Aims

The abundance of neutrophils (polymorphonuclear cells, PMNs) in the oral cavity both in health and disease suggests that PMNs play a significant role in maintaining oral immune equilibrium. This cross-sectional, in vitro study characterizes the surface epitopes, oxidative burst, and migratory properties of oral (oPMNs) and circulating (cPMNs) neutrophils in healthy volunteers.

Methods

cPMNs were isolated from whole blood samples using gravity sediment separation; oPMNs were obtained from filtrated saliva. Surface epitopes and oxidative burst were analyzed with flow cytometry. PMN surface morphology was visualized with scanning electron microscopy (SEM) with gold labeling. A µ-slide chemotaxis assay was conducted.

Results

oPMNs showed significantly higher LOX-1 expression than cPMNs (~1000 times). In contrast to oPMNs, cPMNs could be stimulated with PMA or TNF-α, resulting in a significant LOX-1 increase. PMA induced a stronger increase of LOX-1 than TNF-α. Only stimulation with TNF-α or PMA resulted in the upregulation of CD11b and CD66b and the downregulation of CD62L on cPMNs. SEM showed an abundance of CD66b and diverging surface morphologies between oPMNs and cPMNs. Unlike cPMNs, oPMNs did not show chemotactic migration along an fMLP-gradient.

Conclusion

Our study highlights the high expression of LOX-1 on oPMNs associated with a lack of further stimulability and absent in vitro chemotactic migration, indicating that these cells are in a higher state of activation than cPMNs. These findings provide new insights into the phenotypic and functional characteristics of oral neutrophils.

Summary

Background

PMNs play a key role in maintaining healthy homeostasis in the oral cavity. However, many aspects, especially the nature and alteration of PMN surfaces, remain to be fully elucidated.

Added value of this study

To the best of our knowledge, this is the first study to document a naturally elevated expression of LOX-1 on PMNs in the oral cavity. The different expression of the surface epitopes CD11b, CD66b, and CD62L additionally support a higher state of activation of oral PMNs. LOX-1 appears to serve as a marker for PMNs that exhibit suppressive immune characteristics due to ongoing bacterial stimulation in the oral environment.

Clinical implications

This observation of surface changes, which was also visualized using scanning electron microscopy, improves our understanding of innate immunology in physiology. It also provides an essential basis for identifying changes in disease and potential therapeutic targets in pathological conditions (such as periodontitis).