Critical Role of SerpinB1 in Regulating Inflammatory Responses in Pulmonary Influenza Infection

The serine proteases elastase, cathepsin G, and proteinase-3 are increasingly recognized as modulators of inflammation (reviewed in [1]). These enzymes, commonly called neutrophil serine proteases (NSPs), enter the lungs of infected mice as components of recruited monocytes and neutrophils. At the molecular level, they are related enzymes with distinct but, to a large extent, overlapping activities; they have broad capacity to regulate protective immune functions but, when neutrophils or monocytes are in excess, are key agents of inflammatory damage. We characterized SerpinB1 as a specific and efficient inhibitor of elastase, cathepsin G, and proteinase-3, all 3 of the NSPs [2]. We also deleted the murine gene encoding this common inhibitor to create a model in which the activities of the 3 NSPs would be coordinately increased [3]. Study of the serpinB1−/− mice makes it possible to identify pathological parameters of an infection that have their basis in exaggerated NSP action. Innate immune cells, particularly myeloid cells, are extremely important in containing influenza [4]. Neutrophils can clear the virus in vitro and in vivo [5, 6]. Depletion of either alveolar macrophages or neutrophils before sublethal infection with a pandemic virus resulted in uncontrolled viral replication and increased mortality [7, 8]. However, myeloid cells can be detrimental to the host, contributing to morbidity and mortality through inflammatory injury [7, 9, 10]. Infection by highly pathogenic strains, such as 1918 H1N1 and avian H5N1, caused massive recruitment of neutrophils and monocytes and/or macrophages [11, 12]. These cells interfere with air exchange and rapidly release high levels of proinflammatory cytokines, including TNF-α, IL-1β, and IL-6, as part of the cytokine storm [7, 10, 13] associated with severe lung pathology in patients infected with avian H5N1 [11, 14]. A previous study of gene-targeted mice revealed a critical role for serpinB1 in protecting pulmonary antibacterial defense [3]. In response to Pseudomonas aeruginosa infection, serpinB1−/− mice showed defective clearance of bacteria, increased neutrophil death, elevated proinflammatory cytokines, and depletion by proteolysis of surfactant protein D (SP-D). This pulmonary collectin (collagenous lectin) opsonizes bacteria, neutralizes strains of influenza virus bearing high mannose carbohydrates on hemagglutinin, and maintains the anti-inflammatory environment of the lung [15–17]. In the present study, we evaluated the role of serpinB1 in influenza infection with the SP-D sensitive strain A/Philadelphia/82 (H3N2) (Phil/82), a human isolate that replicates injurious features of highly pathogenic influenza, including high-level recruitment of neutrophils and monocytes.