve as a common tumor-associated antigen, as the tumor suppressor gene p53 is mutated and the p53 protein is often over-expressed in tumor cells. We report that effective immunity to p53 can be induced through an idiotypic network by immunization of mice with a monoclonal antibody (PAb-240) specific for mutated p53, or with a peptide derived from the complementarity determining region (CDR) 3 of the variable domain of the light chain (VL) of this antibody. The immunized mice produced IgG antibodies to p53 and mounted a cytotoxic reaction to a tumor line bearing mutated p53. The idiotypically immunized mice were resistant to challenge with the tumor cells. Thus antibodies to p53 might serve as immunogens for activating resistance to some tumors. At the basic level, these findings indicate that a network of p53 immunity may be organized naturally within the immune system.

Glimpses at the recognition of peptide/MHC complexes by T-cell antigen receptors

More than a decade after the first description of the primary structure of a T-cell antigen receptor (TCR), the recent determination of the crystal structure of several unliganded TCR ectodomains and of two TCRs complexed to peptide-MHC ligand provides a structural basis for understanding the initial event that triggers T-cell activation. This review focuses on the topology of the variable (V) domains found in TCRs and immunoglobulins and attempts to delineate the structural features that may render the TCR complementarity-determining regions particularly suited to dock on the peptide/MHC surface. Finally, the available TCR structures provide an opportunity to re-evaluate the molecular basis for intrathymic positive selection as well as the mechanisms that make a given TCR neither infinitely specific, nor so flexible that it engages productively any MHC-binding peptides.

Expression of accessory molecules and cytokines in acute EAE in marmoset monkeys (Callithrix jacchus)

Accessory molecules and cytokines are involved in the immunopathogenesis of multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE) in rodent models, and are potential targets for immunotherapy. Evaluation of such experimental therapies requires appropriate animal models. Therefore, we analysed the expression of selected accessory molecules and cytokines in the brain of marmoset monkeys (Callithrix jacchus) with acute EAE, a newly described non-human primate model for MS. All animals experienced active disease clinically and histopathologically with strong resemblance to MS. Perivascular infiltrates of mononuclear cells showed abundant expression of CD40. CD40 was expressed on macrophages, indicating that T cell priming and macrophage effector functions may result from local CD40-CD40L interactions. CD40 ligand (CD40L) and B7-2 (CD86) were also expressed, but to a lower extent, while B7-1 (CD80) expression was limited. Both pro-inflammatory and anti-inflammatory cytokines were produced within individual lesions during active disease (IFN-alpha, IFN-gamma, TNF-alpha, IL-1alpha, IL-1beta, IL-2, IL-4, IL-10 and IL-12). This suggests that relative levels rather than sequential expression of Th1- and Th2-type cytokines determine disease activity. These findings demonstrate the value of EAE in marmoset monkeys as a model to assess the role of accessory molecules and cytokines in multiple sclerosis, and to evaluate targeted intervention.

Mechanisms for induction of autoimmunity in humans

The triggering or immunogenic stimulus for human autoimmune diseases is unknown. It is not even known whether the stimulus is endogenous, i.e. truly "self" or exogenous, "non-self". Many autoimmune diseases are human lymphocyte antigen (HLA)-associated and demonstrate linkage to the HLA chromosome in family investigations. For most of these diseases, evidence is strong that the association is directly dependent on specific HLA class I or II molecules rather than on other genes located in the HLA region. Since HLA polymorphic HLA molecules have so far only been shown to have two distinct functions, both of which are immunological, a HLA association supports the notion that a particular disease is autoimmune. Furthermore, an association to a specific HLA allele implies that the immunogenic stimulus for autoimmunity would be one specific HLA-binding peptide and that, at least initially, autoimmunity is dependent on the reactivity of one or a limited number of potentially autoaggressive T cell clones. These findings are encouraging and formin the basis for future preventive measures. One current theory is that autoimmune disease is precipitated by an environmental agent, such as a viral infection. Several different mechanisms to explain how a viral infection could induce autoimmune disease in humans are described and one specific example is presented for a virus-induced autoimmune disease in humans. The question of whether ITP could also be dependent on such a mechanism is briefly discussed.

T cell recognition motifs of an immunodominant peptide of myelin basic protein in patients with multiple sclerosis: structural requirements and clinical implications

Myelin basic protein (MBP)-reactive T cells may play an important role in the pathogenesis of multiple sclerosis (MS). The T cell response to the 83-99 region of MBP represents a dominant autoreactive response to MBP in MS patients of DR2 haplotype. In this study, a large panel of DR2- and DR4-restricted T cell clones specific for the MBP83-99 peptide were examined for the recognition motifs and structural requirements for antigen recognition using alanine-substituted peptides. Our study revealed that although the recognition motifs of the T cell clones were diverse, the TCR contact residues within the 83-99 region of MBP were highly conserved. Two central residues (Phe90 and Lys91) served as the critical TCR contact points for both DR2- and DR4-restricted T cell clones. Single alanine substitution at residue 90 or residue 91 abolished the responses of 81-95 % of the T cell clones while a double alanine substitution rendered all T cell clones unresponsive. It was also demonstrated in this study that the substituted peptides altered the cytokine profile of some, but not all, T cell clones. Some MBP83-99-specific T cell clones were able to sustain alanine substitutions and were susceptible to activation by microbial antigens. The study has an important implication in designing a peptide-based therapy for MS.

Identification of mimicry peptides based on sequential motifs of epitopes derived from 65-kDa glutamic acid decarboxylase

Insulin-dependent diabetes mellitus (IDDM) is an autoimmune disease with a predominantly non-hereditary etiology that results in a destruction of pancreatic beta cells by autoaggressive T lymphocytes. Neither the mechanism of initial stimulation of these T cells nor the nature of the environmental factors implicated in the disease have so far been identified. However, both issues are taken into account by the hypothesis of initial T cell activation by viral or bacterial mimicry peptides with sequence similarities to pancreatic self antigens. We determined sequential epitope motifs to search for mimicry peptides stimulating T cell lines specific for two epitopes derived from the IDDM autoantigen 65-kDa glutamic acid decarboxylase (GAD65). These were GAD65 (88-99), presented by HLA-DRB1*0101, and GAD65 (248-257), presented by HLA-DRB5*0101. T cell stimulation by peptides with substitutions in HLA anchor or T cell contact positions was analyzed to establish degenerate epitope motifs for database searching. Out of 28 tested candidate mimicry peptides derived from bacterial, viral and human proteins, 3 stimulated T cell lines and a T cell clone specific for epitope GAD65 (248-257). Our results demonstrate that mono- and polyclonal GAD65-specific T cells from IDDM patients can be stimulated by viral and bacterial peptides with little apparent sequence homology with autoantigenic epitopes. Moreover, in a synopsis with related published studies, our findings suggest that simple degenerate search motifs comprising principal T cell contacts plus HLA class II binding motifs may suffice to identify most mimicry peptides.

Myelin reactive T cells in the autoimmune pathogenesis of multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) leading to demyelination. Although it is widely accepted that demyelination in MS results from an active inflammatory process, the cause of the inflammation is still not completely resolved. Findings in experimental autoimmune encephalomyelitis (EAE), an animal model of MS, and observations in human MS have led to the hypothesis that MS is an autoimmune disease mediated by autoreactive T cells with specificity for myelin antigens. The identity of the brain antigen(s) which is (are) the primary target(s) of the autoimmune process is not known, but current evidence indicates that myelin basic protein (MBP) is a likely candidate. In this paper we will overview some of the experimental evidence suggesting that MBP reactive T cells hold a central position in the pathogenesis of MS, and discuss some of the currently tested therapeutic strategies in MS which are directed towards the pathogenic MBP reactive T cells. Although there appears to be no direct correlation between anti-MBP T cell responses and clinical disease activity, some recent observations suggest that monitoring of anti-MBP T cell responses could be helpful to study immunological efficacy of experimental immunotherapies in MS.

Interactions with multiple peptide ligands determine the fate of developing thymocytes

Thymocytes are positively or negatively selected depending on interactions between their T cell receptors (TCR) and peptides presented by major histocompatibility complex molecules. We have previously shown that apoptosis of thymocytes from an alpha beta TCR-transgenic mouse (F5), induced by antigenic peptide, can be inhibited specifically by an antagonist peptide variant in an in vitro culture model. We have now extended these experiments by demonstrating that the antagonist peptide can inhibit natural negative selection of maturing thymocytes, induced by endogenously expressed antigen, in fetal thymic organ cultures (FTOC). This inhibition resulted in the rescue and maturation of thymocytes that would otherwise have been deleted. Mature T cells generated in these cultures were able to respond to antigen by producing limited quantities of interferon-gamma, but unlike T cells from control FTOC, they required exogenous interleukin-2 to generate cytolytic effector cells. Interestingly, the antagonist peptide also accelerated the development of F5 thymocytes in the absence of the negatively selecting ligand. These data suggest that the developmental fate of a thymocyte may be determined by the recognition of multiple distinct peptide ligands during thymic selection. Alterations in the profiles of selecting peptides presented in the thymus would thus have profound effects on the size and autoreactive potential of the T cell repertoire generated.

Quantitative aspects of T-cell recognition: from within the antigen-presenting cell to within the T cell

T lymphocytes circulate continually throughout the peripheral lymphoid organs, where they scrutinize the surface of cells to detect the presence of nonself protein fragments. During the last years, many facets of T-cell function have been unravelled. After being bound by major histocompatibility complex (MHC) molecules, peptides derived from nonself as well as from self proteins are delivered to the cell surface. A few copies of a nonself peptide "presented" at the cell surface in the context of an MHC molecule can be detected by specific T cells, and suffice to trigger T-cell activation. This paper reviews the requirements imposed on T cells to fulfill this exquisite sensitivity.

Heat shock proteins as potential targets in the therapy of inflammatory arthritis

Whether heat shock proteins (hsp) will be therapeutic targets in arthritis depends on their role in pathogenesis. In this article, three possibilities are considered. Firstly, an excessive immune response to bacterial hsp could be arthritogenic - as may occur in reactive arthritis. In these circumstances therapy would be directed to down-regulating this immune response, or altering the nature of the immune response e.g. by changing cytokine production from interferon-g to IL-4. However this approach depends on the immune response to bacterial hsp not being critical for control of the bacterial infection. Secondly, an immune response to bacterial hsp may induce autoimmunity by cross-reactivity, e.g. with the homologous human. This could also be modulated in the same way with a lower likelihood of interfering with control of the infectious agent, since only a component of the immune response against the bacterial hsp will be cross-reactive with self. Thirdly, recent experiments raise the possibility that joint inflammation might be controlled by T cells which recognizes self hsp, particularly hsp60. Therapies might enhance this response; protection from experimental arthritis by prior immunization with hsp60 is well established. Whether similar approaches will be viable after arthritis is established remains to be seen.

Cutting Edge: Predictable TCR Antigen Recognition Based on Peptide Scans Leads to the Identification of Agonist Ligands with No Sequence Homology

The potential of CD4+ T cells for cross-recognition of self and foreign Ags has important implications for the understanding of thymic selection, lymphocyte survival, and the occurrence of autoimmune diseases. Here, we define the extensive flexibility of Ag recognition for three human CD4+ autoreactive T cell clones (TCC) by using ligands with single and multiple amino acid (aa) substitutions. Our results demonstrate that the spectrum of tolerated ligands and the resulting stimulatory potency of peptides for a TCC can be predicted by the relative influence of each aa. Using this approach, we have identified stimulatory ligands not sharing a single aa in corresponding positions with the Ag used to establish the TCC. These results argue for an independent contribution of each aa in the peptide sequence to the affinity of the MHC/peptide complex to the TCR.

Probing degeneracy in T-cell recognition using peptide combinatorial libraries

Recent studies have demonstrated flexibility of the T-cell receptor (TCR) with respect to recognition of peptide bound to self major histocompatibility complex (MHC). With the introduction of peptide combinatorial libraries, it has become possible to dissect the extent of degeneracy in T-cell recognition. On the basis of these novel findings, Bernhard Hemmer and colleagues propose a conceptual framework for lymphocyte selection and survival and the occurrence of autoimmunity.

Genetically determined failure of activation-induced apoptosis of autoreactive T cells as a cause of multiple sclerosis

I postulate that multiple sclerosis is an autoimmune disease that involves genetically determined failure of activation-induced apoptosis of autoreactive T cells in the central nervous system. Activation of central-nervous-system-reactive T cells in peripheral lymphoid organs by exposure to crossreacting antigens or superantigens derived from common infectious agents may trigger attacks of multiple sclerosis. In normal individuals these activated T cells are deleted by activation-induced apoptosis, but in individuals predisposed to multiple sclerosis they survive, proliferate, and damage the central nervous system. The clinical course of multiple sclerosis may vary according to the antigens in the central nervous system being targeted: targeting of myelin antigens leads to a relapsing-remitting course of clinical recovery due to remyelination or other mechanisms; targeting of axonal antigens leads to a progressive course from onset because axonal regeneration is limited in the central nervous system. This hypothesis can account for many characteristics of multiple sclerosis and has predictions that can be tested.

Promiscuous recognition of major histocompatibility complex class II determinants in cyclosporine-induced syngeneic graft-versus-host disease: specificity of cytolytic effector T cells

BACKGROUND

Administration of the immunosuppressive drug cyclosporine after syngeneic/autologous bone marrow transplantation paradoxically elicits a systemic autoimmune syndrome resembling graft-versus-host disease (GVHD). This syndrome, termed autologous or syngeneic GVHD, is associated with the development of a highly restricted repertoire of cytolytic T lymphocytes that promiscuously recognizes major histocompatibility complex class II determinants, including self.

METHODS

Vbeta8.5+CD8+ effector lymphocytes and T-cell clones were isolated from Lewis rats with cylosporine-induced syngeneic GVHD. The specificity of the effector T cells and T-cell clones was examined in vitro. The pathogenicity of the T-cell clones was confirmed in vivo using a local graft-versus-host reaction assay.

RESULTS

Clonal analysis reveals that the pathogenic effector T cells recognize a peptide from the invariant chain termed CLIP in association with major histocompatibility complex class II determinants. Moreover, there appears to be an additional interaction between the N-terminal flanking region of CLIP and the Vbeta segment of the T cell receptor.

CONCLUSION

The results suggest that recognition of this highly conserved peptide along with the additional interaction between the flanking region and the T cell receptor may account for the promiscuous activity of the autologous/syngeneic GVHD autoreactive T cells.

Cytotoxic T cells from human immunodeficiency virus type 2-infected patients frequently cross-react with different human immunodeficiency virus type 1 clades

Knowledge of immune mechanisms responsible for the cross-protection between highly divergent viruses such as human immunodeficiency virus type 1 (HIV-1) and HIV-2 may contribute to an understanding of whether virus variability may be overcome in the design of vaccine candidates which are broadly protective across the HIV subtypes. We demonstrate that despite the significant difference in virus amino acid sequence, the majority of HIV-2-infected individuals with different HLA molecules possess a dominant cytotoxic T-cell response which is able to recognize HIV-1 Gag protein. Furthermore, HLA-B5801-positive subjects show broad cross-recognition of HIV-1 subtypes since they mounted a T-cell response that tolerated extensive amino acid substitutions within HLA-B5801-restricted HIV-1 and HIV-2 epitopes. These results suggests that HLA-B5801-positive HIV-2-infected individuals have an enhanced ability to react with HIV-1 that could play a role in cross-protection.

Peptide mimics of a tumor antigen induce functional cytotoxic T cells

The ability to mimic peptide/peptide and/or peptide/carbohydrate structures may be important in generating cross-reactive antibodies for autoimmune and other diseases. We show that the peptide sequence DAHWESWL can mimic the conformation of the unrelated MUC1 peptide SAPDTRPAP(G). Mice immunized with mannan-MUC1-peptides make cytotoxic T lymphocytes (CTLs) and are protected from MUC1+ tumors. We show that the same specific anti-MUC1 responses can be produced by immunizing with the DAHWESWL peptide; furthermore, specific tumor protection is obtained in a manner similar to that with MUC1 immunization. The DAHWESWL peptide immunization leads to CTLs that recognize H2Dd and H2Ld but not H2b or human leukocyte antigens-group A (HLA-A) *0201 presented MUC1 peptides. However, mutation of the DAHWESWL peptide to a more HLA-A*0201-compatible structure with appropriate anchors (DLHWASWV), leads to the production of CTLs in HLA-A*0201 mice.

Molecular mimicry by herpes simplex virus-type 1: autoimmune disease after viral infection

Viral infection is sometimes associated with the initiation or exacerbation of autoimmune disease, although the underlying mechanisms remain unclear. One proposed mechanism is that viral determinants that mimic host antigens trigger self-reactive T cell clones to destroy host tissue. An epitope expressed by a coat protein of herpes simplex virus-type 1 (HSV-1) KOS strain has now been shown to be recognized by autoreactive T cells that target corneal antigens in a murine model of autoimmune herpes stromal keratitis. Mutant HSV-1 viruses that lacked this epitope did not induce autoimmune disease. Thus, expression of molecular mimics can influence the development of autoimmune disease after viral infection.

Structural basis of plasticity in T cell receptor recognition of a self peptide-MHC antigen

The T cell receptor (TCR) inherently has dual specificity. T cells must recognize self-antigens in the thymus during maturation and then discriminate between foreign pathogens in the periphery. A molecular basis for this cross-reactivity is elucidated by the crystal structure of the alloreactive 2C TCR bound to self peptide-major histocompatibility complex (pMHC) antigen H-2Kb-dEV8 refined against anisotropic 3.0 angstrom resolution x-ray data. The interface between peptide and TCR exhibits extremely poor shape complementarity, and the TCR beta chain complementarity-determining region 3 (CDR3) has minimal interaction with the dEV8 peptide. Large conformational changes in three of the TCR CDR loops are induced upon binding, providing a mechanism of structural plasticity to accommodate a variety of different peptide antigens. Extensive TCR interaction with the pMHC alpha helices suggests a generalized orientation that is mediated by the Valpha domain of the TCR and rationalizes how TCRs can effectively "scan" different peptides bound within a large, low-affinity MHC structural framework for those that provide the slight additional kinetic stabilization required for signaling.

Decreased dependence of myelin basic protein-reactive T cells on CD28-mediated costimulation in multiple sclerosis patients. A marker of activated/memory T cells

Although multiple sclerosis (MS) patients and healthy individuals have similar frequencies of myelin basic protein (MBP)-specific T cells, the activation state of these cells has not been well characterized. Therefore, we investigated the dependence of MBP-reactive T cells on CD28-mediated costimulation in MS patients, healthy controls, and stroke patients. MBP-reactive T cells from healthy controls and stroke patients failed to proliferate efficiently when costimulation was blocked using anti-CD28, consistent with a naive T cell response. In contrast, MBP-specific T cell proliferation was not inhibited, or was only partially inhibited when CD28-mediated costimulation was blocked in MS patients. Blockade of CD28 failed to inhibit tetanus toxoid-specific T cell proliferation in both the controls and MS patients, demonstrating that memory cells are not dependent on CD28-mediated costimulation. Limiting dilution analysis indicated that the frequency of MBP-reactive T cells was significantly decreased in healthy controls compared with MS patients when CD28-mediated costimulation was blocked. These data suggest that MBP-reactive T cells are more likely to have been activated in vivo and/or differentiated into memory T cells in MS patients compared with controls, indicating that these cells may be participating in the pathogenesis of MS.

Mechanism of ovarian autoimmunity: induction of T cell and antibody responses by T cell epitope mimicry and epitope spreading

Autoimmune diseases are often manifested as organ inflammation with loss of function, and detectable autoreactive T cell and autoantibody responses. In the proper genetic context, we have shown that these parameters of autoimmunity can result from a single pivotal event: the induction of a strong and persistent T cell response for a foreign or unrelated self peptide that mimics the target self peptide. This may apply to organ-specific and systemic autoimmunity, independent of whether the tissue inflammation results from T cell immune mechanism or antibodies. T cell peptide mimicry, through sharing of critical residues or by a less defined mechanism, can result in autoimmune disease. Once triggered, the helper T cell response leads rapidly to a concomitant autoantibody response spreading to distant B cell determinants of the self protein antigen. Evidently, with T cell help, endogenous antigens can stimulate B cells to provoke a functional autoantibody response against conformational antigenic determinants. These findings are based on recent studies on a novel autoimmune ovarian disease model induced by a self peptide with well-defined T and B cell epitopes. However, studies reported on systemic lupus erythematosus models have shown that similar events may result in autoantibody response in systemic autoimmunity.

CD4+CD25+ T Cells Inhibit Both the Induction and Effector Function of Autoreactive T Cells and Represent a Unique Lineage of Immunoregulatory Cells

Thymectomy of susceptible strains of mice on day 3 of life results in a spectrum of organ-specific autoimmunity that can be prevented by reconstitution of the thymectomized animals early in life with normal adult lymphocytes. The effectors and suppressors of autoimmunity in this model have been convincingly shown to be CD4+ T cells. It has been demonstrated recently that the regulatory CD4+ T cells that prevent disease coexpress CD25. We have further characterized the population of CD4+CD25+ immunoregulatory cells and demonstrated that they can suppress not only the induction of disease post-thymectomy, but can also efficiently suppress disease induced by cloned autoantigen-specific effector cells. Furthermore, the CD4+CD25+ T cells appear to be members of a unique lineage of regulatory T cells, as the induction of CD25 expression on a monospecific population of T cells derived from TCR transgenic SCID mice did not result in suppression of post-thymectomy autoimmunity. In addition, the TCR transgenic SCID mice were highly susceptible to autoimmune disease induced by the cloned line of autoantigen-specific effectors, while normal mice were relatively resistant. The capacity of the cloned line to transfer disease to nu/nu recipients could be inhibited by normal spleen cell populations containing CD4+CD25+ cells and by purified CD4+CD25+ cells. Although the target Ag(s) and mechanism of action of the CD4+CD25+ T cells remain to be determined, it is likely that they also play an important role in modulating other autoimmune diseases that are mediated by activation of “ignorant” self-reactive T cells present in the normal peripheral lymphocyte pool.

Expansion of Autoreactive T Cells in Multiple Sclerosis Is Independent of Exogenous B7 Costimulation

Multiple sclerosis (MS) is an inflammatory disease of the myelinated central nervous system that is postulated to be induced by myelin-reactive CD4 T cells. T cell activation requires an antigen-specific signal through the TCR and a costimulatory signal, which can be mediated by B7–1 or B7–2 engagement of CD28. To directly examine the activation state of myelin-reactive T cells in MS, the costimulation requirements necessary to activate myelin basic protein (MBP) or tetanus toxoid (TT)-reactive CD4 T cells were compared between normal controls and MS patients. Peripheral blood T cells were stimulated with Chinese hamster ovary (CHO) cells transfected either with DRB1*1501/DRA0101 chains (t-DR2) alone, or in combination with, B7–1 or B7–2. In the absence of costimulation, T cells from normal subjects stimulated with the recall antigen TT p830–843 were induced to expand and proliferate, but stimulation with MBP p85–99 did not have this effect. In marked contrast, T cells from patients with MS stimulated with MBP p85–99 in the absence of B7–1 or B7–2 signals expanded and proliferated. Thus, MBP-reactive CD4 T cells in patients with MS are costimulation independent and have been previously activated in vivo. These experiments provide further direct evidence for a role of activated MBP-specific CD4 T cells in the pathogenesis of MS.

Insights into the aetiology and pathogenesis of multiple sclerosis

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system, and the most common neurological disease affecting young adults. Multiple sclerosis is a clinically heterogeneous disorder. It is believed to be an autoimmune disease, with cell-mediated and humoral responses directed against myelin proteins. This hypothesis largely comes from pathological parallels with an animal model, experimental autoimmune encephalomyelitis (EAE). Autoimmunity to myelin proteins in humans may be inadvertently triggered by microbes which have structural homologies with myelin antigens (molecular mimicry). As with other autoimmune diseases, susceptibility to MS is associated with certain MHC genes/haplotypes. Full genomic screening of mutiplex families has underscored the role for MHC genes as exerting moderate but the most significant effects in susceptibility. The primary target autoantigen in MS has yet to be definitively identified, but as well as the major myelin proteins, it is now clear that minor myelin components, such as myelin oligodendrocyte glycoprotein (MOG) may play a primary role in disease initiation. This review examines the current knowledge about the aetiology and pathogenesis of MS, and the important similarities with EAE. A better understanding of the molecular mechanisms of autoimmune pathology will provide the basis for more rational immunotherapies to treat MS.

Cytokine Production and Surface Marker Expression in Acute and Stable Multiple Sclerosis: Altered IL-12 Production and Augmented Signaling Lymphocytic Activation Molecule (SLAM)-Expressing Lymphocytes in Acute Multiple Sclerosis

Ag-stimulated IL-2 production and mitogen-stimulated type 1 and type 2 cytokine production by PBMC, as well as expression of Th1- and Th2-associated phenotypical markers, of B7-1, B7-2, and CD95 (Fas) on the surface of immune cells, and the serum concentration of soluble Apo-1/Fas were evaluated in multiple sclerosis (MS) patients with either acute (AMS) or stable (SMS) disease and in healthy controls (HC). Results showed that 1) Ag-stimulated IL-2 production is reduced in MS patients compared with that in HC; 2) mitogen-stimulated type 1 cytokine production is increased, and IL-10 production is reduced in MS patients compared with those in HC, and in AMS patients compared with those in SMS; 3) whereas production of the metabolically active p70 heterodimers is comparable in SMS, AMS, and HC, production of the p70 heterodimer and the p40 chains (total IL-12) is increased in SMS compared with that in AMS and HC; 4) CD4+, CD4+SLAM+, and CD4+CD7+ lymphocytes (preferentially type 1 cytokine-producing lymphocytes) are increased in MS compared with levels in HC; 5) B7-2- as well as Fas+-expressing monocytes are augmented in MS compared with those in HC, and serum soluble Apo-1/Fas is augmented in AMS compared with SMS and HC. These results confirm that a complex imbalance in both cytokine production and the Fas system is present in MS and indicate that different cytokine profiles may be observed in patients with acute or stable disease. The data also suggest that peculiar phenotypic populations are over-represented in MS patients, and for the first time show that SLAM expression is correlated with dysregulation of type 1 and type 2 cytokine production in human pathology.

Differential display cloning of a novel human histone deacetylase (HDAC3) cDNA from PHA-activated immune cells

The nucleosomal histones can be modified through reversible acetylation by histone acetyltransferases (HATs) and deacetylases (HDACs). HATs induce nucleosomal relaxation and allow DNA-binding by transcriptional activators. HDACs from corepressor complexes which negatively regulate cell growth. However, the HDAC inhibitors butyrate and Trichostatin A block T cell proliferation, suggesting that not all effects of HDACs lead to repression. Using mRNA differential display and 5'RACE we isolated human HDAC3, a novel gene that is upregulated in PHA-activated T cell clones. HDAC3 is homologous to other human HDACs and yeast RPD3. In peripheral blood mononuclear cells (PBMCs), activation by PHA, PMA and alpha-CD3 increased HDAC mRNA but no effect was seen with IFN-gamma, LPS, or IL-4. In contrast, GMCSF downregulated PBMC levels of HDAC3 mRNA. All HDACs were found to be ubiquitously expressed in immune and non-immune tissues. In human myeloid leukemia THP-1 cells, HDAC3 transfection resulted in increased size, aberrant nuclear morphology and cell cycle G2/M cell accumulation. Functional activity of the expressed HDAC3 protein was confirmed in alpha-HDAC3 antibody immunoprecipitates by a histone deacetylase assay. Our study suggests the participation of HDACs in cell cycle progression and activation.

Molecular and Genetic Requirements for Preferential Recruitment of TCRBV8S2+ T Cells in Lewis Rat Experimental Autoimmune Encephalomyelitis

The underlying mechanisms behind the preferential expression of select TCRBV products in certain autoimmune illnesses, such as multiple sclerosis and some models of experimental autoimmune encephalomyelitis (EAE), have principally remained enigmatic. In this study, we examined the mutual role of nonself- vs self-origin of antigenic myelin basic protein (MBP) peptides and given MHC haplotypes in relation to the relative frequency of activated TCRBV8S2+ T lymphocytes in the Lewis (LEW) rat EAE model. Inbred MHC (RT1) congenic LEW rats (LEW (RT1l), LEW.1AV1 (RT1av1), and LEW.1W (RT1u)) were immunized with the 63 to 88 peptide of the guinea pig MBP (MBPGP63-88). Additionally, LEW rats were immunized with the corresponding autologous rat sequence (MBPRAT63-88). Although EAE ensued in all MBP peptide/LEW rat strain combinations, only LEW rats immunized with the heterologous MBPGP63-88 peptide elicited T cell responses encompassing a bias toward TCRBV8S2 expression, as determined by flow cytometric analyses. Reduction of TCRBV8S2+ T cells led to mitigation of disease severity in LEW rats immunized with MBPGP63-88, but not with MBPRAT63-88, indicating that critical encephalitogenic characteristics are associated with this T cell subset. We conclude that the preferential recruitment of TCRBV8S2+ T cells in the LEW rat EAE model is due to selective, high-avidity recognition of the nonself-MBPGP63-88 in the context of the RT1.Bl molecule. This inference lends support to the notion that the highly restricted TCR repertoire of the self-MBP-reactive T cells in certain genetically predisposed multiple sclerosis patients may have its source in a multistep molecular mimicry event.

CD8+ Myelin Peptide-Specific T Cells Can Chemoattract CD4+ Myelin Peptide-Specific T Cells: Importance of IFN-Inducible Protein 10

The demyelination process that occurs in the central nervous system (CNS) of patients with multiple sclerosis (MS) is due, in part, to an inflammatory response in which CD4+ and CD8+ T cells and macrophages infiltrate white matter. While it is thought that the inflammatory and demyelination process in MS is the product of Th1-associated cytokines secreted by CD4+ myelin protein-specific T cells present in the CNS, the mechanisms that are responsible for the recruitment and maintenance of these myelin-reactive CD4+ T cells in the CNS have not been elucidated. We have shown previously that CD8+CTL that recognize peptides derived from sequences of the myelin proteolipid protein (PLP) presented by HLA class I molecules can be generated in vitro, and that these PLP-specific CD8+CTL secrete the proinflammatory chemokines macrophage-inflammatory protein-1α and -1β, IL-16, and IP-10. In this study, we demonstrate that soluble products of these PLP-specific CD8+CTL can chemoattract CD4+ T cells that are specific for a myelin basic protein peptide and a PLP peptide, and that the majority of this chemotactic activity is mediated by IFN-inducible protein 10. These results demonstrate that PLP-specific CD8+ T cells can play a role in the recruitment and retention of myelin-derived peptide-specific CD4+ T cells, and indicate that they may play a proinflammatory role in the pathogenesis of MS.

Altered peptide ligand modulation of experimental allergic encephalomyelitis: immune responses within the CNS

An altered peptide ligand (analog) of the encephalitogenic epitope of proteolipid protein residues 139-151 (p139-151) in which residues 144 and 147 are substituted with leucine and arginine, respectively (LR), protects from clinical but not histological experimental allergic encephalomyelitis (EAE). To understand in situ events associated with this protection, T cells from brains of mice immunized with either native p139-151, the analog LR or a combination of the two were isolated and characterized. High proportions of cells from co-immunized mice (38%) and LR-immunized mice (58%) reacted to both p139-151 and LR, whereas fewer cells from p139-151 immunized mice (7%) were cross-reactive. T cell clones derived from brains of LR- and co-immunized mice were also cross-reactive in vitro. By reverse transcriptase-based polymerase chain reaction, higher levels of TGF-beta mRNA, and lower levels of TNF-alpha and IFN-gamma mRNA were found in the central nervous system (CNS) tissue of LR and co-immunized mice. Immunohistochemistry demonstrated greater TGF-beta immunoreactivity in CNS inflammatory foci in co-immunized and LR-immunized mice. There were no significant differences in CD4+ or CD8+ cell infiltrates among the groups and differences in other cytokines were not identified by immunocytochemistry. Protection from clinical EAE in LR and co-immunized mice was partially abolished by anti-TGF-beta antibody treatment. Thus, protection from clinical disease following immunization with the analog LR is associated with infiltration into the CNS of a T cell population that could potentially recognize the native PLP peptide and with enhanced TGF-beta production by cells within CNS inflammatory foci.

A restricted T cell response to myelin basic protein (MBP) is stable in multiple sclerosis (MS) patients

The close resemblance of MS to the animal model experimental autoimmune encephalomyelitis (EAE) has provided compelling data sustaining a pathogenic role of circulating T cells reactive against MBP. T cell antigen receptor (TCR) usage in EAE is commonly considered restricted; nevertheless, dynamic changes of TCR usage correlate with the course of EAE, resulting in a limited repertoire during early stages of disease activity followed by the recruitment of other T cells reactive against new determinants. Although a broader TCR repertoire mediates the response to MBP in humans, a restricted intraindividual heterogeneity may occur in some MS patients. In the present study we characterize the response to MBP in MS subjects with relapsing remitting disease from two sampling time points 12 months apart. MBP-specific T cell lines (TCL) were first generated from eight MS individuals and two healthy subjects. New TCL were obtained after 12 months from one control and three MS patients whose response, at the first time point, was directed against a single epitope. Interestingly, these three subjects had a stable and mild disease. Few TCL obtained at two time points from the MS individuals recognized the same immunodominant epitope and shared identical TCR Vbeta sequences. In the control we could not detect a restriction of the repertoire. These findings suggest that in some MS patients with benign disease a predominant T cell response to a single determinant may be detectable at different moments and is mediated by clonally expanded populations.

Cell biology of autoimmune diseases

Autoimmune diseases such as insulin-dependent diabetes mellitus, rheumatoid arthritis, and multiple sclerosis are common in the western world and are often devastating diseases which pose serious health problems. The key feature of such diseases is the development and persistence of inflammatory processes in the apparent absence of pathogens, leading to chronic breakdown of selected tissues. To date, no comprehensive explanation can be given for the onset or persistence of autoimmunity. As a rule, the chronic activation of helper T lymphocytes reactive against self proteins appears to be crucial for fueling the destructive autoimmune process, but why this occurs remains to be established. In this review, we present an overview on the rules that govern activation of T lymphocytes and on the factors that control it. The contribution of both genetic and environmental factors are discussed, clarifying that most autoimmune disease are of multifactorial origin. Special emphasis is given to the contribution of infectious events and the role of stress proteins in the process. In attempts to dissect the mechanisms involved in autoimmunity and to develop ways of blocking disease, experimental animal models are widely employed. We describe the various experimental models that exist for the study of multiple sclerosis, diabetes, and other autoimmune diseases and on the experience that has been gained in such models with experimental therapies to block the activation of self-reactive T lymphocytes. The lessons that can be drawn from these studies provide hope that continued efforts will lead to the successful development of antigen-specific strategies which block the development of autoimmunity also in humans.

Molecular basis for the binding promiscuity of an anti-p24 (HIV-1) monoclonal antibody

Multiple binding capabilities utilized by specific protein-to-protein interactions in molecular recognition events are being documented increasingly but remain poorly understood at the molecular level. We identified five unrelated peptides that compete with each other for binding to the paratope region of the monoclonal anti-p24 (HIV-1) antibody CB4-1 by using a synthetic positional scanning combinatorial library XXXX[B1,B2,B3,X1,X2,X3]XXXX (14 mers; 68,590 peptide mixtures in total) prepared by spot synthesis. Complete sets of substitution analogs of the five peptides revealed key interacting residues, information that led to the construction of binding supertopes derived from each peptide. These supertope sequences were identified in hundreds of heterologous proteins, and those proteins that could be obtained were shown to bind CB4-1. Implications of these findings for immune escape mechanisms and autoimmunity are discussed.

Activation of autoreactive T cells by peptides from human pathogens

Activation of autoreactive T cells is a necessary-but not sufficient-step in the development of T cell mediated autoimmunity. Autoreactive T cells can be activated by viral and bacterial peptides that meet the structural requirements for MHC molecule binding and T cell receptor recognition. Due to the degenerate nature of MHC class II molecule binding motifs and a certain degree of flexibility in T cell receptor recognition, such microbial peptides have been found to be quite distinct in their primary sequence from the self-peptide they mimic.