Thymulin evokes IL-6-C/EBP beta regenerative repair and TNF-alpha silencing during endotoxin exposure in fetal lung explants

Chorioamnionitis is associated with increased risks of perinatal respiratory failure; however, components of the inflammatory acute-phase response are known to actively promote lung maturation. To manipulate this relationship, we examined the effect of the thymic immunomodulator thymulin on fetal lung mesenchyme-epithelial differentiation during exposure to Escherichia coli lipopolysaccharide (LPS). Gestation day 14 fetal rat lung explants were cultured for 96 h at fetal (23 mmHg) or ambient (142 mmHg) PO2. Airway surface complexity (ASC, perimeter/varea2) was greater at fetal vs. ambient PO2; however, exposure to 0.1–50 µg/ml LPS significantly raised ASC at 2 µg/ml in ambient PO2 explants. LPS (50 µg/ml) depressed ASC in both conditions to untreated ambient PO2 control values without changes in necrosis or apoptosis. To manipulate LPS-evoked TNF-a and IL-6 release, we exposed explants and A549 cells to combinations of 50 µg/ml LPS, 10 µM ZnCl2, and 0.1–1,000 ng/ml thymulin at either PO2. Thymulin+Zn2+ suppressed and potentiated LPS-evoked TNF-a and IL-6 release, yielding an IC50(TNF-a) of 0.5 ± 0.01 ng/ml and EC50(IL-6) of 1.4 ± 0.3 ng/ml in A549 cells. This was accompanied by activation of the p38 MAPKMAPKAP-K2 pathway with sustained expression of TNF-a and IL-6 transcripts at ambient PO2. LPS+thymulin+Zn2+-treated explants showed proliferation of CCAAT-enhancer binding protein- (C/EBP) and fibroblast growth factor-9 immunoreactive mesenchyme, which was abolished by IL-6 antisense oligonucleotides. The posttranscriptional suppression of immunogenic TNF-a synthesis coupled with raised IL-6 and C/EBP-dependent mesenchyme proliferation suggests a role for bioactive thymulin in regulating regenerative repair in the fetal lung.