3.1 Principal findings
Neonatal sepsis is the major cause of prematurity and infant mortality. Early diagnosis and effective treatment is the key to save the lives of neonates. In this prospective study, we determined the concentrations of LL37 in umbilical cord blood and other inflammatory factors in peripheral blood in preterm infants. The key findings of this study are the following: (1) The preterm infants at risk of early-onset sepsis had higher levels of antimicrobial peptide LL37, CRP, WBC and MPV, and a lower level of PLT. (2) The expression of antimicrobial peptide LL37 was negatively correlated with PLT level and positively correlated with MPV level, the expression of PLT was negatively correlated with MPV level in preterm infants at risk of early-onset sepsis. (3) The early individually value of antimicrobial peptide LL37 for early-onset sepsis is higher than CRP and WBC.
3.2 Strengths and weaknesses
This study has strengths as follows: First, we measured the concentrations of LL37 in umbilical cord blood in preterm infants, analyzed the relationship between LL37 and neonatal early-onset sepsis, which was not reported previously. LL37 is released as an 18-kDa pro-peptide, produced by hCAP-18, which is significantly increased in inflammation and infection, has the function of amplifying Toll-like receptor signals11–12. Bucki et al confirmed that LL37 rose immediately after infection, and reached a peak within 2 hours, which provided a theoretical basis for early diagnosis of sepsis13. Second, in our study, the expression of antimicrobial peptide LL37 was negatively correlated with PLT level and positively correlated with MPV level in preterm infants at risk of early-onset sepsis. LL37 was demonstrated that be able to inhibit P selectin expression, which is a major platelet alpha-granule protein that is highly expressed on the platelet surface during activation14. However, P-selectin can lead activated platelets adhere to neutrophils and monocytes15, with innate immune response of LL37. There may be a link between the anti-inflammation activity of LL37 and its antiplatelet aggregation activity. However, this study also has some limitations. First, the number of early-onset sepsis cases was limited and the efficacy needed to expand the sample size for further evaluation. Second, LPS activates inflammatory cells to secrete TNF-a, IL-1β, IL-6 and other cytokines, which mediate the development of sepsis16. LL37 was proved to be able to bind to LPS and suppress the interaction between LPS and LPS binding protein as well as bind to macrophage CD14 receptors, thus inhibiting LPS-induced TNF-a expression17–18. We may conduct a larger study to assess the mechanism of LL37 and sepsis. Third, the study found the correlation of LL37 with PLT and MPV. However, We didn’t know specific signals or pathways that LL37 influenced them. Wen et al. reported that LL37 also could reduce phosphorylation of Src kinase and AktSer473, indicating that the modulation of Src/PI3k/Akt signaling pathway involved in the antiplatelet activity of LL3714. Src kinase plays an important role in activating platelet19, and phosphatidylinositol 3-kinase (PI3K) is the most crucial one among the downstream effectors of Src kinase, Akt is the most well-known activation marker of
PI3K. Therefore, we may conduct an animal study to explore how the LL37 influence platelet in early-onset neonatal sepsis.
3.3 Comparison with previous studies
In the present studies, it is not surprisingly to find significant elevation in the serum level of CRP, PCT, and WBC in the newborn with sepsis. CRP is the most common laboratory tests in the diagnosis of neonatal sepsis20, which will take a long time for level to increase, so making it low sensitive for early diagnosis of neonatal sepsis21. As well as this study provides low sensitivity (80.0 %) with limited specificity (80.0%) for CRP. However, conditions where there is spurious increase in level of CRP such as meconium aspiration syndrome, premature infant exposure to glucocorticoids, maternal fever during labor, fetal distress, prolonged labor, perinatal asphyxia and intraventricular hemorrhage (IVH), thus making it a nonspecific biomarker for diagnosis of neonatal sepsis21–22.
Leucopenia has been proved to have low sensitivity (29%) but high specificity (91%) for diagnosis of neonatal sepsis23, which defined as WBC count less than 5000 to 7500/mm3. But in our study, there is a different sensitivity (80.0%) and specificity (75.0%). Besides WBC, absolute neutrophil count (ANC), and immature to total leukocyte ratio (I:T) also have been used to diagnosis of neonatal sepsis. Hornik et al. found that low WBC counts, low ANC and high I: T neutrophil ratios were associated with increased odds of infection24. But all have significant limitations in the diagnosis of neonatal sepsis.
Procalcitonin (PCT) is an acute phase reactant protein and has been reported to be associated with immunomodulation associated with systemic inflammatory response syndrome (SIRS). The levels of PCT increase significantly during systemic
bacterial infection such as early-onset or late-onset sepsis25–26, and Chiesa et al. also showed in diagnosis of EOS, PCT had sensitivity of 92%, specificity of 97%26. However, false increase of PCT is seen in many conditions such as premature newborn, intracranial hemorrhage, birth asphyxia, neonatal hypoxemia, and healthy infants born to mothers with chorioamnionitis27. Therefore, PCT must to be studied further in larger groups of infants so as to improve its diagnostic accuracy. As there are limited cases in our study, PCT showed no statistical difference in two groups, and without increasing significantly in early-onset sepsis.
With this study, we investigated the levels of LL37 in umbilical cord blood and found that it was significantly higher in preterm with early-onset sepsis than that in healthy control group. The sensitivity and specificity of LL37 were 90.0% and 80.0%, respectively, values that were higher than the most obtained for the other markers. The inflammatory mediators (interleukin IL-6、IL-1, tumor necrosis factor TNF) released after severe infection, activating the IF-IL-6 site on the antimicrobial peptide LL37 by Kinase/signal and transcription activator (JAK/STAT) pathway or Ras-dependent mitogen-activated protein kinase (MAPK) pathway, so significantly increasing the expression of LL3728. This may be the main mechanism.