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 0.1μg of PA/Alhydrogel was identified which resulted in an ELISA titer 2weeks after a primary immunization of only 0.168μg 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) 2weeks after a booster immunization with 0.1μg of PA/Alhydrogel at 14days (n=10; 40.9μg anti-PA IgG per ml; 522 TNA ED50) and 28days (n=10; 63.8μg 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 4days (40% survival), to 8days (50% survival), and to 12days (80% survival) before challenge. The partial protection of rabbits, even in the absence of protective antibody titers (0.9μg anti-PA IgG per ml and 26 TNA ED50) when the booster immunization was administered 4days before challenge, suggested a protective potential for serologic memory.