Salmonellae are a significant public health threat in both developing and developed countries, causing Typhoid fever or gastroenteritis, and recently have been observed as emerging pathogens causing community acquired bacteremia in sub-Saharan Africa. Salmonellae use virulence factors, including type III secretion systems, to manipulate host cell physiology. The innate immune system detects perturbations in the cytosol during infection. An important family of cytosolic pattern recognition receptors that mediate this detection is the Nod- like receptors (NLR). Some NLRs, including NLRP3 and NLRC4, form inflammasomes that recruit and activate Caspase-1, a protease that subsequently cleaves the inflammatory cytokines IL-1 and IL-18 to their mature, secreted forms. NLRP3 and NLRC4 detect Salmonella typhimurium infection, and the resulting Caspase-1 activation reduces bacterial burden in vivo primarily through the activities of IL-18. In this application, we will analyze the molecular determinants of detection by NLRC4 (Aim 1) and NLRP3 (Aim 2), and the IL-18 response (Aim 3) during S. typhimurium infection. NLRC4 responds to flagellin and rod protein secreted into the macrophage cytosol. We will dissect the molecular determinants of this detection and examine the relative importance of flagellin and rod protein detection in vivo. NLRP3 responds to a variety of cellular perturbations, and we will investigate the hypothesis that NLRP3 detects the prolonged presence of undigested bacteria within the phagosome. We will compare the anatomic localization of NLRP3 and NLRC4 detection, testing the hypothesis that NLRC4 has a discrete window of time to detect S. typhimurium that have recently emigrated from the gut lumen, while NLRP3 detects bacteria after dissemination. Finally, we will define the role of NK cells in the IL-18 response. These studies will provide insight into the complex interplay of innate immune detection through two inflammasomes that respond to different cytosolic perturbations triggered by S. typhimurium. Our results will be instructive for designing live attenuated vaccines as well as vaccine adjuvants. The general mechanisms revealed by these studies will also facilitate the understanding of inflammatory disease.
The immune system can discriminate between bacteria which cause disease and those which cannot by using sensors that detect the disease causing proteins used by the bacteria. This allows the immune system to effectively combat infection, but when the system is activated at the wrong time, it can cause inflammatory disease. Understanding the mechanisms of this system will lead to the design and production of medicines to combat inflammatory disease. It will also aid in the design of new vaccines to prevent infectious diseases.
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