It Eradicated Smallpox, But How?

Researchers are collecting clues about the protection afforded by the smallpox vaccine, the gold standard of vaccines

By Andreas von Bubnoff

One riddle currently confronting researchers is identifying the types of immune responses an AIDS vaccine would have to induce to protect against HIV. They are far from alone on this type of quest. For many vaccines, the immune responses that are actually responsible for providing protection against a pathogen, referred to as the immune correlates of protection, elude researchers even after the vaccine has been in use for decades (see VAX November and December 2006 Primers on Understanding Immune Correlates of Protection Part I and II).

Once researchers find a vaccine that works, there is little interest in figuring out why. But there are benefits to understanding how an effective vaccine affords protection. “We should really know how the things that work, work,” says Shane Crotty, an associate professor for vaccine discovery at the La Jolla Institute of Allergy and Immunology.

Take smallpox, a disfiguring and often deadly disease caused by variola virus. A vaccine, called Dryvax, that protects against this virus led to the eradication of the disease in the late 1970s. Crotty refers to the smallpox vaccine as the gold standard since it is the only vaccine that has ever led to the eradication of a disease. Yet, for several reasons, the immune correlates of protection for the smallpox vaccine are still unknown. When smallpox was eradicated, many of the modern methods used by researchers to measure immune responses weren’t yet available (see VAX February 2009 Primer on Understanding How Immune Responses to AIDS Vaccine Candidates are Measured). At that time, researchers could not measure T-cell responses, says Mark Slifka, associate professor at Oregon Health & Science University. Most of the data on how the smallpox vaccine works were collected from observational studies, and because there are no naturally occurring smallpox infections anymore, it would be impossible to do a randomized clinical trial of a smallpox vaccine today to study the immune correlates of protection.

However, there is now a renewed interest in trying to understand how the smallpox vaccine works. This is driven, in part, by the need to develop a new vaccine, with fewer side effects, which could be used to guard against a potential bioterrorism attack, Crotty says. Dryvax can cause serious side effects in people with compromised immune systems, including people with AIDS, according to D. Huw Davies, a project scientist at the University of California in Irvine. Another recently approved smallpox vaccine called ACAM2000 is a safer version of Dryvax, but it too causes side effects in immunocompromised people, Davies says. Researchers are therefore working on developing another smallpox vaccine and this has led them to try to figure out just how Dryvax provides such strong and long-lasting protection.

While identifying the exact immune correlates of protection for the smallpox vaccine may never be possible, researchers are now starting to collect clues about the way it protects by studying vaccinated individuals and people who have survived an infection, as well as using animal models. So far they have found that the smallpox vaccine primarily works by inducing neutralizing antibodies against the virus. These Y-shaped molecules can bind to the virus and inactivate or neutralize it before it has the chance to infect its target cells (see VAX February 2007 Primer onUnderstanding Neutralizing Antibodies). Antibody responses are considered a critical component in the protection provided by most, if not all, vaccines that are currently in use. Researchers have observed that the antibody responses induced by the first smallpox vaccine are surprisingly variable and redundant. They are now trying to identify certain markers in the antibody response that they hope will help them to predict whether a safer, alternative vaccine will also be protective.

The principles learned from a vaccine that protects against smallpox are unlikely to apply directly to development of an AIDS vaccine, but if anything, dissecting the lifelong protection afforded by the smallpox vaccine illustrates the critical role that antibodies play in vaccine-mediated protection.

Searching for the correlates

“It’s been thought for quite some time that the smallpox vaccine does work on the basis of neutralizing antibodies, but it was really just [a few] years ago that that was directly shown,” says Crotty, referring to a study completed in 2005 that provided evidence in animal experiments that antibodies are required for protection by the smallpox vaccine. “That experiment nailed it,” Crotty says.

In that study, researchers vaccinated monkeys with the human smallpox vaccine and then inhibited either their antibody or cellular immune responses to determine which of these was required for protection against the monkey version of the smallpox virus. They found that inhibiting the antibody response eliminated the protective effect of the vaccine.

While cellular immune responses, namely T cells, play a role in protection against smallpox, says Slifka, an antibody response may almost be completely sufficient for protection against infection with the virus that causes smallpox. He is now studying the antibody and cellular immune responses in a cohort of smallpox survivors and people who received a smallpox vaccination to see if the vaccine induces an immune response similar to that in natural infection.

Davies and Crotty have found that antibody responses to the smallpox vaccine are surprisingly variable among vaccinated people. The antibody responses also appear to be redundant, suggesting that there is not a single mechanism or one magic antibody that is required for protection against smallpox. It seems that as long as the antibodies that are induced by the vaccine cover the surface of the virus, they are able to neutralize it, and thereby protect against infection. “[It’s like] throwing a net over the virus,” says Crotty.

Predicting protection

Researchers are also using animal experiments to identify markers in the antibody response that might help them predict protection for new smallpox vaccine candidates. They will use these markers to evaluate samples from a Phase I clinical trial of a new smallpox vaccine that utilizes a modified vaccinia Ankara (MVA) virus as a vector to see if the MVA-based candidate can protect as well as Dryvax and therefore offer a safer alternative to this existing vaccine. AIDS vaccine researchers are also exploring MVA-based vaccine candidates.

Now that the immune response elicited by the smallpox vaccine has been rather clearly described, Crotty says, the next big question is how it provides such long-lasting protection. “Why is it that you can give one immunization with this vaccine and you get a fantastic protective antibody response and it lasts for life?” he asks.

Lessons for AIDS vaccines?

There are many differences between the smallpox virus and HIV, including their size. HIV has only a single protein covering its surface to which most antibodies would bind, and is made up of nine genes. Comparatively, the smallpox virus is very large, with about 200 genes and dozens of surface proteins. However, HIV is a more complicated pathogen to combat because of its nearly unrivaled ability to change or mutate to avoid the immune responses mounted against it.

Given these differences, understanding how the smallpox vaccine works may not be the best example to guide development of an AIDS vaccine. “We have been applying the rules of conventional vaccinology to HIV since it emerged in 1983,” says Davies, “but this has largely failed us.” While antibodies are likely important for protection to both smallpox and HIV, something very different from conventional vaccines needs to be developed against the rapidly evolving HIV, Davies adds.

Still, there are some general lessons. If there is anything to be learned from understanding the smallpox vaccine, “it’s that neutralizing antibodies are so key for protection,” says Crotty. “It’s yet another piece of information that suggests that you probably need to be able to make neutralizing antibodies.”

The AIDS vaccine candidates tested in clinical trials to date have been largely unsuccessful at inducing broadly neutralizing antibodies against HIV. To develop candidates that are more likely to generate an antibody response, researchers are now increasingly focusing their efforts on looking for new broadly neutralizing antibodies from HIV-infected individuals and studying the handful that have already been identified. The big challenge then is figuring out how to design immunogens—non-infectious fragments of HIV that are included in vaccine candidates—that can induce these antibodies in people (see Primer, this issue).