Distinctive anti-influenza properties of recombinant collectin 43

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Abstract
Collectins play important roles in innate defence against viral, fungal and bacterial pathogens. CL-43, a bovine serum collectin, which appears to have evolutionarily evolved from surfactant protein D (SP-D), shows unique structural and functional properties. In the present study, we describe the initial characterization of a recombinant CL-43 expressed in mammalian cells. Like natural CL-43, the recombinant is secreted as trimeric forms that show a preference for mannose and N-acetyl mannosamine. The natural and recombinant proteins have significantly higher haemagglutination-inhibiting activity against influenza A virus (IAV) than recombinant trimeric forms of SP-D. In contrast with the more highly multimerized forms of SP-D, namely conglutinin or mannose-binding lectin, CL-43 did not induce viral or bacterial aggregation and did not enhance IAV-induced neutrophil H2O2 generation. Like SP-D, CL-43 also strongly enhanced neutrophil uptake of IAV. However, the mechanism of this enhanced internalization is different from that of SP-D in that it did not require viral aggregation. These studies establish that the trimeric structure of CL-43 is specified by its primary sequence and indicate that this naturally occurring trimeric collectin has unique antiviral activities. These findings could facilitate the development of recombinant collectins with novel antimicrobial properties.