Is there a relationship between antiphospholipid antibodies and complement activation in systemic lupus erythematosus?

Systemic lupus erythematosus (SLE) is a relatively uncommon autoimmune disease that can affect all organs of the body, including skin, joints, kidney, lungs, heart, and brain. SLE diagnosis is based on clinical symptoms and laboratory tests. Common tests for SLE measure autoantibodies against nuclear antigens and other molecules. In particular, antinuclear antibodies (ANA), anti-phospholipid antibodies (APL), and antibodies against double-stranded DNA (anti-dsDNA) and Smith antigen (anti-Sm) are important in SLE and are part of classification criteria. However, assays are not standardized and sensitivity or specificity of these biomarkers is low. As complement activation is a hallmark of SLE, complement proteins C3 and C4 are also commonly measured to help the diagnosis of SLE, however also these biomarkers have drawbacks.¹

Because patients with SLE can present with different symptoms and common laboratory biomarkers have suboptimal diagnostic characteristics, the diagnosis of SLE is challenging even for experienced rheumatologists.

Complement activation leads to the formation of cell-bound complement activation products (CB-CAPs). We demonstrated that C4d split fragment bound to erythrocytes (EC4d) and B-cells (BC4d) is a sensitive and specific biomarker of SLE.² In this study, presented at the 69^th AACC annual meeting, we decided to evaluate the relationship between APL and complement activation. We measured CB-CAPs in a large population of SLE patients who were either APL positive or negative. We evaluated also patients with other diseases (47% of whom had rheumatoid arthritis) and normal healthy volunteers (NHV) for comparison.

We measured EC4d and BC4d by flow cytometry using EDTA-anticoagulated blood: positive CB-CAPs consisted of positive EC4d and/or positive BC4d. In terms of APL, we measured anti-cardiolipin IgG, anti-beta-2-glycoprotein 1 IgG, and anti-phosphatidylserine/prothrombin complex IgG antibodies in serum by chemiluminescence (INOVA Diagnostics, San Diego, CA); patients were considered APL positive if any of these 3 autoantibodies was above manufacturer cutoff.

Consistent with previous studies,² CB-CAPs have good sensitivity and specificity. In fact, we measured 61% sensitivity in this study; specificity was 89% in distinguishing SLE from other diseases and 99% in distinguishing SLE from NHV. Overall, SLE patients had APL positivity rate higher than patients with other diseases (40% vs. 17%). In addition, there were more CB-CAPs positive patients in the SLE sub-group that was APL positive compared to the SLE sub-group that was APL negative (77% vs. 51%, p<0.0001 by Fisher’s exact test). A similar trend was observed in patients with other diseases (18% vs. 10%) and in the NHV (7% vs. 0%) (Table).

The finding that CB-CAPs positivity rate is higher in APL positive than APL negative patients indicates that there is a relationship between APL and CB-CAPs. Thus, these data suggest that APL may contribute to complement activation and CB-CAPs formation in SLE.

In conclusion, the results of this and previous studies indicate that CB-CAPs represent a novel biomarker for SLE that can help with the diagnosis and monitoring of this disease.

References

1.        Ramsey-Goldman R, Li J, Dervieux T, Alexander RV. Cell-bound complement activation products in SLE. Lupus Sci Med. 2017:1-8. doi:10.1136/lupus-2017-000236.

2.        Putterman C, Furie R, Ramsey-Goldman R, et al. Cell-bound complement activation products in systemic lupus erythematosus: comparison with anti-double-stranded DNA and standard complement measurements. Lupus Sci Med. 2014;1(1):e000056. doi:10.1136/lupus-2014-000056.