Role of tryptase in immediate cutaneous responses in allergic sheep

In this study, we used a specific tryptase inhibitor, APC-366 [N-(1- hydroxy-2-napthoyl)-L-arginyl-L-prolinamide hydrochloride] to investigate the effect of intradermally administered tryptase and tryptase released by antigen challenge on the immediate cutaneous reaction (ICR) in allergic sheep. The surface areas of cutaneous wheals produced by intradermal injections (0.05 ml) of 1 and 10 ng tryptase alone, tryptase combined with 3 U heparin (tryptase-heparin), or Ascaris suum antigen (10^-5 dilution) with or without pretreatment with APC-366 (1 mg/ml) were measured at 20 and 60 min after challenge. Intradermal injections of 1 and 10 ng tryptase alone (n = 7) produced an ICR of ≤20% of that obtained after injection of histamine (5% wt/vol). Intradermal injection of tryptase-heparin (n = 7), however, resulted in 50 (1 ng) and 82% (10 ng) of the ICR to histamine (both, P < 0.05 vs. tryptase alone). APC-366 inhibited (P < 0.05) the ICR to 1 and 10 ng tryptase-heparin by ≥70% at all times (n = 8) but had no effect on the histamine-induced ICR (n = 3). A combination of the histamine H₁ antagonist chlorpheniramine (2 mg/kg iv) and the H₂ antagonist metiamide (3 mg/kg iv) given 40 min before challenge (n = 8) inhibited the response to 1 and 10 ng tryptase-heparin by 42 and 62% at 20 min and by 96 and 86% at 60 min, respectively (all, P < 0.05). APC-366 also blocked the ICR to A. suum antigen by 68% (P < 0.05) in nine sheep. These results indicate that intradermal injection of tryptase-heparin can induce an ICR. This ICR can be inhibited by APC-366 or a combination of the histamine H₁ and H₂ antagonists, suggesting that the tryptase response is mediated by histamine. APC-366 also blocks the mast cell-mediated ICR to intradermally injected A. suum antigen. Collectively, these results suggest that tryptase may modulate mast cell histamine release.