Soluble CD163 as a Non-Invasive Biomarker in Autoimmune Nephrological and Rheumatological Diseases

Autoimmune nephrological and rheumatological diseases involve macrophage-driven inflammation, yet disease activity is often assessed using invasive or non-specific measures. Soluble CD163 (sCD163), released from activated monocytes and macrophages, is emerging as a biomarker of macrophage-mediated inflammation in these conditions. This narrative review summarizes current evidence on the diagnostic, prognostic, and disease-monitoring potential of sCD163 measured in blood, urine, and synovial fluid in autoimmune nephrological and rheumatological diseases. This review is based on a narrative analysis of selected publications investigating the clinical utility of sCD163 in autoimmune kidney and rheumatic diseases, with emphasis on correlations with disease activity, histopathological findings, and clinical outcomes. Urinary sCD163 shows excellent diagnostic accuracy for active lupus nephritis (area under the receiver operating characteristic [AUROC] 0.89-0.998), correlates with histological activity index (but not chronicity), and distinguishes ongoing inflammation from chronic damage during treatment. In IgA nephropathy, it predicts remission failure and greater benefit from corticosteroids. In ANCA-associated vasculitis, it identifies active renal involvement (AUROC 0.95 in multicenter cohorts). In rheumatoid arthritis (RA), serum sCD163 correlates with early disease activity, predicts radiographic progression, and detects subclinical macrophage activation in remission. In spondylarthritis, synovial fluid sCD163 reflects a disease-specific M2-polarized macrophage phenotype distinct from RA. Utility is compartmentalized: urinary levels indicate intrarenal macrophage activation, synovial fluid local joint inflammation, and serum systemic activation. sCD163 is a promising macrophage-specific biomarker across autoimmune diseases, but its compartmentalized nature requires context-specific measurement. Before clinical implementation, assay standardization, multicenter validation, and interventional trials showing the benefit of sCD163-guided management are needed.