Designing an effective vaccine to protect people from infection with HIV (human immunodeficiency virus) or from becoming ill if already infected by the virus is a high priority among worldwide efforts to control the epidemic.
The ideal HIV vaccine would be inexpensive, easy to store and administer, and would elicit strong, appropriate immune responses that confer long-lasting protection against HIV infection by exposure to infected blood and by sexual contact. The ideal vaccine would also protect against exposure to many different strains of HIV. Despite extraordinary advances in understanding both HIV and the human immune system, such a vaccine continues to evade researchers.
The most rational way to design an effective vaccine is to learn which immune responses protect against the specific infection and to construct a vaccine that stimulates those responses. Although scientists have found clues about these correlates of immunity or correlates of protection for HIV, they have not yet identified these correlates with precision, and are still trying to design vaccines to induce the appropriate immune responses necessary for protection. Once the correlates of immunity are defined, it will be necessary to have standardized and validated immunological assays to measure and compare immune responses among various strategies and candidates of HIV vaccines.
Unlike other viral diseases for which investigators have made successful vaccines, there are no documented cases of complete recovery from HIV infection. Therefore, HIV vaccine researchers have no human model of protection to guide them. Indeed, whether a natural protective state against HIV can exist remains unknown.
Now that the pandemic has matured, however, long-term survivors -- those who remain clinically asymptomatic and maintain a CD4+ T cell count greater than 200 for at least 10 years following infection -- provide ample evidence that some people appear better able than others to resist progression of HIV infection or developing AIDS. Long-term survivors can be divided into two groups:
Researchers have been studying individuals who remain uninfected despite repeated exposure to HIV. If researchers prove these multiply exposed but uninfected individuals have resisted HIV through active immune mechanisms, they would represent the natural protective state upon which a vaccine could be modeled.
Another clue to why some people resist HIV infection has come from studies of the cellular receptors for HIV. Scientists have found that individuals who inherited two copies of a mutated gene coding for one of these receptors, CCR5, appear to be protected from infection with HIV strains using this co-receptor. Interventions targeting this receptor may be capable of preventing HIV infection.
To determine the factors that influence the body's response to HIV exposure and infection, investigators are analyzing the differences between long-term HIV-infected survivors with people who quickly became infected or sick. Leading areas of research include the genetic background, variations in the immune response, and exposure to or infection by less deadly HIV variants. Such studies will help clarify what contributes to protective immunity against HIV.
Immunogenicity refers to the ability of a vaccine or a component of a microorganism to stimulate immune responses. The two main types of immune responses are humoral and cellular immunity.
Humoral (antibody-mediated) immunity refers to protection provided by antibodies, the secreted products of one type of white blood cell called a B lymphocyte. Antibodies are custom-made proteins that circulate in body fluids (primarily blood and lymph), and they specifically recognize foreign bacterial or viral components. B lymphocytes (B cells) produce antibodies in response to a specific foreign invader like HIV or a vaccine.
Antibodies can have different properties. Antibodies can simply attach to part of HIV and may or may not have antiviral effects. Other antibodies actually do something more; for example, neutralizing antibodies inactivate HIV or prevent it from binding and infecting cells.
Scientists have identified the outer envelope of HIV as important for stimulating neutralizing antibodies. Multiple copies of a protein called gp160 form the HIV envelope. Using recombinant technology, gp160 and gp120, a component of gp160, along with various molecularly engineered versions of these proteins, have been produced, purified and tested as vaccines. These recombinant vaccines are designed to induce immune responses specific against the envelope protein.
The second type of immunity, cellular (cell-mediated) immunity, refers to activities of T lymphocytes. Cytotoxic T lymphocytes (CTLs), nicknamed killer T cells, directly destroy HIV-infected cells. CTLs (CD8+ T cells) bear receptors on their surfaces that target cells that are producing HIV. Other CD8+ T cells can suppress HIV replication without necessarily killing the infected cell. CD8+ T cells appear to be critical to resisting HIV infection.
T helper cells, another component of cellular immunity, direct antibody- and cell-mediated immune responses, like a conductor leading a symphony orchestra, through the secretion of molecules called cytokines that have important effects on B cells, CTLs and other immune cells. The helper T cell, also happens to be HIV's main target. The virus attaches to the cell through a receptor on the cell's surface called CD4. Hence, helper T cells are called CD4+ T cells.
A subset of helper T cells, memory T cells, is induced by first exposure to an invading organism. The name "memory" reflects their function, which is to create a criminal record on that virus or microorganism. If the virus enters the body again, memory T cells will quickly stir the immune system into action faster and more potent than the original immune response. The most common way to measure memory T cells is by a test called the lymphocyte proliferation assay, which indicates the strength of such cellular responses to HIV.
To be effective, an HIV vaccine may also have to stimulate immunity (B cells and T cells) at the mucous membranes that line the rectal and genital tract and induce what is called mucosal immunity. Scientists do not fully understand how immune cells lining the genital tract and other HIV portals into the body protect the body, but the cells may be important to blocking HIV transmission.
HIV continually evolves because of genetic mutation and recombination. Thus, researchers must estimate the significance of strain variation within individuals and among populations when developing HIV vaccines. Initially, a person is infected with only one or a limited number of HIV variants. Once HIV infection becomes established, however, the virus continually undergoes changes, and many variants may arise within an infected person.
Whenever a drug or immune response destroys one variant, a distinct but related resistant variant can emerge. In addition, certain variants may thrive in specific tissues or become dominant in an individual because they replicate faster than others. Any of these changes may yield a virus that can escape elimination by the immune system.
The genes encoding the envelope and core proteins of HIV isolates obtained from patients, have been analyzed and compared. On this basis, scientists have grouped HIV isolates worldwide into three groups, M, N, and O. The M (Major) group can be further divided into 9 subtypes. Each subtype within a group is about 30 percent different from any of the others. If an individual is infected with two different subtypes, a new (recombinant) form of virus can result that contains gene fragments from both parental viruses. Hence, there are an infinite number of HIV variants circulating worldwide, and a successful vaccine will need to induce an immune response that protects against a large portion of these variants. In contrast, successful vaccines for other viruses have only had to protect against one or a limited number of virus subtypes.
The first HIV vaccines made were derived from laboratory-adapted versions of a particular strain of virus known as the LAI strain (also known as IIIB or LAV). Other vaccines have been based on the SF-2 and MN isolates, which belong to the same subtype as LAI; subtype B that is prominent in the U.S. and Europe. Recently, it has been shown that these and other laboratory-adapted viruses are more sensitive to neutralization than viruses in the wild, due to the laboratory adaptation process. Newly developed vaccines are now based on wild-type HIV-1 strains, and many vaccines are being designed based on the subtypes most prevalent in Asia and Africa.
Given that a preventive HIV vaccine will need to generate immune responses that protect uninfected individuals from all the different HIV subtypes (clades) and recombinant forms to which they may be exposed, scientists are looking for conserved regions of HIV genes, those common to all or most subtypes. Unless such common regions can be identified, a cocktail vaccine comprising several proteins or peptides from different HIV strains may be necessary to invoke broad-based immunity.
Unlike some other viruses, HIV can be transmitted and can exist in the body not only as free virus but also within infected cells. Thus, a successful vaccine against HIV may need to stimulate the two main types of immune responses. Humoral immunity uses antibodies to defend against free virus while cellular immunity directly or indirectly results in the killing of infected cells. A major unanswered question is how important each type of immunity is for protection from HIV. Data from animal models and long-term HIV survivors and human clinical trials of experimental HIV vaccines may offer clues. However to date, there are no HIV vaccines that induce broadly neutralizing antibodies.
According to the World Health Organization, more than 90 percent of all HIV transmission worldwide occurs sexually. Thus, to be effective, an HIV vaccine also may need to stimulate mucosal immunity. Mucosal immune cells that line the respiratory, digestive, and reproductive tracts and those found in nearby lymph nodes are the first line of defense against infectious organisms. Unfortunately, relatively little is known about how the mucosal immune system protects against viral infection.
The most difficult challenges for vaccine researchers are: 1) the major target of HIV is the immune system itself, as HIV infects the key CD4+ T cells that regulate the immune response, modifying or destroying their ability to function, and 2) once the virus infects CD4+ T cells, its genetic material is permanently integrated into the cell's chromosomes, establishing permanent latency.
After infection, HIV incorporates its genetic material into the host cells. If a cell reproduces itself, each new cell also contains the integrated HIV genes. There the virus can hide its genetic material for prolonged periods until the cell is activated and makes new viruses. Other cells act as HIV reservoirs, harboring intact viruses that may remain undetected by the immune system.
Scientists at the National Institute of Allergy and Infectious Diseases (NIAID) and elsewhere have shown that no true period of biological latency exists in HIV infection. After entering the body, the virus rapidly disseminates, homing to the lymph nodes and related organs where it replicates and accumulates in large quantities. Paradoxically, the filtering system in these lymphoid organs, so effective at trapping pathogens and initiating an immune response, actually helps destroy the immune system. As CD4+ T cells travel to the lymph organs in response to HIV infection they are infected by the HIV that is harbored there.
Basic research in immunology, epidemiology, natural history and vaccine trials in animal models and humans all contribute to a greater understanding of the immune system breakdown and of how new vaccines may be designed to prevent or slow down the progress of HIV disease.
Because of safety concerns, most candidate HIV vaccines are based on one or more proteins of HIV, not the whole infectious virus. As a result, these vaccines cannot cause HIV infection. Traditionally vaccines have been made from whole viruses that have been inactivated or attenuated. Inactivated HIV or SIV (HIV's close cousin that infects monkeys) when tested as a vaccine has not been shown to induce a protective immune response in animal models, hence researchers have been hesitant to test this potentially dangerous vaccine type in humans. Attenuated SIV has been shown to induce the most promising protection in monkeys, but thus far attenuated HIV has been deemed too dangerous for human testing.
To augment the immune responses elicited by these and other vaccines, scientists use immunologic adjuvants that can increase the type, strength, and durability of immune responses evoked by a vaccine. Some vaccine adjuvant combinations can induce cellular immune responses in animals, even if the vaccine antigen by itself does not. Some adjuvants also stimulate mucosal immunity.
Currently, only one adjuvant -- alum, first discovered in 1926 -- is incorporated into vaccines licensed for human use by the U.S. Food and Drug Administration (FDA). An adjuvant may work well with one experimental vaccine but not another. Therefore, the FDA licenses the combination, rather than the adjuvant alone. Alum primarily increases the strength of antibody responses generated by the vaccine antigen. Because of alum's limited activity, other adjuvants now being evaluated in animal models and human studies may be better suited for the newer candidate HIV vaccines.
Improving vaccines to generate more robust and long-lasting T cell response is important for both humoral and cell-mediated immunity. An effective way to enhance immune responses to HIV is to combine vaccines. Researchers first prepare or prime the immune systems of volunteers with one vaccine, such as a live vector vaccine (a bacterium or virus genetically engineered to contain a synthetic HIV gene) and then boost these responses with a different vaccine, such as gp120 or gp160 subunits.
The best studied live vector vaccine is vaccinia virus, formerly used to immunize people against smallpox. Vaccinia is engineered to carry the foreign HIV gene(s) into the body. There, the vaccine directs cells to make the HIV protein, which in turn, stimulates production of protective antibodies and T cells. Later, the volunteers may receive booster shots of a different vaccine containing the same HIV protein carried by the vaccinia vaccine.
By itself, a gp160-containing vaccinia virus vaccine stimulates production of memory T cells but few antibodies. The prime-boost combination, however, can stimulate a strong cellular immune response -- including persistent killer CD8+ T cells -- as well as antibodies that neutralize the virus. Because of concerns that the standard vaccinia-based vaccine might cause serious vaccinia infection in some people with compromised immune systems, such as people already infected with HIV, researchers are developing and evaluating other more-attenuated vector vaccines, including attenuated vaccinia.
Several experimental vector vaccines made from a canarypox virus, which closely resembles vaccinia, are in clinical trials. Canarypox virus infects but does not reproduce in human cells, and therefore, should be much safer. Another example of a vector under development for HIV vaccines is Salmonella, a bacterium that infects the human gut.
At the NIAID Dale and Betty Bumpers Vaccine Research Center (VRC), an experimental adenoviral vector HIV vaccine is scheduled to be tested in the near future in human clinical studies. Although viral vectors may be an effective approach to designing vaccines, many individuals already have pre-existing immunity to certain vectors, such as adenovirus, from prior immunizations to other vaccines. Pre-existing immunity may blunt the desired immune response to an HIV vaccine. Thus, research is also underway to design alternative adenoviral vector serotypes for which pre-existing immunity is low or absent.
For the past 6 years, scientists have been evaluating DNA vaccines. DNA vaccines are direct injections of genes coding for specific HIV proteins and have been shown to induce cellular immune responses in animals. When the DNA is injected, the encoded viral proteins, such as HIV gp160, are produced, just as with live vectors. Scientists are actively pursuing the potential of this vaccine concept.
Animal studies can answer critical questions that cannot be answered either in humans, because of undue risk, or by using computer modeling or laboratory tests. For example, animals can be inoculated with an experimental vaccine and then exposed to a virus to test the vaccine's effectiveness -- a study that would be unethical to conduct in humans. Although AIDS researchers lack an ideal animal model, several animal studies have provided relevant information.
Chimpanzees can be infected with HIV, but only a few chimpanzees have developed disease, making it difficult to extrapolate findings to humans. Moreover, chimpanzees are an endangered species and are difficult and expensive to maintain.
Investigators use macaque monkeys in most non-human primate AIDS research. Macaques can be infected with SIV, a virus similar to HIV that causes an AIDS-like disease. The genetic and physical structures of SIV differ enough from those of HIV, however, that the results of SIV experiments may not be fully applicable in humans. Nonetheless, investigators have obtained important information from studies involving monkeys and chimpanzees. Experiments in both species have demonstrated the feasibility of developing a protective vaccine.
Moreover, studies conducted in macaques have used a chimeric virus (SHIV) that is based on SIV but enclosed within an HIV envelope. Because SHIV mimics HIV infection and causes serious illness in macaque monkeys, it allows researchers to study the reactions of the immune system to the vaccines and the virus. These types of studies are extremely valuable for evaluating candidate HIV vaccines.
Another team of NIAID-funded investigators have researched a combination or "prime boost" HIV vaccine approach that has been effective when tested in monkeys. Although the vaccine did not prevent infection, it kept the virus at undetectable levels for several months after immunization. One of the vaccines contained a piece of DNA designed to carry genes for both HIV and SIV. The second vaccine added the same genes to a virus called MVA, a modified version of vaccinia virus. Both triggered an immune response against SHIV. These positive results led scientists to make HIV versions of the DNA and MVA vaccines to test this concept in human trials.
At the NIAID Vaccine Research Center, a similar concept is currently being tested in monkeys. This vaccine consists of an initial DNA prime representing several genes of the SIV and HIV viruses, and a follow up boost using a corresponding non-replicating adenovirus vector. When administered successively, as a prime-boost strategy (i.e., DNA followed by adenovirus), both a reduction in viral load, and also improved levels of CD4+ counts are elicited.
Inevitably, testing vaccines requires individuals who are willing to participate in clinical trials. The three major phases (I-III) allow researchers to test a new vaccine to evaluate its safety, safe dosage range, side effects, immunogenicity, and effectiveness in standardized conditions. All clinical trials are carefully monitored by Data and Safety Monitoring Boards (DSMBs) to ensure the safety of the participants and that the trial is moving in the right direction. Community advisory boards (CABs) advise and provide another perspective on whether the trial is ethical and reasonable based on the concerns and needs relevant to the community. Currently, one of the biggest challenges is the low participation of women, minorities, and high-risk populations in government-sponsored clinical trials. These groups are also the most in need of an HIV vaccine because they are disproportionately affected by HIV/AIDS. Their participation is needed to ensure that a potential vaccine is safe and effective in all groups of people. Moreover, with recent opportunities within the developing world, conducting HIV vaccine trials in international settings will require greater commitment for funding, training, mobilizing and implementing trials of large capacities while working with foreign governments and communities. This is essential as many countries outside of the United States are heavily afflicted with HIV/AIDS.