Early on, circulating endothelial cells have been detected on the basis of cell morphology in smears of peripheral blood (1). The next important step was to establish immunocytochemistry of these cells. Using sickle cell anaemia as an example, elevated cell numbers were demonstrated in active disease (2). This approach, however, was still cumbersome and more elegant techniques were eagerly awaited. In this regard, immunomagnetic isolation was an important step forward (3, 4). This approach had already been used to detect rare cells, such as tumour cells, in peripheral blood. The technique seemed particularly appropriate because circulating endothelial cells are also rare in peripheral blood. Using this approach, circulating endothelial cells were demonstrated in patients with rickettsial infection (5) and intracellular rickettsial antigens were detected. Patients with acute coronary syndrome also had elevated cell numbers (6). Since then, circulating endothelial cells have been detected in a broad variety of vascular diseases (7). Little, if anything, is currently known about pathogenetic implications of these cells. Mechanisms of detachment are also unknown (8). Figure 1 highlights fields of uncertainty.
We have studied the vascular biology of ANCA-associated small-vessel vasculitis since 2000 (1). In our first study, we detected markedly elevated cell numbers in patients with active disease. We could demonstrate that cell numbers decline during the course of immunosuppressive treatment. Preliminary studies showed a necrotic phenotype in the vast majority of cells (2). In a subsequent study, we evaluated patients with relapse and granulomatous disease. Our results indicate that elevated counts of circulating endothelial cells discriminate between vasculitis and granulomatous inflammation (3).
In this setting, microvascular endothelial cells are important for several reasons. First, endothelial cells serve as a barrier between donor cells within the graft and circulating leukocytes of the recipient. Secondly, endothelial cells of the recipient are targets of drug toxicity and cardiovascular disease is a major cause of morbidity and mortality in these patients. Finally, endothelial cells may become the target of immune events in acute vascular rejection (figure 2). We first studied a group of renal transplant recipients and demonstrated markedly elevated cell numbers in vascular rejection. Surprisingly, all other renal transplant recipients had elevated cell numbers as well (1). We speculated that this finding reflects calcineurin inhibitor toxicity. We therefore studied renal transplant recipients who were not on calcineurine inhibitors and compared cell numbers with a group of matched renal transplant recipients who were receiving these drugs. We were able to demonstrate significantly lower cell numbers in patients without calcineurine inhibitors (2). These finding is in keeping with our hypothesis.
Microvascular endothelial cells are crucial in this setting because a variety of complications occur here. Veno-occlusive disease of the liver, capillary leak syndrome and thrombotic microangiopathy come to mind (figure 3). Graft-versus-host disease has been demonstrated to be a disorder of microvascular endothelial cells. We were able to demonstrate markedly elevated circulating endothelial cells after conditioning, indicating severe endothelial damage. Interestingly, we noted a dose effect in that patients with reduced intensity conditioning had significantly lower cell numbers (1). Further studies must now show whether elevated cell numbers predict vascular complications (2).