Characterization of polymorphonuclear neutrophils in the oral cavity A double-edged sword

Oral health is a continuous dynamic process, which is often confused with the absence of oral disease. Although the oral cavity is heavily colonized with microorganisms, overt infections rarely occur as long as the active equilibrium between microbiota and host immune responses is maintained. With the polymorphonuclear neutrophil (PMN) being the predominant leukocyte effector cell, evaluating their contribution in the healthy oral cavity can help understanding their role in the pathological changes associated with oral conditions.
This thesis represent a number of experiments that sought to characterize oral PMNs (oPMNs) in the healthy and diseased oral cavity. Using transmission electron microscopy, chemiluminescence, flow cytometry and next-gen sequencing, the author showed that the oPMNs play a co‐controlling part in the maintenance of the oral equilibrium. Interestingly, an inappropriate oPMN functional activity and disproportionate infiltration of the activated oPMNs, can result in excessive release of antimicrobial enzymes and ROS production into the extracellular environment and thereby unwillingly damage host tissues. Furthermore, the oPMNs’ transcriptome profile and their functioning showed variations between individuals, which may also result in clinical differences regarding their microbial interactions and possibly differences in treatment responses. The oPMN characteristics showed a beneficial role and detrimental role, which may consequently affect the oral cavity like a double-edged sword. Thus, only the synergy of the hosts’ defense arsenal can provide a peaceful co‐existence, e.g. balance, of the host with its oral environment.