Human mannan-binding lectin inhibits the infection of influenza A virus without complement

Mannan-binding lectin (MBL) is a C-type serum lectin that is believed to pl ay an important role in innate immunity. It is one of the collectin family, which is characterized by having a collagenlike sequence and a carbohydrat e recognition domain. MBL can bind to sugar determinants of several micro-o rganisms, neutralize them and inhibit infection by complement activation th rough the lectin pathway and opsonization by collectin receptors. Bovine co nglutinin and mouse MBL inhibit the infective and haemagglutinating activit ies of influenza A viruses. To identify the direct antiviral activity of hu man MBL against influenza A viruses that does not depend on complement acti vation or opsonization, we isolated native MBL from human serum and produce d a recombinant MBL in Chinese hamster ovary (CHO) cells using a pNOW/CMV-A expression vector system. Native and recombinant human MBL exhibited neutr alization activity against A/Ibaraki/1/90 (H3N2), with the plaque focus red uction assay at the viral attachment phase. Their activities were inhibited by EDTA, mannose and anti-human MBL antibody. Furthermore, at the viral ex pansion phase both MBL in culture medium prevented viral spreading from pri mary infected cells to neighbour cells. A virus recovery study using EDTA i ndicated that interaction between MBL and virus was reversible and non-dama ging to the virus. Lectin blot and immunohistochemistry assays showed that these antiviral activities involved binding between MBL and two viral envel ope proteins, haemagglutinin and neuraminidase. These findings suggest that human MBL can play an important role in innate immunity by direct viral ne utralization and inhibition of viral spread, as well as an indirect role th rough opsonization and complement activation.