Endotoxin induces rapid metalloproteinase-mediated shedding followed by up-regulation of the monocyte hemoglobin scavenger receptor CD163.

CD163, a monocyte and macrophage-specific surface glycoprotein, which is increased by interleukin-10 and glucocorticoids, is a scavenger receptor for hemoglobin/haptoglobin complexes. We report a rapid and highly reproducible rise in soluble CD163 in the plasma of human volunteers given intravenous lipopolysaccharide (LPS). We also show that LPS induces shedding of CD163 from the surface of isolated monocytes, identifying shedding from monocytes and macrophages as a likely mechanism for the endotoxemia-associated rise in plasma CD163 in vivo. Studies using the inhibitor TAPI-0 indicate that a metalloproteinase is responsible for LPS-mediated shedding of CD163. Finally, we demonstrate a marked increase in surface CD163 expression on circulating monocytes 24 h following experimental endotoxemia. These findings show that CD163 is rapidly mobilized in response to bacterial endotoxin. As hemoglobin can bind LPS and enhance its toxicity, it will be important to determine how cell surface and soluble CD163 influence inflammatory processes during sepsis.

Regulation and localization of endogenous human tristetraprolin.

Tumor necrosis factor (TNF) has been implicated in the development and pathogenicity of infectious diseases and autoimmune disorders, such as septic shock and arthritis. The zinc-finger protein tristetraprolin (TTP) has been identified as a major regulator of TNF biosynthesis. To define its intracellular location and examine its regulation of TNF, a quantitive intracellular staining assay specific for TTP was developed. We establish for the first time that in peripheral blood leukocytes, expression of endogenous TTP is confined to the cytoplasm. Baseline expression of TTP was higher in monocytes than in lymphocytes or neutrophils. After in vitro incubation with lipopolysaccharide (LPS), leukocyte TTP levels increased rapidly, peaking after approximately 2 hours. Monocytes showed the greatest response to LPS stimulation and lymphocytes the least. TTP levels were also studied in leukocytes isolated from healthy volunteers infused with a bolus dose of LPS. TTP expression and initial upregulation in response to LPS infusion were consistent with the in vitro data. Neutrophil TTP levels responded first, reaching an initial peak within 1 hour, monocyte levels peaked next at 2 hours, followed by lymphocytes at 4 hours. This response paralleled plasma TNF levels, which peaked 2 hours after infusion and were no longer detectable after 12 hours. A second rise in intracellular TTP levels, which did not parallel plasma TNF levels, was observed in all leukocyte populations, starting 12 hours after infusion. These data establish the cytoplasmic location of TTP, supporting a major role for this protein in regulating TNF production, and suggest that TTP levels are not regulated solely by TNF.