Adjuvant conditioning shapes the adaptive immune response and promotes trained immunotolerance via NLRP3/IL-1

Trained immunity enhances responsiveness of the innate immune system upon restimulation. Although adjuvants are used to enhance immune responses, we showed that repeated administration of alum, termed adjuvant conditioning (AC), establishes an immunosuppressive environment that delays allogeneic graft rejection by expanding myeloid-derived suppressor cells (MDSCs). Here, we show that AC-induced MDSCs suppress antigen specific adaptive responses both in vitro and in vivo, and that the immunosuppression is abolished in the absence of NLRP3 and IL-1 signaling. Allogeneic pancreatic islets transplanted into AC-treated NLRP3-/- mice are not protected, demonstrating that AC requires NLRP3 signaling. Finally, AC also has an immunosuppressive effect on human PBMCs. Overall, our data show that AC establishes an immunosuppressive milieu via the NLRP3/IL-1 axis, leading to trained immunosuppression, or trained tolerance. Our findings give a potent mandate to explore the possibility to target the NLRP3/IL-1 pathway as a new promising strategy to condition transplant recipients and promote allograft tolerance.