Introduction Myasthenia gravis MG is an autoimmune disorder

Introduction Myasthenia gravis (MG) is an autoimmune disorder characterized by muscle weakness, which is mainly due to autoantibodies reducing the number or function of postsynaptic JWH 015 receptors (AChRs) [1], [2]. Animal models have been developed to uncover the immunopathogenesis of MG. Experimental autoimmune myasthenia gravis (EAMG) is induced by immunization of mice or rats with AChR in complete Freund’s adjuvant and mimics clinical and pathological features of MG [3], [4]. The activation of AChR-specific B cells by CD4+ T helper (Th) cells culminates in production of complement-fixing anti-AChR IgG isotypes ultimately leading to membrane attack complex (MAC) formation and impaired neuromuscular junction (NMJ) transmission [3], [4], [5], [6], [7], [8]. Activation of AChR-reactive B cells by T helper cells requires the interaction of MHC class II and CD4 molecules. MHC class II gene-disrupted or CD4 knockout (KO) mice fail to generate anti-AChR antibodies and do not develop clinical EAMG when immunized with the whole native AChR pentamer [4], [8].
Extraocular muscle involvement in EAMG Ocular symptoms are observed almost in every MG patient while extraocular muscle weakness is not a significant characteristic feature of EAMG [1], [2], [9]. Moreover, around 10–20% of MG patients display isolated extraocular muscle weakness and never progress to generalized muscle weakness [1], [2], [9]. Additionally, the antibody positivity of purely ocular MG (40–70%) is lower than generalized MG (85–90%) and AChR antibody positive ocular MG patients are more likely to develop generalized MG [9]. Notably, wild-type (WT) C57BL/6 (B6) and C57BL/10 (B10) mice and HLA-transgenic mice with the B6 or B10 background immunized with an E. coli plasmid expressing the recombinant human AChR α-, γ- or ε-subunit (unfolded form containing non-conformational linear epitopes) develop ptosis suggesting the involvement of levator palpebrae muscle. By contrast, this muscle is spared in the EAMG model induced by immunization with the whole native AChR pentamer [10], [11], [12], [13]. Similar to human MG, ocular muscle weakness precedes generalized muscle weakness in human AChR-subunit immunized mice. Also in resemblance to human MG patients, 20–30% of human AChR-subunit immunized mice never develop generalized muscle weakness and remain with only ocular symptoms [10], [11], [12], [13]. By contrast, both HLA-transgenic and WT mice immunized with the native AChR (made up of 5 subunits and carrying conformational epitopes) only exhibit generalized muscle weakness and very exceptionally develop extraocular muscle weakness [14]. Muscle samples of mice immunized with AChR subunits show NMJ complement and antibody deposits just like mice immunized with the whole AChR molecule. Generation of ocular symptoms by human AChR γ-subunit immunization might be explained by presence of this subunit in extraocular muscles. However, absence of the γ-subunit in the upper eyelid elevator levator palpebrae muscle [15], which is most profoundly affected in the ocular EAMG model, contradicts this assumption. Also, although α and ε subunits of the AChR are expressed by all striated muscles, mice immunized with recombinant human AChR α or ε subunits might display isolated ptosis without accompanying generalized muscle weakness [10], [11], [12]. Therefore distribution of weakness (ocular vs. generalized) in MG cannot be explained by expression pattern differences.
Lymphocyte subsets involved in ocular EAMG MHC class-II deficient mice in the B6 background develop extraocular muscle weakness, when immunized with the recombinant human AChR α-subunit [10], indicating that ocular symptoms might be induced by T cell independent activation of B-cells. On the other hand, EAMG-resistant CD4 KO mice display AChR-binding B cells in peripheral blood following AChR immunization. Although levels of these cells are higher in AChR-immunized CD4 KO mice than AChR-immunized WT mice, CD4 KO mice show very low serum AChR antibody levels [16] suggesting that AChR specific B cells of AChR-immunized CD4 KO mice are not deleted but inhibited from differentiating into antibody secreting plasma cells. Alternatively, plasma cells of CD4 KO mice might have short life spans making accumulation of high levels of pathogenic AChR antibodies unlikely. Nevertheless, EAMG experiments performed with MHC class-II and CD4 KO mice corroborate that AChR antibodies can form by T cell independent mechanisms and by activation of distinct B cell differentiation pathways. These antibodies might induce ocular symptoms but they are evidently not capable of generating severe generalized weakness. T cell dependent mechanisms are required for production of elevated levels of high affinity AChR antibodies, which have a robust MG and EAMG generating action.