4. Discussion
Psoriasis, a common chronic inflammatory skin disease, is primarily
mediated by the pathological crosstalk between immune cells and
epidermal keratinocytes (Lowes, Suarez-Farinas & Krueger, 2014), which
include infiltrating T-cells,
macrophages,
dendritic
cells
(DCs), MDSCs, and neutrophils (Chang et al., 1995; Liang, Sarkar, Tsoi
& Gudjonsson, 2017; Soler et al., 2016). The
IL-23/IL-17A
Th17 axis has a crucial role in the development of psoriasis (Wu et al.,
2018; Zhu et al., 2017).
MDSCs were originally identified by the
CD11b+Gr1+ phenotype in
tumour-bearing mice (Serafini, Borrello & Bronte, 2006). However, as
the Gr1 gene homolog is lacking in humans, the surface markers of human
MDSCs are different from those of mice; for instance,
CD14+HLA-DR-/low (Filipazzi et al.,
2007).
Cao
et al. reported an increase in the number of M-MDSCs in patients with
psoriasis and further studied the abnormal effects on their regulated
function (Cao et al.,
2016).
Other
studies have revealed a significant increase in the number of MDSCs in
psoriasis; however, these cells also lack sufficient immunosuppressive
functions (Ilkovitch & Ferris, 2016; Soler et al., 2016; Turrentine et
al., 2014). Peng et al. reported that MDSCs play a proinflammatory role
in IMQ-induced psoriasis-like skin inflammation by regulating the
infiltration of CD4+ T-cells. The depletion of MDSCs
by gemcitabine significantly suppressed the IMQ-mediated psoriatic
phenotype (Chen et al., 2020), suggesting that targeting MDSCs might
serve as a novel strategy for the treatment of psoriasis. Consistent
with other studies, we also found an increased number of peripheral
blood
CD11b+CD14+HLA-DR− /lowMDSCs in patients with psoriasis compared with that in healthy controls.
Concomitantly, we also found a significant increase in the number of
MDSCs in the spleen and skin lesion of mice in the IMQ-induced psoriasis
model.
IL-35 has been reported to play a critical role in several
immune-associated diseases, such as autoimmune diseases, viral and
bacterial infections, and tumours. Wirtz et al. reported that enteritis
symptoms in an IBD mouse model were significantly alleviated following
the vector-mediated overexpression of
IL-35
(Wirtz, Billmeier, McHedlidze, Blumberg & Neurath, 2011). However, few
studies have explored the role of IL-35 in the pathogenesis of
psoriasis. Li et al. revealed that the serum levels of IL-35 were higher
in patients with psoriatic arthritis than in patients with psoriasis and
healthy controls (Li et al., 2017). Cardoso et al. did not find any
differences between the levels of IL-35 in the serum of Brazilian
patients with psoriasis and healthy controls (Cardoso et al., 2016). Wei
et al. found that the expression of IL-35 in the peripheral blood of
patients with psoriasis with vulgaris was lower than that in the control
group (Chen, Du, Han & Wei, 2021).
Similarly,
Deng et al. revealed that IL-35 concentrations in plasma were lower in
patients with psoriasis than in healthy individuals (Li et al., 2018).
However, Placek et al. observed that the levels of IL-35 were higher in
the serum of patients with psoriasis but without any statistically
significant relationship with PASI scores (Owczarczyk-Saczonek,
Czerwinska, Orylska & Placek, 2019). These studies primarily detected
the expression of IL-35 in patients with psoriasis; however, there are
few studies on its specific role. In this study, we found a significant
increase in the serum levels of IL-35 in patients with psoriasis. In
addition,
the number of EBI3+p35+ cells was
significantly increased in the peripheral blood
of
psoriatic skin (Fig. 1). As an anti-inflammatory cytokine, IL-35
expression was significantly increased in patients with psoriasis,
possibly to combat the severe inflammatory response of psoriasis.
We previously revealed that the overexpression of the IL-35 gene
significantly inhibited the expression of proinflammatory factors in anin-vitro model of psoriasis and ameliorated the disease indexes
of psoriasis in mouse models (Zhang et al., 2016). Similarly, we also
found that the administration of IL-35 recombinant protein improved the
severity of psoriasis in mice; however, the specific mechanism of IL-35
recombinant protein was not explored further (Wang et al., 2018). Our
team previously found that IL-35 gene therapy can exert
immunosuppressive functions by inhibiting the recruitment of macrophages
in psoriatic mice and regulating the ratio between M1 and M2 to
significantly improve the pathogenesis in the psoriatic mouse model. In
addition, we found that IL-35 expression can significantly inhibit the
proportion of CD11b+ myeloid cells (data not shown),
suggesting that IL-35 may regulate myeloid cells. Therefore, in this
study, we focused on MDSCs.
In
this study, a significant increase in the number of MDSCs was observed
in the IMQ-induced psoriasis model (Supplemental Fig. 3). In contrast,
the number of MDSCs was significantly decreased in mice with IMQ-induced
psoriasis after treatment with IL-35 recombinant protein. Furthermore,
we found that G-MDSC and M-MDSC counts were reduced in both the spleen
and skin tissues in the IL-35-treated group compared with that in the
control group (Fig. 5).
Despite
the significant reduction in the populations of MDSCs in IL-35-treated
mice with IMQ-induced psoriasis, IL-35 did not play a direct role in the
differentiation of MDSCs, which indicated that MDSC differentiation is
potentially due to the regulation of the immune response in mice.
As mentioned, MDSCs are known to play deleterious roles in the
progression of cancer and infectious diseases; however, their role in
autoimmune diseases appears to be
more
complex (Veglia, Perego & Gabrilovich, 2018). Herein, to further study
whether the role of MDSCs is to promote or inhibit inflammation, we
conducted experiments of adoptive transfer of MDSCs from IMQ-induced
mice during treatment with IL-35. We found that the adoptive transfer of
MDSCs weakened the effect of the treatment of IMQ-induced psoriasis with
IL-35 (Fig. 6). The results further support the study by Cao et al. (Cao
et al., 2016). iNOS and IL-10 levels were significantly elevated in
tumour-induced MDSCs, which indicates their immunosuppressive function
in cancer (Hart, Byrne, Molloy, Usherwood & Berwin, 2011; Redd et al.,
2017). However, there are few studies on the role of IL-10 and iNOS
secreted by MDSCs in autoimmune diseases. Herein, we found that the
population of MDSCs secreting iNOS, but not IL-10, were significantly
increased in the IMQ-induced mouse psoriasis model. Furthermore, the
adoptive transfer of MDSCs from IMQ-induced mice weakened the
anti-inflammatory effects of the treatment with IL-35 in IMQ-induced
psoriasis; however, this weakened effect was reversed by MDSCs from
IMQ-induced iNOS-/- mice. We consider that the
elevated numbers of MDSCs do not necessarily play an immunosuppressive
function in the IMQ-induced psoriasis mouse model. In addition, it is
plausible that the immunosuppressive functions of MDSCs are
impaired
by secreting iNOS, leading to the promotion of immune reactions.
In summary, the administration of IL-35 attenuated the psoriasis-like
skin inflammation in
psoriasis
mice. Mechanistic studies have revealed
that
IL-35 reduces the severity of psoriasis in mice by inhibiting
pro-inflammatory cytokines in the skin microenvironment, suppressing the
recruitment of MDSCs and the population of MDSC-secreting iNOS. This
role of IL-35 has not been reported thus far. We hypothesized that IL-35
may inhibit the recruitment of MDSCs and the ability of MDSCs to secrete
iNOS by affecting inflammatory factors in the skin microenvironment,
corresponding mechanism diagram as illustrated in Figure 9. Hence, we
conclude that supplemental treatment with IL-35 would be helpful in the
management of psoriasis, and this study highlights a new therapeutic
strategy for other cutaneous inflammatory diseases.