The effect of anti-spore antibody responses on the use of spores for vaccine delivery

Among the different types of bacteria being exploited as live vaccines Bacillus endospores have a number of distinct advantages most importantly being their heat stability. Typically antigens are displayed on the spore surface or expressed in the germinating spore, i.e., the vegetative cell. How antigens are delivered by spores can significantly affect the nature of the resulting immune response. When antigens are expressed in the germinating spore, then, after the first dose of recombinant spores, ensuing humoral responses are abruptly arrested. We have investigated this phenomenon by first demonstrating that while immune responses against the encoded antigen are impaired this is not the case for anti-spore responses that progressively increase until hyperimmunity is reached. We reasoned that anti-spore IgG or sIgA could be responsible for inhibiting spore germination, thus preventing expression of the vegetatively expressed antigen. In this work we have demonstrated that antisera from both immunised and naïve animals can bind to spores non-specifically yet only IgG or sIgA from immunised animals can bind specifically. Both immune and naïve antibodies were found to inhibit germination in vitro and most probably non-specific binding could account for this. On the other hand only immune antibodies were capable of opsonising spores and enhancing their uptake by macrophages and we reason that specific antibody-spore binding is required. Opsonophagocytosis could be an important tool to reduce spore germination and subsequent outgrowth. With regard to use for heterologous antigen delivery though, this severely curtails the use of a repetitive dosing regime.