## 4 Discussion  
The results shown in Figures 1 and 2 confirmed that NPs are useful for the cellular uptake of hydrophobic substances. Compared with PLGA NPs, PLGA-PEG-PLGA NPs had lower cellular uptake. We considered that steric hindrance by PEG prevented the interaction of NPs with the cell membrane [33], and improved hydrophilicity, which made it difficult to be taken up into the cells [34], was responsible for the decreased cellular uptake of PLGA-PEG-PLGA1004 NPs. In the skin permeation test, as shown in Figure 5, it was found that PLGA-PEG-PLGA NPs reached deeper into the skin of psoriasis-like skin than PLGA NPs did. It is known that psoriatic skin has a faster turnover cycle than normal skin [35]. As a result, intercellular lipids are not sufficiently produced in psoriatic skin compared with normal skin, resulting in decreased barrier function and increased transepidermal water loss [36]. Due to these factors, the intercellular spaces in psoriatic skin are wider than those in normal skin, and it was reported that 40 nm FluoSpheres(r) were delivered deep into the skin in the psoriasis model mouse [37]. Therefore, we considered that PLGA-PEG-PLGA1004 NPs were useful for delivering CsA to the vicinity of cells rather than inside cells.  
Figure 8: Concentrations of TNF-\(\alpha\) (**A**), IL-22 (**B**), and IL-17A (**C**) in the skin after psoriasis treatment experiments, quantified using ELISA (mean \(\pm\) S.D., \(n\) = 5, \({}^{\star}\)\(p\) < 0.05, \({}^{\star\star}\)\(p\) < 0.01, Tukey’s test).  
The therapeutic study using the psoriasis model mouse suggested that PLGA-PEG-PLGA1004 NPs have a high therapeutic effect on psoriasis. Th1 and Th17 are mainly involved in psoriasis, and CsA exerts its therapeutic effect by suppressing TNF-\(\alpha\) (Th1), IL-17A (Th17), and IL-22 (Th17), produced by these cells. Symptoms are also caused by TNF-\(\alpha\), IL-23, IL-17A, and IL-22 in the psoriasis model mouse. TNF-\(\alpha\) activates Th17 cells as well as dendritic cells and has the effect of promoting the proliferation of epidermal cells via IL-22. In addition, TNF-\(\alpha\) is also produced by psoriatic lesion epidermal cells and is known to be significantly involved in the pathology of psoriasis [38]. The effects of IL-17A on epidermal cells include an increased expression of antibacterial peptides such as LL-37, the induction of inflammatory cytokine and chemokine production from epidermal cells, the promotion of neutrophil migration to the epidermis, and the proliferation of epidermal cells [39]. L-22 is significantly involved in epidermal thickening and cooperates with Th17 to activate Stat3 and promote keratinocyte proliferation [40]. Suppressing the cytokines (IL-17A, IL-22) produced by Th17 is effective in treating psoriasis in humans as well as in the psoriasis model mouse. Therefore, the reduction in the levels of these cytokines observed in this treatment experiment may correlate with therapeutic efficacy in humans [41]. We also confirmed that hair grew due to the administration of NPs. It is also known that applying CsA to mouse skin has a hair growth effect [42; 43]. These results suggest the usefulness of NPs for the treatment of psoriasis.