Introduction:
Limited studies have suggested that calprotectin may take part in the pathophysiology of community-acquired pneumonia (CAP). Nevertheless, there is no clinical study analysing the role of S100A8 in CAP patients. The objective of this study was to analyse the association of serum S100A8 with the severity of CAP and determine the cut-off values of S100A8 for predictive power based on a cross-sectional study.

Material and methods:
A total of 200 CAP patients and 100 normal subjects were recruited. Demographic data, clinical information, and serum were collected on admission. S100A8 and inflammatory cytokines were detected using ELISA and RT-PCR. All statistical analyses were performed with SPSS 19.0.

Results:
Serum S100A8 was increased in CAP patients on admission. Serum S100A8 was gradually increased in parallel with CAP severity scores. Serum S100A8 was positively correlated with CAP severity scores, blood routine parameters, and inflammatory cytokines. Furthermore, univariate and multivariate logistical regression revealed that there were positive associations between serum S100A8 with CRB-65, PSI, and CURXO. Moreover, the predictive capacity of serum S100A8 was determined by ROC curve analysis. The area under the curves of S100A8 for CAP and CAP severity were 0.855 and 0.893, respectively. Mechanistic analysis found that S100A8 knockdown alleviated streptococcus pneumoniae-evoked inflammatory cytokines in A549 cells.

Conclusions:
Serum S100A8 on admission was positively associated with the severity of CAP. S100A8 knockdown alleviates streptococcus pneumoniae-evoked inflammatory cytokines in A549 cells, indicating that S100A8 may exert a significant effect on the pathophysiology of CAP and could be an early serum diagnostic biomarker for CAP.