Innate Sensing of Cell Death and its Immune Consequences

The innate immune system responds to perturbations in tissue homeostasis resultant from infection but also dying cells and tissue damage. The goals of this program are to identify the immune consequences of cell death. Cells die by different modes of programmed cell death including apoptosis, pyroptosis and necroptosis. Cell death by apoptosis is a normal component of healthy tissue physiology that is balanced by cell division and maintains normal tissue size and function. Cell death by apoptosis is tolerogenic and non-inflammatory, and contrasts with necroptosis or pyroptosis, which are pathological and inflammatory.

We have developed several mouse models for studying the consequences of cell death on immunity, resolution of inflammation and tissue repair.  We have chosen to study these processes in the intestinal epithelium.  The intestinal epithelium continually undergoes a tightly controlled process of cell death and renewal.  Excessive cell death leads to chronic inflammation as in inflammatory bowel disease (IBD), while deficient death leads to cellular transformation and cancer.  Intestinal epithelial cells can also die as a result of infection.  The professional phagocytes within the intestinal lamina propria have been well characterized making it possible for us to identify which the impact of cell death on these important innate immune players and how their functions are modulated by the sampling and phagocytosis of dying intestinal epithelial cells.

Our first foray into the gut was through our investigations of the immune consequences of recognizing infected apoptotic cells. We had shown that apoptosis of the colonic intestinal epithelium as a result of infection with a rodent pathogen equivalent to the enteropathogenic and enterohemorrhagic E. coli in humans, is a physiological signal for T helper 17 (TH17) cell differentiation (See Torchinsky, M.B. et al. Nature 2009; Torchinsky and Blander, Cell Mol Life Sci 2010; Torchinsky, M.B. et al. Curr Opin Immunol 2010).  Within the intestinal TH17 response, we found antigen specificities to both damaged tissue and infecting pathogen suggesting that infections associated with significant cell death could break immune tolerance and prime self-reactive CD4 T cells and drive their differentiation into TH17 cells.  Once primed, these TH17 cells can cause intestinal pathology long after the infection is cleared (See Campisi, L. et al. Nat Immunol 2016; Blander J.M. et al. Immunol Res 2012).

Ongoing work: Innate recognition and immune consequences of necroptosis

Necroptosis involves the formation of a complex of receptor interacting serine/threonine protein kinases RIPK1, RIPK3 and the necroptosis effector mixed lineage kinase domain-like protein (MLKL). Up until recently, necroptosis was thought to function primarily in the control of infection and mainly as a fail-safe strategy that counters viral blockade of apoptosis. Studies in mouse models, however, have shown that deficiency in components of the apoptosis machinery such as caspase-8 or FADD leads to embryonic lethality driven by necroptosis and dependent on RIPK3 and MLKL. Other studies using conditional deletion of caspase-8 or FADD in the intestinal epithelium revealed that blocking the pathways that mediate homeostatic apoptosis precipitates intestinal inflammation associated with elevated levels of RIPK3 and necroptosis. Indeed, a notable increase in programmed cell death of intestinal epithelial cells (IEC) has been reported in patients with inflammatory bowel disease (IBD), and this damage is associated with heightened inflammation and increased levels of tumor necrosis factor (TNF)-a, an important mediator of cell death. Using a novel mouse model where we can inducibly trigger necroptosis of IEC, we are working to determine how IEC necroptosis impacts intestinal homeostasis and we will define the nature of the inflammatory response.