Investigating Human Cell Type-Specific Inflammasome Responses To Salmonella Infection

Nawar Naseer, University of Pennsylvania

Abstract

Diarrheal diseases are a major global public health threat. The Gram-negative bacterial pathogen Salmonella enterica serovar Typhimurium causes diarrheal disease in humans. Immune responses to Salmonella have been extensively studied in mice, where it causes a systemic typhoid-like disease, but our understanding of the human innate immune responses to Salmonella remain limited. Salmonella uses molecular syringe-like machinery called type III secretion systems (T3SS) to inject effectors into the host cell cytosol. These T3SSs enable Salmonella to invade and replicate within host cells such as intestinal epithelial cells (IECs) and macrophages. It is therefore critical to understand how IECs and macrophages, which serve both as targets of infection by Salmonella, as well as the first line of defense against invading enteric pathogens, are sensing and responding to infection. Host cells can recognize T3SS components via cytosolic receptors termed inflammasomes. One such inflammasome, called the NAIP/NLRC4 inflammasome, recognizes bacterial T3SS components and flagellin. Inflammasome activation leads to proinflammatory cytokine release and cell death, alerting nearby cells of the infection and eliminating the pathogen’s replicative niche. In mice, Salmonella infection triggers activation of the NAIP/NLRC4 inflammasome and this response is critical for protecting mice from Salmonella. In this dissertation, we investigated inflammasome responses to Salmonella in human macrophages and IECs. We found that like mice, the NAIP/NLRC4 inflammasome in human macrophages is activated upon Salmonella infection. Two additional inflammasomes, the NLRP3 and CASP4/5 inflammasomes are also activated in human macrophages during Salmonella infection. Importantly, NAIP and NLRP3 inflammasome activation mediates restriction of Salmonella replication in human macrophages, highlighting the importance of these pathways in host defense. In contrast to macrophages, human IECs do not engage the NAIP or NLRP3 inflammasome pathways. Instead, human IECs partially require caspase-1 and absolutely require caspase-4 for inflammasome responses to Salmonella infection. Our findings highlight the cell type-specific inflammasome pathways that are activated during Salmonella infection in humans. Importantly, we demonstrate how different the responses in human cells are compared to murine cells, underscoring the importance of studying human-specific immune responses to infection.