Sickle Cell Patietns with a High Percentage of HBF-Containing RBC (F Cells) Have Shorter Survival of RBC that Lack HBF.

Percent fetal hemoglobin (HbF) is an important determinant of clinical severity in sickle cells disease (SCD). There is a dichotomous distribution of HbF in sickle cells, with one population containing 20–25% HbF (F cells) and another in which HbF is not detectable (nonF cells). Increased HbF in SCD is due to two factors: 1) Increased HbF synthesis, which also occurs in other conditions having markedly increased erythropoiesis; 2) Longer survival of F cells in the circulation compared to nonF cells, which appears to be of great importance in SCD. We previously showed the lifespan of biotin-labeled F cells in the circulation to be about three times longer than nonF cells. We now examine these differences in greater detail, focusing on two issues. The first is whether the range of HbF content (HbF per F cell, pg) that is presumably present in F cells influences cell survival. The second is whether the survival of F and/or nonF cells is dependent upon the fraction of F cells in the circulation. To address these questions, we used the previously described biotin label for RBC. Up to 10 ml of autologous RBC were labeled and reinfused, and overall RBC survival was defined by the time-dependent disappearance of labeled RBC from the circulation. At selected time points after reinfusion, HbF was evaluated in two ways: 1) The percentage of biotin-labeled cells that were F cells was determined by flow cytometry; 2) The biotin-labeled cells were isolated with streptavidin-coated magnetic beads and the percent HbF determined by HPLC. These two assays can be used to determine the individual survival of F and nonF cells and to calculate the HbF per F cell of labeled RBC as a function of time after reinfusion. There were 12 studies in 10 patients, including 2 who were studied before and after hydroxyurea (HU). A total of 4 patients were taking HU at the time of study. F cells ranged from 4 to 90%, including 2 patients with HU and one without who had greater than 88%. There was a time-dependent linear increase in HbF per F cell with a slope of 0.09 ± 0.07 pg/day (n = 11). This is consistent with longer survival of the F cells with higher HbF content. NonF cell survival was lower in patients with a higher percentage of F cells. Subjects with < 50% F cells had an S30 (time until 30% of the labeled RBC remain in the circulation, days) for nonF cells of 14.6 ± 2.9 days (1 SD, n = 5), whereas subjects with > 50% F cells had an S30 for nonF cells of 7.7 ± 2.9 days (P < 0.005). The range of S30 values was from 5 to 17 days, and there was a linear correlation between the S30 of nonF cells and % F cells with R² = 0.65. For a given % F cells, there appeared to be no dependence on HU. These data indicate that the survival of nonF cells (but not F cells) is dependent on the percentage of F cells. Possible reasons for this include 1) the presumed higher oxygen affinity of F cells, leading to lower venous PO2 and thus increased sickling and decreased survival of the nonF cells, and 2) a more sensitive detection of damaged RBC by the RES when RBC turnover is lower due to a high percentage of F cells, again leading to decreased survival of the nonF cells. This could have important implications in chronic transfusion therapy, in which overall RBC turnover is decreased by the presence of donor cells with relatively high oxygen affinity.