Intravenous BCG vaccination in non-human primates induces superior serum antibody titers with enhanced avidity and opsonizing capacity compared to the intradermal route.

A new and more effective tuberculosis (TB) vaccine is urgently needed, but development is hampered by the lack of validated immune correlates of protection. Bacillus Calmette Guérin (BCG) vaccination by the aerosol (AE) and intravenous (IV) routes has been shown to confer superior levels of protection from challenge with Mycobacterium tuberculosis (M.tb) in non-human primates (NHP) compared with standard intradermal (ID) administration. This finding offers a valuable opportunity to investigate which aspects of immunity are associated with improved control of M.tb and may represent biomarkers or correlates of protection. As TB vaccine research to date has focused largely on cellular immunity, we aimed to better characterize the poorly-understood serum antibody response to BCG administered by different routes of vaccination in NHP. We demonstrate superior M.tb-specific IgG, IgA, and IgM titers in serum following IV BCG vaccination compared to the ID or AE routes. We also observe improved capacity of IgG induced by IV BCG to opsonize the surface of mycobacteria, and report for the first time that M.tb-specific IgG from IV BCG vaccinated animals is of higher avidity compared with IgG from ID or AE BCG vaccinated animals. Notably, we identified a significant correlation between IgG avidity and measures of protection from aerosol M.tb challenge. Our findings highlight a potential role for antibodies as markers and/or mediators of the superior vaccine-induced protection IV BCG confers against TB and suggest that quality, as well as quantity, of antibodies should be considered when developing and evaluating TB vaccine candidates.