Chronic Exposure to Porphyromonas gingivalis Disrupts Macrophage Innate Immune Responses

Introduction: Porphyromonas gingivalis (Pg) is a pathogenic, “red-complex” bacteria found in higher concentrations in advanced gingivitis and periodontitis. Macrophages are capable of adopting numerous phenotypes depending on the surrounding environment. Classic M1 macrophages are the result of inflammatory signals including IFNγ. This study investigated whether naïve M0 macrophages can transition into classic M1 macrophages following stimulation with traditional agonists after cells have been exposed to formalin killed P. gingivalis.Methods: The THP-1 monocytes were differentiated into naïve macrophages (M0) using PMA treatment. The M0 cells were challenged with formalin killed P. gingivalis for 24 hours. After prolonged exposure to P. gingivalis, the cells were stimulated by M1 producing agonist IFNγ and Escherichia. coli lipopolysaccharide (LPS, 20ng). The resulting cytokine gene expression profiles and secreted protein concentrations of IL-6, IL-1β and TNF-α were determined. Results: Our study investigated the impact of prolonged P. gingivalis exposure on macrophage differentiation into the M1 phenotype, characterized by high IL-6, TNF-α, and IL-1β expression. While P. gingivalis and S. gordonii initially induced a modest increase in IL-6 mRNA and protein levels, P. gingivalis exposure prior to M1 stimulation significantly suppressed IL-6 expression, whereas S. gordonii did not. Similarly, P. gingivalis exposure inhibited the expected M1-associated increase in TNF-α mRNA and protein levels, reducing TNF-α secretion to levels comparable to unstimulated controls. In contrast, S. gordonii treatment resulted in robust TNF-α induction, similar to M1 stimulation. However, IL-1β expression and secretion were significantly increased by both P. gingivalis and S. gordonii, exceeding levels seen in M1-stimulated macrophages, and prior exposure to either bacterium did not alter IL-1β induction upon M1 stimulation. These findings suggest that P. gingivalis selectively suppresses IL-6 and TNF-α responses while allowing sustained IL-1β production, potentially contributing to immune evasion and chronic inflammation in periodontal disease. Conclusions: Our research identified a significant inhibitory effect of P. gingivalis on the differentiation of naïve (M0) macrophages, preventing their proper polarization into the pro-inflammatory M1 phenotype. Specifically, macrophages exposed to P. gingivalis exhibited an impaired response to M1 stimuli, suggesting that the bacterium actively disrupts classical inflammatory activation. This effect appears to be further amplified in a chronic infection state, contributing to immune dysregulation and persistent inflammation. While our study focused on key inflammatory markers, including TNF-α, IL-6, and IL-1β, the broader impact of P. gingivalis on other critical cytokines and chemokines involved in periodontitis remains to be fully elucidated. These findings highlight the bacterium’s ability to evade immune clearance and drive chronic inflammation, reinforcing its role as a keystone pathogen in periodontal disease.