Carrageenan is a potent inhibitor of papillomavirus infection

Abstract

Certain sexually transmitted human papillomavirus (HPV) types are causally associated with the development of cervical cancer. Our recent development of high-titer HPV pseudoviruses has made it possible to perform high-throughput in vitro screens to identify HPV infection inhibitors. Comparison of a variety of compounds revealed that carrageenan, a type of sulfated polysaccharide extracted from red algae, is an extremely potent infection inhibitor for a broad range of sexually transmitted HPVs. Although carrageenan can inhibit herpes simplex viruses and some strains of HIV in vitro, genital HPVs are about a thousand-fold more susceptible, with 50% inhibitory doses in the low ng/ml range. Carrageenan acts primarily by preventing the binding of HPV virions to cells. This finding is consistent with the fact that carrageenan resembles heparan sulfate, an HPV cell-attachment factor. However, carrageenan is three orders of magnitude more potent than heparin, a form of cell-free heparan sulfate that has been regarded as a highly effective model HPV inhibitor. Carrageenan can also block HPV infection through a second, postattachment heparan sulfate-independent effect. Carrageenan is in widespread commercial use as a thickener in a variety of cosmetic and food products, ranging from sexual lubricants to infant feeding formulas. Some of these products block HPV infectivity in vitro, even when diluted a million-fold. Clinical trials are needed to determine whether carrageenan-based products are effective as topical microbicides against genital HPVs.

Figure 1. Polysaccharide Structures

Idealized average structures of the disaccharide repeat units of various sulfated polysaccharides. Ac = acetyl, R = [H or SO₃ ⁻], R′ = [H, Ac, or SO₃ ⁻]. Structures adapted from [13], http://www.lsbu.ac.uk/water/hycar.html, and http://www.sigmaaldrich.com.

Figure 2. Capsid Dose Influences Carrageenan IC₅₀

Inhibition assays were performed using a standard dose of GFP-expressing HPV16 PsV in the presence of increasing doses of cold capsids. Overall capsid dose is given as the final concentration of the major capsid protein, L1, in the culture medium.

Figure 3. Capsids Bind Carrageenan

Carrageenan beads were incubated with HPV16 or HPV5 capsids in buffers with the NaCl concentration shown. The beads were washed, then bound capsids were eluted and visualized in stained SDS-PAGE gels. Bovine serum albumin (BSA, 65 kDa) was used as a control for nonspecific binding to the beads. For HPV16, an L2 band can be seen above the L1 band.

Figure 4. Standardized Carrageenan IC₅₀ for Various Papillomavirus Types

Points represent carrageenan IC₅₀ of infectivity, except for empty red triangles, which represent the carrageenan IC₅₀ of cell binding for HPV16 capsids covalently linked to a fluorescent dye. Empty circles represent carrageenan IC₅₀ of infectivity observed using HaCaT cells instead of HeLa cells. Error bars represent the 95% CI for the IC₅₀.

Figure 5. Carrageenan Addition Time Course

Cells were incubated with HPV16 PsV for 2 h, followed by washout of the virus inoculum. Carrageenan was added at the timepoints shown, where time zero represents initial PsV inoculation.