Complement 5a receptor-mediated neutrophil dysfunction is associated with a poor outcome in sepsis

Abstract

Complement 5a (C5a) has been implicated in the pathogenesis of sepsis by inducing the functional impairment of neutrophils; however, the utility of C5a receptors (C5aRs; C5aR and C5L2) as biomarkers for the management of sepsis is uncertain. This study investigated the dynamic expression of C5aR and C5L2 on neutrophils and their effects on neutrophil function. We found that sepsis patients displayed low expression levels of C5aR and C5L2 on neutrophils compared to healthy and systemic inflammatory response syndrome (SIRS) subjects, and this expression pattern was correlated with disease severity. Additionally, the expression levels of C5aR and C5L2 were associated with the survival of sepsis patients. In vitro, the addition of C5a significantly reduced C5aR and C5L2 expression levels and IL-8 production in neutrophils from sepsis patients. Those findings suggest that the reduced expression of C5aRs was associated with the functional impairment of neutrophils and a poor prognosis for sepsis patients. Overall, these findings may help establish C5aRs expression levels as early markers to predict the severity of sepsis.

Figure 1

The expression of C5aR and C5L2 on neutrophils in sepsis patients. (a) The percentages of neutrophils, defined as CD66b-positive leukocytes in HC, SIRS and sepsis patients. (b) Representative dotplots show the expression of C5aR on CD66b-positive cells in patients and health controls. (c) The pooled data of the MFI of C5aR expression on CD66b-positive cells in HC, SIRS and sepsis patients. (d) Representative dotplots show the expression of C5L2 on CD66b-positive cells in patients and health controls. (e) The pooled data of the percentage of C5L2 expression in HC, SIRS and sepsis patients. (f) The pooled data of the MFI of C5L2 expression in HC, SIRS and sepsis patients. (g) Differential expression of C5L2 on CD66b-positive cells in blood and fluid from sepsis patients. Each dot represents an individual. *P<0.05; **P<0.01. C5aR, complement 5a receptor; HC, healthy control; MFI, mean fluorescence intensity; SIRS, systemic inflammatory response syndrome.

Figure 2

Reductions in C5aR and C5L2 expression are positively associated with the severity of sepsis. (a–c) The C5aR and C5L2 levels were correlated with the SOFA score of septic patients. (d and e) The C5aR and C5L2 levels were associated with mortality in septic patients. Each circle represents an individual. *P<0.05; **P<0.01. C5aR, complement 5a receptor; SOFA, sepsis-related organ failure assessment.

Figure 3

C5aR levels are prognostic markers for survival in sepsis patients. (a) Receiver-operating characteristic curve analysis of C5aR MFI to predict overall survival in sepsis patients. (b) Receiver-operating characteristic curve analysis of C5L2 percentage to predict overall survival in sepsis patients. (c) Receiver-operating characteristic curve analysis of C5L2 MFI to predict overall survival in sepsis patients. C5aR, complement 5a receptor; MFI, mean fluorescence intensity.

Figure 4

The dynamics of C5aR and C5L2 expression on neutrophils in patients at various phases of disease. (a) The increased C5aR expression on CD66b-positive cells was correlated with improvement in sepsis patients. (b and c) The increased C5L2 expression on CD66b-positive cells was correlated with improvement in sepsis patients. Each circle represents an individual. *P<0.05; **P<0.01. C5aR, complement 5a receptor.

Figure 5

Changes in C5aR and C5L2 expression on neutrophils in response to C5a in vitro. (a) Representative dotplots show the expression of C5aR on CD66b-positive cells. (b) Representative dotplots show the expression of C5L2 on CD66b-positive cells. (c) The pooled data of the C5aR expression on CD66b-positive cells when exposed to C5a in vitro. (d) The pooled data of the C5L2 expression on CD66b-positive cells when exposed to C5a in vitro. Data indicate the means and standard deviations. **P<0.01. C5aR, complement 5a receptor.

Figure 6

Neutrophils from septic patients exhibit a reduced ability to produce IL-8 when exposed to C5a. (a) Representative dot plots show the co-expression of CD66b and IL-8 in neutrophils in vitro. (b) The pooled data of IL-8 expression on CD66b-positive cells in HC, SIRS and sepsis patients when exposed to C5a. **P<0.01. C5a, complement 5a.