New biomarkers for sepsis

XIE Li-xin

Sepsis is the most important cause of morbidity and mortality in the intensive care unit (ICU), but it lacks specific clinical manifestations. As a result, sensitive and specific indicators of infection that can be easily collected and that accurately reflect infection severity and prognosis are anxious to have and are clinically important. Currently, commonly used clinical indicators of infection include pyrexia, white blood cell (WBC) counts, C-reactive protein (CRP), and procalcitonin (PCT). However, in clinical settings, the limited application of CRP and PCT in assessing the severity and in predicting the prognosis of sepsis may affect the evaluation of the change in septic patients' general condition. Therefore, looking for new biomarkers with high sensitivity and speci f icity is one of the main research f ields in sepsis[1-2].

1 Promising biomarkers

1.1 TREM-1 Triggering receptor expressed on myeloid cells-1 (TREM-1), expressing on polymorphonuclear granulocytes and mature monocytes, is a recently discovered member of the immunoglobulin superfamily. Bacterial or fungal infections may induce the expression of TREM-1. Soluble TREM-1(sTREM-1) is a soluble form of TREM-1 that may be released into body fluids upon the up-regulated expression of TREM-1[3]. An increasing number of studies indicated that there were increased level of sTREM-1 in body fluid samples obtained from following diseases and/or conditions, such as sepsis, pneumonia, pleural effusion, septic arthritis,meningitis, peritonitis, and uterine cavity infection[4-6]. This suggests that sTREM-1 may be a valuable diagnostic indicator for making distinctions between infectious and non-infectious diseases. It has also been found that septic shock patients had higher level of serum sTREM-1 which was closely related to the severity of infection, and sTREM-1 had a excellently positive correlation with the Sequential Organ Failure Assessment (SOFA) score[7-8]. With regard to sepsis prognosis, dynamic changes in serum sTREM-1 may provide warnings concerning death of patients[1,9]. Additionally, urine sTREM-1 is more sensitive than WBC counts, serum CRP,and serum PCT for early diagnosis of sepsis, as well as for dynamic assessment of severity and prognosis. It also provides an early warning of possible secondary acute kidney injury(AKI) in sepsis patients[10]. In terms of diagnostic value for ventilator-associated pneumonia (VAP), the combination of sTREM-1 and Clinical Pulmonary Infection Score (CPIS) improve the ability to diagnose VAP. Moreover, researchers by using logistic regression analysis indicated that sTREM-1 was an independent risk factor for VAP[11].We alsofound that sTREM-1 was of no use in determining bacteremia-caused, new fever in ICU patients, but sTREM-1 levels were cor related wi th the prognosis of pat ients wi th bacteremia[2].

Due to the superior value of sTREM-1 in diagnosis of sepsis and its prognosis as discussed above, we speculate the changes of TREM-1 gene may also af fect i ts protein expression.

If this assumption does exist, single nucleotide polymorphism (SNP) of TREM-1 may be of great use in early prognostic assessment.

We sequenced four exons of TREM-1 gene on 79 patients with sepsis and 80 healthy subjects and obtained a very interesting result. We found that TREM-1 gene locus rs2234237 was associated with prognosis and influenced survival time of patients with sepsis. If this locus changed in patients with sepsis, the incidence of poor prognosis would greatly increase[12]. This finding not only explains the mechanism underlying the increase of sTREM-1 in patients with poor prognosis, but also confirms that screening the gene polymorphism markers of sepsis is a new f ield and wor th studying.

1.2 CD163 CD163 is a transmembrane molecule, hitherto only discovered on the membrane of mononuclear phagocytes.As a specific scavenger receptor for hemoglobin and heme inside the body, it is capable of specific recognition of the hemoglobinhaptoglobin complex. Soluble CD163 (sCD163) comes f rom CD163 molecules that peel of f the membrane of mononuclear cells[12-14]. Blood levels of sCD163 have prognostic value for several inflammatory diseases and may be useful in clinical applications as a biomarker for inflammatory diseases. Our prospective and clinical practice has already confirmed that the serum sCD163 level might have potential value for the diagnosis of sepsis and severe sepsis, and its performance is superior to PCT and CRP levels.sCD163 also has advantages for dynamically monitoring pyemic development and prognosis and has favorable prospects in clinical application[15]. In a previous study, we compared sTREM-1, sCD163 and other clinical parameters for their assessment value for sepsis. On the day of ICU admission, the sepsis group displayed higher level of serum sTREM-1, sCD163, PCT and CRP than the Systemic Inflammatory Response Syndrome (SIRS) group did (P＜0.05). Although PCT, sTREM-1 and SOFA score were identified as good markers identifying the severity of sepsis, sTREM-1 was the most reliable one among these 3 markers. In addition, we have also detected the value of sCD163 in urine of sepsis patients. The results suggested that urine sCD163 may be valuable in the diagnosis of sepsis and in sepsis-associated AKI, and determination of prognosis of these patients[16]. As a noninvasive detection index, urine sCD163 levels may have potential clinical value, although this remains to be proved by multi-center study with a larger c l inical sample size.

1.3 microRNAs MicroRNAs (miRNAs) are a type of endogenous non-coding smal l RNAs that are about 22 nucleotides in length[17-18]. They play important biological roles by inhibiting the expression of messenger RNAs (mRNAs)[19]. As with mRNAs,some miRNAs are dif ferentially expressed among tissues or developmental stages. Unlike some widely expressed miRNAs,these tissue- or developmental stage-specific miRNAs are likely to play key roles in regulating specific processes involved in the development and function of individual tissues[20]. A significant amount of miRNAs has been observed in various body fluids, and outside of the cells. These cell-free miRNAs in body fluids are stable under harsh conditions such as boiling, acidic environment(low pH) or basic environment (high pH), and multiple freeze-thaw cycles[21-22]. Circulating miRNAs have been recently identified as biomarkers for sepsis. The level of miR-150, firstly identified as a prognostic marker for sepsis significantly varied in leukocytes between healthy controls and sepsis patients. In the plasma of sepsis patients, the level of miR-150 is correlated with the level of SOFA score, and the plasma level ratiofor miR-150/interleukin-18 can be used to evaluate severity of sepsis[23]. Sepsis is a complex disease that involves various tissues and organs. A simple screen for miRNAs differentially expressed in leukocytes may miss many miRNAs secreted by other type of cells. Hence, a genome-wide method is applied to screen for differentially expressed miRNAs between the surviving and non-surviving groups of sepsis patients. According to the results, two novel prognostic biomarkers,miR-297 and miR-574-5p, are identified by microarray screening and quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR). Moreover, miR-297 is found to be more closely correlated with survival from sepsis, whereas miR-574-5p is correlated with death from sepsis. Although being identified as diagnostic biomarkers for sepsis, the expression level of these markers are not only found to be differentially expressed between sepsis patients and healthy controls, but also between sepsis patients and SIRS pat ients.

The levels of miR-146a and miR-223 in sepsis patients' sera significantly decreased when compared with that in SIRS patients and healthy controls, which are of higher sensitivity than IL-6 in the diagnosis of sepsis[24-25]. miR-15a and miR-16 are also newly identified as diagnostic markers for sepsis. The levels of these 2 miRNAs in sepsis and SIRS patients are both significantly higher than that in normal controls. And miR-15a may serve as marker to distinguish sepsis patients from SIRS patients, which is more valuable compared with CRP and PCT[26]. Although the function of these miRNAs has not been completely elucidated, their clinical value has been confirmed. New biomarkers also mean novel treatment targets. Hence, target genes of these miRNAs may emerge as potent ial t reatment targets for sepsis pat ients.

2 Emerging technologies: proteomics, metabolomics and t rans-omics

Recently, mass spectrometry (MS) technologies have made considerable progress and been used to search for new biomarkers.A large number of differentially expressed proteins and small molecule metabolites have been identified and reported as potential biomarkers for diagnosis and prognosis of several different diseases. Some researchers have put MS technique toward to unraveling new discoveries in the field of critical care medicine and draw some interesting conclusions[27]. Our results of serum proteomics of septic patients indicated that the most important changes in the coagulation-bradykinin and the complement system were noticed when sepsis occured. The urine proteomics results suggested that cadherin-1 (CDH-1), haptoglobin (HPR), complement3 (C3),alpha-1-antitrypsin (SERPINA1) and ceruloplasmin (CP) may reflect the occurrence of sepsis; selenium binding protein-1 (SBP-1), heparan sulfate proteoglycan-2 (HSPG-2), alpha-1-B glycoprotein (A1BG), haptoglobin (HPR), lipocalin (LCN), lysosomeassociated membrane proteins-1 (LAMP-1) and dipeptidyl peptidase-4 (DPP-4) may be able to predict the prognosis of sepsis. In addition, the results of metabolomics study prompted that glycerophospholipid metabolic products and arachidonic acid metabolites were the most important stuffs in the occurrence and development of sepsis. Currently, the results aforesaid are still under subsequent analysis and exper imental ver i f icat ion.

With constant development of high-throughput technologies and improvement of the systems biology, it has been increasingly recognized that we can not simply take from only one perspective to learn and explain a complex biological phenomenon and essence. Therefore, the trans-omics have been put forward to solve this problem. Trans-omics is to explore how to integrate the genome, transcriptome, proteome and metabolomics data together and explain the function, pathogenesis of the disease and their relevant mechanisms. More and more researchers intend to combine the results of exon sequencing, miRNA sequencing, proteomics and metabolomics studies in order to elucidate the molecular mechanisms of sepsis. In theory, new discoveries and ideas in the pathogenesis of sepsis could be put forward wi th the help of the analysis methods of t rans-omics.

3 Conc lusion

In conclusion, in search of new biomarkers for determining the severity of sepsis patients and predicting the prognosis is a very impor tant, interest ing and chal lenging work, which may provide new insights to conf ront sepsis.

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(Edi ted by SHEN Ning, ZHANG Jin-tong)