Muscle-specific kinase (MuSK) myasthenia gravis (MG) is a neuromuscular autoimmune disorder caused by predominantly IgG4 antibodies targeting the MuSK protein. IgG4 has the unique ability to... Show moreMuscle-specific kinase (MuSK) myasthenia gravis (MG) is a neuromuscular autoimmune disorder caused by predominantly IgG4 antibodies targeting the MuSK protein. IgG4 has the unique ability to exchange half-molecules with other IgG4s, resulting in monovalent binding to their antigen. To investigate if MuSK-antibody valency influences its pathogenicity, recombinant bivalent and functionally monovalent MuSK antibodies were generated from B-cell receptor sequences isolated from MuSK MG patients. Passive transfer studies revealed that monovalency amplifies MuSK antibody pathogenicity in vivo. This may be because monovalent MuSK antibodies inhibit MuSK signaling (antagonist), while bivalent MuSK antibodies activate MuSK signaling (agonist) in vitro. The binding epitope on MuSK further influences the consequences and pathogenicity of MuSK antibodies. Collectively, these results suggest that the pathophysiology in individual patients depends on their unique antibody composition and that class-switching to IgG4 is a critical step in developing MuSK MG. Furthermore, the IgG4 response against MuSK does not appear to result from a global increase in IgG4 responses, as MuSK MG patients only had mildly elevated serum IgG4. Instead, it is thought to be driven by the antigen itself. Importantly, MUSK and other MG associated genes are also expressed outside skeletal muscle. These locations are at risk for non-motor symptoms caused by autoantibodies or mutations, or for side-effects of targeted therapeutic strategies. Show less
Muscle-specific kinase (MuSK) myasthenia gravis (MG) is a neuromuscular autoimmune disease belonging to a growing group of IgG4 autoimmune diseases (IgG4-AIDs), in which the majority of pathogenic... Show moreMuscle-specific kinase (MuSK) myasthenia gravis (MG) is a neuromuscular autoimmune disease belonging to a growing group of IgG4 autoimmune diseases (IgG4-AIDs), in which the majority of pathogenic autoantibodies are of the IgG4 subclass. The more prevalent form of MG with acetylcholine receptor (AChR) antibodies is caused by IgG1-3 autoantibodies. A dominant role for IgG4 in autoimmune disease is intriguing due to its antiinflammatory characteristics. It is unclear why MuSK autoantibodies are predominantly IgG4. We hypothesized that MuSK MG patients have a general predisposition to generate IgG4 responses, therefore resulting in high levels of circulating IgG4. To investigate this, we quantified serum Ig isotypes and IgG subclasses using nephelometric and turbidimetric assays in MuSK MG and AChR MG patients not under influence of immunosuppressive treatment. Absolute serum IgG1 was increased in both MuSK and AChR MG patients compared to healthy donors. In addition, only MuSK MG patients on average had significantly increased and enriched serum IgG4. Although more MuSK MG patients had elevated serum IgG4, for most the IgG4 serum levels fell within the normal range. Correlation analyses suggest MuSK-specific antibodies do not solely explain the variation in IgG4 levels. In conclusion, although serum IgG4 levels are slightly increased, the levels do not support ubiquitous IgG4 responses in MuSK MG patients as the underlying cause of dominant IgG4 MuSK antibodies. Show less
Myasthenia gravis (MG) is an acquired autoimmune disorder caused by autoantibodies binding acetylcholine receptors (AChR), muscle-specific kinase (MuSK), agrin or low-density lipoprotein receptor... Show moreMyasthenia gravis (MG) is an acquired autoimmune disorder caused by autoantibodies binding acetylcholine receptors (AChR), muscle-specific kinase (MuSK), agrin or low-density lipoprotein receptor-related protein 4 (Lrp4). These autoantibodies inhibit neuromuscular transmission by blocking the function of these proteins and thereby cause fluctuating skeletal muscle weakness. Several reports suggest that these autoantibodies might also affect the central nervous system (CNS) in MG patients. A comprehensive overview of the timing and localization of the expression of MG-related antigens in other organs is currently lacking. To investigate the spatio-temporal expression of MG-related genes outside skeletal muscle, we used in silico tools to assess public expression databases. Acetylcholine esterase, nicotinic AChR alpha 1 subunit, agrin, collagen Q, downstream of kinase-7, Lrp4, MuSK and rapsyn were included as MG-related genes because of their well-known involvement in either congenital or autoimmune MG. We investigated expression of MG-related genes in (1) all human tissues using GTEx data, (2) specific brain regions, (3) neurodevelopmental stages, and (4) cell types using datasets from the Allen Institute for Brain Sciences. MG-related genes show heterogenous spatio-temporal expression patterns in the human body as well as in the CNS. For each of these genes, several (new) tissues, brain areas and cortical cell types with (relatively) high expression were identified suggesting a potential role for these genes outside skeletal muscle. The possible presence of MG-related antigens outside skeletal muscle suggests that autoimmune MG, congenital MG or treatments targeting the same proteins may affect MG-related protein function in other organs. Show less
Human immunoglobulin (Ig) G4 usually displays antiinflammatory activity, and observations of IgG4 autoantibodies causing severe autoimmune disorders are therefore poorly understood. In blood, IgG4... Show moreHuman immunoglobulin (Ig) G4 usually displays antiinflammatory activity, and observations of IgG4 autoantibodies causing severe autoimmune disorders are therefore poorly understood. In blood, IgG4 naturally engages in a stochastic process termed "Fab-arm exchange" in which unrelated IgG4s exchange half-molecules continuously. The resulting IgG4 antibodies are composed of two different binding sites, thereby acquiring monovalent binding and inability to cross-link for each antigen recognized. Here, we demonstrate that this process amplifies autoantibody pathogenicity in a classic IgG4-mediated autoimmune disease: muscle-specific kinase (MuSK) myasthenia gravis. In mice, monovalent anti-MuSK IgG4s caused rapid and severemyasthenicmuscleweakness, whereas the same antibodies in their parental bivalent form were less potent or did not induce a phenotype. Mechanistically this could be explained by opposing effects onMuSK signaling. Isotype switching to IgG4 in an autoimmune response thereby may be a critical step in the development of disease. Our study establishes functional monovalency as a pathogenic mechanism in IgG4-mediated autoimmune disease and potentially other disorders. Show less
Muscle-specific kinase (MuSK) plays a critical role in establishing and maintaining neuromuscular synapses. Antibodies derived from immunizing animals with MuSK were important tools to help detect... Show moreMuscle-specific kinase (MuSK) plays a critical role in establishing and maintaining neuromuscular synapses. Antibodies derived from immunizing animals with MuSK were important tools to help detect MuSK and its activity. The role of antibodies in MuSK-related research got an extra dimension when autoantibodies to MuSK were found to cause myasthenia gravis (MG) in 2001. Active immunization with MuSK or passive transfer of polyclonal purified IgG(4) fractions from patients reproduced myasthenic muscle weakness in a range of animal models. Polyclonal patient-purified autoantibodies were furthermore found to block agrin-Lrp4-MuSK signaling, explaining the synaptic disassembly, failure of neuromuscular transmission and ultimately muscle fatigue observed in vivo. MuSK autoantibodies are predominantly of the IgG4 subclass. Low levels of other subclass MuSK antibodies coexist, but their role in the pathogenesis is unclear. Patient-derived monoclonal antibodies revealed that MuSK antibody subclass and valency alters their functional effects and possibly their pathogenicity. Interestingly, recombinant functional bivalent MuSK antibodies might even have therapeutic potential for a variety of neuromuscular disorders, due to their agonistic nature on the MuSK signaling cascade. Thus, MuSK antibodies have proven to be helpful tools to study neuromuscular junction physiology, contributed to our understanding of the pathophysiology of MuSK MG and might be used to treat neuromuscular disorders. The source of MuSK antibodies and consequently their (mixed) polyclonal or monoclonal nature were important confounding factors in these experiments. Here we review the variety of MuSK antibodies described thus far, the insights they have given us and their potential for the future. Show less
Muscle-specific kinase (MuSK) myasthenia gravis (MG) is a neuromuscular autoimmune disease that is hallmarked by fatigable muscle weakness of in particular the cranial and bulbar muscles. The... Show moreMuscle-specific kinase (MuSK) myasthenia gravis (MG) is a neuromuscular autoimmune disease that is hallmarked by fatigable muscle weakness of in particular the cranial and bulbar muscles. The disease is caused by auto-antibodies binding MuSK on the post-synaptic muscle membrane. MuSK orchestrates formation, differentiation and maintenance of neuromuscular synapses. The research outlined in this thesis aimed at identifying the pathomechanism of MuSK MG. In passive transfer studies with immune incompetent mice we showed that IgG4 subclass antibodies from these patients exclusively induced myasthenia. We furthermore showed that anti-MuSK IgG4 causes the disease by inhibiting MuSK-Lrp4 interaction which results in loss of acetylcholine receptor clustering and synaptic disassembly. Our retrospective longitudinal study in 53 patients from three different cohorts identified the N-terminal Ig-like domain of MuSK as the main immunogenic region of the protein and showed that titers against this domain correlate with disease severity. Epitope patterns did not correlate with sensitivity to acetylcholine esterase inhibitors. Lastly we observed that, next to MuSK MG, 12 other antibody-mediated autoimmune diseases are hallmarked by predominant involvement of IgG4 antibodies. As IgG4 antibody-mediated autoimmune diseases differ in several aspects from other autoimmune disease they might form a new niche in antibody-mediated autoimmune diseases Show less
Myasthenia gravis (MG) is hallmarked by acquired and fluctuating weakness of voluntary muscles. In the majority of patients, weakness is caused by autoantibodies to the postsynaptic acetylcholine... Show moreMyasthenia gravis (MG) is hallmarked by acquired and fluctuating weakness of voluntary muscles. In the majority of patients, weakness is caused by autoantibodies to the postsynaptic acetylcholine receptor (AChR) in the neuromuscular junction. Approximately 10% of MG patients are seronegative (SNMG). In 2001, antibodies to muscle-specific kinase (MuSK) were discovered within this group. This dissertation describes the epidemiology, clinical characteristics and immunological aspects of this new disease. In the Netherlands, MuSK MG is rare. Incidence was 0.17 per million person-years. Prevalence was 2.8 per million on January 1st 2004. Weakness was more often bulbar, and lead to frequent respiratory crises. MuSK MG was linked to the HLA-DR14-DQ5 haplotype and the disease severity was associated to antigen-specific IgG4 antibodies instead of IgG1. This is in contrast to AChR MG in which autoantibodies are mainly of the IgG1 and IgG3 subclass, and the disease is linked to HLA-B8-DR3. In SNMG patients, no antibodies to postsynaptic ErbB receptors were found. A case-report describes the transmission of MuSK autoantibodies from mother to her newborn child, causing transient weakness in the infant. A second case-report illustrates the effect of MuSK antibodies on both pre- and postsynaptic signal transmission in the neuromuscular junction. Show less
