Discussion
Over the past years, the chemokine has emerged to be an important inflammatory mediator of various human diseases. In the present report, we identified CXCL16 to be centrally involved in the pathogenesis of sepsis. We made the following key observations: I. Sepsis patients had a marked and sustained elevation of soluble CXCL16. II. Admission levels of soluble CXCL16 levels correlated with disease severity and mortality in human patients with sepsis. III. CXCL16 was found to be an independent predictor of 28-day mortality in the patients with sepsis on day of ICU admission. IV. CXCL16 supplementation in the presence of sepsis could worsen outcome of CLP-induced nonsevere sepsis. V. Neutralization of the CXCL16 activity with antibody against CXCL16 could produce substantial survival benefit in CLP-induced severe sepsis.
It has been suggested that soluble CXCL16 could serve as a reliable marker of inflammation. 16 Indeed, soluble levels of serum CXCL16 were correlated significantly with the disease activity of patients with inflammatory bowel disease (IBD) [22]. Consistent with this pro-inflammatory profile, soluble CXCL16 levels were independently associated with coronary atherosclerosis, particularly acute coronary syndromes. 16 In patients with an acute coronary syndrome, CXCL16 levels obtained within 24 hours of admission are associated with long-term mortality after adjustment for other risk factors [23]. Furthermore, admission level of CXCL16 is independently related to adverse clinical outcomes in patients with an acute coronary syndrome, mainly driven by an association to cardiovascular death [18]. Therefore, CXCL16 measurement may enhance risk stratification in patients with acute coronary syndrome. However, to our knowledge CXCL16 has never been evaluated in the patients with sepsis. This study revealed dramatically increased soluble CXCL16 levels in septic patients on day of ICU admission as compared to healthy controls. Further, we observed that admission levels of soluble CXCL16 significantly differed between survivors and non-survivors in the patients with sepsis. Moreover, admission levels of CXCL16 were positively correlated with SOFA score and different laboratory outcome parameters such as WBC, CRP, IL-6, IL-8, IL-27 and TNF-α. We therefore tested for an additive prognostic value of soluble CXCL16 levels in septic patients. We demonstrated, for the first time, the association between increased admission levels of soluble CXCL16 and higher risk of death in the patients with sepsis. Although the AUC of CXCL16 was lower than that of SOFA score in predicting 28-day mortality, it was superior to PCT and CRP for predicting 28-day mortality in septic patients. In the logistic regression analysis, CXCL16 was found to be independent predictors of 28-day mortality in septic patients. These findings suggest that CXCL16 might be added to the list of risk markers that could identify a group of septic patients presenting with higher risk of mortality beyond that of other risk indicators, such as soluble ST2 [24], presepsin [25], soluble IL-7 receptor [26], and apoptosis inhibitor of macrophage (AIM) [27].
CXCL16 up-regulation has been shown to participate in the inflammatory response elicited during bacterial and viral infections, which was associated with either beneficial or detrimental effects [6, 7]. To further investigate the role of CXCL16 in the pathogenesis of sepsis, we established a clinically relevant murine model of sepsis induced by CLP. Consequently, we found that supplementation with recombinant CXCL16 decreased survival and increased tissue inflammation and tissue injury in mice after non-severe CLP, while neutralization of CXCL16 resulted in substantial survival benefit after severe-CLP. Interestingly, CXCL16 is dispensable for controlling bacterial infection in CLP-induced polymicrobial sepsis. These results are consistent with a previous study showing that neutralization of CXCL16 resulted in reduced cerebrospinal fluid pleocytosis in an animal model of pneumococcal meningitis, but it did not influence bacterial outgrowth in the central nervous system irrespective of the time of administration [28]. In a murine model of collagen-induced arthritis (CIA), treatment with anti-CXCL16 monoclonal antibody has also been shown to significantly reduce the clinical arthritis score and reduce infiltration of inflammatory cells and bone destruction in the synovium of mice with CIA [9]. Neutralizing CXCL16 by injection of anti-CXCL16 antibody ameliorated the extent of hepatic inflammation and steatosis development in experimental metabolic injury [29]. Furthermore, administration of anti-CXCL16 antibody could ameliorate colonic inflammation in the experimental model of colitis [22]. In fact, sepsis was associated with a complex network of pro- and anti-inflammatory modulators, the balance of which determines the outcome of the sepsis. Taken together, sepsis was associated with elevated production of CXCL16, and CXCL16 had a limited antibacterial role in sepsis. However, CXCL16 could cause aberrant inflammatory response (increase of leukocyte infiltration including monocytes/macrophages, neutrophils and lymphocytes, and elevation of inflammatory cytokines/ chemokines including IL-6, TNF-α, KC and IL-27), resulting in exaggerated inflammation, which can cause tissue injury and increased lethality. Therefore, targeting CXCL16 might therefore represent a promising novel therapeutic approach for the treatment of sepsis.
Some limitations of this study should be noted. First, this is a single-center study with a limited sample size, and our results should be validated in a larger ideally multicentric study. Second, the AUC of CXCL16 was lower than that of SOFA score in predicting 28-day mortality of septic patients, suggesting that incremental further studies are required to investigate whether the combination of CXCL16 with other risk biomarkers might provide additional prognostic value in septic patients. Finally, further investigation using CXCL16 knockout mice is required to elucidate the underlying molecular mechanisms by which CXCL16 amplifying local and systemic inflammation during sepsis.
Collectively, we here firstly establish that human septic patients had a stable and sustained elevation of soluble serum CXCL16 levels, which correlate with disease severity and mortality. Measuring admission levels of soluble serum CXCL16 could represent a promising biomarker for risk stratification and evaluation of prognosis in the patients with sepsis. In complementary experimental studies, administration with recombinant CXCL16 could aggravate nonsevere sepsis, while anti-CXCL16 treatment could ameliorate severe sepsis. Targeting the chemokine CXCL16 is a promising strategy in the treatment of sepsis. Taken together, CXCL16 emerges as a promising biomarker for disease severity and as a potential target for therapeutic intervention in human sepsis.