Platelet-activating factor drives eotaxin production in an allergic pleurisy in mice

Abstract

1. The activation of eosinophils via G-protein-coupled seven transmembrane receptors play a necessary role in the recruitment of these cells into tissue. The present study investigates a role for PAF in driving eotaxin production and eosinophil recruitment in an allergic pleurisy model in mice. 2. The intrapleural injection of increasing doses of PAF (10(-11) to 10(-9) moles per cavity) induced a dose- and PAF receptor-dependent recruitment of eosinophils 48 h after stimulation. 3. Intrapleural injection of PAF induced the rapid (within 1 h) release of eotaxin into the pleural cavity of mice and an anti-eotaxin antibody effectively inhibited PAF-induced recruitment of eosinophils. 4. Eosinophil recruitment in the allergic pleurisy was markedly inhibited by the PAF receptor antagonist UK-74,505 (modipafant, 1 mg kg(-1)). Moreover, recruitment of eosinophils in sensitized and challenged PAF receptor-deficient animals was lower than that observed in wild-type animals. 5. Blockade of PAF receptors with UK-74,505 suppressed by 85% the release of eotaxin in the allergic pleurisy. 6. Finally, the injection of a sub-threshold dose of PAF and eotaxin cooperated to induce eosinophil recruitment in vivo. 7. In conclusion, the production of PAF in an allergic reaction could function in multiple ways to facilitate the recruitment of eosinophils -- by facilitating eotaxin release and by cooperating with eotaxin to induce greater recruitment of eosinophils.

Figure 1

Dose-dependent effects of PAF on the recruitment of eosinophils to the pleural cavity of mice. In dose-response experiments, phosphate-buffered saline (PBS, 100 μl) or PAF (10⁻¹¹ to 10⁻⁹ moles per cavity) were administered into the pleural cavity of naïve mice and the number of infiltrating eosinophils assessed 48 h after injection. The results are expressed as mean±s.e.mean of six mice in each group. *P<0.01 when compared to PBS-injected animals.

Figure 2

Time-course of the release of eotaxin into the pleural cavity after administration of PAF and effects of an anti-eotaxin polyclonal antibody on PAF-induced eosinophil recruitment. (A) Naïve mice were given an i.pl. injection of PAF (10⁻⁹ moles per cavity) and at different times after challenge (1 – 24 h), the pleural cavity of animals were washed, the cells centrifuged, and the supernatants used for the determination of eotaxin using a specific ELISA. (B) Naïve mice were pretreated with non-immune IgG (100 μg, i.p.) or purified anti-eotaxin polyclonal antibody (100 μg, i.p.) 30 min before the i.pl. injection of PAF (10⁻⁹ moles per cavity) and the number of infiltrating eosinophils assessed after 48 h. Results are expressed as mean±s.e.mean of six mice in each group. *P<0.01 when compared to PBS-injected mice and #P<0.01 when compared to animals treated with non-immune IgG.

Figure 3

Effects of the PAF receptor antagonist, UK-74,505, and of PAF receptor deficiency on the eosinophil recruitment induced by allergen challenge in sensitized mice. (A) Mice were pretreated with UK-74,505 (1 mg kg⁻¹, i.p.) 60 min before the i.pl. injection of antigen (OVA, 1 μg per cavity) in sensitized mice and the number of infiltrating eosinophils assessed after 48 h. (B) Immunized wild-type or PAFR^−/− received an i.pl. injection of antigen (OVA, 1 μg per cavity) and the number of infiltrating eosinophils assessed after 48 h. Results are expressed as means±s.e.mean of six mice in each group. *P<0.01 when compared to PBS-injected mice and #P<0.01 when compared to animals treated with drug vehicle.

Figure 4

Effects of the PAF receptor antagonist, UK-74,505, on the release of eotaxin induced by allergen challenge in sensitized mice. Mice were pretreated with UK-74,505 (1 mg kg⁻¹, i.p.) 60 min before the i.pl. injection of antigen (OVA, 1 μg per cavity) in sensitized mice and the levels of eotaxin on the pleural wash assessed after 6 h. Results are expressed as means±s.e.mean of six mice in each group. *P<0.01 when compared to PBS-injected mice and #P<0.01 when compared to animals treated with drug vehicle.

Figure 5

Cooperative effects of the injection of sub-threshold doses of eotaxin and PAF on eosinophil recruitment into the pleural cavity of mice. Mice were injected with phosphate-buffered saline (PBS, 100 μl), eotaxin (10 ng), PAF (10⁻¹¹ moles) or with PAF and eotaxin and the number of infiltrating eosinophils assessed after 48 h. The results are expressed as means±s.e.mean of six mice in each group. *P<0.01 when compared to mice injected with PBS or the mediators alone.