Myeloperoxidase-antineutrophil Cytoplasmic Antibodies with Cytoplasmic Fluorescence Pattern

INTRODUCTION

Autoantibodies causing a C-ANCA (cytoplasmic) pattern are generally directed against proteinase-3 (PR-3) and can be specifically detected with the anti-PR-3 enzyme-linked immunosorbent assay (ELISA).^[1] In contrast, P-ANCA antibodies can be produced by a variety of different autoantibody specificities. Originally, it was suspected that myeloperoxidase (MPO) was the target antigen of P-ANCA, but later it became evident that a P-ANCA staining pattern can also be produced by other antibodies.^[2]

C-ANCA with MPO specificity accounts for fewer than 10% of all patients with Wegener's granulomatosis (WG).^[3] P-ANCA with PR3 specificity is even rarer. The two cases that we are presenting here demonstrate rare findings with the C-ANCA fluorescence pattern seen with ELISA-proven MPO-ANCA positivity.

CASE REPORT

DISCUSSION

MPO-ANCA-positive sera are known to produce a characteristic perinuclear pattern on IIF of ethanol fixed neutrophil preparations.^[2] In isolated cases only MPO-ANCA may be detected even in sera showing atypical ANCA and C-ANCA patterns.

False positive MPO-ANCA may occur which can be due to the presence of dsDNA antibodies resulting in binding via DNA to MPO. Both of our patients were ANA negative as well as dsDNA negative thereby excluding the possibility of cross reactivity. We have also shown that sera from these patients did not react at all with proteinase 3 in ELISA, thereby also ruling out the possibility of cross reactivity with PR3.

Segelmark et al. have previously reported that antibodies to certain epitopes on MPO produce a cytoplasmic pattern.^[4] They have clearly shown in their experiments that all myeloperoxidase do not relocate toward the nucleus after ethanol fixation (some MPO has to remain in the granules or in the cytoplasm) and that C-ANCA and P-ANCA epitopes exist simultaneously on the same MPO molecule. They also proposed that two immunofluorescence patterns arise due to different availabilities of the epitopes in the microenvironment where myeloperoxidase is present.^[4]

It has been previously shown that patients with MPO-ANCA have a tendency towards more frequent renal involvement than patients with PR3-ANCA, thereby substantiating a stronger renal pathogenic effect of MPO-ANCA compared to PR3-ANCA.^[5] Both the cases that we have presented here showed MPO-ANCA in ELISA with normal renal function tests. However, in the absence of a renal biopsy, we cannot exclude confidently the absence of renal pathology in these cases.

It further needs to be reinforced that the diagnosis of WG should be based on ACR criteria and not on ANCA positivity. In a country like India, where tuberculosis (TB) is endemic and also in view of potential phenotypic similarities of TB and WG, it is necessary that TB should be ruled out by confirming AFB negativity from sputum, body fluids or aspirates and biopsy material or even giving a trial of ATT to check for response to therapy. Though ANCA is an important serologic tool to assist in the diagnosis of WG and other small vessel ANCA-associated vasculitis (SVAAV), it should always be interpreted in conjunction with clinical features. Moreover, clinical definitions of WG, microscopic polyangiitis, and Churg-Strauss syndrome do not include the demonstration of ANCA or target antigen specificity. A number of previous reports confirm the presence of this autoantibody in known cases of tuberculosis and leprosy.^[6-8] A medical rarity known to exist is Wgener's tuberculosis described by Gordon et al.^[9]

The two cases that we have shown here present a rare phenomenon of MPO-ANCA presenting as a C-ANCA pattern on IIF. It is important to be aware of such a situation while reporting and assessing the clinical relevance of ANCA positivity.