Sickle cell anemia as an inflammatory disease.

The classical view of sickle cell anemia has always focused on the primary genetic defect — the abnormal sickle hemoglobin that polymerizes when deoxygenated. Polymerization within the red cell causes it to deform, to become rigid, to obstruct blood flow, and to produce acute and chronic tissue damage because of poor perfusion. A more holistic view sees the sickle red cell with its abnormal contents and membrane in a larger context — as it interacts with, damages, and stimulates the vascular endothelium and surrounding tissues and cells. This view allows examination of the hypothesis that the sticky, stiff, oxidizing sickle red cell is an irritant that provokes an inflammatory response as it obstructs flow. The tissues are not only underperfused, but also exposed to inflammatory cytokines, growth factors, and the actions of the activated cells that respond to and produce them. The article by Kaul and Hebbel in this issue of the JCI (1) suggests that reperfusion injury plays a major role in sickle pathophysiology and provides new insights that contribute to this broader view of the disease process. Using a transgenic sickle mouse model and directly visualizing the microcirculation in a living open cremaster muscle preparation, the authors make three key observations that implicate reperfusion injury (1). First, after 3 hours of mild hypoxia followed by reoxygenation, the vascular bed of the sickle mouse (but not control) showed a distinct inflammatory response with increased leukocyte rolling, adhesion, and emigration. Second, the reoxygenation resulted in conversion in endothelial cells of the oxidant-sensitive probe dihydrorhodamine to the fluorescent compound rhodamine, suggesting local production of H2O2. Third, the abnormal rolling, adhesion, and migration of leukocytes were prevented by infusion with a monoclonal murine P-selectin antibody before reoxygenation. This theme of reactive oxygen species generation and Pselectin–sensitive leukocyte adhesion and migration is consonant with a variety of reperfusion injury models. The implication is that once oxygen is restored to ischemic tissues, oxygen radicals are formed, and inflammatory endothelial and tissue injury occurs. This is a particularly attractive model for sickle cell anemia — especially for the chronic subliminal organ damage that typically occurs in the spleen, lung, and kidney. In the vascular beds of these organs, young reticulocytes periodically attach to endothelial cells and, if conditions are right, cause a transient logjam of relatively rigid deoxygenating mature red cells in their wake (2). As the obstruction eventually clears, the reperfusion pathophysiology plays out, escalating the likelihood of additional rounds of reticulocyte adhesion to the inflamed endothelium that is becoming increasingly decorated with large unyielding leukocytes. These repetitive episodes of localized ischemia and reperfusion can set up a low-grade chronic inflammatory tissue-injuring state.