Von Willebrand Factor Activates the Alternative Complement Pathway In Vivo in a Mouse Model of Complement Thrombotic Microangiopathy

Introduction

Thrombotic microangiopathy (TMA) is a group of disorders presenting with microvascular thrombosis, microangiopathic hemolytic anemia, thrombocytopenia, and microvascular endothelial injury ultimately leading to end organ damage. Atypical hemolytic uremic syndrome (aHUS) and bone marrow transplantation-associated TMA (TA-TMA) are two types of TMA known as "complement TMA," as they are associated with dysfunction of the alternative complement pathway (AP), part of the innate immune system. Complement TMA episodes are frequently initiated during inflammation, and the role of inflammation in these TMAs may be in part related to the finding that the inflammatory cytokine TNF up-regulates complement components, down-regulates complement regulators, and causes AP activation in human glomerular microvascular endothelial cells (GMVECs) in vitro. However, the precise mechanism of AP activation in complement TMA is poorly understood. It has been demonstrated that von Willebrand factor (VWF) serves as a surface for AP activation in vitro. Inflammatory cytokines are stimulatory to endothelial cells, leading to excessive VWF secretion; in fact, patients with inflammatory disorders such as complement TMA demonstrate elevated plasma VWF levels, and this abundance of VWF may lead to excessive AP activation. We hypothesized that in complement TMA, VWF released from endothelial cells during inflammation activates the AP. The objective was to investigate whether VWF-mediated AP activation is initiating episodes of complement TMA in vivo in a complement TMA mouse model.

Methods

A mouse model of complement TMA was developed. To provoke complement TMA, wild-type (WT) C57BL/6J mice (purchased from Jackson Laboratories) were injected intraperitoneally with 5 mg/kg of the inflammatory endotoxin lipopolysaccharide (LPS). Twenty-four hours after injection of either LPS or saline (control), markers of complement TMA-lactate dehydrogenase (LDH), creatinine, platelet count, and complement activation by a rabbit erythrocyte (E_R) lysis assay-were measured. E_R specifically activate the AP in mammalian plasma in the presence of Mg/EGTA, and the resulting lysis is an indicator of the extent to which complement components have been consumed in vivo: low E_R lysis in vitro indicates high complement activation in vivo.

To assess the role of VWF in AP activation in vivo, VWF^-/- mice (purchased from Jackson Laboratories on a C57BL/6J background) were injected with 5 mg/kg LPS to induce complement TMA. Twenty-four hours after injection, plasma E_R lysis was measured and compared to E_R lysis in LPS-injected WT C57BL/6J mice. Other markers of complement TMA, including LDH, creatinine, and platelet count, were also measured in both LPS-injected VWF^-/- and WT C57BL/6J mice.

Results

Compared to saline-injected WT C57BL/6J mice, the LPS-injected mice demonstrated significantly: A) higher LDH (hemolysis); B) higher creatinine (renal dysfunction); C) lower platelet count; and D) lower E_R lysis (increased AP activation) (Fig. 1). These results confirm the establishment of a complement TMA murine model. Furthermore, LPS-injected VWF^-/- mice demonstrated significantly higher E_R lysis values than the LPS-injected WT C57BL/6J mice, indicating less AP activation (Fig 2). Because VWF^-/- mice do not produce any VWF, these data suggest that VWF contributes to AP activation in vivo in complement TMA. Finally, the LPS-injected VWF^-/- mice had less severe TMA as demonstrated by lower LDH levels, lower creatinine levels, and higher platelet counts compared to the LPS-injected WT C57BL/6J mice (Fig. 3). The LPS-injected VWF^-/- mice still had a mild degree of AP activation and TMA compared to saline-injected WT C57BL/6J mice, implying that there are complex mechanisms of AP activation in complement TMA that rely heavily on VWF.

Conclusions

In this study we demonstrated that VWF was important in complement activation and provocation of TMA in a complement TMA murine model. These data are the first step toward a better understanding of VWF's role in complement TMA and may lead to the development of therapeutics that inhibit inflammation, VWF release, or VWF-complement binding to curb AP activation in aHUS and TA-TMA, thereby preventing devastating outcomes. Further study is necessary to delineate the precise mechanisms of VWF-mediated AP activation and TMA development so that these treatment modalities may be explored.

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