Determinants of Foam Cell Formation by Human Monocyte-derived Macrophages Infected with Porphyromonas gingivalis

We have previously reported that infection of human monocyte-derived macrophages with Porphyromonas gingivalis increases foam cell formation. Objectives: In order to explore some of the mechanisms involved in this process, we sought to establish the effect of biological parameters such as low density lipoprotein (LDL) concentration and multiplicity of infection, and to elucidate the role of P. gingivalis adhesion/invasion potential in foam cell formation. Methods: Peripheral blood mononuclear cells were isolated from human healthy blood and cultured in RPMI medium containing 10% fetal calf serum to allow for differentiation. On day 9, macrophages were exposed for 6 hours to P. gingivalis FDC381, or its fimbriae-deficient mutant DPG3, with or without human LDL in different concentrations. Experiments were performed in duplicate wells for each condition, and were repeated two to four times. Macrophages were fixed in 4% paraformaldehyde and stained with Oil Red O to visualize lipid accumulation. Mean counts were determined by viewing lipid-laden macrophages under a microscope interfaced with a videocamera and image analysis software. Results: In the presence of LDL, P. gingivalis infection increased foam cell formation 2- to 4-fold compared to LDL incubation alone. Infection with DPG3 did not have such an effect. Increasing the concentration of LDL from 25 to 200µg/ml increased foam cell formation in a dose-dependent manner. Altering the multiplicity of infection (from 50 to 300 bacteria per macrophage) did not appear to have an effect on the number of foam cells formed. Conclusions: P. gingivalis induces foam cell formation in human macrophages, a process dependent on the degree of hyperlipidemia. In addition, properties of P. gingivalis that render it capable of adhering to/invading other cell types may be operative in macrophages and play an important role in its atherogenic potential.