Human monocyte CD163 expression inversely correlates with soluble CD163 plasma levels

BACKGROUND: CD163 is a monocyte/macrophage-restricted receptor involved in the clearance of hemoglobin-haptoglobin complexes and regulation of inflammatory processes. CD163 is shed from the cell surface and exists as a soluble form in plasma (sCD163). Monocyte CD163 and sCD163 are potential diagnostic tools in variety of disease states. METHODS: We determined the relation between plasma sCD163 levels by enzyme-linked immunosorbent assay, membrane expressions of CD163, CD64, and CD14 on blood monocytes by flow cytometry, and monocyte counts in 129 random blood samples. RESULTS: A strong inverse correlation was found between membrane CD163 expression and sCD163 levels (r = -0.65, P < 0.001). Monocyte CD163 expression and SCD163 levels did not correlate with the monocyte absolute count. CONCLUSIONS: The inverse relation between monocyte surface CD163 expression and sCD163 levels in human blood suggests that plasma sCD163 is derived from circulating monocytes, in addition to an unknown component from tissue macrophages. The lack of correlation with the absolute monocyte number suggests that such a balance is driven by the functional state of monocytes, rather than simply by numerical changes in circulating cells. We propose that further clinical evaluations of CD163 as a diagnostic parameter should include simultaneous measurements of soluble and cell-bound forms of this antigen.