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CUTTING EDGE |



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Department of Medicine, University of Wales, College of Medicine, Cardiff, United Kingdom;
Sanofi-Synthelabo, Labège, France; and
Institute of Immunology and Transfusion Medicine, Ernst-Moritz-Arndt-University, Greifswald, Germany
| Abstract |
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| Introduction |
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CD14 plays a pivotal role in mediating cell activation induced by
bacterial cell wall components (5). Soluble forms of CD14
(sCD14) are found in normal human plasma at 23 µg/ml concentration
(6, 7, 8), exceeding by one or two logs that of the cell
membrane-bound receptor. Increased sCD14 levels were found in a number
of pathological states (9, 10, 11); however, the physiological
role of sCD14 has remained unclear. Recently, we demonstrated that
sCD14 interacts directly (without LPS) with activated human T cells,
decreasing Ag, and mitogen-induced proliferation (12).
This effect results from inhibition of IL-2 production, and IFN-
and
IL-4 are similarly affected. These findings revealed a novel function
of sCD14: its capacity to regulate T lymphocyte activation and
function. In the course of our studies, we observed that sCD14 bound to
B cells in PBMC. Here, we have studied the interaction of sCD14 with
human B cells and tested the functional consequences for the
T-dependent humoral immune response.
| Materials and Methods |
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Human rsCD14 8 aa shorter at the C terminus (sCD14(348)), the full-length protein (sCD14wt), alanine substitution mutants sCD14 (9194, 96)A and sCD14 (97101)A, prepared by two-step PCR-based mutagenesis, and biotinylated sCD14 were prepared as described (12, 13).
Cells, cultures, and flow cytometry
Tonsillar B cells were prepared from normal individuals
undergoing tonsillectomy, as described (14). The purity of
the B cell preparation was
97% as determined by flow cytometry using
anti CD19, CD14, and CD3 mAb. Tonsillar B cells,
EBV+ B cell lines (Akata and Rael), murine L
fibroblasts transfected with human CD40L (tCD40L) or CD32 (tCD32)
(donated by Prof. M. Rowe, University of Wales) and PBMC from healthy
donors were cultured in RPMI 1640 medium supplemented with 2 mM
L-glutamine, 10 mM HEPES buffer, antibiotics, and 10% FCS
(sCD14 from FCS was estimated to contribute about 10% to the total
amount used in the experiments). For flow cytometry, cells were stained
with biotinylated sCD14 or BSA as control (3 µg/ml), the
FITC-conjugated or purified anti-CD mAb indicated in
Results, or their isotype-matched controls and analyzed with
a FACScalibur cytometer (Becton Dickinson, San Jose, CA)
(12).
Cell stimulation and Ig and cytokine determinations
Total tonsillar B cells (1 x
105/well) were cultured for 10 days with 90%
confluent
-irradiated tCD40L cells (5:1, B/fibroblasts) in the
presence or absence of sCD14. Cultures were supplemented with
anti-human IgM-coated beads (25 µg/ml; anti-µ-chain, Irvine
Scientific, Santa Ana, CA), IL-4 (600 IU/ml), and IL-2 (100 IU/ml) as
indicated in Results. Culture supernatants were tested
for IgG1, IgE, or IL-6 by ELISA using specific matched-paired Abs
(IgG1, PharMingen, San Diego, CA; IgE, Serotec, Oxford, U.K.;
IL-6, R&D Systems, Abingdon, U.K.). PBMC (2 x
105/well) were Ag stimulated (varicella-zoster
virus (VZV), Oka strain, vaccine preparation, 0.05% of a
104 PFU solution) in the presence or absence of
sCD14. At day 2, culture supernatants were tested for IL-2 (ELISA; R&D
Systems), and at day 10 were tested for IgG1 and IgE.
I
B-
analysis
B cell lines (5 x 105
cells/well) were stimulated by coculturing with tCD40L or tCD32 cells
as described above, in the presence or absence of sCD14 (3 µg/ml). At
the indicated time points, cytoplasmic extracts were prepared, equal
amounts (40 µg) run on 15% SDS-PAGE, and analyzed by Western
blotting with an anti-I
B-
Ab (FL Ab, Santa Cruz, CA) as
described (12).
| Results |
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We consistently observed that, depending on the donor, between 24
and 60% of the CD19+ cells, but not resting
CD3+ cells in PBMC, bound sCD14 (Fig. 1
A). Tonsillar B cells were
stained with the activation marker CD38 and also tested for sCD14
binding. sCD14 bound 4592% (depending on the donor) of
CD38-ve/low B cells and a smaller proportion (
45%) of
CD38+ve/high cells (Fig. 1
B). Tricolor
flow cytometric analysis (Fig. 1
C) demonstrated that sCD14
bound CD38-ve/low, IgD+,
and IgD- B cell subsets as well as
CD38+ve/high, IgD+, and
IgD- subsets. This phenotype marker expression
is consistent with that reported for naive
(CD38-ve/low, IgD+),
memory (CD38-ve/low,
IgD-), and germinal center (GC)
(CD38+ve/high, IgD+, and
IgD-) B cells (15). Naive and
IgD+ GC cells showed the highest percentage of
sCD14 binding, (48.7 and 56.2%, respectively, in Fig. 1
C);
however, sCD14-binding IgD+ GC cells represented
a small percentage (11.2%) of the CD38+ve/high
cells. Binding of sCD14 to B cells was saturable and the sCD14 (9194,
96)A, but not sCD14 (97101)A, mutant showed reduced binding (Fig. 1
D), indicating specific interaction of sCD14 with a
putative cellular receptor.
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Total tonsillar B cells were stimulated by culturing on
surrogate-activated T cells (tCD40L cells) in the presence of either
anti-µ Ab-coated beads or the T cell cytokines IL-4 and IL-2; the
cultures were supplemented or not with sCD14 (3 µg/ml) and tested for
IgG1 production (Fig. 2
A). At
every time point, IgG1 production was significantly increased in the
presence of sCD14, irrespective of the B cell stimulating condition.
The magnitude of this effect depended on the donor and was sCD14 dose
dependent (Fig. 2
B, a). We also tested the
effect of sCD14 on IgE production by activated tonsillar B cells (Fig. 2
C). Surprisingly, the time course of IgE production showed
a strong inhibitory effect of sCD14 irrespective of the stimulatory
conditions.
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60%. These results
indicated that use of the truncated form of sCD14 is not critical for
its effect on B cells and that this is a genuine effect of sCD14. Of
note, sCD14 (97101)A binds to B cells (Fig. 1
We asked whether T-dependent Ig production by Ag-stimulated PBMC is
affected by sCD14. Fig. 2
D shows significant increase in
IgG1 and profound inhibition of IgE production when PBMC were
stimulated in the presence of sCD14. Importantly, the sCD14-treated
cultures showed reduced levels of IL-2 (not shown), in agreement with
our previous observations (12).
Kinetics of CD40-triggered degradation of I
B-
is slowed by
sCD14
We studied in human B cell lines the kinetics of CD40-triggered
degradation of I
B-
(Fig. 3
A), which we reported is
affected by sCD14 in T cells (12). CD40 ligation in B
cells by tCD40L, but not by tCD32 cells, induced a progressive and
marked reduction in the level of cytoplasmic I
B-
, which started
after 5 min, and by 50 min the I
B-
polypeptide was almost
undetectable. CD40-activated B cells in the presence of sCD14 showed a
slower kinetics of degradation starting after 15 min, and by 50 min
I
B-
was still detectable. Of note, CD40 B cell-surface expression
was not affected by sCD14 (data not shown).
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IL-6 plays an important role in Ig production by increasing the
CD40- and IL-4-dependent IgE synthesis and promoting Th2 skewing
(16, 17, 18). We tested whether sCD14 affects the CD40- and
IL-4-induced IL-6 secretion by tonsillar B cells (Fig. 3
B).
The kinetics of IL-6 release in the presence or absence of sCD14 was
similar; however, the level of IL-6 at every time point was lower in
the sCD14-treated cultures.
CD40L expression is affected by sCD14
The transient expression of CD40L was followed over the time
in Ag-stimulated PBMC in the presence or absence of sCD14
(Fig. 3
C). Maximal CD40L expression and percentage of
CD40L-expressing cells were observed after 8 h and 24 h in
the presence and absence of sCD14, respectively, and were about 2-fold
and 30% higher, respectively, in the sCD14-treated cultures; these
cultures also showed higher IgG1 and reduced IgE levels, as shown in
Fig. 2
D.
| Discussion |
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sCD14 bound preferentially to CD38-ve/low B cells. This binding profile was consistent with that of B cells in PBMC, because here mainly CD38-ve/low resting B cells (naive and memory cells) are present. However, all main tonsillar B cell subsets bound sCD14, suggesting that non-Ag-selected and Ag-specific B cells are targets for the sCD14 regulatory activity, thus securing control of primary and secondary immune responses.
The interference with the CD40 signaling pathway should, at least, contribute to the regulatory effect of sCD14 on Ig production. Especially in view of the role that signaling through CD40 has in B cell activation, isotype switching, and memory and plasma cell differentiation. Regarding the latter, it has been demonstrated that interruption/blockage of CD40 signaling favors plasma cell differentiation (19); this would explain the high level of IgG1 in the sCD14-treated cultures. However, the mechanism by which sCD14 increases IgG1 while inhibiting IgE production deserves further investigation. Of note, in experiments not shown here, sCD14 did not affect a number of CD40-mediated B cell responses including proliferation or CD23, CD86, and very late Ag-4 up-regulation (2). These processes would entail induction of an array of transcription factors. It is tempting to speculate that sCD14 interferes preferentially with those CD40-mediated signaling pathways related to Ig class switching. However, the CD40-mediated production of IL-6 was affected. This may contribute to the inhibition of the CD40- and IL-4-induced IgE synthesis.
sCD14 also regulated Ig production by Ag-stimulated PBMC. In addition
to the direct effect on B cells, the sCD14-induced reduction of the
Th1-like cytokines IL-2 (not shown) and IFN-
(12), the
concomitant reduced T-cell proliferation (12), together
with the putatively increased costimulation via CD40L, a consequence of
its increased expression in T cells (Fig. 3
), may favor humoral immune
response. Moreover, the inhibitory effect on IL-4 (12) and
that on IL-6 reported here should contribute to reduce IgE isotype
switching, as indicated by the work of Vercelli et al.
(17).
In conclusion, these findings show that sCD14 is capable of regulating humoral immune response and, together with the previously reported effects on T cells, support the contention that sCD14 may act as a physiological regulator of the immune response in concert with the activity of other regulatory mechanisms. The CD14-deficient mice model will be ideally suited to test this hypothesis. Furthermore, in light of these findings, it deserves to be evaluated the role that the elevated level of sCD14 plays in a number of pathological states, including HIV-1 infection (9) and sytemic lupus erythematosus (20), as well as the significance of the reported inverse correlation between sCD14 levels and total serum IgE (21).
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Mario O. Labéta, Department of Medicine, Tenovus Building, University of Wales, College of Medicine, Heath Park, Cardiff CF4 4XX, United Kingdom. E-mail address: ![]()
3 Abbreviations used in this paper: CD40L, CD40 ligand; BCR, B-cell Ag receptor; GC, germinal center; sCD14, soluble CD14; sCD14(348), truncated sCD14; sCD14wt, wild type sCD14; tCD40L, CD40L tranfectants; tCD32, control tranfectants; VZV, varicella-zoster virus. ![]()
Received for publication October 25, 1999. Accepted for publication February 2, 2000.
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