Designing an effective vaccine to protect people from infection with HIV, from becoming ill if infected by the virus, or to reduce viral load following infection, 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 elude researchers.

DEVELOPING IMMUNE RESPONSES

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. 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 specifically recognize foreign components, such as those found in bacteria or viruses.

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 parts of 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 and other recombinant vaccines have been designed to induce immune responses specific against the HIV envelope proteins.

Cellular (cell-mediated) immunity, the second type of immunity, refers to activities of T lymphocytes. Cytotoxic T lymphocytes (CTLs), nicknamed killer T cells, directly destroy HIV-infected cells. The CTLs (also known as CD8+ T cells) bear CD8 receptors on their surfaces that help them target other body cells that are producing HIV. Cells (such as HIV infected cells) expressing foreign peptide-complexes on their cell surface can be recognized and destroyed by CTLs through very specific interactions involving T-cell receptors (TCR) that can bind specifically to a particular peptide complex. 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.

Helper T cells (HTLs), another component of cellular immunity, functioning like a conductor leading a symphony orchestra, promotes direct antibody- and cell-mediated immune responses through the secretion of molecules called cytokines, which have important effects on B cells, CTLs, and other immune cells. Unfortunately, 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, which explains why these cells are named 'CD4+ T cells'.

A subset of helper T cells, memory T cells, are induced during initial exposure to an invading organism and will eventually serve as the biologic record of exposure to that virus or microorganism. If the virus enters the body again, memory T cells specific for that pathogen will induce a quicker and more potent immune response than before. The most common way to measure memory T cells is by a test called the lymphocyte proliferation assay, which measures the strength of cellular responses to HIV.

To be effective, an HIV vaccine may also have to stimulate immunity 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 protect the body, but the cells may be important to blocking HIV transmission.

HIV STRAIN VARIATION

HIV continually evolves because of genetic mutation and recombination. Thus, researchers will need to account for 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 genetic 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 identification and attack by the immune system.

Using HIV isolates obtained from patients around the world, the genes encoding their envelope and core proteins 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 nine subtypes, or clades. 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 develop that contains gene fragments from both parental viruses. Hence, since there are a vast number of HIV variants circulating worldwide, 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, like polio, 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), a subtype B virus. Other vaccines have been based on the SF-2 and MN isolates, which also belong to subtype B, the prominent HIV subtype in the United States and Europe. Recently, it has been shown that these and other laboratory-adapted viruses are more sensitive to neutralization than viruses in nature, 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 and recombinant forms to which they may be exposed, scientists are looking for conserved regions of HIV genes; that is, 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 - or directing the body to internally make such proteins - may be necessary to evoke broad-based immunity.

HIV TRANSMISSION IS COMPLEX

According to the World Health Organization, more than 90 percent of all HIV transmission worldwide occurs sexually. Thus, an effective 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 often the first line of defense against infectious organisms. Unfortunately, relatively little is known about how the mucosal immune system protects against viral infection.

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 from humans (long-term HIV survivors and participants in human clinical trials of investigational HIV vaccines) may offer clues. To date, however, there are no HIV vaccines that predictably induce broadly neutralizing protective antibodies in humans.

IMMUNE SYSTEM BREAKDOWN

The most difficult challenges for HIV vaccine researchers are

• 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.
• Once the virus infects CD4+ T cells, the virus' genetic material is permanently integrated into the cell's chromosomes, establishing permanent latency within infected cells.

After infection, HIV incorporates its genetic material into the host cell DNA. If a cell reproduces itself, each new cell also contains the integrated HIV genes. 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, proceeding 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 healthy 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.

METHODS TO ENHANCE IMMUNE RESPONSES TO HIV

Because of safety concerns, no candidate HIV vaccines contain the whole 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 simian immunodeficiency virus (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. Attenuated HIV, however, is 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. An adjuvant may work well with one experimental vaccine but not another. Therefore, the Food and Drug Administration (FDA) licenses the combination, rather than the adjuvant alone.

Currently, only one adjuvant—alum, first discovered in 1926—is incorporated into vaccines licensed for human use by FDA. 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 the immune responses to HIV is to combine vaccines. Researchers may first prepare, or 'prime', the immune system with one vaccine, such as a live vector vaccine (a relatively non-virulent bacterium or non-HIV virus that has been genetically engineered to contain one or more synthetic HIV genes), and then 'boost' the subsequent immune response with a different vaccine, such as a gp120 or gp160 subunit recombinant vaccine.

One of the best-studied live vectors for recombinant vaccines is vaccinia virus, formerly used to immunize people against smallpox. Vaccinia has been 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 encoded by the vaccinia vaccine.

By itself, an HIV gp160-containing vaccinia virus vaccine stimulates production of memory T cells but few antibodies. The prime-boost combination vaccination schedule, however, can stimulate a strong cellular immune response - including persistent killer CD8+ T cells - as well as antibodies that neutralize the virus in laboratory assays. 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 recombinant live vector vaccines made from poxviruses, such as canarypox and MVA, are in clinical trials. These do not reproduce in human cells and, are therefore 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 multiclade and multigene adenoviral vector HIV vaccine is being developed as a part of a DNA prime, adenoviral boost vaccine strategy. The first Phase I study of the boosting vaccine (VRC 006) was launched in July 2004 at the National Institutes of Health Clinical Center. 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 six 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 human clinical trials. When the DNA is injected, the encoded viral proteins, such as HIV gp160, are produced just as with live vectors. An initial clinical study of a multiclade, multigene DNA vaccine developed by VRC demonstrated that it was safe, well-tolerated, and frequently induced human HIV-specific immune responses. VRC’s HIV prime-boost candidate is designed to elicit immune responses to HIV sequences from clades A, B, and C, which together cause about 90 percent of incident HIV infections around the world. The vaccine is also currently being tested in Uganda, and plans are underway for expanded testing at several international sites.

ANIMAL STUDIES

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 AIDS-like 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. The genetic and physical structures of SIV differ considerably from those of HIV, and therefore, 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.

NIAID-funded investigators have researched a combination '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 MVA. 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.

CLINICAL TRIALS

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 to ensure the safety of the participants and the progress of the trial. Community advisory boards 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.

Source:

National Institute of Allergy and Infectious Diseases, USA.