Discussion
Previous studies found that mepolizumab decreased symptoms of
CRSwNP.15-17 However, those studies did not
investigate mepolizumab-induced changes to eosinophilic inflammation in
target sinonasal tissue. There is also negligible data about the
response of local tissue to mepolizumab in other eosinophilic
diseases.12, 13
A key finding in the current study was a reduction in tissue eosinophils
with mepolizumab. This study found a reduction in tissue eosinophils of
59% versus a reduction in blood eosinophils of 83%. This finding is
consistent with the only other study – an asthma trial – to have
assessed the drug’s effect on airway tissue
eosinophils.12 The magnitude of the
mepolizumab-mediated decrease in airway eosinophil numbers in the
present study (59%) was similar to the reduction (55%) in the asthma
study.12 The paucity of tissue eosinophils at Week 24
may be largely attributable to reduced eosinophil production in bone
marrow and reduced trafficking of the cell to tissue, which is
demonstrated by reduced blood eosinophil levels. IL-5 promotes
eosinophilopoiesis in bone marrow,26, 27 and
introducing an anti-IL-5 agent curtails bone marrow production of the
cell.26 Eosinophils in tissue may live up to several
weeks,28-30 compared to the lifespan of an eosinophil
in circulation, which is approximately 24 h.31 Given
the study duration is sufficiently long to account for the difference in
half-life between tissue eosinophils and blood eosinophils, factors
other than half-life presumably account for fewer circulating
eosinophils and a relatively higher number in tissue. Resident tissue
eosinophils, with an immunosuppressive and regulatory role in airway
tissue, may account for the cells persisting in tissue, despite very low
circulating eosinophils.32 However, it remains unclear
whether the remaining eosinophils found in sinonasal tissue of treated
patients are inflammatory or resident tissue eosinophils.
The finding of a reduced tissue eosinophil count with mepolizumab is
clinically salient because high tissue eosinophil counts are strongly
associated with polyp recurrence.33
A study component was the investigation of eosinophil degranulation in
view of its purported role in eCRS pathophysiology.34,
35 Activation and degranulation of eosinophils involves the
extra-cellular release of pre-formed granules, including cytotoxic
proteins (major basic protein, EPX and eosinophil cationic protein),
which mediate sinonasal pathology through epithelial damage and
fibrosis.35-37 In contrast to cationic protein and
major basic protein, EPX is an eosinophil-specific granule
protein.36, 38, 39 Interestingly, the current study
found no change in EPX staining following mepolizumab. In an asthma
study, sputum EPX concentration also did not change in responders to
mepolizumab, but was elevated if symptoms worsened
post-treatment.40
This study was the first to investigate mepolizumab’s effect on cytokine
concentrations in any disease-specific tissue. In line with this study’s
finding of increased IL-5 in tissue, increased IL-5 after mepolizumab
has been found in bronchiolar lavage fluid12, 13 and
plasma of asthma patients.41-43 Sputum IL-5 was also
elevated in asthma patients who showed inadequate response to
mepolizumab.40 IL-5 signalling impacts on key stages
of the eosinophil lifecycle, including mobilisation from bone marrow,
migration in circulation, and trafficking to and survival in tissue,
which generates tissue eosinophilia.30, 44-47 It is
possible that the assay in the current study measures IL-5 bound to
mepolizumab. A future direction would involve evaluating the
concentration of IL-5 bound to mepolizumab when a commercially
anti-mepolizumab antibody becomes available. Elevated levels of IL-5 may
also suggest inadequate neutralisation of this key eosinophilic
cytokine, and may indicate treatment underdosing, which has been raised
as a concern in a previous report.48 A recent
study40 found elevated IL-5 immunoglobulin-G
immune-complex deposition in the sputa of asthma patients who worsened
on mepolizumab therapy – the autoimmune-mediated pathology may have
sustained the eosinophilic inflammation and prevented symptom
resolution.40
There are several possible explanations for the increase in IL-5 in
sinonasal tissue in the current study. First, the functional absence of
IL-5 (bound to mepolizumab) and/or reduction in eosinophils may trigger
a feedback loop, stimulating local tissue cells known to release IL-5,
such as ILC2 cells, Th2 cells, and mast cells.49, 50 A
similar feedback loop may also occur systemically, given reports of
increased plasma IL-5 with mepolizumab in other eosinophilic
diseases.41-43, 51 Second, a decreased rate of
elimination of IL-5 from tissue, when bound in a mepolizumab-IL-5
complex compared with free ligand, may be
responsible.52-54
The study found a significant increase in the tissue concentration of
other type-2 cytokines IL-4, IL-13 and GM-CSF with mepolizumab. Similar
to the possible mechanism for IL-5 upregulation after mepolizumab, the
decrease in airway eosinophils may have stimulated local production of
other type-2 cytokines in a feedback pathway. Increased type-2 cytokines
after mepolizumab, including IL-4, IL-13 and GM-CSF, may provide another
explanation for the lack of complete removal of blood and tissue
eosinophils.12, 13, 55, 56 Cytokines other than IL-5
have an ability, albeit weaker than IL-5, to stimulate bone marrow and
local tissue eosinophilopoiesis.12, 26, 45, 57-59 For
the first time, the present study provides evidence that, after
mepolizumab administration, increased type-2 cytokines in tissue may be
partly responsible for continued bone marrow eosinophilopoiesis, via
distant signalling, and local tissue eosinophilopoiesis. The study also
found increased IL-10 in sinonasal tissue. Evidence suggests that IL-10
acts as an anti-inflammatory cytokine through its inhibition of release
of cytokines such as GM-CSF and IL-5, thereby reducing eosinophilic
inflammation.60
The upregulation of type-2 cytokine concentrations, particularly the
large increase in IL-5 concentration, raises concerns that ceasing
mepolizumab might exacerbate type-2 inflammation. In a clinical trial of
another anti-IL-5 drug (reslizumab), there was a rebound increase in
eosinophil levels following treatment
discontinuation.61 In patients treated with an
anti-IL-4 and anti-IL-13 drug (dupilumab), a transient increase in blood
eosinophils was not associated with disease
worsening.62 Therefore, while the reduced level of
eosinophils found in the current study may be clinically beneficial,
this reduction is not the complete answer to disease control. There has
been no rebound effect described for mepolizumab in the 12-month period
following its stoppage.63 Nevertheless, monitoring of
patients after mepolizumab cessation in further drug trials and clinical
practice is advisable in light of the potential risk of recurrent
disease.
There was no change in non-type-2 cytokines IL-1β and TNF-α after
mepolizumab therapy. TNF-a and IL-B are proinflammatory cytokines that
enhance inflammation.64 IL-4 has been reported to
suppress IL-1B and TNF-a production.65, 66 Elevated
levels of IL-4 and IL-10 may have contributed to the lack of change in
IL-1β and TNF-α.
Therefore, it is suggested that in patients with inflammatory airway
disease, mepolizumab’s antagonism of IL-5-mediated eosinophilopoiesis
and eosinophil trafficking to tissue is responsible for reduced
sinonasal eosinophils. The novel finding of upregulation of type-2
cytokines, particularly IL-5, in sinonasal tissue indicates that
mechanistic factors, such as a possible mepolizumab-induced feedback
loop, may explain incomplete response to therapy, highlighting the need
for more data to improve current treatments.