- Email: [email protected]
CD14
PERGAMON
The International Journal of Biochemistry & Cell Biology 31 (1999) 545±549
Molecules in focus
CD14 Christine SchuÈtt * Institut fuÈr Immunologie und Transfusionsmedizin, UniversitaÈt Greifswald, Diagnostikzentrum Sauerbruchstraûe, 17487 Greifswald, Germany Received 7 September 1998; accepted 20 November 1998
Abstract The GPI-anchored 55 kDa glycoprotein CD14 is expressed on monocytes/macrophages and to a lesser extent on granulocytes. Engagement of CD14 by ligands like lipopolysaccharide, intact bacteria or apoptotic cells can result in either pro- or anti-in¯ammatory responses. Since the CD14 molecule does not have a membrane spanning domain it cannot transmit a signal into the cell. Some as yet unidenti®ed accessory protein is thought to be involved. It will be important to clarify the signalling systems involved since they may provide a therapeutic target for sepsis intervention strategies. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Endotoxin receptor; Innate immunity; Sepsis
1. Introduction At the First Leukocyte Typing Conference in 1982 dierent monoclonal antibodies that selectively bound to human monocytes/macrophages were grouped into the cluster of dierentiation No. 14 (CD14). The molecule involved, CD14 (mCD14), therefore serves as a useful marker to identify cells of the monocyte/macrophage lineage. In addition to these cells, activated granulocytes also weakly express CD14. The 55 kDa glycoprotein was identi®ed in 1990 as a receptor for the endotoxin lipopolysaccharide (LPS) [1]. * Tel.: +49-3834-865-470; fax: +49-3834-865-490; e-mail: [email protected].
LPS is a major component of the outer membrane of Gram-negative bacteria. It consists of an O-speci®c chain, a core oligosaccharide and a lipid component, termed lipid A [2]. Immune cells of the host recognise minute amounts of LPS and rapidly mount in¯ammatory defense mechanisms. However an excessive response can induce an overwhelming release of cytokines leading to septic shock.
2. Structure, synthesis and degradation The human CD14 gene consists of two exons and codes for a 356 amino acid protein with multiple leucine-rich repeats. The mature protein is expressed as a glycosyl-phosphatidylinositol
1357-2725/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 7 - 2 7 2 5 ( 9 8 ) 0 0 1 5 3 - 8
546
C. SchuÈtt / The International Journal of Biochemistry & Cell Biology 31 (1998) 545±549
(GPI)-anchored cell surface molecule, lacking a transmembrane domain. Several groups have sought to de®ne the LPS binding site of the CD14 molecule. The results are rather variable and depend on the method used. Among 23 alanine substitution mutants covering amino acids 1±152 of human CD14, only mutation of amino acids 39±44 results in loss of LPS-binding [3]. Little is known about expression or inducible CD14 expression in dierent species in vivo. In mice it was shown that LPS challenge induced a transient increase in CD14 mRNA levels in myeloid cells as well as in epithelium of the liver and kidney. LPS, TNFa and IFNg upregulate CD14 expression on human myeloid cells in vitro whereas IL4 reduces it. CD14 levels can also be regulated by proteolytic cleavage or phospholipase D induced shedding which lead to the release of a soluble form of CD14 (sCD14). In patients suering from paroxysmal nocturnal haemoglobinuria (PNH), a GPI-anchoring defect, direct release of sCD14 is also possible. Normal human serum contains approximately 3±4 mg/ml sCD14. 3. Biological function LPS is bound in plasma to a protein called LPS binding protein (LBP), which facilitates the rapid binding of nano/picomolar amounts of LPS to CD14. Binding of LPS to membrane bound CD14 results in cellular activation: the production of in¯ammatory cytokines, e.g. TNFa, IL1, IL6, IL8, IL12 or oxygen radicals, NO, tissue factor or others, as well as anti-in¯ammatory cytokines like IL10 or TGFb. It has recently become evident that a number of phosphorylation cascades including MAP kinase pathways and the NFkB activation pathway are initiated after exposure of the cells to LPS [4]. Since the GPI-anchored mCD14 is unable to function as a signal transducing molecule some means must exist to transmit the signal into the cell. One possibility is that an accessory protein is involved. Alternatively Joseph et al. [5] favoured the hypothesis that after binding by CD14, LPS will be integrated into the phospholi-
pid layer of the cell membrane. By mimicking ceramide LPS stimulates cells by interacting with membrane bound ceramide-activated protein kinase. Another possible mechanism of signalling is the association of GPI-linked proteins, like CD14, with b2 integrins as `public' transducers for a panel of `private' GPI-linked receptors [6]. Although our knowledge about the activation pathways induced by LPS/LBP-ligated CD14 are limited, the LBP±LPS interactions themselves have been studied in great detail. LBP like CD14 recognises lipid A and LBP seems to be the only serum component which can catalyse LPS activation of cells via mCD14. Immunodepletion of LBP from rabbit plasma inhibits the secretion of TNF from monocytes in response to LPS. Serum from LBP knock out mice is unable to mediate LPS-induced oxidative burst responses in mouse peritoneal exudate cells [7]. In transferring LPS to HDL particles LBP seems to act as a cofactor in neutralisation of LPS also (Fig. 1). Multicellular organisms had to develop defense mechanisms that start with the recognition of pathogen-associated molecular patterns. Such pattern recognition receptors (PRR) can be divided into three groups: humoral proteins circulating in the plasma, receptors for phagocytosis and signalling receptors on cells of the innate immune system [8]. CD14 and LBP seem to be such PRR. Just as LPS is essential for the survival of Gram-negative bacteria, so is teichoic acid absolutely essential for the Gram-positive microbe's survival and peptidoglycan is essential for all bacteria. In line with its role as a PRR it has been shown that CD14 can interact in vitro with lipoteichoic acid, high doses of soluble peptidoglycan, muramyldipeptide, polymannuronic acid, lipoarabinomannan and other microbial products [9]. Whether these proposed ligands bind to dierent regions of the molecule is unknown so far. CD14 not only binds soluble LPS but also whole Gram-negative bacteria which are then internalised by an LBP dependent (complement independent) pathway [10]. It thus appears that liberation of endotoxin from bacteria is not essential for CD14 recognition. Whether mCD14
C. SchuÈtt / The International Journal of Biochemistry & Cell Biology 31 (1998) 545±549
547
Fig. 1. Role of mCD14 and sCD14 in endotoxic response in vitro. Soluble CD14 and LBP are pivotal players in neutralising or mediating toxic eects of LPS. EC, endothelial cells; HDL, high density lipoprotein; mo, monocytes.
is involved in phagocytosis of bacteria in vivo remains to be elucidated. Recently the CD14 story was extended by the observation that anti-CD14 mAb blocked phagocytosis of apoptotic cells. Phagocyte recognition of `apoptotic self' is essential in protecting tissue from in¯ammatory injury. Several reports demonstrate that phagocytosis of apoptotic cells does not induce or may even inhibit release of proin¯ammatory mediators from phagocytes. This is exactly the opposite of what ligation of PRR evokes. Binding of most ligands to CD14 is proin¯ammatory but there are exceptions like certain derivatives of LPS (synthetic compound 406) or anionic phospholipids like those exposed on apoptotic cells [11]. These recent ®ndings may provide a means of studying how this GPI-anchored molecule engages pro-, non- and anti-in¯ammatory signalling pathways. Macrophages that have ingested apoptotic cells in vitro inhibit proin¯ammatory cytokine production through autocrine TGFb, PGE2 and PAF release [12]. There is growing evi-
dence that certain intracellular pathogens exploit the paracrine or autocrine eects of anti-in¯ammatory cytokines like IL10, TGFb or IL4. The biological function of soluble CD14 is so far unknown. In vitro an excess of sCD14 inhibits LPS binding to mCD14 and hence blocks cellular activation. In contrast sCD14±LPS complexes can activate cells which do not themselves express mCD14. For example endothelial cells can be activated by LPS±sCD14 complexes to release proin¯ammatory mediators. In addition sCD14 may act as a shuttle molecule like LBP to transfer LPS to HDL, thus neutralising its toxic eects (Fig. 1). Yu et al. [13] showed that both LBP and sCD14 may catalyse the exchange of dierent phospholipids including phosphatidylinositol, phosphatidylcholine and phosphatidylethanolamine, suggesting a broader physiological role in lipid exchange mechanisms. In patients soluble CD14 serum levels are elevated 3±5-fold in polytrauma, sepsis, AIDS, SLE, malaria, psoriasis and after hemodialysis. It is not clear, whether the increase of sCD14 is of
548
C. SchuÈtt / The International Journal of Biochemistry & Cell Biology 31 (1998) 545±549
pathophysiological relevance or a mere epiphenomenon, caused by shedding from activated cells. Moreover, it remains to be elucidated whether in vivo sCD14 can support LPS activation of CD14-negative cell or whether it is involved in neutralisation of LPS (see Fig. 1). The CD14-de®cient mice [14] which lack both mCD14 and sCD14 were found to be more than 100-fold less sensitive to shock induced by LPS. Surprisingly these animals showed reduced levels of E. coli bacteraemia suggesting a role for sCD14 in dissemination of bacteria. Dierent species vary in their response to LPS. Rodents are relatively resistant to LPS whereas others like pig, sheep or humans are extremely sensitive. Surprisingly, in in vitro experiments mouse and human macrophages are similar in their sensitivity to LPS. Therefore it is dicult to draw any generally applicable conclusion from animal experiments. The situation is made more complex by the fact that minute amounts of endotoxin can render macrophages tolerant to a subsequent challenge with LPS. Tolerance is believed to be
due to reprogramming of LPS-primed macrophages by autocrine pathways acting via IL10. On the other hand a hypersensitivity occurs after bacterial infection, adrenalectomy or hepatotoxic agents like D-galactosamine.
4. Possible medical applications Septic shock is a major problem in medical care. Anti-CD14 antibodies as well as soluble CD14 have both been considered as candidates for a therapeutic approach. In primates antiCD14 mAb's were useful in preventing experimentally induced endotoxic shock. Much remains to be done to show whether this strategy will be helpful in bacterial infection models as well. In this complex system anti-CD14 antibodies applied at the wrong time may worsen antibacterial defense (like anti-TNFa) while application at an appropriate time point may improve the outcome by blocking CD14 binding to its ligands.
Fig. 2. LPS binding or bacterial docking to mCD14 can lead to several contradictory events in the macrophage. Additionally, this is in¯uenced by LBP and sCD14.
C. SchuÈtt / The International Journal of Biochemistry & Cell Biology 31 (1998) 545±549
Thus, CD14 blockade may result in contradictory eects (see Fig. 2) depending on the immediate in vivo situation [15]. For soluble CD14, a protective eect preventing lethality in mouse was shown after a single bolus application of LPS. Mutant forms of sCD14 lacking the capacity to mediate endothelial cell activation by LPS may turn out to be of therapeutic value. However, the role of sCD14 application in bacteraemia has to be looked at in greater detail because in patients there is no clear cut distinction between endotoxaemia and bacteraemia. Therefore we need clear diagnostic tools with which to identify patients at risk who may bene®t from this therapeutic regime.
[6]
[7]
[8]
[9]
[10]
References [1] S.D. Wright, R.A. Ramos, P.S. Tobias, R.J. Ulevitch, J.C. Mathison, CD14, a receptor for complexes of lipopolysaccharide (LPS) and LPS binding protein, Science 249 (1990) 1431±1433. [2] E.T. Rietschel, T. Kiriakae, F.U. Schade, U. Mamat, G. Schmidt, H. Loppnow, A.J. Ulmer, U. ZaÈhringer, U. Seydel, F. Di Padova, M. Schreier, H. Brade, Bacterial endotoxin: molecular relationships of structure to activity and function, FASEB J. 8 (1994) 217±225. [3] F. Stelter, M. Bernheiden, R. Menzel, R.S. Jack, S. Witt, X. Fan, M. P®ster, C. SchuÈtt, Mutation of amino acids 39±44 of human CD14 abrogates binding of lipopolysaccharide and Escherichia coli, Eur. J. Biochem. 243 (1997) 100±109. [4] H.S. Downey, J. Han, Cellular activation mechanisms in septic shock, Front Biosci. 3 (1998) 468±476. [5] C.K. Joseph, S.D. Wright, W.G. Bornmann, J.T. Randolph, E.R. Kumar, R. Bittmann, J. Lin, R.N. Kolesnick, Bacterial lipopolysaccharide has structural similarity to ceramide and stimulates ceramide-activated
[11] [12]
[13]
[14]
[15]
549
protein kinase in myeloid cells, J. Biol. Chem. 269 (1994) 17606±17610. H.R. Petty, R.F. Todd, Integrins as promiscuous signal transduction devices, Immunol. Today 17 (1996) 209± 211. R.S. Jack, X. Fan, M. Bernheiden, G. Rune, M. Ehlers, A. Weber, G. Kirsch, R. Menzel, B. FuÈrll, M. Freudenberg, G. Schmitz, F. Stelter, C. SchuÈtt, Lipopolysaccharide binding protein is required to combat a murine Gram-negative bacterial infection, Nature 389 (1997) 742±745. R. Medzhitov, A. Janeway Jr.., Innate immunity: the virtues of a nonclonal system of recognition, Cell 91 (1997) 295±298. J. Pugin, D. Heumann, A. Tomasz, V.V. Kravchenko, Y. Akamatsu, M. Nishijima, M.P. Glauser, P.S. Tobias, R.J. Ulevitch, CD14 is a pattern recognititon receptor, Immunity 1 (1994) 509±516. U. Grunwald, X. Fan, R.S. Jack, G. Workalemahu, A. Kallies, F. Stelter, C. SchuÈtt, Monocytes can phagocytose Gram-negative bacteria by a mechanism dependent on CD14, J. Immunol. 157 (1996) 4119±4125. J. Savill, Phagocytic docking without shocking, Nature 392 (1998) 442±443. V.A. Fadok, D.L. Bratton, A. Konowal, P.W. Freed, J.Y. Westcott, P.M. Henson, Macrophages that have ingested apoptotic cells in vitro inhibit proin¯ammatory cytokine production through autocrine/paracrine mechanisms involving TGF-beta, PGE2 and PAF, J. Clin. Invest. 101 (1998) 890±898. B. Yu, E. Hailman, S.D. Wright, Lipopolysaccharide binding protein and soluble CD14 catalyze exchange of phospholipids, J. Clin. Invest. 99 (1997) 315±324. A. Haziot, E. Ferrero, F. Kontgen, N. Hijiya, S. Yamamoto, J. Silver, C.L. Stewart, S.M. Goyert, Resistance to endotoxin shock and reduced dissemination of Gram-negative bacteria in CD14-de®cient mice, Immunity 4 (1996) 407±414. H.D. Volk, P. Reinke, D. Krausch, H. Zuckermann, K. Asadullah, J.M. MuÈller, W.D. DoÈcke, W.J. Kox, Monocyte deactivation: rationale for a new therapeutic strategy in sepsis, Int. Care Med. 22 (1996) 474±481.
The International Journal of Biochemistry & Cell Biology 31 (1999) 545±549
Molecules in focus
CD14 Christine SchuÈtt * Institut fuÈr Immunologie und Transfusionsmedizin, UniversitaÈt Greifswald, Diagnostikzentrum Sauerbruchstraûe, 17487 Greifswald, Germany Received 7 September 1998; accepted 20 November 1998
Abstract The GPI-anchored 55 kDa glycoprotein CD14 is expressed on monocytes/macrophages and to a lesser extent on granulocytes. Engagement of CD14 by ligands like lipopolysaccharide, intact bacteria or apoptotic cells can result in either pro- or anti-in¯ammatory responses. Since the CD14 molecule does not have a membrane spanning domain it cannot transmit a signal into the cell. Some as yet unidenti®ed accessory protein is thought to be involved. It will be important to clarify the signalling systems involved since they may provide a therapeutic target for sepsis intervention strategies. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Endotoxin receptor; Innate immunity; Sepsis
1. Introduction At the First Leukocyte Typing Conference in 1982 dierent monoclonal antibodies that selectively bound to human monocytes/macrophages were grouped into the cluster of dierentiation No. 14 (CD14). The molecule involved, CD14 (mCD14), therefore serves as a useful marker to identify cells of the monocyte/macrophage lineage. In addition to these cells, activated granulocytes also weakly express CD14. The 55 kDa glycoprotein was identi®ed in 1990 as a receptor for the endotoxin lipopolysaccharide (LPS) [1]. * Tel.: +49-3834-865-470; fax: +49-3834-865-490; e-mail: [email protected].
LPS is a major component of the outer membrane of Gram-negative bacteria. It consists of an O-speci®c chain, a core oligosaccharide and a lipid component, termed lipid A [2]. Immune cells of the host recognise minute amounts of LPS and rapidly mount in¯ammatory defense mechanisms. However an excessive response can induce an overwhelming release of cytokines leading to septic shock.
2. Structure, synthesis and degradation The human CD14 gene consists of two exons and codes for a 356 amino acid protein with multiple leucine-rich repeats. The mature protein is expressed as a glycosyl-phosphatidylinositol
1357-2725/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 7 - 2 7 2 5 ( 9 8 ) 0 0 1 5 3 - 8
546
C. SchuÈtt / The International Journal of Biochemistry & Cell Biology 31 (1998) 545±549
(GPI)-anchored cell surface molecule, lacking a transmembrane domain. Several groups have sought to de®ne the LPS binding site of the CD14 molecule. The results are rather variable and depend on the method used. Among 23 alanine substitution mutants covering amino acids 1±152 of human CD14, only mutation of amino acids 39±44 results in loss of LPS-binding [3]. Little is known about expression or inducible CD14 expression in dierent species in vivo. In mice it was shown that LPS challenge induced a transient increase in CD14 mRNA levels in myeloid cells as well as in epithelium of the liver and kidney. LPS, TNFa and IFNg upregulate CD14 expression on human myeloid cells in vitro whereas IL4 reduces it. CD14 levels can also be regulated by proteolytic cleavage or phospholipase D induced shedding which lead to the release of a soluble form of CD14 (sCD14). In patients suering from paroxysmal nocturnal haemoglobinuria (PNH), a GPI-anchoring defect, direct release of sCD14 is also possible. Normal human serum contains approximately 3±4 mg/ml sCD14. 3. Biological function LPS is bound in plasma to a protein called LPS binding protein (LBP), which facilitates the rapid binding of nano/picomolar amounts of LPS to CD14. Binding of LPS to membrane bound CD14 results in cellular activation: the production of in¯ammatory cytokines, e.g. TNFa, IL1, IL6, IL8, IL12 or oxygen radicals, NO, tissue factor or others, as well as anti-in¯ammatory cytokines like IL10 or TGFb. It has recently become evident that a number of phosphorylation cascades including MAP kinase pathways and the NFkB activation pathway are initiated after exposure of the cells to LPS [4]. Since the GPI-anchored mCD14 is unable to function as a signal transducing molecule some means must exist to transmit the signal into the cell. One possibility is that an accessory protein is involved. Alternatively Joseph et al. [5] favoured the hypothesis that after binding by CD14, LPS will be integrated into the phospholi-
pid layer of the cell membrane. By mimicking ceramide LPS stimulates cells by interacting with membrane bound ceramide-activated protein kinase. Another possible mechanism of signalling is the association of GPI-linked proteins, like CD14, with b2 integrins as `public' transducers for a panel of `private' GPI-linked receptors [6]. Although our knowledge about the activation pathways induced by LPS/LBP-ligated CD14 are limited, the LBP±LPS interactions themselves have been studied in great detail. LBP like CD14 recognises lipid A and LBP seems to be the only serum component which can catalyse LPS activation of cells via mCD14. Immunodepletion of LBP from rabbit plasma inhibits the secretion of TNF from monocytes in response to LPS. Serum from LBP knock out mice is unable to mediate LPS-induced oxidative burst responses in mouse peritoneal exudate cells [7]. In transferring LPS to HDL particles LBP seems to act as a cofactor in neutralisation of LPS also (Fig. 1). Multicellular organisms had to develop defense mechanisms that start with the recognition of pathogen-associated molecular patterns. Such pattern recognition receptors (PRR) can be divided into three groups: humoral proteins circulating in the plasma, receptors for phagocytosis and signalling receptors on cells of the innate immune system [8]. CD14 and LBP seem to be such PRR. Just as LPS is essential for the survival of Gram-negative bacteria, so is teichoic acid absolutely essential for the Gram-positive microbe's survival and peptidoglycan is essential for all bacteria. In line with its role as a PRR it has been shown that CD14 can interact in vitro with lipoteichoic acid, high doses of soluble peptidoglycan, muramyldipeptide, polymannuronic acid, lipoarabinomannan and other microbial products [9]. Whether these proposed ligands bind to dierent regions of the molecule is unknown so far. CD14 not only binds soluble LPS but also whole Gram-negative bacteria which are then internalised by an LBP dependent (complement independent) pathway [10]. It thus appears that liberation of endotoxin from bacteria is not essential for CD14 recognition. Whether mCD14
C. SchuÈtt / The International Journal of Biochemistry & Cell Biology 31 (1998) 545±549
547
Fig. 1. Role of mCD14 and sCD14 in endotoxic response in vitro. Soluble CD14 and LBP are pivotal players in neutralising or mediating toxic eects of LPS. EC, endothelial cells; HDL, high density lipoprotein; mo, monocytes.
is involved in phagocytosis of bacteria in vivo remains to be elucidated. Recently the CD14 story was extended by the observation that anti-CD14 mAb blocked phagocytosis of apoptotic cells. Phagocyte recognition of `apoptotic self' is essential in protecting tissue from in¯ammatory injury. Several reports demonstrate that phagocytosis of apoptotic cells does not induce or may even inhibit release of proin¯ammatory mediators from phagocytes. This is exactly the opposite of what ligation of PRR evokes. Binding of most ligands to CD14 is proin¯ammatory but there are exceptions like certain derivatives of LPS (synthetic compound 406) or anionic phospholipids like those exposed on apoptotic cells [11]. These recent ®ndings may provide a means of studying how this GPI-anchored molecule engages pro-, non- and anti-in¯ammatory signalling pathways. Macrophages that have ingested apoptotic cells in vitro inhibit proin¯ammatory cytokine production through autocrine TGFb, PGE2 and PAF release [12]. There is growing evi-
dence that certain intracellular pathogens exploit the paracrine or autocrine eects of anti-in¯ammatory cytokines like IL10, TGFb or IL4. The biological function of soluble CD14 is so far unknown. In vitro an excess of sCD14 inhibits LPS binding to mCD14 and hence blocks cellular activation. In contrast sCD14±LPS complexes can activate cells which do not themselves express mCD14. For example endothelial cells can be activated by LPS±sCD14 complexes to release proin¯ammatory mediators. In addition sCD14 may act as a shuttle molecule like LBP to transfer LPS to HDL, thus neutralising its toxic eects (Fig. 1). Yu et al. [13] showed that both LBP and sCD14 may catalyse the exchange of dierent phospholipids including phosphatidylinositol, phosphatidylcholine and phosphatidylethanolamine, suggesting a broader physiological role in lipid exchange mechanisms. In patients soluble CD14 serum levels are elevated 3±5-fold in polytrauma, sepsis, AIDS, SLE, malaria, psoriasis and after hemodialysis. It is not clear, whether the increase of sCD14 is of
548
C. SchuÈtt / The International Journal of Biochemistry & Cell Biology 31 (1998) 545±549
pathophysiological relevance or a mere epiphenomenon, caused by shedding from activated cells. Moreover, it remains to be elucidated whether in vivo sCD14 can support LPS activation of CD14-negative cell or whether it is involved in neutralisation of LPS (see Fig. 1). The CD14-de®cient mice [14] which lack both mCD14 and sCD14 were found to be more than 100-fold less sensitive to shock induced by LPS. Surprisingly these animals showed reduced levels of E. coli bacteraemia suggesting a role for sCD14 in dissemination of bacteria. Dierent species vary in their response to LPS. Rodents are relatively resistant to LPS whereas others like pig, sheep or humans are extremely sensitive. Surprisingly, in in vitro experiments mouse and human macrophages are similar in their sensitivity to LPS. Therefore it is dicult to draw any generally applicable conclusion from animal experiments. The situation is made more complex by the fact that minute amounts of endotoxin can render macrophages tolerant to a subsequent challenge with LPS. Tolerance is believed to be
due to reprogramming of LPS-primed macrophages by autocrine pathways acting via IL10. On the other hand a hypersensitivity occurs after bacterial infection, adrenalectomy or hepatotoxic agents like D-galactosamine.
4. Possible medical applications Septic shock is a major problem in medical care. Anti-CD14 antibodies as well as soluble CD14 have both been considered as candidates for a therapeutic approach. In primates antiCD14 mAb's were useful in preventing experimentally induced endotoxic shock. Much remains to be done to show whether this strategy will be helpful in bacterial infection models as well. In this complex system anti-CD14 antibodies applied at the wrong time may worsen antibacterial defense (like anti-TNFa) while application at an appropriate time point may improve the outcome by blocking CD14 binding to its ligands.
Fig. 2. LPS binding or bacterial docking to mCD14 can lead to several contradictory events in the macrophage. Additionally, this is in¯uenced by LBP and sCD14.
C. SchuÈtt / The International Journal of Biochemistry & Cell Biology 31 (1998) 545±549
Thus, CD14 blockade may result in contradictory eects (see Fig. 2) depending on the immediate in vivo situation [15]. For soluble CD14, a protective eect preventing lethality in mouse was shown after a single bolus application of LPS. Mutant forms of sCD14 lacking the capacity to mediate endothelial cell activation by LPS may turn out to be of therapeutic value. However, the role of sCD14 application in bacteraemia has to be looked at in greater detail because in patients there is no clear cut distinction between endotoxaemia and bacteraemia. Therefore we need clear diagnostic tools with which to identify patients at risk who may bene®t from this therapeutic regime.
[6]
[7]
[8]
[9]
[10]
References [1] S.D. Wright, R.A. Ramos, P.S. Tobias, R.J. Ulevitch, J.C. Mathison, CD14, a receptor for complexes of lipopolysaccharide (LPS) and LPS binding protein, Science 249 (1990) 1431±1433. [2] E.T. Rietschel, T. Kiriakae, F.U. Schade, U. Mamat, G. Schmidt, H. Loppnow, A.J. Ulmer, U. ZaÈhringer, U. Seydel, F. Di Padova, M. Schreier, H. Brade, Bacterial endotoxin: molecular relationships of structure to activity and function, FASEB J. 8 (1994) 217±225. [3] F. Stelter, M. Bernheiden, R. Menzel, R.S. Jack, S. Witt, X. Fan, M. P®ster, C. SchuÈtt, Mutation of amino acids 39±44 of human CD14 abrogates binding of lipopolysaccharide and Escherichia coli, Eur. J. Biochem. 243 (1997) 100±109. [4] H.S. Downey, J. Han, Cellular activation mechanisms in septic shock, Front Biosci. 3 (1998) 468±476. [5] C.K. Joseph, S.D. Wright, W.G. Bornmann, J.T. Randolph, E.R. Kumar, R. Bittmann, J. Lin, R.N. Kolesnick, Bacterial lipopolysaccharide has structural similarity to ceramide and stimulates ceramide-activated
[11] [12]
[13]
[14]
[15]
549
protein kinase in myeloid cells, J. Biol. Chem. 269 (1994) 17606±17610. H.R. Petty, R.F. Todd, Integrins as promiscuous signal transduction devices, Immunol. Today 17 (1996) 209± 211. R.S. Jack, X. Fan, M. Bernheiden, G. Rune, M. Ehlers, A. Weber, G. Kirsch, R. Menzel, B. FuÈrll, M. Freudenberg, G. Schmitz, F. Stelter, C. SchuÈtt, Lipopolysaccharide binding protein is required to combat a murine Gram-negative bacterial infection, Nature 389 (1997) 742±745. R. Medzhitov, A. Janeway Jr.., Innate immunity: the virtues of a nonclonal system of recognition, Cell 91 (1997) 295±298. J. Pugin, D. Heumann, A. Tomasz, V.V. Kravchenko, Y. Akamatsu, M. Nishijima, M.P. Glauser, P.S. Tobias, R.J. Ulevitch, CD14 is a pattern recognititon receptor, Immunity 1 (1994) 509±516. U. Grunwald, X. Fan, R.S. Jack, G. Workalemahu, A. Kallies, F. Stelter, C. SchuÈtt, Monocytes can phagocytose Gram-negative bacteria by a mechanism dependent on CD14, J. Immunol. 157 (1996) 4119±4125. J. Savill, Phagocytic docking without shocking, Nature 392 (1998) 442±443. V.A. Fadok, D.L. Bratton, A. Konowal, P.W. Freed, J.Y. Westcott, P.M. Henson, Macrophages that have ingested apoptotic cells in vitro inhibit proin¯ammatory cytokine production through autocrine/paracrine mechanisms involving TGF-beta, PGE2 and PAF, J. Clin. Invest. 101 (1998) 890±898. B. Yu, E. Hailman, S.D. Wright, Lipopolysaccharide binding protein and soluble CD14 catalyze exchange of phospholipids, J. Clin. Invest. 99 (1997) 315±324. A. Haziot, E. Ferrero, F. Kontgen, N. Hijiya, S. Yamamoto, J. Silver, C.L. Stewart, S.M. Goyert, Resistance to endotoxin shock and reduced dissemination of Gram-negative bacteria in CD14-de®cient mice, Immunity 4 (1996) 407±414. H.D. Volk, P. Reinke, D. Krausch, H. Zuckermann, K. Asadullah, J.M. MuÈller, W.D. DoÈcke, W.J. Kox, Monocyte deactivation: rationale for a new therapeutic strategy in sepsis, Int. Care Med. 22 (1996) 474±481.