Preclinical evaluation of the immunogenicity of C-type HIV-1-based DNA and NYVAC vaccines in the Balb/C mouse model

As part of a European initiative (EuroVacc), we report the design, construction, and immunogenicity of two HIV-1 vaccine candidates based on a clade C virus strain (CN54) representing the current major epidemic in Asia and parts of Africa. Open reading frames encoding an artificial 160-kDa GagPolNef (GPN) polyprotein and the external glycoprotein gp120 were fully RNA and codon optimized. A DNA vaccine (DNA-GPN and DNA-gp120, referred to as DNA-C), and a replication-deficient vaccinia virus encoding both reading frames (NYVAC-C), were assessed regarding immunogenicity in Balb/C mice. The intramuscular administration of both plasmid DNA constructs, followed by two booster DNA immunizations, induced substantial T-cell responses against both antigens as well as Env-specific antibodies. Whereas low doses of NYVAC-C failed to induce specific CTL or antibodies, high doses generated cellular as well as humoral immune responses, but these did not reach the levels seen following DNA vaccination. The most potent immune responses were detectable using prime:boost protocols, regardless of whether DNA-C or NYVAC-C was used as the priming or boosting agent. These preclinical findings revealed the immunogenic response triggered by DNA-C and its enhancement by combining it with NYVAC-C, thus complementing the macaque preclinical and human phase I clinical studies of EuroVacc.

Better protective effects in rhesus macaques by combining systemic and mucosal application of a dual component vector vaccine after rectal SHIV89.6P challenge compared to systemic vaccination alone

In this study we investigated the efficacy of a multigenic DNA prime/modified vaccinia Ankara (MVA)boost vaccine approach, followed by mucosal challenge with highly pathogenic simian-human immunodeficiency virus (SHIV) 89.6P, using different routes for vaccine delivery. After three times of DNA priming (SIVmac239, GagPol, and SHIV 89.6P Env) one vaccine group of monkeys was immunized with MVA systemically via intramuscular (IM) and intradermal (ID) application, and in another vaccine group the MVA booster immunization comprised the IM, ID, and atraumatic oral route. Although all vaccinees became infected after intra-rectal challenge with SHIV 89.6P, substantial protection as indicated by lower peak and set point viral loads and unambiguous preservation of CD4 T cells could be achieved. As we could only transiently detect low levels of neutralizing antibodies in some vaccinees, these antibodies did not seem to add to the protection in the vaccinees. Our results indicate that both preventive multigenic DNA prime/MVA booster immunization strategies promote the control of virus replication and protect from disease progression. We also demonstrated that combining mucosal and systemic vaccination mediated better protective effects compared to systemic vaccination alone.

Capsid stability and replication of human immunodeficiency virus type 1 are influenced critically by charge and size of Gag residue 183

Structural data support a model where - following proteolytic cleavage--the amino-terminal domain of human immunodeficiency virus type 1 (HIV-1) capsid protein refolds into a beta-hairpin/helix tertiary structure that is stabilized by a buried salt bridge forming between the positively charged primary imino group of a proline residue and the negatively charged carboxyl group of a conserved aspartate. In order to evaluate the contribution of either side-chain length or charge to the formation of infectious virus capsids, aspartate 183 was substituted for glutamate or asparagine in the viral context. It was found that both modifications abolished infectivity of the corresponding viruses in permissive T lymphocytes, although none of particle assembly and release, RNA encapsidation, incorporation of Env glycoproteins and packaging of cyclophilin A were impaired. However, whereas biophysical analyses of mutant virions yielded wild-type-like particle sizes and densities, electron microscopy revealed aberrant core morphologies that could be attributed to either increased (D183N) or reduced (D183E) capsid stability. Although the two amino acid substitutions had opposing effects upon core stability, both mutants were shown to exhibit a severe block in early reverse transcription, underscoring the importance of correct salt-bridge formation for early steps of virus replication.

Generation and immunogenicity of novel HIV/AIDS vaccine candidates targeting HIV-1 Env/Gag-Pol-Nef antigens of clade C

Recombinants based on the attenuated vaccinia virus strains MVA and NYVAC are considered candidate vectors against different human diseases. In this study we have generated and characterized in BALB/c and in transgenic HHD mice the immunogenicity of two attenuated poxvirus vectors expressing in a single locus (TK) the codon optimized HIV-1 genes encoding gp120 and Gag-Pol-Nef (GPN) polyprotein of clade C (referred as MVA-C and NYVAC-C). In HHD mice primed with either MVA-C or NYVAC-C, or primed with DNA-C and boosted with the poxvirus vectors, the splenic T cell responses against clade C peptides spanning gp120/GPN was broad and mainly directed against Gag-1, Env-1 and Env-2 peptide pools. In BALB/c mice immunized with the homologous or the heterologous combination of poxvirus vectors or with Semliki forest virus (SFV) vectors expressing gp120/GPN, the immune response was also broad but the most immunogenic peptides were Env-1, GPN-1 and GPN-2. Differences in the magnitude of the cellular immune responses were observed between the poxvirus vectors depending on the protocol used. The specific cellular immune response triggered by the poxvirus vectors was Th1 type. The cellular response against the vectors was higher for NYVAC than for MVA in both HHD and BALB/c mice, but differences in viral antigen recognition between the vectors was observed in sera from the poxvirus-immunized animals. These results demonstrate the immunogenic potential of MVA-C and NYVAC-C as novel vaccine candidates against clade C of HIV-1.

Qualitative T-helper responses to multiple viral antigens correlate with vaccine-induced immunity to simian/human immunodeficiency virus infection

Evidence is accumulating that CD4(+) T-helper (Th) responses play a critical role in facilitating effector responses which are capable of controlling and even preventing human immunodeficiency virus (HIV) infection. The present work was undertaken to determine whether immunization with multiple antigens influenced individual Th responses and increased protection relative to a single antigen. Rhesus macaques were primed with DNA and boosted (immune-stimulating complex-formulated protein) with a combination of regulatory and structural antigens (Tat-Env-Gag) or with Tat alone. Immunization with combined antigens reduced the magnitude of the responses to Tat compared to the single-antigen immunization. Interestingly, the Th immune responses to the individual antigens were noticeably different. To determine whether the qualitative differences in vaccine-induced Th responses correlated with vaccine efficacy, animals were challenged intravenously with simian/human immunodeficiency virus (strain SHIV(89.6p)) 2 months following the final immunization. Animals that developed combined Th1- and Th2-like responses to Gag and Th2 dominant Env-specific responses were protected from disease progression. Interestingly, one animal that was completely protected from infection had the strongest IFN-gamma and interleukin-2 (IL-2) responses prior to challenge, in addition to very strong IL-4 responses to Gag and Env. In contrast, animals with only a marked vaccine-induced Tat-specific Th2 response (no IFN-gamma) were not protected from infection or disease. These data support the rationale that effective HIV vaccine-induced immunity requires a combination of potent Th1- and Th2-like responses best directed to multiple antigens.

HIV-1 gag expression is quantitatively dependent on the ratio of native and optimized codons

There is a significant variation of codon usage bias among different species and even among genes within the same organisms. Codon optimization, this is, gene redesigning with the use of codons preferred for the specific expression system, results in improved expression of heterologous genes in bacteria, plants, yeast, mammalian cells, and transgenic animals. The mechanisms preventing expression of genes with rare or low-usage codons at adequate levels are not completely elucidated. Human immunodeficiency virus (HIV) represents an interesting model for studying how differences in codon usage affect gene expression in heterologous systems. Construction of synthetic genes with optimized codons demonstrated that the codon-usage effects might be a major impediment to the efficient expression of HIV gag/pol and env gene products in mammalian cells. According to another hypothesis, the poor expression of HIV structural proteins even without HIV context is attributed to the so-called cis-acting inhibitory elements (INS), which are located within the protein-coding region. They consist of AU-rich sequences and may be inactivated through the introduction of multiple mutations over the large regions of gag gene. In our work, we evaluated expression of hybrid HIV-1 gag mRNAs where wild-type (A-rich) gag sequences were combined with artificial sequences. In such "humanized" gag fragments with adapted codon usage, AT-content was significantly reduced in favor of G and C nucleotides without any changes in protein sequence. We show that wild-type gag sequences negatively influence expression of gag-reporter, and the addition of fragments with optimized codons to gag mRNA partially rescues its expression. The results demonstrate that the expression of HIV-1 gag is determined by the ratio of optimized and rare codons within mRNA. Our data also indicates that some wtgag fragments counteract the influence of the other wtgag sequences, which cause the inhibition of gag expression. The presented data do not contradict the concept of INS; yet, it makes the definition of INS more complex. This supports the idea of a broader role of the selected codon usage in influencing the expression of HIV proteins in mammalian cells.

Rev-independent expression of synthetic gag-pol genes of human immunodeficiency virus type 1 and simian immunodeficiency virus: implications for the safety of lentiviral vectors

The safety of lentiviral vectors for clinical applications is still a major concern. The gag-pol expression plasmids and the lentiviral vectors used in previous studies contain homologous regions, which constitute a risk for recombination events. Synthetic gag-pol genes of human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) were therefore constructed, in which the codon usage was optimized for expression in human cells without altering the amino acid sequences. The synthetic gag-pol genes allowed efficient expression of these genes in the absence of Rev and the 5' untranslated leader region. Both the HIV-1 and the SIV synthetic gag-pol expression plasmids could mediate transduction of an SIV vector into nondividing human cells with titers of about 10(6) transducing units/ml. Similar titers were obtained with a four-plasmid vector-packaging system based on HIV-1. Using a biological assay, homologous recombination events between the synthetic gag-pol expression plasmids and an SIV vector were undetectable and in comparison with a previously used gag-pol expression plasmid at least approximately 100-fold less frequent. By eliminating regions of homology and sequences involved in packaging, synthetic gag-pol genes should improve the safety profile of lentiviral vectors.

Concerted action of multiple cis-acting sequences is required for Rev dependence of late human immunodeficiency virus type 1 gene expression

Based on the human immunodeficiency virus type 1 (HIV-1) gag gene, subgenomic reporter constructs have been established allowing the contributions of different cis-acting elements to the Rev dependency of late HIV-1 gene products to be determined. Modification of intragenic regulatory elements achieved by adapting the codon usage of the complete gene to highly expressed mammalian genes resulted in constitutive nuclear export allowing high levels of Gag expression independent from the Rev/Rev-responsive element system and irrespective of the absence or presence of the isolated major splice donor. Leptomycin B inhibitor studies revealed that the RNAs derived from the codon-optimized gag gene lacking AU-rich inhibitory elements are directed to a distinct, CRM1-independent, nuclear export pathway.

Proline residues in the HIV-1 NH2-terminal capsid domain: structure determinants for proper core assembly and subsequent steps of early replication

Recent analyses suggest that the p24 capsid (p24(CA)) domain of the HIV-1 group-specific antigen (Gag) may be divided into two structurally and functionally distinct moieties: (i) an amino-terminal portion, previously shown to bind the cellular chaperone cyclophilin A, and (ii) a carboxy-terminal domain, known to contribute to the interaction of the Gag and Gag-Pol precursors during the early assembly process. In order to gain deeper insight into the role of the amino-terminal domain of the p24(CA) protein during viral replication, eight highly conserved proline residues known to promote turns and to terminate alpha-helices within the p24 tertiary structure were replaced by a leucine residue (P-position-L). Following transfection of the proviral constructs in COS7 cells, the majority of the mutants resembled wild-type viruses with respect to the assembly and release of virions. However, although the released particles contained wild-type levels of genomic viral RNA, the mature products of the Gag and Gag-Pol polyproteins as well as the Env glycoproteins-all of them, except mutant P225L-were either noninfectious or severely affected in their replicative capacity. Entry assays monitoring the process of viral DNA synthesis led to the classification of selected provirus mutants into four different phenotypes: (i) mutant P225L was infectious and allowed complete reverse transcription including formation of 2-LTR circles; (ii) mutants P149L, P170L, and P217L failed to form 2-LTR circles; (iii) mutant P222L displayed a severe defect in binding and incorporating cyclophilin A into virions, was delayed with respect to DNA polymerization, and failed to form a 2-LTR replication intermediate; and (iv) mutant P133L was unable even to synthesize a first-strand cDNA product. All replication-defective mutants were characterized by severe alterations in the stability of virion cores, which were in two cases reflected by visible changes in the core morphology. These results suggest that proline residues in the NH(2)-terminal capsid domain represent critical structure determinants for proper formation of functional virion cores and subsequent stages of early replication.