Four HLA-DR-restricted HIV-derived Th lymphocyte (HTL) epitopes cross-reactive with the murine I-Ab class II molecule were used to evaluate different vaccine design strategies to simultaneously induce multiple HTL responses. All four epitopes were immunogenic in H-2b mice, demonstrating the feasibility of murine models to evaluate epitope-based vaccines destined for human use. Immunization with a pool of peptides induced responses against all four epitopes; illustrating immunodominance does not prevent the induction of balanced multispecific responses. When different delivery systems were evaluated, a multiple Ag peptide construct was found to be less efficient than a linear polypeptide encompassing all four epitopes. Further characterization of linear polypeptide revealed that the sequential arrangement of the epitopes created a junctional epitope with high affinity class II binding. Disruption of this junctional epitope through the introduction of a GPGPG spacer restored the immunogenicity against all four epitopes. Finally, we demonstrate that a GPGPG spacer construct can be used to induce HTL responses by either polypeptide or DNA immunization, highlighting the flexibility of the approach.

Cytotoxic T lymphocytes (CTLs) recognize peptide antigens associated with cell surface major histocompatibility complex (MHC) molecules. The identification of tumor cell-derived peptides capable of eliciting anti-tumor CTL responses would enable the design of antigen-specific immunotherapies. Our strategy to identify such potentially therapeutic peptides relies on selecting high-affinity MHC binders from known tumor-associated antigens. These peptides are subsequently tested for their ability to induce CTLs capable of killing tumor cells. With this strategy, we have identified a nine-residue epitope, derived from the product of the tumorassociated gene MAGE-3, which has the capacity to induce in vitro CTLs that kill melanoma and other tumor cell lines. These results show the primary in vitro induction of tumor-specific human CTLs and illustrate the feasibility of ex vivo antigenspecific approaches to the immunological therapy of cancer.

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Epitope-based vaccines designed to induce CTL responses specific for HIV-1 are being developed as a means for addressing vaccine potency and viral heterogeneity. We identified a set of 21 HLA-A2, HLA-A3, and HLA-B7 restricted supertype epitopes from conserved regions of HIV-1 to develop such a vaccine. Based on peptide-binding studies and phenotypic frequencies of HLA-A2, HLA-A3, and HLA-B7 allelic variants, these epitopes are predicted to be immunogenic in greater than 85% of individuals. Immunological recognition of all but one of the vaccine candidate epitopes was demonstrated by IFN-γ ELISPOT assays in PBMC from HIV-1-infected subjects. The HLA supertypes of the subjects was a very strong predictor of epitope-specific responses, but some subjects responded to epitopes outside of the predicted HLA type. A DNA plasmid vaccine, EP HIV-1090, was designed to express the 21 CTL epitopes as a single Ag and tested for immunogenicity using HLA transgenic mice. Immunization of HLA transgenic mice with this vaccine was sufficient to induce CTL responses to multiple HIV-1 epitopes, comparable in magnitude to those induced by immunization with peptides. The CTL induced by the vaccine recognized target cells pulsed with peptide or cells transfected with HIV-1 env or gag genes. There was no indication of immunodominance, as the vaccine induced CTL responses specific for multiple epitopes in individual mice. These data indicate that the EP HIV-1090 DNA vaccine may be suitable for inducing relevant HIV-1-specific CTL responses in humans.

Chronic hepatitis B virus (HBV) infection, which occurs in 1 to 5% of adult infections and up to 30% of pediatric infections, is characterized by high levels of viral replication averaging a daily production of about 10 11 viral particles, hepatic inflammation, necrosis, and ultimately liver failure (36). An estimated 350 million individuals are classified as chronically HBV infected, with the highest concentrations of infection occurring in large parts of Asia and Africa (23).Chronic HBV can be treated with nucleoside analogues that inhibit polymerase activity. Lamivudine was the first licensed polymerase inhibitor, and it results in significant suppression of HBV DNA levels. However, this response, similar to the loss of hepatitis B virus e antigen, is often not sustained upon discontinuation of treatment (11,28). The emergence of viral resistance in 15 to 20% of treated patients per year clearly pinpoints the limitations of this treatment.Newer drugs such as adefovir dipivoxil, entecavir, and telbivudine can result in less resistance, increased suppression of DNA levels, or in somewhat higher levels of hepatitis B virus e antigen loss. Real long-term treatment data with these drugs are, however, limited, and it is unclear how well these responses would be sustained if therapy were withdrawn.