New Hopes for HIV/AIDS Vaccine
Kara Thompson American Journal of Science
Scientists know more about HIV--the human immunodeficiency virus that causes AIDS--than any other virus. Yet designing a
vaccine able to protect against it remains as much of a challenge today as when the virus was discovered. Part of the problem is that,
unlike the body's response to most acute viral infections, the natural immune response does not destroy HIV. This failure makes it
difficult for investigators to know what type of immune activity an effective
vaccine should evoke.
At the same time, researchers have to be extremely cautious about using the preparations that have become standard for warding off
other infectious diseases--such as whole, killed viruses or live, attenuated versions. If HIV vaccines in these forms managed to cause
infections, the consequences could be devastating. Vaccinologists therefore have had to search for alternative ways to immunize
people against HIV.
Vaccines protect individuals by priming the immune system to recognize disease-causing organisms when they are encountered. In
the case of HIV, a successful vaccine should be able to eliminate incoming virus and destroy quickly any cells that become infected.
Most vaccines activate what is called the humoral arm of the immune system, stimulating formation of protective antibodies:
molecules that mark free virus (which circulates outside cells) for destruction. The antibodies recognize and bind to a unique part of
the infectious agent. This unique structure, called an antigen, is often a protein on the viral surface.
Foreign antigens on an invading virus or in a vaccine activate two types of white blood cells involved in antibody manufacture. After
contacting antigens, cells known as B lymphocytes mature and produce antibodies. In addition, helper, or CD4, T lymphocytes direct
B cells to manufacture more antibodies or to take the form of memory B cells. The memory cells do not produce antibodies
immediately but respond vigorously to subsequent exposures. Following vaccination, the long-term production of small amounts of
antibody and the persistence of memory cells allow the body to mount a rapid defense if ever it encounters the virus.
No vaccines have been designed specifically to stimulate the other arm of the immune system, known as the cellular component. But
many AIDS researchers are working on just that aim because, thus far, vaccines designed to generate antibodies to HIV have failed to
elicit immunity against the strains of the virus commonly found in infected patients.
In cellular immunity, activated white blood cells called cytotoxic T lymphocytes (CD8 T cells) multiply and cruise through the
bloodstream and tissues, searching for and eliminating virus-infected cells. Some also become memory cells, ready to leap into action
after a later exposure to a pathogen. Unlike antibodies, cytotoxic T lymphocytes recognize infected cells, rather than the infectious
agent itself. Like B cells in the humoral arm of the immune system, however, cytotoxic T cells are activated in part by signals from