Septin: a factor in plasma that opsonizes lipopolysaccharide-bearing particles for recognition by CD14 on phagocytes.

Abstract

SnmmJaryWe have previously reported that lipopolysaccharide (LPS) binding protein (LBP) opsonizes endotoxin (LPS) for recognition by CD14 on phagocytes. Here we show that normal human plasma contains high titers of an activity that also binds LPS (Re, 595) and mediates recognition by CD14. Opsonization of LPS-coated particles with plasma enables the particles to be bound by phagocytes. Further, opsonization with plasma also enables subnanogram-per-milliliter concentrations of LPS to induce dramatic alterations in the function of leukocyte integrins on polymorphonudear leukocytes and to induce secretion of tumor necrosis factor by monocytes, suggesting that opsonization by factors in plasma may be important in responses of cells to endotoxin. The opsonic activity in plasma appears distinct from LBP since it is not blocked by neutralizing antibodies against LBP. Surprisingly, the opsonic activity of plasma is not present in a single protein species, but at least two species must be combined to observe activity. Further, the opsonic activity of plasma for LPS is blocked by addition of protease inhibitors, suggesting that proteolytic activity or activities are required for opsonization. These properties are suggestive of the action of a protease cascade, but opsonic activity of plasma is not affected by blockade or depletion of either the complement or dotting cascades. We propose the name "septin" to describe this novel LPS-opsonizing activity in plasma.ukocytes recognize bacterial LPS (endotoxin) through several mechanisms (1). CD18 antigens (2) and the scavenger receptors (3) on leukocytes have been shown to bind particulate and dispersed LPS, respectively. This binding may result in uptake and degradation of LPS but is not associated with stimulation of the ceUs (3, 4). We have recently described a different means of recognizing LPS in which a soluble protein, rather than a membrane-bound molecule, performs the initial binding to LPS. LPS binding protein (LBP) 1 binds to both dispersions of LPS (5) and to LPS on the surface of particles (6). The resulting LPS/LBP complexes are recognized by the receptor CD14 on PMN (7), monocytes, and macrophages (MO) (8). "Opsonization" of bacteria with LBP enables phagocytes to bind and phagncytose the particles (6) in a CD14-dependent fashion (S. D. Wright, unpublished observations), thus resulting in clearance of LPS.CD14 may also initiate the dramatic responses of leukocytes to picogram-per-milliter concentrations of endotoxin. PMN show the most rapid responses with priming for enhanced oxidative activity (9) and enhanced activity of leukocyte integrins (7) observed after 15-30 min of exposure to LPS. Both of these responses of PMN require LBP (7, 10) or serum (7, 11), and both can be blocked by anti-CD14 mAbs (7, and M. J. Pabst, personal communication). Monocytes and MO respond to LPS by synthesizing cytokines such as TNF, Ibl, and IL-6 within 1-2 h. TNF production by rabbit MO in response to sub nanogram-per-milliliter doses of LPS requires LBP (12), and blockade of CD14...

Ascaris suum-Derived Products Induce Human Neutrophil Activation via a G Protein-Coupled Receptor That Interacts with the Interleukin-8 Receptor Pathway

“…Treatment of ABF with polymyxin B-Sepharose did not affect its efficiency as a priming agent. Furthermore, priming of neutrophils by LPS is strongly dependent on the presence of mediators found exclusively in serum or plasma (49) (e.g., LPS-binding protein [69] and septin [75]), which were absent in our assays. We therefore think that putative bacterial contaminants cannot be responsible for the strong priming of neutrophils by ABF.…”

Lipopolysaccharide binding protein and soluble CD14 catalyze exchange of phospholipids.

“…As a consequence, very little LPS associates with cells in proteinfree buffers, and responses generally require very high concentration of LPS (micrograms per milliliter to milligrams per milliliter). The amount of LPS needed to produce a cellular response is reduced by up to three logs by the addition of plasma, serum or serum proteins (1,2). Two serum proteins, LPS binding protein (LBP) 1 and soluble CD14 (sCD14), have been shown to play a role in enhancing responses to LPS (3,4), and work from our laboratory has established that these two proteins act to transport individual LPS molecules (5-7).…”

Induction and characterization of endotoxin tolerance in equine peripheral blood mononuclear cells in vitro

“…LPS is then bound by phagocytes and induces dramatic alterations in the function of leukocyte integrins on polymorph nucleated cells and secretion of TNF-α by monocytes. Septin activity is blocked by addition of protease inhibitors since proteolytic activities are required for opsonization by septin, whereas the function of LBP is not affected (Wright et al 1992). Septin appears to be important for cellular responses to low concentrations of endotoxin that occur in blood during sepsis (LPS < 0.1 ng/ml) (Wright et al 1992).…”

“…Septin activity is blocked by addition of protease inhibitors since proteolytic activities are required for opsonization by septin, whereas the function of LBP is not affected (Wright et al 1992). Septin appears to be important for cellular responses to low concentrations of endotoxin that occur in blood during sepsis (LPS < 0.1 ng/ml) (Wright et al 1992). There are no studies that have evaluated the role of septin in horses.…”