The use of small molecule high-throughput screening to identify inhibitors of the proteinase 3-NB1 interaction.

Anti-neutrophil cytoplasmic antibodies (ANCA) to proteinase 3 (PR3) are found in patients with small-vessel vasculitis. PR3-ANCA bind strongly to membrane PR3 (mPR3) that is presented by the NB1 receptor. We performed high-throughput screening using a small molecule library to identify compounds that inhibit PR3-NB1 binding. We established a human embryonic kidney (HEK293) cell-based system, where approximately 95 +/- 2% of the NB1-transfected cells expressed the NB1 receptor on the cell surface. Addition of 0.1 microg/ml human PR3 to 10(4) NB1-expressing HEK293 cells resulted in PR3 binding that was detected by immunofluorescence using a fluorescence plate reader assay. We identified 13 of 20 000 molecules that inhibited PR3 binding by >70%. Seven of 13 substances showed reproducible inhibition in four additional validation experiments. Two selected compounds (27519 and 27549) demonstrated a dose-dependent inhibition over a range from 6.25 to 100 microM as measured by the plate reader assay. We used flow cytometry as a second assay, and found that both compounds reproducibly inhibited PR3 binding to NB1-transfected HEK293 cells at 50 microM (inhibition to 42 +/- 4% with compound 27519 and to 47 +/- 6% with compound 27549 compared to the dimethylsulphoxide control). Furthermore, compounds 27519 and 27549 also inhibited binding of exogenous PR3 to human neutrophils. In contrast, the compounds did not decrease mPR3 expression on resting neutrophils, but reduced the tumour necrosis factor-alpha-mediated mPR3 increase on NB1(pos) neutrophils when present continuously during the assay. The findings suggest that small inhibitory compounds provide a potential therapeutic tool to reduce mPR3 by preventing its binding to NB1.

Neutrophil surface presentation of the anti-neutrophil cytoplasmic antibody-antigen proteinase 3 depends on N-terminal processing.

The neutrophil serine protease proteinase 3 (PR3) is a main autoantigen in anti-neutrophil cytoplasmic antibody-associated vasculitis. PR3 surface presentation on neutrophilic granulocytes, the main effector cells, is pathogenically important. PR3 is presented by the NB1 (CD177) glycoprotein, but how the presentation develops during neutrophil differentiation is not known. An N-terminally unprocessed PR3 (proPR3) is produced early during neutrophil development and promotes myeloid cell differentiation. We therefore investigated if PR3 presentation depended on NB1 during neutrophil differentiation and if PR3 and proPR3 could both be presented by NB1. In contrast to mature neutrophils, differentiating neutrophils showed an early NB1-independent PR3 surface display that was recognized by only two of four monoclonal anti-PR3 antibodies and occurred in parallel with proPR3, but not PR3 secretion, suggesting that the NB1-independent surface PR3 was proPR3. PR3 gene expression preceeded NB1. When the NB1 receptor was detected on the surface, a mode of PR3 surface display similar to mature neutrophils developed together with the degranulation system. Ectopic expression studies showed that NB1 was a sufficient receptor for PR3 but not proPR3. ProPR3 display on the plasma membrane may influence the bone marrow microenvironment. NB1-mediated PR3 presentation depended on PR3 N-terminal processing implicating the PR3-N-terminus as NB1-binding site.