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IMMUNOBIOLOGY
The normal cellular prion protein is strongly expressed by myeloid dendritic cells
John Burthem, Britta Urban, Arnab Pain, and David J. Roberts
Abnormal isoforms of the prion protein
(PrPSc) that cause prion diseases are
propagated and spread within the body
by “carrier” cell(s). Cells of the immune
system have been strongly implicated in
this process. In particular, PrPSc is known
to accumulate on follicular dendritic cells
(FDCs) in individuals affected by variant
Creutzfeld-Jakob disease. However, FDCs
do not migrate widely and the natural
history of prion disorders suggests other
cells may be required for the transport of
PrPSc from the site of ingestion to lymphoid organs and the central nervous
system. Substantial evidence suggests
that the spread of PrPSc requires bone
marrow-derived cells that express normal
cellular prion protein (PrPC). This study
examined the expression of PrPC on bone
marrow–derived cells that interact with
lymphoid follicles. High levels of PrPC are
present on myeloid dendritic cells (DCs)
that surround the splenic white pulp.
These myeloid DCs are ontologically and
functionally distinct from the FDCs. Consistent with these observations, expression of PrPC was strongly induced during
the generation of mature myeloid DCs in
vitro. In these cells PrPC colocalized with
major histocompatibility complex class II
molecules at the level of light microscopy. Furthermore, given the close anatomic and functional connection of myeloid DCs with lymphoid follicles, these
results raise the possibility that myeloid
DCs may play a role in the propagation of
PrPSc in humans. (Blood. 2001;98:
3733-3738)
© 2001 by The American Society of Hematology
Introduction
The recent emergence of variant Creutzfeld-Jakob Disease (vCJD)
in humans has raised the specter of large-scale primary and
secondary infections in human populations.1-4 In late stages of
vCJD infection, abnormal prion protein (PrPSc) accumulates,
particularly in brain, leading to a distinctive clinical and pathologic
disorder.5 However, preventive and therapeutic strategies will be
most effective if targeted at the initial pathways through which
ingested PrPSc is propagated within the body and transferred to
nerve tissue.
It is now widely accepted that follicular dendritic cells (FDCs)
have an important role in the pathogenesis of prion disease.
Histopathologic studies of mice and other species exposed to PrPSc
have highlighted the accumulation of PrPSc on the FDCs in
lymphoid follicles.6-8 Furthermore, in a mouse model, it has now
been shown that FDCs are required for the replication of PrPSc that
precedes neuroinvasion.9,10 In support of this view, B-cell–deficient
mice, which fail to form functional lymphoid follicles,11,12 are
resistant to PrPSc infection. However, involvement of FDCs alone
is not sufficient to explain how infective forms of the prion protein
are transferred from the gastrointestinal tract to the lymphoid
organs and the central nervous system (CNS). FDCs reside within
B-cell areas of lymphoid organs where they bind and retain
antigens on their cell surface over months or even years.13 The cells
have a slow turnover and are not believed to recirculate. Moreover,
cellular (PrPC) has not been found to be associated with the FDCs
of enteric tissue.14
It is important therefore to understand how FDCs in central
lymphoid organs become exposed to PrPSc and how PrPSc may be
transferred from FDCs to other tissues. In this regard, evidence
suggests that circulating cells may have a role. Results from animal
models have emphasized that infective material can be isolated
from the cellular fraction of the spleen soon after the ingestion of
PrPSc,15 whereas in mice, marrow-derived cells have been shown to
be required for the propagation and spread of PrPSc.16
Attention has therefore focused on the circulating cells that
express normal PrPC and that may therefore act as carrier cells for
the spread and circulation of its abnormal isoform.17,18 In humans,
peripheral B and T lymphocytes express PrPC at low levels,
whereas higher levels of PrPC have been found on subsets of
natural killer (NK) cells, platelets, and monocytes.19-21 However,
the expression of PrPC on blood cells in the peripheral circulation
does not provide a full description of the possible routes of
propagation and spread of PrPSc. Recent evidence suggests that the
expression of PrPC may vary with the developmental stage of cells and
with their activation state21,22; thus the expression of PrPC on circulating
cells will not necessarily reflect the expression of the molecule on the
derived, differentiated, or activated cells within tissues.
Understanding the cellular expression of the normal form of the
prion protein (PrPC) within lymphoid tissues may provide important clues to the pathogenesis of prion disorders. We have therefore
studied the expression of PrPC on cells within human spleen,
focusing particularly on those cells that interact closely with
lymphoid follicles and with FDCs. We describe a distinctive
cellular and intracellular distribution of PrPC that may have
From the Nuffield Department of Biochemistry and Cellular Science, University
of Oxford, and National Blood Service, John Radcliffe Hospital, Oxford, United
Kingdom; and Department of Biomedical Sciences, UMIST, Manchester,
United Kingdom.
Howard Hughes International Research Scholar.
Submitted August 23, 2000; accepted July 31, 2001.
The publication costs of this article were defrayed in part by page charge
payment. Therefore, and solely to indicate this fact, this article is hereby
marked ‘‘advertisement’’ in accordance with 18 U.S.C. section 1734.
Supported by the University of Oxford, the National Blood Service, and the
Wellcome Trust. B.C.U. was supported by the Sir E. P. Abraham Trust (Oxford),
and D.J.R. was a Wellcome Trust Senior Fellow in Clinical Science and a
BLOOD, 15 DECEMBER 2001 䡠 VOLUME 98, NUMBER 13
Reprints: David J. Roberts, National Blood Service, Oxford Centre, John
Radcliffe Hospital, Headington, Oxford OX3 9DU, United Kingdom; e-mail:
[email protected].
© 2001 by The American Society of Hematology
3733
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3734
BLOOD, 15 DECEMBER 2001 䡠 VOLUME 98, NUMBER 13
BURTHEM et al
important implications for its normal function and the spread of its
abnormal isoforms(s).
Materials and methods
Immunohistochemistry and immunocytology
Human tissue samples were obtained from the Pathology Department at the
John Radcliffe Hospital, Oxford. Formalin-fixed tissue sections were
dewaxed and rehydrated. Sections were heat-treated with 2 mM HCl and
PrPC was visualized using antihuman PrPC monoclonal antibody (clone
3F4). Signal was amplified and visualized using the ABComplex system
(Dako, Cambridge, United Kingdom).
Cryostat tissue sections of spleen (8 ␮m) were fixed in acetone and
examined using dual-color immunofluorescence. Cell surface antigens were
detected with the antibodies shown in Table 1.
Antibody-labeled cells were then detected using class-specific goat
antisera to either mouse or rabbit immunoglobulin (fluorescein isothiocyanate [FITC] or Texas red conjugated as indicated) (Cambridge Bioscience,
Cambridge, United Kingdom). Fluorescence-labeled sections were viewed
and images captured using standard immunofluorescence microscopy and
video capture equipment.
The immunocytology of monocyte-derived dendritic cells (DCs) was
also studied. DCs were cultured (see below), washed 4 times with cold
phosphate-buffered saline (PBS), and allowed to attach and spread on glass
slides or coverslips for 60 minutes. Preparations were air dried and fixed in
acetone/methanol. DCs were immunostained and visualized as described
above for cryostat sections.
Western blot analysis
Tissue samples or centrifuged cell pellets were lysed directly in boiling
sample buffer. Protein concentrations were compared using scanning
densitometry of Coomassie-stained gels. Human serum was diluted 1:2 in
PBS and mixed with double-strength sample buffer. Samples were run on
10% sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDSPAGE) and transferred to polyvinylidene difluoride (PVDF) membrane.
PrPC was detected using Mab 6H4 and goat antimouse horseradish
peroxidase (HRP) conjugate (Dako) and visualized using enhanced chemoluminescence (Amersham Life Sciences, Amersham, United Kingdom)
according to the manufacturer’s instructions.
Table 1. Antibodies used to detect cell surface antigens
Antigen
Antibody/use*
Source†
PrP
6H4 (fl, cyt, WBs)
1
PrP
R29 (cryo)
1
PrP
3F4 (paraffin)
2
Generation and assessment of DCs
Immature myeloid DCs were derived from peripheral blood cells using
standard procedures.23,24 Briefly, monocytes were cultivated in RPMI 1640
supplemented with 2 mM glutamine, 50 ␮g/mL kanamycin, 1% nonessential amino acids (Gibco BRL, Glasgow, United Kingdom), 10% human AB
serum, and 50 ng/mL each of interleukin 4 (IL-4; specific activity
⬎ 2 ⫻ 106 U/mg; Peprotech, London, United Kingdom) and granulocytemacrophage colony-stimulating factor (GM-CSF; specific activity
⬎ 1 ⫻ 107 U/mg; Schering-Plough, Welwyn Garden City, United Kingdom). After 6 days, DCs were matured with 100 ng/mL lipopolysaccharide
(LPS; Salmonella typhimurium) for 48 hours. PrPC present in culture
medium was assessed using Western blotting and densitometry and was
found to be less than 10% of that expressed by the harvested DCs. Immature
DCs expressed the surface molecules CD54 (clone 6.5B5), CD40 (clone
LOB7/6), CD86 (clone BU63), and HLA DR (clone BF-1) as confirmed by
staining with the respective monoclonal antibodies and fluorescenceactivated cell sorter (FACScan; Becton Dickinson, Oxford, United Kingdom) analysis. Expression of CD14 was low. On maturation with LPS, 98%
of DCs expressed CD83 (clone HB15b) (Serotec, Oxford, United Kingdom) and the levels of expression of CD54, CD40, CD86, and HLA-DR
were increased. Early endocytotic vesicles in immature DCs were labeled
by exposure of cells to FITC-dextran (1mg/mL; Molecular Probes, Leiden,
Netherlands) for 7 minutes and for 15 minutes at 4°C and 37°C.
Amplification of the PrP gene
Total RNA was prepared from immature myeloid DCs generated from
monocytes (7 days after culture with IL-4 and GM-SCF as described
above), mature LPS-stimulated myeloid DCs, and on 2 independent
preparations of monocytes freshly isolated from peripheral blood. The gene
encoding PrP was amplified by reverse transcription-polymerase chain
reaction (RT-PCR) using the Titan One Tube RT-PCR Kit (Roche), used
according to manufacturer’s instructions or by PCR using Taq polymerase
(Roche). RNA (200 pg) or genomic DNA (50 ng) was used in the respective
reactions. The primers 5⬘-GGCAGTGACTATGAGGACCGTTAC-3⬘ and
5⬘-GGCTTGACCAGCATCTCAGGTCTA-3⬘ were used to amplify the
PrP gene to generate a 528-bp product. The primers 5⬘-CCATGTTCGTCATGGGTGTGAACCA-3⬘ and 5⬘-GCCAGTAGGCAGGGATGATGATGTT-3⬘
were used to amplify a 251-bp product by RT-PCR from the glyceraldehyde3-phosphate dehydrogenase (G3PDH) gene in RNA prepared from all cell
types as a positive control (data not shown).
Results
Recognition
PrPC is strongly expressed by myeloid DCs in spleen
CD3
3D4 (cryo)
3
T cells
CD11c
KB40 (cryo)
3
DC and macrophage
CD13
WM47 (cryo)
3
Myeloid origin (DC
CD20
L26 (cryo)
3
B cells
and macrophage)
CD21
CR4/23 (cryo)
3
FDCs (CD21 isoform)
CD68
PGM1 (cryo)
3
Macrophages (high)
CD83
HB15 (cryo)
3
DCs
HLA II
DK22 (cyt)
2
fl indicates indirect immunofluorescence and flow cytometry; cyro, indirect
immunofluorescence on frozen sections; paraffin, indirect immunofluorescence on
paraffin section; WBs, Western blots.
*Antibodies used in the detection of PrPc in tissues or slide preparations, together
with antibodies used for cellular recognition. A wider range of antibodies for the
characterization of DCs by indirect immunofluorescence is given in “Materials and
methods” under generation and assessment of DCs.
†Sources of antibodies were 1, Prionics (Zurich, Switzerland); 2, Dako (Cambridge, United Kingdom); and 3, LRF Immunodiagnostics Unit (Oxford, United
Kingdom).
The cellular expression of the normal form of prion protein PrPC
was assessed in normal human spleen using immunohistochemistry. The expression of PrPC was weak or absent on most cell types
and on stromal elements, and in contrast to the reported accumulation of PrPSc within lymphoid follicles in infected tissue, strong
expression of PrP was not detected within lymphoid follicles in
these normal spleen sections. However, cells that strongly expressed PrPC were scattered throughout the splenic red pulp and
particularly concentrated adjacent to the marginal zone of the white
pulp (Figure 1). To establish the identity of these cells we
performed immunocytochemistry with lineage-specific antibodies.
Double immunofluorescence staining revealed that these PrPC⫹
cells also expressed CD13, CD11c, and CD86, but did not express
markers of FDCs (CR4/23), T cells (CD3), B cells (CD20 and
CD79), or macrophages (CD68) (Figure 2 and data not shown). We
concluded that the distribution of the PrPC-expressing cells and
their expression of lineage-defining antigens was consistent with
the distribution and phenotype of myeloid DCs.
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BLOOD, 15 DECEMBER 2001 䡠 VOLUME 98, NUMBER 13
MYELOID DENDRITIC CELLS AND NORMAL PRION PROTEIN
3735
We were also interested in whether the glycoforms of PrPC
expressed by myeloid DCs derived in vitro resembled the in vivo
glycoforms of PrPC found in spleen. PrPC is glycosylated in a
tissue-specific manner, and these differences are reflected by
corresponding changes in the apparent molecular weight of these
species on SDS-PAGE.26 When molecular weights of forms of PrPC
found in lysates from monocyte-derived DCss were compared with
those from brain or spleen, the distributions of apparent molecular
weights most closely resembled those of spleen (Figure 3D).
Finally, nonquantitative RT-PCR of total RNA from myeloid DCs
showed that the PrP gene was transcribed in these cells (Figure
3E). We concluded that the expression of PrPC was strongly
up-regulated during the differentiation and maturation of myeloid
DCs in vitro.
The intracellular localization of prion protein
In mice, previous studies have suggested a functional role for PrPC in
T-cell activation. We were, therefore, interested in determining the
Figure 1. Localization of PrPC-expressing cells in tissue from human spleen.
The bulk of strongly PrPC-expressing cells (clone 3F4, red) are found in the red pulp
(RP) immediately adjacent to the white pulp (WP). Tissue sections were counterstained with hematoxylin.
Prion protein expression is specifically up-regulated during the
generation of myeloid DCs in vitro
To confirm our findings that myeloid DCs strongly express prion
protein, we generated DCs from monocytes. Such cells are widely
used as a model of myeloid DCs23,24; these DCs generated in vitro
cannot fully model cells in tissues that receive different stimuli and
may vary in phenotype. However, DCs of splenic red pulp and
marginal zone share many features with cells of monocyte lineage,25 and thus monocyte-derived DCs were selected as a relevant
system to confirm our in vivo findings. Freshly isolated peripheral
blood monocytes express PrPC at low levels.20 We found that
during the differentiation of monocytes to immature DCs in vitro
the surface expression of PrPC increased 4-fold; this increased
further when the DCs were matured with LPS to give an
approximately 8-fold increase compared to monocytes (Figure
3A,B). When the total cellular expression of PrPC was determined
by SDS-PAGE and Western blotting a similar trend of increased
expression was observed; however, increases in total expression
were much greater than for surface expression, suggesting a
significant intracellular pool of PrPC in myeloid DCs (Figure 3C).
By contrast, macrophages generated from monocytes in vitro and
subsequently exposed to LPS showed only a 3-fold increase in the
surface expression of PrPC (Figure 3B) and a small increase in the
total level of expression of PrPC (Figure 3C).
Figure 2. The human spleen cells that strongly express PrPC have an immunophenotype consistent with myeloid DCs. (A-H) Identification of PrPC-expressing
cells by dual-color immunolocalization. PrPC was identified using rabbit antiserum
R26 and detected using Texas red-labeled antirabbit immunoglobulin (red) in all
cases. Lineage-restricted mouse monoclonal antibodies (as indicated) to normal
splenic cell types were detected using FITC-labeled antimouse immunoglobulin
(green). Cell nuclei were identified by DAPI (blue). Sensitivity of the detection system
was adjusted to identify only the strongly expressing cells. Strong PrPC-expressing
cells are rarely found in the T-cell area (panel A, CD3) or B-cell area (panel B, CD20);
the antigen is not strongly expressed on FDCs (panel C, CR4/23). Cells strongly
expressing PrPC are clearly separate from surrounding monocytes/macrophages
(panel D, CD68). Unstained cells (top right) represent an adjacent lymphoid area.
Coexpression of PrPC with CD13 (panel E and detail panel G) and CD11c (panel F
and detail panel H) confirms myeloid origin and is indicated by an orange-yellow tone.
Note that for CD11c (panel H) the cells coexpress PrPC and CD11c, but the
intracellular localization is not identical. CD13⫹ or CD11c⫹, which do not express
PrPC, are likely to represent monocyte-macrophage cells (panels E and F).
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3736
BURTHEM et al
BLOOD, 15 DECEMBER 2001 䡠 VOLUME 98, NUMBER 13
that of MHC class II molecules in immature and mature DCs, a striking
colocalization was revealed at the level of light microscopy (Figure 4C).
Discussion
Figure 3. The expression of cellular PrPC is substantially up-regulated during
differentiation of monocytes to myeloid DCs. Flow cytometric analysis of PrPC
expression during the maturation of DCs (A) or macrophages (B). Adherent
peripheral blood mononuclear cells (PBMCs) were differentiated to DCs (A) or
macrophages (B) in vitro. PrPC surface expression was monitored by staining PrPC
(monoclonal antibody 6H3) and subsequent flow cytometry at days 2, 5, 7, and 10 of
cell culture. PrPC expression is progressively up-regulated on successive days
(d2-d10) for DCs (A), but there is no substantive change for macrophages (B). Isotype
controls are indicated by i. (C) Western blots confirm the substantive up-regulation of
total cellular PrPC during differentiation of adherent PBMCs to DCs (left lanes, days 2,
5, 7, 10), but not in macrophages (right lanes, days 2, 5, 7, 10). Note that the
high-molecular-weight species visible at day 10 of DC maturation (left-hand lanes)
represents a minor band only visible at very high levels of expression. (D) Molecular
weight species of PrPC in lysates prepared from cultured human DCs compared with
lysates of tissue from human brain and human spleen. The protein loading has been
adjusted to allow comparison of tissues. DCs and spleen contain equal protein
loading; brain loading is 10% of the total protein level of splenic tissue (determined by
scanning densitometry of Coomassie-stained gels). Myeloid DCs generated in vitro
and spleen express similar glycoform species. (E) Agarose gel electrophoresis of
RT-PCR products of the PrP gene. A 528-bp fragment of the gene coding for PrP was
amplified by RT-PCR from messenger RNA from myeloid DCs (lane 2), LPSstimulated myeloid DCs (lane 3), and monocytes (lanes 4 and 5). No amplification of
the PrP gene in the same samples by Taq polymerase (lanes 6-9) shows that no DNA
was present in these samples. Amplification of the PrP gene from genomic DNA is
shown for positive controls (lanes 10 and 11). One-Kb standard ladders (Gibco-BRL)
are shown (lanes 1 and 12).
intracellular localization of PrPC within different functional compartments of the myeloid DCs, in particular those associated with adhesion
or antigen processing. We allowed cultured monocyte-derived DCs to
adhere and spread on glass slides, and used dual-fluorescence immunostaining to determine the colocalization of PrPC with cytoskeleton,
major histocompatibility complex (MHC) class II, or endosomes within
those cells. These experiments showed that PrPC distribution was
separate from that of adhesion plaque components mediating contact
with the substratum or adjacent cells. PrPC was also absent from
endocytotic vesicles visualized by the uptake of FITC-dextran (Figure
4A,B). In contrast, when the distribution of PrPC was compared with
In tissue from animals infected by transmissible spongiform
encephalopathy, and in humans suffering vCJD, PrPSc is particularly concentrated on FDCs.6-8 In the present study we show that in
the absence of prion disease, high levels of expression of the
normal form of prion protein in human spleen occur principally on
myeloid DCs immediately adjacent to the white pulp, whereas
FDCs do not strongly express PrPC. In prion diseases FDCs heavily
express PrPSc. PrPC is also found on FDCs in most, but not all,
studies of mice.27,28 We are not aware of previous studies of
myeloid DCs, but recent reports in sheep have identified PrPSc
on myeloid cells with a “dendritic morphology”29; also PrPC
expression has been identified on related microglial cells within
the CNS.30
The distinction between FDCs and myeloid DCs is important
because the 2 cell types are ontologically and functionally distinct.26 Myeloid DCs are derived from bone marrow precursor cells
or from monocytes and their precursors and are readily identified
within circulating cell populations. In spleen, myeloid DCs are
found in the red pulp and immediately adjacent to the white pulp.
The cells migrate into the lymphoid areas after receiving a
maturation stimulus where they are powerful mediators of T-cell
activation. The origin of FDCs remains controversial; although
evidence is accumulating that they too may be derived from bone
marrow cells,31 their turnover and repopulation is slow, and they
are not believed to recirculate.32 In spleen, FDCs are confined to
lymphoid follicles and present surface-bound antigen to maturing
B cells. The high levels of expression of PrPC on myeloid DCs, but
not on surrounding macrophage cells, suggests that PrPC is selectively up-regulated during the ontogeny of myeloid DCs in vivo.
Figure 4. PrPC colocalizes with the MHC class II compartment in monocytederived DCs. Adherent immature monocyte-derived DCs were allowed to ingest
FITC-dextran for 15 minutes to reveal endocytic vesicles (A), stained for talin to
reveal adhesion plaques (B), or stained for MHC class II molecules to reveal the MHC
class II compartment (C). PrPC is shown in red in each instance. Colocalization of
green-red signal gives yellow fluorescence.
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BLOOD, 15 DECEMBER 2001 䡠 VOLUME 98, NUMBER 13
Our data showing a distinct cellular and subcellular expression
of PrPC in antigen- presenting cells may help shed light on the, as
yet, undefined functional role of PrPC. PrPC is a glycosylphosphatidylinositol (GPI)–linked molecule that may be expressed at the cell
surface or be associated with intracellular vesicles.33,34 It has
recently been shown that GPI-linked molecules localize within
lipid rafts where they may play an important role in costimulatory
functions, including activation of T cells (reviewed by Ilangumaran
and coworkers35). The protein has a high-affinity copper-binding
site and has structural homology with signal peptidases,36 although
no enzyme function has been demonstrated for PrPC. The protein
has a widespread tissue distribution, but is particularly expressed in
neurologic tissue and on cells of the immune system. Recent
reports have shown an increase in PrPC expression in monocytes
stimulated by ␥-interferon.21 Taken together with our data these
findings suggest that, at least in the immune system, PrPC expression is controlled during cellular development and activation.
Consistent with such a role, we have shown that the prion
protein was colocalized with MHC class II molecules in immature
and mature myeloid DCs. The colocalization of PrPC with MHC
class II molecules reported in the present study does not define the
function of PrPC; indeed, the MHC class II compartment is large,
and our study does not precisely identify the role of PrPC within
that compartment. Nevertheless, the coexpression of PrPC with
MHC class II in monocyte-derived DCs is consistent with a
potential role for PrPC in the stimulation of a variety of immunologic cells. Indeed, in mice, PrPC has been indirectly linked to a role
in T-cell activation by observations that peptides from PrPC may
directly induce T-cell activation.37,38 Although there have been no
detailed reports of immune function in PrPC⫺/⫺ knockout mice, in
the absence of PrPC concanavalin-A–induced activation of T cells
is reduced.39 DCs are believed to be central to the ability of
concanavalin-A to activate T cells.40 These data, taken together
with our observations that PrPC is colocalized with MHC class II
molecules in myeloid DCs, are consistent with an indirect or direct
role for PrPC in T-cell activation by antigen-presenting cells.
Recent data has suggested a role for PrPC in the processes of
morphogenesis and adhesion41,42 and that the protein distributes to
dendritic extensions on neurites.43 We did not, however, detect a
link with adhesion structures or a distribution to dendritic extensions in our studies. The molecule is also known to be endocytosed
in relation to various stimuli44,45; we did not however detect the
presence of PrPC in early endocytotic vesicles.
MYELOID DENDRITIC CELLS AND NORMAL PRION PROTEIN
3737
Our findings may be relevant to the spread of abnormal forms of
prion protein. The spread of PrPSc in an infected host must require
the conversion of PrPC to PrPSc. In mice high titers of infectivity
occur in the spleen soon after intraperitoneal inoculation with
PrPSc, and subsequent spread to the CNS can be retarded, though
not prevented, by splenectomy.15 PrPSc has been detected on FDCs,
B cells, and T cells in the lymphoid follicles of mice, and recently,
FDCs have been shown to be required for transmission of PrPSc in
mice.9 However, splenic infectivity can be restored in PrPC
knockout mice using transplanted normal bone marrow, suggesting
an importance for a rapidly repopulated bone marrow-derived cell
population,16 and in some mouse models spread of PrPSc from
peripheral tissues to the CNS occurs in the absence of functional
FDCs or T lymphocytes.46 These findings strongly suggest that
PrPSc transmission and propagation is a complex process requiring
the interaction of several cell types. In the present study we have
established that the PrPC expressed by myeloid DCs is the major
reservoir of normal prion protein in human spleen. The relationship
between levels of PrPC expression, and the susceptibility of a cell to
PrPSc infection, is not simple; overexpression of PrPC within an
organism confers an increased susceptibility to prion diseases,47,48
but this is not true for all tissues or cell types. This suggests the
characteristics of the cell(s) expressing PrPC are also important.
Dendritic cells are derived from precursors within the bone
marrow and migrate widely.49-52 They make early contact with
ingested or inoculated antigen and can transport it to secondary
lymphoid organs, where there is extensive interaction with other
immune cell types including B cells and other cells within the
germinal centers.53 Given our novel findings we suggest these
myeloid-derived DCs may be important in the spread of prion
disease and that monocyte-derived DCs may provide a model for
the study of PrPC function in human cells.
Acknowledgments
We would like to thank Professor K. Gatter for his help and
encouragement in this work, Dr John Kurtz for helpful comments
on the manuscript, Robin Roberts-Gant and Kingsley Micklem for
their skilled and dedicated help in the preparation of the figures and
other members of the Cellular Science Research Laboratory and
Medical Informatics Unit at the John Radcliffe Hospital in Oxford
for their help and advice.
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2001 98: 3733-3738
doi:10.1182/blood.V98.13.3733
The normal cellular prion protein is strongly expressed by myeloid dendritic
cells
John Burthem, Britta Urban, Arnab Pain and David J. Roberts
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