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Proteomic analysis of the pentraxin 3 (PTX3) complex circulating in the blood of patients with sepsis—a potential new marker for sepsis

The proteomic profile of circulating pentraxin 3 (PTX3) complex in sepsis demonstrates the interaction with azurocidin 1 and other components of neutrophil extracellular traps.
Daigo K, Yamaguchi N, Kawamura T, Matsubara K, Jiang S, Ohashi R, Sudou Y, Kodama T, Naito M, Inoue K, Hamakubo T.
Mol Cell Proteomics. 2012 Jan 25. [Epub ahead of print]
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Figure 1: Flowchart showing proteomic analysisPentraxin 3 (PTX3) is a secretory protein that plays an important role in innate immune responses as a soluble pattern-recognition receptor.*1 PTX3 is almost nonexistent in the blood under normal conditions; however, it appears in the blood when inflammation or various types of infection occurs, and it is considered to recognize certain bacteria. In sepsis, in particular, the level of PTX3 in the blood is known to be about several hundred-folds (~200 ng/mL) higher than the normal level. In addition, artificial expression of PTX3 is known to increase survival in septic mice; however, the exact mechanism has not been sufficiently clarified. To elucidate the role of PTX3 in sepsis, we have analyzed proteins that bind PTX3 in the blood of patients with sepsis.

Figure 2: Proteins present in the PTX3 complex circulating in the blood of patients with sepsisWe first isolated PTX3 complexes from the blood of patients with sepsis by using anti-PTX3 antibody-bound magnetic beads and then identified the complexes by shotgun proteomics for relative quantitation by using a high-sensitivity mass spectrometer system (Figure 1). After detailed analysis, we discovered that component proteins of neutrophil extracellular traps (NETs)*2 are contained in the proteins identified (Figure 2).

We further continued our analysis focusing on the formation of the complex of PTX3 and NET component proteins discovered by proteomic analysis and newly found that PTX3 was actually bound to the NET component proteins azurocidin 1 (AZU1) and myeloperoxidase (MPO). We clarified the detailed mechanism of binding of PTX3 with AZU1, in particular, and determined that PTX3 and AZU1 are co-localized (Figure 3) in NETs.

Figure 4: Composition of the PTX3 complex inside NETs (image)Figure 3: Co-localization of PTX3 and AZU1 in NETs On the basis of these results, we have proposed the following novel host-defense mechanism of PTX3 in sepsis: PTX3 is bound with antibacterial proteins, such as AZU1 and MPO, inside NETs and plays a role in recognizing bacteria and enhancing bactericidal efficiency (Figure 4). Furthermore, the levels of PTX3–AZU1 and PTX3–MPO complexes in the blood are expected to serve as novel markers for the diagnosis or treatment planning of sepsis.

This study was conducted in collaboration with Juntendo University Nerima Hospital, Niigata University, Perseus Proteomics Inc., and JSR Corporation. 


*1 Pattern-recognition receptors is a general term for proteins functioning as part of the innate immune system. They recognize bacteria-specific molecules or signal molecules, which are released by the host in response to an infection and activate innate immune responses. Generally, they can recognize more than one class of molecules. For further details, please refer to the following review (PubMed link)

*2 NETs are secretory materials produced by neutrophils, and they resemble a net. They consist of DNAs and antibacterial proteins released from neutrophils, which work to protect against bacterial infection. NETs function by capturing and killing bacteria. PTX3 is a component protein of NETs. For further details, please refer to the review (PubMed link)