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Immunopathogenesis of psoriasis: Recent insights on the role
Department of Dermatology 1 and Unite´ INSERM U697, Hoˆpital Saint-Louis, Paris, France
Received 24 May 2005; revised 24 May 2005; accepted 14 September 2005
Psoriasis is a frequent chronic inflammatory disorder involving mostly skin and joints. Its characteristic features in the skin consist of in-
flammatory changes in both dermis and epidermis, with abnormal keratinocytic differentiation and proliferation. In recent years, an importantset of knowledge has been provided by works addressing the immunopathogenic mechanisms of the disease. Indeed, recent advances in theknowledge of mechanisms linking innate and adaptive immunity have led to reconsideration of the roles of key players in the pathogenesisof the disease. This review will focus on results from studies performed in vitro and in vivo in patients with psoriasis, and on lessons from re-cently designed animal models which are considered as the most relevant with respect to the human disease. Even more important, these insightsprovide a rational basis for the design of new therapeutic strategies aiming at the deletion or the down regulation of activated T cells, or at thesuppression of pathogenic cytokines such as TNF-a. Some of these new biotherapeutic tools have been successfully used in vivo in clinical trials,providing a confirmation of such concepts.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Psoriasis; T lymphocyte; Keratinocyte; TNFa
1. The role of adaptive immunity in psoriasis
cytokines and to enhance the proliferation of psoriatic kerati-nocytes, but not of keratinocytes from non-psoriatic individu-
Several lines of evidence obtained in vitro or in vivo sup-
als . These studies revealed an important pathogenic role
port the key contribution of T cells to the psoriatic phenotype,
for T-cell-derived cytokines, mostly interferon-g . These re-
including the hyperproliferation of epidermal cells. First, both
sults have been further confirmed by the development of ani-
CD4C and CD8C T cells have respectively been shown to pre-
mal models of psoriasis, mostly immunodeficient mice bearing
dominantly colonize dermis and epidermis in lesional skin
the SCID mutation, engrafted with human psoriatic skin,
from patients with psoriasis, and these subsets are present in
which demonstrated that the transfer of CD4C T lymphocytes
early stages of plaque formation The pathogenicity of ef-
from patients with psoriasis was mandatory for the induction
fector CD4C T cells infiltrating skin lesions is now widely ac-
and the maintenance of epidermal psoriatic changes Fi-
cepted. This notion relies first on results from in vitro studies
nally, the therapeutic efficacy of immunosuppressive drugs
which showed that lesional skin-derived CD4C T cell lines
such as cyclosporin humanized anti-CD4 or more re-
were characterized by their ability to secrete mostly TH1
cently, LFA3-Ig chimeric molecules or humanized anti-CD11a monoclonal antibodies is dependent on the majorpathogenic role of CD4C helper T lymphocytes. Recently,
* Institut de Recherche sur la Peau, Unite´ INSERM U697, Pavillon Bazin,
knowledge concerning mechanisms of T-cell expansion has
Hopital Saint-Louis, 1 avenue Claude Vellefaux, 75475 Paris cedex 10,
been provided by several studies investigating the diversity
France. Tel.: C33 1 42 49 98 17; fax: C33 1 53 72 20 51.
of the T-cell repertoire of the skin infiltrate in comparison
0896-8411/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
H. Bachelez / Journal of Autoimmunity 25 (2005) 69e73
with peripheral blood. These studies, based on TCRVb CDR3
receptors This model allows the development of psoriatic
length and sequence analysis, showed evidence of a restricted
lesions without any transfer of exogenous CD4C T cells. Se-
diversity of the skin infiltrate, without topographic discrepan-
quential studies showed evidence for a local proliferation of
cies in a given patient, and the long term persistence of
resident T cells without any detectable recirculation in the pe-
expanded clonotypes . Interestingly, the study investi-
ripheral blood. The relevance of this latter model with respect
gating T-cell repertoire in lesional skin from two monozygotic
to the human disease also relies on the evidence for local pro-
twins with psoriasis revealed common clonotypes, suggesting
duction of TNF-a and its critical role in the conversion of pre-
the recognition of common antigens . Nevertheless, results
psoriatic skin to psoriatic lesions, as treatment of these mice
from most studies do not support the existence of public epit-
with infliximab or ethanercept, two TNF-blocking agents, pre-
opes targeted by lesional skin-infiltrating lymphocytes, and the
vent the formation of plaques In this model, it is likely
nature of antigens targeted by these T-cells remains elusive,
that quantitatively, the main cellular subset involved in the
even though some works suggest that bacterial superantigens
production of TNF is the antigen presenting cell (APC) subset.
might be a triggering factor in some cases .
To conclude on the contribution of effector T cells in psoriasis,
On the other hand, little direct evidence for the pathogenic
it is likely that at least a significant proportion of T-cell clono-
role of CD8C T cells in psoriasis has been reported so far, so
types which persist over the long term are resident T lympho-
that the role of this subset in human disease remains elusive.
cytes which are capable of undergoing local activation and
In situ immunophenotypic studies have shown that CD8C T
proliferation without requiring cutaneous lymphocyte associ-
lymphocytes are predominant among the epidermal infiltrate,
ated antigen (CLA)-dependent homing However, this hy-
whereas the majority of dermal lymphocytes exhibit a
pothesis does not rule out the recruitment of memory T cells
CD4C T cell phenotype. Moreover, epidermal CD8C T cells,
from the CLAC pool after their differentiation in regional
like dermal T-cells, display a restricted repertoire, suggesting
again that an antigen-specific immune response drives the
Recently, several studies have focused on the role of a re-
skin expansion of the subset . Likewise, the hypothesis
cently described subset of T cells called regulatory T cells
of a contribution of CD8C T lymphocytes to the pathogenesis
(Tregs) This subset, characterized by high levels of
of psoriasis is reinforced by findings in forms of the disease
CD25 expression, a chain of the interleukin-2 receptor, and
occurring in individuals infected with the human immunodefi-
a high level of mRNA expression of a transcription factor be-
ciency virus . In this model, even though the incidence and
longing to the forkhead/winged helix family, is called foxp3
severity of HIV-related psoriasis has been lowered by the wide
. It has been shown that CD4CCD25high cells, the so-
usage of highly active antiretroviral therapy, the parallel wors-
called naturally occurring Tregs, are involved in the control
ening courses of skin symptoms and of quantitative and qual-
of CD4C and CD8C effectors at the periphery, which include
itative alterations affecting CD4C T cells, and the striking
autoreactive T cells escaping from thymic deletion, since their
predominance of CD8C T lymphocytes both in dermis and
absence due to mutations of the Foxp3 gene is responsible for
epidermis argue against the exclusive deleterious role of TH1
multiple autoimmune manifestations both in mouse and in hu-
cells Furthermore, we recently found evidence in HIV-
mans, notably skin inflammatory changes Naturally oc-
associated psoriasis that T lymphocytes are predominantly
curring Tregs exert their function through both contact- and
CD8C cytotoxic T cells displaying an oligoclonal T-cell
cytokine-dependent mechanisms, mostly through the produc-
receptor pattern, with persistence of some clonotypes in the
tion of IL-10 and TGF-b, and are involved in the tolerance
long term (Kemula M. et al., manuscript in preparation).
to self and tumor-associated antigens, and in the regulation
Analysis of T-cell repertoire in both skin and peripheral blood
of immune response to pathogens . A recent study in-
by using complementary determining region 3 (CDR3) length
vestigated the presence and the functionality of Tregs in the
polymorphism suggests that a dysregulated CD8 cytotoxic
skin and in the peripheral blood of patients with plaque-type
T-cell response towards skin autoantigens underlies psoriasis
psoriasis . The results indicate that Tregs are present at
the site of lesions, and suggest that their regulatory function
Altogether, studies investigating the T-cell repertoire based
would be altered when compared to that of peripheral Tregs
on TCRVb CDR3 length and sequence analysis reveal the re-
from healthy individuals . Similar findings have been re-
stricted diversity of T cells infiltrating psoriatic skin lesions
ported in patients with rheumatoid arthritis, and it remains
both in the epidermis and in the dermis, suggesting that the ac-
to be determined whether this reflects a transient hypofunc-
tivation of both CD4C and CD8C cells in skin lesions are trig-
tionality which might be observed in many inflammatory
gered at least partly through antigen-driven mechanisms. So
chronic disorders, or an intrinsequal abnormality of the Treg
far, the self or non-self nature or the identity of epitopes tar-
population. In the case of RA, functional alterations of Tregs
geted by CD4C and CD8C T cells colonizing skin lesions
reversed during remission induced by TNF-blocking agents,
of plaque-type psoriasis remains unelucidated.
suggesting that mechanisms related to this cytokine are in-
The pathogenic role of T cells has also been demonstrated
in a mouse model of psoriasis, which relies on the graft of hu-
Although the mechanisms controlling T cells with regulatory
man symptomless psoriatic skin onto AGR 129 mice, which is
function in vivo remain elusive, innovative therapeutic tools
characterized by an invalidation of the recombination-activat-
have been shown to enhance the function of the subset. A hu-
ing gene 2, and by a deficiency in type I and type II interferon
manized anti-CD3 monoclonal antibody (mAb) with two
H. Bachelez / Journal of Autoimmunity 25 (2005) 69e73
alanine substitutions at residues critical for binding to Fc recep-
keratinocyte function are associated with a psoriatic pheno-
tor (FcR) has reduced the mitogenicity of OKT3 and favored the
type, including the local expansion of CD4C and CD8C T
production of IL-10 . Clinical trials in patients with type
cells Moreover, the constitutive expression of Stat3 in
1 diabetes provided evidence of a clinical benefit which paral-
keratinocytes is associated with epidermal overreactivity in re-
leled the induction of CD4C CD45ROC IL10C T cells, some
sponse to physical stimulation, a feature typical of human pso-
of which produce TGFb In the context of psoriasis, a re-
riasis called Koebner’s phenomenon. Finally, the authors
cent open-labeled study conducted in patients with psoriatic ar-
showed that CD4C T lymphocytes are critically involved in
thritis suggest that hOKT3g1(Ala-Ala) induced clinical
this phenomenon, and that inducing a downregulation of
Stat3 signaling in vivo provides improvement of the skin con-dition in this transgenic mouse model Thus, it remains
2. The role of cytokines belonging to the interleukin-12
possible that constitutive abnormalities of keratinocytes would
family as a link between innate and adaptive immunity
lead to striking perturbations of keratinocyte-T lymphocyte in-teractions, and consequently to an uncontrolled inflammatory
Recently, several studies have focused on the role of cyto-
reaction with consistent recruitment of activated T cells.
kines belonging to the interleukin-12 (IL-12) family. Interleu-kin-12 is a heterodimeric cytokine which has been shown to
4. Clinical applications: immunotherapeutic strategies
induce and favor the differentiation of naive CD4C and
CD8C T cells into cells preferentially producing IFN-g andIL-2, and is thus involved in Th1-mediated immune responses
The increasing knowledge about mechanisms allowing the
Recently, the role of IL-12 in inflammatory T-cell medi-
control of skin immune responses, mostly pathways linking in-
ated disorders has been reconsidered in light of the character-
nate and adaptive immunity and those involved in the control
ization of new dimeric cytokines belonging to the same
of activated effector T cells at the periphery, provides a rational
family, and regulating IFN-g production by T cells. In this
basis for immunotherapy of skin diseases using T-cell-directed
field, several recent studies have focused on IL-23, which is
therapies. Thus, clinical studies reporting the short-term, par-
composed of the association of p40 with a p19 subunit, confer-
tial benefit of anti-CD4 monoclonal antibodies (mAbs) in pa-
ring specific functions to this cytokine which differ from those
tients with psoriasis can be explained by the non-univocal role
of IL-12, with evidence of enhanced cutaneous immune
of CD4 T helper-type 1 cells in the pathogenesis of the dis-
responses as shown in a transgenic mouse model resulting
ease, and by the contribution of other lymphocyte subsets,
from the skin-directed overexpression of p40 Interestingly,
mostly cytotoxic T lymphocytes (CTLs), to skin inflammation.
it has recently been shown that IL-23 and not IL-12 is
On the other hand, the anti-CD11a mAb efalizumab, which in-
predominantly expressed in lesional psoriatic skin . These
terferes with LFA-1/ICAM-1 interactions, provides some ben-
findings raised a new therapeutic strategy relying on the use of
efit in psoriatic patients. Finally, the properties of IL-10 and
anti-p40 neutralizing Abs. Results from a phase I study in
IL-4 to reverse the TH1 polarization of T lymphocytes have
patients with psoriasis showed that a single administration of
been used to induce a shift from a IFN-g-producing (TH1)
an anti-p40 mAb induced striking clinical responses, with a
to a IL-4-producing pattern in vivo, with evidence of clinical
remission of lesions which was correlated with a reduction of
responses Finally, the conclusions from animal mod-
els stressing the key pathogenic role of TNF-a lead to the ther-apeutic use of monoclonal antibodies (infliximab) or chimeric
3. Lessons from recently developed animal models of
receptors (etanercept) in order to neutralize the proinflamma-
These past few years, the design of several genetically en-
gineered murine models provided phenotypes more or less
reminiscent of the human psoriatic disease The modelsaddressing the contribution of T cells as key effectors have al-
While the hypothesis of the pathogenicity of T-cells in pso-
ready been discussed. Among recently designed murine mod-
riasis has now been validated, the specificity of T cell effectors
els, some consisted of the targeted expression of a transgene in
and the mechanisms underlying their uncontrolled activation
keratinocytes and addressed the consequences of the primary
are far from being fully understood. However, it is striking
keratinocytic event on cutaneous inflammatory and immune
that recent immunogenetic studies led to the characterization
responses. Such a model, recently reported by Sano et al., is
of two candidate genes, SLC9A3R1 and NAT9, which partici-
based on the overexpression in keratinocytes of Stat3 trans-
pate in the regulation of the immunological synapse at the in-
gene, a member of the signal transducers and activators of
terface between T cells and APC . The hypothesis that
transcription under the command of the keratin 5 promoter
these polymorphisms might be involved in tolerance breaking
The rationale of the model is based on the finding that
in psoriasis is very appealing, and will be discussed in another
Stat3 is overexpressed by keratinocytes in lesional psoriatic
paper in this issue. On the other hand, the central role of TNF-
skin, but not in other inflammatory skin diseases such as atopic
a, which is produced following activation of many receptors of
dermatitis or in prurigo. In this transgenic model, alterations in
innate immunity, raised the alternative scenario of a disease
H. Bachelez / Journal of Autoimmunity 25 (2005) 69e73
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