Haptoglobin is synthesized during granulocyte differentiation, stored in specific granules, and released by neutrophils in response to activation

Haptoglobin (Hp) is a plasma protein synthesized primarily by hepatocytes. It exerts a broad range of anti-inflammatory activities and acts indirectly as a bacteriostatic agent and an antioxidant by virtue of its ability to bind free hemoglobin (Hb) and to facilitate its immediate clearance by macrophages. We identified Hp as a novel specific granule protein of neutrophils by means of immunoelectron microscopy, subcellular fractionation, and exocytosis studies. Consistent with these findings, blood cells from a patient with specific granule deficiency (SGD) lacked neutrophil-derived Hp. Neutrophils contained a large amount of highly glycosylated Hp (beta-chain 45-65 kDa) synthesized in neutrophil precursors and stored in specific granules and a small amount of Hp (beta-chain 39 kDa) endocytosed from plasma and stored in secretory vesicles. Subsequent binding studies revealed that Hp from specific granules binds to Hb. Finally, the CCAAT enhancer binding protein-epsilon (C/EBPepsilon) induced Hp transcription in a myeloid cell line, suggesting that Hp expression in myeloid cells, as in hepatocytes, is at least partially regulated by members of the C/EBP transcription factor family. Collectively, these findings demonstrate that Hp is stored in specific granules and is released by neutrophils in response to activation. Hence, neutrophil-derived Hp might reduce tissue damage and bacterial growth at sites of infection or injury by propagating anti-inflammatory activities and Hb clearance.