Cathepsin S is required for murine autoimmune myasthenia gravis pathogenesis

Because presentation of acetylcholine receptor (AChR) peptides to T cells is critical to the development of myasthenia gravis, we examined the role of cathepsin S (Cat S) in experimental autoimmune myasthenia gravis (EAMG) induced by AChR immunization. Compared with wild type, Cat S null mice were markedly resistant to the development of EAMG, and showed reduced T and B cell responses to AChR. Cat S null mice immunized with immunodominant AChR peptides showed weak responses, indicating failed peptide presentation accounted for autoimmune resistance. A Cat S inhibitor suppressed in vitro IFN-gamma production by lymph node cells from AChR-immunized, DR3-bearing transgenic mice. Because Cat S null mice are not severely immunocompromised, Cat S inhibitors could be tested for their therapeutic potential in EAMG.     

Experimental myasthenia gravis — a new autoimmune model

Antibodies raised in experimental animals against fish acetylcholine receptor bind to the animal's own neuromuscular receptors and result in the development of a ‘myasthenia gravis’ type illness. Auto-antibodies to acetylcholine receptor have now been found in the serum of patients with myasthenia — but how relevant are they to the pathogenesis of the disease?     

C5 gene influences the development of murine myasthenia gravis

The influence of the C5 gene and C5 deficiency on murine experimental autoimmune myasthenia gravis (EAMG) susceptibility was evaluated. Two co-isogenic strains, B10.D2/nSn (C5 sufficient) and B10.D2/oSn (C5 deficient), which are genetically identical except for the C5 gene locus, were immunized with acetylcholine receptors (AChR) in CFA to induce myasthenia gravis. Both strains had equivalent concentration of serum autoantibodies to muscle AChR and antibodies bound to muscle AChR. C5-sufficient B10.D2/nSn, but not C5-deficient B10.D2/oSn, demonstrated increased incidence of clinical disease and death and lost significant amounts of muscle AChR. Therefore, C5 deficiency in B10.D2/oSn prevented EAMG. C5 gene, which codes for C component C5, may influence EAMG pathogenesis through activation of the terminal lytic C sequence (C5 to C9) required for muscle AChR destruction, which is the primary pathology.     

Split tolerance in a novel transgenic model of autoimmune myasthenia gravis

Because it is one of the few autoimmune disorders in which the target autoantigen has been definitively identified, myasthenia gravis (MG) provides a unique opportunity for testing basic concepts of immune tolerance. In most MG patients, Abs against the acetylcholine receptors (AChR) at the neuromuscular junction can be readily identified and have been directly shown to cause muscle weakness. T cells have also been implicated and appear to play a role in regulating the pathogenic B cells. A murine MG model, generated by immunizing mice with heterologous AChR from the electric fish Torpedo californica, has been used extensively. In these animals, Abs cross-react with murine AChR; however, the T cells do not. Thus, to study tolerance to AChR, a transgenic mouse model was generated in which the immunodominant Torpedo AChR (T-AChR) alpha subunit is expressed in appropriate tissues. Upon immunization, these mice showed greatly reduced T cell responses to T-AChR and the immunodominant alpha-chain peptide. Limiting dilution assays suggest the likely mechanism of tolerance is deletion or anergy. Despite this tolerance, immunization with intact T-AChR induced anti-AChR Abs, including Abs against the alpha subunit, and the incidence of MG-like symptoms was similar to that of wild-type animals. Furthermore, evidence suggests that this B cell response to the alpha-chain receives help from T cells directed against the other AChR polypeptides (beta, gamma, or delta). This model offers a novel opportunity to elucidate mechanisms of tolerance regulation to muscle AChR and to clarify the role of T cells in MG.     

Anti-CTLA-4 antibody treatment triggers determinant spreading and enhances murine myasthenia gravis

CTLA-4 appears to be a negative regulator of T cell activation and is implicated in T cell-mediated autoimmune diseases. Experimental autoimmune myasthenia gravis (EAMG), induced by immunization of C57BL/6 mice with acetylcholine receptor (AChR) in adjuvant, is an autoantibody-mediated disease model for human myasthenia gravis (MG). The production of anti-AChR Abs in MG and EAMG is T cell dependent. In the present study, we demonstrate that anti-CTLA-4 Ab treatment enhances T cell responses to AChR, increases anti-AChR Ab production, and provokes a rapid onset and severe EAMG. To address possible mechanisms underlying the enhanced autoreactive T cell responses after anti-CTLA-4 Ab treatment, mice were immunized with the immunodominant peptide alpha(146-162) representing an extracellular sequence of the ACHR: Anti-CTLA-4 Ab, but not control Ab, treatment subsequent to peptide immunization results in clinical EAMG with diversification of the autoantibody repertoire as well as enhanced T cell proliferation against not only the immunizing alpha(146-162) peptide, but also against other subdominant epitopes. Thus, treatment with anti-CTLA-4 Ab appears to induce determinant spreading, diversify the autoantibody repertoire, and enhance B cell-mediated autoimmune disease in this murine model of MG.     

Prospects for specific immunotherapy in myasthenia gravis

Myasthenia gravis is an autoimmune disease resulting from a breakdown in T and B cell tolerance to acetylcholine receptor (AChR). Autoantibodies to AChR mediate the disease. Recent advances in experimental immunotherapy of autoimmune disease provide several possibilities for specific intervention in this well-characterized condition.     

Deficient T-cell mitogen response in murine experimental autoimmune myasthenia gravis: A defect in the adherent cell population

T-Lymphocyte number and functions are often reduced, while B-lymphocyte function is often increased in patients with autoimmune disorders. To study the mechanisms responsible for these T-cell malfunctions in autoimmunity we adapted the murine experimental autoimmune myasthenia gravis (EAMG) model. Splenocytes from C57BL/6 mice immunized with acetylcholine receptors (AChR) in complete Freund's adjuvant (CFA) produced approximately half the amount of concanavalin A (Con A)-induced interleukin 2 (IL-2) as did splenocytes of CFA-inoculated controls. Further, AChR plus CFA-immunized splenocytes showed a marked reduction in T-cell proliferative responses induced by Con A or phytohemagglutinin when compared with CFA-inoculated controls. By contrast, lipopolysaccharide-induced B-cell function is preserved. Deficient Con A splenic T-cell response is seen early after secondary inoculation with CFA or AChR in CFA. T-Cell recovery occurs in CFA-inoculated mice but not in AChR plus CFA-inoculated mice. Defective Con A splenic T-cell response seen early after secondary immunization with CFA or AChR in CFA is due to the presence of a defective splenic adherent cell population. Moreover, defective Con A splenic T-cell response seen after established autoimmunity to AChR in EAMG is also due to the presence of a defective splenic adherent cell population.