Paradoxical inhibition of the haemoglobin-scavenging mechanism in SCD

Principal Collaborators:

Kim Quimby (CDRC), Clive Landis (CDRC), Andre Greenidge (CDRC), Marvin Reid (Sickle Cell Unit, Jamaica)

Background:

Sickle Cell Disease (SCD) is an inherited condition which is characterized by intravascular haemolysis. Potentially, 10gm of plasma free Hb can be accumulated per day. Extreme pressure is therefore placed on the haemoglobin scavenging pathway to neutralise the pathogenic properties of free heme.

Objective:

Having proven that the induction of the haemoglobin scavenging mechanism, anti-oxidant phenotype (CD163highintracellularHO-1high) will evolve in circulating CD14pos monocytes during CPB associated haemolysis, the aim was to investigate the protective phenotype in sickle cell disease. This was done as a collaborative effort between the CDRC and the TMRI sister unit, the SCU.

Methods:

Seventeen participants (13 with HBSS and 4 non-HBSS controls) were enrolled at the SCU, Mona. Whole blood was collected in EDTA and shipped to CDRC, Barbados. A pilot study verified the stability of the macrophage phenotype at room temperature for 48 hours. Three-colour and intracellular flow cytometry was performed at the CDRC in Barbados as detailed above. Comparisons were made by an un-paired t-test (GraphPad Prism Software, Inc., San Diego, CA).

Results:

The mean fluorescent intensity (MFI) of CD163 was down-regulated in HBSS [5.2±0.4 (mean ± S.E.M.) vs.7.7±0.8 in HBAA; p=0.01] (Figure). The MFI of intracellular HO-1 protein was also down-regulated in HBSS [69.0±4.0 vs. 47.0±5.4], although not to statistical significance (n=2 HBAA; 7 HBSS).

Interpretation:

These preliminary data suggest that the haemoglobin-scavenging mechanism is not induced in individuals with HBSS.

Impact/Expected Impact:

The consequential increase in plasma haemoglobin in HBSS had been linked to vascular dysfunction e.g. limited vasodilatation and induction of vascular adhesion markers; and adverse clinical outcome e.g. ‘sickle foot’ and pulmonary hypertension. Linkage with this molecular aberration would add to the understanding of the patho-physiology of vascular dysfunction in SCD and could offer a possible target for future therapeutic regimes. A larger case-control study with targeted clinical outcomes would be needed.