Abstract
Low-pathogenicity avian influenza H9N2 remains an endemic disease worldwide despite continuous vaccination, indicating the need for an improved vaccine strategy. Bacillus subtilis (B. subtilis), a gram-positive and endospore-forming bacterium, is a non-pathogenic species that has been used in probiotic formulations for both animals and humans. The objective of the present study was to elucidate the effect of B. subtilis spores as adjuvants in chickens administered inactivated avian influenza virus H9N2. Herein, the adjuvanticity of B. subtilis spores in chickens was demonstrated by enhancement of H9N2 virus-specific IgG responses. B. subtilis spores enhanced the proportion of B cells and the innate cell population in splenocytes from chickens administered both inactivated H9N2 and B. subtilis spores (Spore + H9N2). Furthermore, the H9N2 and spore administration induced significantly increased expression of the pro-inflammatory cytokines IL-1β and IL-6 compared to that in the H9N2 only group. Additionally, total splenocytes from chickens immunized with inactivated H9N2 in the presence or absence of B. subtilis spores were re-stimulated with inactivated H9N2. The subsequent results showed that the extent of antigen-specific CD4+ and CD8+ T cell proliferation was higher in the Spore + H9N2 group than in the group administered only H9N2. Taken together, these data demonstrate that B. subtilis spores, as adjuvants, enhance not only H9N2 virus-specific IgG but also CD4+ and CD8+ T cell responses, with an increase in pro-inflammatory cytokine production. This approach to vaccination with inactivated H9N2 together with a B. subtilis spore adjuvant in chickens produces a significant effect on antigen-specific antibody and T cell responses against avian influenza virus.
Highlights
Avian influenza has been a global problem because it infects wild and domestic birds and because it can be transmitted to humans
Enhancement of H9N2 virus‐specific Immunoglobulin G (IgG) production in chickens immunized with inactivated H9N2 with B. subtilis spores To determine the adjuvant effect of B. subtilis spores on antigen-specific antibody responses, chickens were intramuscularly immunized with inactivated H9N2 with or without B. subtilis spores according to the immunization schedule (Figure 1A)
It has been reported that oil adjuvants are not appropriate for inducing T cell responses, which are essential for memory T or B cell generation [35]; novel adjuvants and formulations of vaccine adjuvants are essential
Summary
Avian influenza has been a global problem because it infects wild and domestic birds and because it can be transmitted to humans. Lee et al Vet Res (2020) 51:68 animals immunized against avian influenza [3], implying that the current vaccination strategy is in need of advancement for improved performance. Bacillus subtilis, a gram-positive bacterium, is a non-pathogenic species that has been used as a probiotic for both animals and humans as feed [7] or health food [8], respectively. It is classified as a generally regarded as safe microorganism by the Food and Drug Administration. The immature spore stage (i.e., the forespore) is surrounded by a double membrane of the mother cell and develops into the mature spore [10]
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