Myeloperoxidase (MPO) is the most abundant proinflammatory enzyme stored in the azurophilic granules of neutrophils, accounting for 5% of their dry mass. It is also present to a lesser degree in the lysosomes of monocytes. MPO is a component of the intracellular microbicidal system of phagocytes and part of innate immunity [1, 2]. MPO is a heme enzyme that is released during phagocytosis, which uses the superoxide (O${}_2$${}^{-}$) and hydrogen peroxide (H${}_2$O${}_2$) generated by the neutrophil oxidative burst (NADPH-oxidase) to produce highly reactive hypochlorous acid (HOCl) and other reactive oxidants (hypobromous, hypothiocyanous acids). Unlike other peroxidases, MPO has a chlorinating activity (hypochlorous acid production) in addition to its regular peroxidative activity, which means it can use hydrogen peroxide and superoxide to oxidize numerous biological compounds [3, 4].

Saliva is a valuable yet complex diagnostic medium, as it is a mix of salivary gland secretions, serum transudate from gingival crevices, oral mucosa cells, and oral microorganisms [5]. Neutrophil-derived MPO constitutes a significant proportion of the total salivary peroxidase activity [6], while the number of neutrophils that migrate to the oral cavity determines the MPO activity in saliva. Due to its potentially defensive role, MPO is found to be increased in conditions associated with oxidative stress and inflammation [7, 8]. Indeed, high salivary MPO levels have been reported in patients with oral inflammation [9]. However, much less is known about the impact of systemic conditions and medication on salivary MPO. Some studies have reported that MPO could be a mediator of inflammatory processes, promoting tissue damage and contributing to the pathogenesis of diseases, such as cardiovascular disease, rheumatoid arthritis, and neurodegenerative disorders [10, 11, 12]. In this respect, Polizzi et al. [13] detected increased levels of MPO in saliva from patients with coronary artery disease. We have recently observed that, compared to healthy individuals, patients with ulcerative colitis that are not responding to combined immunosuppression treatment had significantly decreased salivary MPO levels [14], which subsequently increased when patients responded satisfactorily to biologic therapy [15]. However, changes in the salivary MPO levels from either oral or systemic factors have not currently been investigated in healthy subjects.

Thus, in the present study, we aimed to determine how oral health status and anthropometric factors affect salivary MPO levels in apparently healthy adults. Additionally, we aimed to assess whether biochemical alterations in saliva or demographic factors could be confounders in these relationships.

The myeloperoxidase concentration in the saliva of the examined individuals ranged from 3 to 720 ng/mL (median of 297 ng/mL). No differences in salivary MPO concentrations were found between either men and women (Fig. 1A) or between smokers and non-smokers (Fig. 1B). Therefore, all patients were grouped, and all subsequent analyses were performed on all patients as a single group. In this group, the median BMI was 27.5 kg/m${}^2$, while patients with a BMI above 25 kg/m${}^2$ had significantly higher MPO levels (Fig. 1C). Table 1 shows the recorded clinical and laboratory values. Then, these parameters were correlated with salivary MPO levels (Table 1).

The main observation in the present study was that the salivary MPO levels in apparently healthy adults predominantly correlate to their BMI. While this association does not imply causality, one may envisage a number of mechanisms through which an increased body mass can contribute toward an increase in saliva MPO levels.

Firstly, a higher BMI is associated with inferior gingival status, poorer oral hygiene, and increased levels of proinflammatory cytokines in the saliva. Although the impairment of oral health from the observed parameters appeared to be minimal in the population, the primary cause of these associations remains unknown; however, the resulting chronic microinflammation may lead to an increase in neutrophil migration to the oral cavity, their lysis in a hypotonic environment, and MPO release [18].

Secondly, the level of salivary MPO may be partly related to the level of systemic MPO, which has been reported to increase in accordance with obesity [19]. To the best of our knowledge, this represents the first study to clearly identify that the MPO levels found in saliva correlate to BMI in healthy subjects.

Previous studies on MPO and obesity only analysed the MPO levels found in the blood. For example, Qaddoumi et al. [20] found that plasma MPO concentrations were elevated in obese patients, both with and without diabetes, compared to non-obese subjects. These MPO levels correlated clearly with both BMI and inflammatory markers, such as CRP, TNF$\alpha$, and IL-6. The authors speculated that MPO could regulate obesity independently of diabetes.

Moreover, Choromańska et al. [21] found that serum MPO concentrations were significantly elevated and could be used to differentiate obese patients with metabolic syndrome from those only suffering from obesity. These findings showed that MPO activity rose in parallel with the progression of metabolic disturbances in patients with obesity. Additionally, Sparks et al. [22] observed that a twelve-week aerobic exercise intervention promoted a reduction in BMI and a decrease in plasma MPO concentration.

Only a few previous studies have measured the level of MPO in saliva, and these have been performed primarily in the context of periodontium inflammation. In a three-week experimental study by Nascimento et al. [23], the rapid development of gingival inflammation was associated with an increase in salivary MPO.

Klangprapan et al. [24] observed that compared to healthy controls, salivary MPO was significantly increased in patients with periodontitis, and to a lesser extent, in those with gingivitis. Nizam et al. [25] confirmed a close relationship between MPO levels in the saliva and periodontal indices of patients with generalised periodontitis.

Similarly, Lahdentausta et al. [26] showed that using salivary MPO concentrations could clearly distinguish patients with periodontitis from healthy subjects. Interestingly, Polizzi et al. [13] found that salivary MPO levels were higher in patients with periodontitis and coronary heart disease than in those solely with periodontitis. Based on the multivariate regression analysis, the authors concluded that plasma C-reactive protein and total cholesterol levels were the best predictors of salivary MPO levels in these patients.

Meschiari et al. [27] observed that salivary MPO activity was significantly higher in patients with periodontitis and decreased only marginally following three months of non-surgical therapy.

In contrast, Hernández et al. [28] found a significant decrease in MPO activity in the gingival crevicular fluid (GCF) after two months of treatment for chronic periodontitis. Over et al. [29] found that the highest MPO concentrations in patients with rapidly progressive periodontal disease were in the saliva and GCF. Moreover, Saloom et al. [30] observed an increase in GCF secretion during orthodontic treatment and a simultaneous increase in MPO levels, which were associated with a greater rate of tooth movements, especially in obese adolescents.

The strengths of the current study are that a cross-sectional character was performed to investigate the potential relationships between local and systemic parameters and the antioxidant status in healthy adult subjects, while such data were determined in a non-invasive manner. Despite an absence in previously justifying the sample size used, this study determined the correlations and regressions with a high testing power. However, the limitations of the study include the relatively good periodontal condition of the included patients, who reported for regular routine oral examinations; however, this fact excluded the potential confounding of poor oral conditions on systemic dependencies. Thus, in order to investigate further relationships, such as considering body mass and age as concurrent confounders, the size of the subgroups should be increased to provide these analyses with higher statistical powers.

Moreover, additional studies should be performed to consider the correlations between salivary MPO levels and oral, demographic, and anthropometric factors and how these relate to potential alterations in serum MPO levels. Currently, salivary markers are a promising branch of laboratory diagnostics and offer the possibility of determining reference ranges in the future, since these have not yet been determined and large populations are required for testing.

Salivary levels of myeloperoxidase in healthy adults with no clinically detected signs of inflammation in the oral cavity correlated predominantly with the patient’s body mass index. Additionally, a higher body mass index was also associated with reduced oral hygiene, lower gingival status, and less salivation. Therefore, these factors should be considered when interpreting increased salivary MPO concentrations.

Data are available on request from the corresponding author.

KN is responsible for conceptualisation, methodology, formal analysis, investigation and resources, writing the original draft, review and editing, visualisation. AL is responsible for investigation and resources. RR is responsible for investigation and resources. KK is responsible for conceptualisation and methodology. JW is responsible for conceptualisation, methodology, writing review and editing, visualisation and supervision. AS is responsible for conceptualisation, writing review and editing and supervision. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript.

The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Bioethics Committee of Poznan University of Medical Sciences (189/14). Informed consent was obtained from all subjects involved in the study.

Not applicable.

This research received no external funding.

Given the role as Guest Editor, Kacper Nijakowski had no involvement in the peer-review of this article and has no access to information regarding its peer-review. Full responsibility for the editorial process for this article was delegated to Pier Paolo Piccaluga.