Progress on new vaccine strategies against chronic viral infections (original) (raw)
The basis for current HIV vaccine strategies. There is little direct evidence for immune correlates of protection against HIV in humans since no individual has mounted an immune response capable of spontaneously clearing the infection, even though there are some long-term nonprogressors who have remained infected without developing AIDS. Nevertheless, there is much evidence in animal studies and indirect evidence in humans that CD4+ and CD8+ T cells, broadly neutralizing antibodies, and innate immunity all play an important role in the control of infection with HIV and its close cousin, simian immunodeficiency virus (SIV), in macaques.
Antibodies neutralizing AIDS viruses clearly play an important role in protection. Passive transfer of IgG1 monoclonal antibodies was shown to be sufficient to protect macaques against i.v. challenge or against mucosal transmission (16, 17). However, a high level of monoclonal antibody is required to achieve complete protection while partial protection could be achieved with a lower-antibody titer. Therefore early studies primarily focused on the HIV envelope protein gp160 as the primary target of neutralizing antibodies. However, while it was possible to achieve neutralizing antibodies against a specific virus strain grown in the laboratory, the difficulty of obtaining antibodies that neutralized a broad array of strains, particularly primary isolates, provided incentive both to devise novel approaches for the induction of the relevant antibodies and to target T cell immunity as an alternative strategy.
A number of lines of evidence implicate CD8+ T lymphocytes (especially CTLs) in controlling HIV or SIV infection (reviewed in refs. 18–21). The acute viremia in both HIV and SIV was found to decline concomitant with the rise of the CTL response and prior to the appearance of neutralizing antibodies. Many HIV-infected long-term nonprogressors have expressed a high level of HIV-specific CTLs, and African sex workers that had been exposed to HIV but remained uninfected possessed high CTL responses. However, the most direct evidence comes from studies of HIV-infected chimpanzees and SIV-infected macaques, in which depletion of CD8+ T cells in vivo led to increases in viral load that were later reversed when the T cells reappeared (22–24). For this reason, most strategies studied in nonhuman primates today are based on eliciting an effective HIV- or SIV–specific CTL response. Although CTLs can be elicited by peptides and other constructs, the most straightforward approaches have involved agents that induce endogenous expression of the viral antigens in a professional antigen-presenting cell, such as a dendritic cell, because the most efficient and natural way to load class I MHC molecules with peptides for presentation to CD8+ T cells is endogenous expression of the protein within the cell. This can be accomplished with DNA vaccines or with viral vectors, which can introduce the antigen gene into antigen-presenting cells. A recent study indicates that a DNA vaccine augmented by IL-2 can induce a strong HIV-specific CTL response and can control pathogenic viral challenge and prevent AIDS in rhesus macaques (25).
Compared to repeated DNA vaccination or viral-vector–based vaccination alone, one of the most effective strategies for eliciting HIV-specific immunity is a heterologous prime and boost regimen, which involves administration of a DNA vaccine followed by a viral-vector vaccine, which induces a stronger CTL response than can be achieved by priming and boosting with the same agent (26), or a recombinant protein boost, which induces a significant level of neutralizing antibodies (27). HIV DNA vaccines were shown to be effectively boosted by a recombinant vaccinia virus, such as modified vaccinia Ankara (MVA) (28) or replication-deficient recombinant adenovirus (29), which induced high frequencies of CD8+ CTLs and neutralizing antibodies and protected against viral challenge. MVA is a highly attenuated vaccinia virus that has lost the ability to replicate in primate cells and can be considered a safe vaccine. In a study comparing SIV gag recombinant MVA and recombinant adenoviral vectors in prime-boost regimens in macaques, DNA prime–recombinant adenovirus boosting was most effective at eliciting long-lasting CD8+ IFN-γ responses and protection against viral challenge (30). However, mutation of AIDS viruses, leading to escape from immune control mediated by CTLs (31), and the breadth of the protection against more distant strains of challenge virus remain concerns. Indeed, several of these studies used the highly pathogenic simian-human immunodeficiency virus strain 89.6 (SHIV 89.6P) as the challenge virus as it has an atypically rapid disease course, but this virus may not be representative. Protection against heterologous viral challenge will be critical to demonstrating the breadth of vaccine efficacy.
These difficulties led to the development of new vaccine strategies against AIDS viruses as described below (reviewed in ref. 32), including but not limited to the following: (a) creation by sequence modification of enhanced epitopes that bind with higher affinity to MHC molecules; (b) targeted induction of mucosal immunity; (c) use of synergistic combinations of cytokines, chemokines, or costimulatory molecules to enhance the immune response; (d) relief of negative regulatory or suppressive mechanisms that inhibit the immune response; (e) use of dendritic cells as vaccine vehicles; (f) induction of alloimmunity for protection against HIV; and (g) formulation of the vaccine to incorporate agents inducing innate immunity (Figure 1).
New strategies for second generation vaccines based on cellular immunity. CD4+ helper T cells mature and activate APCs through recognition of epitopes presented by class II MHC molecules (MHC II) and interaction of CD40 and CD40 ligand (CD40L). The CD40-CD40L interaction causes the APC to upregulate expression of costimulatory molecules such as CD80 and CD86 and to secrete cytokines IL-12 and IL-15. The costimulatory molecules interact with CD28 on the CD8+ CTL to provide a second CTL activation signal in addition to T cell receptor (TCR) recognition of an antigenic peptide presented by a class I MHC molecule (signal 1). IL-12 also contributes to activating the CTL and polarizing the T helper cell to produce Th1 cytokines, such as IFN-γ. IL-15 contributes to induction and maintenance of CTL memory and longevity. Regulatory T cells, including NK T cells and CD25+CD4+ T cells, can dampen or inhibit the CTL response in order to prevent autoimmunity, but also reduce the immune response to the vaccine. Various strategies may be employed to improve the natural T cell response. Epitope enhancement of class I or class II MHC–binding peptides can increase their affinity for the respective MHC molecules and their immunogenicity. Incorporation of (a) cytokines such as IL-15 to recruit more memory CTLs; (b) IL-12 to steer the T helper cell population towards a Th1 response; (c) GM-CSF to recruit dendritic cells; or (d) costimulatory ligands such as CD40L to activate and induce maturation of the dendritic cells recruited by the GM-CSF can synergistically amplify the immune response. In addition, CpG-containing oligonucleotides can act through toll-like receptor 9 (TLR 9) to activate dendritic cells. Increased levels of costimulatory molecules can selectively induce higher-avidity CTLs that are more effective at clearing virus infections. Agents that block factors secreted or induced by regulatory T cells, such as IL-13 and TGF-β, can synergize with other strategies to allow the CTL response to reach its full potential. Similarly, blockade of the inhibitory receptor, CTLA-4, on the T cell, can increase the T cell response. See multiple references in the text for the other strategies.
Mucosal vaccination of mice with an HIV peptide induced systemic and mucosal CTL responses, while parental vaccine administration induced predominantly systemic CTL responses (33). Systemic CTLs were not sufficient to protect against mucosal virus transmission, so it is important that the CTLs be locally present in the mucosa (34). Mucosal delivery of the vaccine is usually the most effective route to induce mucosal immunity although, interestingly, transcutaneous immunization can also induce mucosal CTLs and may provide an alternative vaccination option (35). Moreover, mucosal vaccination against AIDS viruses, which induced CD8+ CTLs in the gut mucosa of immunized Rhesus macaques, more effectively cleared the major reservoir for SIV replication in the gut and thus reduced plasma viral load below the level of detection. The same vaccine administered subcutaneously was less effective, leaving residual viremia (36). Also, control of SIV infection was associated with mucosal CTLs (37). These findings make a strong argument for mucosal delivery of an AIDS vaccine even if some partial protection against mucosal challenge can be observed with a systemic vaccine (38).
Cytokine and chemokine gene codelivery along with DNA-encoding immunogens can modulate the direction and magnitude of immune responses (39) and can improve vaccine efficacy compared to delivery of DNA alone (25). For example, RANTES coinjection induced high levels of CD8+ CTLs, as did a synergistic combination of GM-CSF and CD40 ligand when negative regulatory mechanisms were blocked (40).
Overall it is widely believed that induction of both antibodies and T cells will be needed for an effective AIDS vaccine. Several major strategies are being studied for development of a preventive vaccine against AIDS viruses (reviewed in ref. 41). Most of these approaches focus on the generation of either neutralizing antibodies or CTL responses, but ultimately some combination of these approaches may be needed.
Another major hurdle for HIV vaccine development is the extraordinary diversity of the virus and its ability to rapidly mutate within each infected individual. The genetic subtypes of HIV, clades A–E, are responsible for the main epidemics in different parts of the world: clade B is prevalent in North America and Europe; clades A, C, and D in Africa; and clades E and B in Thailand. In addition to the diversity of viral subtypes that must be targeted by a vaccine, the high level of mutability, due to the error-prone nature of reverse transcriptase, facilitates escape mutation. Some of the major neutralizing epitopes of HIV are so variable that it is hard to find broadly cross-reactive neutralizing antibodies, but the existence of a handful of monoclonal antibodies that are broadly neutralizing implies that it is possible, in principle, to do so. These antibodies bind to at least three different sites on the HIV envelope protein: the CD4 binding domain, the chemokine receptor–binding domain, and the stalk of gp41 that must change conformation in order to mediate fusion with the cell membrane (Figure 2) (42). In addition, the conserved conformation of the V3 loop, despite high sequence variability, has allowed production of broadly cross-reactive antibodies to this principal neutralizing region (43, 44). Thus, with respect to antibodies, a goal is to develop vaccines that direct the immune response to conserved sites such as these that cannot vary without a resulting loss of function, but this has not proven straightforward through the use of existing forms of recombinant envelope proteins. In the case of T cells, which can target internal viral proteins and are not limited to neutralizing epitopes, one approach has been to focus on sequences that are conserved for functional or structural reasons and cannot tolerate modification by escape mutations. This approach is supported by the finding that more cross-clade reactivity has been observed among T cells than antibodies (45, 46).
Interactions of HIV envelope glycoproteins, CD4, and chemokine receptors CCR5 or CXCR4 trigger fusion and entry of HIV. These interactions determine critical regions of the HIV envelope glycoprotein against which neutralizing antibodies could be raised. After the envelope protein interacts with CD4 on the target cell (A and B), it undergoes a conformational change allowing its interaction with a chemokine receptor (C). This second interaction induces a further conformational change in the gp41 portion of the envelope glycoprotein that mediates the fusion event (D and E). Blockade of any of these three steps can prevent viral entry.
HIV-1 vaccine clinical trials. This year heralds the deployment worldwide of multiple vaccine trials in an effort to stem the ongoing HIV epidemic. Safety and immunogenicity data for multiple vaccines and immunization platforms currently moving into phase I/II studies will establish the criteria for phase III efficacy studies. A listing of ongoing preventive trials of HIV vaccines is available at the International AIDS Vaccine Initiative website (http://www.iavi.org/trialsdb/basicsearchform.asp), and the ongoing and planned protocols of the HIV Vaccine Trials Network in association with the National Institute of Allergy and Infectious Diseases can be found at http://chi.ucsf.edu/vaccines/.
Therapeutic vaccine trials. Early clinical studies tested the ability of HIV-1 clade B envelope protein (gp160 or gp120) vaccines or peptide vaccines based on the V3 loop of gp120 that had been identified as the principal neutralizing determinant (PND), in order to elicit neutralizing antibodies. Other vaccine trials in HIV-1 infected individuals initiated during the pre–highly active antiretroviral therapy (pre-HAART) era attempted to elicit both cellular and humoral immune responses using synthetic peptides containing promiscuous CD4+ T helper cell epitopes linked to the PND located at the crown of the V3 loop (47, 48). Various strategies (e.g., the use of adjuvants and multimeric and multivalent immunogens) were employed to increase vaccine immunogenicity and cross-reactivity. Although these approaches proved capable of eliciting high-titered type-specific neutralizing antibodies to tissue culture lab–adapted virus strains in addition to some lymphoproliferative responses in immunized patients, these antibodies failed to neutralize primary viral isolates (49–51).
An early approach targeting specific cellular immunity used gp120-depleted, whole, killed virus (52). The idea was to elicit cellular immunity to internal viral proteins while avoiding induction of so-called enhancing antibodies, specific for the envelope protein, that facilitate virus uptake by cells, as well as avoiding induction of other potential deleterious effects of the envelope protein. A large multicenter, double-blind, placebo-controlled, randomized trial was conducted on a whole inactivated Zairian HIV-1 isolate in incomplete Freund’s adjuvant given intramuscularly at 12-week intervals. A total of 1262 subjects of 2527 HIV-1 infected patients received the HIV-1 immunogen. There were no statistically significant differences between groups in plasma HIV RNA loads although patients in the vaccine group had an increase in average CD4+ T cell counts. Recent studies have shown that this vaccine is able to enhance specific CD4+ T cell responses in patients with chronic HIV infection (53, 54). However, the clinical benefit of enhancing such responses in the therapy or prevention of HIV infection is yet undetermined.
Current therapeutic vaccines entering phase I/II trials are aimed at boosting immune responses in HIV-1–positive patients where plasma viral load is controlled by antiretroviral therapy. These include immunization with a recombinant canarypox vector (VCP1452) expressing clade B gag, protease, reverse transcriptase, gp120 and nef, and immunization with gag, pol, and nef lipopeptides, in addition to peptide-pulsed autologous dendritic cell immunization plus IL-2.
Preventive vaccine trials. Recombinant vaccinia virus vectors have proven effective at eliciting CD8+ T cell responses in small animal models; however, concern about the effect of disseminated vaccinia on immunocompromised patients and the effect of prior smallpox vaccination on immunogenicity of the vaccine led to the development of a number of live, attenuated pox-vectored HIV vaccines that do not replicate in human cells and can be administered repeatedly. In a study by Evans et al. (55), recombinant canarypox expressing only gp160 of HIV-1MN (also known as vCP125) elicited anti-HIV env CD8+ CTLs in 24% of low-risk subjects. Anti–HIV-1 env CTLs were detected in 12 subjects and anti–HIV-1 gag CTLs were detected in 7 of the 20 vaccine subjects receiving canarypox virus (known as ALVAC) expressing HIV-1 env, gag, and protease (vCP205) vaccine alone or with clade B HIV-1 strain SF-2 recombinant gp120 protein (called rgp120 SF) (56, 57). Coadministration with SF-2 rgp120 vaccine enhanced lymphocyte proliferation in response to HIV-1 envelope glycoprotein and broadened envelope-stimulated cytokine secretion. Belshe et al. (58) reported a cumulative positive response frequency of 33% for anti–HIV-1 env or gag CTLs among 170 subjects in a phase II trial of vCP205. The vaccines were safe, and all patients developed binding antibody to monomeric gp120; approximately 60% developed antibody to gag p24.
Overall, lymphoproliferative responses to gp120 varied among ALVAC vCP205 studies, with from 50–100% of vaccinated subjects demonstrating CD4+ T cell proliferation. Fifteen to twenty percent of vaccinees developed CD8+ CTL responses, mostly against the envelope protein, with cross-clade reactivity seen in some subjects (45, 59). The maximum positive CTL response (35/84; 42%) was observed after four immunizations (57). It is noteworthy that the first successful phase I vaccine study initiated in Africa involved vaccinating uninfected volunteers in Uganda with clade B ALVAC vCP205 (60). Future vaccine strategies involving variations of the canarypox vector currently being developed as combination vaccines include replacing the clade B env sequences in vCP205 with clade A or clade E env sequences or sequences from a primary clade B isolate and the addition of reverse transcriptase (RT) and nef epitope sequences.
Heterologous prime-boost strategies. DNA vaccines used alone have not proven as immunogenic in humans and nonhuman primates as in mice; however, strategies involving DNA priming and boosting with a viral vector are capable of eliciting potent CD8+ and modest CD4+ T cell and antibody responses in macaques (28, 29). In an effort to improve immunogenicity results obtained with ALVAC vectors, current studies employing multiple viral gene products in complex combinations of DNA and viral vectors such as MVA, attenuated Vaccinia Copenhagen strain with deletions in virulence genes (NYVAC), fowlpox, adenovirus, and Venezuelan equine encephalitis virus–like replicon particles are underway. Major considerations in the development of recombinant viral vectors are ways to circumvent preexisting immunity and the production of high-titered stable vectors. Current phase I placebo-controlled trials are aimed at defining optimum vaccination regimens for eliciting cellular immune responses by varying the dose (dose escalation), number of doses, intervals between doses, and routes of administration. Results of phase I adenovirus trials in humans show safe, strong, long-lasting CD8+ T cell responses measured by IFN-γ ELISPOT. The University of New South Wales, Australia, has recently initiated a phase I trial of a prime-boost protocol using DNA and a fowlpox vector each expressing clade B gag-pol (without integrase), tat, nef, and gp160 env coding sequences. It is encouraging that recent clinical trials evaluating heterologous prime-boost regimens with HIV-1 and a recent malaria sporozoite protection trial with pre-erythrocytic Plasmodium falciparum immunogens have demonstrated the ability of these regimens to elicit strong IFN-γ–secreting CD8+ T cell responses equivalent to or better than those achieved during natural infection (61). Current vaccines moving into clinical trial that incorporate multiple immunogenic viral gene products are designed to address the issues of HLA polymorphism and escape mutation, and to identify correlates of immune protection. Second-generation DNA and viral-vectored vaccines include multiclade gag, pol, and env and may include nef and the accessory gene products tat and vpu. In addition, several novel agents are currently in phase I trials. One examines modified HIV envelope immunogens (e.g., ØCFI [cleavage site deletion (C), fusion peptide deletion (F), deletion of interspace between gp41 (N), and C heptad repeats (I)]) and clade A, B, and C DNA envelope immunogens developed by the Vaccine Research Center. A second examines a V2-deleted trimeric gp140 protein developed at Chiron Corp. Two other studies (at St. Jude’s Children’s Hospital, Memphis, Tennessee, USA; and University of Massachusetts Medical School, Worcester, Massachusetts, USA) test multiclade, multienvelope DNA, recombinant vaccinia virus with protein boost strategies in an effort to elicit broadly neutralizing antibody in addition to CD4+ and CD8+ T cell responses. In the Vaccine Research Center phase I DNA vaccine trial with a gag-pol-nef and multiclade env vaccine, CD4+ T cell responses were more frequent than CD8+ T cell responses and were primarily directed toward env but not the gag-pol-nef fusion protein. Coadministration of plasmid DNA expressing IL-2–Ig to enhance cellular immune responses is currently being tested with the Vaccine Research Center DNA vaccine (clade B gag-pol-nef and multiclade env) in a phase I trial. Cytokines IL-12 and IL-15, which have been shown to enhance induction of cellular immune responses and memory to vaccine antigens in small animal models, are scheduled for testing in human phase I HIV-1 vaccine trials in the near future.
Although cross-clade CD8+ CTL responses have been reported with clade B immunogens (45, 46), the importance of clade diversity will only be definitively addressed in phase III trials that compare vaccine candidates in parallel trials in different geographic regions. A recent study of HIV-1 subtype C–specific immune responses during natural infection in individuals in Botswana emphasizes the need to match vaccine epitopes to immunodominant epitopes detected in the target population based upon HLA frequencies (62). Recently, the first phase I HIV vaccine trial to be conducted simultaneously in Africa and the United States (sites in Gabarone, Botswana; Boston, Massachusetts, USA; and St. Louis, Missouri, USA) was initiated to test a DNA vaccine developed by Epimmune, composed of a promiscuous helper T cell epitope, pan-DR epitope (PADRE), and 21 specific epitopes optimized to elicit CD8+ CTL responses in individuals expressing one of three HLA alleles: HLA-A2, HLA-A3, and HLA-B7. Results obtained from this polyepitope study regarding immunogenicity will provide information for future vaccine design since the immunogen is not specifically selected for epitopes or matched for HLA types prevalent in this African population. Results of a previous single phase I trial in Africa employing a DNA vaccine expressing clade A gag and a stretch of 25 CTL epitopes known to be expressed in the vaccinated population revealed modest CD8+ T cell IFN-γ responses to gag, but no responses to the individual epitopes included in the vaccine were seen. After a single MVA boost (5 × 107 pfu) 6 months to 1 year later, CD8+ IFN-γ ELISPOT responses were detected in 19 of 26 individuals. In addition, an increase in the breadth of responses was seen after boosting. A phase II trial of this vaccine is in progress (63).
Lessons for the future. Only one HIV vaccine construct has yet progressed through large-scale phase III studies testing efficacy. A phase III trial completed in early 2003 (known as VAX003) in the United States, Canada, and the Netherlands and another in Thailand completed in 2004 (known as VAX004), both testing bivalent formulations of gp120 protein subunit vaccine (AIDSVAX B/B and B/E; VaxGen Inc.) aimed at targeting neutralizing antibodies, failed to demonstrate efficacy. Although a difference in the infection rate of African-American placebo recipients (9/116; 7.8%) versus African-American vaccine recipients (2.6%; 6/233) was found, based upon the small number of infections, further analysis is necessary to determine the significance of these differences. This suggestive result in a retrospective stratification emphasizes the need to adequately power future phase III trials to address differences in immune responses based upon gender and ethnicity. Phylogenetic analysis representing the overall diversity of viral isolates from the complete VAX004 data set showed no differences in any treatment group based upon race, gender, or geography (64). Results from these trials were consistent with those of previous studies, in which monomeric gp120 was not proven effective at eliciting broadly cross-reactive neutralizing antibodies.
A critical component for future vaccine prime-boost regimens is the inclusion of an envelope immunogen capable of eliciting broadly cross-reactive neutralizing antibodies against primary HIV-1 isolates. Although HIV-1–infected individuals are capable of developing neutralizing antibodies to primary viral quasispecies, serial escape occurs; consequently, the neutralizing antibody response lags one step behind the evolution of the viral envelope (65–67). The majority of cross-reactive neutralizing antibodies directed against HIV-1 glycoproteins have been mapped to conserved regions within the CD4 binding site and CD4 inducible epitope, V2, V3, the carboxy-terminus of C5, the leucine zipper-like region of gp41, and the ELDKWAS motif in the transmembrane region of gp41. Monoclonal antibodies directed to these epitopes neutralize primary isolates from multiple clades to varying degrees. Monoclonal antibodies, 2F5 and 4E10, directed to membrane proximal domains in gp41 are the most potent, in that they cross-neutralize 67% and 100%, respectively, of all clade isolates tested (68). The inability of primary isolates to elicit cross-reactive, neutralizing antibody may be explained by the low immunogenicity of these epitopes, resulting from conformational dynamics within the viral envelope that maintain a structure that makes these sites inaccessible or only transiently exposed. The rational design of new envelope immunogens should focus on engineering structures that expose, and direct antibody responses to, conserved epitopes on native trimers that are recognized by broadly cross-reactive neutralizing antibodies, and that at the same time prevent induction of dominant non-neutralizing antibodies.
An effective HIV-1 vaccine will require both potent and durable cell-mediated immune responses as well as effective neutralizing antibody responses. In the coming years, phase III efficacy trials of vaccines of proven immunogenicity will determine the need to employ immunization strategies focusing on eliciting mucosal immune responses, as noted above, since delivery of current vaccines primarily targets induction of systemic responses. Another important issue is whether sterilizing immunity, in which a vaccine is completely successful in preventing infection and which is likely to require high titers of broadly neutralizing antibodies, is essential or whether a vaccine based only on inducing cellular immunity, which controls viral loads so as to both prevent disease in the individual and reduce the risk of transmission to others (69, 70), will be sufficient to contain this pandemic.