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Influenza virus A (IAV) causes annual epidemics and intermittent pandemics that affect millions of people worldwide. Potent inflammatory responses are commonly associated with severe cases of IAV infection. The complement system, an important mechanism of innate and humoral immune responses to infections, is activated during primary IAV infection and mediates, in association with natural IgM, viral neutralization by virion aggregation and coating of viral hemmagglutinin. Increased levels of the anaphylatoxin C5a were found in patients fatally infected with the most recent H1N1 pandemic virus. In this study, our aim was to evaluate whether targeting C5 activation alters inflammatory lung injury and viral load in a murine model of IAV infection. To address this question C57Bl/6j mice were infected intranasally with 104 PFU of the mouse adapted Influenza A virus A/WSN/33 (H1N1) or inoculated with PBS (Mock). We demonstrated that C5a is increased in bronchoalveolar lavage fluid (BALF) upon experimental IAV infection. To evaluate the role of C5, we used OmCI, a potent arthropod-derived inhibitor of C5 activation that binds to C5 and prevents release of C5a by complement. OmCI was given daily by intraperitoneal injection from the day of IAV infection until day 5. Treatment with OmCI only partially reduced C5a levels in BALF. However, there was significant inhibition of neutrophil and macrophage infiltration in the airways, Neutrophil Extracellular Traps (NETs) formation, death of leukocytes, lung epithelial injury and overall lung damage induced by the infection. There was no effect on viral load. Taken together, these data suggest that targeting C5 activation with OmCI during IAV infection could be a promising approach to reduce excessive inflammatory reactions associated with the severe forms of IAV infections.

Background: Viral infection including pandemic influenza may be associated with severe acute respiratory distress syndrome (ARDS) characterised by rapidly developing alveolitis with dense neutrophil infiltration. Complement activation has been suggested as a trigger but until now evidence has been lacking. Here we show that blockade of the complement system at the C5 level substantially reduces the severe respiratory effects of H1N1 infection in a mouse model and that this inhibition is independent of the severity of the viral infection. Methods: BalbC mice were infected with a sub-lethal (104 PFU) or a lethal dose (106 PFU) of human H1N1 virus or sham infected. Severe respiratory infection developed within 4 days in low inoculum animals and more rapidly in the high inoculum group. Mice were injected intraperitoneally with coversin, a C5 antagonist, at the time of infection and daily thereafter. At Day 6 (low inoculum group) and Day 3 (high inoculum group) animals underwent broncholaveolar lavage (BAL) pre-sacrifice. BAL fluid was inspected for total cells, neutrophils, protein and cytokines. Results: Vehicle treated mice in both high and low inocula groups had significantly elevated total cells, protein, neutrophils and cytokines (IL-1b, IL-6 and CXCL2) compared to sham treated. Mice treated with coversin had significantly lower elevation of all inflammatory parameters. In particular there was 69% inhibition of the neutrophil response in the low inoculum group and 71% in the high inoculum group compared to vehicle. These differences were significant at 104 PFU (p < 0.01) and at 106 PFU (p < 0.001). Conclusions: Treatment of mice infected with H1N1 virus with a complement C5 inhibitor significantly attenuated respiratory inflammation. The alveolar inflammatory response and the inhibition appeared to be independent of the viral load. This suggests that the response is an all-or-none phenomenon which further supports the possibility that complement activation is invol

Because histamine receptor type I blockade attenuates allergic asthma, we asked whether complete neutralization of histamine by an arthropod-derived, high affinity histamine-binding protein (EV131) would prevent allergic asthma. Intranasal administration of EV131 given before Ag challenge in immunized mice prevented airway hyperreactivity by 70%, and abrogated peribronchial inflammation, pulmonary eosinophilia, mucus hypersecretion, and IL-4 and IL-5 secretion. Saturation with histamine abrogated the inhibitory effect of EV131 on bronchial hyperreactivity. The inhibitory effect of EV131 on bronchial hyperreactivity was comparable to that of glucocorticosteroids. These results demonstrate that histamine is a critical mediator of allergic asthma. Therefore, complete neutralization of histamine, rather than specific histamine receptor blockade, may have a profound effect on allergic asthma.