The total IgG concentration decreased from a mean
The total IgG concentration decreased from a mean of 997 mg/100 ml before treatment to a mean of 696 after treatment in the MuSK+MG patients, and from a mean of 1736 mg/100 ml to a mean of 802 mg/100 ml in the AChR+MG patients (normal values 723–1685 mg/100 ml). This decrement was also statistically significant (Wilcoxon test, p=0.04). IgG4 values went from 37 mg/100 ml to 21 mg/100 ml in the MuSK+MG group and from 24 mg/100 ml to 9.6 mg/100 ml in the AChR+MG group (normal values 6–121 mg/100 ml), without statistical significance (Wilcoxon test, p=0.08).
Discussion In this series of six seropositive MG patients with severe symptoms who did not respond to first or second-line immunosuppressive drugs and who were treated with Rituximab, the final outcome was excellent regardless of whether they were AChR+MG or MuSK+MG patients. However, some differences were observed in terms of clinical response and decline of antibody titers; the improvements were significantly better and more persistent in the MuSK+MG group. The need for Rituximab in a higher percentage of MuSK+MG patients supports the notion that these patients are more resistant to conventional therapies than seropositive MG patients. Rituximab produced a radical change in the quality of life of the six patients who all had severe or life-threatening bulbar and respiratory symptoms. All patients were able to return to a normal life within 6 months of treatment. A similar clinical improvement has been reported in single cases of patients with AChR+MG or MuSK+MG (Baek et al., 2007, Gajra et al., 2004, Hain et al., 2006, Thakre et al., 2007, Wylam et al., 2003, Zaja et al., 2000). Interestingly, we were able to reduce the dose of immunosuppressive agents in the three MuSK+MG patients and none of them relapsed. However, in the AChR+MG group of patients, therapy could be tapered to a lesser degree because, although they clearly improved, significant bulbar clinical manifestations persisted. A clinical improvement after immune therapy is commonly associated with a decrease in the AChR-Ab and it YM 58483 has also been reported for MuSK-Ab when the values are analyzed individually (Bartoccioni et al., 2006, Lindstrom et al., 1976). Accordingly, in a unique case that was recently reported, we observed high titers of both antibodies simultaneously. Prednisone reduced the levels of AChR-Ab within the first months but did not have any effect on the MuSK-Ab titers. Treatment with Rituximab improved the patient's clinical symptoms and the MuSK-Ab decreased 81% while the AChR-Ab decreased 52% 2 months after infusion (Diaz-Manera et al., 2007). These percentages are very similar to those observed in the six patients with single reactivities. Serial testing for anti-MuSK-Ab shortly after the initiation of therapy with prednisone and other second-line immunosuppressive agents would perhaps allow early detection of poor responders to these drugs. The pathogenic effect of AchR-Ab has been demonstrated and there is emerging evidence for pathogenicity of MuSK-Ab (Buckley and Vincent, 2005). Although we do not know the reasons for this different effect on antibody titer, it could be related to the different etiological and pathological mechanisms implicated in the responses to MuSK and AChR. While IgG4 is the main IgG subclass produced in response to MuSK (“Th2-like”), IgG1 is the main IgG subclass produced in response to AChR (“Th1-like response”) (McConville et al., 2004). The therapeutic efficiency of Rituximab could be due not only to its ability to deplete CD20+ B cells but also to its ability to downregulate CD40 and CD80 costimulatory molecules on B cells, suggesting that disturbance of T-cell activation through these molecules could induce the generation of regulatory T cells that could help to control the autoimmune process (Tokunaga et al., 2005). Although none of the current treatments specifically target the autoimmune etiology of MG, therapeutic approaches using monoclonal antibodies such as Rituximab are a useful new tool to further treat patients with MG resistant to other established treatments, both AChR+MG and MuSK+MG (Sieb, 2005). If good tolerance and lack of side effects are confirmed in a greater number of patients in future studies, Rituximab could be used earlier in the treatment of severe, refractory MG, especially in the case of MuSK+MG patients.