http://www.ilovefoodsomuch.com/1-full-ripe-banana-a-day-can-increase-immunity/
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Banana Lectin (BanLec) prevents attachment of the human immunodeficiency virus (HIV) to immune cells and is well adapted to HIV tropism (the viral targeting of diverse immune cell receptors), thus it might become a microbicide against HIV infection.
Like in the battle against the influenza virus, glycoscience may come for help to prevent the spread of HIV – currently debilitating 33 million people worldwide –, albeit in a different way. gp120, a glycoprotein carrying many mannose-rich glycans, occurs abundantly on the surface of HIV and is pivotal for T cell attachment of the virus. gp120 glycans could be targeted by mannose-binding plant lectins, which would put the virus in a strait jacket, thereby blocking HIV attachment to immune cells and virus replication.
To prevent HIV infection, a microbicide to apply internally after unprotected sexual intercourse should be non-toxic and effective against the dreaded tropism of HIV. In 2009, the lectin Griffithsin (GRFT) was shown to exhibit these properties.
Whereas GRFT requires expression in genetically modified tobacco plants to gain sufficient amounts, BanLec can just be isolated from ripe bananas.
Swanson et al. infected TZM-bl cells which emit light upon viral entry and replication. They found that TZM-bl preincubated with 5nm BanLec decreased infection with various HIV strains by 90%. This suggests that BanLec is well adapted to deal with HIV tropism.
A 50% reduction of HIV attachment to the cells (termed IC50 value) was achieved with 3nm BanLec, which compares well to the values for GRFT (0.4nm) and snowdrop lectin (34nm).
BanLec prove its anti-viral strength even after the virus had bound to but not yet fused with the cell. Its IC50 value was 20nm, alike to the commercial available fusion inhibitor Maraviroc. Importantly, 25nm pre-incubation of macrophages with BanLec shielded them from HIV entry over a period of two weeks.
As vaginal macrophages can become HIV reservoirs resistant to antiviral therapy, this finding indicates that BanLec may be able to stop HIV infection from scratch, and that it maintains its inhibitory property under physiological conditions such as the temperature of the human body.
Closing in to the glycan binding properties of BanLec, Swanson et al. showed that BanLec was fully active against two common HIV subtypes whereas only one of them was recognized HIV-1 budding (in green) from cultured lymphocyte by the 2G12 antibody known to bind to mannose glycans at three different glycosylation sites. Moreover, methyl-α-D-mannose, a known ligand for BanLec, abolished its binding to gp120.
These observations reveal the glycan specificity of BanLec and its versatility towards glycosylation sites.
However, as retroviruses are glycosylated by host glycosyltranferases, autoimmune effects of GRFT and BanLec as well as the gp120-binding lectin candidates Cyanovirin-N and Scytovirin should be tested in vivo. Swanson et al. draw interesting conclusions from their observations.
First, mutations the HIV coat proteins may not diminish lectin binding as they flexibly target the proteins’ glycans; so resistance to lectins as microbicides, particularly if used in combination, is less likely to develop.
Second, glycosylation mutations can render HIV vulnerable to the immune system, particularly when glycans are shed from the viral envelope proteins.
Last, the results of O'Keefe and Swanson et al. encourage testing lectins against enveloped viruses such as Ebola and hepatitis C. Mirko von Elsterman Alexandre KB et al.
Mannose-rich glycosylation patterns on HIV-1 subtype C gp120 and sensitivity to the lectins, Griffithsin, Cyanovirin-N and Scytovirin. Virology 12 April 2010. Article O'Keefe BR et al.
Scaleable manufacture of HIV-1 entry inhibitor griffithsin and validation of its safety and efficacy as a topical microbicide component. Proc Natl Acad Sci U S A 2009, 106, 6099-6104. Article Swanson MD, Winter HC, Goldstein IJ, Markovitz DM.
A lectin isolated from bananas is a potent inhibitor of HIV replication. J Biol Chem. 2010, 285, 8646-8655. Article
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