Heterogeneity of F cells in ß-thalassemia.

ABSTRACT

BACKGROUND: Fetal hemoglobin (HbF), which is largely replaced after birth by the adult Hb, is concentrated in a few "F cells." Their number significantly increases in certain physiologic and clinical situations, including in ß-thalassemia (ß-thal). Their quantification is used to detect fetal-maternal hemorrhage (FMH), where fetal cells enter the maternal circulation. We were confronted with a pregnant woman with ß-thal who was suspected to have FMH. To establish the usefulness of a flow cytometric procedure to differentiate between fetal cells and the maternal F cells, we screened adult ß-thal patients. STUDY DESIGN AND METHODS: Blood samples were simultaneously stained with fluorescent antibodies to HbF and to carbonic anhydrase (CA) isotype II, which is specific to adult red blood cells (RBCs). RESULTS: A heterogeneous distribution of RBCs with respect to HbF and CA expression was observed adult non-F cells (CA+HbF-) and F cells (CA+HbF+/HbF++) as well as F cells with characteristics of fetal cells (CA-HbF++). CONCLUSIONS: The presence of CA-HbF++ RBCs in nonpregnant women, and even men, with thal indicates that the CA/HbF method is inappropriate for detection of FMH. The coexistence of F cells carrying fetal or adult markers suggests that they originate from two types of stem cell, adult and fetal, lineages. Normally, the fetal lineage is insignificant, but in ß-thal, as HbF-containing RBCs have a selective advantage, the "fetal" lineage gains significance.