Elevated antilysosomal-associated membrane protein-2 antibody levels in patients with adult Henoch-Schönlein purpura

Abstract

Henoch-Schönlein purpura (HSP) is characterized by IgA-containing immune complexes within leucocytoclastic vasculitis. Lysosomal-associated membrane protein-2 (LAMP-2) was first identified as part of a systematic search for antineutrophil cytoplasmic antibody (ANCA) antigens expressed on neutrophils and endothelial cells. To investigate the presence of ANCA in patients with adult HSP and microscopic polyangiitis (MPA), and to measure serum LAMP-2 antibody levels in these patients. Twenty-four adult patients with HSP, eight with MPA and 24 normal healthy controls were examined. ANCA detection was performed using indirect immunofluorescence (IIF), a direct enzyme-linked immunosorbent assay (ELISA) and a capture ELISA specific for myeloperoxidase (MPO) and proteinase 3 (PR3). We measured other ANCA-associated antibodies including anti-LAMP-2 antibody in serum using ELISA. Immunohistochemical (IHC) staining was used for anti-LAMP-2 antibody expression in patient skin biopsies. To determine the cut-off value of the serum anti-LAMP-2 antibody, a receiver operating characteristic (ROC) curve was constructed using statistical analysis software (JMP 8·0·2; SAS Institute Inc., Cary, NC, U.S.A.). The sera of all patients with HSP were negative for MPO-ANCA and PR3-ANCA by direct ELISA and by capture ELISA. However, ANCA was present in 17 (71%) of the 24 patients with HSP based on IIF. In contrast, we found MPO-ANCA in all eight patients with MPA using both ELISA methods. We found serum anti-LAMP-2 antibody levels in HSP significantly higher than in MPA and in healthy individuals (P = 0·002 and P = 0·00167, respectively). The area under the curve of serum anti-LAMP-2 antibody between HSP and MPA was 0·8698 by ROC analysis. The optimal cut-off point was 0·267 U mL(-1) (sensitivity 1·000, specificity 0·583). We found a significant positive correlation between serum anti-LAMP-2 antibody levels and serum IgA levels in HSP (r(s) = 0·731, P = 0·00226). Anti-LAMP-2 antibody overexpression in IHC staining was present in 20 (83%) of the patients with HSP. The overexpression was observed within the neutrophils and endothelial cells of leucocytoclastic vasculitis. There was a significant positive correlation between IHC staining score and positive serum anti-LAMP-2 antibody. The 24 patients with HSP and the eight patients with MPA were negative for antiazurocidin antibodies, antibactericidal permeability increasing protein antibodies, anticathepsin G antibodies, antielastase antibodies, antilactoferrin antibodies and antilysozyme antibodies. We suggest that anti-LAMP-2 antibody could play some role in the pathogenesis of adult HSP, and have excluded a role for MPO-ANCA and PR3-ANCA. We propose that measuring serum anti-LAMP-2 antibody could be a feasible method of differential diagnosis between HSP and MPA.