An Epitope of Bacillus anthracis Protective Antigen That Is Cryptic in Rabbits May Be Immunodominant in Humans

Abstract

In a recent article, Oscherwitz et al. endeavor to enhance the immunogenicity of a multiple antigenic peptide (MAP) vaccine that targets a loop-neutralizing determinant (LND) of Bacillus anthracis protective antigen (PA) (1). They showed in a previous study (2) that this MAP, consisting of four copies of amino acids 305 to 319 of PA (PA 305-319) extending from a lysine core, can elicit humoral immunity in rabbits that is specific and strongly neutralizing for the 2β2-2β3 loop in domain 2 of PA. However, LND-specific antibodies were not detected in rabbits immunized with whole PA, and it was concluded that PA 305-319 may be a cryptic epitope. T cell assays in PA 305-319-immunized mice showed that this peptide appeared to lack activity as a T helper cell epitope. We have good evidence that humans exposed to PA, either following cutaneous anthrax infection or through vaccination, generate long-term, T cell memory to both whole PA and PA 305-319. In a study including samples from naturally exposed patients recovered from cutaneous anthrax, from vaccine-hyperimmunized AVP (anthrax vaccine precipitated) donors, and from recombinant PA-vaccinated donors, positive T cell gamma interferon (IFN-γ) enzyme-linked immunospot (ELISPOT) assay responses were detected across all groups on restimulation of peripheral blood mononuclear cells (PBMC) with both whole PA and PA 305-319 (Table 1; unpublished data). This shows that humans exposed to B. anthracis and whole PA recognize and generate memory to PA 305-319, suggesting that it is immunodominant rather than cryptic. Donors expressed a range of HLA haplotypes, with a majority being HLA-DR4 positive. While many important discoveries on the immunology of infection are made in experimental models, the human T cell response to PA in this respect differs from that of both rabbits and mice. In this light, the epitope used by Oscherwitz and colleagues may indeed be more relevant to eliciting appropriate human immunity than previously envisaged.