Phenotypic commitment of monocytes towards a protective hemoglobin scavenging phenotype...

Intravascular hemolysis may cause tissue injury directly or via a systemic inflammatory response. Under physiological conditions, extracorpuscular hemoglobin (Hb) is bound by haptoglobin(Hp) and the complex internalized via the hemoglobin scavenger receptor CD163 on monocytes, prior to catabolism via heme-oxygenase-1 (HO-1). Recently, a novel subset of CD68(pos)CD163(high)HLA-DR(low) macrophages with high expression of HO-1 was recognized in hemorrhagic areas of atherosclerotic plaques, distinct from CD68(pos)CD163(low)HLA-DR(high) foam cell macrophages with low- HO-1 content. Considering the hemolytic insult during CPB, we hypothesized that an equivalent compensatory CD163(high)HLA-DR(low) phenotype will evolve in circulating CD14(pos) monocytes post surgery.

Twelve patients undergoing elective surgery with CPB were enrolled with informed consent.Whole-blood samples were collected in EDTA at predetermined time-points, pre- intra-, and postoperatively. Whole-blood was evaluated by three-color flow cytometry for expression of CD14, CD163, and HLA-DR; CD14(pos) cells were also permeabilized to detect intracellular HO-1 protein. Plasma [Hp-Hb] concentration was determined by ELISA.

A striking phenotypic switch from CD163(low)HLA-DR(high) preoperatively to CD163(high)HLA-DR(low) postoperatively at 24 h was observed on circulating CD14(pos) monocytes. Intracellular HO-1 protein was also significantly up-regulated at 24 h after declamping. These phenotypic changes were preceded by peak Hb-Hp levels observed at 2 h.

We have shown for the first time, a phenotypic commitment of monocytes towards a protective CD14(pos)CD163(high)HLA-DR(low) population with increased intracellular HO-1 occurring in the circulation during the recovery phase of CPB. These findings have implications for monitoring of anti-inflammatory interventions and linkage to clinical outcomes.