Sexually Transmitted Diseases
- Disease Burden
- Live Attenuated Vaccines
- Subunit Vaccines
- Live Recombinant Vaccines
- Other Vaccinal Approaches
The acquired immunodeficiency syndrome (AIDS) emerged in the human population in the summer of 1981. There now is convincing evidence that its agent, the human immunodeficiency virus (HIV), probably crossed the simian-human species barrier before the middle of the 19th century. At the end of 2004, the number of adults and children living with HIV/AIDS was estimated by WHO/UNAIDS to have reached 39 million worldwide. An estimated 4.8 million people (including 600 000 children less than 15 years of age) becomes infected each year, 95% of whom live in developing countries, and an annual 2.9 million people die of the disease.
Today, HIV/AIDS is the leading cause of death in sub-Saharan Africa and the fourth biggest killer in the world. The number of HIV infections is equally distributed between men and women, but infection rates in young women in today’s Africa are close to three times higher than those among young men, reflecting the degree to which gender inequities are driving the epidemic, as many women in developing countries lack socio-economic independence, education and access to health information and services, and have difficulty avoiding exposure to the virus.
Sub-Saharan Africa remains the hardest-hit region in the world, with at least 25 million infected people, accounting for 70% of the people living with HIV/AIDS and 77% of AIDS deaths worldwide. The overwhelming majority of HIV transmission in the region stems from sexual behavior. In some African countries, overall prevalence in the adult population can be greater than 10%, with figures reaching up to 38.8% in some areas. Among the most severely hit countries are South Africa, with more than 5.6 million infected people, together with Bostwana, Mozambique, Tanzania and Zimbabwe. Highest infection rates are found among commercial sex workers, truck drivers and seasonal migrant workers. Sub-Saharan Africa also is home to an estimated 500 000 infants who contracted HIV each year before the onset of prevention of vertical transmission by use of antiretroviral drugs: transmission from mother-to-child can occur in utero, at birth or as a result of breastfeeding. In addition, sub-Saharan Africa faces numerous wars and civil conflicts, producing large numbers of refugees who are at heightened risk of contracting HIV. A remarkable success story in the fight against AIDS was undertaken in Uganda, which was facing a severe HIV epidemic in the mid 1980s. Through voluntary HIV counselling, expanded treatment of STDs, awareness campaigns and community mobilization encouraging delayed initiation of sexual activity, monogamy and use of condoms, the level of infection declined significantly since 1992 – from nearly 30% to 11.2% in prenatal settings in Kampala and from 13% to 5.9% in clinics outside major urban areas.
The estimated number of people living today with HIV in Asia and the Pacific Region is more than seven million, but the accuracy of the figure is questionable, in view of the fast pace at which the epidemic is expanding. It has been projected that the region will contribute 40% of all new infections by the end of the decade, with China reaching 10 million infected persons, from an official 800 000 today, and India 20–25 million, from 5.1 million today. Increasing sex trade, use of illicit drugs, and rates of sexually transmitted infections contribute to an increased vulnerability in the region. Injection drug use and heterosexual intercourse are the primary modes of transmission, although improper blood donation practices in China and unsafe injection practices in health-care settings in India and surrounding countries have resulted in hundreds of thousands of infections. Substantial transmission also occurs in men who have sex with men, with prevalence rates of 14–20% reported in male homosexual communities in Cambodia, India and Thailand. Gender inequities play a major role in the epidemic as young girls are frequently steered toward sex work by their families.
The estimated number of adults and children living with HIV in Latin America and the Caribbean at the end of 2003 was two million. While in some countries HIV infections remain concentrated mainly in men who have sex with men and injecting drug users, others are experiencing increasing rates of heterosexual transmission. The Eastern European countries continue to experience one of the sharpest increases in the number of new HIV infections, most of which occur among injecting drug users. The number of people with HIV/AIDS in the region is estimated to be 1.3 million.
In industrialized countries, highly active antiretroviral treatment (HAART) has considerably reduced disease progression to AIDS and transformed HIV/AIDS from a deadly disease to a somewhat manageable chronic disease. However, successes in treatment and care are not being matched by progress in prevention. Each year, some 75 000 individuals become infected with HIV in industrialized countries, where an estimated 1.6 million people are living with HIV/AIDS (1 million in North America alone), and where new evidence of rising HIV infection rates is emerging, particularly in marginalized communities.
The human immunodeficiency virus (HIV), together with the simian, the feline, and the bovine immunodeficiency viruses (SIV, FIV, and BIV, respectively), the Visna virus of sheep, the caprine arthritis-encephalitis virus (CAEV) and the equine infectious anaemia virus (EIAV), belongs to the genus Lentivirus in the family Retroviridae. These enveloped RNA viruses produce characteristically slow, progressive infections. Their replication depends on the presence of an active reverse transcriptase responsible for the transformation of the RNA genome into a DNA copy that integrates into the host cell chromosome in the form of a provirus. The provirus is eventually transcribed into a set of mRNAs that encode the viral proteins and into progeny genomic RNA. The genome of HIV is a single-stranded positive sense RNA molecule, 9.5 kb in length, which encodes the typical retrovirus proteins Gag (further cleaved into Matrix, Capsid and Nucleocapsid proteins), Pol (itself cleaved into Protease, Reverse Transcriptase and Integrase) and Env (a 160 kD glycoprotein eventually cleaved into a gp120 external subunit and a gp41 transmembrane subunit that form together trimeric spikes on the surface of the virion). In addition, the genome encodes a variety of nonstructural proteins, such as regulatory proteins Tat and Rev and accessory proteins Nef, Vif, Vpr and Vpu. The gp120 subunit binds the CD4 receptor and CCR-5 or CXCR-4 co-receptors on the surface of target cells, whereas gp41, which anchors the spikes in the viral envelope and maintains their trimeric organization, plays a major role in fusion of the virus and cell membranes. Neutralizing human monoclonal antibodies have allowed the identification of several neutralization epitopes on gp120 that overlap the receptor or co-receptor binding sites, but they appear to be little accessible to the cognate antibodies due to hindrance by the many glycosylation motifs on the molecule as well as by the presence of hypervariable loops that act as antigenic decoys. Fusion-blocking antibodies also have been described, with corresponding epitopes located at the base of the gp41 ectodomain. Contrary to laboratory-adapted virus strains (“X4” strains) against which protection in chimpanzees could readily be obtained with neutralizing antibodies, field virus isolates (“R5” virus strains) have turned out to be extremely difficult to neutralize, casting doubt on the feasibility of a vaccine to elicit protection against infection by induction of neutralizing antibodies alone.
Two types of HIV have been described: HIV-1 and HIV-2, the latter being less virulent and geographically limited to West Africa. HIV-1 is phylogenetically close to SIVcpz, a commensal virus in chimpanzees, whereas HIV-2 is closely related to SIVmac, the agent of simian AIDS, and to SIVsm, a commensal virus in sooty mangabey monkeys. HIV-1 is further divided into three groups, M, N, and O. The vast majority of the HIV-1 strains responsible for the global pandemic belong to group M. These have further been classified in 10 subtypes, also known as clades, which have been designated by letters from A to K. HIV-1 subtype B predominates in industrialized countries as well as in Latin America and the Caribbean. Subtypes A and D are more common in Central Africa. Subtype C accounts for the majority of infections in southern Africa, parts of Eastern Africa and India. Interclade recombinant strains are relatively common and have been designated “circulating recombinant forms” (CRF). Major CRFs are CRF_AG, prevalent in western Africa, CRF_AE, which predominates in south-eastern Asia, and CRF_BC, prevalent in China. Amino acid sequence of the viral envelope glycoprotein shows 25–35% divergence between clades and up to 20% divergence within any given clade, which constitutes a formidable challenge to vaccine development.
The development of a safe and effective vaccine is hampered by the high genetic variability of HIV, the lack of knowledge of immune correlates of protection, the absence of relevant and predictive animal models, and the complexity of the implementation of efficacy trials, especially in developing countries. The first Phase I trial of an HIV vaccine was conducted in the USA in 1987. Since then, more than 30 candidate vaccines have been tested in over 80 Phase I/II clinical trials, involving more than 10 000 healthy human volunteers. Two Phase III trials have been carried to completion and a third one is in progress. Most of the effort to develop and evaluate HIV vaccines is borne by the NIH, CDC and WRAIR in the USA and by ANRS in France, with strong help from the International AIDS Vaccine Initiative (IAVI) in New York, the European Union, initiatives in WHO and UNAIDS, and the recent commitment of the Bill and Melinda Gates Foundation for a Global Enterprise. The HIV Vaccine Trial Network (HVTN) established by NIAID in 2000, with 25 clinical sites in four continents, represents a major resource for clinical HIV vaccine research. The European Union has created the European and Developing Countries Clinical Trials Partnership (EDCTP) with the aim of helping developing countries build up their capacity in testing the efficacy of new drugs, microbicides, and vaccines.
In the absence of identified correlates of immune protection, multiple vaccine concepts are being explored in parallel.
The observation that nef-deleted mutants of SIV could elicit protection against challenge with pathogenic SIV in rhesus macaques served as a model in favour of a live attenuated HIV vaccine approach. The SIV ∆nef mutants, however, establish a lifelong, persistent low grade infection that does not protect the vaccinated monkeys against superinfection with wild-type virus, although the animals seem to be protected against subsequent disease. In addition, the attenuated virus still may cause AIDS when administered orally to infant monkeys. Additional deletions or mutations can further attenuate the virus but at the expense of its protective efficacy. Mostly because of safety concerns, this approach was therefore not pursued.
A subunit HIV vaccine was developed based on monomeric gp120 added with alum (VaxGen). The vaccine was tested in two double-blind, controlled Phase III efficacy trials, one on 5000 volunteers at risk (mostly men who had sex with men) in the USA, with sites in Canada and in the Netherlands, using a mixture of two subtype B gp120s as the immunogen, the other on 2500 volunteers in Thailand (mostly drug users), using a mixture of a subtype E (CRF_AE) and a subtype B gp120s. None of these studies showed a statistically significant reduction of HIV infection in the vaccinees in spite of biannual booster immunizations. A reduction of the number of HIV infections was observed in certain ethnic subgroups in the first study, correlating with a higher level of anti-gp120 antibody, but the numbers were too small to provide statistical confidence. The same subtype E/B gp120 vaccine is planned to be used for booster immunizations in a prime-boost Phase III trial which was launched in late 2003 in Thailand in collaboration between the Ministry of Health of Thailand, WRAIR, Sanofi-Pasteur and VaxGen, and uses for priming a recombinant canarypox virus (ALVAC) that expresses CRF_AE gp120 and subtype B Gag, Pol and Nef antigens. The trial will enrol 16 000 heterosexual volunteers and is expected to last four years.
Other approaches aimed at eliciting HIV neutralizing antibodies are at an early clinical stage. These include the use of:
- trimeric gp140 molecules (gp120 + the ectodomain of gp41) with a deletion of the hypervariable V2 loop in order to expose the neutralization epitopes overlapping the CD4-binding site;
- oligomeric gp140 molecules covalently coupled to synthetic mimics of the CD4 receptor that should expose neutralization epitopes overlapping the coreceptor (CCR5 or CXCR4)-binding site;
- gp120/gp41 trimers internally stabilized by disulfide bond formation (SOS proteins) which should elicit both neutralizing and fusion-blocking antibodies.
Induction of fusion-blocking antibodies by immunization with recombinant oligomeric gp41 molecules is a promising new approach that still is at an early preclinical stage of development.
Rather than attempting to elicit a neutralizing antibody response, recent HIV vaccine approaches have aimed to elicit a T-cell response, especially a CD8+ CTL response, whose role in control of virus load and evolution of disease has been well documented in the monkey model. In addition to perforin-based cellular cytotoxicity, CD8+ T-cell secrete antiviral cytokines (IFN-γ), still unidentified antiviral factors (CAF) and virus entry-blocking β-chemokines that have been correlated with protection against SIV infection in the monkey model, as well as associated to asymptomatic HIV-1 infection in humans and slower disease protection in HIV-2-infected patients. Vaccines that stimulate the T-cell arm of the immune response are however not expected to protect against infection, but rather to control its course and reduce viral loads, thus preventing or at least delaying the occurrence of symptoms. Reduction of viral loads in vaccinated but HIV-infected individuals also would hopefully result in lowering the probability of virus transmission to their partners.
Several prime-boost strategies involving priming with a DNA vaccine followed by boosting with a live recombinant vector-based vaccine have been tested in monkeys against challenge with a lethal dose of simian–human immunodeficiency virus (SHIV) that causes AIDS-like illness in the animals. These strategies resulted in reduction in virus load and provided protection against disease and death in the vaccinated animals. The same approach was however less successful in protection against SIV challenge.
A number of these vaccines also have been tested in Phase I/II trials in humans, including plasmid DNA and poxvirus vectors (MVA, fowlpox or canarypox viruses) expressing a variety of HIV antigens, such as Gag, Env, Pol and Nef.
Replication-defective adenovirus type 5 (Ad5) represents another promising vector: a recombinant Ad5-gag/pol/nef vaccine has entered Phase II trials on some 1200 men and 400 women at high risk who will be followed for 3 years after 3 immunizations at 0, 4 and 26 weeks (Merck).
The list of other vectors is long. It includes, among others:
- BCG (NIH Japan)
- Salmonella (IAVI/Institute for Human Virology, University of Maryland)
- Venezuelian equine encephalitis virus (VEEV; Alphavax)
- adenovirus-associated virus (AAV; IAVI/ Targeted Genetics)
- Sendai virus (NIH Japan)
- vesicular stomatitis virus (VSV; Yale University/Wyeth)
- Newcastle Disease Virus (NDV; Mount Sinai, New York, and Kyoto University, Japan)
- measles virus (Pasteur Institute/GSK).
The immunogenicity of the DNA and poxvirus vector vaccines in humans has usually been relatively weak, with generally less than 35% of the vaccinees scoring positive at any time point, as determined by IFN-γ ELISPOT assays. This was emphasized by the very disappointing results of a recent DNA prime-MVA boost Phase I study in Kenya which showed that the promising immunogenicity data obtained in monkeys with clade A HIV DNA and MVA constructs could not be repeated in human volunteers.
The best results so far, in terms of the percentage of human responders, level of T-cell responses and duration of immune responses have been obtained with the Ad5 vector. However, Ad5-based recombinant vaccines are confronted with the problem of a frequent pre-existing anti-vector immunity in the human population, especially in developing countries, which dampens the immune response to the HIV transgene. This has prompted the development of less prevalent human adenovirus serotypes (Ad35, Ad11, or Ad24) as nonreplicative vectors. Like Ad5, these vectors readily multiply to large yields in PRC-6 cells in fermenters. Nonreplicative chimpanzee adenovirus vectors (AdC68, AdC6 and AdC7) also are being developed. Combinations of different vectors in mixed modality prime-boost regimens will likely be developed in the future.
In rhesus monkeys, responses arising from an Ad5 priming-poxvirus (MVA or ALVAC) boosting regimen were significantly greater than those elicited by homologous regimens with the individual vectors, but this was not observed in human volunteers (Merck and Sanofi-Pasteur).
Induction of persistent HIV Gag-specific CD8+ CTL responses was attempted in a Phase I trial involving immunization with a fusion protein comprising the HIV p24Gag protein and detoxified Bacillus anthracis lethal factor (see 7.1.2.) to target the antigen to antigen presenting-cells (Avant Therapeutics and WRAIR).
Multi-epitopic combinations of peptides, fusion proteins and long lipopeptides also are at an early stage of clinical development, either alone or in prime-boost combinations with live vector-based recombinant vaccines. Lipopeptides whose sequence corresponds to that of CTL epitopes-rich regions in the Gag and Nef viral proteins are in Phase II trials in the USA and in France (NIAID/ANRS).
Finally, a number of candidate vaccines that target nonstructural viral proteins such as Tat, Rev, Vif and Nef are being developed using viral vectors such as MVA or Ad5 (bioMérieux/Transgene), fowlpoxvirus (AVC), DNA (Vical, Istituto superiore dei Sanita/Parexel) recombinant proteins (FIT Biotech, Institute for Human Virology) or fusion proteins (GSK), or polyepitopic peptides (Wyeth/Duke University, Epimmune). Some of these candidate vaccines will be tested as therapeutic vaccines in patients as a complement to antiretroviral therapy. Tat has been shown to act as a viral toxin and to promote apoptosis of uninfected bystander T-cells and secretion of Th2 cytokines.
The population-wide effects of partially effective vaccines that do not prevent infection but only can reduce viral loads are largely unknown. Mathematical models predict that the factor with the greatest impact on reducing infections and deaths will be the degree of virus load reduction. A 90% reduction in viral load, which is a reasonable expectation with current candidate vaccines under development, would significantly reduce HIV mortality within 20 years after introduction of the vaccine.
The development of a safe, effective, and affordable HIV vaccine remains a formidable scientific and public health challenge at the dawn of this century.