Vaccines often require adjuvants to elicit responses to otherwise weakly immunogenic proteins. The TLR4 agonist, monophosphoryl Lipid A (MLA) retains the immunogenicity, but not the toxic inflammatory response, of its parent molecule S. minnesota LPS. We previously reported that MLA is a TRIF-biased agonist of TLR4, and have now determined that a single phosphate group difference is sufficient for this signaling difference, as well as for a loss of IL-1β production. These two characteristics of MLA contribute greatly to its reduced toxicity. In order to determine the relationship between TRIF-biased signaling and the loss of IL-1β, we compared IL-1β production by RT-PCR and ELISA in primary bone marrow-derived dendritic cells. Preliminary studies show that MyD88 signaling is required for post-transcriptional IL-1β processing, indicating that TRIF-bias may cause MLA to produce low levels of IL-1β protein. Discovering the mechanisms responsible for these low-toxicity adjuvant properties of MLA will aid in the generation of safer and more effective vaccines. This project was supported by funding from the NIH/NIAID.