March 16, 2010
A study just published in the Journal of Biochemistry has generated a great deal of press coverage because it reports that a compound derived from bananas can inhibit HIV in a lab dish. The substance, a banana lectin known as BanLec, was first discovered twenty years ago and shown to be a potent mitogen (a substance which non-specifically stimulates T cell proliferation). The mechanism of HIV inhibition appears related to its ability to bind to sugar molecules on the viral envelope, thereby blocking virus attachment and entry into target cells. This mechanism is similar to the broadly neutralizing monoclonal antibody 2G12, although the paper reports that, unlike 2G12, BanLec can inhibit HIV isolates from clade C as well as clade B. The researchers conclude that BanLec might be adapted for use in microbicides, but caution that its mitogenic potential would have to somehow be stymied. Unfortunately many of the press stories have missed this caveat, instead characterizing BanLec as "naturally occurring" as if this is synonymous with safe. But the caveat is crucial, because stimulation of T cell proliferation would carry the risk of increasing susceptibility to HIV infection and causing other untoward inflammation-related toxicities. A Google news search for "BanLec HIV" currently retrieves 156 results, a search for "BanLec mitogen" retrieves none.
J Biol Chem. 2010 Mar 19;285(12):8646-55. Epub 2010 Jan 15.
Michael D. Swanson‡, Harry C. Winter§, Irwin J. Goldstein§ and David M. Markovitz‡¶||
From the ¶Department of Internal Medicine, Division of Infectious Diseases, ‡Program in Immunology, ||Cellular and Molecular Biology Program, and §Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor, Michigan 48109
BanLec is a jacalin-related lectin isolated from the fruit of bananas, Musa acuminata. This lectin binds to high mannose carbohydrate structures, including those found on viruses containing glycosylated envelope proteins such as human immunodeficiency virus type-1 (HIV-1). Therefore, we hypothesized that BanLec might inhibit HIV-1 through binding of the glycosylated HIV-1 envelope protein, gp120. We determined that BanLec inhibits primary and laboratory-adapted HIV-1 isolates of different tropisms and subtypes. BanLec possesses potent anti-HIV activity, with IC50 values in the low nanomolar to picomolar range. The mechanism for BanLec-mediated antiviral activity was investigated by determining if this lectin can directly bind the HIV-1 envelope protein and block entry of the virus into the cell. An enzyme-linked immunosorbent assay confirmed direct binding of BanLec to gp120 and indicated that BanLec can recognize the high mannose structures that are recognized by the monoclonal antibody 2G12. Furthermore, BanLec is able to block HIV-1 cellular entry as indicated by temperature-sensitive viral entry studies and by the decreased levels of the strong-stop product of early reverse transcription seen in the presence of BanLec. Thus, our data indicate that BanLec inhibits HIV-1 infection by binding to the glycosylated viral envelope and blocking cellular entry. The relative anti-HIV activity of BanLec compared favorably to other anti-HIV lectins, such as snowdrop lectin and Griffithsin, and to T-20 and maraviroc, two anti-HIV drugs currently in clinical use. Based on these results, BanLec is a potential component for an anti-viral microbicide that could be used to prevent the sexual transmission of HIV-1.