Graduate School of Biomedical Sciences, Immunology and Microbiology
Dissertations, UMMS; Candida albicans; Caspase 8; Immune System; Immunity, Innate; Inflammasomes; Receptors, Immunologic; Receptors, Pattern Recognition
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.
Ganesan, Sandhya, "Characterization of Anti-Fungal Inflammasome Responses and the Role of Caspase-8 in Innate Immune Signaling: A Dissertation" (2014). University of Massachusetts Medical School. GSBS Dissertations and Theses. Paper 707.