Present status of human HIV vaccine development (2003)

Introduction The AIDS epidemic continues to grow worldwide. UNAIDS and WHO estimate that as of December 2002 some 38.6 million adults and 3.2 million children have been infected with HIV-1, with the largest burden in sub-Saharan Africa and in Asia. This virus has already taken the lives of nearly 25 million adults and children. Intensive chemotherapy has been proven to slow or halt disease progression in most infected individuals. However, these treatments are poorly accessible in developing countries. In addition, the persistence of the viral reservoir permitting the emergence of drug resistance continues being a major obstacle to virus eradication. It is clear that the development of a safe and effective vaccine remains a crucial goal for controlling this epidemic. HIV-1 has proven to be a difficult target for vaccine development for several different reasons (Table 1). HIV-1 genes present various degrees of variation and the envelope proteins show ample diversity allowing escape from the host immune response. HIV infection of memory T cells represents a formidable obstacle for the immune system. Once integrated into these cells genome, the virus can hide from immune attack due to a lack of antigen expression in these cells. Moreover, the high early viral replication rate facilitates the rapid seeding of newly expanded immune cells as well as memory T cells during their generation. Infection of antigen-presenting cells may play a role in defective presentation of HIV antigens. Viral infection and destruction of CD4+ T cells may also modify the anti-HIV immune response, impacting both the CD8-mediated as well as B-cell responses. The complexity of the viral gp160 envelope glycoprotein represents a formidable obstacle for the development of broadly cross-reactive neutralizing antibodies. The target sites for such antibodies appear buried in clefts or perhaps hidden under sugars and the structure necessary for antibody binding may be energetically unfavorable or only transiently available for immune attack.Table 1: HIV vaccine challenges.At present, it is not clear what arm of the immune response correlates with protection from infection or disease. It is widely accepted that cellular immune responses play an important role in controlling HIV-1 replication. Nonetheless, humoral immune responses are believed to be critical for controlling early viral replication and preventing viral entry into target cells. In the present article, recent advances for circumventing these problems resulting in anti-HIV immunity for the development of HIV vaccines and immune therapies are highlighted. Complex strategies for neutralization escape The induction of strong cross-reactive neutralizing antibody responses is considered the ultimate goal for an effective HIV vaccine. Observations originally in chimpanzees [1–3] but more recently in macaque infection models support that humoral immune responses may contribute to the control of HIV or simian immunodefiiciency virus (SIV) replication [4–6]. However, it has been difficult to elicit antibodies with the capacity to neutralize a diversity of primary HIV-1 isolates. The antigenic variation of envelope has been one stated reason for this difficulty; however, recent important studies have focused attention on other even more complex methods of neutralization resistance. Elegant studies by Kwong et al. [7], through measuring the thermodynamics of binding of gp120-reactive antibodies, found that antibodies directed to the CD4-binding site exhibited large negative changes in entropy, which are indicative of protein folding. Taking into account the structure of gp120, these data suggested that the receptor-binding sites (located between the inner and outer domains of gp120) are obstructed by oligomerization and glycosylation, which impose a conformational barrier for antibody binding. These studies suggest that the receptor-binding sites on gp120 are protected by conformational or entropic masking in that antibodies directed against these regions incur an energetic limitation for binding not observed by other non-CD4 binding site anti-gp120 antibodies. Studies indicate that simultaneous binding of gp120 by multiple CD4 receptors generates sufficient avidity to compensate for such conformational masking. These observations illustrate the complexity and stealth HIV uses to protect this critical receptor interaction from antibody neutralization which confounds vaccine development aimed at this crucial target. One approach previously suggested to expose important neutralization conserved epitopes is to generate envelope transition phase intermediates by freezing the envelope in its transition state. A simple and creative attempt in this direction is under evaluation in the laboratory of Anthony DeVico [8]. Covalently crosslinked complexes of soluble human CD4 and HIV env were generated by standard methods. Interestingly, several species of animals including rhesus macaques immunized with these complexes generated antibodies that neutralized a wide range of primary isolates regardless of the co-receptor usage or genetic subtype [8]. Although this data is both intriguing and encouraging, it remains unknown if the neutralizing antibodies elicited are solely targeting the envelope or target CD4 as well. Additionally, other creative methods for generation of envelope intermediate structures through structural analysis and CD4 cavity space filling mutagenesis approaches are worth watching as these studies represent the cutting edge in immunogen vaccine engineering [9,10]. Glycosylation of the HIV envelope and neutralization escape is another important issue. One role proposed for the high levels of glycosylation exhibited by the viral envelope in regions V1/V2 and V4 is to form a shield that protects the virus from immune recognition. Evidence for this effect came originally from the SIV model. Macaques infected with SIV mutants lacking glycosylation in the V1 regions of gp120 generated high titers of antibodies to both mutants and wild-type SIV env proteins [11]. Furthermore, these same glycosylation mutants appear highly sensitive to neutralization [12]. Impressive new studies give a similar picture of the role of glycosylation in the evolution of the host immune response during HIV-1 infection. Several groups have observed that during HIV infection, virus in the blood continues to evolve away from the host neutralizing antibody response [13,14]. Over time early viral populations, which are neutralization sensitive, evolve and are replaced by neutralization-resistant viruses. Elegant studies by Wei et al. [13] determined that the escape virus contained mutations in the env gene involving primarily changes at glycosylation sites, suggesting that HIV often escapes from humoral responses by modifying its ‘heavy glycan shield'. They theorized that this shield prevents neutralizing antibodies from binding to their envelope targets. A creative vaccine spin on this insidious shield has been suggested by Burton and colleagues [15]. In an exciting effort to focus the host immune response to an immunogen, they engineered envelopes, which use extra N-linked glycans, to block irrelevant antigen sites and leave exposed important epitopes from the broadly neutralizing monoclonal antibody b12 site, which targets the CD4-binding site on gp120. These series of gp120 mutants have been proposed as promising immunogenes [15]. The elicitation of broadly cross-reactive neutralizing antibodies by these modified glycoproteins remains to be proven as these studies are early. However, such designer immunogen strategies may be useful in arenas other than HIV. A surprising caveat for the role of glycosylation in HIV immune evasion comes from the studies of Derdeyn et al. [16]. By cloning and sequencing the earliest viruses transmitted in eight clade C transmission pairs, these investigators observed that the transmitted viruses exhibited low levels of glycosylation strongly suggesting that they would all be neutralization sensitive. This virus was different from the more heavy glycosylated virus found in the bloodstream. These important studies illustrate that viral evolution in blood of infected individuals may not be directly related to the isolates transmitted as transmitted virus appears to be sequestered in some specific compartments. In fact, the transmission data suggesting that a single virus establishes infection could be more consistent with cell-mediated transmission rather than free viral transmission model where low levels of free virus would be expected to be more susceptible to innate host immune defenses. These studies also highlight one possible weakness of primate vaccine challenge models where high doses of challenge virus are necessary to ensure infection, thus presenting an artificially high barrier for vaccines under evaluation. Recent studies have refocused interest on perhaps the Gordian Knot of HIV vaccines. The V3 loop of gp120 has long been considered the principal neutralizing HIV-1 domain, evoking type-specific and minimally cross-neutralizing antibodies. However, such responses are often poor at neutralizing primary HIV-1 isolates and its dramatic variability has been the downfall of many creative and complex HIV immunogens targeting this area. This loop plays a key role in determining cell tropism of HIV through differential binding to the CXCR4 and/or CCR5 co-receptors on the target cell. Delineating the three-dimensional structure of this loop, could be a key element for designing vaccine components to more broadly neutralizing responses. by and a V3 loop structure by V3 with monoclonal antibodies that to and neutralize by These studies an between the structure of V3 and of the which are for the HIV that one of of the V3 loop for viruses that to CCR5 and CXCR4 studies suggest that V3 that to CCR5 receptors have that are similar to or the for V3 that to CXCR4 all have a conserved structure that the variability of the V3 loop is not but is to in the virus for co-receptor These studies suggest new for use of structural of the V3 loop in HIV vaccine and are already some of in this that be between viral isolates the same clade or between different has been an One approach proposed for developing vaccine immunogens that may be more cross-reactive is to or of the between available viral This has been suggested in to the of a vaccine response and generate a more methods have been proposed for designing vaccine that the of but the by a the of the on an This approach may have specific are found to be more as in the including multiple or of such to neutralizing antibody responses are in primate models the generated in rhesus macaques a vaccines or by elicited control viral replication and protection from CD4 macaque models have been with interest in vaccine The macaque model several important in HIV vaccine It analysis of vaccine and of in a species more related to that is more than or model are a for the of a vaccine in a or challenge such as the SIV or that as may be another caveat has to focus the of the in vaccine challenge et al. suggested that animals with a to be to disease progression to the most protection studies the groups that vaccine were a of animals in to for a more evaluation of cellular responses by on analysis of responses However, a recent by et al. from their same vaccine in elicited viral control than the observed in their in only was the viral control but the of the response was also that animals have a dramatic response that may not be of human responses to HIV cellular vaccines. 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