The balance between the serum levels of IL-6 and IL-10 cytokines discriminates mild and severe acute pneumonia

Community acquired pneumonia (CAP) is a serious illness of the lower respiratory tract, responsible for high morbidity in children, especially those younger than five years old [1‐3]. The World Health Organization (WHO) uses clinical manifestations as a parameter for definition of severity ratings (pneumonia and severe pneumonia) [4]. The gradient of clinical severity arises from a complex interaction between environmental factors, pathogenicity of the etiologic agent and host characteristics, including immunological competence, malnutrition, anemia and other comorbidities [5‐13]. Clinical data associated to blood count and serum level of the C-reactive protein (CRP) are determinants for therapeutic management of acute respiratory disease in clinical practice [3, 14‐16]. Blood count and serum levels of CRP are low-cost laboratory tests, accessible in health services in countries with limited resources and their results help the differentiation between viral or bacterial respiratory infection [3, 14, 17]. High levels of CRP are caused bythe activity of circulating tumor necrosis factor (TNF) in the liver as a response to the infectious process; therefore, CRP is considered a nonspecific marker [18].

Recently, serum and bronchoalveolar levels of cytokines have been related to severity of pulmonary inflammatory process [9, 19‐21]. During the early stages of pneumonia, the alveolar macrophages produce a variety of cytokines and inflammatory chemokines, which attract and activate polymorphonuclear leukocytes that will mount an appropriate immune response in the lung parenchyma [22‐24]. Circulating levels of TNF, Interleukin (IL)-1, IL-6, IL-8, IL-12 and interferon-gamma (IFN-γ) are found in patients with CAP [7, 9, 19‐21]. The cytokines IL-1, TNF and IL-6 are important for the acute phase response [7, 9, 25], while modulation of inflammatory responses of the airways is related to the expression of the anti-inflammatory cytokines,IL-10 and IL-4 [6, 8, 24]. Studies on the inflammatory process during acute respiratory disease have reported conflicting results. Most studies that seek to relate the severity of acute respiratory disease with cytokines serum levels have evaluated cytokine levels in a single time point and samples were collected from adult patients, who may have different inflammatory responses from those observed in children due to the anatomical-functional differences of the pulmonary alveoli, in addition to the immune system immaturity [7, 9‐13, 21, 26]. The aim of the present study was to evaluate the correlation between the cytokines profile in the serum of children and adolescents with different pneumonia severity, defined by clinical manifestations and alterations in the blood count, in order to identify markers for early diagnosis of severe pneumonia.

Pneumonia is a disease associated to poverty conditions related to the environment, the individual, the infectious agent and the healthcare services [3, 4, 28‐30]. In the present study, we adopted the criteria published in the Brazilian guidelines for community acquired pneumonia in pediatrics, which follows the former guidelines of WHO [3], allowing that patients (5–10% of the cases) with hidden pneumonia, which is characterized by fever and coughing, with or without radiographic changes, and improvement after empirical treatment, may be diagnosed at an early stage [1, 31, 32]. Thus, patients with fever, coughing, tachypnea and radiological changes were diagnosed with pneumonia, and those with subcostal recession were classified with severe pneumonia. In our casuistic, factors such age, anorexia, sibilance, difficult breathing, and vomiting were statistically correlated with pneumonia severity what corroborates with other studies [30, 33‐36]. Presence of abdominal pain in children with pneumonia was observed by us as reported by other authors, but without any association with the disease severity [34, 37]. Anemia has a high prevalence in our midst, as demonstrated by Carvalho et al., who found a prevalence of 92% of healthy children in day care centers in the public health network with hemoglobin levels < 11.0 g/dL; but it was not also associated to severe pneumonia [12, 38]. The elapsed time between the beginning of the disease and its diagnostic, which can be related to the difficult access to the health services, was associated with disease severity, corroborating with other studies carried out in Brazil and Africa [33, 39]. Coincidently, all children under 5 years old were from countryside and presented severe pnuemonia. In this case, two aspects should be considered. First, a possible delay in diagnosis and treatment in the residence’s town may have contribute to an hemodynamic instability and ICU requirement at admission in the hospital; secondly, the innate immunity of young children may be compromised by less bactericidal activity and inflammatory response by immature neutrophils, and poor cytokine response by macrophages and monocytes [40].

Inflammation biomarkers, as cytokines, may be useful in determining the magnitude of the inflammatory response and lung injury in children with pneumonia. However, studies that tried to associate clinical severity in adult [18, 21, 41] and children [42] patients with respiratory diseases and serum levels of cytokines had controversial results. In the present study, the cytokines, TNF, IL-1β, IL-6, IL-8, IL-12p70, IFN-γ, IL-17A, IL-10 and IL-5, were detected in the serum of children with pneumonia and severe pneumonia at hospital admission, and IL-6 was the only cytokine associated with disease severity.

At admission (D0), we observed high serum levels of cytokines, predominantly pro-inflammatory including TNF, IL-1β, IL-6 and IL-8, which are involved in the innate immune response and in the immune response development [22, 43, 44]. In patients with severe pneumonia, the IL-6 levels were positively associated with high levels of leukocytes while the levels of TNF and IL-1β were associated with monocytes, possibly related to the recruitment and differentiation of macrophages at the site of inflammation. According to Bauer et al., TNF and IL-1β are associated with lung injury in acute respiratory distress syndrome and in severe pneumonia, although in a smaller magnitude [21]. A study reported by Kolsuz et al. on inflammatory cytokines (TNF, IL-1β, IL-6 and IL-8) levels in bronchoalveolar material after 24 h of admission for adult patients with pneumonia showed IL-6 as the most important cytokine in the determination of the disease severity in patients with systemic inflammatory response syndrome [45]. In other study, Antunes et al. reported high levels of IL-6, TNF-α, IL-10 and IL-1β detected in the majority of the 24 patients with CAP at admission, but the cytokines levels decreased significantly on days three and five of hospitalization, and IL-6 was the only cytokine associated with the disease severity, as ours findings [9].

High levels of IL-6 were observed in both studied groups at admission, highlighting the importance of this cytokine in the pulmonary inflammatory process regardless the disease severity [9]. Although IL-6 levels have been correlated with vomiting, which is a clinical manifestation of disease severity, other causes, such as the use of medications and inflammatory systemic process, may implicate in the etiology of vomiting and abdominal pain [34, 37]. IL-6 was also positively correlated with dyspnea, which may represent, under the point of view of the pathophysiology, the intensity of the lung’s inflammatory process, which is associated with respiratory disease severity.

An important anti-inflammatory cytokine in pneumonia is IL-10. In the present study, IL-10 levels were higher in the group of patients with pneumonia than in patients with severe pneumonia in admission (1.99 x 3.68 pg/mL; P = 0.074). Although the statistical differences were not significant what may be related to the small number of samples, the increased levels of IL-10 in patients with pneumonia was associated with high levels of lymphocytes, suggesting that IL-10 is an important modulatory factor of the inflammatory response of acute pneumonia [6, 24].

In conclusion, the controversial results of some studies regarding the correlation of IL-6 serum levels and pneumonia severity may be produced by the choice of disease time point analysed and the balance between IL-6 and IL-10 serum levels. Our findings showed the ratio between the serum levels of IL-6 and IL-10 is an important criteria for severity definition at hospital admission, allowing to screening at the emergency room who is at risk of evolving into complication, and evaluation of recovery capacity. Specially in the third day of antibiotic therapy, when the patients’ clinical condition is usually evaluated for possible changing in the treatment protocol, an IL-6:IL-10 ratio higher than 5.0 may predict that 89% of positive cases have still severe pneumonia, indicating that changes in the treatment protocol might be needed. In future, the determination of IL-6 and IL-10 serum levels may be offered by routine laboratory as one more marker to evaluate the severity of infection disease at the diagnosis.