Markers of complement activation in plasma during quiescent phases in patients with catastrophic antiphospholipid syndrome

TO THE EDITOR:

Complement activation has been implicated in the pathogenesis of catastrophic antiphospholipid syndrome (CAPS), a rare life-threatening disease.^1-5  Low C3 and C4 levels that have been identified in patients during acute or relapsing CAPS have, in fact, been considered signs of complement activation and consumption.^1-3  Likewise, complement activation products and high levels of C5b-9 terminal complex have been detected in the plasma of patients with acute CAPS.^2,4  Studies by Chaturvedi et al have further broadened our understanding of the complement system’s role in the pathogenesis of CAPS thrombotic storm by demonstrating that sera from acute phase patients induced complement-dependent cell killing and cell-surface deposition of C5b-9 on PIGAnull TF-1 cells.⁵  No signs of complement activation during quiescent CAPS phases, that is, long before its onset or after its remission, have ever been reported. High C5b-9 levels (1657 U; normal value <200) were recently identified in a patient with CAPS that had severe cardiac involvement immediately before treatment was begun.⁴  She was refractory to the so-called triple therapy (anticoagulation + steroids + plasma exchange/IV immunoglobulins)⁶  and was successfully treated with eculizumab. Unexpectedly, 5 months after a complete recovery (eculizumab had been suspended 3 months earlier), the patient’s C5b-9 plasma levels were found to be similar (1804 U) to those registered during the acute phase.⁴  In the current study, plasma levels of C5b-9 terminal complex and C5a anaphylatoxin during the quiescent phases of CAPS were investigated, compared with control populations, and discussed.

The plasma of 7 patients who had suffered from CAPS between 2009 and 2017 was collected during the quiescent CAPS phases, 3 before (median, 57 months; interquartile range [IQR], 26) and 4 after (median, 38 months; IQR, 24.2) the acute CAPS episode. The samples were stored at −80°C. Plasma concentrations of C5b-9 and C5a were assessed using the MicroVue C5b-9 Plus EIA and the MicroVue C5a Plus EIA, respectively (QUIDEL, San Diego, CA). There were 6 women and 1 man with a median age of 44 years (IQR, 20). Because histological studies were performed only in 2 cases, 2 patients had definite, and 5 had probable CAPS, according to Asherson et al classification criteria.⁷  Eight patients with antiphospholipid syndrome (APS) who had experienced a thrombosis over the preceding year and 8 healthy subjects sex and age matched with the quiescent patients with CAPS were used as the control populations. The median follow-up of the APS patients was 15 years (IQR, 9.2). The study was approved by the regional ethics committee and was carried out in accordance with the 1964 Declaration of Helsinki. Written informed consent was obtained from all the participants.

The patients’ demographic and clinical characteristics along with the time to recovery and antiphospholipid antibody profile at time of diagnosis are outlined in Table 1. All were successfully treated using conventional triple therapy, and none required therapeutic complement inhibition. The results of C5b-9 and C5a analyses during the quiescent phases in the patients with CAPS and in the controls are outlined in Figure 1. The C5b-9 levels in the quiescent CAPS were significantly higher than those in the thrombotic patients with APS (median, 330.2 ng/mL; IQR, 86.6; vs median, 189.7 ng/mL; IQR, 138.9) and in the healthy controls (median, 330.2 ng/mL; IQR, 86.6; vs median, 120.9 ng/mL; IQR, 83.7). Similarly, the C5a levels were significantly higher in the quiescent patients with CAPS with respect to the thrombotic patients with APS (median, 27.6 ng/mL; IQR, 58.4; vs median, 8.9 ng/mL; IQR, 8.3) and the healthy controls (median, 27.6 ng/mL; IQR, 58.4; vs median, 4.9 ng/mL; IQR, 4.9).

There are some investigations in the literature reporting elevated plasma levels of C5b-9^2,4,8,9  and C5a⁹  in the active phase of CAPS^2,4  or thrombotic APS.^8,9  The current study demonstrates for the first time the finding of significantly high C5b-9 and C5a plasma levels during the quiescent phases long before the onset of the acute phase and after CAPS remission. Excessive delivery of complement activation products in the plasma of quiescent patients with CAPS both before and after an acute episode suggests that there may be a defective complement control in these patients. Chaturvedi et al, who identified a high prevalence of rare germline variants in complement regulatory genes in patients with CAPS, speculated that in the presence of antiphospholipid antibodies, patients who also have a pathogenic complement regulatory gene mutation are predisposed to uncontrolled activation of complement leading to CAPS when a trigger factor emerges. No functional data concerning the pathogenic role of the rare germline variants in CAPS were however provided by this study.⁵  The significantly higher C5b-9 and C5a plasma levels during the quiescent phases could be a feature of patients with APS that have a predisposition to developing CAPS in the presence of a precipitating factor. The absence of CAPS over a long follow-up period (median, 15 years) in patients with thrombotic APS and the occurrence of relapses, at times even repeated ones, in patients who had already experienced a CAPS episode seem to confirm a predisposition to the disorder.¹⁰  The low number of patients with CAPS available for our study is one of the study’s limits that can be explained and justified by the rarity of the disease. Furthermore, we did not have serial samples from our patients with quiescent CAPS to draw solid conclusions regarding the persistence of elevated levels of C5b-9 and C5a over time both before and after the CAPS episode and their clinical significance. If our data are confirmed by multicenter studies investigating large CAPS and control populations and repeated samples from each quiescent patient with CAPS, high C5b-9 and C5a levels in patients with APS will be able identify subjects at risk of CAPS who would benefit from complement inhibitor therapy during a CAPS episode. Currently, eculizumab is the most available anticomplement drug; it is a humanized monoclonal antibody targeted against complement C5 that inhibits its cleavage and prevents the generation of prothrombotic and proinflammatory molecules and membrane attack complex C5b-9. Although 12 case reports investigating its use to treat CAPS have been published,^1-4,11-18  there are yet no guidelines for its timing of administration and appropriate dosage in CAPS. Given the rapid clinical improvement noted already at the time of the third injection in the majority of CAPS patients, the dosage used during the induction phase seems to be crucial.¹¹  Although the most common induction dose, which was 900 mg/wk, produced both a good response^12,14,16,17  and a partial remission,^1,11,15  beginning treatment immediately or within 9 days of a CAPS diagnosis was associated with a better outcome^2-4,12,17  with respect to a later start.^1,11,15  The recent report by Yelnik et al represents the first cohort study on the use of eculizumab in CAPS and describes the results obtained in 11 patients.¹⁹  Five of them (45.4%) only had a significant improvement in the few days after the first administration. Unfortunately, in this study, eculizumab was often used in the presence of severe and prolonged organ failures indicative of irreversible injury. There was, in fact, more patients who required permanent hemodialysis prior to receiving eculizumab in the nonresponders with respect to the responders.¹⁹  The efficacy and safety of eculizumab in CAPS remain to be clarify by well-designed multicenter studies that take into account the rapid evolution of CAPS in organ failure and the importance of using eculizumab in the early stages of the disease.^6,20