The early stratification of CAP patients will help optimize hospital resources and formulate a diagnosis and treatment plan [24]. NMI is a regulator involved in various inflammatory diseases by participating with macrophages in the inflammatory response. In this study, we tested NMI levels in patients with CAP in two separate cohorts to explore the role of NMI in predicting severity of CAP. We measured NMI levels in both serum and BALF of CAP patients and analyzed its correlation with clinical outcomes (30-day mortality and ICU admission). We compared the efficiency of NMI levels in predicting CAP severity with other classical CAP severity score systems and biomarkers. Our results showed that NMI is a novel biomarker reflecting the severity of CAP patients.
Many parameters have been studied and widely applied to assess the severity of CAP patients, among which the PSI and CURB65 score are the most recommended in international guidelines [7]. The PSI score can accurately predict the 30-day mortality rate, however, the complexity of its 20 variables limits its clinical application. As simple as CURB-65 is, it underestimates the potential severity of young patients and mistakes elderly CAP patients as severe CAP patients [7]. The expanded-CURB-65 improved the recognition of patients with severe CAP compared with CUBR-65 to some extent, but it didn’t involve specific biomarkers [25]. Our results showed that it’s not sufficient enough to predict 30-day mortality or ICU admission based on the PSI or CURB-65 score alone, and the addition of specific biomarkers may further improve the predictive ability of these scores [26]. A more convenient, highly efficient, and earlier recognition score system is urgently needed (32).
Some biomarkers were showed to be related to CAP, such as PCT, CRP, proADM, and c-terminal vasopressin (copeptin) [12, 13, 27, 28], and they play an important role in estimating the severity, treatment, discontinuation, and etiology in the management of CAP patients [29, 30]. PCT can guide antibiotic treatment or discontinuation in lower respiratory tract infection [29]. However, PCT can’t improve the predictive ability of the PSI/CURB65 score [31, 32] and is insufficient to distinguish bacterial from viral infection [33]. CRP shows only moderate predictive values for mortality of CAP [13, 34, 35]. Some new biomarkers like proADM and copeptin may be useful to infer mortality and severity of CAP, but further verification is needed [7, 36]. Although there are some biomarkers such as CRP that can distinguish bacterial pneumonia, their value in predicting the severity of CAP in the early stage is limited [37, 38].
Compared with the biomarkers mentioned above, our study revealed that NMI concentration at admission is superior in assessing the severity and risk of death of CAP. First, a higher comprehensive efficiency of NMI than CRP and PCT was found in predicting 30-day mortality and ICU admission in CAP patients. Second, the serum NMI level of patients with severe CAP on the day of admission was significantly higher than that of non-severe CAP patients, suggesting that NMI is an early indicator of the severity of CAP. Finally, the AUC was significantly increased after adding NMI to PSI and CURB65 score both for 30-day mortality and ICU admission. All of these indicate the superior potential value of NMI in the clinical application of CAP.
Previous studies have already shown that NMI is associated with various inflammatory diseases [18, 19, 39, 40]. Xiahou et al. found that serum NMI levels in human significantly increased in sepsis patients and were associated with mortality [18]. Wu et al. showed that NMI expression in human lung A549 cells was upregulated after H3N2 SIV infection [40]. NMI levels also elevated in serum and liver tissue of patients with hepatitis B virus-related acute-to-chronic liver failure and the concentrations of NMI decreased when in convalescent stage of disease [19]. Our current results showed that the expression of NMI in both serum and BALF was significantly increased in CAP and that NMI levels were positively correlated with mortality and ICU admission. Together, the results indicated that NMI expression in both serum and local tissue were increased in infectious diseases, and might be related to the prognosis of the disease. Interference with NMI expression presumably relieves inflammation and improves prognosis as experiments confirmed that NMI and IFP35 knockout sepsis mice had reduced inflammation and mortality [18]. Wang et al. also found that the apoptosis induced by foot and mouth disease virus was significantly inhibited after silencing NMI expression [41]. Therefore, NMI can possibly be used not only as a predictor of the severity, but also as a therapeutic target of CAP patients.
Hitherto, the mechanisms of NMI participating in inflammatory response have been explored.
Wang et al. found that after Sendai virus infection, NMI overexpression mice exerted antiviral effects by limiting the overproduction of type I IFN [
42]. Similarly, Hu et al. found that overexpression of NMI reduced the replication of prototype foamy virus [
43]. Nevertheless, Cheng et al. found that 293T cells infected with SARS-CoV promoted the NMI ubiquitin-dependent protein degradation [
21]. Xiahou et al further elucidated that NMI acted as a proinflammatory damage associated molecular pattern in mice with sepsis and promoted macrophages to release inflammatory factors TNF and IL-6 by activating TLR4 signaling [
18]. These evidences indicated that NMI was involved in the process of infectious diseases. However, the mechanisms of NMI involved in the pathogenesis of bacteria and other pathogens need further elucidation.