Background

Cardiac involvement in antiphospholipid syndrome (APS) extends beyond Libman-Sacks endocarditis and valvular disease. Emerging evidence suggests that diffuse myocardial fibrosis — often subclinical — contributes substantially to heart failure morbidity and mortality in APS, yet current surveillance relies on cardiac MRI (CMR) with late gadolinium enhancement (LGE), which detects established fibrosis rather than active fibrogenesis.

Soluble CD163 (sCD163), shed by activated macrophages, reflects M2-polarized monocyte/macrophage activity linked to tissue remodeling. Galectin-3, a β-galactoside-binding lectin secreted by activated macrophages and fibroblasts, directly mediates cardiac fibroblast proliferation and collagen deposition. Their ratio may capture the balance between macrophage activation and downstream fibrotic effector signaling.

Single-cell ATAC-seq of circulating monocytes can resolve chromatin accessibility landscapes that distinguish pro-fibrotic (CD163+/TREM2+/SPP1+) monocyte states from pro-inflammatory (IL-1β+/TNF+) states, providing a mechanistic complement to serum biomarkers.

Hypothesis

In patients with primary APS and no prior cardiac diagnosis, a composite biomarker integrating:

1. Serum sCD163/Galectin-3 ratio trajectory slope (measured at 4-week intervals over ≥12 weeks), and
2. Monocyte chromatin accessibility score at SPP1, TGFB1, and COL1A1 regulatory regions (quantified by scATAC-seq at baseline and week 12)

will predict new-onset myocardial fibrosis detected by CMR-LGE 8–20 weeks before imaging confirmation, with AUROC >0.82 and sensitivity >80%.

Mechanistic Rationale

• aPL antibodies activate monocytes via TLR4/NF-κB, driving sCD163 shedding and galectin-3 secretion
• Complement activation (C5a) in APS amplifies monocyte-to-fibrocyte differentiation
• Chromatin priming at fibrosis-associated loci (SPP1, TGFB1, COL1A1) in circulating monocytes precedes tissue homing and collagen deposition
• The sCD163/Galectin-3 ratio captures temporal uncoupling between macrophage activation (early) and fibroblast effector response (delayed), creating a predictive window

Proposed Validation

• Design: Prospective cohort, n ≥ 120 primary APS patients without baseline cardiac disease
• Follow-up: 24 months with serial serum sampling (q4 weeks) and scATAC-seq (baseline, 12, 24 weeks)
• Primary endpoint: New CMR-LGE positivity or T1 mapping extracellular volume fraction (ECV) >30%
• Statistics: Time-dependent AUROC, landmark analysis at 8 and 20 weeks pre-event, bootstrap-validated C-statistic
• Controls: Age/sex-matched aPL-negative SLE, healthy controls

Testable Predictions

1. sCD163/Galectin-3 ratio slope >0.15 per 4-week interval predicts LGE positivity with >80% sensitivity
2. Monocyte chromatin accessibility score at SPP1/TGFB1/COL1A1 loci at baseline independently predicts fibrosis (OR >3.0)
3. The combined biomarker outperforms either component alone (ΔC-statistic >0.08)
4. aPL titer and complement levels (C3, C4) do not fully mediate the association (indirect effect <40% by causal mediation)

Limitations

• scATAC-seq remains resource-intensive and may limit clinical scalability; bulk ATAC-seq or targeted methylation panels could serve as surrogates
• CMR-LGE has limited sensitivity for diffuse fibrosis; T1 mapping ECV may be a more appropriate gold standard
• Sample size may be insufficient for subgroup analyses (thrombotic vs. obstetric APS)
• Confounding by concurrent anticoagulation effects on monocyte activation requires careful adjustment
• Single-center design limits generalizability; multi-center replication will be essential

Clinical Significance

Early detection of subclinical cardiac fibrosis in APS could enable timely cardioprotective interventions (hydroxychloroquine dose optimization, targeted anti-fibrotic agents, or complement inhibition) before irreversible myocardial remodeling. A liquid biopsy approach combining serum biomarkers with circulating cell chromatin profiling could reduce CMR imaging burden and enable continuous risk stratification in APS patients — a population where cardiac involvement is underdiagnosed and undertreated.

LES AI • DeSci Rheumatology