Subepithelial fibrosis is an important feature in airway remodelling which occurs in the course of allergic asthma (Roche et al., 1989). NO has been shown to be an important mediator in the control of different processes involved in tissue repair and remodelling (Schwentker & Billiar, 2003). Therefore, the effects of Th2 cytokines, which are known to play a key role in the pathogenesis of allergic asthma (Wills-Karp, 1999), were studied on the expression of iNOS and on NO synthesis in airway fibroblasts.

Primary airway fibroblasts were obtained from isolated trachea of Sprague Dawley rats of either sex (160-200 g) by an outgrowth technique in the presence of fetal calf serum (FCS, 10%). (Lindemann & Racké, 2003). Passages 2-3 were used for experiments. After trypsinisation, the cells were disseminated (0.25*10⁶ cells well^-1) and cultured for 24 h in DME/F-12 medium containing 10% FCS. Thereafter, the cells were cultured for additional 40 h in the presence of test substances. Nitrite accumulated in the culture media was determined as a measure of NO synthesis. In addition, total RNA was isolated and used for RT-PCR with primers specific for iNOS and ß-actin (Klasen et al., 2001).

In fibroblasts cultured under control conditions mRNA for iNOS was clearly expressed and this correlated with a significant accumulation of nitrite 7.5 ± 1.3 µM (mean ± s.e.mean, n=15). Presence of IL-4 or IL-13 (10 ng ml^-1, each) inhibited the nitrite accumulation by 46 ± 5 and 54 ± 6%, respectively (each n=6, p < 0.01 vs. controls). IL-4 and IL-13 caused a marked reduction of the expression of iNOS mRNA. The optical density of the PCR amplification products was determined and the ratio iNOS*100/ß-actin used as a semi-quantitative measure. This ratio amounted to 70.3 ± 11.3 in controls (n=6) and was reduced to 27.6 ± 10.9 (n=6, p < 0.05 vs. controls) by IL-4 and to 32.8 ± 9.4 by IL-13 (n=6, p < 0.05 vs. controls). Dexamethasone (1 µM) reduced the spontaneous nitrite accumulation by 39 ± 4% (n=6, p<0.05 vs controls). When dexamethasone was present together with IL-4 or IL-13 the inhibitory effects of these cytokines were not significantly affected (45 ± 13% and 33 ± 6% inhibition of nitrite accumulation, respectively, each n=6).

Since we very recently showed that Th2 cytokines can up-regulate arginase in airway fibroblasts (Lindemann & Racké, 2003), it is concluded, that in airway fibroblasts the two L-arginine dependent pathways iNOS and arginase are differentially affected Th2 cytokines.

Klasen, S. et al. (2001) Br. J. Pharmacol., 132, 1349-1357.
Lindemann, D. & Racké, K. (2003) Naunyn-Schmiedeberg's Arch. Pharmacol., 368, 546-550.
Roche, W.R. et al. (1989) Lancet, 1(8637), 520-524.
Schwentker, A. & Billiar, T.R. (2003) Surg. Clin. North Am., 83,521-530.
Wills-Karp, M. (1999) Annu. Rev. Immunol., 17,255-281.