Multiple sclerosis (MS) is a T cell-mediated autoimmune demyelinating disease, which may be initiated by a virus infection. Theiler's murine encephalomyelitis virus (TMEV), a natural mouse pathogen, is a picornavirus that induces a chronic, CD4+ T cell-mediated demyelinating disease with a clinical course and histopathology similar to that of chronic progressive MS (ref. 3). Demyelination in TMEV-infected mice is initiated by a mononuclear inflammatory response mediated by virus-specific CD4+ T cells targeting virus, which chronically persists in the CNS (ref. 4-6). We show that beginning 3-4 weeks after disease onset, T-cell responses to multiple myelin autoepitopes arise in an ordered progression and may play a pathologic role in chronic disease. Kinetic and functional studies show that T-cell responses to the immunodominant myelin proteolipid protein epitope (PLP139-151) did not arise because of cross-reactivity between TMEV and self epitopes (that is, molecular mimicry), but because of de novo priming of self-reactive T cells to sequestered autoantigens released secondary to virus-specific T cell-mediated demyelination (that is, epitope spreading). Epitope spreading is an important alternate mechanism to explain the etiology of virus-induced organ-specific autoimmune diseases.

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Epitope spreading is a process whereby epitopes distinct from and non-cross-reactive with an inducing epitope become major targets of an ongoing immune response. This phenomenon has been defined in experimental and natural situations as a consequence of acute or persistent infection and secondary to chronic tissue destruction that occurs during progressive autoimmune disease. We have investigated the functional significance of this process in the chronic stages of both autoimmune and virus-induced central nervous system (CNS) demyelinating disease models in the SJL/J mouse. During the relapsing-remitting course of experimental autoimmune encephalomyelitis (R-EAE) induced with defined encephalitogenic myelin peptides, CD4+ T cells specific for endogenous epitopes on both the initiating myelin protein (intramolecular epitope spreading) and distinct myelin proteins (intermolecular epitope spreading) are primed secondary to myelin destruction during acute disease and play a major functional role in mediating disease relapses. Similarly, epitope spreading to endogenous myelin epitopes appears to play a major functional role in the chronic-progressive course of Theiler's murine encephalomyelitis virus-induced demyelinating disease (TMEV-IDD), a virus-induced CD4+ T-cell-mediated immunopathology. In TMEV-IDD, myelin destruction is initiated by virus-specific CD4+ T cells which target virus epitopes persisting in CNS-derived antigen-presenting cells. However, the chronic stage of this progressive disease is associated with the activation of CD4+ T cells specific for multiple myelin epitopes. In both models, the temporal course of T-cell activation occurs in a hierarchical order of epitope dominance, spreading first to the most immunodominant epitope and progressing to lesser immunodominant epitopes. In addition, epitope spreading in R-EAE is regulated predominantly by CD28/B7-1 co-stimulatory interactions, as antagonism of B7-1-mediated co-stimulation using anti-B7-1 F(ab) fragments is an effective ameliorative therapy for ongoing disease. The process of epitope spreading has obvious important implications for the design of antigen-specific therapies for the treatment of autoimmune disease since these therapies will have to identify and target endogenous self epitopes associated with chronic tissue destruction.

Theiler’s murine encephalomyelitis virus (TMEV)-induced demyelinating disease is a chronic-progressive, immune-mediated CNS demyelinating disease and a relevant model of multiple sclerosis. Myelin destruction is initiated by TMEV-specific CD4+ T cells targeting persistently infected CNS-resident APCs leading to activation of myelin epitope-specific CD4+ T cells via epitope spreading. We examined the temporal development of virus- and myelin-specific T cell responses and acquisition of virus and myelin epitopes by CNS-resident APCs during the chronic disease course. CD4+ T cell responses to virus epitopes arise within 1 wk after infection and persist over a >300-day period. In contrast, myelin-specific T cell responses are first apparent ∼50–60 days postinfection, appear in an ordered progression associated with their relative encephalitogenic dominance, and also persist. Consistent with disease initiation by virus-specific CD4+ T cells, CNS mononuclear cells from TMEV-infected SJL mice endogenously process and present virus epitopes throughout the disease course, while myelin epitopes are presented only after initiation of myelin damage (>50–60 days postinfection). Activated F4/80+ APCs expressing high levels of MHC class II and B7 costimulatory molecules and ingested myelin debris chronically accumulate in the CNS. These results suggest a process of autoimmune induction in which virus-specific T cell-mediated bystander myelin destruction leads to the recruitment and activation of infiltrating and CNS-resident APCs that process and present endogenous myelin epitopes to autoreactive T cells in a hierarchical order.

Myasthenia gravis (MG) is a T cellregulated, antibody-mediated autoimmune disease. Two peptides representing sequences of the human acetylcholine receptor ␣-subunit, p195-212 and p259-271, were previously shown to stimulate peripheral blood lymphocytes of patients with MG and were found to be immunodominant T cell epitopes in SJL and BALB͞c mice, respectively. Single amino acid substituted analogs of p195-212 (analog Ala-207) and p259-271 (analog Lys-262) were synthesized. We showed that analogs Ala-207 and Lys-262 inhibited, in vitro and in vivo, the proliferative responses of T cell lines specific to the relevant peptide and lymph node cells of mice immunized to p195-212 and p259-271, respectively. To inhibit T cell responses to both peptides (p195-212 and p259-271), we synthesized dual analogs composed of the tandemly arranged two single (Ala-207 and Lys-262) analogs (dual analog) either sequentially (Ala-207-Lys-262) or reciprocally (Lys-262-Ala-207). In the present study, we report that both dual analogs could bind to major histocompatibility complex class II molecules on antigen-presenting cells of SJL and BALB͞c mice. Analog Lys-262-Ala-207, which bound more efficiently to major histocompatibility complex class II molecules, was found to inhibit the proliferative responses of both p195-212-and p259-271-specific T cell lines. Furthermore, the analog inhibited the in vivo priming of lymph node cells of both SJL and BALB͞c mice when administered i.v., i.p., or per os. The dual analog Lys-262-Ala-207 could also immunomodulate myasthenogenic manifestations in mice with experimental autoimmune MG induced by inoculation of a pathogenic T cell line. Thus, a single peptide that is composed of analogs to two epitope specificities can be used to regulate T cell responses and disease associated with each epitope.Myasthenia gravis (MG) is an autoimmune disease caused by antibody-mediated autoimmune responses to the nicotinic acetylcholine receptor (1, 2). Although autoantibodies play a pivotal role in the development of MG, T cells were shown to be important in the pathogenesis of this disease (3-6). Two peptides representing sequences of the human acetylcholine receptor ␣-subunit, namely p195-212 and p259-271, were previously shown by our laboratory to significantly stimulate peripheral blood lymphocytes of patients with MG and to serve as immunodominant T cell epitopes of SJL and BALB͞c mice, respectively (7,8). T cell lines specific to peptides p195-212 and p259-271 (TCSJL195-212 and TCBALB͞c259-271) were established in our laboratory from lymph node (LN) cells of immunized SJL and BALB͞c mice, respectively, and were found to induce experimental autoimmune MG (EAMG) following inoculation into naive syngeneic mice (9).The current treatment of MG is nonspecific (reviewed in ref. 1). It has been proposed that T cell responses may be inhibited by peptides that bind to major histocompatibility complex (MHC) class II restriction elements but that do not activate specific T cells. Such inhibitory peptides may be u...