Activation thresholds determine susceptibility to peptide-induced tolerance in a heterogeneous myelin-reactive T cell repertoire

Combining antigen and immunomodulators: Emerging trends in antigen-specific immunotherapy for autoimmunity

A majority of current therapies for autoimmune diseases are general immunosuppressants, which can compromise patient response to opportunistic infection and lead to adverse events. Using antigen-specific immunotherapy (ASIT) to selectively disarm autoimmune diseases, without suppressing the global immune response, would be a transformative therapy for patients. ASIT has been used historically in allergy hyposensitization therapy to induce tolerance to an allergen. Similar strategies to induce immune tolerance toward autoantigens responsible for autoimmune disease have been attempted but have yielded limited clinical success. Recent studies of ASIT for autoimmunity have explored combination therapy, combining the disease-causing autoantigen with an immunomodulatory compound. ASIT combination therapy may direct the immune response in an antigen-specific manner, potentially reversing the root cause of autoimmunity while limiting side effects. This review analyzes recent advances in ASIT applied to autoimmune diseases, emphasizing current combination therapies and future strategies.

Control of T cell antigen reactivity via programmed TCR downregulation

The T cell receptor (TCR) is unique in that its affinity for ligand is unknown prior to encounter and can vary by orders of magnitude. How the immune system regulates individual T cells that display highly different reactivity to antigen remains unclear. Here we identified that activated CD4⁺ T cells, at the peak of clonal expansion, persistently downregulate TCR expression in proportion to the strength of initial antigen recognition. This programmed response increases the threshold for cytokine production and recall proliferation in a clone-specific manner, ultimately excluding clones with the highest antigen reactivities. Thus, programmed TCR downregulation represents a negative feedback mechanism to constrain T cell effector function with a suitable time delay, thereby allowing pathogen control while avoiding excess inflammatory damage.

Taming the TCR: antigen-specific immunotherapeutic agents for autoimmune diseases

Current treatments for autoimmune diseases are typically non-specific anti-inflammatory agents that affect not only the autoreactive cells but also the parts of the immune system that are required to maintain health. There is a need for the development of antigen-specific therapeutic agents that can effectively prevent the autoimmune attack while leaving the rest of the immune system functioning as normal. The simplest way to achieve this is using the autoantigen itself as a tolerizing agent; however, there is some risk involved with administering a potentially pathogenic antigen. In this review, we focus instead on the development and use of modified T cell receptor (TCR) ligands, in which the peptide ligand is modified to change the response by the T cell from a disease inducing to a protective response, and still retain the antigen-specificity necessary to target the autoreactive T cells. We review the use of modified TCR ligands as therapeutic agents in animal models of autoimmunity and in human autoimmune disease, and finally consider how they need to be improved in order to use them effectively in patients with autoimmune disease.

Experimental autoimmune encephalomyelitis (EAE) as a model for multiple sclerosis (MS)

Experimental autoimmune encephalomyelitis (EAE) is the most commonly used experimental model for the human inflammatory demyelinating disease, multiple sclerosis (MS). EAE is a complex condition in which the interaction between a variety of immunopathological and neuropathological mechanisms leads to an approximation of the key pathological features of MS: inflammation, demyelination, axonal loss and gliosis. The counter-regulatory mechanisms of resolution of inflammation and remyelination also occur in EAE, which, therefore can also serve as a model for these processes. Moreover, EAE is often used as a model of cell-mediated organ-specific autoimmune conditions in general. EAE has a complex neuropharmacology, and many of the drugs that are in current or imminent use in MS have been developed, tested or validated on the basis of EAE studies. There is great heterogeneity in the susceptibility to the induction, the method of induction and the response to various immunological or neuropharmacological interventions, many of which are reviewed here. This makes EAE a very versatile system to use in translational neuro- and immunopharmacology, but the model needs to be tailored to the scientific question being asked. While creating difficulties and underscoring the inherent weaknesses of this model of MS in straightforward translation from EAE to the human disease, this variability also creates an opportunity to explore multiple facets of the immune and neural mechanisms of immune-mediated neuroinflammation and demyelination as well as intrinsic protective mechanisms. This allows the eventual development and preclinical testing of a wide range of potential therapeutic interventions.

Novel therapeutic strategies targeting the pathogenic T-cells in multiple sclerosis

Multiple sclerosis is a chronic disease in which immune cells incite inflammation in the central nervous system, ultimately resulting in the destruction of the myelin nerve sheath. Pathogenic CD4+ T-cells are believed to be responsible for initiating this process. Recent advances in molecular biology, such as transgenic and knockout animal models, genomics and proteomics, have allowed for a much greater understanding of the cellular and subcellular pathways involved in autoimmunity. The end result is an ever more specific array of potential therapeutic agents, each designed to target one component of the dysregulated immune system and in some cases, specific to each individual patient. The mechanisms, promises and pitfalls of these various strategies for the treatment of multiple sclerosis are the topic of this review.

Conditional superagonist CTL ligands for the promotion of tumor-specific CTL responses

Although it has been demonstrated that CTLs can be raised against tumor-associated self-antigens, achieving consistent and effective clinical responses has proven challenging. Superagonist altered peptide ligands (APLs) can often elicit potent antitumor CTL responses where the native tumor-associated epitope fails. Current methods have identified a limited number of superagonist APLs, including the prototypic 27L mutant of MART-1. However, more comprehensive screening strategies would be desirable. In this study, we use a novel genetic screen, involving recombinant technology and class I Ag cross-presentation, to search for supraoptimal superagonists of the 27L MART-1 mutant by surveying the effectiveness of virtually every single amino acid substitution mutant of 27L to activate human Ag-specific CTL clones recognizing the wild-type MART-1(26-35) epitope. We identify three novel mutant epitopes with superagonist properties that are functionally superior to 27L; however, the ability of a given analogue to act as superagonist varies among patients and suggests that a given superagonist APL may be ideally suited to different patients. These findings endorse the use of comprehensive methods to establish panels of potential superagonist APLs to individualize tumor peptide vaccines among patients.

Manipulating antigenic ligand strength to selectively target myelin-reactive CD4+ T cells in EAE

The development of antigen-specific therapies for the selective tolerization of autoreactive T cells remains the Holy Grail for the treatment of T-cell-mediated autoimmune diseases such as multiple sclerosis (MS) and its animal model experimental autoimmune encephalomyelitis (EAE). This quest remains elusive, however, as the numerous antigen-specific strategies targeting myelin-specific T cells over the years have failed to result in clinical success. In this review, we revisit the antigen-based therapies used in the treatment of myelin-specific CD4+ T cells in the context of the functional avidity and the strength of signal of the encephalitogenic CD4+ T cell repertoire. In light of differences in activation thresholds, we propose that autoreactive T cells are not all equal, and therefore tolerance induction strategies must incorporate ligand strength in order to be successful in treating EAE and ultimately the human disease MS.

Death, adaptation and regulation: The three pillars of immune tolerance restrict the risk of autoimmune disease caused by molecular mimicry

Extensive cross-reactivity in T cell receptor (TCR) recognition of peptide-MHC (pMHC) complexes seems to be essential to give sufficient immune surveillance against invading pathogens. This carries with it an inherent risk that T cells activated during a response to clear an infection can, perhaps years later, respond to a self pMHC of sufficient similarity. This lies at the heart of the molecular mimicry theory. Here we discuss our studies on the disease-causing potential of altered peptide ligands (APL) based on the sequence of a single autoantigenic epitope, the Ac1-9 peptide of myelin basic protein that induces experimental autoimmune encephalomyelitis in mice. These show that the window of similarity to self for induction of disease by cross-reactive non-self peptides is actually quite restricted. We show that each of the three pillars of immune tolerance (death, anergy/adaptation and regulation) has a role in limiting the risk of molecular mimicry by maintaining a threshold for harm.

Feedback regulation of murine autoimmunity via dominant anti-inflammatory effects of interferon gamma

There is a paucity of knowledge concerning the immunologic sequelae that culminate in overt autoimmunity. In the present study, we have analyzed the factors that lead to disease in the model of autoimmunity, murine experimental autoimmune encephalomyelitis (EAE). EAE in H-2(u) mice involves autoreactive CD4(+) T cells that are induced by immunization with the immunodominant N-terminal epitope of myelin basic protein. The affinity of this epitope for I-A(u) can be increased by substituting lysine at position 4 with tyrosine, and this can be used to increase the effective Ag dose. Paradoxically, high doses of Ag are poorly encephalitogenic. We have used quantitative analyses to study autoreactive CD4(+) T cell responses following immunization of mice with Ag doses that are at the extremes of encephalitogenicity. A dose of autoantigen that is poorly encephalitogenic results in T cell hyperresponsiveness, triggering an anti-inflammatory feedback loop in which IFN-gamma plays a pivotal role. Our studies define a regulatory mechanism that serves to limit overly robust T cell responses. This feedback regulation has broad relevance to understanding the factors that determine T cell responsiveness.

CD4+CD25+ regulatory T cells limit the risk of autoimmune disease arising from T cell receptor crossreactivity

The molecular-mimicry theory proposes that immune crossreactivity between microbial and self-antigen is the initiating event in the activation of autoaggressive immune responses leading to autoimmune disease. In support of this possibility, it is now accepted that T cell recognition of antigen is highly degenerate. However, it is to be expected that the immune system would have evolved mechanisms to counter such a potential danger. We studied the influence of CD4(+)CD25(+) regulatory T cells (Treg) on the ability of suboptimal T cell receptor ligands to provoke autoimmunity. By using CD4(+) T cell-driven experimental autoimmune encephalomyelitis as a model, it was found that depletion of CD4(+)CD25(+)Foxp3(+) Treg allowed pathology to develop in response to suboptimal T cell stimulation. These data demonstrate the importance of Treg in raising the threshold of triggering of autoreactive T cell responses, thus limiting the risk of autoimmune disease due to molecular mimicry.

Encephalitogenic and tolerogenic potential of altered peptide ligands of MOG and PLP in Biozzi ABH mice

Altered peptide ligands (APL) are highly effective in inhibiting experimental autoimmune encephalomyelitis (EAE) in rodents although clinical trials in multiple sclerosis reveal severe limitations probably due to the diverse and differential effects of APL in vivo compared to in vitro. Myelin oligodendrocyte glycoprotein (MOG 8-21) induces relapsing EAE in ABH (A(g7)) mice associated with broadening of the autoimmune repertoire thus providing a dynamic system to examine the efficacy of peptide analogues. Subtle changes in MOG 8-21 dramatically influenced disease susceptibility and T cell responses in vitro. Non-encephalitogenic APL that induce production of the 'regulatory' cytokines IL-10 and/or TGFbeta and concomitant low levels of the 'proinflammatory' cytokines IFNgamma and TNFalpha modulated relapsing EAE but were far less effective than the 'proinflammatory' wild-type MOG 8-21 peptide. These data reveal that APL differ greatly in their ability to activate encephalitogenic T cells. The extensive heterogeneity of responses of APL in vitro suggests that selection of APL on this criteria is highly unpredictable and probably less effective for therapy than selecting the dominant wild-type epitope and delivering it using a tolerogenic route.