Chemical Syntheses of Lipid As and their Biological Evaluation

Gram-negative bacteria have an outer membrane containing lipopolysaccharides (LPS), which play a crucial role in immune system interactions, primarily through their lipid A component. While high levels of lipid A can lead to severe immune activation and septic shock, moderate activation is beneficial for immune protection and may be used in vaccine adjuvants. The immune system recognizes lipid A through pathogen recognition receptors (PRRs) such as TLR4/MD-2, triggering two signaling pathways: MyD88 and TRIF. TRIF activation promotes adaptive immunity, which is crucial for long-term protection. In Chapter 1, the interaction of lipid A with the innate immune system and its historical synthesis is discussed. Lipid A activates the TLR4 receptor, leading to immune responses that are useful for developing vaccines. However, traditional adjuvants like alum activate Th2 cells, not the adaptive immune system, while newer adjuvants such as monophosphoryl lipid A (MPLA) favor TRIF activation, reducing inflammation. The 2009 discovery of the TLR4/MD-2 complex with lipid A clarified the importance of lipid A’s structure, particularly its anomeric phosphate group. Chapter 2 introduces a novel synthetic route for MPLA, improving upon earlier methods that used hydrogenolysis, which had limitations when working with certain functional groups. The new method employs DDQ for oxidative cleavage, overcoming previous issues and offering better control over deprotection conditions. Chapter 3 investigates the role of acyl chains on lipid A, focusing on how truncating the acyl chain at the reducing end amine affects immune activation. The chapter reveals a structure-activity relationship, demonstrating that altering the lipid length can modulate cytokine release and help control immune responses, important for creating safer vaccine adjuvants. Truncating the acyl chain significantly reduced cytokine release, with the monophosphoryl lipid A derivatives showing more noticeable differences in activity compared to bisphosphoryl forms. Chapter 4 explores the lipid A of Bacteroides fragilis, a commensal bacterium suggested of inducing a beneficial antiviral state through TRIF-mediated TLR4 activation. The synthetic lipid A of B. fragilis was created and tested for cytokine production, revealing partial antagonism of E. coli lipid A, suggesting it could dampen excessive pro-inflammatory cytokine responses. This synthetic lipid A, although not inducing IFN-β, showed a reduced IL-6 response when pre-incubated with E. coli lipid A, indicating its potential use in modulating immune responses for therapeutic applications. Overall, the research highlights the complexity of lipid A’s role in immune activation and the potential of modifying its structure to develop safer, more effective vaccine adjuvants.