Abstract:
Psoriasis is a common inflammatory skin disease affecting an estimated 2-3% of the population. This skin disease is a complex disorder that can be triggered by genetic, environmental and immunological factors, but the exact cause is still unknown. Psoriasis is characterized by persistent inflammation associated with uncontrolled keratinocyte hyperproliferation and abnormal differentiation. The inflammation is triggered by a broad spectrum of cytokines, such as IL-8, IL-23, IL-17, and IL-36. These cytokines are produced by many different cell types, but one of the most important producers in psoriasis are the keratinocytes. In this process, the keratinocytes produce not only cytokines, but also chemokines and antimicrobial peptides to attract and activate immune cells, which then also release cytokines that activate the keratinocytes and lead to hyperproliferation of these cells. This feedback loop amplifies the response and increases the complexity and severity of psoriasis. Furthermore, the severity of psoriasis is associated with the frequency of neutrophil extracellular traps (NETs). These NETs can not only capture and kill microorganisms, they also promote inflammation. In this context, NET-associated RNA (naRNA) was first described by Herster et al. in 2020. This naRNA is a potential trigger of the proinflammatory processes, however, not much is known about naRNA or its influence on immune responses. Therefore, the aim of this work was to investigate the influence of naRNA on cells in the skin, to identify the receptors responsible for it and to analyze the possible role in the progression of psoriasis.
It could be shown that naRNA leads to self-amplification of NET-formation in neutrophils and that this is mediated by the receptor TLR8 in human or TLR13 in mice. Thereby this work could show that naRNA recognition in macrophages leads to TLR8-dependent IL-8 release. Human keratinocytes, the most abundant cell type in skin, respond to naRNA in a dose-dependent manner and subsequently release IL-8. This has also been demonstrated in physiologically relevant 3D models of the skin. This recognition of naRNA is not only TLR8, but MyD88-, NLRP1- and MAVS-independent. This activation of keratinocytes is mediated by the NOD2-RIPK2 signaling pathway. This was demonstrated by inhibition of RIPK2 and knockdown of NOD2 using siRNA. NOD2-mediated activation by naRNA thereby induces the expression of proinflammatory cytokines and antimicrobial peptides associated with psoriasis, such as IL17C, IL36G, CXCL8, CAMP and RNASE7. The activation of these genes could also be detected in 3D models of the skin. Furthermore, the induced expression of IL17C, IL36G, CXCL8, and CAMP was found to be mediated by NOD2, but not RNASE7. In addition, inhibition of IκBζ was found to lead to a reduction in many of these cytokines, suggesting that IκBζ lies downstream of NOD2. NaRNA-mediated activation of mouse keratinocytes was also examined. This also revealed NOD2-dependent activation of mouse keratinocytes by RNA, leading to induction of psoriasis-associated genes. Fibroblasts, the second most abundant cell type in the skin, also respond to RNA in NETs, but the cell line used responded in a RIPK2 independent manner.
In summary, naRNA was shown to be a major proinflammatory component of NETs that is recognized by all cell types tested. However, the signaling pathways for recognition differ. The NOD2-mediated activation of keratinocytes and the resulting activation of psoriasis-associated genes could be a key to further understanding this disease but also provide new potential therapeutic approaches.
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