Ultrastructural demonstration of intracellular bacteria in xanthogranulomatous pyelonephritis.

Xanthogranulomatous pyelonephritis is considered to be an unusual cellular response to bacterial invasion. This assumption is based on circumstantial evidence and experimental studies. In our electron microscopic study of five cases of xanthogranulomatous pyelonephritis demonstrated a stratification of the cellular components, somewhat resembling that described in the experimental lesions. The center of the xanthogranulomas was occupied by purulent exudate. Bacteria were identified mainly in polymorphonuclear leukocytes and extracellularly in four cases in which central area were available for electron microscopic studies. Many bacteria were located in cytoplasmic vacuoles. more peripherally, histiocytes predominated, and their cytoplasm contained numerous lipid droplets with and without limiting membranes and phagolysosomes. The latter contained electron dense, granular, and membranous structures. The outermost layer of the lesion demonstrated macrophages with a decreased number of lipid droplets and larger intralysosomal particles. Lymphocytes, plasma cells, and fibroblasts represented an increasing proportion of the infiltrate in this layer. Our study suggests that xanthogranulomatous pyelonephritis is a bacteria induced process. Its histological appearance may be related to incomplete bacterial degradation and altered host response.