Susceptibility to ovalbumin-induced airway inflammation and fibrosis in inducible nitric oxide synthetase-deficient mice: Mechanisms and consequences

In a previous study, we showed that BALB/c mice demonstrate significant increases in accumulation of airway collagen after 4 weeks of exposure to ovalbumin aerosol. In the current study we examined the response to ovalbumin aerosol of a different strain of mice, C57BL/6, and compared this response to an otherwise isogenic C57BL strain (iNOS^-/-) in which the gene for inducible nitric oxide synthetase (iNOS) had been knocked out. We hypothesized that C57BL mice, a Th-1-responsive strain, would be relatively resistant to ovalbumin exposure compared with our previous observations in the BALB/c strain, a Th-2 responder. Our results are consistent with this hypothesis, especially with respect to the accumulation of collagen in the airways of the mice exposed to ovalbumin and increased airway reactivity to challenge with methacholine, as measured by the Penh response. Since NO participates in multiple signal transduction pathways, there was no a priori reason to predict whether iNOS^-/- mice would be more or less susceptible to allergen-induced airway inflammation than their parental wild-type strain. Responses to ovalbumin exposure of the Th-1-responsive C57BL animals were significantly less (or slower) than those we observed with the iNOS^-/- mice. Significant increases in airway collagen content were seen only after 6 weeks of exposure of the C57BL mice, as contrasted with 4 weeks in the iNOS^-/- animals. At each time point examined, Penh values for the iNOS^-/- mice were significantly increased, while no increases were observed with the C57BL strain. Thus, the iNOS^-/- mice are more susceptible to ovalbumin-induced airway inflammation and fibrosis than the C57BL strain, giving results intermediate between the previous observations in BALB/c mice and our current findings in C57BL animals with the various assays performed. We also asked whether the effects of knocking out the iNOS gene were exerted before or after the release of TGF-β₁ by eosinophils and other effector cells in the lung. We measured the response of C57BL and iNOS^-/- mice to direct intratracheal challenge with TGF-β₁. There was no apparent response of C57BL mice to TGF-β₁ at 4 or 11 days after TGF-β₁ challenge, as evaluated by bronchoprovocation testing. On the other hand, the observed Penh values were significantly greater in iNOS^-/- mice that had also received TGF-β₁ 4 days previously. These results strongly support the hypothesis that the increased sensitivity of iNOS^-/- mice to ovalbumin is at least partially dependent on pathways that come into play subsequent to the release of TGF-β₁ by effector cells in the lungs of mice exposed to ovalbumin aerosol.