Red cell adhesion molecules, foetal haemoglobin and endothelial factors in sickle cell disorders
Sickle cell anaemia (SS) is a haemoglobinopathy involving production of sickle haemoglobin (HbS, β⁶Glu-->Val), which is able to polymerise leading to vaso-occlusion. Hydroxyurea (HU) treatment increases foetal haemoglobin (HbF) levels but decreases vaso-occlusion and red cell adhesion molecule (AM) expression, and therefore improves clinical symptoms. In this thesis, the contribution of AMs, HbF and endothelial factors to the severity of sickle cell disease has been studied. Flow cytometry for measurement of HbF-containing red cells (F⁺cells), AM-expressing red cells (AM⁺cells) and reticulocytes (retics) was developed and validated using single-, double- and triple-colour staining procedures. The AMs examined were CD36, CD41 and CD49d. F⁺cells are increased in SS patients over normal control subjects. In the controls, the percentage of F⁺mature red cells (%F⁺MRCs) is equal to %F⁺retics leading to an enrichment ratio (ER = %F⁺MRCs / %F⁺retics) close to 1.0, indicating no survival advantage of F⁺cells. However in SS patients, the ER is about 1.5-2.5 indicating a survival advantage of F⁺cells. AM⁺cells in SS patients are also increased over the controls. In both the controls and SS patients, the AM⁺cell depletion ratio (AMDR = %AM⁺MRCs / %AM⁺retics) is decreased to less than 0.05, indicating a rapid shedding of AMs from reticulocytes during maturation. This shedding is confirmed by a reduction of %AM⁺cells to undetectable levels after 5 days of reticulocytes in culture. In HU-treated SS patients, F⁺cell increment and AM⁺cell reduction are found. The %AM⁺retics for pre- and post-HU treatment is not different, suggesting that AM⁺cell reduction is dependent on the reduction of reticulocytes. In the controls, proportion of F⁺cells expressing AMs is higher than F⁺cells, indicating that F⁺cells are more primitive. In SS patients, these proportions are similar. However, in HU-treated SS patients, the proportion of F⁺cells expressing AMs is lower than F⁺cells, confirming that F⁺cells survive longer so that they have more time to shed more AMs. This finding is similar in both reticulocyte and MRC populations. Vascular endothelial growth factor (VEGF) may be involved in vasoocclusion. It is significantly increased in SS patients. VEGF is able to induce nitric oxide metabolite (NOx) release. However in this thesis, NOx levels were not increased in SS patients and no correlation was found between NOx and VEGF levels. No relationship was observed between VEGF and either erythropoietin or Hb levels, suggesting that VEGF increment may not be due to generalised anaernia. Increases in soluble endothelial selectin (sE-Selectin) and soluble vascular cell adhesion molecule 1 (sVCAM-1) found in SS patients indicate endothelial activation. Beta-thromboglobulin (BTG) and platelet factor 4 (PF4) were also increased in SS patients, due to local platelet activation in vivo. Platelets as well as endothelial cells contain VEGF, therefore the increased VEGF levels in SS could be a consequence of local ischaernia resulting from vaso-occlusion and local platelet and endothelial cell activation.