Báo cáo khoa học: Differential post-translational modification of CD63 molecules during maturation of human dendritic cells potx

Differential post-translational modification of CD63 molecules
during maturation of human dendritic cells
Anneke Engering
1,3
, Lotte Kuhn
1,2
, Donna Fluitsma
3
, Elisabeth Hoefsmit
3
and Jean Pieters
1,2
1
Basel Institute for Immunology, Basel, Switzerland;
2
Biozentrum, University of Basel, Basel, Switzerland;
3
Department of Cell Biology and Immunology, Vrije Universiteit, Amsterdam, The Netherlands
The capacity of dendritic cells to initiate T cell responses is
related to their ability to redistribute MHC class II mole-
cules from the intracellular MHC class II compartments to
the cell surface. This redistribution occurs during dendritic
cell development as they are converted from an antigen
capturing, immature dendritic cell into an MHC class II-
peptide presenting mature dendritic cell. During this matu-
ration, antigen uptake and processing are down-regulated
and peptide-loaded class II complexes become expressed in
a stable manner on the cell surface. Here we report that
the tetraspanin CD63, that associates with intracellularly
localized MHC class II molecules in immature dendritic
cells, was modified post-translationally by poly N-acetyl

lactosamine addition during maturation. This modification
of CD63 was accompanied by a change in morphology of
MHC class II compartments from typical multivesicular
organelles to structures containing densely packed lipid
moieties. Post-translational modification of CD63 may be
involved in the functional and morphological changes of
MHC class II compartments that occur during dendritic
cell maturation.
Keywords: antigen presentation; poly N-acetyl lactosamine
addition; dendritic cells; tetraspanins; MHC class II.
Dendritic cells have the unique feature to induce T cell
responses in lymphoid organs against antigens captured in
peripheral tissues (for review, see [1,2]). Immature tissue
dendritic cells use several mechanisms to internalize a broad
array of antigens via the endosomal–lysosomal pathway
including fluid phase endocytosis, macropinocytosis and
several receptor-dependent mechanisms [3–5]. Peptides
derived from internalized antigens are loaded onto class II
molecules in MHC class II compartments [6–9]. After
migration to lymph nodes upon inflammation or an
infection, mature dendritic cells present these MHC
class II-peptide complexes to T lymphocytes.
Several co-ordinated changes enable efficient presenta-
tion of epitopes generated at sites of inflammation to
T lymphocytes for prolonged periods of time [10,11].
During maturation of dendritic cells, the number of MHC
class II-peptide complexes that are generated is increased,
both by a transient up-regulation of synthesis as well as by
an increase in half-life of MHC class II molecules [10]. In
addition, uptake and processing of antigen is down-

regulated and MHC class II molecules are redistributed to
the cell surface. Trafficking of MHC class II-peptide com-
plexes is in part regulated by the protease, cathepsin S, that
is activated upon maturation of dendritic cells [12,13]. This
protease removes the sorting signal in the cytoplasmic tail of
the invariant chain, a protein involved in targeting newly
synthesized MHC class II molecules to MHC class II com-
partments [14–16], thus allowing MHC class II molecules
to exit these organelles. Moreover, maturation induces a
reduction of internalization and degradation of cell surface
MHC class II molecules [17]. These mechanisms result in
the stable expression of MHC class II-peptide complexes
on the cell surface of mature dendritic cells [10,11,18].
Recently, more insight has been gained into the
transport-routes of MHC class II molecules to the plasma
membrane [19,20]. Direct fusion of multivesicular
MHC class II compartments has been shown to occur,
resulting in secretion of the internal MHC class II-con-
taining vesicles, so-called exosomes [21,22]. However, this
route is down-regulated upon maturation of dendritic cells
and may represent only a minor pathway of MHC class II
transport to the cell surface [22,23]. Recently, using GFP-
tagged MHC class II molecules in living murine dendritic
cells, it was shown that upon a maturation stimulus,
tubular MHC class II-containing endosomes extend from
MHC class II compartments and can fuse directly with
the plasma membrane [24,25]. Interestingly, these tubules
were directed towards the contact phase with a T cell in
an antigen-dependent manner [24]. In human dendritic
cells, immuno-electron microscopy also demonstrated the

appearance of MHC class II-containing tubules and vesi-
cles upon induction of maturation [26]. These structures
were suggested to represent transport intermediates
between MHC class II compartments and the plasma
membrane [26,27]. The underlying mechanisms of trans-
porting MHC class II-containing vesicles to the cell
surface remain unclear.
Correspondence to J. Pieters, Biozentrum, University of Basel,
Klingelbergstrasse 50–70, 4056 CH Basel, Switzerland.
Fax: + 41 61 267 21 49, Tel.: + 41 61 267 21 51,
E-mail:
Abbreviations: CD, cluster of differentiation; endo H, endoglyco-
sidase H; LAMP, lysosomal-associated membrane proteins; LPS,
lipopolysaccharide; MHC, major histocompatibility complex.
(Received 18 February 2003, accepted 7 April 2003)
Eur. J. Biochem. 270, 2412–2420 (2003) Ó FEBS 2003 doi:10.1046/j.1432-1033.2003.03609.x
We have reported recently that distinct tetraspanins
associate with MHC class II molecules at different sites in
immature dendritic cells [28]. CD9, CD53 and CD81
associate with surface MHC class II molecules, whereas
CD63-MHC class II complexes are present exclusively in
MHC class II compartments. Similarly, in B cells, CD63 as
well as the tetraspanin CD82 were shown to associate with
MHC class II molecules intracellularly [29]. In this paper,
we describe that CD63 is modified post-translationally upon
maturation of dendritic cells. This modification was attri-
buted to the addition of poly N-acetyl lactosamine groups
onto CD63. Interestingly, CD63-positive organelles chan-
ged morphologically during dendritic cell maturation –
from compartments with a multilaminar appearance into

structures containing multiple condensed lipid layers. The
increase in lactosaminoglycans on CD63 may be involved
in the changes that occur in MHC class II compartments
during dendritic cell maturation.
Materials and methods
Antibodies, reagents and cells
The following antibodies were used: I98 (anti-CD63, IgG
1
),
anti-CD63 (IgG
1
, CLB, Amsterdam), and a polyclonal
antibody against MHC class II (kind gift of H. Ploegh,
Harvard Medical School, Boston, MA, USA).
Dendritic cells were generated from human peripheral
blood monocytes cultured for 4–8 days in RPMI-1640
medium supplemented with 10% fetal bovine serum
(Hyclone), 50 ngÆmL
)1
recombinant GM-CSF (Leucomax,
Sandoz) and 1000 UÆmL
)1
recombinant IL-4 [4,30]. Buffy
coats were from healthy blood donors, upon written
consent. To induce maturation, dendritic cells were stimu-
lated with LPS for 40 h (1 lgÆmL
)1
, S. abortus equi,Sebak).
The human melanoma cell line, Mel JuSo [31] was grown in
RPMI-1640 supplemented with 10% fetal bovine serum;

cells were stimulated by culturing for 48 h in 500 UÆmL
)1
Interferon-c (IFN-c)(Pharmingen).
Metabolic labeling and immunoprecipitation
Prior to metabolic labeling, cells were cultured in RPMI
without methionine and cysteine for 20 min. Cells were
labeled for the times indicated in the same medium
containing 0.1–0.2 mCiÆmL
)1
[
35
S]methionine/cysteine and
10% dialyzed fetal bovine serum. Cells were washed and
chased in complete medium, supplemented with 2 m
M
methionine and cysteine, or lysed directly. Lysis buffer
contained 20 m
M
Hepes (pH 7.5) with 100 m
M
NaCl, 5 m
M
MgCl
2
, 1% Triton X-100 with protease inhibitors [32].
For immunoprecipitation, lysates were incubated with
the indicated antibodies for 2–12 h at 4 °C, followed by 1 h
incubation with 30 lL protein A-Sepharose (Pharmacia).
The immune complexes were washed as described [33],
eluted from the protein A-Sepharose beads by incubation at

95 °C for 5 min in Laemmli sample buffer [34] and
subjected to SDS/PAGE, fluorography and autoradio-
graphy. When indicated, half of the immune complexes
were incubated prior to elution with 10 mU endo-b-
galactosidase (Bacteroides fragilis, Boehringer Mannheim)
in 50 m
M
sodium acetate (pH 5.8) with 0.2 mgÆmL
)1
bovine
serum albumin for 24 h at 37 °C. As a control, the enzyme
was omitted.
Two-dimensional gel electrophoresis
Two-dimensional IEF/SDS/PAGE was performed accord-
ing to O’Farrell [35] with described modifications [36].
Resolyte pH 4–8 (BDH) was used for IEF.
Immunocytochemistry
Dendritic cells were fixed at room temperature in 2%
paraformaldehyde and 0.5% glutaraldehyde in NaCl/P
i
for
2 h. Cells were pelleted and resuspended in 2% para-
formaldehyde at 4 °C. Subsequently, samples were infused
with 2.3
M
sucrose and frozen quickly in liquid nitrogen.
Ultrathin cryosections were labeled with specific primary
antibodies as indicated, followed by colloidal gold particles
coupled to protein A. To minimize cross reactivity of
protein A-gold particles, sections were fixed briefly using

1% glutaraldehyde before double-labeling. Cryosections
were analyzed on a CM 100 electron microscope (Philips).
Subcellular fractionation
Subcellular fractionation of dendritic cells was performed
essentially as described [4,7,37]. Dendritic cells were harves-
ted, washed and resuspended in homogenization buffer
(10 m
M
triethanolamine, 10 m
M
acetic acid, 1 m
M
EDTA,
0.25
M
sucrose, pH 7.4) at 10
7
cellsÆmL
)1
. The cells were
homogenized at 4 °C by passing through a 27G3/4 needle.
After removal of the nuclei by centrifugation (840 g,
15 min), the postnuclear supernatant was incubated with
trypsin (25 lgÆmg
)1
protein) for 5 min at 37 °C. Digestion
was stopped by addition of ice-cold soybean trypsin
inhibitor (625 lgÆmg
)1
protein). Membranes were sediment-

ed by centrifugation for 45 min at 100 000 g, resuspended
in 6% Ficoll-70 (Pharmacia) in homogenization buffer and
electrophoresed for 90 min at 10.4 mA [7]. Fractions of
0.5 mL were collected from the top and analyzed for the
different markers. Protein levels were analyzed according to
the Bradford method [38]. The activity of b-hexosaminidase
was assayed as described [39].
Results
Analysis of CD63 expression during dendritic cell
maturation
Upon maturation, dendritic cells undergo a number of
changes that contribute to their capacity to induce T cell
responses [10,11,24–26]. These changes include biochemical
as well as morphological alterations in organelles and
molecules involved in MHC class II-restricted T cell
activation. The tetraspanin CD63 associates with
MHC class II molecules within class II compartments in a
number of antigen presenting cells [29,40], including human
immature dendritic cells [28]. To analyze CD63 expression
during maturation, immature and mature dendritic cells
were pulse-labeled with [
35
S]methionine/cysteine, and CD63
molecules immunoprecipitated and analyzed by SDS/
PAGE and fluorography. After a 20-min labeling period,
Ó FEBS 2003 CD63 glycosylation during dendritic cell maturation (Eur. J. Biochem. 270) 2413
in both immature and mature dendritic cells, a protein of
34 kDa was resolved (Fig. 1A, lane 1 and 3). However, after
a 4-h pulse, in immature dendritic cells, anti-CD63 Igs
immunoprecipitated alongside the 34 kDa polypeptide, an

 50 kDa protein, the molecular mass of which increased
to  70 kDa after maturation of the cells induced by LPS
(Fig. 1A, lane 2 and 4). The increase in molecular mass of
CD63 was independent of the stimulus used to induce
maturation of dendritic cells (not shown). In all other cell
types tested (including B lymphoblastoid, HeLa, Mel JuSo
cells, with or without stimulation by LPS or IFN-c)CD63
antibodies precipitated a 34 and 50 kDa protein (shown for
Mel JuSo, Fig. 1A, lanes 5–8). This indicates that the
34 kDa protein of CD63 either became associated with
distinct proteins during maturation, or that CD63 was
differentially modified post-translationally in immature vs.
mature dendritic cells.
To distinguish between these two possibilities, pulse/chase
analysis was performed. As shown in Fig. 1B, in both
immature and mature dendritic cells, the amount of the
34 kDa form of CD63 disappeared gradually during the
chase period, concomitantly with the appearance of isoforms
of CD63 of  50 and  70 kDa, respectively. Together, these
data indicate that the 70 kDa isoform of CD63 is a result of
post-translational modifications of the 34 kDa isoform
that occurred exclusively in mature dendritic cells.
Characterization of post-translational modifications
on CD63 during maturation of dendritic cells
CD63 contains three putative acceptor sites for N-linked
glycosylation [41]. To analyze the addition of carbohydrates
to these sites in CD63 molecules from immature and mature
dendritic cells, both cell types were labeled for 10 min
with [
35

S]methionine and -cysteine, lysed and proteins were
precipitated using anti-CD63 Igs. Immuno-isolated com-
plexes were treated with endoglycosidase H (endo H). This
enzyme cleaves N-linked glycans of the high mannose form
that are acquired upon synthesis in the endoplasmic
reticulum. As shown in Fig. 2A, endo H treatment resulted
in a reduction of the apparent molecular mass to  25 kDa
in immature as well as in mature dendritic cells, indicating
identical N-linked glycosylation of CD63 in immature as
well as mature dendritic cells.
To analyze the type of modification on CD63 occurring
during maturation, immature and mature dendritic cells
were labeled metabolically for 16 h and immunoprecipi-
tated material analyzed by two-dimensional IEF/SDS/
PAGE followed by fluorography. In immature cells, besides
the 34 kDa form of CD63, eight additional spots of
 50 kDa were resolved. The shapes of these spots indicated
that they represent forms of CD63 that have been modified
post-translationally with carbohydrate residues. Interest-
ingly, after maturation of dendritic cells, the apparent
molecular mass of each of these eight carbohydrate
containing polypeptides was increased by  10 kDa
(Fig. 2B). In mature dendritic cells, the 34 kDa form of
CD63 was not detectable anymore, indicating a more
complete conversion of CD63 to the complex-type carbo-
hydrate form than in immature dendritic cells during the
16 h labeling period. Together, these results indicate that
not only the type or complexity, but the degree of CD63
glycosylation differed in immature vs. mature dendritic cells.
A modification known to result in differences in mole-

cular mass, rather than charge, is addition of poly
N-acetyl lactosamine to N-linked glycans, and this post-
translational modification is known to occur on lysosomal
associated membrane glycoproteins (LAMP) [42,43]. Poly
γ
Fig. 1. CD63 isoforms in immature and mature dendritic cells. (A) Immature dendritic cells, mature dendritic cells (DC) and Mel JuSo cells incubated
for 48 h in the absence or presence of IFN-c were labeled metabolically for 20 min or 4 h using [
35
S]methionine/cysteine and lysed. Proteins were
immunoprecipitated with anti-CD63 Igs. (B) Immature and mature dendritic cells were labeled metabolically for 10 min, washed and cultured for
the times indicated before lysis and immunoprecipitation with anti-CD63. Shown are autoradiographs after SDS/PAGE and fluorography.
2414 A. Engering et al. (Eur. J. Biochem. 270) Ó FEBS 2003
N-lactosamine groups are sensitive to endo-b-galactosidase
[43]. To analyze the presence of polylactosaminoglycans,
immature dendritic cells and maturing cells, cultured for
12 h in the presence of LPS, were labeled metabolically for
4 h, followed by lysis and immunoprecipitation using anti-
CD63 or anti-LAMP Igs. Immune complexes were incuba-
ted for 24 h at 37 °C in the presence or absence of 10 mU
endo-b-galactosidase. As can be seen in Fig. 3A and B in
both immature and maturing dendritic cells, CD63 as well
as LAMP molecules are susceptible to digestion with endo-
b-galactosidase. Therefore, these results indicate that poly-
lactosaminoglycan addition already occurs in immature
cells, and that during maturation these molecules acquire
additional polylactosaminoglycans. Indeed, when immature
dendritic cells were induced to mature by LPS, a gradual
increase in the molecular masss of both CD63 and LAMP
was observed, reaching a maximum after 12–24 h, with a
subsequent decrease of the apparent molecular mass

(Figs 3C,D). As can be seen in Fig. 3C,D, both the
biosynthesis and the polylactosaminoglycan addition is
dependent on the state of maturation, as has also been
observed to occur for other molecules during maturation
[10].
Stability and subcellular localization of CD63 molecules
in immature and mature dendritic cells
Maturation of dendritic cells is known to result in a
differential stability of MHC class II [10,11]. Given the
association of CD63 molecules with MHC class II mole-
cules in MHC class II compartments [28,29], we analyzed
whether the addition of polylactosaminoglycan moieties on
CD63 molecules resulted in an altered stability of CD63. To
that end, immature and mature dendritic cells were pulsed
for1 hwith[
35
S]methionine/cysteine followed by a chase for
thetimesindicatedinFig.4.AsshowninFig.4,bothin
immature as well as mature dendritic cells, CD63 molecules
displayed a similar half life of  15 h, indicating that the
degree of polylactosaminoglycan modification does not
alter CD63 stability.
In immature dendritic cells, CD63 is largely located
intracellularly within MHC class II compartments
[28,29,40]. To investigate whether the post-translational
modification of CD63 molecules by polylactosaminoglycans
may coincide with an altered subcellular localization,
organelles from [
35
S]methionine/cysteine metabolically labe-

led immature as well as mature dendritic cells were
separated by organelle electrophoresis. Upon electropho-
resis, MHC class II compartments and late endosomal
lysosomal organelles shift towards the anode, whereas the
plasma membrane and most other subcellular organelles
do not migrate significantly [4,7]. After fractionation, the
fractions containing the lysosomal marker b-hexosamini-
dase were pooled (ÔPool IÕ) as well as the nonshifted fractions
(ÔPool IIÕ; see Fig. 5A). From these pooled fractions, CD63
as well as LAMP molecules were immunoprecipitated and
analyzed by SDS/PAGE and fluorography. As shown in
Fig. 5B in both immature and mature dendritic cells, CD63
as well as LAMP were largely present within pool I,
indicating that before and after maturation of den-
dritic cells, CD63 remained localized within MHC class II
compartments.
Immunocytochemical localization of CD63 in immature
and mature dendritic cells
The subcellular distribution of CD63 molecules was
analyzed further by immunocytochemistry using immature
and mature dendritic cells. In immature dendritic cells,
CD63 colocalized with class II molecules in MHC class II
compartments, predominantly with a multilaminar appear-
ance (Fig. 6Aa). CD63 was occasionally detected in small
Fig. 2. Two dimensional gel electrophoresis of CD63 isoforms. (A) Core-glycosylation of CD63 molecules. Immature (imm. DC) and mature
dendritic cells (mat. DC) were labeled metabolically with [
35
S]methionine/cysteine for 10 min and lysed. Immune-isolated complexes after preci-
pitation with anti-CD63 Igs were incubated with (+) or without (–) endo-glycosidase H (endo H) for 14 h at 37 °C. An asterix indicates the
deglycosylated forms of CD63. (B) Immature and mature dendritic cells were labeled with [

35
S]methionine/cysteine for 14 h. After lysis, proteins
were immunoprecipitated with anti-CD63 Igs and analyzed by IEF/SDS/PAGE. Shown are autoradiographs after SDS/PAGE and fluorography,
basic end, right; acidic end, left.
Ó FEBS 2003 CD63 glycosylation during dendritic cell maturation (Eur. J. Biochem. 270) 2415
electron dense vesicles, devoid of MHC class II molecules
(Figs 6Aa,c), potentially representing a transport vesicle.
After maturation, MHC class II molecules are redistributed
to the cell surface, whereas CD63 was still predominantly
found intracellularly (Figs 6Bb,d). Interestingly, the mor-
phology of CD63 positive organelles changed upon matur-
ation from the typical multilaminar structure into an
organelle that appeared to contain densely packed lipid
moieties (Fig. 6A). This difference in morphology is further
illustrated in Fig. 6B, that shows several sections labeled for
MHC class II molecules. In mature dendritic cells, densely
packed structures were abundantly present (Figs 6Bb–d)
but were rarely found in immature dendritic cells (Fig. 6Ba).
Taken together, these results indicate that concomitant with
the acquisition of polylactosaminoglycans on CD63, the
MHC class II compartments became altered, both with
respect to their morphology as well as the occupancy.
Discussion
Dendritic cells efficiently present epitopes from antigens
captured at sites of inflammation to naive T lymphocytes in
lymphoid organs by regulating MHC class II distribution
and antigen-internalization mechanisms [1–3,18]. In imma-
ture dendritic cells, antigens are efficiently internalized and
antigenic peptides loaded onto MHC class II complexes
intracellularly [44]. Maturation stimuli induce a transient

enhancement of antigen uptake and peptide loading,
whereas in fully matured dendritic cells, antigen uptake is
down-regulated and peptide-loaded class II complexes are
redistributed to the cell surface [10,11,44]. The molecular
mechanisms regulating these processes are not well under-
stood. In this paper, we describe that during maturation
of dendritic cells, CD63 acquired additional polylactos-
aminoglycans, concomitant with morphological changes
β
Fig. 3. CD63 and LAMP glycosylation in immature and mature dendritic cells Poly N-acetyl lactosaminoglycans on CD63 and LAMP molecules
during maturation of dendritic cells. (A,B): Dendritic cells were incubated with LPS for the times indicated, followed by metabolic labeling with
[
35
S]methionine/cysteine for 4 h. Cell lysates were immunoprecipitated with anti-CD63 (A,C) or anti-LAMP (B,D) Igs. In A and B, immuno-
isolated complexes were incubated in the presence (+) or absence (–) of endo-b-galactosidase (endo-b)for24hat37°C. The differences in
metabolic labeling are probably to be due to the different efficiencies in labeling at the different times after LPS addition. Shown are autoradio-
graphs after SDS/PAGE (A,C: 12% acrylamide gels; B,D: 7.5% acrylamide gels) and fluorography.
Fig. 4. Stability of CD63 molecules in immature and mature dendritic
cells. Immature and mature dendritic cells were labeled metabolically
with [
35
S]methionine/cysteine followed be a chase in normal medium
for the times indicated. At each chase time, cells were lysed and CD63
molecules immunoprecipitated from the detergent lysates followed
by analysis by SDS/PAGE, fluorography and autoradiography.
2416 A. Engering et al. (Eur. J. Biochem. 270) Ó FEBS 2003
in MHC class II compartments from a characteristic
multivesicular/multilaminar appearance to structures with
dense lipid moieties.
CD63 is generally considered as one of the best markers of

MHC class II compartments besides MHC class II mole-
cules themselves [45,46]. CD63 is a member of the tetraspanin
superfamily, consisting of polypeptides with four membrane
spanning domains that associate with a variety of proteins
[47]. On the cell surface, tetraspanins were found to colocalize
in distinct membrane microdomains that are enriched for
specific MHC class II-peptide complexes as well as costim-
ulatory molecules, thereby facilitating T cell activation
[29,48,49]. It has been shown recently that in human
immature dendritic cells and in B cells, CD63 and CD82
form complexes with MHC class II molecules in class II
compartments, whereas another set of tetraspanins associ-
ate with surface MHC class II molecules [28,29]. Here, we
show that in both immature and mature dendritic cells, the
subcellular localization of CD63 molecules was confined to
the intracellular MHC class II compartment. At later time
points of maturation, MHC class II synthesis and recycling
of MHC class II molecules from the cell surface is shut off
[10,17]. In accordance with this, CD63 positive organelles in
mature dendritic cells were found to be depleted from
MHC class II molecules.
CD63 has been localized to a wide variety of distinct
intracellular organelles whose content or membrane mole-
cules are discharged after appropriate stimuli. These include
the cytolytic granules of cytotoxic T lymphocytes [50,51],
the Weibel–Palade bodies of vascular endothelial cells [52],
the secretory granules of neutrophils and basophils [53,54],
as well as those from megakaryocytes and platelets [55].
Interestingly, all of these organelles rely on stimulated
processes to release their content, a process that may be

similar to the regulated redistribution of MHC class II
molecules from the MHC class II compartment to the
plasma membrane, as occurs during dendritic cell develop-
ment [10]. Indeed, dendritic cells secrete exosomes upon
fusion of multivesicular organelles with the cell surface
[21,23]. Exosomes contain high levels of tetraspanins,
including CD63, in agreement with the localization of
CD63 to internal membranes of multivesicular bodies [40].
Dendritic cells, however, contain mainly multilaminar
MHC class II compartments and probably only a minor
part of the MHC class II molecules are secreted in
exosomes. Recent data revealed an additional pathway of
transport of MHC class II molecules to the cell surface,
namely via tubules that emerge from MHC class II com-
partment and fuse with the plasma membrane [24–26].
CD63 was found to be absent from these tubules and
remained associated with MHC class II compartments [26],
in agreement with our results. Although we did not observe
tubular structures in our immuno-electronmicroscopy stud-
ies, both the described modification of CD63 (this study)
and the previously reported appearance of tubules occurs
12–20 h after maturation [26]. Whether there is a direct role
for CD63 in the formation of tubules remains to be
investigated.
Maturation of dendritic cells resulted in an increase in the
CD63 molecular mass by  20 kDa, that could be accoun-
ted for by poly N-lactosaminoglycan addition. This modi-
fication differs from the usual complex-type N-linked
saccharides and is characterized by having side chains of
Galb1–4GlcNAcb1–3 repeats. Poly N-lactosaminoglycans

are present on several membrane proteins, including the
lysosomal associated membrane proteins 1 and 2 (LAMP-1

Fig. 5. Subcellular localization of CD63 and
LAMP molecules in immature and mature
dendritic cells. Immature and mature dendritic
cells were labeled metabolically with
[
35
S]methionine/cysteine prior to homogeni-
zation and subcellular fractionation by
organelle electrophoresis. After electropho-
resis, fractions were pooled as indicated in A,
lysed and CD63 or LAMP molecules
immunoprecipitated and analyzed by
SDS/PAGE. Shown are autoradiographs
after fluorography (B).
Ó FEBS 2003 CD63 glycosylation during dendritic cell maturation (Eur. J. Biochem. 270) 2417
and LAMP-2 [42,43]), but the physiological significance of
this post-translational modification has remained unclear.
In human dendritic cells, LAMP also acquired additional
lactosaminoglycans upon dendritic cell maturation (this
study), with similar kinetics as CD63, whereas cell-surface
localized tetraspanins were not modified (not shown). It will
be interesting to analyze the post-translational modifica-
tions on the dendritic cell-specific lysosomal protein DC-
LAMP during dendritic cell maturation, since this protein
was found to colocalize with MHC class II in tubules in
contrast to LAMP and CD63 [26].
One possible function of the extensive glycosylation of

lysosomal membrane proteins is to protect the lysosomes as
well as lysosome-like organelles from excessive degradative
activities. The changes observed in MHC class II compart-
ments from multilaminar organelles to vesicles with densely
packed membranes might be accompanied with changes in
the function of these organelles. In mature dendritic cells,
macropinocytosis has ceased, but receptor-mediated endo-
cytosis is still ongoing, although it is unclear if internalized
antigens can reach degradative organelles. Interestingly,
preliminary studies revealed reduction in the activity of the
lysosomal enzyme b-hexosaminidase in MHC class II com-
partments, in accordance with the previously reported
disappearance of acidic organelles during dendritic cell
maturation [56].
Neither the stability nor the subcellular localization of
CD63 molecules was altered upon the addition of poly
N-lactosaminoglycans during dendritic cell maturation.
However, the extensive poly N-lactosaminoglycan addition
on CD63 molecules during dendritic cell development, may
be functionally important to maintain the endo–lysosomal
system during dendritic cell development and accompanies
the dramatic change in lysosomal morphology (see Fig. 6).
Although post-translational modification could occur as a
result from the morphological changes observed upon
dendritic cell maturation, the addition of lactosamino-
glycans on CD63 molecules could be involved in the
modulation of class II peptide loading events, possibly
through contributing to a differential distribution of
associated MHC class II molecules during maturation of
dendritic cells.

Fig. 6. Distribution of CD63 and MHC class II molecules in immature and mature dendritic cells by immunocytochemistry. (A) Immature (a) and
mature (b–d) dendritic cells were fixed and embedded as described in Materials and methods. Ultrathin cryosections were labeled with anti-CD63
and anti-MHC class II Igs, followed by 10 nm and 15 nm protein A-gold, respectively. (a–c) Detail of intracellular organelles, (d) overview. Bar,
50 nm. (B) Morphology of MHC class II-containing organelles in immature and mature dendritic cells analyzed by immunocytochemistry.
Immature (a) and mature (b–d) dendritic cells were fixed and embedded and ultrathin cryosections were labeled with anti-MHC class II Igs,
followed by 15 nm protein A-gold. Bar, 50 nm.
2418 A. Engering et al. (Eur. J. Biochem. 270) Ó FEBS 2003
Acknowledgements
We thank M. Cella and A. Lanzavecchia for discussion and critical
review of the manuscript, S. Paniry for photography and H. L. Ploegh
for antibodies. The Basel Institute for Immunology was founded and
supported by F. Hoffmann-La Roche & Co., Ltd, Basel, Switzerland.
This work was supported in part by the Swiss National Science
Foundation.
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