Study Highlights Role of Complement Activation in Antiphospholipid Syndrome

Treatment of patients with antiphospholipid syndrome (APS), a disease characterized by thrombosis and pregnancy morbidity and mortality, is hindered by an incomplete understanding of its underlying mechanisms, but a new study published in Blood suggests that anti-beta-2-glycoprotein-I (anti-β2GPI) antibodies activate, complement, and contribute to disease-related thrombosis. In addition, researchers found that patients with catastrophic APS (CAPS) often have mutations in complement regulatory genes that can lead to uncontrolled activation of the complement pathway, resulting in widespread thrombosis and multi-organ failure. This research also was presented during the plenary session at the 2019 ASH Annual Meeting by lead author Shruti Chaturvedi, MBBS, MS, from Johns Hopkins University School of Medicine.

“Our findings suggest that complement is a major driver for the thrombosis and organ failure that accompanies APS and CAPS,” said corresponding study author Robert A. Brodsky, MD, also from John Hopkins. “Antibodies to antiphospholipid appear to activate the classical or lectin pathway of complement.”

In this study, investigators prospectively recruited patients diagnosed with APS (n=59), CAPS (n=10), or systemic lupus erythematosus (SLE; n=74) between January 2015 and June 2019 from John Hopkins, Cleveland Clinic, and McMaster University. The diagnosis of APS was made based on International Society on Thrombosis and Hemostasis criteria:

  • ≥1 arterial, venous, or small vessel thrombosis clinical episode
  • presence of lupus anticoagulant (LA), anticardiolipin antibody of the IgG/IgM isotype, or anti-β2GPI antibody of the IgG/IgM isotype

In the group of patients with APS, 37 patients had venous thrombosis, 13 had arterial thrombosis, and 9 had both venous and arterial thrombotic events;  37% of participants with APS had recurrent thrombotic events.

Participants’ samples were collected for analysis via complement activation assays and genetic sequencing. Researchers also collected DNA samples from healthy pregnant women and patients with atypical hemolytic uremic syndrome (aHUS) as controls. To assess complement activation by anti-β2GPI antibodies, researchers added patient-derived anti-β2GPI antibodies to normal human serum and performed a modified Ham (mHam) assay.

In terms of complement activity, 36% of patients with thrombotic APS and 86% with CAPS had a positive mHam assay, compared with only 6.8% of patients with SLE (p<0.001). The findings suggest that CAPS is associated with higher complement activation by anti-β2GPI antibodies than APS and SLE, the authors reported.

In patients with APS, mHam positivity also was correlated with triple positivity, defined as the presence of LA, anticardiolipin, and anti-β2GPI antibodies, compared with double- or single-positivity (60% vs. 23% and 10%, respectively; p=0.002). Patients with recurrent thrombosis were also more likely to have a positive mHam assay compared with patients with a single event (67% vs. 33%, respectively).

The investigators also observed that mHam positivity correlated with time since thrombosis: Assays were more likely to be positive when a thrombosis event occurred within a 1-year period before testing, compared with events that occurred more than 1 year after testing (68% vs. 32%; p=0.019). Patients who continued to have positive mHam assays more than 1 year after thrombosis were more likely to have recurrent thrombosis, the researchers added.

To test their hypothesis that CAPS is associated with germline mutations in complement regulatory genes, the investigators then performed targeted sequencing on study participants and controls. Of the 10 patients with CAPS, 60% were shown to have rare variants in genes that regulate the complementary pathway, compared with 22% of patients in the APS cohort and 29% in the SLE cohort. Incidence in the aHUS and control cohorts were also lower (52% and 23%, respectively).

“CAPS is a more severe phenotype because these patients have germline mutations in complement regulatory genes in addition to the antiphospholipid antibodies, essentially offering a second ‘hit,’” Dr. Brodsky explained. “These findings suggest that complement inhibition might be a useful strategy to treat refractory APS and CAPS.” Several patients in the study were treated with eculizumab to manage APS, but it is too early to recommend treatment with the terminal complement pathway inhibitor in this setting.

A limitation of the study included the highly selected group of patients, all of whom were evaluated at one of 3 tertiary care centers. In addition, patients were not consecutively recruited or fully matched to the SLE control group. These limitations may reduce the generalizability of the findings.

Dr. Brodsky and co-authors also noted that this study was unable to elucidate the mechanism of how antibodies activate complement in this patient population. “A future question that needs to be explored is whether or not complement inhibition can prevent recurrent thrombosis and/or pregnancy loss in refractory APS or CAPS,” concluded Dr. Brodsky.

Study authors report no relevant conflicts of interest.


Chaturvedi S, Braunstein EM, Yuan X, et al. Complement activity and complement regulatory gene mutations are associated with thrombosis in APS and CAPS. Blood. 2019 December 8. [Epub ahead of print]