Expression of FcRn in human blood monocytes, NK cells, and PMNs. (A) FcRn mRNA and protein levels in freshly isolated human monocytes, NK cells, and PMNs as measured by real-time quantitative RT-PCR (AU defined as a ratio of ABL expression) and flow cytometry (geometric mean fluorescence). FcRn expression in cells was detected by mAb 22H4C11 in fixed and permeabilized cells. (B) FcRn is localized intracellularly in resting PMNs. Freshly isolated PMNs or paraformaldehyde-fixed and saponin-treated PMNs were stained with mouse anti-FcRn mAb 1G3 and measured by FACS to detect extracellular and intracellular FcRn levels, respectively. (A-B) Data are presented as means plus standard deviations. Data are representative of at least 5 experiments.