Antigen depot is not required for alum adjuvanticity

Area of Science:

• Immunology
• Vaccinology

Background:

• Alum adjuvants have been used for over 80 years to enhance vaccine efficacy.
• The prevailing theory suggests depot formation and slow antigen release are key to alum's adjuvant activity.
• This study investigates the actual role of depot formation in alum's mechanism of action.

Purpose of the Study:

• To determine if depot formation is essential for alum adjuvant activity.
• To investigate the kinetics of antigen presentation and T-cell activation in the presence of alum adjuvants.
• To challenge the long-standing hypothesis regarding alum's mechanism.

Main Methods:

• Utilized EαGFP fluorescent protein to track antigen presentation in situ.
• Employed Y-Ae antibody for assessing antigen presentation.
• Administered alum and/or CpG adjuvants with antigen.
• Monitored antigen persistence in draining lymph nodes.
• Assessed antigen-presenting cell (APC) populations, including B cells and dendritic cells (DCs).
• Evaluated antigen-specific T-cell activation in vivo.
• Performed early removal of the injection site to assess depot impact.

Main Results:

• Antigen uptake was similar with alum or CpG adjuvants.
• GFP signal, indicating antigen, did not persist beyond 24 hours in antigen-presenting cells.
• Antigen presentation was observed sequentially on B cells (6-12h), conventional DCs (12-24h), and plasmacytoid DCs (≥48h).
• Alum and CpG adjuvants did not alter the timing or magnitude of antigen presentation or T-cell activation.
• Early removal of the injection site and alum depot had no significant effect on immune responses.

Conclusions:

• Depot formation is not required for alum adjuvant activity.
• Alum's enhancement of immune responses is independent of slow antigen release from a depot.
• The findings necessitate a re-evaluation of the established mechanisms of alum adjuvants.