Three different classes of Fc receptors for IgG (Fc gamma R) are currently distinguished in humans, of which polymorphonuclear phagocytes (PMN) normally express both low-affinity receptor classes-- Fc gamma RII (CD32) and Fc gamma RIII (CD16). During therapy with granulocyte colony-stimulating factor (G-CSF), neutrophils from patients with various malignancies and different hematologic disorders were found to additionally express high levels of the receptor with high affinity for IgG (Fc gamma RI; CD64). For these patients, the relative fluorescence intensity (rFI) for Fc gamma RI was 5.3 (range, 1.7 to 10.3; n = 19), compared with 1.0 (range, 1.0 to 1.1; n = 8) for healthy donors. The expression of Fc gamma RI during G-CSF therapy could be confirmed by using a panel of six CD64-specific antibodies, and by showing mRNA for Fc gamma RI. So far, three genes for Fc gamma RI have been identified, encoding four distinct transcription products. By reverse transcriptase-polymerase chain reaction technology, transcripts for both membrane-associated isoforms (hFc gamma RIa and hFc gamma RIb2) could be detected. The functional activity of Fc gamma RI on PMN during G-CSF therapy was shown by measuring binding of monomeric human IgG and antibody-dependent cellular cytotoxicity (ADCC). Thus, Fc gamma RI-positive neutrophils displayed enhanced ADCC activity to glioma (A1207), squamous cell (A431), and ovarian (SK-ov3) carcinoma cell lines. The involvement of Fc gamma RI in this increased cytotoxic activity was shown by blocking Fc gamma receptors with monoclonal antibodies, and by using F(ab')2 x F(ab')2-bispecific antibodies with specificities against tumor-related antigens and Fc gamma RI, resulting in solely Fc gamma RI-mediated cytotoxicity. Therapeutically, this additional Fc receptor on PMN may increase the efficacy of experimental antibody therapy.

In this study we have examined hFc gamma RI expression during myelopoiesis. Normal bone marrow (BM) cells were found to express hFc gamma RI up to the metamyelocyte stage. A different Fc gamma RI expression pattern was observed in an in vitro model of myelopoiesis. Purified CD34-positive BM cells, cultured for 12 to 14 days with granulocyte colony-stimulating factor (G-CSF), differentiate into a population of mature granulocytic cells. In these cultures, in which hFc gamma RI was virtually absent on the initial CD34-positive BM cells, hFc gamma RI was strongly induced by G-CSF after only 5 days. During final maturation the cells remained hFc gamma RI positive. This expression was confirmed functionally by antibody-sensitized erythrocytes (EA)-rosette assays. Moreover, the mature myeloid cells were found to express mRNA encoding for hFc gamma RI, whereas reverse- transcriptase polymerase chain reaction analysis showed that both hFc gamma RIA and hFc gamma RIB genes were expressed. In contrast, on peripheral blood (PB) polymorphonuclear neutrophil leukocytes (PMN) the in vitro effect of G-CSF as to hFc gamma RI induction was limited. Therefore, we conclude that, with respect to hFc gamma RI expression on PMN, G-CSF acts on myeloid precursor cells rather than on mature cells. This conclusion could be strengthened by in vivo administration of a single dose of G-CSF to a healthy volunteer. After a 12-hour lag time, hFc gamma RI expressing PMNs were detected in the peripheral blood. This study shows that hFc gamma RI is an early myeloid differentiation marker that is lost during normal final maturation. However, committed myeloid progenitor cells can be strongly induced by G-CSF to express hFc gamma RI, ultimately resulting in mature granulocytic cells expressing the high-affinity receptor for IgG. This expression may have important consequences for the functional capacity of these cells.

In this study we have examined hFc gamma RI expression during myelopoiesis. Normal bone marrow (BM) cells were found to express hFc gamma RI up to the metamyelocyte stage. A different Fc gamma RI expression pattern was observed in an in vitro model of myelopoiesis. Purified CD34-positive BM cells, cultured for 12 to 14 days with granulocyte colony-stimulating factor (G-CSF), differentiate into a population of mature granulocytic cells. In these cultures, in which hFc gamma RI was virtually absent on the initial CD34-positive BM cells, hFc gamma RI was strongly induced by G-CSF after only 5 days. During final maturation the cells remained hFc gamma RI positive. This expression was confirmed functionally by antibody-sensitized erythrocytes (EA)-rosette assays. Moreover, the mature myeloid cells were found to express mRNA encoding for hFc gamma RI, whereas reverse- transcriptase polymerase chain reaction analysis showed that both hFc gamma RIA and hFc gamma RIB genes were expressed. In contrast, on peripheral blood (PB) polymorphonuclear neutrophil leukocytes (PMN) the in vitro effect of G-CSF as to hFc gamma RI induction was limited. Therefore, we conclude that, with respect to hFc gamma RI expression on PMN, G-CSF acts on myeloid precursor cells rather than on mature cells. This conclusion could be strengthened by in vivo administration of a single dose of G-CSF to a healthy volunteer. After a 12-hour lag time, hFc gamma RI expressing PMNs were detected in the peripheral blood. This study shows that hFc gamma RI is an early myeloid differentiation marker that is lost during normal final maturation. However, committed myeloid progenitor cells can be strongly induced by G-CSF to express hFc gamma RI, ultimately resulting in mature granulocytic cells expressing the high-affinity receptor for IgG. This expression may have important consequences for the functional capacity of these cells.