The Merck candidate vaccine showed good HIV-specific immunogenicity in Phase I and II studies (see http://www.hvtn.org/science/1107.html
for the recently released STEP trial results) as measured mostly by a single parameter: the IFN-γ ELISPOT assay. The rAd vaccine also induced long-lasting, multifunctional responses as monitored by polychromatic flow cytometry (http://www.hvtn.org/fgm/1107slides/McElrath.pdf
). Indeed, after homologous prime-boost immunization with a replication-defective adenovirus-based vaccine, a majority of responders had HIV-specific CD8+
T lymphocytes that were capable of producing CD107, macrophage inflammatory protein 1β, IFN-γ, and TNF, and antigen-specific CD4+
T cells that were able to produce IFN-γ, interleukin (IL)-2, and TNF (Casimiro, D., personal communication). The CD8 T cell responses to HIV antigens, however, were not particularly broad. A median of three peptide pools, each consisting of overlapping 9–amino acid peptides spanning a 16–amino acid region of gag, nef, or pol, was recognized by vaccinated subjects (Casimiro, D., personal communication). Thus, by all accounts, the STEP vaccine was safe, immunogenic (by the aforementioned criteria), and induced some level of protection in primate studies. Although some may have predicted disappointing results based on the relatively weak protection in macaques and the lack of a broad T cell response, no one could have predicted the correlation between preexisting vector-specific immunity and an increase in susceptibility to infection—a result that lead to the immediate halting of both the STEP and Phambilli trials.
The new study by Harari et al. clearly shows that the DNA/NYVAC prime-boost regimen induces HIV-specific CD4+
T cell responses (27
). In this regimen, DNA priming was essential for the induction of a strong response to the recombinant virus. The T cell responses were also polyfunctional; nearly 50% of the HIV-specific CD4+
T cells induced in vaccinated subjects produced more than three cytokines. It is also interesting to note that this regimen induced strong T cell proliferation and, more importantly, robust production of the T cell growth factor IL-2 by HIV-specific CD4+
). These responses were also persistent, as some T cell responses were still detectable after 96 weeks (Pantaleo, G., personal communication). A big effort is now being made to standardize immune-monitoring assays in humans and in preclinical models, but it is currently difficult to compare the relative immunogenicity of two vectors outside of a single trial where the two vaccines are compared head-to-head using a common, standardized set of assays to monitor the resulting immune response. Alternatively, immune responses from both trials should be tested in a single assay platform.
One notable difference between the vaccines used in the STEP trial and by Harari et al. was the detection in the latter study of higher frequencies of CD4+
T cells producing a high level of IL-2 or both IL-2 and IFN-γ. These dual-producing cells have previously been associated with better control of viral load after vaccination and in natural history studies (30
). Polyfunctional T cell responses have also been associated with protection against Leishmania major
) and are a feature of immune responses in HIV-infected elite controllers (32
). Moreover, while rAd5-based vaccines elicit strong CD8 responses to gag, the DNA/NYVAC leads to the development of strong env-specific responses, whereas gag-specific responses are poor. Of note, the rAd5 vaccine tested in the STEP trial did not include env constructs.
The DNA/poxvirus immunization strategy may thus provide a promising alternative to adenoviral vector–based approaches. This strategy is also advantageous in that most individuals born after 1974 have not been immunized against smallpox and hence will have little preexisting immunity to these viruses—an important consideration given the results of the STEP trial. In addition, the DNA-priming step appears to enhance the immune response against the poxvirus, as it also does with other vectors, including adenoviruses (33
). Hence, the DNA prime NYVAC–boost strategy not only might bypass the requirement for multiple booster injections with viral vector, but it might also favorably modulate the immune response against the immunogen. Finally, the immunogenicty data obtained with the DNA/poxvirus strategy show that this vector combination can induce potent, multifunctional immune responses, including a large fraction of IL-2–producing cells, which are often endowed with superior memory functions.
In response to the STEP failure, however, it is important to continue to research the criteria that best predict protective T cell immunity against pathogens. It must now be determined whether the potential improvements afforded by DNA prime virus–boost regimens warrant large-scale clinical studies to evaluate protective immunity. More importantly, we should reflect on whether the assays we are currently using to guide vaccine development should be expanded to include additional surrogate markers of efficacy, or whether they need only provide a simple indicator of immunogenicity. The field should quickly develop assays that will help in predicting the development of protective immunity in response to vaccines.