TGFβ and Smad3 link inflammation to progressive fibrosis

Transient adenovirus-mediated gene transfer of active TGFβ1 (AdTGFβ1) induces severe and progressive fibrosis in rodent lung without apparent inflammation. Alternately, transfer of IL-1β (AdIL-1β), a proinflammatory cytokine, induces marked tissue injury and inflammation which develops into progressive fibrosis. This inflammation is associated with an increase in TGFβ1 concentrations in bronchoalveolar lavage (BAL) fluid. This apparent divergence of progressive fibrosis from the presence of inflammation is currently a well-debated topic in clinical medicine, particularly as it applies to Idiopathic Pulmonary Fibrosis. TGFβ1 is a key cytokine in the process of fibrogenesis, using intracellular signaling pathways involving the ALK5 receptor and signaling molecules Smad2 and Smad3. Transient gene transfer of TGFβ1 to Smad3 null mice lung provides little evidence of progressive fibrosis and no fibrogenesis associated genes are induced. To investigate whether inflammation could bypass the requirement for TGFβ and Smad3 signaling and independently induce lung fibrosis we transiently overexpressed the IL-1β gene in Smad3 null mice lung. At day 7 similar levels of IL-1β are seen in BAL in both wild-type (WT) and knockout (KO) mice with marked neutrophilic inflammation and positive phospho-Smad2 staining. By day 35 after transient IL-1β expression, WT mice showed marked fibrosis in peribronchial areas, however, there was no evidence of fibrosis or collagen accumulation in IL-1β-treated KO mice, no difference from KO mice treated with a control adenovector. TGFβ1 and phospho-Smad2 were strongly positive at day 35 in fibrotic areas observed in WT mice, but no such staining was detectable in KO mice. The IL-1β inflammation induced chronic fibrotic response in mouse lungs appears dependent on Smad3 as both KO and WT animals demonstrated a similar inflammatory response to overexpression of IL-1β. These findings indicate that inflammation must link to the Smad3 pathway, likely through TGFβ, to induce progressive fibrosis and tissue scarring.