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Journal of Virology, May 2003, p. 5535-5539, Vol. 77, No. 9
0022-538X/03/$08.00+0 DOI: 10.1128/JVI.77.9.5535-5539.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Sphingosine Kinase-Dependent Migration of Immature Dendritic Cells in Response to Neurotoxic Prion Protein Fragment
Nicole C. Kaneider,1 Arthur Kaser,2 Stefan Dunzendorfer,1 Herbert Tilg,2 and Christian J. Wiedermann1*
Division of General Internal Medicine,1
Division of Gastroenterology, Department of Internal Medicine, University of Innsbruck, A-6020 Innsbruck, Austria2
Received 9 October 2002/
Accepted 5 February 2003

ABSTRACT
The concept that circulating dendritic cells mediate neuroinvasion
in transmissible spongiform encephalopathies received strong
support from recent observations that prion protein is expressed
in myeloid dendritic cells. We observed that prion protein fragment
106-126 is a chemoattractant for monocyte-derived immature but
not mature dendritic cells. Signaling events in chemotaxis involved
enzymes downstream of G
q protein and were inhibited by blockade
of sphingosine kinase, suggesting transactivation of sphingosine-1-phosphate-dependent
cell motility by prion protein.

TEXT
Prion glycoproteins may become unprecedented infectious pathogens
that cause a group of invariably fatal neurodegenerative diseases
by novel mechanisms (
27,
29). New-variant Creutzfeldt-Jakob
disease [(v)CJD] and scrapie, known as transmissible spongiform
encephalopathies, are typically initiated by exposure to the
causative agent and early prion replication in lymphoid tissues
(
20). (v)CJD has raised concerns that bovine spongiform encephalopathies
might be communicable to humans by dietary exposure (
4,
28).
Gut M-cell-dependent transepithelial uptake of dietary prion protein is followed by transcytosis directly to intraepithelial pockets, where key players of the immune system, including dendritic cells (DCs), are located (11). DCs are also able to open the tight junctions between epithelial cells, send dendrites outside the epithelium, and directly sample pathogens in an M-cell-independent way (30). The details of the mechanism by which infective prions are transferred from the gastrointestinal tract to the nervous system are unknown. It is important to understand how central lymphoid organs and peripheral neurons become exposed to infective prion protein (PrPsc).
Evidence suggests that circulating blood cells may have a role in enteral prion infection. Results from animal models have emphasized the fact that infective material can be isolated from the cell fraction of spleen soon after the ingestion of PrPsc (19), whereas in mice, bone marrow-derived myeloid cells have been shown to be required for its propagation and spread (2). It was shown previously that cellular prion protein (PrPc) is strongly expressed in myeloid DCs, which may act as carrier cells for the spread and circulation of the abnormal isoform PrPsc (3). In the absence of prion disease, high levels of expression of PrPc in human spleen occur principally on myeloid DCs immediately adjacent to the white pulp, whereas follicular DCs do not strongly express PrPc; myeloid DCs are found in the red pulp of the spleen, and cells migrate into its lymphoid areas after receiving a maturation stimulus (3). Moreover, DCs can be found in the peripheral and central nervous system (9, 25). Here we report on the chemotaxis of immature DCs and arrest of mature DCs by a synthetic peptide corresponding to residues 106 to 126 of human PrP (PrP106-126). Signal transduction mechanisms that may be involved in directed migration of monocyte-derived DCs toward PrP106-126 are described.
PrP106-126, which is toxic to neurons, increases chemotaxis, oxygen free radical release, and intracellular calcium concentration in neutrophils and monocytes (5). To determine whether PrP106-126 is a chemoattractant of monocyte-derived DCs (17), chemotaxis experiments in modified multiwell Boyden chambers (Neuroprobe, Gaithersburg, Md.) using nitrocellulose micropore filters (Sartorius, Göttingen, Germany) were performed as previously described (6). DCs were prepared as described previously (6, 7, 17, 18). Distinction between mature and immature DCs was made by fluorescence-activated cell sorting analyses (Fig. 1).
Immature DCs migrated for 4 h toward PrP
106-126 (Bachem, Bubendorf,
Switzerland) in a concentration-dependent manner, whereas PrP
106-126 was not chemotactic for mature DCs (Fig.
2). Maximum chemotactic
activity of PrP
106-126 for immature DCs was seen at concentrations
of 0.1 to 10 nmol/liter and was comparable in its potency to
that of RANTES [20 ng/ml] (Peprotech, London, United Kingdom).
As a control, chemotaxis toward scrambled PrP
106-126 and PrP
118-135 was monitored. Neither the scrambled form nor PrP
118-135 exerted
chemotactic effects on immature DCs (Fig.
2). Checkerboard analysis
revealed that the migration of immature DCs toward PrP
106-126 is true concentration gradient-dependent chemotaxis (Table
1).
The influence of PrP
106-126 on 6Ckine-induced chemotaxis of
mature DCs was tested. Combination of 6Ckine (1 µg/ml)
with PrP
106-126 (10 fM to 1 µM) in the lower wells of
the chemotaxis chamber deactivated mature DC migration. The
mean distance of random migration was 45 ± 5.2 µm,
the mean 6Ckine-induced migration was 100 ± 8.4 µm,
and the mean when 6Ckine was combined with PrP
106-126 was 78
± 6.3 µm (
P < 0.05). The effects of PrP
106-126 and 6Ckine alone are shown in Fig.
1.
Tyrphostin-23 (a tyrosine kinase inhibitor), bisindolylmaleimide
(GFX; a protein kinase C inhibitor), wortmannin (WTN; a phospholipase
3 inhibitor), rolipram (a phosphodiesterase inhibitor; Sigma
Chemical Co., St. Louis, Mo.), and dimethylsphingosine (DMS;
a sphingosine kinase inhibitor) were used for blocking signaling
enzymes; pertussis toxin (PTX) and cholera toxin (CTX; Sigma
Chemical Co.) were used for testing involvement of G
i/G
0 protein
and G
s protein, respectively. Patterns of migration toward other
DC attractants, including
N-formyl-methionyl-phenylalanine (fMLP;
Sigma Chemical Co.) and substance P (Neosystem, Strasbourg,
France), were compared with that of PrP
106-126. Signaling studies
revealed that PrP
106-126-induced chemotaxis is sensitive to
GFX, tyrphostin-23, rolipram, WTN, and PTX. This signaling pattern
mimicked that of substance P, another peptide attractant of
DCs (
8), but differed from that of fMLP. CTX did not affect
chemotaxis to any of them (Table
2).
Activation of protein kinase C leads to phosphorylation of sphingosine
kinase and sphingosine-1-phosphate production (
24), which induces
G-protein-dependent cell migration (
16). Sphingosine-1-phosphate-induced
chemotaxis of DCs has been reported recently (
14). As PrP
106-126-induced
migration of DCs was inhibited by the inhibitor of protein kinase
C and by blockade of G
i/G
0 (Table
2), effects of the sphingosine
kinase inhibitor DMS (Biomol, Plymouth Meeting, Pa.) on PrP
106-126-induced
chemotaxis were tested. Migration of DCs toward optimal concentrations
of PrP
106-126 was inhibited in a concentration-dependent manner
by pretreatment with DMS, whereas chemotaxis of immature DCs
toward fMLP was not inhibited (Fig.
3).
Results of this in vitro study demonstrate that PrP
106-126 induces
migration of monocyte-derived immature DCs in a dose-dependent
manner. In tissues of animals infected by transmissible spongiform
encephalopathy agent and in humans suffering from (v)CJD, PrP
sc is particularly concentrated in follicular DCs, whereas high
levels of PrP
c are present in myeloid DCs (
3), which are ontologically
and functionally distinct from follicular DCs (
15). Myeloid
DCs are derived from bone marrow precursor cells or from monocytes
and their precursors and are readily identified within circulating
cell populations. In the spleen, myeloid DCs are found in the
red pulp and immediately adjacent to the white pulp. The cells
migrate into the lymphoid areas, where they are powerful mediators
of T-cell activation. Given the close anatomic and functional
connection of myeloid DCs with lymphoid follicles, these results
raise the possibility that migration of myeloid DCs toward prion
protein may play a role in the propagation of PrP
sc in humans
(
3). It has been observed that injection of prion-infected DCs
induced scrapie without accumulation of prions in the spleen
in a model of RAG-1 knockout mice, indicating that DCs can propagate
prions from the periphery to the central nervous system in the
absence of any additional lymphoid element (
1). Migratory bone
marrow-derived DCs, entering the intestinal wall from blood,
sample antigens from the gut lumen and carry them to the lymph
nodes. Huang et al. showed that DCs acquire PrP
sc in vitro and
transport intestinally administered PrP
sc directly into lymphoid
tissue in vivo, suggesting that DCs are a cellular bridge between
the gut lumen and the lymphoid transmissible spongiform encephalopathy
replication machinery (
13).
In the present study, it was observed that the prion protein fragment is able to induce a chemotactic response in immature DCs and to arrest mature DCs, suggesting that DC trafficking may directly depend on the infective agent. DCs might be attracted toward high concentrations of prion protein, as are found in the gut epithelial venules or splenic lymphoid follicles, where uptake and accumulation of pathogens take place as a prerequisite of prion spread. When stimuli are received as a result of pathogen uptake, maturation might occur and DCs might subsequently be arrested at the sites of prion uptake.
We further observed an inhibition of migration after pretreatment of cells with DMS, which was used as specific inhibitor of sphingosine kinase. Previously it was suggested for monocytes that PrP106-126 might use the formyl peptide-like receptor 1, a pattern recognition seven transmembrane receptor for chemotaxis also expressed in DCs (12). This receptor is coupled to G proteins and may be sufficient for migration induction (22). The observation that PrP106-126-induced chemotaxis is sphingosine kinase dependent suggests that transactivation of chemotaxis toward sphingosine-1-phosphate, a lysophospholipid that activates cytoskeletal remodeling and motility via endothelial-differentiation-gene receptors (EDG) (21), is also involved in prion protein fragment-dependent effects in DCs (23, 24, 26, 31). Immature and mature DCs express the mRNA for different sphingosine-1-phosphate receptors (EDG-1, -3, -5, and -6), where sphingosine-1-phosphate induces further chemotaxis (14). Activation of sphingosine kinase and production of sphingosine-1-phosphate increases intracellular Ca2+ concentrations, an effect known to take place in PrP106-126-induced apoptosis of several cell lines (10). These observations strengthen the concept that migration of antigen-presenting cells toward high concentrations of PrP106-126 involves sphingosine kinase.
Since in dietary prion disease, PrPsc is incorporated via the lymphatic system of the gut, migration of circulating DCs toward high concentrations of prion protein and their accumulation for prion uptake may be important pathophysiological mechanisms that involve a chemotactic lysophospholipid. Interference with sphingosine kinase-dependent pathways might become a novel pharmaceutical target for preventing prion-dependent DC trafficking and the associated spread of infection.

FOOTNOTES
* Corresponding author. Mailing address: Department of Internal Medicine, University of Innsbruck, Anichstrasse 35, A-6020 Innsbruck, Austria. Phone: 43-512-5044180. Fax: 43-512-504674180. E-mail:
Christian.Wiedermann{at}uibk.ac.at.


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Journal of Virology, May 2003, p. 5535-5539, Vol. 77, No. 9
0022-538X/03/$08.00+0 DOI: 10.1128/JVI.77.9.5535-5539.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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