A Phase I Study Evaluating the Safety and Immunogenicity of a New TB Vaccine, MVA85A, in Asymptomatic Volunteers Who Are Infected With Either Mycobacterium Tuberculosis (M.tb.), Human Immunodeficiency Virus (HIV) or Both

Prime-boost immunization strategies Heterologous prime-boost immunization strategies involve giving two different vaccines, each encoding the same antigen, several weeks apart. Using a DNA prime-recombinant modified vaccinia virus Ankara (MVA) boost induces higher levels of antigen specific CD4+ and CD8+ T cells than using homologous boosting with the same vector in a number of different disease models (Schneider, 1998; McShane, 2001). Given the protective efficacy of BCG in childhood, ideally BCG would be the priming immunization in such a prime-boost strategy. In order to do this, we have focused on antigen 85A as a candidate antigen. Antigen 85A is highly conserved amongst all mycobacterial species and is present in all strains of BCG. Antigen 85A is a major secreted antigen from M. tuberculosis which forms part of the antigen 85 complex (A, B and C). This complex constitutes a major portion of the secreted proteins of both M.tb and BCG. It is involved in fibronectin binding within the cell wall and has mycolyltransferase activity. Antigen 85A is immunodominant in murine and human studies and is protective in small animals (Huygen, 1996).

Recombinant modified vaccinia virus Ankara (rMVA). Many viruses have been investigated as potential recombinant vaccines. The successful worldwide eradication of smallpox via vaccination with live vaccinia virus highlighted vaccinia as a candidate for recombinant use. The recognition in recent years that non- replicating strains of poxvirus such as MVA and avipox vectors can be more immunogenic than traditional replicating vaccinia strains has enhanced the attractiveness of this approach. MVA (modified vaccinia virus Ankara) is a strain of vaccinia virus which has been passaged more than 570 times though avian cells, is replication incompetent in human cell lines and has a good safety record. It has been administered to more than 120,000 vaccinees as part of the smallpox eradication programme, with no adverse effects, despite the deliberate vaccination of high risk groups (Stickl, 1974; Mahnel, 1994). This safety in man is consistent with the avirulence of MVA in animal models. MVA has six major genomic deletions compared to the parental genome severely compromising its ability to replicate in mammalian cells (Meher, 1991). No replication has been documented in non- transformed mammalian cells. Viral replication is blocked late during infection of cells but importantly viral and recombinant protein synthesis is unimpaired even during this abortive infection. The viral genome has been proven to be stable through a large series of passages in chicken embryo fibroblasts. Replication-deficient recombinant MVA has been seen as an exceptionally safe viral vector. When tested in animal model studies recombinant MVA's have been shown to be avirulent, yet protectively immunogenic as vaccines against viral diseases and cancer. Recent studies in severely immunosuppressed macaques have supported the view that MVA should be safe in immunocompromised humans (Akira, 2001; Stittelaar, 2001).

Safety of rMVAs in immunosuppression. There is now considerable preclinical and clinical data demonstrating the safety and immunogenicity of MVA as a viral vector in immunosuppression and HIV infection. Recombinant MVAs expressing HIV antigens have been administered to immunosuppressed macaques with no serious adverse events (Stittelaar et al, 2001). In addition, the safety of recombinant MVAs expressing HIV antigens and epitopes have now been evaluated in several Phase I clinical trials of HIV-infected subjects, both on and off antiretroviral therapy, with no serious adverse events (Cosma et al, 2003; Harrer et al 2005).

Preclinical data supporting this BCG prime-MVA85A boost strategy In BALB/c mice, using BCG as the priming immunization and then boosting with MVA85A induces higher levels of both antigen specific IFN-γ secreting CD4+ and CD8+ T cells and significantly greater levels of protection against aerosol challenge than after BCG alone (Goonetilleke et al, 2003). This regime has now been further evaluated in the more sensitive guinea pig aerosol challenge model with very encouraging results. Guinea pigs vaccinated with BCG and boosted with MVA85A, and then further boosted with a second recombinant viral vector expressing antigen 85A, fowlpox-85A (FP85A) showed significantly greater protection against challenge than guinea pigs vaccinated with BCG alone (Williams et al, 2005). This regime is also immunogenic in rhesus macaques, and protective (Verrek, personal communication).

Clinical data to date with MVA85A UK studies Over the last 3 years HM has established a clinical trial programme to evaluate the safety and immunogenicity of this BCG prime-MVA85A boost vaccination strategy in a series of Phase I studies. MVA85A was the first candidate TB vaccine to enter clinical trials anywhere in the world in September 2002, and is currently the only one in clinical trials in Africa. The design of these Phase I studies with MVA85A allowed for sequential vaccination of volunteer groups with a step-wise increase in mycobacterial exposure, to minimize the possibility of a Koch reaction. Trials were also conducted sequentially in the UK and The Gambia, as there is a greater degree of exposure to both environmental mycobacteria and M.tb in The Gambia. A Koch reaction describes the development of immunopathology in a person or animal with tuberculosis, when an exaggerated immune response to M.tb is stimulated. It was described in patients with TB disease when Koch performed his original studies employing mycobacteria as a type of therapeutic vaccination. It has now been demonstrated in the mouse model of therapeutic vaccination27. Available animal data suggest that these reactions do not occur in mice latently infected with M.tb, suggesting that such reactions may correlate with high bacterial load and that the Koch phenomenon may not pose a problem for vaccination of asymptomatic albeit latently infected humans25.