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

Abstract

Mannan-binding lectin (MBL) is a C-type serum lectin that is believed to play an important role in innate immunity. It is one of the collectin family, which is characterized by having a collagen-like sequence and a carbohydrate recognition domain. MBL can bind to sugar determinants of several micro-organisms, neutralize them and inhibit infection by complement activation through the lectin pathway and opsonization by collectin receptors. Bovine conglutinin and mouse MBL inhibit the infective and haemagglutinating activities of influenza A viruses. To identify the direct antiviral activity of human MBL against influenza A viruses that does not depend on complement activation or opsonization, we isolated native MBL from human serum and produced a recombinant MBL in Chinese hamster ovary (CHO) cells using a pNOW/CMV-A expression vector system. Native and recombinant human MBL exhibited neutralization activity against A/Ibaraki/1/90 (H3N2), with the plaque focus reduction assay at the viral attachment phase. Their activities were inhibited by EDTA, mannose and anti-human MBL antibody. Furthermore, at the viral expansion phase both MBL in culture medium prevented viral spreading from primary infected cells to neighbour cells. A virus recovery study using EDTA indicated that interaction between MBL and virus was reversible and non-damaging to the virus. Lectin blot and immunohistochemistry assays showed that these antiviral activities involved binding between MBL and two viral envelope proteins, haemagglutinin and neuraminidase. These findings suggest that human MBL can play an important role in innate immunity by direct viral neutralization and inhibition of viral spread, as well as an indirect role through opsonization and complement activation.

Virus complement evasion strategies

“…Also, opsonization of virions and infected cells by the deposition of C3b may lead to phagocytic uptake by leukocytes. Furthermore, interaction of complement components with virion surfaces on itself has been reported to be neutralizing for certain viruses (Ikeda et al, 1998;Kase et al, 1999).…”

Anti-Influenza A Virus Activities of Mannan-Binding Lectins and Bovine Conglutinin

“…Recently, we have focused on the direct anti-influenza virus activities of human MBL independent of complement activation and opsonization, and showed that human MBL had an ability to inhibit viral spreading from primary infected cells to noninfected cells, which was designated as viral growth inhibition (GI) activity, as well as inhibit viral hemagglutination and infectivity directly [19]. However, whether other animal serum collectins exert direct inhibitory effects against influenza A virus has not been definitely determined.…”

“…Bovine MBL, rabbit MBL and human MBL were purified from each serum sample using mannan-agarose (SigmaAldrich Corp., St. Louis, MO, U.S.A.) as previously described [19][20][21]. BKg was purified from heat-inactivated (56°C, 30 min) bovine serum [29].…”

“…The hemagglutination inhibition (HI) test was done by a standard microtiter method [19]. Influenza A virus A/Ibaraki/1/90 (H3N2) used was sensitive to human MBL [19].…”

“…Influenza A virus A/Ibaraki/1/90 (H3N2) used was sensitive to human MBL [19]. Viral solution (4 HA units) was incubated with serial dilutions of collectins for 30 min at 37°C, and then mixed with 0.5% chicken erythrocytes.…”