Mechanisms of endothelial cell dysfunction in Wegener's granulomatosis

Abstract

Vasculitides are a heterogeneous group of disorders that share a common feature of blood vessel inflammation. A major cell type of the blood vessel affected is the endothelial cell (EC). Wegener´s granulomatosis (WG) is one type of vasculitis of unknown aetiology, involving granulomatous inflammation and necrosis that most frequently targets the small and medium-sized vessels of the upper respiratory tract, lower respiratory tract and kidneys. A crucial event in the initiation, localization and propagation of EC injury in WG involves activation of the EC by various stimuli, one of which is antiendothelial cell antibodies (AECA). However, the exact mechanisms by which EC in WG are damaged are not known.

Important criteria for distinguishing the various vasculitides are the size of the vessels involved and the organs supported by the inflamed vessels. Thus, use of the relevant EC as target cells for studies involving AECA carries implications for understanding the clinically important mechanisms underlying the pathogenesis/progression of WG. However such studies are lacking. Therefore, in order to understand some of the mechanisms by which EC in WG are damaged we performed the following studies:

In paper I, we studied the frequency and interaction of AECA with EC isolated from the clinically relevant small blood vessels of the nose, lung and the kidneys. We demonstrated, as compared with other patient groups, that WG was significantly associated with non-cytotoxic AECA that selectively bind surface antigens on unstimulated nasal, kidney, and lung EC. However AECA binding was lost/decreased when cytokine activated EC were used. We also demonstrated that EC from various organs are characterized by heterogeneity in morphological/functional aspects, marker proteins of cell activation, and responsiveness to cytokines.

In paper II, we report a novel finding that demonstrates the occurrence of two heterogeneous populations of EC within the nasal microvasculature. One EC population exhibited classic vascular endothelial markers with cobblestone-like morphology, while the other was sinusoidal in nature, possessing fenestrae. We also established novel protocols for the isolation and culture of these EC.

In paper III, we suggest a novel mechanism by which EC dysfunction in WG may perpetuate vasculitis. Our findings suggest that inflammatory EC (IECs) may detach from the inflamed organ and enter the circulation, and via production of soluble factors may have an inhibitory effect on the repair function of EC progenitors (EPCs). We demonstrated that during active WG disease the number of circulating IECs was significantly higher as compared to WG patients in remission and normal controls. Furthermore, IECs but not EPC expressed two novel EC inflammatory markers; vascular adhesion protein 1 (VAP-1) and MHC class-I related chain A (MICA). These markers were also highly expressed in kidney biopsies of WG patients during active disease.

In paper IV, we studied the functional role of WG AECA on kidney EC. We report that isolated IgG fractions from WG patients induced a rapid calcium flux, up-regulation of MICA, and production of neutrophil/monocyte activating chemokines. Furthermore, western blot analysis of immunoprecipitated kidney EC proteins with WG IgG revealed three bands of: 190-200 kDa, 70-73 kDa and 50-53 kDa.

Our data suggest that AECA may play an important role in the dysfunction of EC in WG. AECA per se may not be cytotoxic, but may act as modulators of the immune responses. Thus, a pro-inflammatory loop may exist between the binding of AECA to EC and the possible recruitment of inflammatory cells via stress/adhesion molecules and production of chemokines resulting in inflammation and EC dysfunction. Furthermore, circulating IECs may parallely contribute to the pathogenesis/progression of WG by interfering with the functional capacity for vessel wall repair by EPCs.