Factors involved in the pathogenesis of neutrophilic vasculitis in MRL/Mp-lpr/lpr mice: a model for human microscopic angiitis.

Anti-neutrophil cytoplasm antibodies (ANCA) directed against myeloperoxidase (MPO) are detected in patients with microscopic angiitis. Human MPO autoantibodies stimulate neutrophil degranulation in vitro and are thought to be pathogenic. We have previously shown that MRL-lpr mice with MPO autoantibodies have a higher incidence of vasculitis than their seronegative littermates. The aim of the present study is to determine the relationship between MPO autoantibodies and microscopic angiitis. The neutrophil binding properties of anti-MPO monoclonal antibodies (mAbs) from MRL-lpr mice were tested using murine heterophils (neutrophils) present in blood and induced peritoneal exudates. MRL anti-MPO mAbs selectively bind activated neutrophils which express MPO in vitro. The pathogenicity of an IgG2b anti-MPO mAb, C6, was investigated in vivo. Anti-MPO mAb, C6 was administered to young MRL mice which had been primed with exogenous TNF alpha to induce neutrophil activation and expression of MPO. Neutrophilic vasculitis similar to microscopic angiitis occurred in 33% of MRL mice which had been treated with anti-MPO mAb. The lesions were mainly restricted to sites of previous endothelial insult which suggests an active role for injured endothelium in this pathology.