Cutting Edge: Inflammasome Activation by Alum and Alum’s Adjuvant Effect Are Mediated by NLRP3

Abstract

Alum is the only adjuvant approved for routine use in humans, although the basis for its adjuvanticity remains poorly understood. We have recently shown that alum activates caspase-1 and induces secretion of mature IL-1β and IL-18. In this study we show that, in human and mouse macrophages, alum-induced secretion of IL-1β, IL-18, and IL-33 is mediated by the NLR (nucleotide-binding domain leucine-rich repeat-containing) protein NLRP3 and its adaptor ASC, but not by NLRC4. Other particulate adjuvants, such as QuilA and chitosan, induce inflammasome activation in a NLRP3-dependent fashion, suggesting that activation of the NLRP3-inflammasome may be a common mechanism of action of particulate adjuvants. Importantly, we demonstrate that Ag-specific Ab production elicited by vaccines that contain alum is significantly impaired in NLRP3-deficient mice. Our results demonstrate for the first time a role for the NLRP3-inflammasome during development of the immune response elicited by alum-enhanced vaccination and suggest that therapeutic intervention aimed at NLRP3 may improve adjuvant efficacy.

Alum adjuvanticity: Unraveling a century old mystery

“…Using knockout mice, Eisenbarth et al [20] show that all inflammasome components (Nlrp3, Asc and caspase-1) are required for alum adjuvanticity following OVA subcutaneous immunization and in an Ovalbumin (OVA) asthma model. Similar results where obtained by Li et al [22] using OVA and diphtheria toxin antigens. These data suggest that direct activation of Nlrp3 by alum is required for adjuvanticity; however, since alum is not capable of activating cytokine transcription in vitro, it is not clear how Nlrp3 activation alone can trigger adjuvanticity.…”

“…During the preparation of this commentary, a third work by Li et al [22] confirms the model described above and extended it to the human inflammasome complex. This study [22] also shows that Nlrp3 is targeted by two additional adjuvants of unknown function: Chitosan, a polysaccharide derived from chitin, and QuilA, a saponin extracted from the bark of Quillaja saponaria. These findings suggest that activation of the inflammasome may be a common theme in the mechanism of action of particulate adjuvants.…”

“…Questions that arise from the three studies discussed [19,20,22] include whether multiple modes of action of the inflammasome contribute to alum adjuvanticity and how one should interpret the conflicting data. One answer to these questions is that the models for adjuvanticity are not standardized and that the outcome of vaccination depends on many variables including vaccine formulation (alum adsorption), route of administration, dose of antigen, dose of adjuvant, choice and purity of antigen and the immunization schedule.…”

“…While these three recent reports [19,20,22] agree on the molecular mechanism of alum-TLR agonists crosstalk in vitro, they propose conflicting models for the contribution of Nlrp3 activation to alum adjuvanticity. Using knockout mice, Eisenbarth et al [20] show that all inflammasome components (Nlrp3, Asc and caspase-1) are required for alum adjuvanticity following OVA subcutaneous immunization and in an Ovalbumin (OVA) asthma model.…”

TLR, NLR Agonists, and Other Immune Modulators as Infectious Disease Vaccine Adjuvants

“…Although the mechanism of action of alum is still unclear, recent reports have implicated the NALP3 inflammasome in its immunostimulatory activity [49][50][51][52]. The adjuvant action of alum was shown to require NALP3 activation in a mouse model of allergic airway disease in which alum was administered intraperitoneally with ovalbumin [50].…”