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Killed Bacillus subtilis spores as a mucosal adjuvant for an H5N1 vaccine
Manki Song1, Huynh A Hong, Jen-Min Huang
1Laboratory Science Division, International Vaccine Institute, Seoul 151-818, Republic of Korea.
Abstract:
Heat killed spores of the Gram-positive bacterium Bacillus subtilis have been evaluated as a vaccine delivery system with mucosal adjuvant properties for influenza. Killed spores were able to bind H5N1 virions (NIBRG-14; clade 1) and, when intra-nasally administered to mice, resulting immune responses, both humoral and cell mediated, were enhanced compared to immunization with the virion alone. Levels of both systemic IgG and mucosal sIgA specific to the virion were elevated. Levels of IgG2a (a Th(1) antibody type) were strongly enhanced when the virion was co-administered with killed spores. Cytokine induction in stimulated splenocytes was also apparent indicating balanced T(h)1 and T(h)2 responses. Evidence of cross-neutralization of clade 2.2 viruses was shown. In a challenge experiment mice dosed two times with spores adsorbed with just 20 ng HA (hemagglutinin) of inactivated NIBRG-14 were fully protected against challenge with 20 LD(50) of H5N2 virus. Interestingly, partial protection (60%) was observed in animals dosed only with killed spores. Mice dosed only with killed spores were shown to be fully protected against H5N2 (5 LD(50)) infection indicating that innate immunity and its stimulation by spores may play an important role in protection. Supporting this killed spores were (i) shown to stimulate TLR-mediated expression of NF-κB, and (ii) able to recruit NK cells into lungs and induce maturation of DCs. This work demonstrates the potential and underlying mechanism for the use of bacterial spores as an adjuvant for H5N1 vaccination.
Insights
Heat-killed Bacillus subtilis spores enhance immune responses against influenza when used as a vaccine delivery system. These bacterial spores show potential as an adjuvant for H5N1 vaccination, offering protection through both adaptive and innate immunity.
Area of Science:
- Immunology
- Vaccinology
- Microbiology
Background:
- Influenza viruses, particularly H5N1, pose a significant global health threat.
- Development of effective vaccine delivery systems and adjuvants is crucial for improving vaccine efficacy.
- Bacillus subtilis spores possess inherent properties that make them suitable for mucosal delivery and immune stimulation.
Purpose of the Study:
- To evaluate heat-killed Bacillus subtilis spores as a vaccine delivery system for influenza.
- To investigate the adjuvant properties of these spores in enhancing immune responses to H5N1 influenza.
- To elucidate the mechanisms underlying spore-mediated immune protection.
Main Methods:
- Heat-killed Bacillus subtilis spores were used to adsorb H5N1 virions.
- Intra-nasal administration of spore-virion complexes in a mouse model.
- Humoral (IgG, sIgA) and cell-mediated (cytokine production, splenocyte stimulation) immune responses were assessed.
- Innate immune responses, including TLR signaling, NF-κB activation, NK cell recruitment, and dendritic cell maturation, were analyzed.
- Vaccine efficacy was determined through viral challenge experiments.
Main Results:
- Spore-virion complexes significantly enhanced systemic IgG and mucosal sIgA responses compared to virion alone.
- Co-administration with spores strongly boosted IgG2a levels, indicating a Th1-biased response.
- Balanced Th1/Th2 cytokine induction was observed, suggesting a well-rounded immune activation.
- Full protection against H5N2 challenge was achieved with low doses of spore-adsorbed H5N1 antigen.
- Partial protection was observed even with killed spores alone, highlighting the role of innate immunity.
- Spores stimulated TLR-mediated NF-κB activation, recruited NK cells to the lungs, and promoted dendritic cell maturation.
Conclusions:
- Heat-killed Bacillus subtilis spores serve as an effective delivery system and mucosal adjuvant for influenza vaccines.
- Spore-mediated enhancement of immune responses involves both adaptive immunity and the stimulation of innate immune pathways.
- Bacterial spores demonstrate significant potential for developing novel and effective vaccination strategies against influenza.
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