Expanding strategies to resolve psoriasis-like inflammation: Non-canonical NF-κB signaling controls IL-23 production in dendritic cells

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Inflammation is a crucial process employed by the immune system to control pathogens and other invading stimuli. After clearance, inflammation is resolved and controlled by multiple mechanisms to prevent unwanted and excessive inflammatory processes. In cases of failure of these control mechanisms, chronic inflammatory diseases can arise. A classic example of a chronic inflammatory disease is psoriasis, which clinically manifests in the skin as scaly, erythematous lesions and affects approximately 2%–3% of people worldwide.¹ In psoriasis, the interleukin (IL)-17/IL-23 axis has been shown to be a major signaling pathway in disease pathology, and multiple immunotherapies targeting this pathway have been developed.² While immunotherapies have good clinical efficacy, they globally inhibit effector responses, making patients susceptible to opportunistic infections.³ Hence, there would be great benefit in developing therapies aimed at reinstating immune homeostasis in the tissue. In a recent article published in Molecular Therapy, Xia et al.⁴ highlight that this could be possible by targeting ANKRD22, an ankyrin repeat-containing protein family member. Using a mouse model of psoriasis, the authors show that ANKRD22 is lost in inflamed psoriatic skin and is able to bind nuclear factor κB (NF-κB)-inducing kinase (NIK) and target it for degradation, leading to downregulation of non-canonical NF-κB signaling and subsequent resolution of inflammation. This pathway specifically affects dendritic cells’ (DCs’) ability to produce IL-23, which in turn causes a decrease in IL-17A-producing T cells, thereby blocking the initiation and maintenance of the IL-17 inflammatory loop associated with psoriasis.

The NF-κB pathway is a major downstream signaling target of multiple cytokines and chemokines. The transcription factors activated by this pathway consist of five monomeric proteins (p65/RelA, RelB, cRel, p50, and p52), which form homo- or heterodimers upon activation.⁵ Signaling can be induced via two different pathways: the canonical, NF-κB essential modulator (NEMO)-dependent pathway and the non-canonical, NEMO-independent pathway.⁶ Traditionally, the canonical pathway is considered to be induced upon inflammatory stimuli, while the non-canonical pathway is usually associated with developmental signals and tumor necrosis factor (TNF) receptor signals.⁷ The non-canonical pathway uses NIK as a signal transducer, which requires its stabilization and accumulation downstream of TRAF2-dependent cIAP activation and subsequent degradation of TRAF3. Ultimately, the non-canonical pathways results in nuclear translocation of the RelB:p52 transcription factor dimer.⁸ NIK, like other NF-κB components, is tightly regulated by inhibitory proteins (termed IκBs) containing ankyrin repeat domains.⁹ While some of these proteins have been characterized extensively in the canonical pathway, control of NIK signaling by non-IκB ankyrin domain-containing proteins is not yet well described and represents a potential novel avenue for specifically targeting non-canonical NF-κB signaling in inflammation.

In their study, Xia et al.⁴ characterize one such novel ankyrin repeat domain-containing protein: ANKRD22. The authors use imiquimod (IMQ) to chemically induce psoriasiform inflammation in mouse back skin to show that ANKRD22 expression is lost in hematopoietic cells upon IMQ application but increased upon resolution of inflammation. Global knockout of ANKRD22 aggravated IMQ-induced inflammatory cytokine production in the skin, suggesting that expression of this protein serves a protective role in skin inflammation. As IL-17A is considered to be a main driver of the psoriatic inflammatory loop, the authors first assessed dermal γδT cells, the main source of IL-17A in mouse skin. These cells showed increased IL-17A production in ANKRD22^−/− mice. However, upon adoptive transfer of wild-type or ANKRD22^−/− γδT cells, no difference was observed in pathology scores of IMQ-induced psoriasis. This led the authors to investigate the cells inducing differentiation of naive effector T cells into IL-17A-producing cells: DCs (Figure 1).

Figure 1.

ANKRD22 controls resolution of psoriasis-like inflammation via the non-canonical NF-κB pathway

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ANKRD22 was most highly expressed in dermal DCs. Deletion of DCs via CD11c-DTR and subsequent adoptive transfer of wild-type or ANKRD22^−/− bone-marrow-derived DCs (BMDCs) showed that mice lacking DCs had significant resistance to IMQ-induced psoriasis. Transfer of ANKRD22^−/− DCs conferred increased susceptibility, suggesting that DCs and their expression of ANKRD22 are major determinants of psoriasis initiation. The authors further show that BMDCs and dermal DCs from ANKRD22^−/− mice show increased expression of IL-23—the cytokine inducing IL-17A expression in T cells. IL-23 induction is known to be regulated via the NF-κB pathway. Using a series of western blots and chromatin immunoprecipitation for multiple components of the canonical and non-canonical pathways of NF-κB signaling, the authors show here that ANKRD22 deficiency led to an increase in non-canonical signaling via NIK stabilization by increased nuclear translocation of p52 and RelB and subsequent IL-23 induction. Based on these findings, the authors aimed to investigate the potential therapeutic benefits of targeting ANKRD22 in psoriasis. Using publicly available datasets, a negative correlation between ANKRD22 expression and IL23A expression, as well as PASI score, was found, showing potential translatability of the findings in the mouse model. The direct effect of ANKRD22 on non-canonical NF-κB signaling in humans was then mechanistically demonstrated by small interfering RNA-mediated knockdown in PMDCs, which showed increased NIK accumulation, p52 and RelB translocation, and IL23A expression. Lastly, IMQ-treated mice were injected subcutaneously with an adeno-associated virus (AAV) containing an ANKRD22-overexpression vector, which showed clinical improvement of the disease by day 10 compared to empty AAV.

Together, the results convincingly show that non-canonical NF-κB signaling in dermal DCs can be regulated by ANKRD22 in the context of IMQ-induced psoriasis and can potentially be translated to humans. Multiple mouse models for psoriasis exist, with IMQ being one of the most frequently used. Application of IMQ phenocopies most features associated with psoriasis and the underlying molecular pathology of the IL-17/IL-23 axis.¹⁰ IMQ acts primarily through the Toll-like receptor 7, although alternative signaling pathways have been suggested. This notion is further supported here since a high level of activation of the non-canonical NF-κB pathway via TNFRII is induced in DCs upon IMQ treatment. However, using genetic mouse models, which mimic the more chronic form of psoriasis and are not as dependent on dermal γδT cells for IL-17A production, could be beneficial since these cells are not considered the primary pathogenic cell type in humans due to their absence in human skin.¹¹ However, this study provides additional evidence that targeting the IL-17A inducing cytokine IL-23 could prove highly beneficial in skin inflammation by promoting resolution of inflammation rather than blocking final effector cytokines. However, further studies investigating potential side effects such as the occurrence of opportunistic bacterial and viral infections when targeting this pathway need to be undertaken and have not been addressed in this study. The IL-23 locus and members of the NF-κB family have previously been associated with psoriasis on a genome-wide level in humans.¹² Furthermore, psoriasis severity has been linked to a combinatorial genetic effect of HLA-C∗06, NF-κB1, IL-23R, and IL-23A.¹³ Therefore, developing drugs specifically targeting IL-23 and non-canonical NF-κB signaling may give additional benefit to patients suffering from IL-17-driven skin diseases.