Platelet depletion abolishes leukocyte recruitment to the peritoneum in a murine model of inflammation

Pitchford SC, Banks M, Kornerup K, Page C. The Sackler Institute of Pulmonary Pharmacology, GKT School of Biomedical Sciences, King’s College, London. SE1 9RT.

Platelet depletion has been shown to inhibit leukocyte recruitment in animal models of allergic inflammation (Coyle et al., 1990; Pitchford et al., 2003), although a requirement for platelets to facilitate leukocyte recruitment in other inflammatory diseases is unknown, although platelets form complexes to monocytes in patients with rheumatoid arthritis and atherosclerosis (Joseph et al., 2001; Furman et al., 1998), and activated platelets promote monocyte arrest to murine atherosclerotic carotid arteries in vivo (Huo et al., 2003). The effects of thrombocytopaenia, induced using either an immunological or non-immunological method of platelet depletion on monocyte recruitment were therefore studied in a murine model of interleukin-1 (IL-1) induced inflammation to the peritoneum.

Female BALB/C mice (n= 4-6) were administered IL-1, 10 µg/ mouse i.p. in 0.2ml saline or saline alone. A preliminary time course revealed monocyte recruitment peaked at 24 hours post IL-1 administration, and this time point was thus used in successive experiments. Some groups were selectively depleted of platelets via busulfan administration (non-immunological method of platelet depletion. 25mg/kg in 12.5%PEG400 0.2ml i.p.) on days –12, -10, and -7 before IL-1 administration, or via anti-CD41 (immunological method of platelet depletion, rat-anti-mouse alpha II beta, MWReg30 Santa Cruz biotech USA) administered i.m. into the hind limb 24 hours before IL-1 induced peritonitis. Peritoneal lavage fluid samples were taken 24 hours following IL-1 administration and total and differential leukocyte counts were obtained. Data were analysed using one way ANOVA and Bonferroni multiple comparison tests between groups.

IL-1 induced a significant increase in monocyte recruitment to the peritoneal cavity at 24 hours post administration (Busulfan experiment, saline: 9.9±2.6x106 cells ml-1 vs IL-1+ PEG400: 23.3±4.3x106 cells ml-1 p<0.05; anti-CD41 experiment: saline: 6.7±1.3x106 cells ml-1 vs IL-1+ Control IgG: 11.7±1.5x106 cells ml-1 p<0.05). Monocyte accumulation was significantly reduced in mice treated with busulfan (12.9±2.1x106 cells ml-1 p<0.05) and anti-CD41 (6.6±1.4x106 cells ml-1 p<0.05). Both busulfan and anti-CD41 treated mice exhibited >90% platelet depletion without affecting circulating leukocyte numbers (IL-1+PEG400: 1.2±0.6x107 cells ml-1 vs IL-1+Busulfan: 0.9±0.3x107 cells ml-1 ns; IL+control IgG: 1.0±0.3x107 cells ml-1 vs IL-1+antiCD41: 0.9±0.2x107 cells ml-1 ns).

This study reveals a requirement for platelets in monocyte recruitment in a murine model of peritonitis.

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This work was funded by Pfizer Ltd.