Anthrax - Host immune responses
Characterization of human immune responses: Surprisingly little work has been undertaken to characterize the basis of the human immune response to immunization or infection with anthrax (Baillie et al., 2003;2004). Fully understanding this response will be key to the validation of current vaccine targets and the identification of new ones.
Aims of Project
There is a pressing need to identify human surrogate markers of protection since a new anthrax vaccine is likely to be licensed on the basis of animal studies. In collaboration with colleagues in Turkey, the UK Ministry of Defense (MOD) and across the US we have been working to characterize human B and T cell response of immunized and infected individuals to anthrax lethal toxin. This major virulence factor is an A/B type subunit toxin comprises two elements, protective antigen (PA) which is non-toxic and lethal factor (LF) which is toxic. The role of PA is to bind to the cell and transport LF into the host cell cytoplasm. The key protective immunogen of the current licensed US human vaccine is PA. The vaccine licensed for use in the U.K. also contains PA and trace amounts of LF, which like PA, is thought to contribute to protection through the stimulation of toxin neutralizing antibodies (Baillie et al.,2004; Galloway and Baillie., 2004). Recent studies in our laboratory have shown that LF is considerable more immunogenic than PA in humans and that it is a more robust diagnostic indicator of infection (manuscript in preparation).In addition to characterizing the overall human antibody response to anthrax lethal toxin we are also in the process of mapping the B cell epitope recognition sites of previously isolated protective PA and LF specific toxin neutralizing human monoclonal antibodies. These “protective epitopes” will underpin the development new, epitope based vaccines (see below) and laboratory assay’s capable of assessing the degree of protective immunity stimulated following vaccination. Indeed we intend to use these assays to support studies in collaboration with the UK MOD to optimize the dosing schedule of the current UK licensed vaccine. In preliminary studies we have shown that a more robust response can be achieved by increasing the interval between the first three vaccinations (Hepburn et al manuscript in preparation).While the ability to inhibit toxin activity is thought to be a major mediator of protection it is not the only mechanism by which antibodies contribute to immunity. In recent studies we have shown that PA and LF specific humans antibodies can bind to the anthrax spores and promote uptake and subsequent killing by the macrophages (Kang et al., 2005). The ability to kill the organism before it has under gone significant gene expression would offer an additional element of protection particularly in the context of genetically engineered threats. Studies are in progress to determine the biological mechanism underlying this observation, to identify the protective epitopes and to develop an assay with which to determine the contribution of these antibodies to protection.
B cell responses: Although anti-LF toxin antibodies are important immunological correlate of protection persisting PA and LF-specific IgG memory B cells may also contribute to protection due to their ability to proliferate and differentiate rapidly into antibody–secreting plasma cells following infection with B.anthracis. Recently published data suggests that quantitative analysis of the PA specific IgG B cell memory population may prove to be a useful predictor of the duration of protection against anthrax and a surrogate marker protection (Quinn et al., 2004). In collaboration with researchers at Emory Medical School in Atlanta and the UK MOD we are in the process of characterizing the PA and LF specific B cell memory response of individuals who have received the UK licensed anthrax vaccine.
T cell responses: If a vaccine is to stimulate long lasting protection it must contain epitopes capable of stimulating memory T cells. One obvious challenge is the need to know which epitopes elicit the best immune responses to a particular agent. To address this issue, the National Institute of Allergy and Infectious Diseases (NIAID) has established the Large-Scale Antibody and T Cell Epitope Discovery Program, which is aimed at identifying relevant immune epitopes, particularly of category A-C bioterrorism agents. It is hoped that this initiative will lead toward development of new diagnostics, therapeutics and vaccines (Sette et al., 2005).
Under this program my laboratory is collaborating with researchers from Imperial College, the University of Newcastle Medical School, UK and the UK MOD to identify CD4 T cell epitopes relevant to humans within the principal vaccine antigens of B. anthracis and Y. pestis. Through these studies, we have been able to identify potential promiscuous human CD4+ T cell epitopes within LF by mapping the T cell responses of immunized human transgenic HLA-DR1 and DQ8 mice and human volunteers who have received the UK licensed anthrax vaccine. Studies are in progress to identify similar epitopes in PA. It is expected that once defined these CD4 T cell epitopes will be included in multi-epitope vaccine constructs.