The unique lipopolysaccharide composition of Salmonella Typhimurium ST313 dampens pyroptosis and inflammasome activation and suppresses host cell death

In humans, non-typhoidal Salmonella (NTS) serovars, such as Salmonella enterica Typhimurium cause self-limiting gastroenteritis. However, invasive non-typhoidal Salmonella (iNTS) isolates ST313 have emerged in sub-Saharan Africa that cause a severe bacteremia in immunocompromised individuals including HIV-positive adults. Despite a decade of studies comparing ST313 to the classical diarrheal S. Typhimurium ST19 pathovariant, the molecular basis of the differences of pathogenesis between these bacteria remains unclear. Here we used primary human macrophages infected with the invasive S. Typhimurium ST313 D23580 isolate and the reference ST19 4/74 isolate as an in vitro model of infection. Transcriptomic profiling revealed that primary human macrophages infected with ST313 bacteria had distinct patterns of host cell death and inflammation responses compared to ST19-infected cells. Specifically, infection with ST313 isolate induced significantly less pyroptotic cell death which was associated with minimal processing of Gasdermin-D. We discovered that S. Typhimurium ST313 stimulated reduced IL-1{beta} release and a low-level activation of the upstream inflammasome signaling cascade with reduced Caspase-4 and Caspase-1 cleavage. To investigate the molecular basis of this phenotype we purified lipopolysaccharide (LPS) species from ST313 to identify physical properties and structures that were distinct from ST19 LPS. The ST313-derived LPS was less pro- inflammatory than LPS from ST19, mirroring the low inflammatory phenotype of the live bacteria. Together, our data reveal that the unique LPS of ST313 is responsible for the reduced level of pyroptosis in human macrophages. This phenotype is consistent with the intracellular persistence and dissemination of these pathogenic bacteria. AUTHOR SUMMARYNon-typhoidal Salmonella (NTS) is a foodborne microbial pathogen that causes a self- limiting gastroenteritis in humans. However, in recent decades invasive non-typhoidal Salmonella (iNTS) isolates have emerged in sub-Saharan Africa, and are currently causing severe bloodstream infections responsible for about 66,000 deaths each year, largely in immunocompromised individuals. To investigate differences in pathogenicity between iNTS and NTS strains, we studied the infection of primary human macrophage with an iNTS ST313 isolate from Malawi in comparison to a gastroenteritis-associated ST19 reference strain. Primary macrophages play a key role in clearing Salmonella infections by recognizing bacterial components such as lipopolysaccharide (LPS). LPS induces immune responses including activation of cytokine release, inflammation and host cell death for effective bacterial elimination. Our study revealed that the iNTS isolate survived better in human primary macrophages than the NTS strain due to reduced inflammatory cell death, weakened inflammasome activation and lower cytokine release. We suggest that the distinct LPS structure is responsible for these dampened immune responses which aid the spread and persistence of S. Typhimurium ST313 in the human host.