Modifications of the Coagulation System Alter Inflammation in Murine Sickle Cell Disease.

There is evidence for increased activity of both the coagulation and inflammatory pathways in patients with sickle cell disease (SCD). The interrelationships between inflammation and coagulation likely set up a vicious cycle that further propagates the end organ pathology observed in SCD. The aim of this study was to determine whether alterations of the coagulation system change the inflammatory system in mice with SCD. For this study, we generated SCD mice that were deficient in either tissue factor (SCD-TF^Low mice) or fibrinogen (SCD-Fib−/− mice) by transplantation of hematopoietic stem cells from Berkeley SCD fetal livers into tissue factor-deficient (TF^Low) mice or fibrinogen-deficient (Fib−/−) mice. Because tissue factor and thrombin have potent pro-inflammatory effects, we hypothesized that SCD-TF^Low mice (low in both tissue factor and subsequent thrombin generation) have decreased inflammation while SCD-Fib−/− mice (with increased thrombin levels due to afibrinogenemia) have increased inflammation. We measured levels of the following markers of vascular and systemic inflammation: (1) pro-inflammatory or Th1 cytokines (IL-17, TNF-α, IFN-γ, and IL-2); (2) a Th2 cytokine that may limit inflammation (IL-4); and (3) soluble vascular cell adhesion molecule-1 (sVCAM), a measure of endothelial injury. Plasma levels of these markers were measured with commercially available ELISA kits (R&D systems) or BioPlex cytokine assay kits. We compared plasma from SCD-TF^Low mice, SCD-Fib−/− mice, transgenic SCD mice (SCD mice), and mice that express only normal human hemoglobin (HbA mice). In comparison to control HbA mice, SCD mice had elevated levels of IL-17, TNF-α, IL-2, and sVCAM, consistent with a pro-inflammatory state. SCD-Fib −/− mice had further elevations of these inflammatory markers. In contrast, SCD-TF^Low mice had decreased levels of IL-17, TNF-α, IL-2, and sVCAM in comparison to SCD mice, suggesting that tissue factor deficiency decreases inflammation. IL-4 remained low for all groups of mice, suggesting that these manipulations of the hemostatic pathway did not affect this potentially anti-inflammatory cytokine.

This pattern of change in the plasma levels of these selected inflammatory markers provides evidence that

1. at baseline, SCD is a pro-inflammatory state;

2. afibrinogenemia has the potential to worsen inflammation in SCD, likely through increased thrombin generation previously described in human and murine afibrinogenemia; and

3. tissue factor deficiency decreases systemic and vascular inflammation in SCD. These data indicate that modifications of the coagulation system may affect the inflammatory state in SCD, and suggest the potential for therapeutic manipulation of coagulation in order to reduce inflammation in SCD.