A combination of the TLR4 agonist CIA05 and alum promotes the immune responses to Bacillus anthracis protective antigen in mice

Anthrax is an infectious disease caused by Bacillus anthracis. The currently licensed human anthrax vaccines contain protective antigen (PA) as a major protective component and alum as an adjuvant. In this study, we investigated whether CIA05, a TLR4 agonist, is able to promote the immune response to an anthrax vaccine adjuvanted with alum. BALB/c mice were immunized intraperitoneally three times at 2-week intervals with a recombinant B. anthracis PA alone or in combination with CIA05 in the absence or presence of alum, and immune responses were determined 2 or 3 weeks after the third immunization. The results showed that the combination of CIA05 and alum significantly increased both serum anti-PA IgG antibody and toxin-neutralizing antibody titers, and the adjuvant effects were greater when lower antigen doses were used for immunization. Both CIA05 and alum stimulated PA-specific splenocyte secretion of interleukin (IL)-4, IL-5, and IL-6. A combination of the two yielded synergistic effects on IL-4 secretion, but CIA05 tended to repress IL-5 and IL-6 secretions induced by alum. Co-administration of CIA05 and alum also increased GL7 expression in B220+CD24+ splenic cells, indicating the ability to activate B cells. These data suggest that CIA05, combined with alum, could be used to achieve higher immune responses to PA, leading to the development of an effective anthrax vaccine.

Research Highlights► The adjuvant activity of a TLR4 agonist to anthrax protective antigen was studied. ► A combination of CIA05 and alum enhanced antibody responses to the antigen. ► The effects were greater when lower antigen doses were used for immunization. ► Increased GL7 level in B220+CD24+ splenic cells indicated B cell activation. ► CIA05 may serve as a good adjuvant for anthrax vaccines.