Mechanisms of activity of the combination TLR4 agonist and emulsion adjuvant GLA-SE

GLA-SE is a safe and potent clinical stage vaccine adjuvant consisting of the synthetic TLR4 agonist glucopyranosyl lipid adjuvant (GLA) formulated in a squalene-in-water emulsion (SE). GLA-SE augments both TH1 and IgG2-skewed antibody responses to a variety of vaccine antigens. The adjuvanticity of GLA is critically dependent on its formulation in SE. Individually GLA promotes IgG2 responses with little detectable TH1 induction, whereas SE promotes IgG1 and TH2 responsiveness. The combination of GLA-SE mediates TH1 induction via MyD88 and TRIF signaling and activation of the inflammasome to produce IL-18. The TH1 induction activity also depends on an early burst of type I and II interferons, the latter of which is made by memory CD8 T cells, NK cells, and neutrophils. In addition to TH1 responses GLA and SE synergize to promote the numbers of antigen-specific B cells and TFH cells quickly after immunization. This is reflected in a substantial increase in the IgG2-skewed antibody response. Promotion of B cell and TFH responses depends on IL-18 signaling and the presence of lymph node resident macrophages that take up the adjuvant. Disruption of either of these axes is sufficient to ablate the antigen-specific B cell and antibody responses. Surprisingly the generation of TFH cells did not depend on lymph node resident macrophages, whereas TH1 responses were dramatically reduced under these conditions. We identify GLA-SE as an adjuvant capable of inducing robust B cell and antibody response that operates through interaction with LN macrophages and dependent on IL-18 signaling but independent of the number of antigen-specific TFH cells.