GM-CSF and IL-4 in combination cleave membrane-bound M-CSFR in monocytes. (A) Cell lysates and culture supernatants were harvested after culturing monocytes for 48 hours in the presence or absence of GM-CSF and IL-4 in combination. M-CSFR immunoreactivity was analyzed by immunoblotting with an antibody against an M-CSFR extracellular domain. (B) Monocytes isolated from healthy donors (n = 12) were cultured for 48 hours in the presence or absence of GM-CSF and IL-4 in combination. Soluble M-CSFR levels in the culture supernatants were measured by ELISA. Data are expressed as means ± SD; *P < .05. (C) M-CSFR mRNA expression in monocytes cultured for 6 hours in the absence or presence of GM-CSF and IL-4 in combination was quantified by real-time PCR.