Introduction Obliterative Bronchiolitis (OB) is characterised by fibrotic obliteration of small airways which adversely affecting graft function and survival after lung transplantation. It has been shown in vitro and in vivo that primary bronchial epithelial cells (PBEC) from the transplanted lung can undergo epithelial to mesenchymal transition (EMT) and this process may contribute to the development of OB. We have shown that activated macrophages can disrupt epithelial wound repair by accentuating TGF-β1-driven EMT. We hypothesised that this effect might be limited to macrophages with an M1 phenotype and that their secretory products might be a target for limiting the inflammatory accentuation of EMT.

Methods and materials The THP-1 monocytic cell line was stimulated with clinical isolates of Pseudomonas aeruginosa (PA) and the effect of the activated cells or conditioned media on TGF-β1-driven EMT assessed in PBEC (Western blotting, confocal microscopy). In addition, THP-1 cells were differentiated to an M1 phenotype by treatment with IFNγ and an M2 phenotype with IL-4/IL-13 and cytokine release (ELISA) and their effect on TGF-β1 driven EMT assessed. The effect of blocking TNFα secreted from activated THP-1 cells on EMT was assessed using an anti-TNFα antibody.

Results Treatment with TGF-β1+activated THP-1 cells had no effect on EMT marker expression (p>0.05 n=6). However, co-treatment with TGF-β1+conditioned media from activated THP-1 cells dramatically accentuated TGF-β1-driven EMT (p<0.05 n=6). M1 differentiated THP-1 cells released 8.4-fold more TNFα and 8.1-fold more IL-1β than M2 cells (p<0.05, n=3). Conditioned media from M1, but not M2, cells dramatically accentuated TGF-β1 driven EMT (p<0.05 n=6). Blocking TNFα in the conditioned media from THP-1 cells significantly inhibits the decrease in E-cadherin (39%±4%) and the increase in vimentin (59%±18%) and fibronectin (72%±14%) expression (p<0.05, n=5).

Conclusion The secretory products of M1, but not M2, macrophages significantly accentuate TGF-β1 driven EMT. TNFα appears to be a major constituent of this accentuating action. This raises the possibility that either TNFα targeted therapies or modulation of macrophage phenotype may inhibit the inflammatory accentuation of EMT in airway epithelium.