Tartrate-resistant acid phosphatase 5a in sarcoidosis: Further evidence for a novel macrophage biomarker in chronic inflammation

Background/Purpose Tartrate-resistant acid phosphatase (TRACP) 5a is expressed strongly in inflammatory macrophages (MΦ). Serum TRACP5a is elevated in rheumatoid arthritis patients with extra-articular manifestations of rheumatoid nodules, in a percentage of patients with end-stage chronic kidney disease, and may be a risk marker for acute myocardial infarction. This proof-of-concept study was undertaken in patients with sarcoidosis to further substantiate our hypothesis that TRACP5a protein is a biomarker for macrophages in other chronic inflammatory diseases. Methods Immunohistochemical staining for TRACP5a and CD68 was performed in tissues of 19 patients with sarcoidosis. We also measured circulating TRACP5a protein and other inflammation biomarkers including interkeukin-6, angiotensin-converting enzyme, and C-reactive protein in 13 patients. Twenty healthy age-matched nonsmoking individuals were used as the reference group. Results All sarcoidosis tissues showed strong staining for TRACP5a and CD68 in the non-caseating granulomatous lesions and localized specifically to MΦ, multinucleate giant cells, and epithelioid MΦ. Serum TRACP5a protein was elevated significantly in active sarcoidosis patients compared with the control group, and levels fluctuated with disease activity in one patient studied longitudinally. Conclusion TRACP5a protein is expressed abundantly in the granulomatous tissues and may be elevated in a significant proportion of sarcoidosis patients. These findings further support our hypothesis that serum TRACP5a is derived from systemic inflammatory MΦ and thereby may be a biomarker of inflammation for sarcoidosis and also reflect its disease activity.