Identification of a gap junction communication pathway critical in innate immunity

Abstract

The innate immune system is the first line of host defense, and its ability to propagate antimicrobial and inflammatory signals from the cellular microenvironment to the tissue at-large is critical for survival. In a remarkably complex microenvironment, cells are constantly processing external cues, initiating convoluted intracellular signaling cascades, and interacting with neighboring cells to generate a global, unified response. At the onset of infection or sterile injury, individual cells sense danger or damage signals and elicit innate immune responses that spread from the challenged cells to surrounding cells, thereby establishing an overall inflammatory state. However, little is known about how these dynamic spatiotemporal responses unfold. Through the use GFP reporters, in vitro transplant coculture systems, and in vivo models of infection and sterile injury, this thesis describes identification of a gap junction intercellular communication pathway for amplifying immune and inflammatory responses, and demonstrates its importance in host innate immunity. The first section describes development of stable GFP reporters to study the spatiotemporal activation patterns of two key transcription factors in inflammation and innate immunity: Nuclear factor-KappaB (NFKB) and Interferon regulatory factor 3 (IRF3). Stimulation of NFKB-GFP reporters resulted in a spatially homogeneous pattern of activation, found to be largely mediated by paracrine action of the pro-inflammatory cytokine TNFa. In contrast, the activation of IRF3 was spatially heterogeneous, resulting in the formation of multicellular colonies of activated cells in an otherwise latent background. This pattern of activation was demonstrated to be dependent on cell-cell contact mediated communication between neighboring cells, and not on paracrine signaling. The second section describes the discovery of a gap junction intercellular communication pathway responsible for the formation of IRF3 active colonies in response to immune activation. Cell sorting and gene expression profiling revealed that the activated reporter colonies, collectively, serve as the major source of critical antimicrobial and inflammatory cytokines. Using in vitro transplant coculture systems, colony formation was found to be dependent on gap junction communication. Blocking gap junctions with genetic specificity severely compromised the innate immune system's ability to mount antiviral and inflammatory responses. The third section illustrates an application of the gap junction-induced amplification of innate immunity phenomenon in an animal model of sterile injury. Drug-induced liver injury was shown to be dependent on gap junction communication for amplifying sterile inflammatory signals. Mice deficient in hepatic gap junction protein connexin 32 (Cx32) were protected against liver damage, inflammation, and death in response to hepatotoxic drugs. Co-administration of a selective pharmacologic Cx32 inhibitor with hepatotoxic drugs significantly limited hepatocyte damage and sterile inflammation, and completely abrogated mortality. These finds suggests that co-formulation of gap junction inhibitors with hepatotoxic drugs may prevent liver failure in humans, and potentially limit other forms of sterile injury. In summary, this thesis demonstrates the development of novel tools for investigating the spatiotemporal dynamics of cellular responses, describes how these tools were utilized to discover a basic gap junction communication pathway critical in innate immunity, and provides evidence for the clinical relevance of this pathway in sterile inflammatory injury.