Elucidation of the cell signaling pathways mediating innate immunity and host-pathogen interactions

Abstract

The ability to generate a robust immune response is integral to organismal homeostasis. Cells of the innate immune system are considered the first responders of immunity, and are therefore responsible for sensing both pathogens and endogenous danger signals and initiating a protective inflammatory response. To appropriately sense pathogens and danger signals, cells have developed intricate mechanisms for transducing signals from the extracellular environment into the cell. The integration of these signals is complex, resulting from crosstalk between many signaling pathways, but is critical to generating a coordinated biological response. In additional to the specialized mechanisms of innate immune cells to respond to antigens, these cells (like most) have evolved a complex set of adaptive mechanisms that maintain homeostasis during cell stress. Activation of innate immunity via pathogen invasion or the presence of danger signals can be considered an especially intense form of cell stress, thereby implicating these homeostatic pathways as components of the innate immune response.

The work presented in this thesis relates to the molecular mechanisms by which cells of the innate immune system integrate signals from the microenvironment to produce a coordinated biological response. The aim was to elucidate the mechanisms by which innate macrophages transduce extracellular signals to activate important effector pathways, and to describe crosstalk between cell signaling pathways that mediate adaptive responses to cell stress. Finally, we looked to extend our understanding to pathophysiological settings, and investigated the mechanisms by which pathogens that cause cell stress generate an aberrant inflammatory response. In doing so, we described novel components of these signaling pathways, which may be exploited in designing novel therapeutics.

In paper I, Gαi2 was identified as a critical signaling molecule in macrophage phenotype determination, functioning to transduce signals from the microenvironment to fine tune macrophage propensity towards an M1 inflammatory or M2 anti-inflammatory phenotype. In paper II, the immune receptor CD38 was shown activate the master transcriptional regulation of the autophagic/lysosome machinery, TFEB. We further identified the large kinase LRRK2 as essential in signal transduction downstream of CD38. In paper III, we described adaptive crosstalk between TFEB, an essential component of the cell stress response, and the typically proliferative WNT signaling pathway. Finally, in paper IV we describe how the SARS-Coronavirus open reading frame-3a causes multimodal necrotic death by activating multiple cell stress and innate immune pathways, resulting in aberrant inflammation.

In summary, the work presented in this thesis extends our current understanding of the molecular mechanisms mediating the integration of signals in innate immune cells. We have identified several novel signaling mechanisms, which could lay the foundation for the development of targeted therapeutics.