PGE₂ and its receptors during macrophage maturation. (A) PGE₂ is present during macrophage differentiation. Freshly flushed bone marrow cells were cultured for up to 6 days in the presence of 30% L929 supernatant; after 3 days, cultures were replenished with new medium totaling 50% of original volume. Day 0 indicates 30% L929 supernatant alone in the absence of cells. Each day of culture, an aliquot of cell supernatant was aspirated and analyzed for PGE₂ concentration by ELISA. Data are expressed as the mean ± SEM from 6 to 8 experiments, each using cells from a single mouse. (B) EP2 is most highly expressed among EP receptors during macrophage maturation. At each of days 3 to 6 of cell culture (as described in panel A), adherent cells were collected and analyzed for the expression of EP receptors with qRT-PCR. Data presented in all panels are expressed as the mean ± SEM from 3 to 4 experiments, each using cells from a single mouse (*P < .05, # indicates significant [P < .05] difference from all other conditions).