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From the SRHI (CEA,DSV-DRM),*
Commissariat à
lEnergie Atomique; the Institut de Recherche sur la
Peau,
Hôpital Saint-Louis; and the
Unité de Dermatologie,
Hôpital
Tenon (Ap-Hp), Paris, France
| Abstract |
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, a major
cytokine in psoriasis, we asked whether HLA-G and its receptor
might be expressed in this disease. Specific RNAs for HLA-G1 and HLA-G5
were consistently found in lesional skin specimens, soluble
HLA-G5 transcripts being found only in psoriasis. HLA-G protein was
found in all psoriatic sections, but never in normal skin
controls. Double labeling demonstrated that HLA-G-positive cells were
CD68+, CD11c+ macrophages. The NKR ILT2
was also present in psoriatic skin, the T
CD4+-infiltrating cells expressing indeed ILT2. The
demonstration of HLA-G and ILT2 expression in psoriatic skin suggests
that this pathway may act as an inhibitory feed back aimed to
down-regulate the deleterious effects of T-cell infiltrate in this
disease.
| Introduction |
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(IFN-
).10,11
Altogether, these data have led to a
general agreement about the major role of T cells, rather than
epidermal cells, in the pathogenesis of psoriasis.
Besides these data, recent studies have emphasized the potential role
of natural killer receptors (NKRs) in
psoriasis. These receptors were indeed
found in human psoriatic lesions6
as well as in an animal
model,12
but not in normal skin.6
These NKRs
recognize various class I alleles, such linking being able to
down-regulate several lymphocytic pathways. NKR binding may therefore
play an important role in the regulation of the disease pathways. HLA-G
is a nonclassical class I MHC molecule, whose expression is restricted
to a few tissues such as cytotrophoblasts and thymic epithelial
cells.13
HLA-G interacts with different NKRs such as
ILT214,15
KIR2DL4,16
and p4917
and is able to inhibit both natural killer (NK)18
and
T-cell cytotoxicity as well as to inhibit T-cell
proliferation.19,20
There are six isoforms for this
molecule: four are membrane-bound (HLA-G1, -G2, -G3, and -G4) and two
are soluble (HLA-G5 and -G6);13
their expression being
enhanced by IFN-
.21
HLA-G has been shown to play a
crucial role in the protection of the fetus toward the mothers immune
cells.22
In addition, it has also been recently suggested
that the expression of HLA-G in melanoma may participate in the
mechanisms of escape of the tumor from the immune surveillance of the
host.23,24
HLA-G seems therefore to be able to
down-regulate several lymphoid reactions in various situations. Because
psoriasis is a disease characterized by the secretion of cytokines also
able to up-regulate HLA-G, and because psoriatic infiltrating cells
express NKR receptors, we asked whether HLA-G and its receptor might be
expressed in this disease.
| Materials and Methods |
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To be eligible for inclusion in this study, patients had to 1) display typical lesions of chronic plaque type psoriasis, 2) did not receive any systemic or local therapy for at least 14 days before biopsy, and 3) give informed consent. Control skin was obtained from healthy females who underwent breast reduction surgery. The obtained specimens were cut into two parts. One part was snap-frozen in liquid nitrogen and used for reverse transcription-polymerase chain reaction (RT-PCR) whereas the other was placed in OCT for immunohistochemical analysis.
RT-PCR
RNA isolation followed by RT-PCR was performed according to the manufacturers recommendations. Briefly, the skin biopsies stored at -80°C were put in RNA Now reagent (Biogentex, Seabrook, TX) and homogenized using ultraturrax (IKA Labortechnic, Staufen, Germany) according to the manufacturers recommendations. The quality of RNA was checked by electrophoresis on 1.5% agarose gel. Five µg of mRNA was reverse-transcripted into cDNA using oligo (dT)12-18 primer (Life Technologies, Cergy Fontoise, France), and Moloney murine leukemia virus reverse transcriptase (Life Technologies) at 42°C for 1 hour. The cDNA obtained was used for PCR. Amplifications were performed as previously described using the primers G.257 (5'-GGA AGA GGA GAC ACG GAA CA) and G3U 55'-(GGC TGG TCT CTG CAC AAA GAG A). These primers amplify all HLA-G isoforms. Specific amplification of the soluble HLA-G5 transcript was performed using primers G.526 (5'-CCA ATG TGG CTG AAC AAA GG) and G.i4b (5'-AAC GGA GAA GGT GAG GG) primers. The PCR was conducted for 35 cycles, consisting of 1 minute at 90°C, 90 seconds at 65°C (61°C for G5), and 2 minutes at 72°C. PCR products were run on 1.5% agarose gels. Co-amplification of ß-actin cDNA was performed in each sample, using the ß-actin amplimer set (Life Technologies) for 16 cycles to assess that equal amounts of material were loaded in each well.
Radioactive Hybridization of PCR Products
PCR products were then transferred by alkaline blotting onto nylon
membranes (Hybond-N+, Amersham) and hybridized with a
(
-32P)ATP end-labeled oligonucleotide probe.
The GR probe that recognizes all HLA-G-isoforms has the following
sequence: 5'-GGT CTG CAG GTT CAT TCT GTC. The HLA-G5-specific probe
sequence is: 5'-GAG GCA TCA TGT CTG TTA GG. The bands were quantified
by densitometry.
Immunohistochemistry
Four-µm cryostat sections were taken from frozen specimens, dehydrated in acetone at -20° for 10 minutes, and then air-dried. The DAKO EnVision+ System, peroxidase (amino ethyl carbazol) (DAKO, Carpinteria, CA) was used. Briefly, after rinsing sections in phosphate-buffered saline with 0.1% saponin, endogenous peroxidases were inhibited using H2O2, and then the samples were incubated for 1 hour at room temperature with the specific or irrelevant antibodies. The sections were then put in a solution containing a peroxidase-labeled polymer conjugated to a goat anti-mouse immunoglobulin for 30 minutes. The diaminobenzidine plus substrate-chromogen was finally used for revealing the antibody fixation. The antibodies used were: 87G and O1G (both specific for HLA-G1 and G5); 16G1 (specific for the soluble G5 isoform) (these antibodies being kindly provided by Dan Geraghty, Fred Hutchinson Cancer Research Center, Seattle, WA); 4H84, recognizing denatured HLA-G isoforms (kindly provided by Dr. M. Mc Master, San Francisco, CA); anti-ILT2 that recognizes a NKR involved in HLA-G binding (kindly provided by Dr. Marco Colonna, Basel, Switzerland); w6/32 (anti-MHC class I) (Sigma Chemical Co., St. Louis, MO); CD3, CD14, and a mouse-IgG2a as control (Sigma).
Double Labeling
To assess which cells were expressing HLA-G and ILT2, double-labeling experiments were used with 87G and anti-CD68; 87G and anti-CD11c, anti-ILT2, anti-CD3, anti-CD4, and anti-CD8. These were performed follows. After incubation with normal goat serum, lesional sections were incubated with mouse monoclonal antibody 87G for 60 minutes, rinsed, and incubated with goat anti-mouse IgG conjugated with Texas Red. After rinsing, sections were incubated for 30 minutes with fluorescein isothiocyanate (FITC)-conjugated CD68 or FITC-conjugated CD11c antibodies. Controls consisted in same studies with replacement of 87G with IgG. In the same way, double immunostaining with anti-ILT2 and FITC-conjugated anti-CD3, anti-CD4, or anti-CD8 were performed to evaluate the presence of T cells expressing the ILT2 receptor. Controls consisted in same studies with replacement of anti-ILT2 with IgG. The sections were examined using confocal laser microscopy.
| Results |
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|
|
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The amounts of HLA-G mRNA obtained from six psoriatic plaque
specimens and four healthy skin controls were analyzed by RT-PCR. HLA-G
isoforms were consistently found in all psoriatic specimens, but in
only two of the four normal control skin biopsies (Figure 1)
. With the exception of one specimen
(case 3), only the bands corresponding to the HLA-G1 and HLA-G5
isoforms were found in psoriatic lesions. Patient 3 had a severe
resistant disease, with psoriatic arthritis. Finally, the level of the
HLA-G1/G5 transcripts appeared more intense in the psoriatic specimens
compared to normal controls.
|
The soluble isoform HLA-G5 was studied in the same specimens using
primers and probe that are specific for this isoform. A band
corresponding to HLA-G5 was demonstrated in three of six psoriatic
specimens whereas it was constantly absent from normal skin (Figure 2)
.
|
When analyzing nine psoriatic biopsies by immunohistochemistry, a
labeling of cells within the papillary dermis was consistently found
using the specific HLA-G antibodies 87G and O1G (Figures 3 and 4)
.
The numbers of HLA-G-positive cells was however variable, being very
dense in some cases (Figure 4)
. In only two cases were the stained
cells not only within the papillary dermis but also in cells of the
epidermis (Figures 4B and 6A)
. Using 16G1, an antibody that recognizes
only the soluble HLA-G5 isoform, positive cells were also found in
papillary dermis (Figure 3)
. In contrast, normal skin never expressed
HLA-G (Figure 5)
. To determine which
types of infiltrating cells were expressing HLA-G, serial sections
labeled with 87G antibodies, CD3 and CD14 antibodies were performed.
The HLA-G-positive cells never co-localized with T lymphocytes, but
were rather close to the CD14+ cell distribution
(Figure 3)
.
|
|
|
|
In view of the above-detailed serial section analysis, we
performed double labeling with the HLA-G antibody 87G and anti-CD68 and
anti-CD11c antibodies that recognize macrophages. Nearly all
HLA-G-positive cells were also labeled with CD68 and CD11c (Figures 6 and 7)
,
demonstrating that macrophages were the source for HLA-G in these
specimens.
|
Immunohistochemistry realized on psoriatic sections incubated with
anti-ILT2 demonstrated the labeling of a dense infiltrate in the
superficial dermis (Figure 4)
. In contrast, almost no positive cells
were found in the dermis of normal skin (Figure 5)
.
Immunohistochemistry with anti-CD3 and anti-ILT2 antibodies showed
afterward the presence of several double-positive cells in papillary
dermis (Figure 8)
. In addition, double
immunofluorescence with anti-ILT2 and FITC-labeled anti-CD4 or anti-CD8
antibodies showed that the T cells expressing ILT2 in psoriasis were
only T CD4+ cells. CD8-positive cells were not
labeled with ILT2.
|
| Discussion |
|---|
|
|
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, a
cytokine constantly found in psoriasis;9,10
we asked
whether HLA-G may be implicated in the control of this disease. Our
results demonstrate that HLA-G was present at lesional sites of
psoriasis. Indeed, specific RNAs for membrane bound and soluble
isoforms HLA-G1 and HLA-G5 were consistently found in lesional skin
specimens, their levels seeming to be higher than that of controls. In
addition, soluble HLA-G5 transcripts were only found in psoriasis.
Overall, HLA-G protein expression was found in all psoriatic sections
examined, but never in sections from normal skin. This expression was
demonstrated by three different monoclonal antibodies. The expression
of the soluble protein HLA-G5 was also shown at the same
sites.26
Our results are slightly different from those of
Ulbrecht and colleagues27
who have suggested that the
levels of HLA-G transcripts may be decreased in psoriasis. However,
only two specimens of normal skin were analyzed by RT-PCR, the levels
of these two specimens being very different, making a clear-cut
conclusion difficult to assess. In addition, analysis of soluble
isoforms and protein analysis was not done in this study.
The expression of HLA-G was always found within lining infiltrating
cells in the papillary dermis. In psoriatic lesions, previous studies
have shown the presence of a subset of spindle-shaped macrophages
lining below the dermal-epidermal junction.28,29
This
distribution is similar to the HLA-G+ cells
observed in this study. Because the spindle-shaped psoriatic
macrophages express CD11c+ and
CD68+, double-labeling experiments disclosed that
the HLA-G+ cells were indeed
CD68+ and CD11c+
macrophages. These cells are derived from the monocyte-macrophage
lineage, known to be able to express HLA-G in certain circumstances,
particularly, when incubated with IFN-
.21,30
Therefore,
it is tempting to hypothesize that the psoriatic macrophages expressed
HLA-G on stimulation by IFN-
secreted by infiltrating T cells.
Finally, in only two cases were HLA-G+ cells also
found in the epidermis. However, these cells were either infiltrating
cells as these were grouped within isolated small foci close to the
dermal labeled infiltrate (Figure 6)
or corresponded to few
keratinocytes (Figure 4B)
. In this view, this presence may correspond
to the diffusion of soluble HLA-G as it was found only closely to
dermal HLA-G-producing cells (Figure 4B)
. In contrast, this presence
may also theoretically have resulted from IFN-
-induced secretion by
keratinocytes because these cells in culture may express HLA-G when
stimulated with this cytokine (S Aratingi, personal communication).
However, this hypothesis is difficult to admit because expression of
HLA-G in keratinocytes was found in only one biopsy suggesting that the
in vitro results do not correlate with in vivo
results.
HLA-G is a nonclassical MHC molecule able to induce tolerance. Indeed, the transfection of this molecule in cells sensitive to polyclonal NK lysis allows the inhibition of cytolysis.18 In the same way, trophoblasts expressing HLA-G inhibit decidual NK lysis.31 These data strongly suggest that the expression of this molecule by the cytotrophoblasts is one of the major tools for the establishment of tolerance of the fetus during pregnancy.22 More recent studies have shown that HLA-G transcripts and proteins may be found in some melanoma biopsies.23,24,32 Here too, the transfection of HLA-G in a melanoma cell line that did not express this molecule inhibited in vitro T-cell and NK-cell cytotoxicity.23,32 Other experiments have shown that HLA-G was able to inhibit the allogeneic proliferative T-cell response.19,20 This inhibition is mediated by the binding of HLA-G to various inhibitory NKRs such as ILT2, ILT4, p49, and KIR2DL4.14-17 NKRs, originally characterized on NK cells can also be expressed on T-cell subsets. The presence of ILT2 in T CD4+ psoriatic cells found here suggest that pathogenic T cells in this disease can be regulated via the HLA-G pathway. Interestingly, in a very recent study done in an animal model for psoriasis, other NKRs such as CD94, CD158a, CD158b, and NKB1 were found in dermal and epidermal infiltrating T cells;12 the ratio of cells expressing NKRs being higher than the one expected from peripheral blood findings.12 These receptors were found in human psoriatic skin lesions, but not in normal skin.6 The authors proposed that in psoriasis, T cells expressing NKR could be viewed as an early warning back-up system.12,33 Of note, Fournel and colleagues34 have recently shown that soluble HLA-G was able to trigger apoptosis of activated CD8 cells, but not resting CD8 cells. CD8+ cytotoxic cells are constantly present in psoriatic epidermis.25 Despite the presence of these cells, no evidence of cytopathic effects on adjacent keratinocytes is found in psoriasis suggesting that suppressor signals may be implicated.12 The soluble HLA-G molecule, that we identified in psoriasis, could therefore constitute at least one of the negative pathways that inhibit T-infiltrating cells. This action may target CD4+ cells as suggested in this study, but possibly also CD8+ cells, since a recent work demonstrated that newer antibodies, not yet available, detected ILT2-positive T cells that appeared previously negative with the classical antibodies that we used here.35 In view of all these findings, the above demonstration of HLA-G and ILT2 expression in psoriasis suggests that this pathway may act as an inhibitory feedback aimed to down-regulate the deleterious effects of initial T-cell infiltrate in this disease. Future analysis, such as functional studies in animal models, will be needed to ultimately assess the role of HLA-G in psoriasis.
| Acknowledgements |
|---|
| Footnotes |
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Accepted for publication March 1, 2001.
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domain of HLA-G1 and HLA-G2 inhibits cytotoxicity induced by natural killer cells: is HLA-G the public ligand for natural killer inhibitory receptors? Proc Natl Acad Sci USA 1997, 94:5249-5254
. J Immunol 1996, 156:4224-4231[Abstract]
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