Antiphospholipid syndrome (APS) is an autoimmune disease characterized by thrombosis and pregnancy morbidity in the persistent presence of antiphospholipid antibodies (APL). A rapidly progressive form of catastrophic APS (CAPS) may also occur with multiorgan thrombosis and thrombotic microangiopathy.1 The pathogenesis of thrombosis and pregnancy morbidity in APS is not completely understood, and it remains unclear why some patients have more severe phenotypes including recurrent thrombosis or CAPS.2 Those who are triple positive for lupus anticoagulant, anti-cardiolipin antibody, and anti-β2-glycoprotein-1 antibody are at highest risk of recurrent thrombosis despite anticoagulant therapy.3
In this article, Dr. Shruti Chaturvedi and colleagues investigate the role of complement dysregulation in the pathophysiology of thrombosis and CAPS in APS. Patients with thrombotic APS (n = 59) and CAPS (n = 22) from three tertiary centers were prospectively recruited, along with patients with systemic lupus erythematosus (SLE; n = 74) for comparison. Serum samples were collected and assessed for evidence of complement activation using a modified Ham’s (mHam) assay to assess cell killing of PIGA-negative cells after exposure to patient sera. The mHam assay was also performed with the addition of both eculizumab (terminal complement blockade) and novel factor D inhibitor (selective inhibition of alternative complement pathway). Evidence of cell surface complement (C5b-C9) deposition was confirmed via flow cytometry. Finally, targeted sequencing for germline mutations of 15 genes involved in complement regulation (including CFH) was also completed in several patients, along with samples from patients with atypical hemolytic uremic syndrome (aHUS).
The mHam assay was positive in a greater proportion of patients with CAPS (86%) than those with thrombotic APS (36%) and SLE (7%). mHam positivity was also more strongly associated with triple positivity (60%) than with patients who were double- (23%) or single-positive only (10%). Moreover, mHam positivity was more likely in those who had a recurrent thrombotic event despite therapeutic anticoagulation (43%) than those with a single event (33%).
APS patient sera were found to induce C5b-C9 deposition seen by flow cytometry on the test PIGA-negative cells. Moreover, the addition specifically of anti-β2-glycoprotein-1 antibody from patients was also shown to induce C5-C9 deposition. This complement activation induced by patient sera was inhibited by treatment with both eculizumab but not inhibited by a factor D inhibitor (selective inhibitor for alternative pathway), suggesting that complement activation in APS occurred via the classical pathway. Finally, germline mutations in complement regulatory genes were seen in a higher proportion of patient with CAPS (6/10, 60%) compared with thrombotic APS (12/55, 22%), and SLE (6/21, 29%). The rate of germline mutations in CAPS was similar to that of a sample of patients with aHUS (17/33, 52%).
In Brief
These data were initially presented during the Plenary Scientific Session at the 2019 ASH Annual Meeting.4 What is the significance of these findings? They confirm that complement activation likely plays an important role in the pathogenesis of thrombosis in APS, particularly in more severe phenotypes including recurrent thrombosis and CAPS. This complement activation likely occurs via the classical complement pathway, and may be induced by patient-specific APL. This data supports a “two-hit” pathogenic model in which APL may activate complement, and those with germline mutations may be susceptible to uncontrolled complement activation in the setting of a trigger (infection, surgery, autoimmune disease, etc.), rendering them susceptible to the development of CAPS.
Limitations of this study include referral bias, as these cases were highly selected for referral to tertiary care. There was also no control group matched for patient characteristics or treatments, limiting the ability to draw comparisons. Nevertheless, these data provide important insight into the pathogenesis of immunothrombosis in APS. They also provide a possible rationale for the use of complement blockade in treating refractory CAPS, which has been reported in case series after the failure of standard therapy with steroids, plasma exchange, and anticoagulation.5 Future studies should focus on clarifying the functional significance of germline mutations and investigating complement blockade for CAPS.
References
Competing Interests
Dr. Tseng indicated no relevant conflicts of interest.
