There is an increasing body of data to indicate that priming with rBCG in heterologous regimens that boost with attenuated viral or protein modalities represents an effective strategy for the induction of passenger immunogen-specific T-cell responses (4
). Here, by using a more immunogenic and safer rBCG (57
) in prime-boost regimens with rMVA and novel rOAdV (5
) vaccines, we demonstrated (i) enhanced T-cell induction in mice by using regimens that incorporated BCG.HIVA401
priming (with BCG.HIVA401
priming for increased responses not formally observed in rhesus macaques); (ii) the induction in rhesus macaques of high-frequency HIV-1-specific T-cell responses, which recognized multiple HIV-1 Gag-derived epitopes; and (iii) the effective recruitment in rhesus macaques of insert-specific T-cell clonotypes into diverse compartments of the memory pool. These are desirable features of HIV-1 vaccine-induced T cells.
This is the first immunogenicity study of BCG.HIVA401
and, indeed, of the parental AERAS-401 BCG strain in nonhuman primates. BCG.HIVA401
alone induced strong T-cell responses to PPD, yet only weak responses specific for the HIV-1 transgene product HIVA (Fig. and ). This might represent a positive feature of BCG-vectored vaccine usage during the priming phase of more complex regimens because the nature of priming events can dramatically influence the differentiation and fate of the elicited CD8 T cells (60
). Indeed, it was reported previously that recombinant mycobacterium-induced T lymphocytes that were skewed toward durable antigen-specific memory CD8 T cells and rapidly expanded by heterologous virus-vectored vaccines sharing the same immunogen (28
). This quality memory induction is possibly a consequence of the strong CD4 T cell help that mycobacteria elicit (43
). Similarly, despite the induction of specific T cells at or below the limit of detection, DNA priming increased the consistency of the subsequent MVA.HIVA boost to 100% in a group of 8 human volunteers (14
The MO and OM boosts yielded T-cell responses of similar overall frequencies. The first viral boost resulted in only a small augmentation of HIV-1-specific T-cell responses, possibly due to the accelerated regimen. Thus, after 4 weeks, mycobacteria might still persist at high levels, thereby driving a cytokine storm and ongoing activation of innate responses that could decrease vaccine take. Nevertheless, the second viral vaccination delivered a substantial T-cell boost and induced broadly targeted responses capable of rapid expansion upon antigenic reexposure. Small, but significant differences were detected between the two heterologous boosts in macaques, with OAdV.HIVA inducing higher frequencies of HIV-1-specific T cells than MVA.HIVA (Fig. ). Greater T cell induction by OAdV.HIVA over MVA.HIVA was also detected in mice (Fig. ). The overall T-cell immunogenicity of our two regimens is similar to a recently published regimen consisting of two rBCG primes boosted by an rHAdV-5 vaccine (8
). Clearly, further experiments are necessary to optimize the timing, dosage, and administration routes of the BCG.HIVA401
, MVA.HIVA, and OAdV.HIVA vaccination regimen. However, the availability of other HIVA vaccine modalities provides the opportunity for further augmentation of vaccine-induced HIV-1-specific T-cell frequencies (53a
Optimized rBCG is a very attractive priming component for adult prophylactic HIV-1 vaccines. We have argued that rBCG is also a logical means of preventing HIV-1 transmission from infected mothers to breastfeeding infants (32
), because BCG is an integral component of the EPI given to infants at birth or upon the first contact with a health care worker and is followed by other EPI vaccines between 4 and 6 weeks later, depending on the national guidelines. In immunocompromised individuals, BCG can cause disseminated disease and is, therefore, not recommended for HIV-1-infected infants. Nevertheless, because of the risk of tuberculosis in impoverished countries, WHO guidelines do recommend BCG vaccination for healthy asymptomatic babies of unknown HIV-1 status (64
). Thus, rBCG expressing an HIV-1 immunogen would serve as a dual-priming platform for vaccines against M. tuberculosis
) and HIV-1. In this respect, the demonstrably increased safety in SCID mice of AERAS-401 relative to its parental BCG strain SSI-1331 (57
) is an important reassurance for such an approach. Here, we have tested the immunogenicity of BBMO and BBOM regimens delivered 4 weeks apart with the view that it is critical in infants to elicit protective HIV-1-specific responses as soon after birth as possible to prevent breast milk transmission, although in infants, only one dose of rBCG would be used. While robust anti-HIV-1 T-cell responses were induced by the 4-wk-gap regimen, these might be higher still if more commonly used intervals between individual doses were employed (8
Ovine atadenovirus (OAdV) is a novel and underexplored vaccine vector, which is distinct from the more commonly used mastadenoviruses (2
). Here, we demonstrated that OAdV is capable of antigen presentation for the induction of effective CD8 and CD4 T cell responses in heterologous prime-boost regimens. In addition, OAdV avoids the problems and possible risks associated with preexisting immunity to HAdV vectors, such as HAdV-5, in target populations. This work, which is the first demonstration of the safety and T-cell immunogenicity of OAdV in nonhuman primates, justifies its further development as a vaccine vector for HIV-1, other infections, and cancer.
The HIVA construct (21
) has been an extremely useful model immunogen both for clinical (14
) and preclinical (30
) vaccine development. The epitopes recognized in mice and Mamu-A*01+
rhesus macaques continue to be mapped, and the corresponding CD8 T-cell responses are being characterized in greater detail. However, there is no appropriate challenge available for the vaccinated animals because HIVA is designed for humans and is, therefore, derived from HIV-1 antigens; HIV-1 does not replicate in rhesus macaques. As for human efficacy, broad Gag-specific responses have been associated with good control of HIV-1 replication in chronically infected patients (27
), responses to HIVA have been readily detected in exposed uninfected children in Kenya (55
), and it has been demonstrated that HIVA boosts specific CD8 and CD4 T cell responses effectively in patients infected with a variety of HIV-1 clades (10
). Thus, HIVA as a Gag-based immunogen concurs at several levels with the emerging correlates of T-cell-mediated control of HIV-1 replication.
In conclusion, although patients have benefited from highly active antiretroviral treatment, the HIV-1 pandemic continues unabated and an effective vaccine remains the best solution for halting the spread of this virus in resource-poor areas. New, safer, and more immunogenic vaccine vectors in combination are necessary for the definition of an optimal strategy. Here, we tested for the first time in nonhuman primates a unique vector combination of two novel vaccines based on modified BCG and sheep atadenovirus and demonstrated their potential utility for further HIV-1 vaccine development.