FIGUREure 8. Yinxieling(YXL) inhibits the Langerhans cell(LC)
differentiation in vitro . A: Gating strategy, flow cytometry scatter
plot and frequency of cytokine induced LC; B: YXL inhibits the
Langerhans cell(LC) differentiation in vitro. n = 6, repreated at least
twice with similar results. Comparing to induced LC group, *
p <0.05.
DISCUSSION
Psoriasis
is not just a disFIGUREuring and refractory skin disease, but also a
systemic disease associated with metabolic syndrome and cardiovascular
diseases [21]. In recent years, the incidence of
psoriasis has gradually increased, which has brought great pressure to
patients’ physical and mental health and economy[22].
In
2013, the annual meeting of the World
Psoriasis Federation reached agreement on the new treatment strategy of
psoriasis [23]. The main treatment strategies are
to control the development, reduce recurrence, prevent complications and
improve the quality of life, which is in line with the characteristics
of traditional Chinese medicine in the treatment of diseases.
Prescription Yinxieling has been used in clinic for more than 20
years with good curative effect and no obvious adverse reactions. Based
on this, this study used IMQ-induced psoriasis-like mouse model to
investigate the pharmacological effects of YXL[24]. Light red spots appeared on the second day
of modeling. With the continuous effect of modeling drugs, the red
patches became darker, the scales increased and the skin thickened, and
reached the peak on the fifth day, covering almost all the bare skin. It
can be seen from the PASI score curve that both CsA and YXL can
effectively improve the clinical symptoms of psoriasis in mice, and the
PSAI score is almost lower than that of IMQ group, especially YXL-M. The
experiment confirmed that although the peak time of psoriasis-like
symptoms in YXL group was basically the same as that in IMQ group, both
erythema and scales were relieved in the later stage, indicating that
YXL did not delay the inflammatory eruption time of psoriasis-like skin
lesions induced by IMQ, but alleviated psoriasis by inhibiting the
intensity of inflammatory eruption in this model skin lesions.
DCs stimulate naive T cells through their antigen presentation function
to initiate acquired immunity and trigger inflammation. The plasmacytoid
dendritic cell B220+DCs, when stimulated by bacterial
CpG or viral infection, is a potent producer of interferon-α
(IFN-α ) and
interferon-β (IFN-β ) [25]. Classical
dendritic cells(cDC), otherwise known as myeloid dendritic cells, are
the most abundant dendritic cell type in the dermis of healthy mice and
are classified by expression of CD11b. Part of the dermal
CD11b+cDCs in mice express Aldh1a2, the sole
rate-limiting factor for retinoic acid production. Retinoic acid is a
vitamin A metabolite that promotes the production of Treg cells from
primitive CD4+T cells. The other part expressed the
C-type lectin Langerin (also known as CD207 and CLEC4K), including
CD103+ and CD103- cells. They
developmentally related to CD8α +cDCs in
secondary lymphoid tissues. When cDCs are activated by proinflammatory
cytokines or pathogens, CD103 cDCs mainly secretes IL-12 and CD11b cDCs
mainly secretes IL-23. These two cytokines share IL23p40 subunit and are
involved in the development of psoriasis [26].
The experimental results showed that CsA had no effect on the total DCs
ratio, but reduced the proportion of B220+ cells and
CD207-CD11B+ cells, and promoted the
proliferation of CD207-CD11B- cells.
However, CsA had no obvious effect on
CD103+CD207+ and
CD103-CD207+ subsets. These results
indicate that CsA may inhibit the secretion of IL-23 by reducing CD11b+
cells, while YXL has no effect on cDCs in lymph nodes. LC are stellate
cells that protrude their dendrites through tight junctions toward the
cuticle and thus can detect antigens of the epidermis without disrupting
the permeability barrier. After birth, they acquire a typical DC
phenotype of histocompatibility complex II (MHC-II) molecule and CD207.
LC is the main source of IL-23 in psoriasis, and the reduction or
absence of LC may lead to the attenuation of psoriasis-like lesions[27]. IL-23 /Th17 axis is the main known
pathogenesis of psoriasis. IL-23 is a cytokine essential for the
expansion and survival of pathogenic Th17 cells. IL-23 is highly
expressed in psoriatic skin and mouse models, including models that rely
on the local application of TLR7 agonist IMQ [28].
It was also found that the expression of LC in IMQ group was much higher
than that in normal group.
LC differentiation is closely
related to the expression of the transcription factor PU.1, an ETS
family member transcription factor encoded by the Sfpi1 gene, which is
an important regulator of many aspects of early hematopoietic and bone
marrow cell differentiation [29]. Recently, PU.1
has been shown to control the expression of Flt3 in a dose-dependent
manner, promoting the differentiation of cDC and pDC, and
synergistically promoting the expression of RUNX3 with TGF-β ,
which further promotes the differentiation of LC[30]. Therefore, MHC-II and CD207 were used to
label LC. The results showed that the ratio and number of LC and PU.1
were highly expressed in psoriatic lesions, while YXL and CsA
significantly down-regulated the number and ratio of LC, but only
decreased the number of PU.1, indicating that PU.1 was directly
proportional to LC. Therefore, YXL may inhibit the expression of LC by
reducing PU.1.
LC, as a unique antigen presenting
cell in the epidermis, captures exogenous antigens and then passes them
to T cells in the skin draining lymph nodes, activating
CD8+T cells and CD4+T cells, further
inducing the production of proinflammatory factors and leading to
inflammation. The antigen presentation function of LC is powerful, and
its ability to transfer from epidermis to local lymph nodes is important
for inducing skin immune response [31].
Psoriasis is just an immune-mediated skin disease, one of its
pathological features is keratinocyte proliferation[28]. It has
been observed that keratinocytes under stress rapidly up-regulate
ligands of lymphocyte activating receptor natural killer group 2D
(NKG2D), leading to migration of LC populations out of the epidermis and
subsequent emergence of αβ +T cells in the
epidermis. Enhanced expression of the murine NKG2D ligand Rae-1εwas observed in injured skin, while mice lacking NKG2D showed delayed
skin wound healing, confirming the importance of keratinocyte and LCs
interactions during skin regeneration [32].
The activated LC expresses CCR7 and
then travels through dermal lymphatics along the CCL19 and CCL21
chemokine gradients into parcutaneous cortical draining lymph nodes[33]. LC
migration is associated with the synergistic effect of inflammatory
cytokines IL-1β , IL-18, and TNF, so mice treated with these
cytokines or with blocking antibodies against Casp1-/-, IL-1B -/-, and
Tnfr2-/- reduced LC migration. Therefore, abnormal proliferation of
epidermal keratinocytes in psoriasis may promote LC migration[34].
The LC carrying the neoantigen ”descends” into the dermis and, together
with the draining lymph, drains into the local lymph node, forming a
nest that attracts antigen-specific lymphocytes. These lymphocytes are
activated in the lymph nodes, proliferate, and then propagate
systemically back primarily to the skin to destroy keratinocytes[35]. The experimental data showed that in IMQ
group, the proportion of epidermal LC migrating to lymph nodes
increased, and the epidermal LC with antigen-presenting function
reduced. LC carrying antigen induced T cell differentiation and produced
proinflammatory factors to aggravate psoriasis. After YXL
administration, the LC migrating to lymph nodes decreased significantly,
while the LC maintaining antigen presentation function in epidermis
increased dramatically.
Therefore, YXL may inhibit the differentiation of LC by inhibiting the
expression of TGFβ, blocking the migration and maturation of LC and its
antigen presentation, thereby inhibiting the release of inflammatory
factors IL-23 and alleviating the development of psoriasis.
CONCLUSIONS
Yinxieling
significantly alleviates psoriasis-like skin inflammation induced by IMQ
in mice. Although it has no obvious effect on DC in skin draining lymph
nodes, it can significantly reduce the expression of LC in skin lesions,
inhibit the migration and maturation of LC, and prevent its antigen
presentation by TGFβ1/PU.1 signaling axis, so as to effectively
alleviates the psoriasis-like skin inflammation on mice.