Abstract Rationale Heterogeneous host responses to infection remain a major barrier to developing effective therapies for sepsis and acute respiratory distress syndrome (ARDS). Gram-negative rod (GNR) bacteria cause approximately 30% of sepsis cases, with pathogenesis largely driven by a key component of the GNR outer membrane, lipopolysaccharide (LPS). Extracellular LPS is recognized by Toll-like receptor 4 (TLR4) in both humans and mice, whereas intracellular LPS is sensed by Caspase-4 and -5 (CASP4/5) in humans and by Caspase-11 (Casp11) in mice, leading to the formation of the CASP4/5/11-inflammasomes that trigger pyrotopsis, an inflammatory form of cell death. Many Gram-negative pathogens evade immune detection by modifying their LPS structure from the classical proinflammatory hexa-acylated form to less immunostimulatory tetra- or penta-acylated variants. While the biological properties and responses of murine Casp11 to cytosolic LPS are well characterized, those of its human orthologs, CASP4 and CASP5, remain poorly understood. Moreover, how LPS structural variations influence CASP4/5/11-inflammasome activation and contribute to the heterogeneous host responses to infection remains largely unexplored. Methods Using Casp11-/- bone marrow-derived macrophages reconstituted with Casp11, CASP4, or CASP5, we assessed individual caspase responses to a panel of LPS structural variants. Furthermore, we employed transgenic mice expressing human CASP4 in lieu of murine Casp11 to study human CASP4-inflammasome activation in host defense and sepsis in vivo. Results Early CASP4-responses to tetra-acylated LPS during Francisella novicida pulmonary infections promoted antibacterial defense in vivo, but excessive activation by tetra-acylated LPS delivered via a non-pathogenic bacteria led to sepsis. Conversely, murine Casp11’s tolerance to tetra-acylated LPS facilitated F. novicida escape but prevented sepsis development despite a high burden of tetra-acylated LPS. Notably, human CASP4 and CASP5 responded to distinct, non-overlapping LPS structures, suggesting non-redundant roles in LPS-sensing and host defense during gram-negative bacterial infections. Conclusions Our findings highlight that during Gram-negative infections, LPS is not a single structural entity but rather, when dynamically modified by pathogens, can influence host inflammasome activation and contribute to the heterogeneous host responses observed in sepsis and ARDS. This abstract is funded by: NHLBI, NIAID
Wu et al. (Fri,) studied this question.