Oral NETs: the deadly kiss

In this issue of Blood, Mohanty et al have identified a novel mechanism that protects us from pathogens where we encounter them first: in the oral cavity.¹ 

Everyday we are exposed to microbes, whether we inhale air, expose our skin, or simply eat. As soon as foreign material enters our body, pathogens get their chance to invade and cause infection. Therefore, our mucosal immune system has evolved specific strategies to defend us at host-environment interaction zones. At the skin, the gut, and the lung, innate immunity has been studied in more detail, but the oral cavity, one of the most heavily colonized parts of the human body, remains surprisingly poorly understood.

Both cellular and noncellular mechanisms shield us from infection. Neutrophils are the first immune cells recruited and employ 3 major weapons to destroy pathogens: phagocytosis, granule release, and the formation of neutrophil extracellular traps (NETs). NETs have attracted increased attention because of the extraordinary nature of this defense mechanism.²  Using “beneficial suicide” as a defensive measure, neutrophils expel their own DNA to entrap and destroy pathogens extracellularly. By doing so, NETs can combat microbes that are physically larger than the neutrophils, such as fungi.³  NET formation (NETosis) has been studied in a variety of diseases and NET structures were also found at sites of periodontitis, but their functional role remained enigmatic.⁴  As the central entry point to the human body, the oral cavity has one main defense shield, the saliva, which contains a plethora of antimicrobial components, such as lysozyme and mucins.⁵  However, how these extracellular salival proteins cooperate with the cellular immune response has not been defined precisely.

Mohanty et al now broaden our view at this intimate host-pathogen interaction area by demonstrating that NETs are present in the oral cavity, are induced by saliva, and are highly bactericidal. Experiments were performed to uncover the factors that were responsible for saliva-induced NETosis, revealing that salival mucins triggered NET formation through sialyl Lewis^X- and l-selectin–mediated signaling. This identified pathway turned out to be novel, as it was rapid and did not require reactive oxygen species/reduced nicotinamide adenine dinucleotide phosphate-oxidase or elastase activities that are usually involved in NETosis.⁶ 

When NETs capture bacteria, two major factors determine how this interaction ends: DNases (destroy NETs, can be produced by certain microbes⁷ ) and the antimicrobial efficacy of the extruded DNA web. The abundance of NETs present in the oral cavity is somewhat unexpected based on the high DNase activity present in saliva.⁸  However, saliva-induced NETs are unique in 2 aspects: (1) they are resistant to DNases in contrast to conventional NETs (induced by phorbol myristate acetate or bacteria), which are degraded, and (2) they destroy bacteria more efficiently than conventional NETs do.

So what is the clinical relevance of these findings? Neutrophils are continuously migrating to the oral mucosa and seem to be important there, since quantitative (neutropenia) or qualitative (functional) neutrophil deficiencies are commonly associated with gingivitis, ulcerations, periodontitis, and a disturbed oral microflora.⁹  The authors further showed that saliva-induced NETosis was deficient in patients with Behçet disease (BD), an inflammatory condition characterized by recurrent oral ulcerations, indicating that this cellular mechanism has clinical impact. Nevertheless, the generalizability of this NET-related mechanism for other oral pathologies, such as recurrent aphthous stomatitis independent from BD, needs to be investigated in future studies.

NETosis is becoming more complex the more we learn about it. This study suggests that salival NET formation is essential in maintaining a healthy oral microbiota and pharmacotherapeutic strategies to enhance NET formation could represent a novel approach for oral manifestations of inflammatory diseases. Whereas progress in cell biology in the NETosis field is striking, there is, however, still little progress on how to apply this information therapeutically. In particular, it seems challenging to selectively activate NET formation at sites where this may be beneficial (such as the oral cavity), while simultaneously avoiding excessive and harmful NET release that has been associated with autoimmune, pulmonary, and vascular pathologies¹⁰ ; this is a motivation to dig deeper into the manifold roles of NETosis.