In this study, we identified significant differential expression of peptides between sepsis patients, healthy individuals, and individuals with general infections through plasma peptidomics analysis. These differences were primarily observed in processes such as humoral immune response, complement, and coagulation cascades. Among these, peptides related to Serum Amyloid A1 (SAA1) exhibited the most notable changes.
The Serum Amyloid A (SAA) gene family comprises four distinct loci: SAA1 and SAA2 encode acute-phase SAAs (A-SAAs), while SAA3 and SAA4 encode other types of SAA proteins. A-SAA is a crucial acute-phase reactant that plays a significant role in the immune response, including the induction of cytokine production, promotion of leukocyte migration and adhesion, and activation of the complement system[16, 17]. A-SAA can activate the nuclear factor-kappa B (NF-κB) signaling pathway by binding to Toll-like receptors (TLR) 2 and TLR4, thereby inducing cytokine production and participating in the inflammatory response[18].
Studies have shown that combining SAA1 with other inflammatory markers can enhance the diagnostic accuracy for sepsis. For example, a predictive model combining SAA1, C-reactive protein (CRP), and procalcitonin (PCT) achieved an area under the receiver operating characteristic (ROC) curve (AUC) of 0.89 [95% CI 0.82–0.95] for diagnosing sepsis, significantly outperforming the individual predictive values of CRP, PCT, and interleukin-6 (IL-6) [19]. In neonatal sepsis, SAA has also demonstrated good diagnostic and monitoring value[20]. Moreover, SAA1 can directly bind to pathogens, such as Gram-negative bacteria, thereby enhancing the phagocytic and bactericidal activities of macrophages and neutrophils[18]. Research has also found that SAA has a protective role in lipopolysaccharide (LPS)-induced inflammation and tissue damage by binding to LPS and promoting its clearance[21].
SAA1 is an important inflammatory marker that reflects the degree and activity of inflammation. SAA1 levels rise rapidly upon infection or injury and decrease quickly after antigen clearance, making it valuable for early diagnosis, risk assessment, treatment monitoring, and prognostic evaluation in clinical practice. Our study found that SAA1-related peptides were significantly elevated in children with sepsis, distinguishing them from healthy and generally infected patients, with the peptide RSFFSFLGEA showing the most significant difference. This finding further supports the diagnostic value of SAA1 in pediatric sepsis.
The complement system is a part of the immune system that initiates inflammatory responses by recognizing pathogens and damaged cells. Activation of the complement system can lead to cell lysis, release of inflammatory mediators, and recruitment of immune cells. Complement component 3 (C3) is a key protein in the complement system, involved in pathogen marking and clearance, as well as in regulating the inflammatory response[22]. In this study, we found that peptides related to C3 and Complement Factor B (CFB) were significantly elevated in children with sepsis, with the peptides IITPNILRLESEE and IEGVDAEDGHGPGEQQ showing the most notable differences. The activation of the complement system not only enhances the immune response but may also exacerbate inflammation, leading to more severe pathological states.
Research has revealed that the Carboxypeptidase B1 (Cpb1)-C3-C3a receptor (C3aR) pathway significantly impacts the severity of endotoxemia. Deficiency of C3aR or the use of C3aR inhibitors can reduce the expression of inflammatory mediators and weight loss in mice with endotoxemia, thereby improving their survival rates. C3aR plays an important role in primary human macrophages by enhancing the inflammatory response, and its expression is associated with increased caspase-5 expression in peripheral blood mononuclear cells of sepsis patients. The complement pathway amplifies caspase-11-dependent cell death through the Cpb1-C3-C3aR signaling pathway, which is crucial for the severity of Gram-negative bacterial infections and endotoxemia. Thus, C3aR may serve as an early therapeutic target for sepsis, and C3aR and caspase-5 transcripts could be potential biomarkers for diagnosing sepsis[23].
In addition, this study found that hemoglobin (Hb)-related peptides, specifically those related to hemoglobin subunit beta (HBB) and hemoglobin subunit gamma-2 (HBG2), were significantly elevated in pediatric sepsis patients. Among these, the peptides WGKVNVDEVGGEALGRLL and GHFTEEDKATITSL exhibited the most notable differences. The destruction of erythrocytes induced by infection may be an important early marker event in sepsis.
Erythrocyte destruction leads to elevated levels of free hemoglobin (free-Hb). Verma et al. found through Raman spectroscopy that free-Hb was observed in a lipopolysaccharide (LPS)-induced endotoxin shock model, but not in a thioglycolate-induced sterile peritonitis model. This study suggests that hemolysis is a marker of systemic inflammation rather than local inflammation[24]. The onset of systemic inflammatory response is an early event in sepsis; hence, hemolysis-related markers induced by infection have potential value for the early diagnosis of sepsis. Additionally, free-Hb plays an important role in the progression of sepsis. Elevated levels of free-Hb increase mortality and lung injury in septic shock animals. The mechanism of action of free-Hb may be related to the generation of a cytokine storm and its nutritional role for bacteria. Free-Hb can significantly elevate the levels of cytokines and chemokines, promoting the occurrence of a cytokine storm. Free-Hb may also provide iron that promotes bacterial growth, further exacerbating the condition[25, 26].
Notably, our previous research identified haptoglobin (HP), another important protein involved in hemolysis-related injury, as an independent risk factor for mortality in pediatric sepsis[27]. HP can specifically bind to free-Hb, facilitating its transfer to the liver for catabolism, thus mitigating the toxic effects of free-Hb and inhibiting bacterial growth[26, 28]. HP also binds to high mobility group box 1 protein (HMGB1) and mediates the production of interleukin-10 (IL-10) and heme oxygenase-1 (HO-1) through the CD163 signaling pathway, thereby reducing inflammation and oxidative stress[29]. Consequently, HP is known as a natural scavenger of hemolytic toxic effects within the body. In this study, we found that HP-related peptides were significantly elevated in sepsis, with the peptide YVGKKQLVE showing the most significant difference. This finding further supports the evaluative value of HP in sepsis severity.