HUS and atypical HUS

Abstract

Hemolytic uremic syndrome (HUS) is a thrombotic microangiopathy characterized by intravascular hemolysis, thrombocytopenia, and acute kidney failure. HUS is usually categorized as typical, caused by Shiga toxin-producing Escherichia coli (STEC) infection, as atypical HUS (aHUS), usually caused by uncontrolled complement activation, or as secondary HUS with a coexisting disease. In recent years, a general understanding of the pathogenetic mechanisms driving HUS has increased. Typical HUS (ie, STEC-HUS) follows a gastrointestinal infection with STEC, whereas aHUS is associated primarily with mutations or autoantibodies leading to dysregulated complement activation. Among the 30% to 50% of patients with HUS who have no detectable complement defect, some have either impaired diacylglycerol kinase ε (DGKε) activity, cobalamin C deficiency, or plasminogen deficiency. Some have secondary HUS with a coexisting disease or trigger such as autoimmunity, transplantation, cancer, infection, certain cytotoxic drugs, or pregnancy. The common pathogenetic features in STEC-HUS, aHUS, and secondary HUS are simultaneous damage to endothelial cells, intravascular hemolysis, and activation of platelets leading to a procoagulative state, formation of microthrombi, and tissue damage. In this review, the differences and similarities in the pathogenesis of STEC-HUS, aHUS, and secondary HUS are discussed. Common for the pathogenesis seems to be the vicious cycle of complement activation, endothelial cell damage, platelet activation, and thrombosis. This process can be stopped by therapeutic complement inhibition in most patients with aHUS, but usually not those with a DGKε mutation, and some patients with STEC-HUS or secondary HUS. Therefore, understanding the pathogenesis of the different forms of HUS may prove helpful in clinical practice.

Figure 1.

Predisposing factors, promoters, and triggers of aHUS and secondary HUS. Under physiological conditions, regulation of complement is always at a higher level than the activation challenge. (Right) Regulation consists of the role of the membrane regulators CD35, CD46, CD55, and CD59, and the plasma regulators factors H and I. Regulation is compromised by mutations in complement regulators, autoantibodies against factor H, or potentially some infections in which microbes remove sialic acids from the surface of self cells. Regulation is enhanced by upregulation of complement protein expression in liver (eg, because of acute phase reaction or pregnancy) or introduction of additional regulators by plasma exchange. (Left) Activation of complement consists of continuous alternative pathway activation and occasional (or longer standing) physiological or pathological activation. The level of complement activation can be increased by mutations in complement proteins C3 or factor B, infections, iatrogenic phenomena (eg, transplantation, plasma exchange, or certain drugs), pregnancy, or malignancy. The net result of all the activation and regulation boosting or inhibiting effects may dictate whether activation overrides regulation leading or contributing to a pathological process.

Figure 2.

Consequences of complement activation. (A) The complement system can be activated via 3 pathways: classical, lectin, and alternative. All pathways lead to formation of powerful enzymes, the C3-convertases, followed by activation of the terminal cascade. The main effector functions of complement (promotion of opsonophagocytosis by opsonization and chemotaxis and formation of lytic membrane attack complexes) aim to destroy harmful agents such as microbes. Lysis of target cells can lead to damage-induced enhancement of complement activation. (B) Alternative pathway activation is based on continuous, low-level covalent deposition of C3b molecules onto practically all surfaces in contact with plasma. If the C3b molecule is allowed to form an enzyme (shown in red), new C3b deposits will be formed around the enzyme leading to rapid amplification of the activation. If the regulator factor H binds to C3b, the convertase enzyme is inactivated and no complement activation follows. The simultaneous interaction of factor H with both C3b and cell surface sialic acids (or possibly glycosaminoglycans [GAGs]) is essential for proper regulation on self red cells, platelets, and endothelial cells. If this fails, disbalance between activation and regulation may lead to pathogenesis of atypical HUS. iC3b, C3b molecule incapable of forming an enzyme with factor B.

Figure 3.

Schematic presentation of the main links between the complement and coagulation systems and platelets in formation of microthrombi in aHUS. Complement activation leads to release of the C5a peptide, inducing tissue factor activity on endothelial cells leading to a procoagulative state of the endothelium. Activation of the coagulation cascade leads to generation of active thrombin that is able to cleave not only fibrinogen but also complement C5, which thereby enables coagulation-enhanced complement activation. Formation of membrane attack complexes on endothelial cells and platelets can cause endothelial cell damage and platelet activation. Finally, activation of the coagulation system leads to platelet activation via various mechanisms. Together, the coagulation system and platelet activation/aggregation lead to formation of microthrombi. The importance of complement in this process in aHUS is clearly demonstrated by rapid inhibition of microvascular thrombosis by therapeutic complement inhibition.

Figure 4.

A model of the parallel pathogenic processes in HUS. Excess complement activation on endothelial cell, platelet, and red cell surfaces leads to C5a release and membrane attack complex (MAC) formation. This leads to enhanced tissue factor (TF) activity on the endothelium, activation and aggregation of platelets, and release of hemoglobin and reduction of nitric oxide (NO) in plasma. These phenomena lead to a procoagulative state, coagulation, and thrombosis-mediated tissue damage. There are several feedback loops in this process. These loops can be seen as a cycle that may be initiated at several points and where several phenomena may take place in parallel. This may explain why STEC-HUS, secondary HUS, and aHUS share clinical features although the processes start in various ways. DGKε, diacylglycerol kinase ε.

Figure 5.

Schematic model of the role of complement activation, cell damage, and thrombosis in various severe diseases or conditions with major thrombotic problems. Activation of complement can occur via the alternative pathway (in the absence of antibodies) or the classical pathway (in the presence of target-bound antibodies). In each of the indicated diseases or conditions, endothelium, red cells, or platelets are damaged. This may contribute to coagulation and thrombosis, and direct procoagulative effects may also participate (red arrows). Thrombosis can lead to further tissue damage and increasing complement activation. The process can enhance itself via positive feedback loops and form a vicious cycle. Although complement is involved in each of these diseases, its impact needs to be clarified in clinical studies. CAPS, catastrophic antiphospholipid syndrome, DIC, disseminated intravascular coagulation; PLG, plasminogen; THBD, thrombomodulin.