Ninjurin-1 Negatively Regulates Humoral and Cellular Immune Responses Induced by the Saponin-Based Adjuvant Quil-A in Mice

Adjuvants are co-administered with antigens to enhance vaccine-induced protection. Saponins are plant-derived compounds with adjuvant properties, some of which are used in licensed vaccines. Macrophages and dendritic cells (DCs) exposed to saponin-based adjuvants have been reported to exhibit NLRP3-dependent interleukin-1 beta (IL-1β) release, and NLRP3 signaling has been shown to limit their adjuvant activity. Saponin-based adjuvants also induce plasma membrane rupture (PMR) and the release of high-molecular-weight intracellular molecules; however, the molecular mechanisms that mediate PMR and its impact on adjuvant-induced immune responses remain unclear. Here, we investigated the involvement of Ninjurin-1 (NINJ1), a key executor of PMR, in Quil-A-induced PMR and its immunological consequences. Upon stimulation with the saponin mixture Quil-A, peritoneal macrophages and bone marrow-derived dendritic cells (BMDCs) showed NLRP3-independent PMR but NLRP3-dependent IL-1β release. Quil-A-induced PMR was almost completely suppressed in Ninj1^-/- peritoneal macrophages and BMDCs compared with wild-type cells, whereas IL-1β release remained unaffected by NINJ1 deficiency. Immunization with Quil-A and ovalbumin (OVA) increased OVA-specific serum immunoglobulin G (IgG), IgG2b, and IgG2c levels in Ninj1^-/- mice compared with wild-type mice. Splenocytes from Ninj1^-/- mice produced higher levels of interferon-gamma upon stimulation with class I- and class II-restricted OVA peptides than those from wild-type mice. Ninj1^-/- mice also showed a higher frequency of OVA-bearing cells, particularly monocyte-derived DCs, in the draining lymph nodes. These results demonstrate that NINJ1 is critical for Quil-A-induced PMR and that NINJ1-mediated PMR negatively regulates Quil-A-induced humoral and cellular immune activation by restricting antigen delivery via antigen-presenting cells.