The ISD map contents: a graphical review of mechanisms

The ISD map was built based on the review of AD- and PsO-related articles and, as such, the exploration of the contents of this map leads naturally to a graphical review of established molecular mechanisms of these diseases. We summarise the main molecular and cellular aspects of AD and PsO as shown in the side-by-side overview map and the mechanisms at the intercellular and intracellular levels. We provide here hyperlinks that allow readers directly access some of the mentioned region of the ISD map.

Side-by-side overview

In AD, initial trigger factors elicit keratinocytes (KCs) to release proinflammatory molecules that activate immune cells such as ILC2 cells (see “ILC2 cells activation by keratinocytes” in the map at the MINERVA platform). Independently of KCs, some antigens can also be involved in the onset of AD by directly activating Langerhans cells (LCs) and conventional dendritic cell (cDCs). In response to these antigens, LCs and cDCs drive the differentiation of naïve T cells into specific T helper (Th) cells (see “Langerhans and dendritic cells-driven T cell polarization” in the map at the MINERVA platform) that sustain the disease progression. Th cells sustain the progress of AD via expression of cytokines IL4, IL13, IL17A, IL22, IL31 and IFNG. While IL31 activates sensory nerve endings (SNEs) and promotes itch in AD, all other cytokines disturb KC proliferation and differentiation and stimulate release of more proinflammatory molecules (see “Th-derived interleukins effects on keratinocytes behavior and itch induction in AD” in the map at the MINERVA platform). While disturbances in the KCs proliferation and differentiation cause the typical histopathology manifestations of AD, the KCs-derived proinflammatory molecules promote increased differentiation of T naïve cells into Th cells, sustaining skin inflammation. In AD, Th2 cells-derived interleukins also activate B cells and eosinophils (see “Th2 cells-activated B cells and eosinophils” in the map at the MINERVA platform).

In PsO, initial trigger factors promote KCs to release proinflammatory molecules that activate plasmacytoid (pDCs) and conventional dendritic cell (cDCs). KCs also recruit neutrophils to epidermis in response to initial trigger factors. Some antigens can also be involved in the onset of PsO by directly activating pDCs (see “Keratinocyte-dependent or independent activation of dendritic cells and neutrophils in PsO” in the map at the MINERVA platform). In response to these antigens and the KCs-derived proinflammatory molecules, pDCs express IFNA1 that, in turn, makes cDCs drive the differentiation of naïve T cells into specific Th cells and induce the activation of ILC3 cells (see “Dendritic cells- driven T cell polarization and ILC3 activation in PsO” in the map at the MINERVA platform). Th cells sustain the progress of PsO via expression of IL17A, IL22 and IFNG. These cytokines disturb KC proliferation and differentiation and stimulate release of more proinflammatory molecules (see “Th-derived interleukins effects on keratinocytes behavior in PsO” in the map at the MINERVA platform). Defects in KCs proliferation and differentiation cause the typical histopathology manifestations of PsO, and the KCs-derived proinflammatory molecules, such as TNF, promote increased differentiation of T naïve cells into Th cells, sustaining skin inflammation (see “TNF-mediated inflammatory circuit in PsO” in the map at the MINERVA platform).

Intercellular communication in AD and PsO

The intercellular communication view for AD includes 19 cell types interconnected by cytokines and their receptors (see the map at the MINERVA platform. These are KCs, LCs, SNEs and inflammatory dendritic epidermal cells (IDECs) in the epidermis, and Th and B cells, granulocytes, cDCs, monocytes, macrophages, ILC2 cells and fibroblasts in dermis. The activation of this network in AD starts with the disruption of the skin barrier homeostasis, leading to a release of alarmins and allowing foreign substances to encounter LCs, cDCs and IDECs. In response, these cells release interleukins and chemokines that promote differentiation and recruitment of Th2, Th1, Th17 and Th22 cells (see “Activation of T cells by innate immune cells” in the map at the MINERVA platform). Th cell interactions drive (i) the emergence of epidermal hyperplasia, lichenification, sensory perception of itch, scratching and inflammatory response and (ii) the maintenance of the skin barrier dysfunction.

Lichenification, one of the AD hallmarks, is characterised by epidermal hyperplasia, i.e. pathological multiplication of KCs. It emerges from stimulatory effects of KCs by IFNG, IL17A and IL22 released, respectively, by Th1, Th17 and Th22 cells. IL17A also plays a role in the maintenance of skin barrier dysfunction via downregulation of proteins BTC, CLDN1 and FLG in KCs. Th2 cells-derived granzyme B (GZMB) also takes part in skin barrier dysfunction (see “T cells-driven skin barrier dysfunction and keratinocytes hyperplasia” in the map at the MINERVA platform). Both skin barrier dysfunction and lichenification are influenced by scratching, promoted by an increased sensory perception of itch. This increase perception of itch, in turn, is generated by an overstimulation of sensory nerve endings (nociceptor sensory neurons) in response to a combination of IL4, IL13 and IL31 derived from Th2 cells, KC-derived TSLP, histamine released by mast cells, eosinophils-derived IL5-induced BDNF – a neurotrophin that promotes sensory nerve branching - and the expression of serotonin receptor HTR7 in SNEs and histamine receptors HHR1 and HHR4 in Th2 cells (see “Itch induction” in the map at the MINERVA platform). Itching elicits scratching, a behaviour that stimulates an AD-associated inflammatory response via IL4, IL13 and IL5 released by Th2 cells, IL17A released by Th17 cells and mast cell-derived GZMB. These proteins influence the activities of KCs, B cells, fibroblasts, eosinophils, Langerhans cells, and SNEs (see “Scratching-induced inflammatory response” in the map at the MINERVA platform). Importantly, the elevated production of IgE (IGHE), a hallmark in AD, is due to IL4 and IL13 stimulation of B cells. IgE induces the production of interleukins and chemokines in mast cells and may trigger LCs, cDCs, macrophages, basophils and eosinophils to produce interleukins and chemokines, contributing to the continued skin inflammation (see “IgE effects on skin inflammation” in the map at the MINERVA platform).

The intercellular communication view for PsO is comprised by 17 different cell types interconnected mainly by cytokines and their receptors that are currently known to be involved in psoriasis: KCs, LCs, cDCs, plasmacytoid dendritic cells (pDCs), T cytotoxic (Tc), Th and B cells, granulocytes, ILC3 cells, melanocytes and fibroblasts (see the map at the MINERVA platform). The activation of this PsO-related cellular network can be linked to skin injury, causing KCs to release double-stranded DNA (dsDNA) and RNA (dsRNA) fragments complexed with antimicrobial peptides, notably CAMP (LL37). These complexes can stimulate several cells, including KCs themselves, neutrophils and antigen-presenting cells (APCs) such as LCs, cDCs and pDCs. These stimulated cells express, mainly via TLR signalling pathways, proinflammatory molecules able to recruit and activate both innate and adaptive immune cells. Neutrophils respond to the dsRNA-CAMP complex by secreting neutrophil extracellular traps (NETs) with IL17A, a cytokine playing a critical role in the pathogenesis of psoriasis. In addition to IL17A, NETs also contain other proteins, that promote inflammatory responses in KCs; these cells, in turn, produce neutrophil-attracting chemokines, such as CXCL8, CXCL1 and LCN2, which leads to more neutrophils infiltrating the epidermis. In addition, as NETs also contain dsRNA and CAMP, neutrophils are further activated and more NETs are released, creating an early self-amplifying inflammation cycle (see “Early innate immune-led self-amplifying inflammation cycle” in the map at the MINERVA platform).

In addition to this early KCs-neutrophils crosstalk, the activation of pDCs is essential for the development of PsO. Once activated, pDCs produce IFNA1 that stimulates cDCs to express TNF, IL1B, IL6 and IL23. While TNF acts on cDCs, LCs and KCs by promoting the expression of proinflammatory proteins, including CCL20, the chemoattractant of the Tc17 and Th17 cells, cDCs-derived IL1B, IL6 and IL23 act together to stimulate differentiation of Tc17 and Th17 cells with subsequent expression of IL17A, IL17F and IL22. These interleukins drive KCs to express more proinflammatory proteins, including chemokines, such as TNF and CCL20, and promote KCs hyperproliferation. This sustained cDCs-Tc17-Th17-KCs crosstalk creates an additional self-amplifying inflammation cycle in PsO (see “Innate to adaptive immune self-amplifying inflammation cycle” in the map at the MINERVA platform). This cycle can also include LCs, γδ T, ILC3 and mast cells, as they produce IL17A and IL22 and, except for LCs, are also attracted to skin via KC-derived chemokines. Finally, besides KCs, there are two other non-specialised immune cells: melanocytes and fibroblasts. Melanocytes are targets of not only TNF, but also Tc17 cells in PsO. Melanocytes express major histocompatibility (MHC) class I proteins that present ADAMTSL5 fragments in their surface, targeted for a noncytotoxic Tc cell–mediated autoimmune response. In turn, fibroblasts play a dual role in PsO. First, by expressing chemerin (RARRES2), they attract pDCs to skin in the earliest phases of the disease. Second, by expressing tenascin C (TNC), they promote neurite outgrowth.

Intracellular mechanisms of AD and PsO

Role of non-specialised immune cells in AD and PsO

In AD and PsO, KCs are the most important non-specialised immune cells. KCs are involved not only in the onset of AD and PsO, but also in the maintenance of these two ISDs. Although they are non-bone marrow-derived cells, KCs play a critical role in innate immune reactions and participate in the activation of adaptive immunity in the skin. Both AD and PsO maps show this aspect: KCs respond to a variety of endogenous and exogenous ligands and multiple interleukins derived from innate and adaptive immune cells by expressing chemokines and interleukins that attract and activate both innate and adaptive immune cells. In both diseases, this response takes place through the JAK-STAT, MEK-ERK and NFKB signalling pathways as we can observe in both AD and PsO maps. In AD, the chemokines CCL17 and CCL22, expressed in response to IFNG via JAK-STAT, MEK-ERK and NFKB pathways attract Th2 cells to dermis, while interleukin TSLP, expressed in response to TLRs, IL4/IL13 and IFNG, activates Th2 cells. In this case, KCs bridge two different adaptive immune cells, i.e., Th1 and Th2 cells. In PsO, KCs bridge innate and adaptive immune cells (e.g., ILC3 cells and Th17 cells) and an innate immune cell (neutrophil). For instance, CXCL8, a neutrophil chemoattractant, is produced in response to IL17A via the NFKB signalling pathway.

T cell activity in AD and PsO

At least five types of T cells - γδ T, Th1, Th2, Th17 and Th22 cells - play a role in AD or PsO. Here we describe in detail the intracellular pathways in Th1 and Th2 cells in AD and Th17 and γδ T cells in PsO.

Th2 cells are key in AD and are involved in both acute and chronic phases of this disease. As can be observed in the AD map, in response to IL33, IL25, TSLP and IL4 stimulation, Th2 cells express interleukins known to influence skin barrier dysfunction (IL4, IL13), eosinophil activation (IL5), keratinocyte differentiation and sensory perception of itch (IL4, IL13, IL31). IL33 promotes the activation of four transcription factors - ATF2, GATA3, JUN and the NFkB complex - via the MYD88-TRAF6-TAK1(MAP3K7)-TABs (MTTT) signalling cascade. Downstream p38 MAPK (MAPK14) activates ATF2 and GATA3. In parallel, downstream JNK signalling cascade activates JUN. Finally, parallel downstream IKKs activate the NFkB complex. IL25 also promotes the activation of the NFkB complex via TRAF6-TAK1(MAP3K7), but with TRAF3IP2 (ACT1) as the adaptor protein. Additionally, IL25 also activates a JAK-STAT signalling pathway to regulate downstream genes, as well as TSLP and IL4.

As for Th2 cells, Th1 cells also play a role in both phases of AD. In response to KCs-derived IL1B, Th1 cells express mainly IFNG, a proinflammatory protein that negatively influences KCs differentiation and positively influences inflammatory response. IFNG is expressed because of the activation of the NFkB complex via the MTTT cascade - the same used by IL33 in Th2 cells as previously discussed. IFNG is also upregulated by an autocrine loop in which IFNG itself stimulates IFNG expression via JAK-STAT signalling and the transcription factor TBX21.

Like Th2 cells in AD, Th17 cells in PsO are considered one of the most critical pathogenic factors. As a response to IL1B, IL6 and IL23 stimulation, Th17 cells express, among other proteins, IL17A and IL22 that, in turn, stimulate KC proliferation and expression of inflammatory proteins. IL1B binds to IL1RI and induces expression of the transcription factor IRF4 that, in turn, upregulates expression of RORC. The RORC-dependent transcription is then activated by IL6 and IL23 via their receptors and JAK-STAT3 signalling. RORC in turn can induce transcription of IL17A, IL17F, IL21, IL22, IL23R, as well as CCR6, a chemokine that influences Th17 cell migration to sites of inflammation. STAT3 itself is also able to directly induce expression of IL17A, IL17F and IL22.