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.