Sean P. McBurney

Viral sequence diversity: challenges for AIDS vaccine designs

Among the greatest challenges facing AIDS vaccine development is the intrinsic diversity among circulating populations of HIV-1 in various geographical locations and the need to develop vaccines that can elicit enduring protective immunity to variant HIV-1 strains. While variation is observed in all of the viral proteins, the greatest diversity is localized to the viral envelope glycoproteins, evidently reflecting the predominant role of these proteins in eliciting host immune recognition and responses that result in progressive evolution of the envelope proteins during persistent infection. Interestingly, while envelope glycoprotein variation is widely assumed to be a major obstacle to AIDS vaccine development, there is very little experimental data in animal or human lentivirus systems addressing this critical issue. In this review, the state of vaccine development to address envelope diversity will be presented, focusing on the use of centralized and polyvalent sequence design as mechanisms to elicit broadly reactive immune responses.

Association between Magnitude of the Virus-Specific Plasmablast Response and Disease Severity in Dengue Patients

Dengue is a globally expanding disease caused by infection with dengue virus (DENV) that ranges from febrile illness to acute disease with serious complications. Secondary infection predisposes individuals to more severe disease, and B lymphocytes may play a role in this phenomenon through production of Ab that enhance infection. To better define the acute B cell response during dengue, we analyzed peripheral B cells from an adult Brazilian hospital cohort with primary and secondary DENV infections of varying clinical severity. Circulating B cells in dengue patients were proliferating, activated, and apoptotic relative to individuals with other febrile illnesses. Severe secondary DENV infection was associated with extraordinary peak plasmablast frequencies between 4 and 7 d of illness, averaging 46% and reaching 87% of B cells, significantly greater than those seen in mild illness or primary infections. On average >70% of IgG-secreting cells in individuals with severe secondary DENV infection were DENV specific. Plasmablasts produced Ab that cross-reacted with heterotypic DENV serotypes, but with a 3-fold greater reactivity to DENV-3, the infecting serotype. Plasmablast frequency did not correlate with acute serum-neutralizing Ab titers to any DENV serotype regardless of severity of disease. These findings indicate that massive expansion of DENV-specific and serotype cross-reactive plasmablasts occurs in acute secondary DENV infection of adults in Brazil, which is associated with increasing disease severity.

Human immunodeficiency virus-like particles with consensus envelopes elicited broader cell-mediated peripheral and mucosal immune responses than polyvalent and monovalent Env vaccines.

Envelope (Env) sequences from human immunodeficiency virus (HIV) strains can vary by 15-20% within a single clade and as much as 35% between clades. Previous AIDS vaccines based upon a single isolate often could not elicit protective immune responses against heterologous viral challenges. In order to address the vast sequence diversity in Env sequences, consensus sequences were constructed for clade B and clade C envelopes and delivered to the mouse lung mucosa on the surface of virus-like particles (VLP). Consensus sequences decrease the genetic difference between the vaccine strain and any given viral isolate. The elicited immune responses were compared to a mixture of VLPs with Envs from primary viral isolates. This polyvalent vaccine approach contains multiple, diverse Envs to increase the breadth of epitopes recognized by the immune response and thereby increase the potential number of primary isolates recognized. Both consensus and polyvalent clade B Env VLP vaccines elicited cell-mediated immune responses that recognized a broader number of clade B Env peptides than a control monovalent Env VLP vaccine in both the systemic and the mucosal immune compartments. All three clade C Env vaccine strategies elicited similar responses to clade C peptides. However, both the consensus B and C Env VLP vaccines were more effective at eliciting cross-reactive cellular immune responses to epitopes in other clades. This is the first study to directly compare the breadth of cell-mediated immune responses elicited by consensus and polyvalent Env vaccines.

Developing broadly reactive HIV-1/AIDS vaccines: a review of polyvalent and centralized HIV-1 vaccines.

The development of an HIV/AIDS vaccine requires consideration of the large diversity of viral isolates. In 2005, there were 5 million new cases of HIV infection and over 4 million deaths due to AIDS. An HIV vaccine is needed to prevent the spread of this virus. One of the greatest challenges to developing a preventative HIV vaccine is the diversity of HIV-1 isolates. Env sequences can differ by as much as 35% between isolates from different clades and by as much as 10% within a clade. Two main strategies to address viral diversity for HIV vaccine development are the use of polymeric- or centralized-based immunogens. Polymeric-based vaccines, which have been used for polio and pneumococcus vaccines, use components from a range of viral isolates to increase the breadth of immune recognition. Centralized sequences decrease the sequence diversity by encoding the most common amino acid at each position from a diverse pool of viral isolates. These sequences are derived using the consensus, center-of-the-tree, or ancestral methods. The use of polyvalent- and centralized-based vaccines induce broadly reactive immune responses, however it is unclear whether the use of these sequences will increase protection against diverse HIV-1 infection. This review will summarize the current uses of polyvalent and centralized vaccines to increase immune breadth that may determine future directions for HIV-1 vaccine development.

Lentivirus-Like Particles Without Reverse Transcriptase Elicit Efficient Immune Responses

Following infection by HIV or SIV, reverse transcriptase (RT) directs the conversion of the single-stranded RNA genome into a double-stranded DNA molecule that integrates into the host cell genome. RT encodes for several immunogenic epitopes that are desirable for inclusion in a human vaccine for HIV infection, however, issues of safety have dampened enthusiasm for inclusion of an enzymatically-active RT molecule into an AIDS vaccine. In this study, virally-regulated, replication-incompetent lentiviral particles were expressed from DNA plasmids. The sequences for integrase, Vpr, Vif, Nef, and the long terminal repeats (LTRs) were deleted and mutations were engineered into capsid to decreases RNA packaging. Virus-like particles incorporated no RT (HIV-VLP DeltaRT or SHIV-VLP DeltaRT) or contained a full-length enzymatically-inactivated RT molecule (HIV-VLP or SHIV-VLP). Each secreted VLP was enveloped with a lipid bilayer derived from primate cells with embedded, native viral envelopes in similar concentrations as infectious virions. BALB/c mice were vaccinated (weeks 0, 3, and 6) with purified VLPs via intranasal inoculation in the presence of cytosine-phosphate-guanosine oligodeoxynucleotides (CpG ODNs). All VLPs, with or without RT, elicited both robust humoral and cellular immune responses to Gag, Pol, and Env antigens. Therefore, the lack of RT enhances the safety of these VLPs for use in future human clinical trials without a significant reduction in the overall immunogenicity of these VLP immunogens.

C1q binding to dengue virus decreases levels of infection and inflammatory molecules transcription in THP-1 cells

Dengue virus infection elicits a spectrum of clinical presentations ranging from asymptomatic to severe disease. The mechanisms leading to severe dengue are not known, however it has been reported that the complement system is hyper-activated in severe dengue. Screening of complement proteins demonstrated that C1q, a pattern recognition molecule, can bind directly to dengue virus envelope protein and to whole dengue virus serotype 2. Incubation of dengue virus serotype 2 with C1q prior to infection of THP-1 cells led to decreased virus infectivity and modulation of mRNA expression of immunoregulatory molecules suggesting reduced inflammatory responses.

Attenuated nef DNA vaccine construct induces cellular immune response: role in HIV-1 multiprotein vaccine.

HIV-1 positive patients generate Nef-specific CTL response, indicating that Nef is a potent immunogen. However, Nef is also known to down regulate the expression of CD4 and MHC-I molecules, thereby protecting virally infected target cells. We compared the immunogenicity of non-functional nef vaccine constructs to wild type functional nef as potential immunogen. Mice were immunized with different nef constructs and assessed for their ability to induce cellular immune responses. Evaluation of T cell immune responses in mice showed that non-functional nef vaccine constructs are capable of inducing a significant T cell immune response measured by IFN-gamma ELISPOT. Further epitope mapping studies indicate that one of our attenuated constructs, Nef R-38, has multiple CTL epitopes spanning throughout the gene. Our results indicate that functionally attenuated Nef antigen might be a better candidate for future multiprotein HIV-1 vaccine.