GPRC6A mediates Alum-induced Nlrp3 inflammasome activation but limits Th2 type antibody responses.

Alum adjuvanticity is still an unknown mechanism despite the frequent use as vaccine adjuvant in humans. Here we show that Alum-induced inflammasome activation in vitro and in vivo is mediated by the G protein-coupled receptor GPRC6A. The Alum-induced humoral response in vivo was independent of the inflammasome because Nlrp3-/- and ASC-/- mice responded normally to Alum and blockade of IL-1 had no effect on antibody production. In contrast, Alum adjuvanticity was increased in GPRC6A-/- mice resulting in increased antibody responses and increased Th2 cytokine concentrations compared to wildtype mice. In vitro activation of GPRC6A-/- splenic B cells also induced increased IgG1 concentrations compared to wildtype B cells. For the first time, we show GPRC6A expression in B cells, contributing to the direct effects of Alum on those cells. B cell produced immunostimulatory IL-10 is elevated in GPRC6A-/- B cells in vitro and in vivo. Our results demonstrate a dual role of GPRC6A in Alum adjuvanticity. GPCR6A activation by Alum leads to the initiation of innate inflammatory responses whereas it is an important signal for the limitation of adaptive immune responses induced by Alum, partially explained by B cell IL-10.