Plasmin regulation of acute cytokine storm

In this issue of Blood, Shimazu et al report that cytokine release and activation is mediated through a previously unrecognized TLR9/plasmin/MMP9 axis.¹ 

Macrophage activation syndrome (MAS) is a life-threatening complication associated with systemic juvenile idiopathic arthritis or with adult-onset Still disease.²  This syndrome has several attributes common to hemophagocytic lymphohistiocytosis (HLH), because both of these life-threatening conditions are characterized by a supraphysiological elevation of cytokines, an increase in inflammation, multiorgan dysfunction, and death.²  Although MAS is generally treated with corticosteroids, this treatment is not effective in all patients. Refractory patients have been treated with prototypical immune modulators, including cyclosporine A, tumor necrosis factor (TNF) inhibitors, or anti–interleukin-1 or -6 therapies,²  with marginal benefit. In search of the pathogenesis of MAS and HLH, Shimazu et al studied the function of plasmin activation in response to an acute cytokine storm that is the basis of and driver for many of these diseases. Their results demonstrate how plasmin activation contributes to the regulation of the inflammatory response through the activation of TNFα among other cytokines.

To establish an in vivo model of MAS, cytosine guanine (CpG) bacterial DNA was injected into mice.³  However, this model developed a mild inflammatory response associated with the activation of Toll-like receptor 9 (TLR-9) by CpG that did not reproduce the exacerbated inflammation observed in patients with MAS. This can be achieved by the coinjection of CpG with D-galactosamine (DG) to induce lethal hepatitis in mice through a dramatic increase in TLR-9–mediated TNFα activation, as has been previously described.⁴  Using this model system of CpG/DG coinjection, Shimazu et al not only were able to phenocopy the onset of MAS syndrome but also have uncovered the role of plasmin in the dysregulation of inflammation and coagulation that leads to MAS.

Plasmin is the active form of the enzyme plasminogen and is involved in the processing and degradation of many blood plasma proteins, including fibrin clots. Although the role of plasmin in inflammation has been previously described,⁵  its role in the development of inflammatory syndromes is undefined. Therefore, the use of plasmin-deficient mice has been useful in the characterization of its role in mediating inflammatory reactions. However, a major disadvantage of plasmin-deficient mice is the development of diverse pathologies associated with fibrin deposition.⁶  The chemical inhibition of plasmin in wild-type mice offers an alternative model. Notably, the authors observed that the chemical inhibition of the active center of plasmin with trans-4-aminomethylcyclohexanecarbonyl-Tyr(O-Pic)-octylamide (YO-2), after CpG/DG administration, was more effective when administrated at early stages of inflammation, decreasing the inflammatory markers in blood and inflammatory cells in the liver, spleen, and bone marrow, leading to an increase in survival. This finding suggests that plasmin could contribute to the rapid onset of inflammatory response, although further studies should determine its contribution to chronic inflammatory conditions.

Although pharmacological inhibition of plasmin controlled acute inflammation and liver damage, it did not diminish the activation of the coagulation pathway. Although these results could be due to a different pathway involving fibrin deposition and fibrin-associated inflammatory response as suggested by the authors, the administration of CpG/DG also activates a TLR-9 response in endothelial cells that triggers the activation of the NFκB pathway.⁷  This latter pathway has been related to the activation of the coagulation cascade⁸  (see figure). Furthermore, the genetic deletion of plasmin in mice generates spontaneous thrombosis,⁶  thereby limiting the opportunity for therapeutic intervention. Nonetheless, despite these limitations, the global decrease of the inflammatory response and the increase in survival were significant achievements.

The authors noted that TLR-9–driven TNFα activation was mediated through a plasmin/matrix metalloproteinase 9 (MMP9) axis (see figure). They show how in MMP9- deficient mice, the activation with CpD/DG was significantly diminished, resulting in improved survival. Importantly, these data are supported by previous reports, demonstrating that MMP9 deficiency has a protective effect in response to lipopolysaccharide challenge by reducing the inflammatory response.⁹  Moreover, similar results have been recently described using a blocking antibody against TLR-9 after the administration of CpG/DG,¹⁰  confirming the importance of the TLR-9/plasmin/MMP9 axis in the control of the cytokine storm response.

In summary, the article by Shimazu et al describes a relevant model of MAS in which TLR-9 stimulation contributes to the activation of plasmin, augmenting the inflammatory response through the control of MMP9. This study reveals the importance of plasmin in the control of acute inflammatory cytokine production and opens the door for the development of alternative therapeutic approaches for treating syndromes associated with an acute cytokine storm, such as MAS or HLH.