A Reduced Capacity To Generate Activated Protein C and the Role of PAI-1 Deficiency on Coagulation Activation and Fibrin Formation during Murine Influenza Pneumonia.

Background: Recent studies have shown that influenza activates coagulation in vitro. We aimed to study influenza and post-influenza pneumonia induced hemostatic changes in vitro and to study the role a reduced capacity to generate activated protein C (APC) and PAI-1 deficiency on the prothrombotic state.

Methods: C57BL/6 mice were inoculated with influenza or PBS (control). After 14 days these mice received S. pneumoniae. PAI-1^−/− , TM ^pro/pro and control mice were inoculated with influenza. Controls for the influenza-infected PAI-1^−/− and TM ^pro/pro mice were C57BL/6 mice and wild-type littermates, respectively. TATc, PAI-1, D-dimer levels, fibrin deposition and pro-inflammatory cytokines were studied on day 0, 2, 4, 8, 14, 15 and 16.

Results: Influenza-infected C57BL/6 mice showed a dose dependent increase in TATc, PAI-1 and fibrin deposition compared to non-infected controls. Median TATc levels increased from 5.3 at baseline to 12.8 ng/ml at day 4 and PAI-1 levels increased from 4.0 to 16.0 IU/ml (both p<0.05). TATc and PAI-1 levels remained unchanged in non infected controls. Mice recovered from influenza infection were highly susceptible to pneumococcal pneumonia as reflected by a 3-fold increase in cytokines compared to non-influenza infected controls. In addition, an increase in TATc and PAI-1 levels was seen (both p<0.05). Furthermore, influenza-infected TM ^pro/pro mice showed higher TATc levels (16.2 vs. 12.0 ng/ml, p<0.05) and higher D-dimer levels (2.6 vs. 2.0 μg/l, p<0.05) compared to infected controls 4 days after infection. In influenza-infected PAI-1^−/− mice TATc levels were similar as in infected controls, but D-dimer levels were increased (4.0 vs. 2.2 μg/l, p< 0.05).

Conclusion: This study demonstrates that influenza and post-influenza pneumonia exert a prothrombotic state in vivo. Mice with a reduced capacity to generate APC show increased thrombin generation. PAI-1 deficiency does not effect coagulation activation but results in a hyperfibrinolytic state.