流行性感冒病毒感染是世界上高傳染性的疾病之一。通常流感病毒的感染會造成細胞激素的分泌和病毒複製所引起的急性發炎，例如支氣管炎、肺炎或是嚴重致死的急性肺炎。這些細胞激素會吸引嗜中性白血球、巨噬細胞和淋巴細胞到感染部位，導致末端氣管窄化和造成肺部組織的損傷。另外，流感病毒外套膜的兩個糖蛋白hemagglutinin (HA)和neuraminidase (NA)，在流感病毒的感染上也扮演重要角色。HA可以幫助流感病毒和目標細胞的唾液酸(sialic acid)受器結合，然後進入細胞內進行複製，而絲胺酸蛋白酶(serine protease)會將HA0切割成HA1和HA2，增進病毒的感染力，這對病毒的致病力很重要。目前已知kallikrein為絲胺酸蛋白酶家族之一員，在肺炎中會大量增加。而kallistatin是kallikrein的抑制劑，且己被証實具有抗發炎的功能。在本篇研究裡，我們研究kallikrein在HA切割上的影響，同時也探討kallistatin在流感病毒感染上的主要角色。首先，我們評估kallistatin和kallikrein對於流感病毒感染的影響。結果顯示kallikrein可促進HA切割，增加流感在MDCK (Madin-Darbin Canine kidney)細胞的複製與感染能力，然而kallistatin卻可以避免此現象。進一步在流感病毒感染下，給予攜帶人類kallistatin之慢病毒載體(lentiviral vector)LV-HKBP治療的小鼠存活率比給予攜帶綠色螢光蛋白之慢病毒載體LV-GFP組的小鼠高。另外在LV-HKBP治療後的小鼠血清中，介白素六號 (interleukin-6, IL-6)表現下降，而肺的損傷減緩。綜合以上結果， kallistatin可減少流感病毒的感染和降低病毒所引起的免疫病理變化，因此有潛力發展為抗流感病毒的藥物。

Influenza virus infection is one of the most pandemic infectious diseases in the world. In general, influenza virus infection induces bronchitis, pneumonia, and even severe lethal pneumonia caused from extensive cytokine secretion and acute inflammation at areas of viral replication. These cytokines are associated with the recruitment of neutrophils, macrophages and lymphocytes to the infection site resulting in narrow of the terminal bronchi and lung damage. In addition, two envelope glycoproteins of influenza virus, hemagglutinin (HA) and neuraminidase (NA), play an important role in virus infection. HA mediates influenza attachment to sialylic acid cell receptors and virus entry into target cells. The cleavage of HA0 into HA1/HA2 with secreted serine proteases activates virus infectivity and is important for the pathogenesis of influenza virus. Kallikrein is a serine protease that increases in pneumonia. Kallistatin, a serine protease inhibitor, has been shown to reduce inflammation. In this study, we investigated the effect of kallikrein on HA cleavage and the potential role of kallistatin in influenza infection. First, we evaluated the effects of kallistatin and kallikrein on influenza virus infection in mice. Our results showed that kallikrein could promote HA cleavage and increase influenza replication and infection in MDCK (Madin-Darbin Canine kidney) cells, which could be abrogated by kallistatin. Furthermore, in mice infected with influenza viruses, the survival rate of LV-HKBP, a lentiviral vector encoding human kallistatin, was higher than that of the mice treated with LV-GFP, a lentiviral vector encoding green fluorescent protein. Besides, the level of interleukin-6 (IL-6) was lower in the serum and the lung injury was alleviated in the LV-HKBP-treated mice than those in LV-GFP-treated mice. Taken together, these results show that kallistatin can decrease influenza virus infection and hence reduce immunopathology. Furthermore, kallistatin may have potential to act as an anti-influenza agent.

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