br Thymic expression of AChR Both linear

Thymic expression of AChR Both linear unfolded epitopes of AChR subunits and the intact AChR are avidly expressed in thymus, particularly by thymic epithelial cells and myoid cells [17], [18], [19]. AChR-antibody positive MG patients often show thymic hyperplasia, characterized by lymphoid follicles with germinal centers in the thymic medulla. These centers exhibit infiltrating B cells, dendritic cells and macrophages and show an abundance of complement deposits and complement regulators indicating the active ongoing inflammation [20]. Moreover, thymic lymphocytes of MG patients produce AChR Cy7 maleimide (non-sulfonated) synthesis [21] and immunization of experimental animals with thymic myoid cells induces AChR antibody production without generating generalized muscle weakness [22]. These studies have implicated that thymus, and particularly thymic myoid cells, might possibly trigger autoimmunity against AChR.
A testable hypothesis Completion of T cell independent B cell activation requires additional signals such as activation of toll-like receptors or B cell receptors by microorganisms such as Epstein-Barr virus that is found in high prevalence in myasthenic thymic tissue or any other non-specific pathogen coincidentally found in the MG patient [23]. Possibly, at this initial stage, only low levels of AChR antibodies with a low affinity are produced by the immune system. Extraocular muscles have an increased susceptibility to AChR-antibody induced and complement mediated NMJ dysfunction since they have lower endplate safety factor, higher energy requirement due to continuous activity and reduced expression of complement inhibitors [24]. As a result, these initial low level and low affinity AChR antibodies might generate ocular symptoms. They might also bind AChR expressed by myoid cells, activate the complement system and initiate formation of germinal centers. The resulting antigen-antibody complexes and recruitment of professional antigen presenting cells increase the exposure of native AChR molecules. As the immune system gains access to native AChR molecules originating from destroyed tissues during this initial attack, helper T cells are primed. It is well known that by epitope spreading mechanism, immunization with short linear epitopes of a large molecule might end up with a full scale autoimmune reaction against the entire native molecule [25]. By this way, primed T helper cells might facilitate production of B cells producing high affinity antibodies against a broad range of AChR epitopes thus initiating generalized muscle weakness, attacking myoid cells in thymus and enhancing thymic germinal center formation. This two-step process has already been proposed for thymic tissue alterations in MG [26], however the impact of this process on distribution of weakness (ocular vs. generalized) has not been emphasized. To test this hypothesis, an initial step could be measuring antibody levels against recombinant human AChR subunits and the native human AChR pentamer at different stages of purely ocular and generalized MG. Antibodies directed against conformational epitopes can ideally be demonstrated by human embryonal kidney (HEK) cells expressing AChR molecules and this method has already been established [27]. A second approach could be measuring lymphocyte proliferation and intracellular cytokine production levels in response to recombinant human AChR subunit and native human AChR pentamer stimulation in purely ocular and generalized MG patients at earlier and later stages of the clinical course. Accurate identification of the chain of events leading to extraocular and limb muscle weakness is a crucial step in developing specific treatment methods that could act by interrupting disease processes at an earlier stage before generalized muscle weakness ensues.
Introduction Myasthenia gravis (MG) is a T-cell dependent antibody-mediated autoimmune disease characterized by muscle weakness, which develops as a result of impaired neuromuscular transmission [1], [2], [3], [4]. MG is a chronic affliction lasting many years [5], [6]. Typically young females and older people are affected. In most patients, MG is caused by pathogenic autoantibodies to muscle nicotinic acetylcholine receptors (AChRs), while in a minority of patients, it is caused by autoantibodies against other muscle endplate proteins, such as muscle-specific kinase (MuSK) and lipoprotein-related protein 4 (LRP4) [7], [8]. What initiates the autoimmune response to AChR is unknown [9]. Evidence suggests that a complex interaction between multiple genotypes of low penetrance and several, largely unidentified, environmental factors contributes to the pathogenesis of MG [10]. The pathogenic role of antibodies to AChR has been clearly established by the development of EAMG when injected into laboratory animals, as well as by the transient clinical improvement of patients following plasmapheresis [11], [12], [13], [14]. More than half of the autoantibodies in MG are directed at the main immunogenic region (MIR) on AChR α1 subunits [15]. Autoantibodies to AChR in MG impair neuromuscular transmission primarily by two mechanisms. First, the autoantibodies cause focal complement-mediated lysis of the postsynaptic membrane, which destroys AChRs and disrupts synaptic morphology [16], [17]. Second, cross-linking AChRs by the autoantibodies on the surface of postsynaptic membrane results in an increase of endocytosis and lysosomal destruction of AChRs (this is termed antigenic modulation) [18]. Some autoantibodies inhibit AChR function directly, but these are usually a small fraction of the total [19].