A role for endogenous histamine in interleukin‐8‐induced neutrophil infiltration into mouse air‐pouch: investigation of the modulatory action of systemic and local dexamethasone

Abstract

1 When injected into a 6-day-old mouse air-pouch, human recombinant interleukin-8 (IL-8; 0.03–3 μg) induced, in a dose-dependent fashion, an accumulation of neutrophils which could be reliably assessed 4 h after the injection. No protein extravasation was measured above the values obtained with the vehicle alone (carboxymethylcellulose, CMC, 0.5% w/v in phosphate-buffered solution, PBS). 2 The IL-8 effect (routinely evaluated at 1 μg dose) was inhibited neither by local administration of actinomycin D (1 μg) nor by systemic treatment with indomethacin (1 mg kg⁻¹, i.v.), BWA4C (5 mg kg⁻¹, p.o.), methysergide (6 mg kg⁻¹, i.p.) and RP67580 (2 mg kg⁻¹, i.p.). 3 Treatment of mice with the H₁ antagonist, mepyramine (1–10 mg kg⁻¹, i.p.) resulted in a dose-dependent inhibition of the cell accumulation elicited by the chemokine, with a maximal reduction of approximately 50–60%. The mepyramine effect was not due to a non specific reduction of neutrophil function, since treatment with this drug (6 mg kg⁻¹, i.p.) did not modify the cell infiltration measured in response to a challenge with interleukin-1β (20 ng) or with the vehicle CMC to any extent. Moreover, treatment of mice with mepyramine did not modify cell counts in a peripheral blood film with respect to controls. Two other H₁ antagonists, chemically unrelated to mepyramine, diphenhydramine (9 mg kg⁻¹, i.p.) and triprolidine (0.5 mg kg⁻¹, i.p.), inhibited IL-8-induced migration to a similar extent (∼ 50–60%), whereas the H₂ antagonist, ranitidine (5 mg kg⁻¹, i.p.) was without effect. 4 The concept that endogenous histamine could be involved in the IL-8 effect was strengthened in two ways: (i) addition of histamine (0.2–2 μg) to a small dose of IL-8 (0.3 μg) potentiated the cell elicitation induced by the chemokine without having any effect on its own; (ii) IL-8-induced neutrophil accumulation was greatly impaired in animals depleted of mast cell amines by sub-chronic (5 day) treatment with compound 48/80 according to an established protocol. 5 The glucocorticoid dexamethasone (Dex; 1–50 μg per mouse, i.v., corresponding approximately to 0.03–1.5 mg kg⁻¹, given i.v. 2 h prior to challenge with IL-8) potently inhibited neutrophil infiltration with an approximate ED₅₀ of 5 μg per mouse (∼ 0.3 mg kg⁻¹, i.v.). Passive immunisation of mice with a polyclonal sheep serum raised against the steroid-inducible anti-inflammatory protein lipocortin 1 (LC1) abolished the inhibitory action of Dex whereas a control serum was without effect. 6 Local administration of Dex at a dose which was ineffective when given systemically (1 μg) also reduced neutrophil migration induced by IL-8, either alone or in combination with histamine. This local inhibition (∼ 50%), also seen with hydrocortisone (30 μg), was prevented by the concomitant administration of the steroid antagonist RU38486 (10 μg) indicating the involvement of glucocorticoid receptor in the response. 7 These findings characterize further the mechanisms underlying PMN recruitment induced by IL-8 in vivo, and point to a role for histamine. The anti-inflammatory action of the glucocorticoids, as in some other models, appears to be LC1-dependent when these drugs are given systemically and LC1-independent when the steroids are given locally.