Analysis of the CD4+ T cell immune response to Anthrax Lethal Factor

Abstract

The Gram-positive bacterium Bacillus anthracis is the causative agent of the potentially fatal illness, anthrax. A major determinant of B. anthracis pathogenicity is a binary toxin composed of Protective Antigen (PA) and one of two subunits; Lethal Factor (LF) or Edema Factor (EF). Translocation by PA into the host cell cytosol allows LF, which is a zinc metalloprotease, to inactivate the mitogen-activated protein kinase (MAPK) pathway. The expected consequences for the immune response include subversion of antigen presentation and T cell priming. In contrast to this, long term CD4+ T cell responses to LF were identified following natural human anthrax infection and vaccination, indicating that this toxin component is a principle B. anthracis antigen which may confer protective immunity. It has been observed that polymorphism in the HLA class II alleles at the DR and DQ loci affects susceptibilty to infectious disease outcome. In order to map the human response within a defined genetic background, transgenic mice expressing individual HLA heterodimers in the absence of endogenous MHC class II were utilised. HLA-DR1, HLA-DR15, HLA-DR4, HLA-DQ8 and HLA-DQ6 (the latter generated as part of this project) transgenic mice were compared in terms of response magnitude to LF and HLA expression levels. This was correlated with survival following live anthrax challenge. Immunodominant epitopes within LF were elucidated for all HLA-transgenic lines. Immunogenicity in the transgenic model was shown to be primarily restricted to epitopes from domains II and IV. Dominant epitopes, which were common to all HLA types, were identified in domain II. HLA-DR specific epitopes were also identified. T cell responses to cryptic epitopes, revealed following immunisation with the individual peptides, were compared to the immunodominant epitope hierarchy. A peptide epitope of LF was identified with a strong relative binding affinity for HLA-DR15, making it a candidate therapeutic for ‘MHC blockade’ strategies. This epitope was tested for therapeutic blockade in a humanised transgenic mouse model which develops spontaneous paralysis with central nervous system (CNS) pathology similar to human multiple sclerosis (MS). Disease is characterised by an autoimmune response to a DR15 restricted epitope of myelin basic protein (MBP). The LF peptide was found to prevent induction of antigen-specific T cell responses, presumably by HLA class II, reducing the inflammatory response to self-peptide both in vitro and in vivo.