Background
Eosinophilic granulomatosis with polyangiitis (EGPA) is a rare immunological condition characterized by allergic granulomatosis and small- and medium-vessel necrotizing vasculitis associated with peripheral blood eosinophilia and tissue infiltration by eosinophils [
1,
2]. Eosinophils are the most important effector cells and contribute to the disease mechanism [
2,
3]. Interleukin (IL)-5, which regulates eosinophil proliferation, maturation, and differentiation, is produced by T-helper (Th)2 cells and type 2 innate lymphoid cells (ILC2s) [
4,
5]. Serum IL-5 or IL-25 levels are more highly correlated with disease activity in patients with active EGPA than in those with asthma [
6,
7], and in these patients they are significantly higher than in healthy controls [
8]. We reported previously that the serum IL-33 concentration and peripheral blood ILC2 count increase in patients with active EGPA at diagnosis and relapse [
9].
The mainstay of treatment for EGPA is systemic corticosteroid therapy; some patients receive additional treatment with other immunosuppressive agents, such as cyclophosphamide, azathioprine, methotrexate, interferon-α [
10,
11], rituximab [
12‐
14], intravenous immunoglobulins (IVIGs) [
15‐
17], and other biologics [
18,
19]. Biologics include omalizumab [
20,
21], mepolizumab [
21‐
28], benralizumab [
29] dupilumab [
30], reslizumab, and lebrikizumab; the latter three are being tested in ongoing clinical trials [
31].
Mepolizumab, an anti-IL-5 monoclonal antibody, reduces blood eosinophil counts, and its efficacy in severe asthma has been established in large-scale trials [
26,
27]. In 2010, mepolizumab administered by 750-mg intravenous infusion every 4 weeks was shown to reduce the peripheral eosinophil count effectively and to be safe and well tolerated in patients with EGPA [
22]. In addition to decreasing the peripheral eosinophil count, mepolizumab improves the signs of vasculitis, such as asthma exacerbation, sinusitis, arthralgia, multiple polyneuropathy, gastrointestinal tract involvement, and skin involvement [
28,
32], and its use reduces the daily systemic corticosteroid dose in patients with refractory or relapsed EGPA [
23,
24]. In 2017, Mepolizumab In Relapsing or Refractory EGPA (MIRRA)—the first randomized, double-blind, placebo-controlled trial of a subcutaneous 300-mg monthly dose of mepolizumab for patients with relapsing or refractory EGPA—led to a significant increase in remission duration in the proportion of patients achieving remission. Moreover, remission occurred in 53% of participants in the mepolizumab group vs. 19% in the placebo group, and at the end of the study the average daily prednisolone (PSL) dose had been reduced to only 4 mg in 44% of participants in the mepolizumab treatment group [
25]. This was the first demonstration of the efficacy of mepolizumab in maintaining remission in EGPA.
Details of the specific effects of mepolizumab on vasculitis symptoms were not fully analyzed in the MIRRA study [
25]. However, in a later study, we found that, among EGPA patients treated with mepolizumab, there are super-responders displaying marked effects and responders displaying weak effects. We found that mepolizumab reduced vasculitis symptoms in many organs, and that the efficient response to mepolizumab in super-responders was correlated with high peripheral blood eosinophil counts at diagnosis and high serum IgG levels before mepolizumab administration [
28].
Since 1994, prognosis and mortality rates for EGPA have improved as a result of better diagnosis in accordance with American College of Rheumatology criteria [
33] and improvements in treatment over the last 20 years. EGPA patients diagnosed after 1996 have a better prognosis than those diagnosed before 1996 [
34]. However, patients with Five Factor Score 2009 criteria of age ≥ 65 years, severe cardiac involvement, severe gastrointestinal tract involvement, severe renal insufficiency, and status without sinusitis ≥ 2 still have a poor prognosis [
35]. Additionally, the 5−, 10−, and 20-year survival rates reported in 2011 by Guillevin et al. were about 90%, 75%, and 45%, respectively [
35], whereas those reported in 2013 by Moosig et al. were 97%, 89%, and 72%, respectively [
36]; we reported in 2017 that these survival rates were 91.1%, 83.7%, and 68.6%, respectively [
15]. EGPA is thus a disease with a poor prognosis in the long term despite the recent improvements.
Long-term biologic treatment of bronchial asthma has been reported only for omalizumab, which has now been administered for as long as 10 years [
37]. Other biologics used in asthma, such as mepolizumab [
38], reslizumab [
38,
39], and benralizumab [
38,
40], have been administered for about 2 years. In contrast, mepolizumab is the only biologic available in Japan for EGPA, and until now there have been no reports of long-term analysis or follow-up.
The clinical efficacy of mepolizumab in EGPA and the disease prognosis with long-term treatment with this drug are unknown. Here, we selected EGPA patients who had received mepolizumab for at least 3 years, and we analyzed the differences in background factors between super-responders and responders. We report the frequency of relapse per year in patients in each group and discuss whether treatment of EGPA with mepolizumab affects the prognosis of the disease.
Discussion
IL-5 is a critical cytokine for the growth, maturation, and differentiation of eosinophils, making it an attractive target in EGPA treatment [
50]. Mepolizumab, a humanized monoclonal antibody targeting IL-5, was developed in the late 1990s [
51], and the results of its first clinical trial for asthma were published in 2000 [
52]. MIRRA was the first randomized controlled trial of mepolizumab for treating refractory or relapsing EGPA [
25]. Other published studies support the use of mepolizumab for the induction and maintenance of remission in refractory, relapsing, or glucocorticoid-dependent EGPA [
25,
53‐
56]. However, it has not been conclusively determined whether mepolizumab treatment is effective against various vasculitis signs in EGPA or in ANCA-positive cases [
57].
We administered mepolizumab to EGPA patients in whom conventional therapy had failed to induce remission. We showed that there were super-responders and responders with respect to the efficacy of mepolizumab; super-responders were characterized by high peripheral blood eosinophil counts at diagnosis and high serum IgG levels before mepolizumab administration [
28]. We also showed that mepolizumab effectively treated vasculitis symptoms in many organs [
28].
Here, we confirmed that all patients for whom conventional therapy could not induce remission received some clinical benefit after being subsequently treated with mepolizumab; among them were several patients with cardiac involvement (Table
1). The period required for the effects of clinical improvement with mepolizumab to become apparent varied, with a mean and standard deviation of 3.4 ± 3.0 months, and about 34.9% of patients experienced improvement on mepolizumab within 1 month. In 8/43 (18.6%) patients it took at least half a year for the improvement to become apparent (Fig.
2). We therefore consider that it is necessary to confirm any improvements after at least 1 year of administration, because a substantial period of time may be required for mepolizumab to take effect.
In one study, a group with a peripheral blood eosinophil count of ≥ 150 cells/μL before administration of mepolizumab achieved remissions lasting 24 weeks, compared with a group with a peripheral blood eosinophil count of < 150 cells/μL [
58]. In that study, and in our previous study [
28], the peripheral blood eosinophil count decreased between the time of diagnosis and the time of initiation of mepolizumab, and it had decreased further in both of our groups by the time of last examination. We considered that the peripheral blood eosinophil count after the start of treatment was affected by the amounts of steroids and immunosuppressants administered up to that point. The treatment-naïve eosinophil count is a predictor of mepolizumab efficacy [
28], and this is supported by the significantly higher blood eosinophil count in the super-responders at diagnosis (see Table
2). We confirmed the expectation that the mechanism underlying the clinical effects of mepolizumab leads to a decrease in the eosinophil count.
Our analysis also showed that the super-responders had a significantly higher peripheral blood eosinophil count at the time of diagnosis and a significantly higher serum IgG value before mepolizumab than the responders. We reported previously that EGPA patients with repeated relapses have activated CD86
+ B cells and a decreased number of CD19
+ B cells. As a result, serum IgG levels in these patients decrease and are not correlated with the concurrent PSL dose [
59]. Our finding that serum IgG levels were maintained in the super-responders might reflect these patients’ ability to maintain their immune function; the effects of therapeutic drugs such as mepolizumab might thus be expressed better than in the responder group with low serum IgG.
Rates of manifestation of vasculitis signs, such as arthralgia or myalgia, were higher at diagnosis in the responders than in the super-responders (Table
2); we consider that these results reflect the small sample size of the analysis. In both groups, the BVAS at diagnosis was significantly lower both before and after mepolizumab administration than at diagnosis (Fig.
5). However, the BVAS values both before starting mepolizumab and at the time of the last visit were significantly lower in super-responders than in responders (Table
2). Most patients in the responder group had received immunosuppressants before administration of mepolizumab, and the PSL dose at their last visit on mepolizumab was significantly higher than that in the super-responder group.
There have been reports that mepolizumab is effective for vascular symptoms as manifested by asthma, arthralgia, and multiple polyneuropathy in EGPA patients [
21,
25,
53‐
56,
60], but there have been no reports regarding its effect on long-term prognosis. In the super-responder group, the relapse rate was significantly lower in the third year and at the last visit (mean and standard deviation, 4.3 ± 1.0 years after the start of mepolizumab) than in the first year after the start of mepolizumab administration (Fig.
6). These results indicate that mepolizumab administration might reduce the relapse rate in the long term and thus improve the prognosis.
On the other hand, in the responder group, the BVAS decreased after the start of mepolizumab, but a reduction in the PSL daily dose was difficult to achieve in the long term, and the relapse rate after prolonged mepolizumab administration did not decrease. Thus, in responders considered to have severe and intractable disease, it is difficult to reduce the daily dose of steroids or other immunosuppressive agents; even if mepolizumab has some effect on vascular symptoms, it is unable to induce long-term remission.
We reported previously that the prognosis of EGPA could be improved by IVIG [
15] via an increase in either the number of CD4
+CD25
+ T cells producing IL-10 [
61] or the number of FOXP3
+ regulatory T cells [
62]. IVIG was administered to 22 of our 25 (88.0%) patients treated with mepolizumab (see Table
2), and it might induce remission by increasing the number of FOXP3 + regulatory T cells [
61‐
63]. IL-33 and ILC2 are involved in the pathogenesis of asthma [
64] and EGPA [
9,
65]. We previously reported that, in EGPA, levels of ILC2 and IL-33 were high during onset and relapse and low during remission [
9]. In addition, we reported that, in super-responders, the interval between IVIG treatments after mepolizumab initiation was significantly longer than that before mepolizumab [
28].
We have also reported that the number of eosinophils in the colonic mucosa is significantly correlated with the number of Th17 cells in blood (CD4
+ T cells producing IL-17) [
48]. Increases in serum IL-17 and IL-22 levels are related to granuloma formation in sarcoidosis [
66]. EGPA combines features of both hypereosinophilic disorders (Th2 activity) and ANCA-associated vasculitis (Th1 activity) [
8]. In granulomatosis with polyangiitis, Th17 lymphocytes are a possible pathogenetic subset, and Treg cells are possible suppressors of the inflammatory process [
67]. On the basis of these and our current results, we conclude that the synergistic effect of multiple drugs, including IVIG, mepolizumab, steroids, and other immunosuppressants, may have durably stabilized immune function and thus durably decreased the relapse rate in the super-responder group.
Biologics may trigger the production of antidrug antibodies against the drug, although one study did not detect any neutralizing antibodies [
68]. There are cases of patients with asthma in whom the efficacy of mepolizumab has been attenuated over time [
69]. However, long-term (> 10 years) administration of omalizumab has been reported in asthma [
37] and there have been no reports of tolerance. Although omalizumab is the asthma biologic that has been on the market for the longest time, 'it is not clear when, or whether, its use should be discontinued in individual patients [
70].
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