Von Willebrand factor in the rat kidney: its localization and the effects of its in vivo interaction with specific antibodies

A complement-dependent model of thrombotic thrombocytopenic purpura induced by antibodies reactive with endothelial cells

Thrombotic thrombocytopenic purpura (TTP) is an immunologically mediated disease characterized by thrombocytopenia, hemolytic anemia, and pathologic changes in various organs, including the kidney, which are secondary to widespread thromboses. Central to TTP is platelet activation, which may occur from a variety of mechanisms, including endothelial cell activation or injury. In this study, injection of K6/1, a monoclonal antibody with widespread reactivity toward endothelia, led to dose-dependent thrombocytopenia in rats. This was magnified if animals were preimmunized with mouse IgG, thereby resulting in an accelerated autologous phase of injury. In this setting, significant anemia also resulted. Rats injected with K6/1 developed renal injury, consisting of tubular damage and glomerular thrombi. Thrombocytopenia and renal morphological abnormalities were eliminated if animals were complement depleted with cobra venom factor prior to K6/1 injection and worsened when the activity of the ubiquitous complement regulator Crry was inhibited with function-neutralizing antibodies. Therefore, we have developed a complement-dependent model of TTP in rats by injecting monoclonal antibodies reactive with endothelial cells. Antibody-directed complement activation leads to stimulation of platelets, through direct interactions with complement fragments and/or indirectly through endothelial cell activation or injury, with the subsequent development of TTP.

Anti-endothelial cell auto-antibodies in hepatitis C virus mixed cryoglobulinemia

BACKGROUND/AIM

Hepatitis C virus (HCV) infection is often associated with mixed cryoglobulins (MC) and may manifest as small-vessel vasculitis. It has been suggested that antibody (Ab) or sensitized T cells to HCV-containing endothelial cells may initiate the vasculitis process. Anti-endothelial cell antibodies (AECA) have been found in various connective tissue disorders, with a high prevalence in systemic vasculitis. The aim of the study was to determine the prevalence of AECA in HCV patients with or without MC-associated vasculitis, and to identify associations with clinical, immunological, virological and liver characteristics.

METHODS

Sixty-nine HCV patients (Group 1), 46 of whom had MC (type II=30, type III=16), and 23 without MC, were prospectively studied. HCV-MC-associated vasculitis was noted in 25 patients who had at least one of the following clinical features: peripheral neuropathy, glomerulonephritis, skin purpura, cerebral vasculitis. Group 2 included 20 patients with non-HCV viral diseases: HHV8 (10), miscellaneous (10). Group 3 included 25 patients with biopsy-proven non-HCV chronic liver diseases: hepatitis B virus (10), miscellaneous (15). Controls were 100 blood donors (Group 4). Sera were adsorbed onto a pellet of A549/8 epithelial cells before being evaluated. AECA were then searched using a cellular ELISA, with a permanent cell line (EA.hy 926) as the substrate. All sera were also examined for the presence of cryoglobulin, antinuclear Ab, anticardiolipin Ab, and rheumatoid factor.

RESULTS

AECA were more frequently found in HCV patients than in blood donors (41% vs 5%, p=0.0001). The prevalence of AECA was lower in non-HCV than in group 1 patients [group 2=15%, p=0.03; group 3=16%, p=0.01]. There was no significant difference in AECA prevalence between groups 2, 3 and 4. In HCV patients, AECA were associated with age (p<0.001), the presence of MC (p=0.008), cryoglobulin level (p=0.016), HCV-associated vasculitis (p=0.04), genotype 1b (p=0.005) and severity of liver histologic damage. AECA isotypes were not different in the 4 groups. AECA were not associated with antinuclear Ab, anticardiolipin Ab, rheumatoid factor or interferon alpha treatment.

CONCLUSION

AECA are a common finding in HCV patients (41%), but not in other viral diseases or in non-HCV chronic liver diseases. In HCV patients, AECA are associated with MC-vasculitis, suggesting that AECA may be a marker for HCV-induced vasculitis.

Specificity, pathogenecity, and clinical value of antiendothelial cell antibodies

OBJECTIVE

To characterize the putative target antigens for antiendothelial cell antibodies (AECA), the possible pathophysiological role of AECA, and the clinical value of these antibodies as markers of disease activity.

METHODS

A structured literature search was done using Medline in combination with a manual search. Two physicians reviewed all articles of special interest.

RESULTS

AECA are a heterogenous group of antibodies directed against a variety of antigen determinants on endothelial cells (EC). The EC antigens can be constitutively expressed, constitutively expressed and modulated by cytokines, or cryptic. In addition, antigen determinants for AECA may also be molecules that adhere to EC ("planted" antigens). However, many AECA antigens are currently not well characterized. AECA are detected in a wide variety of inflammatory disorders. Although probably of limited value in disease diagnosis, the detection of these antibodies may be valuable in following disease activity. In several diseases such as systemic lupus erythematosus and systemic vasculitis, high AECA titers are found during active disease whereas lower titers or disappearence of AECA have been reported during remission. The correlation between changes in AECA titers and disease activity suggests an important role for AECA in processes in which vessel wall damage occurs, although it does not exclude the possibility that AECA are an epiphenomenon of vascular injury. Several recent in vitro studies support a role of AECA in the pathophysiology of these inflammatory disorders. AECA may play a role in the pathophysiology by inducing activation of EC resulting in upregulation in the expression of endothelial adhesion molecules and/or secretion of chemoattractants and cytokines. An alternative mechanism by which AECA could be a trigger in the pathogenesis of some diseases is complement dependent cytotoxicity (CDC) and/or antibody dependent cellular cytotoxicity (ADCC). In experimental animal models, antibodies to antigenic determinants expressed on EC were capable of inducing vascular injury.

CONCLUSION

AECA represent a heterogenous group of antibodies directed against a variety of antigenic determinants on EC. They are present in a variety of inflammatory disorders. The detection of these antibodies may be valuable in following disease activity. Further characterization of putative antigens is needed to better understand their pathophysiological role.