Serpins, viruses, and the virome: New directions in therapy

Serine protease inhibitors, termed serpins, regulate myriad physiological processes in the mammalian body from thrombotic and thrombolytic pathways to inflammation, angiogenesis, hormone transport, and hypertension. The large percentage of serpins among the plasma proteins in the circulating blood as in the case of plasminogen activators, the functional redundancy of serpins, and also the debilitating serpinopathies of antithrombin III (SERPINC1), neuroserpin (SERPINI1), and alpha-1 antitrypsin (SERPINA1) provide evidence of the importance of serpins and their widespread impact in normal physiological homeostasis. Inflammation, also termed innate immunity, interacts closely with and both regulates and is regulated by thrombotic and thrombolytic serine proteases. Activation of the coagulation proteases is, in turn, controlled by serpins. Apoptosis is also modulated by serpins with cross-class inhibitory activity for cysteine and serine proteases. Excessive inflammation and cell death processes are now recognized as interacting with the thrombotic and thrombolytic proteases. Viruses have evolved to communicate and control these processes by encoding their own serpins, which confer on them the ability to evade host immune defenses. This chapter provides an introduction to the viral serpins derived from poxvirus origins that have been shown not only to be essential for successful viral infection but, in some cases, as for Serp-1 and Serp- 2, to have the potential to mitigate inflammatory disease in animal models. Serp- 1 has in fact been successfully tested in a small phase 2A clinical trial in unstable angina patients with coronary stent implants. A tandem discussion of mammalian serpins with actions similar to those of the viral serpins is also presented to emphasize potential evolutionary relationships between viral and mammalian serpins. The anti-inflammatory serpins hold the potential to be effective in disease states such as atheroma, sepsis, cancer, and wound healing given that these conditions are all associated with aberrant inflammatory responses and with dysregulation of thrombotic, thrombolytic, and apoptotic protease cascades. The capacity of viral serpins to provide antiviral protection by modulating the virome as well as possible therapeutic effects of serpin metabolites in inflammation will also be discussed. In summary, viral serpins have evolved over many millions of years and provide a unique and highly potent reservoir for both the study of serpin modulation of inflammatory responses and for new therapeutic approaches to inflammatory and even infectious diseases.