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We have analyzed the effect of cellular density of 5-Lipoxygenase (5-LO), 5-lipoxygenase-activating protein (FLAP) and interleukin-1β (IL-1β) gene expression in neutrophils from healthy subjects under culture conditions of low and high cell density. By using RT-PCR techniques, we have found that 5-LO mRNA accumulation decreased in cells cultured at high density, while FLAP mRNA is not affected. De novo 5-LO synthesis, as well as steady-state levels, were reduced in cells maintained at high density. In contrast, the high density conditions lead to the induction of IL-1β gene at the RNA and protein levels as measured by RT-PCR and by immunoprecipitation. These results suggest that cellular density plays a role in gene modulation when neutrophils are accumulating at an inflammatory site since neutrophils obtained from the synovial fluid of patients with RA exhibit a protein synthesis profile similar to that observed in peripheral blood neutrophils cultured at high density.