Thrombin-Activatable Fibrinolysis Inhibitor (TAFI) Regulates Activated Complement C5a in Vivo.

The latent plasma carboxypeptidase thrombin-activatable fibrinolysis inhibitor (TAFI) is activated by thrombin/thrombomodulin on the endothelial cell surface, and functions in dampening fibrinolysis. We recently showed that TAFIa efficiently inactivates bradykinin, C5a, and thrombin-cleaved osteopontin, suggesting that it may have a broad anti-inflammatory role. In this study, we examined the pulmonary inflammatory responses in a C5a-induced alveolitis model in wild-type (WT) and TAFI deficient mice. TAFI deficient mice had been backcrossed into a pure C57/B6 background. C5a was instilled into the trachea of anesthetized mice, which were then allowed to recover. Six hours later, bronchiole alveolar lavage (BAL) was performed and the extent of pulmonary inflammation determined. C5a caused a dose-dependent increase in cell counts (WBC/ul) in the BAL fluids in WT mice (saline control, 5.3±4.1 (mean±SD); C5a at 0.05 mg/ml, 28.7±8.5, C5a at 0.1 mg/ml, 60.4±23.8; n=6 for saline and low-dose C5a, n=12 for high dose C5a; p < 0.0005 and < 0.0001 for low-dose and high-dose C5a compared to saline control respectively). Significantly increased BAL cell counts in the TAFI deficient mice in response to C5a stimulation were noted (saline control, 3.3±3.9; low-dose C5a 70±11.2; high-dose C5a 159.6±58.7; n=6 for saline and low-dose C5a, n=12 for high dose C5a; p < 0.00001 and < 5E-0.6 respectively), representing ~2.4–2.6 fold increase in BAL cell counts in the TAFI deficient mice. Determinations of the wet/dried lung weight ratios, an index of pulmonary edema, showed similarly significant results (WT mice: 4.1±0.14, 4.25±0.1, 4.52±0.15 and TAFI-deficient mice: 4.27±0.41, 5.3±0.53, 5.77±0.62 for saline control, low- and high-dose C5a respectively). Our data indicate that in the absence of TAFI, the chemotactic and inflammatory effects of C5a in pulmonary inflammation were enhanced. We are in the process of testing whether E229K thrombin, which selectively activates protein C and TAFI, will ameliorate the C5a alveolitis response in WT mice. Our results support our thesis that the function of TAFIa is not restricted to fibrinolysis. It has a broad anti-inflammatory role and may serve as a homeostatic counterbalance to thrombin’s inflammatory functions.