Blockade of IKK signaling induces RIPK1-independent apoptosis in human macrophages.

Regulated cell death in response to microbial infection plays an important role in immune defense and is triggered by pathogen disruption of essential cellular pathways. Gram-negative bacterial pathogens in the Yersinia genus disrupt NF-κB signaling via translocated effectors injected by a type III secretion system, thereby preventing induction of cytokine production and antimicrobial defense. In murine models of infection, Yersinia blockade of NF-κB signaling triggers cell-extrinsic apoptosis through Receptor Interacting Serine-Threonine Protein Kinase 1 (RIPK1) and caspase-8, which is required for bacterial clearance and host survival. Unexpectedly, we find that human macrophages undergo apoptosis independently of RIPK1 in response to Yersinia or chemical blockade of IKKβ. Instead, IKK blockade led to decreased cFLIP expression, and overexpression of cFLIP contributed to protection from IKK blockade-induced apoptosis in human macrophages. We found that IKK blockade also induces RIPK1 kinase activity-independent apoptosis in human T cells and human pancreatic cells. Altogether, our data indicate that, in contrast to murine cells, blockade of IKK activity in human cells triggers a distinct apoptosis pathway that is independent of RIPK1 kinase activity. These findings have implications for the contribution of RIPK1 to cell death in human cells and the efficacy of RIPK1 inhibition in human diseases. Author summary: Programmed cell death is critical for organismal homeostasis and for host defense against microbial infection. Gram-negative bacteria of the genus Yersinia cause diseases ranging from plague (Y. pestis) to severe gastroenteritis (Y. pseudotuberculosis and Y. enterocolitica). Studies in murine models have demonstrated that all pathogenic Yersinia disrupt pro-inflammatory cell signaling, which triggers cell death in murine macrophages. This cell death is mediated by the kinase RIPK1, and RIPK1-mediated cell death is essential for host defense and survival. Although murine studies have provided insight into mechanisms that regulate host defense during Yersinia infection, there are important differences between human and murine immune systems regarding expression and presence of key proteins. Thus, how human cells respond to Yersinia infection remains poorly understood. Here, we report that, in contrast to murine systems, Yersinia infection or chemical blockade of immune signaling in human cells induces a distinct apoptotic cell death that is entirely independent of RIPK1 kinase activity. RIPK1 is implicated in a wide range of systemic disorders and is currently being targeted in clinical trials. Our study provides insight into human-specific cell death signaling and host defense mechanisms, which has implications for understanding human immune responses to infection and therapeutic approaches for treating human diseases. [ABSTRACT FROM AUTHOR]