Biomarkers of early sepsis may be correlated with outcome

Sepsis is one of main causes of death in intensive care units (ICUs), and estimates of its prevalence range from 25% to 70% [1–5]. Sepsis often exhibits two stages concomitantly; systemic inflammatory response syndrome (SIRS) releases inflammatory cytokines, and the compensatory anti-inflammatory system (CARS) raises the concentration of anti-inflammatory mediators [6, 7]. This overwhelming inflammatory response leads to early mortality and tilts the precise balance of inflammation and anti-inflammation towards recovery, and excessive anti-inflammation often leads to secondary infections [6, 8]. The latter is known as immunoparalysis [9]. Cytokine production precedes the expansion of CD14⁺CD16⁺ monocytes [10]. The levels of IL-8, IL-6, and monocyte chemoattractant protein-1 (MCP-1) are associated with early 48-hr and 28-day mortality in sepsis patients [11]. The duration of immunoparalysis is not known; therefore, sepsis management strategies do not differ for patients with the late death or recovery outcomes because prognostic markers for the early stages are not available. However, if these outcomes could be predicted at an early stage, patient management could be tailored to reduce late mortality.

We hypothesized that survivors would have a weaker pro-inflammatory response than non-survivors, while mid-term survivors (which acquire secondary infections) would have a more pronounced anti-inflammatory response (making them susceptible to infection). Therefore, the plasma levels of cytokines and lymphocyte subpopulations from cases of early death, mid-term survivors, and long-term survivors were measured in the early phases of severe sepsis to verify this hypothesis.

The plasma levels of MCP-1, IL-6, and IL-8 in the early death and survival groups were significantly different during the early stages of the sepsis episode. The surviving patients recovered from this infection, while the others died from a secondary infection. The plasma levels of IL-6, IL-8, and MCP-1 in these two groups of patients were significantly different; this is a novel finding of the study. Our data support the hypothesis that sepsis survivors have a weaker pro-inflammatory response than non-survivors and reject the hypothesis that mid-term survivors have a more pronounced anti-inflammatory response. The duration of immunosuppression after SIRS is not known. One MTSG case died on day 176, and one case was hospitalized for more than six months. Therefore, the recovery group was designated as those patients who were healthy at the time of discharge from the hospital and lived longer than six months after sepsis.

A finely tuned balance between pro- and anti-inflammatory events is a prerequisite for better prognosis in sepsis. T-helper (Th)1 response predominates after microbial infection, which activates cells to directly clear an infection. Th2 cytokine secretion is then induced to resolve the pro-inflammatory response and to stimulate a humoral response. However, during sepsis, a Th2 response causes the dysregulation of the cellular immune response instead of resolving infection. Th2 cytokines inhibit the Th1 response, and vice versa. IL- 10, a potent anti-inflammatory cytokine, has a strong suppressive effect on monocytes/macrophages, dendritic cells, neutrophils and T cells [14]. In this study, the plasma levels of IL-10 were not significantly different in the early death group versus the survivors or in the MTSG versus the LTSG. Higher levels of IL-10 were present in the MTSG on day 0, which implies that the MTSG was experiencing immunosuppression. Although the levels of IL-6, IL-8, and MCP-1 in the MTSG and LTSG were significantly different in the early phase, the MTSG patients survived until the secondary infection. High levels of IL-8 and MCP-1 would attract, activate, and promote neutrophil and monocyte migration toward the site of inflammation, as well as to the remote organs. Excessive neutrophil and monocyte infiltration exaggerates inflammation and severe organ injury by releasing pro-inflammatory mediators, such as IL-6, which can lead to shock, multiple organ failure, and even death [15].

Our results indicate that IL-6, IL-8, and MCP-1 were much more potent than IL-10 in promoting death due to a secondary infection. Factors other than IL-10 may be more effective in inhibiting inflammation in the early phase of sepsis. IL-7 has anti-apoptotic properties and induces the proliferation of CD4⁺ and CD8⁺ naïve T cells [15]. Because lymphocyte apoptosis has been reported in sepsis, the level of IL-7 was expected to be higher in the survival group and the LTSG. However, the levels of IL-7 were higher in both the death group and the MTSG, which conflicts with our expectations. A further investigation of this effect is desired.

A distinctive drop in the proportion of CD3⁺ lymphocytes, particularly CD8⁺ lymphocytes in the death group on day 3, was observed in this study. The level of CD19⁺ B lymphocytes in the death group was elevated on day 3; this may be due to the decrease in the proportion of both CD4⁺ and CD8⁺ lymphocytes. CD8⁺ lymphocytes were most likely more fragile and eliminated more on day 3. These effects also require further investigation.

No comparisons of the lymphocyte subpopulations of patients who survive or died due to sepsis or between MTSG and LTSG were conducted in previous studies. In one example, the leukocyte subpopulations of 8 cases of pneumonia-derived sepsis (PDS) with a 37.5% survival rate and 14 cases of intra-abdominal sepsis (IAS) with a 35.7% survival rate were compared to those from normal controls over the course of 4–6 days after the onset of sepsis. Lymphocyte levels were diminished compared to controls in both types of sepsis, and a marked drop in CD3⁺CD8⁺ lymphocytes was observed [16]. Another study reported that the levels of CD3⁺CD4⁺ and CD19⁺ lymphocytes and the CD4⁺/CD8⁺ T cell ratio were significantly lower in 25 septic shock non-survivors (P < 0.01) compared to 27 survivors on the day of hospitalization; however, there was no difference in the proportion of CD3⁺CD8⁺ T lymphocytes between non-survivors and survivors [17]. A discrimination of the MTSG and LTSG was not conducted in their study. The zero time point was also restricted in our study, whereas previous authors defined day zero as the day of hospitalization; one drawback of this approach is that the onset of sepsis may not be clear. The number of cases included in this study was reduced due to our strict definition of the zero point to within 12 hours after the first organ failure due to sepsis. More cases will be needed to validate these results.

One of our inclusion criteria in the present study is that patients were recruited with 12 hours after the first organ failure due to sepsis as the zero time point. It caused that we were not able to recruit enough patients. Thus, the main limitation of the present study is the small sample size. In the future studies, more patients will be needed for validation.

The presence of high levels of MCP-1, IL-6, IL-8, and IL-10 in the death group in our study indicated an imbalance of inflammation and anti-inflammation; this imbalance promotes overwhelming inflammation, leading to death. The levels of MCP-1, IL-6, and IL-8 in the early death and survival groups and between the MTSG and LTSG were significantly different. Pro-inflammatory cytokines were more prominent in the MTSG.

Our data support the hypothesis that sepsis survivors would have a weaker pro-inflammatory response than non-survivors, whereas mid-term survivors did not have a more pronounced anti-inflammatory response. The levels of pro-inflammatory cytokines were significantly different in mid-term and long-term survivors.