Examination of serological memory in rabbits injected with Bacillus anthracis protective antigen adsorbed to Alhydrogel

Serological memory after inoculation of protective antigen (PA) combined with Alhydrogel adjuvant (PA/Alhydrogel) was examined in New Zealand white rabbits, an animal model for anthrax. A threshold dose of 0.1 μg of PA/Alhydrogel was identified which resulted in an ELISA titer 2 weeks after a primary immunization of only 0.168 μg anti-PA IgG per ml and a toxin-neutralizing antibody titer (TNA ED50) of 1.8 (n = 40). A significant increase in anti-PA IgG and TNA ED50 titers were measured (p < 0.0001) 2 weeks after a booster immunization with 0.1 μg of PA/Alhydrogel at 14 days (n = 10; 40.9 μg anti-PA IgG per ml; 522 TNA ED50) and 28 days (n = 10; 63.8 μg anti-PA IgG per ml; 501 TNA ED50). At this threshold dose of PA/Alhydrogel, protection against an aerosol exposure to Bacillus anthracis Ames spores improved as the booster immunization was administered from 4 days (40% survival), to 8 days (50% survival), and to 12 days (80% survival) before challenge. The partial protection of rabbits, even in the absence of protective antibody titers (0.9 μg anti-PA IgG per ml and 26 TNA ED50) when the booster immunization was administered 4 days before challenge, suggested a protective potential for serologic memory.