Comparative investigation of respiratory tract involvement in granulomatosis with polyangiitis between PR3-ANCA positive and MPO-ANCA positive cases: a retrospective cohort study

In the EULAR recommendation, AAV is defined as chronic inflammatory disease which
lasts for more than four weeks, where infection and malignant tumors are excluded,
and characteristic histological findings are observed in biopsy or an ANCA-positive
result is obtained 11]. AAV includes MPA, GPA, and EGPA; however, the proportion of these disorders differs
greatly between Europe/US and Japan 12]. The majority of the AAV patients in Europe and US have GPA, and 80 to 90 % of GPA
patients have PR3-ANCA 13]. In contrast, the prevalence of MPA has been reported to be much higher than GPA
in Japan. In addition, more than 80 % of Japanese AAV patients were MPO-ANCA positive
14]. These epidemiological differences became the background for the establishment of
EMA algorithm 4].

In the present study, 56 % of GPA patients were positive for PR3-ANCA, 38 % were positive
for MPO-ANCA, and the remaining 6 % were positive for both. According to the retrospective
studies regarding Japanese GPA patients diagnosed by EMA algorithm, the percentage
of PR3-ANCA positive patients was 39.5–58.3 % and MPO-ANCA was 33.3–54.6 % 15]–17]. High prevalence of MPO-ANCA positive GPA in Japanese was consistently noted, whereas
the proportion of MPO-ANCA positive patients was 2.6–13 % in Europe 18], 19]. These differences may be due, in part, to genetic factors. A genome-wide association
study of AAVs in European Caucasian patients reported that PR3-ANCA was associated
with HLA-DP and genes encoding ?1-antitrypsin (SERPINA1) and proteinase 3 (PRTN3),
whereas MPO-ANCA was associated with HLA-DQ 20]. Among these, the prevalence of HLA-DPB1*0401 allele was higher in patients with
PR3-ANCA associated vasculitis than in patients with MPO-ANCA associated vasculitis
or healthy controls. This allele is also less frequent in Japan, China and US African
Americans; these are three populations where PR3-ANCA vasculitis is less common than
in Europe 21]. In addition, Watts et al. reported that HLA-DPB1*0401 allele frequencies was associated
with GPA incidence, and may help explain variations in GPA incidence between populations
22].

With regard to the baseline characteristics, the MPO-ANCA positive group included
a greater number of females (67 %). Many studies of GPA showed a male predominance
or almost equal numbers of males and females 1], 18], 19], 23]. However, according to the retrospective multi-center study of Japanese patients
with GPA or MPA 16], 82 % (14/17) of MPO-ANCA positive GPA were female, a significantly greater population
than for PR3-ANCA. Another retrospective study of 24 Japanese patients with GPA also
showed that 87.5 % were female 15], consistent with the results of our study.

In the PR3-ANCA positive cases, the time from onset to first visit was significantly
shorter than the MPO-ANCA positive cases. On the other hand, although not statistically
significant, markers of inflammatory reaction tended to be higher in MPO-ANCA positive
cases. No meaningful difference was observed in hemoglobin or serum albumin, which
reflects the degree of exhaustion. In addition, there were no significant differences
in BVAS. Thus, it cannot be stated which group had higher disease activity and severity.

At the onset of GPA, respiratory tract involvement is usually the most prevalent sign.
Patients with GPA have either upper or lower respiratory tract involvement and majority
of patients have both 1], 18], 19], 23], 24], whereas in patients with MPA, upper respiratory tract involvement does not occur
and pulmonary involvement is usually manifested by alveolar hemorrhage. Characteristics
of respiratory tract involvement in MPO-ANCA positive GPA have not been fully clarified
until now. In the present study, high frequency of upper respiratory tract involvement
was also noted in MPO-ANCA positive cases (67 %) as well as PR3-ANCA positive cases
(89 %). As for BVAS at the time of diagnosis, neither total scores nor the scores
for every internal organ differ between PR3-ANCA positive cases and MPO-ANCA positive
cases. In a retrospective study of 24 Japanese cases of GPA, MPO-ANCA positive cases
had nose and sinus involvement less frequently compared to PR3-ANCA positive cases
15]. On the contrary, retrospective multi-center study of Japanese patients with GPA
or MPA showed that MPO-ANCA positive cases tended to have ear involvement more frequently,
reflected in the fact that otitis media was significantly higher than in PR3-ANCA
positive cases 16]. However, neither of the tendencies was observed in the present study, indicating
that further investigation is required.

With regard to chest CT imaging, no significant differences were observed in the findings
between PR3-ANCA positive cases and MPO-ANCA positive cases. Lohrmann et al. investigated
CT images for 57 cases of Wegener’s granulomatosis and reported that the most frequently
observed finding is nodular shadow at 89 %, followed by thickening of bronchial walls
at 56 % 25]. However, to the best of our knowledge, no report has presented the details of radiological
findings in MPO-ANCA positive GPA or compared them with those of PR3-ANCA positive
cases. Incidence of nodular shadows (78 % in PR3-ANCA positive cases and 100 % in
MPO-ANCA positive cases) and thickening of trachea or bronchial walls (56 % in PR3-ANCA
positive cases and 67 % in MPO-ANCA positive cases) reported in this study is close
to the results reported in the previous studies of GPA. There were no significant
difference in the median number of small/large nodules per person between PR3-ANCA
positive cases and MPO-ANCA positive cases.

On the other hands, less common findings including ground glass opacity, thickening
of interlobular septa, lymphadenopathy, and pleural effusion, were more frequently
observed in MPO-ANCA positive cases in the present study. In addition, bronchial wall
thickning from the main bronchi level to the segmental/sub-segmental bronchi level
tended to be more conspicuous in MPO-ANCA positive cases. Further investigation is
required on whether these differences were accidental occurrence due to the small
sample number.

One report noted that cavitation is observed in approximately 30 to 50 % of nodules
26]; however, in the present study the incidence of cavitation was found to be low. In
some cases, cavities were formed during the course of the treatment, suggesting that
a certain period of time is required for cavities to form in the nodular shadows.
It is also possible that as a result of using EMA algorithm, the disorder is discovered
at an early stage, before the cavities are formed.

With respect to biopsy sections and histological findings, the detection rate of diagnostic
findings, such as granuloma/granulomatous inflammation of an artery/perivascular area,
necrotizing vasculitis/glomerulonephritis, granulomatous inflammation of the respiratory
tract, was the highest for biopsy of the kidney (66 %), followed by the lung (40 %)
and nasal mucosa (29 %). Significant findings leading to the diagnosis of GPA are
rarely seen in specimens from the upper respiratory tract, as previously reported
27]–29]. For lung biopsy, the detection rate of diagnostic findings was 100 % for cases where
video-assisted thoracic surgery (VATS) was performed. TBLB or echo/CT-guided biopsy
resulted in a lower detection rate (20 %, 33 %, and 0 %, respectively). Meaningful
but not diagnostic findings such as vasculitis without necrosis were observed in small
specimens by TBLB and nasal mucosa. These results suggest that sufficient tissue is
necessary for successful pathological verification.

However, histopathological investigation cannot be conducted in some cases; thus,
establishment of useful surrogate marker is desired. In this study, the most common
surrogate markers defined in the EMA algorithm were fixed pulmonary infiltrates, nodules,
or cavitation present for 1 month (78 % in PR3-ANCA positive cases and 86 % in MPO-ANCA
positive cases, respectively), followed by bronchial stenosis (33 % and 50 %, respectively)
and chronic sinusitis, otitis media, or mastoiditis for 3 months (56 % and 67 %,
respectively) (Table 6). The detection rate of these markers in MPO-ANCA positive cases was almost the same
as that of PR3-ANCA positive cases. Thus, investigation of respiratory tract involvement
according to these surrogate markers will assist the diagnosis of GPA in the cases
without histological proof of granuloma/necrotizing vasculitis even in MPO-ANCA positive
cases.

Table 6. Details of the items applied to each patient according to the EMA algorithm

The prognosis of AAV markedly improved with the combination therapy of high-dose steroids
and cyclophosphamide 4]. In the present study, all the patients initially treated with prednisone monotherapy
or combination with prednisone and methotrexate relapsed during the treatment course.
On the other hand, the rate of relapse was only 20 % in patients initially treated
with prednisone and cyclophosphamide.

In addition, the recent randomized trial of rituximab showed that rituximab therapy
was superior to cyclophosphamide treatment in relapsing cases of AAV 30]. In the present study, rituximab was administered in two cases; one case was refractory
to the initial combination therapy with prednisone and cyclophosphamide, and the other
case was refractory to prednisone and cyclophosphamide started after relapse. Both
cases improved immediately by addition of rituximab, and no relapse has occurred as
of this writing. Moreover, rituximab was as effective as continuous conventional immunosuppressive
therapy in the patients with severe AAV 31]. In future, initial intensive treatment including rituximab may lead to favorable
prognosis.

In the present study, the rate of relapse in MPO-ANCA positive cases was lower than
that of PR3-ANCA positive cases (17 % and 56 %, respectively). Relapses are more common
in patients with GPA (25 to 80 % of patients) than in those with MPA, in whom relapse
has been reported in 8 % at 18 months 32]. According to the community-based cohort study of 350 patients with newly diagnosed
AAV, the positivity for PR3-ANCA and involvement of the lung and the upper respiratory
tract were associated with relapse 33]. Even among patients of GPA, MPO-ANCA positive cases may be less likely to relapse
than PR3-ANCA positive cases.

Limitations of the present study include the small number of patients investigated
and possible deviation of patient distribution. Another limitation is insufficient
histopathological investigation in MPO-ANCA positive cases. The short observation
period is also a problem when assessing long-term prognosis.