Plasma Protein Concentrations of Complement Components C4 and C3 Are Inversely Associated with Anticardiolipin Antibodies in APS Patients.
Authors: Savelli Stephanie L. , Roubey Robert A.S. , Buxton Gail S. , Yang Yan , Wu Yeeling , Yu C. Yung 
Published: April 2020

Recently, it has been shown that injection of human antiphospholipid antibodies (aPL) into pregnant mice resulted in fetal resorption and complement proteins were implicated in the pathophysiologic process of the antiphospholipid syndrome (APS). Studies have been unable to determine definitively which complement components are responsible for the human disease. A significant number of systemic lupus erythematosus (SLE) patients have elevated titers of aPL and lupus anticoagulants. Homozygous and heterozygous deficiencies of complement C4A are known risk factors for the development of SLE. Complement C4 is a subunit protein of the C3 and C5 convertases for the classical and lectin activation pathways. C4A is the acidic isotype involved in immunoclearance and immunotolerance whereas C4B, the basic isotype, is important in the propagation of the complement activation pathways. To investigate the possible role of C4 in APS, we examined the phenotypic diversities of C4A and C4B as well as the plasma C4 and C3 protein concentrations. Peripheral blood plasma samples and genomic DNAs were provided by the Antiphospholipid Syndrome Collaborative Registry (APSCORE). Two patient groups were studied: (1) patients with definite APS as defined by the Sapporo criteria (APS) and (2) patients with elevated aPL without clinical sequelae (asymptomatic aPL). The APS group consisted of 184 patients. The mean age was 44 ±13 years. Among them 79% were female and 84% were Caucasian. The asymptomatic aPL group included 166 subjects. The mean age was 43 ± 13 years. Among them 87% were female and 78% were Caucasian. Complement C4A and C4B protein polymorphisms were determined by immunofixation of plasma samples. Plasma C3 and C4 protein concentrations were determined by single radial immunoassays. Factors known to have an effect on plasma C4 levels including body mass index (BMI), race and gender, were also examined by regression analyses. When compared to sex and race-matched normal controls, we found that total C4, C4A and C4B protein levels are significantly lower in the patient study populations (p&lt;0.001). The lowest total C4 levels were observed in the asymptomatic aPL group that has a mean total C4 level of 16.5 mg/dL, compared to 31.2 mg/dL and 23 mg/dL in the control and APS groups, respectively. C4A and C4B levels were also lowest in the asymptomatic aPL subjects. C3 protein levels are positively associated with C4 in both APS and asymptomatic aPL groups (p&lt;0.001); the strongest association was noted in the asymptomatic aPL group (R2=0.324). Anticardiolipin (ACLA) IgM is inversely associated with C4 concentrations in both APS (p=0.006) and asymptomatic aPL (p=0.015). The asymptomatic aPL subjects also showed an association of higher ACLA IgG titers with lower C3 concentrations (p=0.008). We also found that both APS and asymptomatic aPL have higher BMI (APS: 28.6 ± 6.8; asymptomatic aPL: 28.1 ± 6.1; controls: 25.6.± 5.8). Our results underscore an important role of C4 and C3 in human APS. We are currently determining C4A and C4B polygenic variations in these patient groups. The ultimate goal is to elucidate the intricate relationships among APS, asymptomatic aPL, SLE and low plasma levels of total C4, C4A and C4B.