241 - Mesenchyme-derived inflammation results in macrophage recruitment and alters saccular stage lung development

Fibroblasts in the lung mesenchyme direct organization of future airspaces during the saccular stage of lung development. Inflammation in preterm infants alters critical components of this process leading to reduced alveolar formation and bronchopulmonary dysplasia (BPD).

Objective:

To model the effects of inflammatory signaling initiated by fibroblasts in the mouse lung mesenchyme and, thereby, better understand how inflammation contributes to BPD pathogenesis.

Design/Methods:

We used a transgenic mouse model in which human IKKb (an upstream activator of NF-kB) is expressed in Tbx4-positive lung fibroblasts in a doxycycline (Dox) inducible manner (referred to as IKKb^Tbx4). We activated the transgene in neonatal IKKb^Tbx4 mice during the saccular stage from postnatal (PN) day 0-5 by placing lactating dams on Dox (2g/L) and evaluated the effects on the lung at PN5 and PN60. Additionally, we investigated the effect of transgene activation in lung fibroblast cultures generated from IKKb^Tbx4 mice.

Results:

Transgene activation in IKKb^Tbx4 mice resulted in lung inflammation with increased mRNA expression of cytokines and chemokines and recruitment of macrophages (% macrophages/viable lung cells [mean ± SEM]: control: 7.6 ± 0.5, IKKb^Tbx4: 33.2 ± 0.6; P < 0.05) into the lung. Dox treatment of lung fibroblast cultures from IKKb^Tbx4 mice demonstrated similarly increased transcription of macrophage chemokines. Further, conditioned media from these cells stimulated chemotaxis of macrophages in vitro. IKKb^Tbx4 mice had altered saccular stage lung development with increased interstitial thickening and decreased distal airspace area (distal airspace area [% of distal lung, mean ± SEM]: control: 41.2 ± 2.7, IKKb^Tbx4: 26.2 ± 0.3; P < 0.05). Depletion of macrophages with daily injections of intraperitoneal (IP) clodronate from PN 1-4 rescued the saccular lung phenotype in IKKb^Tbx4 mice. Importantly, saccular stage macrophage recruitment had long term consequences on the lung. Adult IKKb^Tbx4 mice at PN60 demonstrated an emphysematous lung phenotype with alveolar simplification (mean linear intercept in µM [mean ± SEM]: control: 22.5 ± 0.7, IKKb^Tbx4: 39.9 ± 2.1; P < 0.05) and altered tissue elastance.

Conclusion(s):

Our findings suggest that fibroblast-macrophage interactions have important roles in the inflammatory cascade leading to abnormal saccular lung development. Delineating mediators of reciprocal fibroblast-macrophage crosstalk and downstream effects of macrophage-dependent inflammation are needed to understand BPD pathogenesis.