Leukocyte Integrin Activation and Alterations in Parathyroid Hormone Levels Following G-CSF Administration

Abstract 2164

Following administration of granulocyte colony-stimulating factor (G-CSF) there is a marked but transient drop in circulating neutrophils. To determine the contribution of integrin activation to these events, a monoclonal antibody (mAb 24) that recognizes the activated α-subunit cation binding domain of the integrin family of receptors was used to evaluate human leukocytes following G-CSF administration. A single 480 mcg dose of human recombinant G-CSF (Neupogen, Amgen) was administered in the upper arm of six healthy human subjects (3 male, 3 female). EDTA and heparinized blood samples were collected prior to and at 15, 30, 60, 90, 120 and 240 minutes after injection. Mean absolute neutrophil count decreased from 2,557 ± 654/μl at baseline to 227 ± 193 (p<0.001) at 30 and 45 minutes. Flow cytometric analysis revealed a dramatic increase in binding of mAb24, which recognizes the occupied divalent cation binding site of the activated integrin α subunit, to neutrophils but not other circulating nucleated cells within 15 to 30 minutes after G-CSF administration, concurrent with a dramatic decrease in circulating neutrophil numbers. This activation, like the drop in neutrophil count, was transient, with a return to baseline status by 60 minutes. Intact parathyroid hormone (iPTH) levels were also evaluated. Within 30–90 minutes following G-CSF injection there was a significant drop in iPTH from a baseline of 52.5 ± 31.1 to a nadir value of 30.6 ± 20.9 pg/ml (p=0.01). A rebound in iPTH levels to 63.4 ± 27.0 pg/ml (p<0.001) occurred at 90 to 240 minutes. There were no significant changes in ionized calcium or magnesium levels. These results demonstrate that the integrin on circulating neutrophils is transiently activated following G-CSF administration and that this activation may mediate the transient neutropenia seen in this setting. In addition, alterations in PTH levels occur following G-CSF administration. As it is known that iPTH influences the hematopoietic stem cell (HSC) niche, fluctuations in iPTH levels following G-CSF administration may play a role in HSC expansion.