| The goal of this dissertation project was to determine the mechanisms behind the observed increases of myeloid dendritic cells (mDCs) in the peripheral blood of patients with chronic periodontitis (CP) and to determine how these mDCs may contribute to chronic inflammation. CP is a chronic infection characterized by frequent bouts of bacteremia, which contain the 'keystone' oral pathogen Porphyromonas gingivalis. Induction of transient bacteremia, by mechanical debridement of plaque in CP patients, was shown to increase the number of blood mDCs within 24 hours, but the mechanisms were not clarified. Previous studies have indicated that the interaction of the 67-kDa fimbrial adhesin (Mfa-1, minor fimbriae) of P. gingivalis with the DC pattern recognition receptor (PRR) DC-SIGN prevented a productive inflammatory response. In previous studies ligation of DC-SIGN has been shown enhance growth factor-mediated differentiation, but to lead to compromised function of DCs. These observations were the basis for developing our hypothesis that the Mfa-1 fimbriae of P. gingivalis drives differentiation of immature DCs from monocytes, a myeloid progenitor of mDCs and monocyte-derived DCs (MoDCs), while inhibiting routine DC apoptosis, by ligating DC-SIGN. To this end, I show that infection of primary human monocytes with low doses of P. gingivalis, which mimics natural bacterial burden rates observed in patients, drives differentiation of immature MoDC populations. These pathogen-differentiated MoDCs, or PDDCs as we termed them, have similar function to conventional immature MoDCs and are capable of initiating immunostimulation after forced maturation, but this depends on the initial P. gingivalis strain of infection. I also show that conventional MoDCs infected with Mfa-1+ P. gingivalis display reduced rates of apoptosis, which was further characterized as an inhibition of caspases-1, -3/7, and -8 activity. Moreover, as P. gingivalis-harbouring DCs are found to be metastasized throughout the body in situ, I hypothesized that Mfa-1/DC-SIGN interaction may also disrupt chemokine receptor switching during DC activation, leading to dysregulated DC migration. I show that immature MoDCs infected with Mfa-1+ P. gingivalis fail to upregulate the lymphoid-homing chemokine receptor CCR7, but instead upregulate the angiogenic chemokine receptor CXCR4. Infected MoDCs are thus not able to migrate through endothelial monolayers in response to the chemokine CCL19, but can migrate through this barrier towards CXCL12. As CP is characterized by a high degree of vascular remodeling and elevated CXCL12 levels, this can potentially target P. gingivalis-harbouring DCs to damaged vasculature. Taken together, I have identified novel mechanisms by which P. gingivalis drives monocyte to MoDC differentiation, inhibits MoDC apoptosis to maintain its protective niche, and dysregulates typical DC migration patterns. These observations highlight the importance of blood DCs in the immune response to CP and the link of these cells with chronic inflammation and systemic health. |
