Stochastic humoral immunity to Bacillus anthracis protective antigen: Identification of anti-peptide IgG correlating with seroconversion to Lethal Toxin neutralization

Abstract

A substantial fraction of individuals vaccinated against anthrax have low to immeasurable levels of serum Lethal Toxin (LeTx)-neutralizing activity. The only known correlate of protection against Bacillus anthracis in the currently licensed vaccine is magnitude of the IgG response to Protective Antigen (PA); however, some individuals producing high serum levels of anti-PA IgG fail to neutralize LeTx in vitro. This suggests that non-protective humoral responses to PA may be immunodominant in some individuals. Therefore, to better understand why anthrax vaccination elicits heterogeneous levels of protection, this study was designed to elucidate the relationship between anti-PA fine specificity and LeTx neutralization in response to PA vaccination. Inbred mice immunized with recombinant PA produced high levels of anti-PA IgG and neutralized LeTx in vitro and in vivo. Decapeptide binding studies using pooled sera reproducibly identified the same 9 epitopes. Unexpectedly, sera from individual mice revealed substantial heterogeneity in the anti-PA IgG and LeTx neutralization responses, despite relative genetic homogeneity, shared environment and exposure to the same immunogen. This heterogeneity permitted the identification of specificities that correlate with LeTx-neutralizing activity. IgG binding to six decapeptides comprising two PA epitopes, located in domains I and IV, significantly correlate with seroconversion to LeTx neutralization. These results indicate that stochastic variation in humoral immunity is likely to be a major contributor to the general problem of heterogeneity in vaccine responsiveness and suggest that vaccine effectiveness could be improved by approaches that focus the humoral response toward protective epitopes in a greater fraction of vaccinees.

Defective Selection of Thymic Regulatory T Cells Accompanies Autoimmunity and Pulmonary Infiltrates in Tcra-Deficient Mice Double Transgenic for Human La/Sjögren’s Syndrome-B and Human La-Specific TCR

“…Strikingly, in contrast to the endogenous Tcra -intact double Tg model, all Tcra −/− 3B5.8/hLa double Tg mice developed high-titer serum IgG antibodies to recombinant hLa antigen but not to an irrelevant foreign antigen from Bacillus anthracis (6xhis-PA) that is immunogenic in H-2 k mice (32) (Fig. 6B).…”

MHC Class II and Non-MHC Class II Genes Differentially Influence Humoral Immunity to Bacillus anthracis Lethal Factor and Protective Antigen

“…Our laboratory has previously reported sequential B cell epitopes of LF [13], EF [30] and PA [31] in A/J mice, but this is our first report of PA epitopes in B6 or B6.H2k mice. The previous reports demonstrated that identification of sequential B cell epitopes in mice is highly reproducible, in terms of both epitopes identified and relative magnitude of responses to individual epitopes, if pools of serum from groups of immunized mice are evaluated.…”

Anthrax Vaccine Precipitated Induces Edema Toxin-Neutralizing, Edema Factor-Specific Antibodies in Human Recipients

“…The source of variation in these responses in unclear. We previously demonstrated in genetically identical mice that substantial epitope variation in the response to immunization with PA is stochastic (59).…”