Neha Nataraj (Mentors: Igor Brodsky, PhD & Sunny Shin, PhD)

“Defining the Regulation of Extrinsic Apoptosis  in Human Macrophages during Yersinia infection”

Neha M. Nataraj, Igor E. Brodsky* and Sunny Shin* (* equal contributors)

Many microbial pathogens suppress the pro-inflammatory response of innate immune cells to evade detection by their hosts. The gram-negative bacterial pathogen Yersinia, which causes diseases from self-limiting gastroenteritis to systemic bacteremia, utilizes a type III secretion system (T3SS) to inject effectors into host cells. These effectors disrupt NF-κB signaling which is critical for cytokine production and anti-microbial defense. We and others have demonstrated that in murine cells, Yersinia blockade of NF-κB signaling triggers apoptosis, involving engagement of the proteins RIP1 and Casp8. RIP1-dependent apoptosis is required for bacterial clearance and host survival, as mice deficient in RIP1 kinase activity or Casp8 cannot control Yersinia burdens and succumb to infection. A key gap in our knowledge is how human cell extrinsic apoptosis promotes cell death and anti-microbial immune defense in response to immunoevasive pathogens. Addressing this gap will provide a basis for therapeutically targeting these pathways in humans in the setting of bacterial infection or inflammatory disease. My preliminary data demonstrate for the first time that, in contrast to murine macrophages, human macrophages do not require RIP1 to undergo apoptosis in response to Yersinia¬-mediated blockade of NF-κB signaling. Moreover, mice only possess one initiator of cell-extrinsic apoptosis, Casp8, as opposed to other vertebrates including humans, who possess two orthologs, CASP8 and CASP10. Further, my data indicate that both Casp8 and 10 are activated during apoptosis of human macrophages induced by NF-κB signaling blockade. Altogether, my strong preliminary data provoke the conceptually novel hypothesis that bacterial blockade of NF-κB triggers a human-specific apoptosis pathway in macrophages that requires Casp8 and 10 but is independent of RIP1. To test this hypothesis, in the following aims, I will use a combination of bacterial and host genetics and pharmacological approaches in primary and in immortalized human macrophages.