Vaccines against Anthrax
Two strategies under investigation:
- DNA vaccination
- Salmonella based vaccine system
Aims of Project
The naked DNA approach is vaccination at its simplest. The gene encoding the vaccine antigen is introduced into the host and expressed in vivo where it stimulates a protective immune response. In previous studies we have demonstrated the ability of separate DNA vaccines encoding anthrax and plague vaccine targets to protect animals against live agent challenge (Williamson et al., 1999,2002; Galloway and Baillie, 2004). The advantages of this technology are obvious to anyone who has attempted to develop robust, GMP compliant protein based expression systems capable of meeting the demanding specifications of the FDA. Indeed the first human anthrax DNA vaccine, which was developed in as little as four years, recently completed NIAID funded phase I clinical trials.
Traditional vaccine antigens such as those used in the anthrax DNA vaccine described above comprise a mixture of epitopes of which only a small number play any role in protection. In contrast an epitope based vaccine is design to include only those regions which contribute directly to immunity. In addition one incorporate more copies of each protective epitope and thus enhance the magnitude of the resulting immune response. Finally the small size of each epitope makes it much easier to construct vaccines capable of conferring protection against multiple agents in a single formulation. Research in my laboratory is focused on developing a human CD4 T cell and B cell epitope DNA based vaccine capable of conferring protection against anthrax and plague in a single formulation.
Identification of new vaccine candidates: Concerns over the ability of an aggressor to genetically alter B.anthracis such that the resulting organism defeats the current vaccine has driven researchers to identify additional vaccine targets capable of conferring sterile immunity. In collaboration with colleagues at the Center for Vaccine Development, University of Maryland at Baltimore we are employing a proteomic based approach to identify immunogenic protein targets by probing whole cell extract of the bacterium with immune sera from previously infected individuals. Proteins identified by this method will be cloned and assessed for protective efficacy in animal models. While not all immunogenic proteins will be protective this approach will provide a first filter to focus our efforts on proteins which are know to be expressed during infection. Once vaccine targets have been identified the next step will be express them from a live Salmonella vaccine strain to enable the development of an oral, single dose vaccine.
Salmonella based vaccine system
The utility of Salmonella as a live oral vaccine for typhoid has resulted in the development of Ty21a as a licensed, FDA approved vaccine. There is considerable interest in building on this approach to develop Salmonella based vaccines capable of conferring protection against a range of infectious agents and cancer. To date attempts to develop Salmonella based vaccines for anthrax have proved problematic. This is thought to be due to poor Salmonella mediated expression of PA. A number of possible reasons have been proposed to account for this failure and include codon usage, toxicity of the expressed protein for the bacterial host, the physiological burden on the bacteria of producing the protein, proteolytic degradation of the expressed protein, poor extra-cellular export and non-specific sequestering of PA by non-antigen presenting cells (Thwaite et al., 2002; Williams et al., 2003).
My lab, in collaboration with colleagues at DSTL Porton Down in the UK are addressing these issues by developing a strain of Ty21a capable of expressing both PA and biologically inactive LF. The system makes use of a heterologous protein export system based on the cytolysin A hemolysin of Salmonella enterica serovar Typhi (Galen et al., 2004). Recent animal data has confirmed the utility of this approach such that we are in the process of applying for NIH funds to support a human phase I clinical trial. It is envisioned that once this platform has been developed it could serve as a vehicle for the expression of vaccine targets from a range of bio-defense organisms.