Changes of red blood cell surface markers in a blood doping model of neocytolysis.

BACKGROUND: Neocytolysis, the selective hemolysis of young circulating red blood cells (RBCs), contributes to the physiologic control of red cell mass and to pathophysiologic phenomena such as anemia of renal disease, anemia after spaceflight, and blood doping by athletes. Progress in understanding the process is hampered by the lack of established markers to distinguish young from older RBC. METHODS: Twelve potentially informative RBC surface markers were assayed by flow cytometry in normal blood samples, and 4 were preferentially expressed in young RBC. To create a model of neocytolysis, 3 normal volunteers had recombinant human erythropoietin (rhEpo) administered until mild erythrocytosis occurred, then were studied upon rhEpo withdrawal. RESULTS: Neocytolysis ensued that most evident from a rapid rise in serum ferritin as the iron from young RBC was transferred back to stores. Five additional volunteers had surface markers monitored during and after rhEpo administration. Three subjects with marginal baseline iron stores had blunted response to rhEpo, no significant neocytolysis, and no change in RBC surface marker expression. Two subjects with adequate baseline iron stores developed erythrocytosis followed by neocytolysis. Decreased expression of CD44 (homing-associated cell adhesion molecule) and CD71 (transferrin receptor) seemed to correlate best with neocytolysis; CD35 (complement receptor) less so. Of note, further studies are needed to determine if these changes are causative of red cell destruction. CONCLUSION: This study begins to establish a human model of neocytolysis, to establish markers differentiating young and old RBC, and to establish a basis for better definition of the process. Although our study is preliminary, the results support the possibility that flow could be useful to detect blood doping because neocytolysis should predictably occur in athletes who surreptitiously blood dope.

Implications of neocytolysis for optimal management of anaemia in chronic kidney disease.

Erythropoietin is the major hormone regulator of erythrocyte production promoting the survival, as well as the differentiation and maturation, of erythroid progenitor cells. In addition to these well-characterized effects, it appears that an erythropoietin-responsive non-erythroid mechanism also mediates the selective destruction of young circulating erythrocytes (neocytes) when red cell mass becomes excessive - a process termed 'neocytolysis'. Endothelial cells appear to respond to a rapid decrease in circulating levels of erythropoietin by secreting cytokines (including TGF-alpha), which signal reticuloendothelial phagocytes to destroy neocytes. The result is a more rapid decrease in red cell mass than can be explained by natural erythrocyte senescence alone. The current pharmacologic approach to treatment of anaemia in chronic kidney disease may cause neocytolysis and could keep therapy from reaching its full potential. Understanding neocytolysis and its relationship to fluctuating serum erythropoietin levels might help to better understand optimal treatment with erythropoietic agents.

The negative regulation of red cell mass by neocytolysis: physiologic and pathophysiologic manifestations.

We have uncovered a physiologic process which negatively regulates the red cell mass by selectively hemolyzing young circulating red blood cells. This allows fine control of the number of circulating red blood cells under steady-state conditions and relatively rapid adaptation to new environments. Neocytolysis is initiated by a fall in erythropoietin levels, so this hormone remains the major regulator of red cell mass both with anemia and with red cell excess. Physiologic situations in which there is increased neocytolysis include the emergence of newborns from the hypoxic uterine environment and the descent of polycythemic high-altitude dwellers to sea level. The process first became apparent while investigating the mechanism of the anemia that invariably occurs after spaceflight. Astronauts experience acute central plethora on entering microgravity resulting in erythropoietin suppression and neocytolysis, but the reduced blood volume and red cell mass become suddenly maladaptive on re-entry to earth's gravity. The pathologic erythropoietin deficiency of renal disease precipitates neocytolysis, which explains the prolongation of red cell survival consistently resulting from erythropoietin therapy and points to optimally efficient erythropoietin dosing schedules. Implications should extend to a number of other physiologic and pathologic situations including polycythemias, hemolytic anemias, 'blood-doping' by elite athletes, and oxygen therapy. It is likely that erythropoietin influences endothelial cells which in turn signal reticuloendothelial phagocytes to destroy or permit the survival of young red cells marked by surface molecules. Ongoing studies to identify the molecular targets and cytokine intermediaries should facilitate detection, dissection and eventual therapeutic manipulation of the process.