How do researchers measure the immune responses induced by AIDS vaccine candidates?
Researchers measure the efficacy of preventive AIDS vaccine candidates in Phase III clinical trials. A candidate is effective if it protects recipients from HIV infection or, in the case of a partially-effective vaccine, if it either slows or prevents disease progression in individuals who subsequently become infected through exposure to the virus (seeVAX May 2007 Primer on Understanding Partially Effective AIDS Vaccines).
In the earlier stages of clinical evaluation, during both Phase I and II trials, researchers are primarily measuring the safety of the vaccine candidates and the extent to which they induce immune responses against HIV, an idea referred to as immunogenicity. Together this information helps researchers prioritize candidates for further evaluation.
There are two main types of immune responses to HIV that are routinely assessed. The first is the presence of HIV-specific neutralizing antibodies that are capable of latching on to the virus and disabling it (see VAX February 2007Primer on Understanding Neutralizing Antibodies). Tests such as the enzyme-linked immunosorbent assay, or ELISA, are used to detect and quantify HIV-specific antibodies that are induced in response to an AIDS vaccine candidate. An ELISA is performed by exposing a blood plasma sample from a vaccinated individual to HIV antigens—the pieces of HIV that are in the vaccine—on a plastic plate. Any antibodies that are present will bind to the HIV antigen. The bound antibodies can then be separated from any other antibodies that were in the blood and the quantity of HIV-specific antibodies can be measured. The ELISA is also commonly used to determine if a person is HIV infected (seeVAX November 2005 Primer on Understanding HIV Testing). More specific assays can also measure the ability of HIV-specific antibodies to successfully neutralize the virus.
The other category of immune responses is cellular immunity, which includes two specific types of immune cells known as CD4+ and CD8+ T cells. The majority of vaccine candidates that are currently in clinical trials primarily induce cellular immune responses and there are several different assays that are used to measure both the quantity and quality of these responses.
An ELISPOT assay is most commonly used to measure the immunogenicity of AIDS vaccine candidates. It works by detecting CD4+ and CD8+ T cells that are producing cytokines, which are a group of proteins secreted by immune cells in response to a virus or bacteria. Cytokines are sometimes referred to as the messengers of the immune system and they can also inhibit a virus from replicating. The interaction of an HIV antigen (from a vaccine candidate) with an immune cell can result in the secretion of many different cytokines and researchers can detect the release of these proteins by using an ELISPOT assay. Usually researchers use an ELISPOT to detect the presence of a specific cytokine called interferon gamma (or IFN-g) that is secreted by both CD4+ and CD8+ T cells as a defense mechanism against viruses.
During clinical trials ELISPOT assays are run in immunology laboratories on blood samples collected from volunteers that have received the AIDS vaccine candidate being tested. From these samples researchers isolate the white blood cells—called peripheral blood mononuclear cells or PBMCs—that are critical to the immune system. These cells are then added on to a plastic plate that is coated with antibodies. When the PBMCs are stimulated with HIV antigens they release different cytokines, including IFN-g, that attach to the antibodies already on the plate. Other antibodies that are tagged with a chemical that produces a strong color are then added, so that wherever there is an immune cell that is producing cytokine a dark spot will appear. The presence of spots shows that there are CD4+ and CD8+ T cells that are responding to the HIV antigen included in the vaccine candidate.
By counting the spots, researchers can see how many cells are releasing IFN-g, for example. This is referred to as the number of spot-forming cells. Although ELISPOTS are most often looking at secretion of IFN-g, they can also look at many other cytokines that are released by immune cells. If the number of spot-forming cells for a vaccinated volunteer is above a threshold set by researchers before the start of the trial, then that volunteer is considered to have responded to the vaccine candidate. For many AIDS vaccine trials in developing countries, ELISPOT assays are run in immunology laboratories that are associated with the clinical trial site.
ELISPOT assays are just one way to measure the activation of the immune system by a vaccine candidate. The difficulty with interpreting the results of these assays is that researchers don't yet know if the production of IFN-g by immune cells, or any other cytokine, correlates with even partial protection against HIV infection. The precise immune responses that correlate with protection against HIV have not yet been identified (see VAX November and December 2006 Primers on Understanding Immune Correlates of Protection, Part I and II). The results of the ELISPOT assay serve only as an indication of immune function. Researchers are currently studying already-licensed vaccines for other diseases that induce cellular immune responses to provide clues about whether or not the ELISPOT assay provides an accurate measure of immunogenicity.
Although imperfect, the results from ELISPOT assays help researchers compare the immunogenicity of different vaccine candidates and therefore decide which should undergo further clinical evaluation. Another laboratory test called flow cytometry analyzes the ability of immune cells to produce several cytokines at once. Researchers are also starting to use assays that test the ability of immune cells to directly suppress the virus. All of these tests help researchers further classify the immunogenicity of different candidates.