Research on the role of the complement system in the pathogenesis of atypical haemolytic uraemic syndrome (aHUS) and other thrombotic microangiopathies (TMAs) has stimulated the development of anti-complement therapy. Eculizumab, the first monoclonal antibody which blocks the complement component C5, is approved for the treatment of paroxysmal nocturnal haemoglobinuria and aHUS. Its efficacy in both native and transplanted kidney highlighted the importance of complement activation in aHUS, a prototype disease, caused by primary alteration of the complement system. However, diagnosis and academic classification by exclusion of different aetiologies and identification of genetic variants soon becomes more complex in light of evidence from situations in which different forms of TMAs converge, and the need to develop categories based on pathogenesis and treatment.

The first experiences which extended the original indication of eculizumab for paroxysmal nocturnal haemoglobinuria and aHUS are based on the scientific evidence of the narrow boundary between primary and secondary TMAs.1 Three concepts modify the classic view of TMA: primary or secondary complement activation, the overlapping in different clinical entities and endothelial dysfunction playing a key role in pathogenesis.

TMA is a complex process, resulting from the imbalance between immunity, clotting and complement, which is altered by precipitating factors (prevalent in secondary TMAs) in patients predisposed by multiple genetic determinants (dominant in aHUS) (Fig. 1).

Fig. 1.

Elements of TMA, complex multifactorial process.

(0.06MB).

Local activation of complement (aHUS, ischaemia/reperfusion) or systemic complement activation, is cause of endothelial lesion (humoral rejection, C3 glomerulonephritis [C3GN], recurrence of post-transplant nephropathy) is present in the primary or secondary form of TMA. Demonstration of the activation of the terminal effector complex (C5b-9) in secondary TMAs makes the complement system a therapeutic target and this is the basis of treatment with eculizumab2 in two scenarios: situations of a difficult differential diagnosis between primary and secondary TMA, and entities with activation of the terminal complement pathway with or without microvascular thrombosis:

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  TMA difficult to diagnose:

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  Triggered by infections (Table 1)

  Table 1.

  Patients with complement-amplifying conditions treated with eculizumab.

  Secondary TMA	Outcome
  Drug-induced: Doxorubicin Gemcitabine Mitomycin	Improves kidney function138 Improves kidney function139 Kidney and haematological remission140
  STEC-HUS STEC-HUS and mutation in the CFH gene	Kidney and neurological remission141 Haematological remission, kidney remission without dialysis or PE142
  TTP resistant to PE, vincristine, steroids and rituximab	Treatment with eculizumab, PE and steroids143 Haematological, kidney and neurological remission143
  Cobalamin deficiency	Eculizumab before diagnosis144

  PE: plasma exchange; STEC-HUS: Shiga toxin-associated E. coli-induced haemolytic uraemic syndrome; TTP: thrombotic thrombocytopaenic purpura.
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  Associated with drugs (Table 1)

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  Associated with glomerulonephritis

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  In systemic diseases (vasculitis, lupus, scleroderma, etc.)

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  In pregnancy and post-partum

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  Malignant hypertension (MHTN)

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  De novo post-solid organ transplantation (SOT-TMA)

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  Microvascular damage with complement activation:

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  TMA in haematopoietic stem cell transplantation (HSCT-TMA)

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  Antibody-mediated rejection (AMR)

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  Damage caused by ischaemia/reperfusion

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  Refractory thrombotic thrombocytopaenic purpura

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  Nephropathy caused by antiphospholipid antibodies.

Patients with TMA are identified quickly in clinical practice, but determining the cause is difficult. The time that elapses between suspicion and true demonstration of the disease may have a negative impact on the efficacy of the drug if the therapeutic decision is delayed. Genetic and molecular studies of the complement system have a long-term prognostic and strategic value, but not it is of limited help for early indication, which is decisive for preserving kidney function. TMA is a catastrophic and fatal process for the endothelium that, regardless of its cause, leads to kidney and systemic damage with a limited response to conventional therapy. Proof of this is that it TMA worsens the prognosis and survival of the patient in all situations in which it complicates the underlying disease: post-partum, kidney transplant and HSCT, MHTN, autoimmune diseases and glomerulopathies.

The objective of this review is to provide current scientific evidence on the treatment of secondary TMAs and the management of these patients from an emergent and promising therapeutic perspective.

There are cases of TMAs associated with a great variety of systemic diseases, in which there is an overlap between entities and mutations of the complement system in up to 33% of patients with HUS associated with autoimmune diseases.3,4 The presence of diverse components of the complement system in kidney biopsies suggests its pathogenic role, and the response to eculizumab indicates that the deregulation of the non-genetic basis of the complement system may play an important role, and be a potential therapeutic target.

The clinical, pathogenic and evolutionary characteristics C3 glomerulopathies (C3G) have been characterised in recent years. The definition is based on the presence of intense, isolated or clearly predominant deposits of C3 by immunofluorescence.36,37 Two types can be distinguished: C3GN and dense deposit disease; the latter is characterised by ribbon-shaped intensely osmiophilic deposits along the basement membrane.36 C3G pathogenesis consists of the abnormal activation of the alternative complement pathway by mutations in the genes coding for complement factors or regulatory proteins (factors H, I, CD46), or for autoantibodies against these regulatory factors.36–38

Various studies have shown that the spectrum of genetic mutations and autoantibodies associated with C3G is very similar to that of patients with aHUS.36–38 It is postulated that the deregulation of the complement system in C3G occurs in the fluid phase, resulting in the accumulation of the complement system's degradation products in the glomerular capillaries, while complement activation in aHUS mainly affects the cell surfaces (endothelium) and causes severe TMA.36,38–40

The reasons why some patients with a certain genetic mutation present with aHUS and others with C3G are only partially known.39,40 However, cases of aHUS/TMA and C3G as coincident processes41 in the same patient39,42–44 have been reported in the literature: TMA in patients previously diagnosed with C3G42 and others who develop aHUS/TMA after a kidney transplant,43,44 and whose end-stage kidney failure had been caused by C3G.

The treatment of C3G is widely debated. Although conventional immunosuppression had been considered ineffective based on some case reports and short series of patients, a recent study showed a favourable effect, especially for that based on steroids and mycophenolate mofetil (MMF).45 Preliminary data from this study indicate that the cases caused by autoantibodies could be especially sensitive to MMF, while those caused by genetic abnormalities would be, in general, resistant. Several patients have been treated with eculizumab, with varied results46–51; however a careful analysis of the cases indicates that eculizumab could be effective in patients with acute and aggressive disease, with no advanced chronic lesions in the kidney biopsy, and with increased serum levels of C5b-9.46–51 To date, we are not aware of any patient with C3G treated with eculizumab, who had developed TMA.

Pregnancy-associated TMA is a rare clinical entity with an estimated incidence of one case per every 25,000 pregnancies and with a high maternal and perinatal morbidity and mortality. During the last decade, two findings have contributed to improve the knowledge of this entity: the acquired or congenital deficiency of ADAMTS13, cause of thrombotic thrombocytopaenic purpura in the second and third trimesters of pregnancy, and the imbalance of regulatory factors of the complement system's alternative pathway as a genetic risk factor for the development of post-partum aHUS. Fakhouri et al. reported that 85% of patients with pregnancy-associated HUS presented abnormalities in the complement factors.64 Although the pathogenic mechanism need to be clarified, it is suggested that in a normal pregnancy with mutations in some the complement genes, there may be an uncontrolled activation counterbalanced by regulatory proteins located on the surface of the trophoblast such as CD55 or DAF (decay-accelerating factor), MCP and CD59,. However, post-partum TMA may be precipitated by numerous triggering factors (inflammatory phenomena, infections, haemorrhages) that could reactivate the complement system's alternative pathway after delivery without the presence of regulatory mechanisms on the placental surface.65 The involvement of the complement system in pregnancy-associated TMA has encouraged the use of eculizumab in seven patients, with or without complement mutations, during pregnancy or post-partum; different regimens were used, but an excellent clinical response was obtained in all cases (Table 4).66–71

Also, complement activation has been recently associated with other entities related to pregnancy, such as pre-eclampsia and haemolytic anaemia syndrome, thrombocytopaenia and elevated liver enzymes (HELLP syndrome). Pre-eclampsia represents the maternal response to an excess of antiangiogenic factors generated by placental hypoperfusion which includes vasculopathic factors, such as Soluble Fms-Like Tyrosine kinase 1 (sFlt-1), a potent antagonist of vascular endothelial growth factor (VEGF) and endoglin (TGF-B inhibitor). The symptoms vary from different degrees of hypertension to HELLP syndrome or eclampsia.65,72 The complement system is key in inflammatory processes. Over activation of the complement system induces the deregulation of angiogenic factors which contribute to the inflammatory process. Based on this notion, Burwick et al. presented the case of a woman with severe HELLP syndrome at 26 weeks of pregnancy, treated with eculizumab, with a favourable clinical response. This made it possible to prolong the pregnancy by 17 days, reducing the likelihood of morbidity and mortality of the newborn.73

HSCT-TMA is a complication with high mortality (up to 90% in patients with severe cases92) and with a risk of chronic nephropathy in patients with less severe cases. It has been estimated that TMA occurs in 10–35% of the haematopoietic stem cell transplantation (HSCTs), particularly after an allogeneic transplant.113–117 Triggering factors include CNIs for the prophylaxis of graft-versus-host disease, which directly cause endothelial lesion, activate the complement and alter the activity of ADAMTS13 (Table 5).97 Although acute graft-versus-host disease is an independent risk factor of HSCT-TMA, no causal relationship has been established.118–120

HSCT-TMA may have a benign course without treatment or with a reduced dose of the CNI. However, some patients develop a systemic lesion with kidney injury, gastrointestinal damage, serositis, pulmonary hypertension and multiple organ failure.121–124 The most common endothelial lesion is the kidney lesion, which causes a decline in glomerular filtration rate, proteinuria and hypertension. Pulmonary involvement causes hypoxaemia or respiratory125,126 and gastrointestinal distress, vomiting, diarrhoea, abdominal pain and bleeding. This overlaps with the signs and symptoms of intestinal graft-versus-host disease, and requires a histological examination to reach a differential diagnosis.

HSCT-TMA should be suspected if there is a sudden increase in serum lactate dehydrogenase (LDH) levels, hypertension and proteinuria. The increase in LDH is secondary to the ischaemic tissue damage related to thrombosis and endothelial lesion.127 The appearance of schistocytes is usually delayed. Table 6 lists the diagnostic criteria for HSCT-TMA.

The determination of the terminal complement activity by the quantification of plasma levels of the terminal effector complex (sC5b-9) would enable patients who may benefit from an anti-complement therapy to be identified. High plasma levels of sC5b-9 and proteinuria have been associated with a decline in survival rate (<20% after a year).128

Plasma levels of ADAMTS13, that normally are moderately reduced, can exclude thrombotic thrombocytopaenic purpura, which is extremely rare after an HSCT.

Lastly, in allogeneic transplant recipients, the genetic study of the complement system should be conducted in non-haematological samples, such as a buccal smear.

Initial treatment consists of removing potential triggering agents (CNI), controlling associated complications (infections, graft-versus-host disease) and an appropriate antihypertensive therapy. However, the response is often limited, especially in patients with severe HSCT-TMA. Current therapeutic options include plasma exchange, defibrotide, rituximab and eculizumab.

The use of plasma exchange is debated and is associated with a response rate of 36% (0–80%). This is probably because it is reserved for severe cases. The improvement of haematological parameters (platelets, haemoglobin, haptoglobin, LDH) may give a false impression of improvement of the underlying disorder. Complement regulatory proteins in infused plasma may produce remission in the short-term, but tissue damage and mortality are not modified.129,130 If plasma exchange is indicated, it should be started early and administered daily until the TMA is resolved.

The anticoagulant defibrotide, approved in Europe, has been used in patients with mild manifestations, and the administration of rituximab, alone or combined with other agents, has also been associated with a favourable response in selected cases.131

HSCT-TMA is a multifactorial disease in which the complement system is activated by the classical or alternative pathway, resulting in tissue damage caused by microvascular thrombosis.132 There is growing evidence of complement system imbalance involved in some cases of HSCT-TMA, and of complete remission with anti-complement therapy with eculizumab. It has been verified that patients with HSCT-TMA have complement activation, anti-factor H autoantibodies and C4d renal deposits,133–135 and may present pathogenic variants of aHUS complement genes. In a paediatric study of six children with acute kidney failure and HSCT-TMA, most presented with deletion of genes for complement factor H-related proteins (CFHR3 and CFHR1), three of them with anti-FH autoantibodies. Response to plasma exchange was poor, and high levels of sC5b-C9 and thrombosis were detected in the kidney biopsy. Of the six patients, four achieved therapeutic plasma levels of eculizumab and a clinical response.136

In a retrospective study in France from 2010 to 2013, 12 patients with severe HSCT-TMA received eculizumab, with a median follow-up of 14 months. The haematological and overall response was 50% and 33%, respectively.137

Both the doses of eculizumab and the period of time required to control HSCT-TMA are greater than the observed in aHUS,. It is recommended to assess the response after at least 4–6 weeks following the induction.

The involvement of the complement system in aHUS and TMA of distinct aetiologies uncover a new therapeutic perspective: control of the complement system to prevent endothelial and inflammatory damage. Experience shows that the range of TMAs is not made up of disjunctive categories, but that there is a continuum in the pathogenesis where two elements intervene and interact with variable intensity: genetic susceptibility and aetiological or predisposing factors. Scientific evidence shows overlap and common mechanisms between aHUS and other secondary TMAs in which complement-mediated endothelial damage occurs. This fact explains the positive response to eculizumab in patients with TMA of distinct aetiology, and makes it a promising therapy. The contribution of the complement system in the pathogenesis of secondary TMAs is well documented in the entities reviewed in this article. Systemic studies analysing the genetic variants and variants of predisposition to activation of the alternative and terminal pathways, the functional consequences of this activation on the disease mechanisms and the functional/clinical effect of blocking the C5 component by eculizumab, which represents a coadjuvant therapy to conventional treatment of the underlying disease in secondary TMAs, need to be carried out. Lack of response to the control of the aetiological factor and conventional treatment in secondary TMAs has supported the therapy with eculizumab in most of the published cases.

The fundamental limitations for evaluating the efficacy of eculizumab in secondary TMAs are: the clinical and pathogenic heterogeneity of generally severe and refractory cases; publication bias; concurrence of other therapies used in the underlying disease; dose variation; frequency and duration of treatments; and difficulty of monitoring using biological and pharmacokinetic markers. Optimal duration of treatment is an important issue, given the economic impact of eculizumab. This is also a limiting factor for early treatment which, as demonstrated in aHUS, is more cost-effective. The administration regimens correspond to the indication for aHUS and paroxysmal nocturnal haemoglobinuria in most cases, or they are modified empirically.

Although presently the overall number of patients treated is not very abundant, a significant number of cases are now treated successfully with eculizumab in post-pregnancy/partum TMA, post-SOT TMA, HSCT-TMA and in the prevention/treatment of AMR. Cases of TMA in systemic diseases associated with drugs, glomerulonephritis and MHTN form the most heterogeneous and difficult to evaluate group.

The involvement of complement activation in the pathogenic mechanisms of secondary TMAs is clear, but it is necessary to promote studies to standardise clinical experience, and to enable common strategies to be designed for different diseases and effective biological markers and clinical parameters to be identified, which make it possible to accurately determine the potential therapeutic indication in each case.

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  TMA is a complex process, derived from the imbalance between immunity, clotting and complement through a combination of aetiological factors (secondary TMAs) and genetic risk factors (dominant in aHUS).

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  The classification of classic TMA does not explain the complexity of the disease mechanisms or reflect the therapeutic objectives. It is therefore necessary to reconsider a pathogenic classification of TMA.

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  There is an overlap between both entities and common mechanisms of complement-mediated endothelial damage which explains the response to eculizumab in patients with TMA of distinct aetiology, making it a promising therapy.

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  Although experience is limited, eculizumab has proven to be effective in post-SOT TMA, HSCT-TMA, prevention and treatment of humoral rejection, pregnancy-associated TMA and systemic diseases. As with aHUS, early treatment is associated with an improved therapeutic benefit.

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  More studies are needed to accurately determine the indication, dosage and duration of treatment as a coadjuvant therapy to aetiological treatment in each case.

Elena Román, Javier de la Rubia, Amparo Sempere, Enrique Morales, Manuel Praga and Ana Ávila have carried out consulting and teaching activities for Alexion Pharmaceuticals. Elena Román, Enrique Morales and Manuel Praga have been members of the expert committees on aHUS. None of the above-mentioned activities has influenced the preparation of this manuscript.

Elena Román, Santiago Mendizábal, Isidro Jarque, Ana Ávila and José Luis Górriz are members of ‘Medicamentos de Alto Impacto Sanitario o Económico’ [Medicinal Products with a Major Health and Economic Impact] (MAISE) for eculizumab of the ‘Conselleria de Sanidad Universal y Salud Pública de la Generalitat Valenciana’ [Universal Healthcare and Public Health Council of the Regional Government of Valencia].