Microvascular injury and repair in acute human bacterial pyelonephritis.

Acute inflammatory cell-capillary endothelial cell interactions, related to injury and repair, were investigated light and electron microscopically in acute human bacterial pyelonephritis. In inflammatory infiltrate-adjacent microvessels, the small capillaries were completely occluded by leukocyte plugs and the large capillaries were densely filled with acute inflammatory cells adhering to the endothelium. Severe damage to small and large capillaries was observed around endothelium adherent, degranulated neutrophil granulocytes containing phagocytosed bacteria. There were spaces in the endothelium, degradation of the vascular basement membrane, of the perivascular interstitial matrix and of collagen fibrils, with fibrin deposition and vessel wall fragmentation. In the small capillaries relatively distant from the interstitial infiltrates, emigration of leukocytes was frequently seen. Around the escaping cells the endothelial lining displayed occasional discontinuities, allowing leakage of vascular fluid into the interstitial space. Some small capillaries not related to the infiltrate were occluded by fibrin thrombi with apparent damage to the endothelial cells and disruption of the capillary wall. Various reparative changes were noticed in association with this change including capillary neovascularization. The findings confirm the existence of polymorphonuclear leukocyte-mediated injury of capillaries during the development of inflammatory responses in acute pyelonephritis.