Towards prediction of degenerate CTL epitope recognition

Performance of a redesigned HIV Selectest enzyme-linked immunosorbent assay optimized to minimize vaccine-induced seropositivity in HIV vaccine trial participants

Vaccine-induced seropositivity (VISP) or seroreactivity (VISR), defined as the reaction of antibodies elicited by HIV vaccines with antigens used in HIV diagnostic immunoassays, can result in reactive assay results for vaccinated but uninfected individuals, with subsequent misclassification of their infection status. The eventual licensure of a vaccine will magnify this issue and calls for the development of mitigating solutions in advance. An immunoassay that discriminates between antibodies elicited by vaccine antigens and those elicited by infection has been developed to address this laboratory testing need. The HIV Selectest is based on consensus and clade-specific HIV peptides that are omitted in many HIV vaccine constructs. The assay was redesigned to enhance performance across worldwide clades and to simplify routine use via a standard kit format. The redesigned assay was evaluated with sera from vaccine trial participants, HIV-infected and uninfected individuals, and healthy controls. The HIV Selectest exhibited specificities of 99.5% with sera from uninfected recipients of 6 different HIV vaccines and 100% with sera from normal donors, while detecting HIV-1 infections, including intercurrent infections, with 95 to 100% sensitivity depending on the clade, with the highest sensitivities for clades A and C. HIV Selectest sensitivity decreased in very early seroconversion specimens, which possibly explains the slightly lower sensitivity observed for asymptomatic blood donors than for clinical HIV cases. Thus, the HIV Selectest provides a new laboratory tool for use in vaccine settings to distinguish the immune response to HIV vaccine antigens from that due to true infection.

A trivalent recombinant Ad5 gag/pol/nef vaccine fails to protect rhesus macaques from infection or control virus replication after a limiting-dose heterologous SIV challenge

It has been suggested that poor immunogenicity may explain the lack of vaccine efficacy in preventing or controlling HIV infection in the Step trial. To investigate this issue we vaccinated eight Indian rhesus macaques with a trivalent replication-incompetent adenovirus serotype 5 vaccine expressing SIV Gag, Pol, and Nef using a regimen similar to that employed in the Step trial. We detected broad vaccine-induced CD8(+) (2-7 pool-specific responses) and CD4(+) (5-19 pool-specific responses) T-cell responses in IFN-γ ELISPOT assays at one week post-boost using fresh PBMC. However, using cryopreserved cells at one and four weeks post-boost we observed a reduction in both the number and magnitude of most vaccine-induced responses. This demonstrates that the time points and conditions chosen to perform immune assays may influence the observed breadth and frequency of vaccine-induced T-cell responses. To evaluate protective efficacy, we challenged the immunized macaques, along with naïve controls, with repeated, limiting doses of the heterologous swarm isolate SIVsmE660. Vaccination did not significantly affect acquisition or control of virus replication in vaccinees compared to naïve controls. Post-infection we observed an average of only two anamnestic CD8(+) T-cell responses per animal, which may not have been sufficiently broad to control heterologous virus replication. While the trivalent vaccine regimen induced relatively broad T-cell responses in rhesus macaques, it failed to protect against infection or control viral replication. Our results are consistent with those observed in the Step trial and indicate that SIV immunization and challenge studies in macaque models of HIV infection can be informative in assessing pre-clinical HIV vaccines.

Focus and breadth of cellular immune responses elicited by a heterologous insert prime-boost vaccine regimen in rhesus monkeys

The global sequence diversity of HIV-1 presents a daunting challenge for vaccine development. We investigated whether a heterologous insert prime-boost regimen could expand global coverage by selectively boosting cellular immune responses to conserved epitopes. Rhesus monkeys were primed and boosted with recombinant adenovirus vectors expressing homologous or heterologous HIV-1 Gag sequences that were optimized to focus responses on highly conserved epitopes. We observed comparable responses directed to specific regions of the Gag protein in all experimental groups without evidence of improved coverage or expanded breadth in the heterologous insert group. These data suggest that antigen-independent factors contribute to the immunodominance patterns of vaccine-elicited cellular immune responses.

Vaccine development against Trypanosoma cruzi and Leishmania species in the post-genomic era

Trypanosoma cruzi and the genus Leishmania are protozoan parasites causing diseases of major public health importance, and the recent completion of the sequencing of their genomes has opened new opportunities to further our understanding of the mechanisms required for protection and the development of vaccines. For example, trans-sialidases, one of the largest protein families from T. cruzi, contain dominant CD8+ T cell epitopes, and their use as preventive or therapeutic vaccines in different animal models has provided encouraging results. A much wider range of antigens and vaccine formulations have been tested against Leishmania, and new correlates for protection are being defined, such as the induction of multifunctional Th1 effector cells capable of producing a complex set of cytokines. Also, while a large number of these vaccine candidates have been rather successful in mouse models, their usefulness in more relevant animal models is still poor, in spite of significant immunogenicity. Novel proteomics and genomics approaches are being used for antigen discovery and the identification of new vaccine candidates, some of which have shown promise for the control of infection. These studies cast little doubt that T. cruzi and Leishmania genomes represent major resources for understanding key aspects of the mechanisms of immune protection against these parasites, and the increasing use of these tools will greatly impact vaccine development.