Mannose binding lectin (MBL) and HIV

The envelope protein (gp120/gp41) of HIV-1 is highly glycosylated with about half of the molecular mass of gp120 consisting of N-linked carbohydrates. While glycosylation of HIV gp120/gp41 provides a formidable barrier for development of strong antibody responses to the virus, it also provides a potential site of attack by the innate immune system through the C-type lectin mannose binding lectin (MBL) (also called mannan binding lectin or mannan binding protein). A number of studies have clearly shown that MBL binds to HIV. Binding of MBL to HIV is dependent on the high-mannose glycans on gp120 while host cell glycans incorporated into virions do not contribute substantially to this interaction. It is notable that MBL, due to its specificity for the types of glycans that are abundant on gp120, has been shown to interact with all tested HIV strains. While direct neutralization of HIV produced in T cell lines by MBL has been reported, neutralization is relatively low for HIV primary isolates. However, drugs that alter processing of carbohydrates enhance neutralization of HIV primary isolates by MBL. Complement activation on gp120 and opsonization of HIV due to MBL binding have also been observed but these immune mechanisms have not been studied in detail. MBL has also been shown to block the interaction between HIV and DC-SIGN. Clinical studies show that levels of MBL, an acute-phase protein, increase during HIV disease. The effects of MBL on HIV disease progression and transmission are equivocal with some studies showing positive effects and other showing no effect or negative effects. Because of apparently universal reactivity with HIV strains, MBL clearly represents an important mechanism for recognition of HIV by the immune system. However, further studies are needed to define the in vivo contribution of MBL to clearance and destruction of HIV, the reasons for low neutralization by MBL and ways that MBL anti-viral effects can be augmented.