Discussion
The present study shows for the first time that chronic alterations of circadian rhythm related to stress in policemen impairs not only the perceived stress levels and sleepiness, but also the lung function and the pulmonary and the systemic immune response.
The association between chronic shiftwork with impaired perceived levels of stress is clearly stablished14, while the impact of such association in different organs and systems are less understood. Therefore, the present study shows for the first time that policemen working in a shiftwork scale for at least 5 years, present increased levels of perceived stress, followed by impaired lung function and impaired systemic and pulmonary immune response.
Significant positive associations have been observed between stress indices and increased incidence of chronic diseases, such as cardiovascular and metabolic diseases7,15, as well as systemic immune dysregulation5. Herein, the present study shows for the first time that compared with civil men working in a continuous 8 hours/day diurnal scale, without increased levels of perceived stress and sleepiness disturbance, policemen presented increased levels of serum IL-2. Increased levels of IL-2 play a major role in the growth and proliferation of many immune cells such NK and T cells and is known as a pro-inflammatory cytokine. The activation of T lymphocytes via interleukin IL-2 and IL-2 receptor plays an important role in pulmonary diseases such as asthma16,17, in which airway obstruction is present at different levels. Furthermore, asthmatic patients have higher pulmonary levels of IL-2 compared to healthy subjects18. In addition, several studies have reported inverse association of pulmonary levels of IL-2 with the FEV1 in asthmatic individuals17,18. Our findings show that police officers have higher levels of lung IL-2 compared to civilians, which were followed by impaired lung function, an event never demonstrated before. Of note, we reinforce that the present study demonstrates that chronic shiftwork scale leads to increased levels of perceived stress and impaired sleepiness, which can be related to increased levels of pulmonary IL-2 associated with reduced lung function.
Interestingly, higher levels of pulmonary IL-2 in asthmatic individuals present inverse association with FEV117,18. In this way, the present study shows that non-asthmatic police officers, beyond increased pulmonary IL-2 levels and reduced FEV1, also presented increased levels of exhaled nitric oxide, a molecule involved with airway inflammation and obstruction (reduced FEV1)19. In fact, the present study showed that police officers presented increased levels of exhaled nitric oxide, slightly above 25 parts per billion (ppb), which significate airway inflammation20. In addition, other sources of airway inflammation beyond allergen-induced asthma, such as nitrogen dioxide (a major traffic-related air pollutant)21, lipopolysaccharide9,22,23, and smoking24 also can increase the levels of nitric oxide. Therefore, the present study showed that policemen working in a shiftwork scale clearly present airway inflammation, characterized by increased levels of exhaled nitric oxide, although the source of such airway inflammation could not be confirmed in this study.
On the other side, increased levels of blood IL-10, an immunomodulatory and anti-inflammatory cytokine has been associated increased and better FEV1/FVC values, which is also an important marker of airway obstruction25. In the present study, it was found similar response, since policemen working in a shiftwork scale presented reduced levels of IL-10 and reduced FEV1. These findings reinforce the findings of previous studies performed in asthmatics25, demonstrating a possible association between lower levels of IL-10 with airway inflammation and airway obstruction. In fact, this concept that low levels of IL-10 can be associated with airway obstruction is strengthened by the present study, since that such phenomena were also found in non-asthmatic patients. In addition, reduced levels of IL-10 has been associated also with increased pulmonary inflammation induced by Mycoplasma pneumoniae 26, obstructive sleep apnea27, and poor progression of COPD patients28, demonstrating that perhaps, police officers could be more susceptible to develop pneumonias, obstructive sleep apnea and even COPD. However, the present study has not evaluated such hypothesis and cannot confirm that policemen could be more susceptible to develop pneumonias, obstructive sleep apnea and COPD.