In vivo–generated thrombin and plasmin do not activate the complement system in baboons

Abstract

Sepsis concurrently activates both coagulation and complement systems. Although complement activation by bacteria is well documented, work in mice and in vitro suggests that coagulation proteases can directly cleave complement proteins. We aimed to determine whether generation of coagulation proteases in vivo can activate the complement cascade in 2 highly coagulopathic models. We compared temporal changes in activation biomarkers of coagulation (thrombin-antithrombin [TAT]), fibrinolysis (plasmin-antiplasmin [PAP]), and complement (C3b, C5a, C5b-9) in baboons infused with factor Xa (FXa) and phospholipids (FXa/phosphatidylcholine-phosphatidylserine [PCPS]) vs LD100 We found that, albeit with different timing, both FXa/PCPS and infusion led to robust thrombin and plasmin generation. Conversely, only challenge activated the complement system, reaching a maximum at 2 hours postchallenge during the peaks of lipopolysaccharide and bacteremia but not of TAT and PAP. Despite inducing a strong burst of thrombin and plasmin, FXa/PCPS infusion did not produce measurable levels of complement activation in vivo. Similarly, ex vivo incubation of baboon serum with thrombin, plasmin, or FXa did not show noticeable complement cleavage unless supraphysiologic amounts of enzymes were used. Our results suggest that in vivo-generated thrombin and plasmin do not directly activate the complement in nonhuman primates.

Complement and coagulation: so close, yet so far

“…1 T heir observation appears in sharp contrast with earlier studies that did identify a potential for thrombin and plasmin to activate the complement system. 2,3 The apparent contradiction thus raises a fundamental question: Is this a case of "in vivo veritas," or are there experimental or physiological grounds that may explain the differences?…”

“…1 What unites these diverse infusion of a lethal dose of E coli induced protracted activation of coagulation and fibrinolytic pathways when compared with the "pure coagulopathy" model, as evidenced by delayed but strong consumption of prothrombin, plasminogen, and fibrinogen. In addition, the sepsis model also fueled strong complement activation.…”

Staphylococcus aureus‐induced complement activation promotes tissue factor‐mediated coagulation

“…Second, lepirudin specifically blocks thrombin, both limiting the available assays of coagulation and masking the contributions of thrombin itself. Thrombin has been postulated to activate complement directly , although a very recent study performed in septic baboons did not detect any complement activation by thrombin (or plasmin) . Likewise, the recent focus on the cross‐talk between the many plasma cascade systems in thromboinflammation has revealed that thrombin can affect both platelet and endothelial cell function .…”

Carboxypeptidase B2 and carboxypeptidase N in the crosstalk between coagulation, thrombosis, inflammation, and innate immunity

“…Various enzymes involved in coagulation and fibrinolysis have been proposed as C5 convertases, with the strongest evidence supporting plasmin , but these models all involve tissue damage as well as protease activation. Baboons with sepsis triggered by either Escherichia coli or lipopolysaccharide (LPS) showed no evidence of complement activation by either thrombin or plasmin, but the role of platelets in these models was not investigated . To determine whether any of the coagulation enzymes can activate complement in vivo , cleavage of C5 to C5a and C5b should be studied in the absence of C3 in the C3‐deficient mouse in an appropriate complement activation‐mediated disease model.…”