Recent clinical observations suggest that viral infection/reactivation may contribute to the development of idiopathic pulmonary fibrosis in susceptible hosts. Viral gene therapy using adenoviral vectors in the lung is limited by immune response directed against the viral vector and/or its foreign gene. We therefore examined the effects of adenoviral vector (E1, E3 deleted) on the course of bleomycin-induced lung injury/fibrosis in mice.
Methods Adenoviral vectors (native and UV-inactivated) expressing luciferase, β-galactosidase, or a human gene were administered intratracheally (IT; 109 PFU/animal) to mice 2 days after bleomycin (4 U/kg IT). Animals were assessed for mortality, lung inflammation (bronchoalveolar lavage [BAL] cell counts and differential), vascular permeability (BAL protein levels) and fibrosis (lung hydroxyproline) over the next 28 days.
Results Combined bleomycin/adenoviral vector administration caused a 3-fold increase in 28 day mortality compared with bleomycin alone (20% to 60%; n = 20/group). The excess mortality was completely abrogated by substituting a UV-inactivated vector and partially abrogated with a null-expressing adenoviral vector. Similarly, combined bleomycin/adenoviral vector administration caused a 2-fold increase in the increment of lung collagen (hydroxyproline) compared with bleomycin alone (150 ± 18 μg/animal vs 300 ± 22 μg/animal; n = 5/group), which was abrogated by substituting a UV-inactivated, or a null-expressing adenoviral vector. Neither IT adenoviral vectors alone nor IT saline alone induced lung fibrosis (by hydroxyproline and trichrome stain). Combined bleomycin/adenoviral vector administration also resulted in increased inflammation (2-fold increment in total BAL cells), and alveolar protein levels compared with bleomycin alone, at 10 days after IT bleomycin. Adenoviral vectors alone had no effect on BAL cell counts or protein levels compared with IT saline-administered controls. The alveolar lymphocytosis was also 2-fold higher in the bleomycin/adenovirally administered group compared with the bleomycin alone group. These data indicate that adenoviral vectors are proinflammatory, enhance lymphocytic recruitment into the alveolar compartment and profibrotic. Further, the dependence of these effects on viral/foreign gene expression implicate antiviral adaptive immunity. Viral-induced adaptive immunity may be one mechanism whereby viral infection will induce a fibrotic response in susceptible hosts with underlying lung inflammation.