The role played by endogenous lipocortin 1 in the anti‐migratory action exerted by dexamethasone (Dex) on monocyte recruitment in an in vivo model of acute inflammation was investigated by use of several neutralizing polyclonal antibodies raised against lipocortin 1 or a lipocortin 1‐derived N‐terminus peptide (peptide Ac2‐26). The efficacy of peptide Ac2‐26 in inhibiting monocyte and polymorphonuclear leucocyte (PMN) recruitment was also tested.

Intraperitoneal (i.p.) injection of zymosan A (1 mg) produced a time‐dependent cell accumulation into mouse peritoneal cavities which followed a typical profile of acute inflammation: PMN influx was maximal at 4 h post‐zymosan (between 15 and 20×10⁶ cells per mouse), and this was followed by an accumulation of monocytes which peaked at the 24 h time‐point (between 10 and 15×10⁶ cells per mouse).

Dex administration to mice reduced zymosan‐induced 4 h PMN infiltration and 24 h monocyte accumulation with similar efficacy: approximately 50% of inhibition of recruitment of both cell types was achieved at the dose of 30 μg per mouse (∼1 mg kg⁻¹, subcutaneously (s.c.)). Maximal inhibitions of 64% and 67% on PMN and monocyte recruitment, respectively, were measured after a dose of 100 μg per mouse (∼3 mg kg⁻¹, s.c.).

Dex (30 μg s.c.) inhibited monocyte (53%) and PMN (69%) accumulation in response to zymosan application in mice which had been treated with a non‐immune sheep serum (50 μl s.c.). In contrast, the steroid was no longer active in reducing cell accumulation in mice which had been passively immunized against full length human recombinant lipocortin 1 (serum LCS3), or against lipocortin 1 N‐terminus peptide.

Treatment of mice with vinblastine (1 mg kg⁻¹, intravenously (i.v.)) produced a remarkable leucopenia as assessed 24 h after administration. This was accompanied by a 60% reduction in 4 h‐PMN influx, and by a 27% reduction in 24 h‐monocyte accumulation, measured after zymosan administration. The inhibitory effect of Dex on monocyte recruitment was not significantly modified in vinblastine‐treated mice, with 36% and 57% of inhibition calculated at the dose of 30 μg Dex, and 70% and 60% of inhibition at 100 μg Dex, in vehicle‐ and vinblastine‐treated mice, respectively.

Treatment of mice with peptide Ac2‐26 dose‐dependently attenuated PMN influx at 4 h post‐zymosan with a significant effect at 100 μg per mouse (45% of inhibition, n=9, P<0.05) and a maximal effect of 61% inhibition at the highest dose tested of 200 μg s.c. (n=14, P<0.05). No effect of peptide Ac2‐26 (200 μg s.c.) was seen on zymosan‐induced 24 h monocyte recruitment. In contrast, administration of 200 μg peptide Ac2‐26 every 6 h was effective in reducing the number of monocytes harvested from the inflamed peritoneal cavities at 24 h post‐zymosan: 9.40±0.58×10⁶ monocytes per mouse (n=13) and 5.74±0.34 monocytes per mouse (n=14) in vehicle‐ and peptide Ac2‐26‐treated mice, respectively (P<0.05).

Finally, peptide Ac2‐26 produced a concentration‐dependent inhibition of the rate of phagocytosis of mouse resident peritoneal macrophages as measured by flow cytometry, with a maximal reduction of 34% at the highest concentration tested of 100 μg ml⁻¹ (n=8 experiments performed in duplicate; P<0.05).

In conclusion, this study suggests that in vivo monocyte recruitment during acute inflammation is, at least in part, under the negative modulatory control of endogenous lipocortin 1 (as seen after administration of Dex by using the specific antisera) and exogenous lipocortin 1 mimetics (as observed with peptide Ac2‐26). In addition to the neutrophil, we can now propose …