Non-conjugated TLR agonists increase the quantity of antibodies but not their quality in a murine immunization model

Bacterial vaccines have largely shifted from whole bacteria to selected proteins or glycans, thereby removing immune-stimulant components such as toll-like receptor (TLR) agonists from the formulation. Therefore, TLR agonists are often added to these vaccines to increase immunogenicity, and their addition has been frequently associated with increased antibody titers. To investigate the effects of TLR agonists at the B cell level, a small exploratory cohort study was performed with a murine immunization model using alum and R-phycoerythrin, with and without different TLR agonists. We observed that, while none of the TLR agonists increased affinity, the addition of TLR2, TLR9, and especially a combination of TLR2/4/9 agonists was able to increase the antibody quantity by increasing the number of secreting cells and/or cellular secretion rates. Our results indicate that the increased antibody titers for various TLR agonists or combinations might stem from an increased antibody magnitude rather than higher affinity antibodies.