Diss. (sammanfattning) Stockholm : Karolinska institutet, 2007. / Härtill 4 uppsatser.

Diss. Stockholm : Karolinska institutet, 2007.

Dissertation (Ph.D.)--University of South Florida, 2007. / Title from PDF of title page. Document formatted into pages; contains 147 pages. Includes vita. Includes bibliographical references.

The innate immune system provides the first line of defense against infection. Pathogens are detected though a variety of Pattern Recognition Receptors (PRRs), which activate downstream signaling cascades. Effector molecules such as cytokines and chemokines are released upon activation and aid in cell recruitment, control of pathogen replication, and coordination of the adaptive immune response. Nucleic acids that are released into the cytosol during viral and bacterial infection are recognized through a special class of PRRs, coined cytosolic nucleic acid sensors. Upon recognition, these receptors induce the production of type I interferons and other cytokines to aid in pathogen clearance. Although many cytosolic nucleic acid sensors have been discovered, it is unclear how they work in concert to mediate these responses. The Interferon Gamma Inducible protein (IFI)16 and its proposed mouse orthologue IFI204 are cytosolic DNA sensors that have been linked to the detection of cytosolic DNA during infection with Herpes Simplex Virus (HSV-1). IFI16 binds dsDNA that has been released into the cytosol during viral infection and engages the adaptor molecule Stimulator of Interferon Genes (STING) leading to TANK binding kinase-1 (TBK1) dependent phosphorylation of interferon regulatory factor 3 (IRF3) and transcription of type I interferons and interferon stimulated genes. In addition to its role as a sensor, in chapter two of this thesis we describe a broader role for IFI16 in the regulation of the type I IFN response to RNA and DNA viruses in anti-viral immunity. In an effort to better understand the role of IFI16 in coordinating type I IFN gene regulation, we generated cell lines with stable knockdown of IFI16 and examined responses to DNA and RNA viruses as well as other inducers of IFN such as cyclic-dinucleotides. As expected, stable knockdown of IFI16 led to a severely attenuated type I IFN response to cytosolic DNA ligands and DNA viruses. In contrast, expression of the NF-κB regulated cytokines such as IL-6 and IL-1β were unaffected in IFI16 knockdown cells, suggesting that the role of IFI16 in sensing these triggers was unique to the type I IFN pathway. Surprisingly, we also found that knockdown of IFI16 led to a severe attenuation of expression of IFN-α and IFN stimulated genes such as RIG-I in response to cyclic GMP-AMP (cGAMP), a second messenger produced in response to cGAS, as well as RNA ligands and viruses. Analysis of IFI16 knockdown cells revealed compromised occupancy of RNA polymerase II on the IFN-α promoter in IFI16 knockdown cells suggesting that transcription of ISGs is dependent on IFI16. Since IFI16 knockdown compromised not only DNA virus driven pathways, we propose additional regulatory roles outside of DNA sensing. Collectively, these results indicate that IFI16 plays a role in the regulation of type I IFN gene transcription and production in response to both RNA and DNA viruses. The role of IFI16/IFI204 has been studied extensively in vitro, however the role of the receptors in vivo has yet to be determined. In chapter three of this thesis, we developed a mouse deficient in IFI204 to explore the role of IFI204 in in vivo immune responses to viruses. We investigated the ability of IFI204 deficient cells to induce type I interferons and other cytokines in response to a panel of DNA and RNA ligands in vitro. IFI204 deficient BMDMs displayed a partial defect in type I interferon induction in response to both DNA and RNA ligands and viruses as compared to WT mice. We also observed that this phenotype is time dependent, since there was no change in type I interferon induction after 12 hours post infection as compared to earlier time points. In contrast to these results, expression of the NF-κB regulated cytokines IL-6 and IL-1β were unaffected in IFI16 knockdown cells. These results suggest that IFI204 plays a partial role in the induction of type I interferons in response to both DNA and RNA ligands. Additionally, IFI204 may work in tandem with other receptors in a sequential manner to amplify the type I interferon response. We also studied the involvement of IFI204 in an in vivo model of HSV-1 infection. IFI204 knockout mice produce less brain and serum IFN-β, IL-6, and IL-1β 72 hours post intraperitoneal infection with HSV-1. Furthermore, IFI204 -/- mice are more susceptible to HSV-1 infection as compared to WT mice. These data indicate that IFI204 mediates the response to HSV-1 in vivo by inducing the production of cytokines that are necessary for the control of viral infection.

The innate immune system is an evolutionarily conserved primary defense system against microbial infections. One of the central components of innate immunity are the pattern recognition receptors which sense infection by detecting various conserved molecular patterns of pathogens and trigger a variety of signaling pathways. In this dissertation, the signaling pathways of several classes of these receptors were dissected. In chapters II and III, the role of two NOD-like receptors, NLRP3 and NLRC4 were investigated in the context of infection with the fungal pathogen, C. albicans. C. albicans is an opportunistic pathogen that causes diseases mainly in immunocompromised humans and innate immunity is critical to control the infection. In chapters II and III, we demonstrate that a multiprotein-inflammasome complex formed by the NLR protein, NLRP3 and its associated partners, ASC and caspase-1 are critical for triggering the production of mature cytokine IL-1β in response to C. albicans. NLRC4, another inflammasome forming NLR that is activated by intracellular bacterial pathogens, was not required for this process in macrophages. Thus, our data indicates that NLRP3 inflammasome responds to fungal infections in addition to its known stimuli such as bacterial and viral infections, toxic, crystalline and metabolic signals. Interestingly, this NLRP3 dependent inflammasome response was maintained even when the pathogen is not viable, and is either formalin fixed or heat-killed (HK). Hence, in chapter III, we examined β-glucans, a structural cell wall component, as the potential immunostimulatory component of C. albicans and dissected the inflammasome responses to β -glucans. We observed that NLRP3-ASC-caspase-1 inflammasome was critical for commercially obtained particulate β-glucans similar to the case of C. albicans. β-glucan sensing C-lectin receptor dectin-1 and the complement receptor CR3 mediated inflammasome activation, IL-1β production in response to the glucan particles. Interestingly, CR3 which recognizes glucans as well as complement opsonized pathogens was strongly required for HK C. albicans induced IL-1β, and partially required for that of live C. albicans, while dectin-1 was not required. Consistent with the receptor studies, blocking of β -glucan receptors by pre-incubating cells with nonstimulatory, soluble glucans led to decreased IL-1β production in response to HK C. albicanswith no effect on IL-1β in response to the live fungus. Dectin-1, CR3 and β-glucan sensing also triggered a moderate dendritic cell death response to β-glucans and HK C. albicans. Live C. albicans induced cell death requires phagocytosis but not the inflammasome, β-glucan sensing, dectin-1 or CR3. The Drosophila caspase-8 like molecule DREDD plays an essential, nonapoptotic role in the Drosophila NF-κB pathway called the ‘IMD’ pathway. Owing to the remarkable evolutionary conservation between Drosophila and mammalian innate immune NF-κB pathways, we explored the potential role of caspase-8 in inflammasomes and in TLR signaling. Using casp8-/- Rip3-/- macrophages and dendritic cells, we observed that caspase-8, specifically augments β-glucan and HK C. albicans induced IL-1β as well as cell death in a caspase-1 independent manner, but not that of live C. albicans, in chapter III. We also found that caspase-8 differentially regulates TLR4 and TLR3 induced cytokine production (chapter IV). Caspase-8 specifically promotes TLR4 induced production of cytokines such as TNF, IL-1β in response to LPS and E. coli. On the other hand, caspase-8 negatively regulates TRIF induced IFNβ production in TLR4 and TLR3 signaling in response to LPS and dsRNA. Caspase-8 executed a similar mode of regulation of the cytokine RANTES in MEFs, in part, by collaborating with RIP3. Strikingly, caspase-8 deficiency alone triggers higher macrophage death and IL-1β production in response to TLR ligands, due to the presence of RIP3. Thus, in addition to its conventional roles in apoptosis, caspase-8 modulates TLR4 and TLR3 induced cytokine production and prevents RIP3 mediated hyper inflammation in response to TLR signals. Together, our findings provide valuable information on fungal pattern recognition and inflammasome pathways and define the contribution of β-glucan sensing to C. albicans induced inflammasome responses. In addition, we demonstrate how caspase-8 adds a layer of specificity to inflammasome as well as TLR signaling. Overall, these results also shed light on the cross talk between death signaling components and innate immune pathways to mount a specific and potentially effective innate immune response against microbial pathogens.

Diss. (sammanfattning) Umeå : Umeå universitet, 2010.