The immunopathology of erythema nodosum leprosum: The role of extravascular complexes

Abstract

Skin biopsies of 20 patients with erythema nodosum leprosum were studied histologically, by acid-fast, silver and immunological methods for the demonstration of bacterial antigen, and by immuno-peroxidase for a variety of immunological factors. The results were compared with those in 10 non-reacting lepromatous patients. At the centre of the ENL lesions there was always disintegration of macrophages and release of bacterial antigen, comprising cell walls and particulate or diffuse components of Mycobacterium leprae. These products were found to combine first with IgM, later with IgG, which together with complement components of the classical pathway were present at the same sites. These complexes were found both extracellularly and in neutrophils and macrophages, and were constant features of acute stage lesions. C-reactive protein and B-lipoprotein were present in varying amounts and were associated partly with connective tissue. It is thought that CRP, and the related SAP, may be factors in the disruption or repair of elastic and collagen, which are conspicuous in some ENL lesions. The results support the view that ENL is an immune complex phenomenon, possibly self-perpetuating, occurring at the site of breakdown of small lepromatous granulomas. The immune complexes are extravascular and in this repect ENL differs from the classical 'serum sickness' described by Arthus.