* Institut National de la Santé et de la Recherche Médicale, U343, Nice 06202, France; Laboratoire Central d'Immunologie,
Centre Hospitalier Universitaire, Nice 06202, France; § Laboratoire d'Anatomie Pathologique, Centre Hospitalier Universitaire,
Nice 06002, France;
Department of Cell Biology, Scripps Research Institute, La Jolla, California 92037; and ¶ Centre National de
la Recherche Scientifique, UPR 412, Lyon 69367, France
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Abstract |
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Laminin 5 (3
3
2) distribution in the human
thymus was investigated by immunofluorescence on
frozen sections with anti-
3, -
3, and -
2 mAbs. In addition to a linear staining of subcapsular basal laminae,
the three mAbs give a disperse staining in the parenchyma restricted to the medullary area on a subset of
stellate epithelial cells and vessel structures. We also
found that laminin 5 may influence mature human thymocyte expansion; while bulk laminin and laminin 2, when cross-linked, are comitogenic with a TCR signal, cross-linked laminin 5 has no effect. By contrast, soluble laminin 5 inhibits thymocyte proliferation induced
by a TCR signal. This is accompanied by a particular
pattern of inhibition of early tyrosine kinases, including
Zap 70 and p59fyn inhibition, but not overall inhibition
of p56lck. Using a mAb specific for
6
4 integrins, we
observed that while
3
1 are known to be uniformly
present on all thymocytes,
6
4 expression parallels
thymocyte maturation; thus a correspondence exists between laminin 5 in the thymic medulla and
6
4 on mature thymocytes. Moreover, the soluble Ab against
6
4
inhibits thymocyte proliferation and reproduces the
same pattern of tyrosine kinase phosphorylation suggesting that
6
4 is involved in laminin 5-induced modulation of T cell activation.
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Introduction |
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LAMININS are the major constituents of basal laminae; together with other extracellular matrix components (ECM)1 they contribute, by binding cell surface receptors such as integrins, to maintain epithelial tissue integrity (3, 38, 76). In addition to their mechanical role, they are important signaling molecules with the ability to strongly influence cellular programs, promoting differentiation and migration, proliferation and activation (13, 39, 40). Thus, they are influential molecules in the development (1, 59, 62) and repair (29, 30, 53) of many tissues.
Laminins are heterotrimers where the chain is critical
for cell/matrix interactions (37). Among laminin isoforms
(4, 67), laminin 5 (epiligrin, kalinin, nicein) is somewhat
unique and is preferentially found in basement membranes underlying squamous and transitional epithelia (4).
From a structural viewpoint, the molecule displays a particular chain composition (
3
3
2) and has been characterized as a 105-nm rod-like molecule from the conditioned
medium of normal human keratinocytes (49). Studies performed with extracts of human amnion revealed that, in
addition to monomeric molecules, much of the laminin 5 isolated is covalently associated with laminin 6 (
3
1
1)
and 7 (
3
2
1) (8). Whether these complexes are located in
other basement membranes remains to be shown. Laminin
5 has the ability to bind the NH2-terminal domain of type
VII collagen and is believed to strongly connect cells to
anchoring fibrils in skin (51). Laminin 5 has been identified as a ligand for the integrins
3
1 (7, 48) and
6
4 (44,
60). From a functional viewpoint, laminin 5 plays a major
mechanical role in maintaining the basal layer of epithelia (1, 3, 6, 7, 14, 15, 23, 49, 51, 52), and it is also increased in
the early stages of wound repair (30, 53). Furthermore, accumulated evidence suggests that laminin 5 is a signaling
molecule that is particularly involved in controlling cell
migration and tumor cell expansion (16, 26, 27, 39, 46).
The thymus can be regarded as a mesh of epithelial cells
surrounding maturing T cells; thus, ECM components and
integrins displayed by thymocytes appear to strongly influence T cell differentiation/maturation (5, 10, 12, 20, 31, 43,
54, 69, 73, 74). Mice deficient in merosin, for example,
suffer from abnormal T cell development (31). Since the
presence of laminin 5 has been found within the human
thymus (25, 42, 72), this work was undertaken to document expression and functional influence of laminin 5 and
its cellular receptors during thymocyte maturation. We
show here that laminin 5 displays a peculiar tissue distribution in the human thymus. In addition, we show that laminin 5, in soluble form, provides mature thymocytes with an
inhibitory signal upon stimulation via the CD3-TCR complex which can be reproduced by an anti-6 mAb specific
for
6
4. We have already shown that ligands to
1 integrins, when used in soluble form, inhibit T cell activation triggered via the CD3-TCR complex (19, 34, 65). We now
report that
6
4 integrins can also trigger a similar effect
after ligation with soluble laminin 5.
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Materials and Methods |
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Materials
Human laminin 5 was obtained by immunopurification of the conditioned
medium of SCC25 cells (squamous cell carcinoma) using the 6F12 mAb
coupled to CNBr-activated Sepharose 4B ().
Before this step, medium was passed sequentially over 25 ml of gelatin-
Sepharose () in order to remove fibronectin.
Molecular mass of the eluted laminin 5 was analyzed by SDS-PAGE and
revealed the presence of the trimer 3
3
2 only. Purity of the molecule
preparation was checked by immunoblotting using a panel of several
monoclonal and polyclonal antibodies as previously described (48, 50).
Laminin 2 was obtained from Chemicon. Human fibronectin, protein
A-Sepharose CL 4B, PMSF, leupeptin, aprotinin, NP-40, and BSA were
all purchased from ; human IL-2 was from R&D Systems Inc. Chemicals for PAGE were from Bio-Rad Laboratories. Antifading mounting medium Citifluor was obtained from Citifluor Chemical Laboratory. [3H]thymidine and 125I were obtained from Commisariat á
l'Energie Atomique. ECL reagents were purchased from .
Antibodies
The BM165 mAb (mouse IgG1), specific for 3 chain of human laminin 5 was produced as previously described (49). The 6F12 mAb, specific for
3
chain of human laminin 5, was produced as previously described (33). The
GB3 mAb (70) specific for
2 chain of human laminin 5, was from Accurate and Specific Corporation. The human CD49f (
6 integrin chain)
mAb S3-41 (mouse, IgG1) was produced as previously described (24). The
human CD49f mAb GOH3 (rat IgG1; 61) was obtained from Immunotech. The human CD49f mAbs 135 13C (rat IgG2a), J8H (mouse IgG1),
BQ16 (mouse IgG1), and 450 30A1 (mouse IgG1) were obtained from the
5th International Workshop of Leukocyte Typing. The CD49c mAb (
3
integrin chain) P1B5 was obtained from . Human
CD104 (
4 integrin chain) mAb 3E1F6 was a gift from Dr. Kennel (Oak
Ridge National Laboratory, Oak Ridge, Tennessee). Anti-p67 high affinity non-integrin laminin receptor mAb MPLR2 was provided by Dr. Colnaghi (Istituto Nazionale Tumori, Milan, Italy) as ascites fluid. CD28
mAb 9.3 (mouse IgG2a) was kindly provided by Dr. Ledbetter (Pharmaceutic Research Institute, Bristol Myers Squibb, Seattle, WA). Human CD29 (
1 integrin chain) mAb K20 (mouse IgG2a), CD3 mAb X3 (mouse IgG2a), and CD2 mAbs GT2 (mouse IgG1) and D66 (mouse IgM) were generated in our laboratory. The anti-Zap 70 rabbit antibody was directed
against amino acids 485 to 499 of the human ZAP 70 sequence as previously described (68). Rabbit anti-human p56lck and anti-p59fyn Abs and
peroxidase (HRP) conjugated anti-phosphotyrosine (4G10) were obtained
from Upstate Biotechnology Inc. The goat anti-mouse, anti-rat, or anti-
rabbit antibodies, alone or coupled with fluorochromes or peroxidase, as
well as irrelevant mouse IgG1, were purchased from Dakopatts.
Cell Culture and Tissue
Normal thymi were obtained from children (<4 yr) undergoing cardiac surgery. Thymocytes were prepared by physical disruption of the tissue and washes in RPMI/10% FCS. Jurkat (T lymphoma), HT29 (human colon carcinoma), and K562 (myelogenous leukemia) cell lines were obtained from the American Type Culture Collection.
Thymocytes, human cell lines Jurkat and K562 were cultured in RPMI 1640 () supplemented with 10% FCS. Human colon carcinoma cell line HT29 was cultured in DME () 5% FCS. All media were supplemented with 50 U/ml penicillin, 50 µg/ml streptomycin, 2 mM L-glutamine, and 1 mM pyruvate (Merck).
Immunohistochemistry
For immunohistochemistry, specimens of human thymi were fragmented,
immediately frozen in liquid nitrogen, and stored at 70°C.
Frozen sections (4 µm thick) were processed for indirect immunofluorescence, using a single label fluorescein technique. Air-dried cryostat sections were incubated for 30 min with the primary antibody diluted in PBS, at room temperature. Antibody binding was then detected by incubating the sections with FITC-conjugated rat anti-mouse secondary antibody (F(ab)'2) fragments (Dakopatts), diluted 1:50 in PBS, for 30 min at room temperature. Cell nuclei were labeled with 0.5% propidium iodide (). After final washing in PBS, the sections were mounted in Citifluor (Plano) and examined with a laser scanning confocal microscope (Ultima Meridian; DGL Bioscience). Indirect double staining of thymic epithelial cells was performed with the anti-laminin 5 mAb BM165 (mouse IgG1) and the anti-keratin mAb CK19 (mouse IgM) using isotype-specific secondary antibodies, i.e., FITC-conjugated goat anti-mouse IgM, obtained from Chemicon, and rabbit anti-mouse IgG1 (Dakopatts) revealed with phycoerythrin (PE)-conjugated goat anti-rabbit Ig (Dakopatts). After final washing in PBS, sections were mounted in Citifluor (Plano) and examined with a laser scanning confocal microscope (Ultima Meridian; DGL Bioscience).
Proliferation Assays
Cell proliferation assays were performed in triplicate in 96-well culture plates (Nunc). Thymocytes at 105 cells/well were cultured in 0.2 ml of RPMI 1640 with 10% FCS. Coating of the CD3 mAb was carried out by overnight incubation at 4°C with CD3 mAb (×3, 10 µg/ml) diluted in PBS. Each well was then washed three times with PBS. To saturate the plastic before using soluble reagents, RPMI with 10% FCS was added for 2 h, then the wells were washed three more times with PBS. Cells were added in culture medium containing soluble laminins or Abs before the addition of rIL-2 (5 ng/ml).
For costimulation assays, CD3 mAb was coimmobilized with another mAb or with an ECM. A first overnight incubation at 4°C was carried out with an ECM component (laminin 5, laminin 2, or fibronectin) or mAbs diluted in PBS followed, after washing, by immobilization of CD3 mAb.
Cells were incubated at 37°C in a 5% CO2 humidified atmosphere for 4 d, and 1 µCi of [3H]thymidine (2 Ci/mmol; CEA) was added for the final 18 h. Cells were harvested onto filter paper using a semiautomatic cell harvester (Skatron) and thymidine incorporation was measured in a liquid scintillation counter ().
Apoptosis Assays
Apoptosis was determined by incubating cells previously cultivated as described above with Hoechst 33342 (Interchim), 30 min at 37°C. Results were analyzed on a FACStar® cytometer ().
Immunofluorescence Analysis of Isolated Cells
Cells were washed three times with PBS and incubated at 4°C for 30 min in the dark in 100 µl PBS, 0.1% NaN3, and 0.1% BSA with saturating concentration of mAbs, washed three times and resuspended at a density of 3 × 106 cells/ml. Indirect labeling was performed by adding goat anti- mouse PE-conjugated F(ab)'2 fragments (Dako). This reagent also binds to rat IgG. Double staining was performed using indirect labeling with a primary mAb revealed with anti-mouse (PE) conjugated F(ab)'2 fragments (Dako) followed, after washes, by staining with (FITC) conjugated CD3. Controls included cells incubated with mouse IgG of the relevant subclasses. Analysis was performed on a FACScan® ().
Radioiodination of Cells, Immunoprecipitation, and Immunoblot Analysis
Surface labeling of cells with 125I (1 mCi; ) was performed by
the lactoperoxidase method, as previously described (11). For immunoprecipitation, cells were washed three times, and lysed in ice-cold lysis
buffer (stop buffer with 1% NP-40, 10 µg/ml leupeptin, 1 mM PMSF, and
1,000 U/ml aprotinin) for 30 min on ice. Immunoprecipitation with indicated mAbs (10 µg) adsorbed on protein A-Sepharose 3 h at 4°C under
agitation was then performed on precleared lysates. Sepharose-bound immune complexes were washed three times in TNN buffer (50 mM Tris [pH
7.8], 250 mM NaCl, 0.1% NP-40), then eluted into reduced Laemmli
buffer. Immunoprecipitated proteins were analyzed by SDS-PAGE on a
7.5% gel and revelation was performed by autoradiography. Unlabeled
immunoprecipitated proteins were analyzed by SDS-PAGE on a 7.5% gel
and then electroblotted onto Immobilon P membrane (). The
blotted membrane was saturated overnight at 4°C in a buffer containing
5% BSA, 100 mM Tris, 1.4 M NaCl, pH 7.4, then incubated overnight with
the anti-6 mAb S3-41. Immunolabeling was revealed by chemiluminescence (ECL) after reaction with goat anti-mouse immunoglobulins in
conjunction with HRP.
Anti-Phosphotyrosine Immunoblot Analysis
Cells at 20 × 106/ml in RPMI 5% FCS were incubated with Abs at 37°C for various time intervals. Activation was stopped by addition of 1 ml of ice-cold stop buffer (20 mM Hepes [pH 7.4], 150 mM NaCl, 100 mM NaF, 10 mM EDTA, 10 mM Na4P2O7, and 2 mM Na3VO4). Cells were spun down and the pellets were resuspended in lysis buffer (stop buffer with 1% NP-40, 10 µg/ml leupeptin, 1 mM PMSF, and 1,000 U/ml aprotinin) and incubated on ice for 30 min. After centrifugation (15,000 g for 15 min) supernatants were incubated with Abs for 4 h at 4°C with shaking, followed by 1 h with RAM adsorbed on protein A-Sepharose. Pellets were washed four times with lysis buffer containing 1% NP-40, then once in stop buffer without detergent before mixing with Laemmli buffer containing 3% (final volume) SDS. For reduction, the loading buffer contained 0.75 M 2 ME. Proteins were analyzed by SDS-PAGE, then electroblotted onto Immobilon P membrane (). The blots were saturated overnight at 4°C in a buffer containing 5% BSA, 100 mM Tris, 1.4 M NaCl, pH 7.4, then incubated overnight with the appropriate antibodies (HRP conjugated anti-phosphotyrosine mAb [4G10], pAb anti-lck, pAb anti-fyn, or pAb anti-Zap 70). For anti-lck, anti-fyn, and anti-Zap 70 immunoblots, membranes were washed and incubated 1 h with a HRP-conjugated goat anti-rabbit antibody. After washing, the immunolabeling was revealed by ECL (enhanced chemiluminescence) analysis system. HRP-conjugated anti-phosphotyrosine Ab was also used followed by ECL. For reprobing, the membranes were submerged in stripping buffer (100 mM 2 NE, 2% SDS, 62.5 mM Tris-HCl, pH 6.7) and incubated at 50°C for 30 min. After washing, the membrane was blocked and immunodetection was performed as described.
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Results |
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Immunohistolocalization of Laminin 5 on Post-natal Human Thymus Sections
We investigated laminin 5 (3
3
2) expression in the human thymus by immunofluorescence, using mAbs specific
for the
3 chain (BM165 [49]; Fig. 1, a and b), for the
3
chain (6F12 [33]; Fig. 1 c), or for the
2 chain (GB3, [35];
Fig. 1 d). Staining of laminin 5 was performed together
with a nuclear counter-staining with propidium iodide
(Fig. 1, a-d; Fig. 2, a and b) in order to recognize the cortical (C) and the medullary (M) regions. All three mAbs stained strongly the medullary area of parenchyma and
the basal laminae from subcapsular cortex, consistent with
the presence of laminin 5 in these areas. More precisely,
laminin 5 was localized in stellate keratin positive epithelial cells (Fig. 2, c and d) and in the basal laminae of vessels,
including small vessels and capillary structures, together
with endothelial cells of the larger vessels (Fig. 1 b, arrows;
Fig. 2, a and b). Of note, a more intense staining at the corticomedullary junction was only visible with the anti-
3
mAb BM165, suggesting that laminin 6 (
3
1
1) and/or 7 (
3
2
1) could be associated with laminin 5 in this area. As to the cortex, mAbs stained very scattered structures that
corresponded for the most part to vascular structures.
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Laminin 5 and Laminin 2 Have Distinct Effects on Thymocyte Proliferation: The Soluble Form of Laminin 5 Inhibits Proliferation of Human Thymocytes Stimulated via the CD3-TCR Complex
Since laminins were shown to influence the functional program of numerous cell types, and in particular bulk preparations of laminins were shown to deliver coactivation signals to T cells (10, 58, 66), we investigated the effects of
purified laminin 5 on thymocytes. When we coimmobilized laminin 5 on plastic, this induced a weak costimulation with a CD3-TCR signal, as compared with coated fibronectin or laminin 2 (2
1
1), another laminin isoform
found in the thymus (10; Fig. 3). We then investigated the
effect of laminin 5 and laminin 2 presented in soluble
form; soluble laminin 5 clearly inhibited the proliferation
of thymocytes stimulated via the CD3-TCR (Fig. 4 a). The
inhibition was dose-dependent reaching a maximum of
50%, in terms of [3H]thymidine incorporation, at 5 µg/ml.
Over six independent experiments we saw a quite significant inhibition down to 1 µg/ml (~30%). We checked the
specificity of laminin 5-induced inhibition by adding the
anti-
3 chain mAb BM165, which recognizes an epitope involved in the interaction of laminin 5 with cells. BM165
abolished the inhibiting effect observed with soluble laminin 5 alone (Fig. 4 a). Conversely, no inhibition was seen
when we added, instead of laminin 5, increasing amounts
of soluble human laminin 2 (Fig. 4 a).
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Inhibition was seen only when T cells were stimulated via the CD3-TCR; we observed no inhibition when thymocytes were induced to proliferate via a mitogenic pair of CD2 mAbs (Fig. 4 b). We verified that inhibition of proliferation was not related to apoptosis by incubating cells with Hoechst 342 30 min at 37°C. Results analyzed on a FACStar® cytometer () did not reveal any sign of apoptosis (not shown).
Inhibition by Soluble Laminin 5 of Early Kinase Phosphorylations Activated via the CD3-TCR Complex
We tested soluble laminin 5 effects on early kinase phosphorylation events. Since Jurkat cells express a low density
of 6
4 we selected by cytofluorometry cells expressing
6
4 in similar density as CD3high thymocytes (not shown).
These cells were activated with a CD3 mAb (×3, 10 µg/ml)
during 2.5 min at 37°C, with or without soluble laminin 5. Immunoprecipitation of Zap 70, p56lck, or p59fyn was carried out on activated cell lysates, after which precipitated proteins were transferred on Immobilon. Membranes were
then incubated with HRP conjugated anti-phosphotyrosine antibody. Results were revealed by ECL system.
We observed that the addition of 1 µg/ml of soluble laminin 5 was sufficient to induce an inhibition of Zap 70 (Fig.
5 a) and p59fyn tyrosine phosphorylation (Fig. 5 c). By contrast, the addition of soluble laminin 5, up to 5 µg/ml, a
concentration fully efficient in proliferation assays, did not
induce a significant inhibition of p56lck phosphorylation
(Fig. 5 b). Membranes were stripped, blocked, and reprobed with the precipitating antibody, i.e., respectively anti-Zap 70 (Fig. 5 a), anti-lck (Fig. 5 b), and anti-fyn (Fig. 5 c), respectively, in order to verify amounts of precipitated material in each lane.
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Identification of an Anti-6 mAb Specific for
6
4:
Evidence for Distinct Forms of
6 Chains Paired with
1 and
4 Chains
Next, we investigated whether the effect of laminin 5 would be mediated by 6
4,
3
1, or both. We tested a battery of anti-
6 mAbs for their reactivity in flow cytometry
with dispersed human thymocytes (135 13C [rat IgG2a];
J8H, BQ16, 450 30A1, S3-41 [mouse IgG1]; reference 24)
and the widely used anti-
6 GOH3 [rat IgG1]; reference
61). We noticed that the mouse mAb S3-41 displayed a peculiar reactivity as compared with other anti-
6 mAbs,
strikingly similar to the reactivity of the anti-
4 mAb
3E1F6 (Fig. 6). S3-41, as 3E1F6, did not react with the
most immature CD3low thymocytes. Moreover, staining by
S3-41 and 3E1F6 increased with the density of CD3, which
indicates the level of maturation of thymocytes (Fig. 6).
This pattern of reactivity was in marked contrast to the
pattern given by the other anti-
6 mAbs tested including
GOH3, which stained immature CD3low thymocytes. Of
note, peripheral blood T lymphocytes were equally stained
with GOH3 and S3-41 (not shown).
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To confirm that S3-41 would react only with the 6
chains linked to
4, we performed sequential immunoprecipitations using the colon carcinoma cell line HT29, which
expresses high densities of both
6
1 and
6
4. Cells were
lysed with 1% NP-40 and lysates were depleted of
1 or
4
integrins by two successive immunoprecipitations either
with the anti-
1 mAb K20 or the anti-
4 mAb 3E1F6. Depleted lysates were then immunoprecipitated with either
the anti-
6 mAbs S3-41 or GOH3 (Fig. 7 a). It can be seen
that GOH3 was still able to immunoprecipitate residual
material when lysates were depleted of either
1 or
4 integrins (Fig. 7 a, middle, lane 3; right, lane 3). In contrast,
S3-41 did not precipitate any residual material when lysates were depleted of
4 integrins (Fig. 7 a, right, lane 2)
whereas it remained fully reactive when lysates were depleted of
1 integrins (Fig. 7 a, middle, lane 2). Thus, if
GOH3 immunoprecipitates
6 chains associated with either
1 or
4 chains, S3-41 recognizes only
6 chains
paired with
4. These results were confirmed by immunoblotting experiments with S3-41. Cell lysates were first immunoprecipitated either with the anti-
1 mAb K20 or the
anti-
4 3E1F6. Fig. 7 b shows that S3-41 recognized only
the
6 chains that coimmunoprecipitated with
4. Note
that these experiments were performed on two distinct cell
lines that expressed different relative amounts of
6
1 and
6
4 (Fig. 7 b, left and right). Taken together, these experiments demonstrate that the anti-
6 mAb S3-41 reacts
with an epitope present only when
6 chains are linked to
4 chains.
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When Used in Soluble Form the 6
4-specific mAb
S3-41 Inhibits Proliferation of Thymocytes Stimulated
via the CD3-TCR Complex
Anti-6 antibodies were used in proliferation assays performed on human thymocytes. To determine which laminin 5 receptor is involved in the modulation of thymocyte
proliferation, we used GOH3 mAb which recognizes all
6
chains and S3-41 mAb which is
6
4 specific. As previously
described (58, 66), the anti-
6 mAb GOH3 immobilized
on culture wells delivered a strong costimulus for thymocyte proliferation in the presence of cross-linked CD3 mAb (Fig. 8 a). Under the same conditions, S3-41 delivered a weak proliferative costimulus to thymocytes when
compared with GOH3. Moreover, there was no additive
or synergistic effect when the two mAbs GOH3 and S3-41
were immobilized on plates with a CD3 mAb. When anti-
6 mAbs were used in soluble form, we observed that only
S3-41 inhibited proliferation of thymocytes stimulated via
the CD3-TCR complex (Fig. 8 b). Inhibition was very efficient with S3-41 (50%). We tested the same soluble mAbs
on thymocytes stimulated with a mitogenic pair of CD2
mAbs (GT2 + D66) (Fig. 9 a), the CD28 mAb and immobilized CD3 mAb (Fig. 9 b), or the CD28 mAb and rIL-2
(Fig. 9 c). Again, an inhibitory effect was seen only when
thymocytes were stimulated via the CD3-TCR pathway.
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We also used the anti-3 mAb P1B5, which recognizes
the
3
1 epitope involved in laminin 5 binding (7, 48).
When added in soluble form in amounts ranging from
0.1 to 10 µg/ml, P1B5 did not inhibit proliferation of
thymocytes stimulated via the CD3-TCR complex (not shown).
As it has been described that the non-integrin receptor
p67 can associate with 6
4 (2), it was of interest to test the effects of an anti-p67 mAb, namely MPLR2, on T cell proliferation. We did not observe a significant inhibition of
soluble MPLR2 on thymocyte proliferation induced via
the CD3-TCR complex (not shown).
Inhibition by Soluble S3-41 mAb of Early Kinase Phosphorylations Activated via the CD3-TCR Complex
We tested the effects of S3-41 and P1B5 mAbs, used in soluble form, on early kinase phosphorylation events (Fig.
10). Jurkat cells expressing 6
4 in similar density as
CD3high thymocytes were activated with a CD3 mAb (×3,
10 µg/ml) during 2.5 min at 37°C, with or without soluble
mAbs. Immunoprecipitation of Zap 70, p56lck, or p59fyn
was carried out on activated cell lysates. Precipitated proteins were transferred onto Immobilon, then membranes
were incubated with an anti-phosphotyrosine antibody
coupled to peroxidase. Results were revealed by ECL. We
observed that the addition of S3-41 (10 µg/ml) induced the
same pattern of inhibition as soluble laminin 5 with inhibition of Zap 70 and p59fyn tyrosine phosphorylation (Fig.
10, a and c, respectively) but no inhibition of overall p56lck
(Fig. 10 b). In contrast, addition of soluble P1B5 (10 µg/ml) did not induce this particular pattern of reactivity, supporting the hypothesis that
6
4 is the laminin 5 receptor
involved.
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Discussion |
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Using mAbs specific for the three chains of laminin 5 (anti-3 mAb BM165, anti-
3 mAb 6F12, anti-
2 mAb
GB3), we have investigated the presence and cellular distribution of laminin 5 on frozen human thymus sections. It
has been mentioned in earlier publications that laminin 5 is expressed in the human thymus (25, 42, 72). Our data focus on laminin 5 expression in thymic parenchyma and link up with functional effects of this molecule on thymocyte proliferation. Since we have found an identical reactivity of the three mAbs, it can be concluded that a complete form of laminin 5 is displayed by the thymus. Indeed,
it is known that
3
2 modules can be expressed without
any detectable
3 chains, such as in choroid plexus (1). We
have observed that laminin 5 has quite a particular expression in the thymus; it can be detected in the medulla where
a subpopulation of epithelial cells displays laminin 5 as revealed by double staining with keratin. For double staining we used the anti-keratin Ab CK19 that recognizes all epithelial cells (71). Whether the laminin 5 epithelial cells
form a specialized set of cells or whether any epithelial
cells in these areas could become laminin 5 positive under
certain circumstances of stimulus remains to be established. However, this feature is additional to the similarities previously described between thymic epithelial cells
and epidermal keratinocytes (45). Thymic vessels are also strongly stained with anti-laminin 5 antibody, particularly
at the level of basal laminae, including capillary structures.
It is more difficult to ascertain that endothelial cells are
also stained; this staining is irregular and is restricted to
larger vessels. Interestingly, preliminary results indicate
that, in humans, small vessels and capillary structures from
other organs appear devoid of laminin 5. In the thymus,
vessels are abundant in the medulla, particularly at the
corticomedullary junction and it is likely that emigration
of thymocytes having reached maturation occurs in this
area. Indeed, laminin 5 has been involved in various cell
migration phenomena in wound repair (63) or tumor invasion (26, 27, 41) involving cleavage by metalloproteases (17). Therefore, the present data raise the possibility of a role for laminin 5 in T cell migration outside the thymus.
The intensity of BM165 staining at the corticomedullary
junction suggests that laminin 6 and/or 7 may be also
present in these areas. Laminin 5 is also present in subcortical flat epithelial basement membrane that is also known
to include hemidesmosome components like collagen type
VII and
6
4 integrins (50, 72). It is thus likely that laminin
5 in this region has a different function to the laminin 5 present in the thymus parenchyma, which would be similar
to the structural role it exerts in the basement membrane
of skin and other epithelia.
Whereas laminin 5 has been identified as a ligand for
both 3
1 and
6
4 integrins in vitro, (7, 44, 48, 60), we have observed that
3
1 and
6
4 have a quite distinct distribution on human thymocytes in vivo. Whereas it was
found that
3
1 is evenly distributed on all thymocyte subsets (43), we have shown here that
6
4 density on the surface of thymocytes parallels their maturation. We have
used the mAb S3-41 (24) because we found that it reacts
only with the
6 chains linked to the
4 chain, and not
with the
6 chains linked to the
1 chain. Although this
mAb was classified as CD49f by the International Workshops on Leucocyte Antigens on the basis of its capacity to
recognize human
6 transfected in RBL rat cells (22), a
particular reactivity was noticed on skin sections, where
6
is associated with the
4 chain, but not on kidney sections,
where
6 is associated with the
1 chain (28, 77). The sequential immunoprecipitations and immunoprecipitations
followed by Western blots that were performed in the
present study confirmed that S3-41 mAb reacts exclusively with the
6 chain bound to
4. We have noticed that, on
total lysates, S3-41 immunoprecipitated much less material
than anti-
6 mAb GOH3. This could be explained either
by a difference of affinity between the two mAbs and/or
by the staining of only a subpopulation of
6 chains with
S3-41. Indeed, S3-41 mAb did not precipitate any material
when lysates were depleted of
4 integrins but it was fully
reactive when lysates were depleted of
1 integrins, and
only
6 chains coimmunoprecipitated with
4 carried the
S3-41 epitope. Moreover, as S3-41 reacted with
6 chains
separated from
4 chains, this suggests that it recognizes
an intrinsic isomorphism of the
6 chains and not a conformational epitope. Thus, strikingly,
6
4 upregulation on
mature thymocytes coincides with laminin 5 expression in
the medullary area, the thymic region where mature thymocytes are located. In mice, it has been described that, on the contrary, downregulation of
4 is simultaneous with
the upregulation of CD4, CD8, and CD3 (73), but differences in integrin expression with T cell maturation between human and mouse have already been observed for
4
1 (55).
Since it was described that bulk preparations of laminin
coated on plastic are strongly mitogenic for T cells (58), it
was of interest to investigate the effect of laminin 5 on human T cell proliferation. We show here that laminin 5 presented in a cross-linked form has only a slight coactivation
effect on thymocytes. However, when presented in soluble
form, a clear inhibition of proliferation triggered via the
CD3-TCR complex was observed. It should be noted that
this effect was reproduced by the anti-6 mAb S3-41; however, it was not reproduced by the typical anti-
3 integrin
mAb P1B5 in agreement with the view that the inhibitory influence of soluble laminin 5 on T cell proliferation is mediated via
6
4. In addition, we did not observe an inhibition on TCR triggered proliferation with soluble mAb
MPLR2, an antibody reacting with the high affinity non-integrin p67 laminin receptor, reported to be associated
with
6
4 (2). Soluble laminin can be detected in the serum
of normal individuals, and its level was found to be
strongly increased in inflammatory conditions and certain
autoimmune disorders (57, 75). Laminin is also present in
the extracellular spaces of secreting cells, and it was found
that a significant proportion (~15%) of Engelbreth Holm
Swarm matrix proteins is present as a soluble form released into the supernatant (47). It is likely that an equilibrium takes place between the soluble laminins, either released by secreting cells or liberated from the ECM on the
one hand, and the laminin deposited as an insoluble fraction in the ECM or on cell surface receptors on the other hand (47). The data presented here disclose a cross-talk
event occurring between the laminin 5 receptor
6
4 and
the T cell receptor. Of note, inhibition induced by laminin
5 is restricted to signaling via the CD3-TCR complex, and
we observed no inhibition when T cells were stimulated
via other pathways such as the CD2 or the CD28 pathways. Inhibition by laminin 5 therefore shares similarities with the inhibition generated by
1 integrin ligands we
have described, either VCAM 1 or mAbs (19, 65, 34). It
has been previously shown by other groups that integrins
can negatively regulate T cell activation (21, 36, 64). Our
data show that
6
4 can also modulate T cell activation.
Consistent with this view, it has been previously suggested
that this integrin could, depending on the cell type, either
increase or block normal cell proliferation (16). Besides,
as an activation pathway was previously described when
6
4 was ligated with coated antibodies or laminin 5 (32), our data obtained with soluble ligand and mAb suggest
also that exposing cells to immobilized or soluble ligands
can trigger different signaling pathways. Such influences,
depending on the form of the ligand, fits well with the
known feature of integrins signaling, whose pattern is critically dependent on sole ligand occupancy, sole integrin aggregation or both ligand occupancy and aggregation (40).
Moreover, we have observed here that laminin 5 inhibits
the phosphorylation of Zap 70 and p59fyn but has no effect
on overall phosphorylation and activation of p56lck, a pattern of kinase inhibition that was also observed in our laboratory by binding 1 integrins with soluble ligand (34). Thus
6
4, under the influence of laminin 5 appears to use,
at least in part, the same intracellular signaling pathway as
1 integrins under the influence of VCAM 1. Whether
these signaling pathways are common or whether they diverge deserves further investigation.
There is increasing evidence that integrins and their ligands, including ECM components, play a critical role in T cell maturation (5, 10, 12, 20, 31, 43, 54, 69, 73, 74). The pattern of expression of the various integrins present in the thymus and the pattern of expression of their ligand are both subtle events that account for their influence at given stages of T cell differentiation/maturation (12, 54). Since, it is clear that thymocyte differentiation involves cycles of proliferation and arrest (18), the coordination of adhesion and proliferation events mediated by laminin sub-types is likely to be significant for T cell maturation. The results reported here suggest that laminin 5 might be an important signaling molecule in the control of expansion of mature thymocytes and possibly their migration outside the thymus, given its presence in vascular structures.
![]() |
Footnotes |
---|
Address correspondence to Alain Bernard, Institut National de la Santé et de la Recherche Médicale, U343, Hôpital de l'Archet, route de Saint Antoine de Ginestière, Nice 06202, France. Tel.: 33 492 157 700. Fax: 33 492 157 709. E-mail: u343{at}hermes.unice.fr
Received for publication 27 July 1998 and in revised form 27 November 1998.
The authors express gratitude to Dr. S. Kennel for the gift of 3E1F6 mAb,
to Hélène Senesi for technical assistance and to Ghislaine Bernard for
helpful discussion.
This work was supported by grants from the Institut National de la Santé et de la Recherche Médicale, the Association pour la Recherche centre le Cancer, the Ligue contre le Cancer, the Programme Hospitalier de Recherche Clinique and the FEGEFLUC.
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Abbreviations used in this paper |
---|
APY, anti-phosphotyrosine; Ab, antibody; ECM, extracellular matrix; GAR, goat anti-rabbit immunoglobulins; IP, immunoprecipitation; pAb, polyclonal antibody; PE, phycoerythrin.
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