Author ORCID Identifier
https://orcid.org/0000-0003-3421-8297
Defense Date
2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy
Department
Oral Health Research
First Advisor
Huizhi Wang
Abstract
Periodontal disease is a polymicrobial infection-induced chronic inflammatory disease and results in irreversible tissue damage and bone loss. While bacteria play a large role in the initiation of this disease, excessive and prolonged inflammatory immune responses are fundamental for its progression, making host directed therapy (HDT) a possible and potentially valuable treatment. Histone Deacetylase 6 (HDAC6) is an enzyme that removes acetyl groups from both histone and non-histone proteins, giving it a potential role in regulating inflammation initiated by pathogenic oral bacteria. Previous studies indicated that HDACs act as a bivalent regulator in inflammatory responses; however, the role that HDAC6 plays in the inflammatory immune response towards oral pathogens is understudied. Using both in vitro and in vivo models, we observed that HDAC6 deletion or inhibition significantly decreased the expression and secretion of several proinflammatory cytokines and chemokines, including IL-6, TNF-α, and MIP-2, in several types of Porphyromonas gingivalis-stimulated immune cells and fibroblasts. Moreover, the activity of the proinflammatory transcription factors NF-κB and FoxO1 and the kinase Erk1/2 was decreased when HDAC6 was not present, indicated by a change in their phosphorylation and/or acetylation upon P. gingivalis challenge. In two different mouse periodontitis models, oral infection with P. gingivalis in HDAC6 KO mice led to an overall lower gingival tissue infiltration of immune cells and decreased levels of alveolar bone loss, as compared to that of WT mice. Additionally, shotgun DNA sequencing analysis, providing species-level resolution, revealed that HDAC6 deficiency and P. gingivalis infection substantially impacted the composition and diversity of bacteria colonized in gingival tissues. Further mechanistic data showed that P. gingivalis infection led to the activation of HDAC6, followed by two distinct mechanisms of FoxO1 activation. The first mechanism is that activated HDAC6 decreases FoxO1 acetylation, leading to its sequestration in the nucleus and promoting its transcription activity, as demonstrated by the use of separated cell fractions and immunofluorescence staining. The other mechanism, which we uncovered for the first time, is through HDAC6-mediated altered expression of rictor, a component of the mTORC2 complex, which led to a downregulated phosphorylation of FoxO1 and subsequent retention in the nucleus. Lastly, we examined the impact of HDAC6 on phagocytes and CD8+ effector T cells in different contexts. We found that HDAC6 deficiency suppressed macrophage polarization toward M1, decreased P. gingivalis-induced PD-L1 expression on dendritic cells, and reduced PD-1 expression on CD8+ T cells. Using a syngeneic mouse oral cancer model, we also found that HDAC6 deletion resulted in decreased tumor size and improved the overall survival of mouse oral cancer 1 (MOC1) injected mice. Altogether, through this dissertation study, we have demonstrated the multifaceted role of HDAC6 in periodontal inflammation and its impact on the alteration of the oral microbiome in a mouse periodontitis model, along with the influence of HDAC6 on the activity of different types of immune cells in various contexts. Our further investigation also elucidated the molecular mechanisms of HDAC6-mediated activation of FoxO1, which occurs through modulation of both the acetylation and phosphorylation of FoxO1 simultaneously. These findings indicate that HDAC6 plays a pivotal role in maintaining the homeostasis of immune responses in the oral cavity, and therefore holds great potential as a legitimate target for controlling periodontal inflammation or enhancing the efficacy of immune-checkpoint blockade therapies in oral cancer.
Rights
© The Author
Is Part Of
VCU University Archives
Is Part Of
VCU Theses and Dissertations
Date of Submission
12-1-2024