Activity of supernatants from monosodium urate (MSU) crystal-stimulated monocytes (SMMs) on CD11b membrane expression on human neutrophils. Monocytes were incubated for 18 hours with MSU crystals (1 mg/mL) with or without alpha-melanocyte-stimulating hormone (α-MSH) (10-6 M) or (CKPV)2 (10-6 M). Purified neutrophils were pretreated with the different SMMs or N-formyl-methionyl-leucyl-phenylalanine (FMLP) (30 minutes at 37°C) before staining with the phycoerythrin-conjugated anti-CD11b monoclonal antibody. The results are expressed as mean fluorescence intensity (MFI) ± standard error of the mean, corrected for non-specific staining, from five separate experiments. °P < 0.001 versus RPMI; *P < 0.05 and **P < 0.01 versus SMMs.