Effects of Differences in Lipid A Structure on TLR4 Pro-Inflammatory Signaling and Inflammasome Activation

The vertebrate immune system exists in equilibrium with the microbial world. The innate immune system recognizes pathogen-associated molecular patterns via a family of Toll-like receptors (TLR) that activate cells upon detection of potential pathogens. Because some microbes benefit their hosts, mobilizing the appropriate response, and then controlling that response is critical in the maintenance of health. TLR4 recognizes the various forms of lipid A produced by Gram-negative bacteria. Depending on the structural form of the eliciting lipid A molecule, TLR4 responses range from a highly inflammatory endotoxic response involving inflammasome and other pro-inflammatory mediators, to an inhibitory, protective response. Mounting the correct response against an offending microbe is key to maintaining health when exposed to various bacterial species. Further study of lipid A variants may pave the way to understanding how TLR4 responses are generally able to avoid chronic inflammatory damage

Reconciliation of IL-1ß loss with TRIF-biased TLR4 signaling by monophosphate lipid A.

Monophosphoryl Lipid A (MPLA), a derivative of LPS endotoxin, is a TLR4 agonist that displays as little as 0.1-1% as much toxicity as its parent molecule while retaining immunostimulatory properties. We discovered that MPLA activates a TRIF-biased pattern of TLR4 signaling, resulting in reduced production of MyD88-dependent pro-inflammatory factors, and credited TRIF-bias for MPLA\u27s reduced toxicity. A contemporary study showed that MPLA fails to promote maturation of the potent inflammatory cytokine IL-1ß. This dissertation seeks to reconcile MPLA\u27s TRIF-biased signaling with IL-1 ß loss, and to determine the ultimate cause of MPLA\u27s reduced toxicity compared to LPS. We find that TRIF-biased TLR4 activation results in weak MyD88-dependent induction of NLRP3, a critical inflammasome component required for IL-1 ß production. MPLA\u27s loss of IL-1 ß results in decreased potentiation of MyD88-dependent inflammatory factors in vivo and reduced IL-1 RI-dependent hepatotoxicity. Ultimately, TRIF-biased TLR4 signaling is the causative factor resulting in MPLA\u27s immunogenicity with low toxicity; however more studies are needed to determine the initial events leading to the TRIF-dependent Signaling cascade. This dissertation describes the mechanisms responsible for MPLA\u27s reduced induction of inflammatory responses and outlines how this signaling may be exploited for therapeutic use