The binding of proteinase 3 antineutrophil cytoplasmic antibodies (PR3-ANCA) varies in different ELISAs

Wegener's granulomatosis with orbital involvement: case report and literature review

Objective: To describe the chronology and the extent of orbital involvement in a case of granulomatosis with polyangiitis.

Methods: Descriptive case report and literature review.

Results: A 45-year-old patient, formerly diagnosed with granulomatosis with polyangiitis due to otorhinolaryngologic manifestations, pulmonary lesions, renal impairment, left knee arthritis and high blood levels of antineutrophil cytoplasmic antibodies, addressed the Ophthalmology Department in November 2020, having the following complaints: left eye mild retro-orbital discomfort, proptosis and epiphora.

On examination, Snellen’s best corrected visual acuity was 6/ 6 in both eyes. The anterior segment of the left eye displayed significant changes: proptosis, upper lid swelling, ptosis, slightly decreased ocular motility, mild conjunctival hyperemia and chemosis, thinning of sclera in the upper quadrant and mild posterior subcapsular cataract. Left eye funduscopy revealed a slightly elevated optic disc, with indistinct margins in the nasal quadrant. Spectral-domain optical coherence tomography (OCT) of the optic nerve confirmed the clinical findings, illustrating an increase in the retinal nerve fiber layers thickness in the nasal quadrant, with no corresponding visual field defect. The orbit magnetic resonance imaging (MRI) unveiled an intraconal mass surrounding the optic nerve on its entire intra-orbital path, confirming the diagnosis of left orbital granuloma.

Conclusion: Considering the relapsing disease and the orbital involvement, the patient is currently a candidate for rituximab, a monoclonal antibody against CD20.

Abbreviations: AAV = ANCA associated vasculitides, ANCA = antineutrophil cytoplasmic antibody, AOM = acute otitis media, BCVA = best corrected visual acuity, CRP = C-reactive protein, CT = computerized tomography, EGPA = eosinophilic granulomatosis with polyangiitis, ENT = otorhinolaryngology/ ear-nose-throat, ESR = erythrocyte sedimentation rate, GPA = granulomatosis with polyangiitis, LE = left eye, MPA = microscopic polyangiitis, MRI = magnetic resonance imaging, OCT = optical coherence tomography, RE = right eye, RNFL = retinal nerve fiber layers, TNF = tumor necrosis factor, WG = Wegener’s granulomatosis

PR3-ANCA: a promising biomarker in primary sclerosing cholangitis (PSC)

Background and Aims

The only recognized biomarker for primary sclerosing cholangitis (PSC) is atypical anti-neutrophil cytoplasmic antibodies (aANCA), which, in addition to having low sensitivity and specificity, is an indirect immunofluorescence (IIF) test lacking the advantages of high throughput and objectivity. Recent reports have shown that antibodies to proteinase-3 (PR3-ANCA) might add diagnostic value in inflammatory bowel disease (IBD), specifically in ulcerative colitis (UC). As PSC is associated with IBD, the objective of this study was to evaluate the frequency and clinical significance of PR3-ANCA in a large cohort of patients.

Methods

A total of 244 PSC and 254 control [autoimmune hepatitis (AIH), primary biliary cirrhosis (PBC), hepatitis C viral infection (HCV), hepatitis B viral infection (HBV), and healthy controls] sera and their clinical correlations were retrospectively analyzed for PR3-ANCA determined by ELISA and a new chemiluminescence immunoassay (CIA). Testing was also performed for aANCA by IIF.

Results

When measured by CIA, PR3-ANCA was detected in 38.5% (94/244) of PSC patients compared to 10.6% (27/254) controls (p<0.0001). By ELISA, PR3-ANCA was detected in 23.4% (57/244) of PSC patients compared to 2.7% (6/254) controls (p<0.0001). PR3-ANCA in PSC patients was not associated with the presence or type of underlying IBD, and, in fact, it was more frequent in Crohn's disease (CD) patients with PSC than previously reported in CD alone. PR3-ANCA in PSC measured by CIA correlated with higher liver enzymes.

Conclusion

PR3-ANCA is detected in a significant proportion of PSC patients compared to other liver diseases including PBC and AIH. PR3-ANCA is associated with higher liver enzyme levels in PSC, and is not solely related to underlying IBD.

Antineutrophil cytoplasmic antibody (ANCA) testing of routine sera varies in different laboratories but concordance is greater for cytoplasmic fluorescence (C-ANCA) and myeloperoxidase specificity (MPO-ANCA)

Most laboratories screen for antineutrophil cytoplasmic antibodies (ANCA) with indirect immunofluorescence (IIF) and confirm cytoplasmic (C-ANCA) and perinuclear (P-ANCA) staining with ELISAs for proteinase 3 (PR3) and myeloperoxidase (MPO) specificities. This study determined the concordance of ANCA test results from 48 diagnostic laboratories participating in a national Quality Assurance programme, that used different assays and methods and varied in expertise. Laboratories were circulated with a questionnaire about their techniques, and provided with 24 sera for testing over a 30 month period. Results for individual sera were compared with the 'observed consensus' found in more than 50% of laboratories. The 23 laboratories (48%) that responded to the questionnaire used 5 different IIF substrates and 11 ELISAs, and differed in other aspects of testing. Concordance for ANCA test results was greater for IIF-positive (n=22, median 96%, range 68%-100%) than an IIF-negative serum (median 64%); for C-ANCA (n=8, median 89%, range 66-100%) rather than P-ANCA (n=10, median 76%, range 52-88%); for MPO-ANCA (n=5, median 100%) rather than PR3-ANCA (n=7, median 89%, range 82-100%); and for strongly-positive (n=2, median 97%, range 96-97%) rather than low positive PR3-ANCA (n=4, median 80%, range 74-86%). Concordance for test results might be improved with further standardisation of testing methodologies.

Antineutrophil cytoplasmic antibody measurement: advantages and disadvantages of a capture PR3 ELISA and a direct PR3 ELISA

AIM

To compare the performance of a capture proteinase 3 enzyme linked immunosorbent assay (PR3 ELISA) with a direct PR3 ELISA in the measurement of PR3 antineutrophil cytoplasmic antibodies (ANCA).

METHOD

The performance of both assays systems was compared using two sets of sera. Sera from patients (n = 49) suffering from Wegener's granulomatosis (WG) and fulfilling the American College of Rheumatology classification criteria (or a modification of those criteria that allowed for ANCA positivity) were used along with sera from a group of patients (n = 48) considered to have a clinically false positive PR3 ANCA result when measured by routine direct ELISA.

RESULTS

Using the assay specific cut-offs, the direct ELISA gave a positive result in 92% on repeat testing and the capture ELISA a positive result in 84% of sera from patients with WG. The capture ELISA was negative in 75% of patients considered to have a false positive PR3 ANCA on initial testing by direct ELISA (27% were negative on repeat testing by direct ELISA). The mean concentration of PR3 ANCA in WG patient sera measured by the capture ELISA was significantly higher than that measured by the direct ELISA. The capture PR3 ELISA had a broader analytical range which was also reflected in PR3 ANCA concentrations measured in serial serum samples from WG patients.

CONCLUSION

In this study the direct PR3 ELISA performed better as a screening test for PR3 ANCA compared with the capture PR3 ELISA, mainly because the cut-off for the capture ELISA needed to be set higher to avoid non-specific binding. In contrast, the improved analytical range of the capture ELISA made it a potentially more useful method for monitoring serial PR3 ANCA concentrations. In specific serum samples the capture ELISA was better able to discriminate 'false positive' PR3 ANCA.

Agreement of anti-neutrophil cytoplasmic antibody measurements obtained from serum and plasma

Serum and plasma are used interchangeably to measure anti-neutrophil cytoplasmic antibodies (ANCA), even though the release of ANCA target antigens during the preparation of serum could affect ANCA assays and cause discrepancies between the results obtained from serum and plasma. To what extent ANCA test results obtained from serum agree and correlate with results from plasma remains unknown. Therefore, a comprehensive comparison was performed using serum and plasma samples which were collected in 175 patients with active Wegener's granulomatosis at enrollment of a recent randomized trial. These paired serum and plasma samples were subjected to parallel ANCA testing by standard indirect immunofluorescence on ethanol-fixed neutrophils, a direct enzyme-linked immunoassay (ELISA) for proteinase 3 (PR3)-ANCA and myeloperoxidase (MPO)-ANCA, and two different capture ELISAs for PR3-ANCA. The concordance of categorical serum and plasma ANCA results was assessed using kappa-coefficients. These were > 0.8 for all assays, indicating a very good concordance between positive and negative serum and plasma results. Spearman's correlation coefficients for serum and plasma PR3-ANCA values obtained by direct ELISA and both capture ELISAs were > or = 0.95 (P < 0.0001). Our study shows that serum and plasma samples can be used interchangeably for measuring ANCA.

A novel capture-ELISA for detection of anti-neutrophil cytoplasmic antibodies (ANCA) based on c-myc peptide recognition in carboxy-terminally tagged recombinant neutrophil serine proteases

Testing for antineutrophil cytoplasmic antibodies (ANCA) reacting with proteinase 3 (PR3) is part of the routine diagnostic evaluation of patients with small vessel vasculitis. For PR3-ANCA detection, capture ELISAs are reported to be superior to direct ELISAs. Standard capture ELISAs, in which PR3 is anchored by anti-PR3 monoclonal antibodies (moAB), have two potential disadvantages. First, the capturing moAB may compete for epitopes recognized by some PR3-ANCA, causing occasional false-negative results. Second, the capture of recombinant PR3 mutant molecules becomes unpredictable as modifications of specific conformational epitopes may not only affect the binding of PR3-ANCA, but also the affinity of the capturing anti-PR3 moAB. Here, we describe a new capture ELISA, and its application for PR3-ANCA detection. This new assay is based on the standardized capture of a variety of different carboxy-terminally c-myc tagged recombinant ANCA target antigens using anti-c-myc coated ELISA plates. Antigen used include c-myc tagged human rPR3 variants (mature and pro-form conformations), mouse mature rPR3 and human recombinant neutrophil elastase. This new anti-c-myc-capture ELISA for PR3-ANCA detection has an intra- and inter-assay coefficient of variation of 3.6% to 7.7%, and 15.8% to 18.4%, respectively. The analytical sensitivity and specificity for PR3-ANCA positive serum samples were 93% and 100%, respectively when rPR3 with mature conformation was used as target antigen, and 83% and 100% when the pro-enzyme conformation was employed. In conclusion, this new anti-c-myc capture ELISA compares favorably to our standard capture ELISA for PR3-ANCA detection, enables the unified capture of different ANCA target antigens through binding to a c-myc tag, and allows capture of rPR3 mutants necessary for PR3-ANCA epitope mapping studies.