Background Antiphospholipid antibodies (APLA) are necessary, but not sufficient for the development of thrombosis in APS. Soluble triggering receptor expressed on myeloid cells-1 (sTREM-1) is an innate-immune receptor found in the blood and reflects innate immune cells activation.

Aim To determine if plasma sTREM-1 can be served as a biomarker for thrombosis in patients with PAPS.

Methods A cross-sectional, case-control study. Plasma level of sTREM-1 was analysed by ELISA in a group of consecutive patients diagnosed with either PAPS (Sapporo criteria) or asymptomatic persistently positive APLA, and healthy controls (HC).

Results The study group comprised of 33 patients with PAPS (age 52.0±17.4 years.), 10 asymptomatic APLA-positive patients (50.6±17.9 years.) and 73 HC (42.6±13.3 years.). The mean plasma sTREM-1 level was significantly higher in the PAPS group compared to HC (316.3±119.3 pg/ml, vs 230.2±85.5 pg/ml, p=0.0002), as well as past obstetric APS (195.12±58.52 pg/ml, p=0.014), and asymptomatic APLA (215.8±pg/ml vs HC, p=0.019). Plasma sTREM-1 in PAPS patients with an acute event of thrombosis was significantly higher than in patients with past thrombotic event (p=0.012), past obstetric APS (p=0.0001) and HC (p<0.0001). Plasma sTREM-1 level was significantly higher in PAPS patients who ever had stroke (p=0.007) or venous thromboembolic event (p=0.018). On receiver operator curve (ROC) analysis, plasma sTREM-1 showed an area under the curve (AUC) 0.7292 in differentiating between thrombotic APS (ever) and non-thrombotic APS or asymptomatic APLA-positivity. A multivariate regression model to predict sTREM-1 level by thrombotic PAPS ever, age and sex found that sTREM-1 level is independently associated with thrombotic PAPS (p<0.004) as well as with female gender (p=0.017) and older age (p=0.0006). Plasma sTREM-1 level was neither associated with anti-cardiolipin, anti-β2 glycoprotein I (IgG/IgM/IgA) Abs’ titers and/or lupus anticoagulant positivity, nor with single-, double- or triple- APLA positivity.

Conclusion Our data show for the first time that plasma sTREM-1 level is significantly elevated in patients with thrombotic PAPS. We suggest that soluble TREM-1 might be used as a biomarker for thrombosis in patients with primary APS and our results support the possible role for the innate immune system in the pathogenesis of thrombosis in PAPS.