January 2004, Vol. 10, No. 1

Biomedical Research
Carraguard May Block HIV by Adhering to Cells

HIV may be ferried deep into the body from the vagina by immune system cells known as macrophages, suggests new research conducted by Population Council virologist David M. Phillips and his colleagues. The research also showed that the Council’s lead candidate microbicide gel, Carraguard^®, is effective at reducing this form of HIV transmission in lab animals. The Population Council is preparing to enter into large-scale efficacy trials to test Carraguard’s efficacy in blocking HIV transmission in 4,000 non-pregnant women. The microbicide will be one of the first products of its kind to enter this advanced phase of research.

The active ingredient in Carraguard is carrageenan, a substance derived from seaweed. Carrageenan compounds are on the U.S. Food and Drug Administration’s list of GRAS (generally recognized as safe) products for consumption and topical application.

The term “microbicide” refers to a range of products, in cream, gel, film, or suppository form, that would substantially reduce the transmission of HIV—and possibly other sexually transmitted infections (STIs)—when applied topically. If proven viable, these products would offer a powerful new prevention tool in the fight against AIDS. Globally, women’s rates of HIV infection are growing more rapidly than men’s, and the current strategies for prevention—mutual monogamy among HIV-negative partners, condom use, and treatment of existing STIs—are not practical for many women. Because of this urgent need for a woman-controlled product, and one that does not necessarily prevent pregnancy, the Population Council has focused to date on a vaginal, non-contraceptive microbicide.

Macrophage trafficking
Previous research by Phillips and others has suggested that macrophages, which ingest invading microbes, can migrate from the intact surface of the inside of the vagina to lymph nodes and other immune system structures inside the body. Some investigators have suggested that HIV-infected macrophages in semen can act as Trojan horses, transporting HIV into the body as they conduct their normal immune system duties.

To confirm whether macrophages could indeed make this journey, Phillips and his colleagues stained live mouse macrophages with an orange dye. They then inserted these cells into the vaginas of mice. Four hours later, the immune system organs were removed from the mice and processed. The investigators discovered an average of 55 orange-dyed macrophages per animal in lymph nodes from the pelvic and groin region. They also found an average of 558 orange-dyed macrophages per animal in the spleen, an immune system organ located just below the diaphragm.

The next step was to test the effectiveness of Carraguard against that of a placebo. The researchers used methylcellulose, a gel similar in appearance and consistency to Carraguard. This time, Phillips and his team treated mice vaginally with either Carraguard or methylcellulose 20 minutes in advance of introducing the macrophages.

In mice that received a vaginal pretreatment with Carraguard, an average of only four orange-dyed macrophages were found in lymph nodes per animal. An average of only 28 such macrophages were discovered in the animals’ spleens. When the mice were pretreated with methylcellulose, an average of 26 macrophages reached the lymph nodes and 153 homed in on the spleen. The difference in the number of macrophages seen in these immune system organs between the Carraguard- and the methylcellulose-treated mice was statistically significant. The difference in these measures between Carraguard-treated and untreated animals was highly significant, whereas the difference between methylcellulose-treated and untreated mice was not significant.

Mechanism of action
Eager to learn how Carraguard kept macrophages out of the body, Phillips and his team placed the cells in culture dishes containing either Carraguard or methylcellulose. They also left some macrophages unexposed to either substance. They wanted to see whether either substance could kill macrophages outright. They found that, although the viability of the macrophages exposed to Carraguard and methylcellulose was reduced somewhat compared to the unexposed macrophages, the exposed cells fared similarly. Some other mechanism must account for the inhibition of HIV transmission seen with Carraguard.

The researchers then sought to determine whether Carraguard prevented macrophage movement by binding to them. They used a technique that detects only carrageenan firmly attached to cells. They found that Carraguard adhered to both macrophages and the cells in the vagina. Because carrageenan is a large molecule that cannot enter the body through the vagina, “this binding is likely the mechanism that reduced the number of macrophages that traveled from the vagina to the lymph nodes and spleen,” says Phillips.

HIV may have several means of entering the body, the scientists note. Thus, it is important to develop microbicides that block HIV transmission in many ways. The Population Council continues to investigate ways of broadening the mechanisms that its candidate microbicides use to reduce HIV transmission.

Source
Perotti, Maria-Elisa, Alessia Pirovano, and David Phillips. 2003. “Carrageenan formulation prevents macrophage trafficking from vagina: Implications for microbicide development,” Biology of Reproduction 69(3): 933–939.

Outside funding
MIUR/University of Milan, the National Institute Allergy and Infectious Diseases, the National of Child Health and Human Development, Rockefeller Foundation

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See Also

• Microbicides

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