Abstract
Few vaccine adjuvants have been approved for human use although several are currently being studied in preclinical and clinical trial. MPL is a toll-like receptor agonist able to trigger a high and persistent antibody response via-TLR-4 while QS-21 activates the NLRP3 inflammasome. Data suggest that there is a cross-talk between Notch and TLR signaling pathways modulating the polarization of the immune response in a MyD88-dependent manner. However, the role of Notch on the mechanism action of immunogenic adjuvants has not been addressed yet. This study aims to evaluate the in vitro toxicity and inflammatory response triggered by MPL and QS-21 using an in vitro human cell co-culture model and to determine whether NFκB or Notch signaling pathways are involved in their mechanism of immunotoxicity. In order to do this, we evaluated the effect of QS- 21/MPL alone or in combination using a co-culture of PBMC and HUVEC using cytotoxicity, surface expression of ECAMs, cell adhesion and cytokine release, NF-κB activation and NOTCH1 expression as observation endpoints. We found that both MPL and QS-21 were cytotoxic at concentrations over 5 μg/mL. Both adjuvants were able to trigger the expression of ECAMs and induce firm adhesion of PBMC to the endothelium. QS-21 and MPL combination demonstrated a synergistic effect on cellular recruitment and cytokine release generating a switch from Th2 to Th1 cytokine profile. Both MPL and QS-21 by themselves were able to generate significant NF-κB activation. However, this effect was not observed when both adjuvants were combined. On the contrary, the adjuvants alone and combined induced an overexpression of NOTCH-1. This is an important finding, as it provides new evidence that these adjuvants could modulate reactogenicity of vaccines through Notch signaling.