As non-inflammatory cells, keratinocytes contribute to lesional inflammation in CLE. An initial trigger, such as UV radiation, smoking or drugs causes keratinocyte apoptosis. UV radiation leads to an upregulation of autoantigens, such as Ro52, in keratinocytes, inducing and activating the proinflammatory pathways [
15,
21]. Apoptotic keratinocytes present antigens, which can be recognized by autoantibodies in autoantibody-positive patients [
22]. Autoantibodies against ribonucleoproteins could also have an independent pathophysiological role, as they trigger the development of lupus lesions in mice [
23]. Interestingly, UV radiation or other damaging triggers initially lead to keratinocytic cell death and chemokine production in the whole epidermal layer [
24], however, later in established CLE lesions, keratinocytic apoptosis and proinflammatory chemokine production is limited to the dermoepidermal junction, resulting in interface dermatitis [
25]. Even keratinocytes from uninvolved (non-lesional) skin of CLE patients are more sensitive to UV radiation-induced cytotoxicity compared with keratinocytes from healthy donors, which leads to the assumption of disease predisposition [
26,
27]. Following the initial keratinocyte damage, secondary necroptosis of keratinocytes further sparks lesional release of nucleic acids and danger-associated molecular patterns (DAMPs). The latter include high mobility group box 1 protein (HMGB1), a proinflammatory cytokine, which can also function as autoantibody in CLE [
28]. UV radiation also leads to DNA damage, generating immune-stimulatory DNA motifs, such as 8-hydroxyguanosine [
29]. Phagocytic clearance of apoptotic cells and nucleic acids can be impaired in CLE [
27]. Nucleic acids are recognized via pattern recognition receptors (PRRs), including MDA5, RIG-I and cGAS–STING, expressed by keratinocytes leading to production of IFN-regulated genes [
29]. The response in keratinocytes is toll-like receptor (TLR)-independent [
1]. Keratinocytes produce IFNκ and IFNλ (type I and type III IFNs), which, by autocrine secretion, further induce the keratinocytic production of IFN-regulated proinflammatory cytokines, including interleukin (IL)-6, and chemokines, including CXCL9, CXCL10 and CXCL11, which are CXCR3 ligands [
29‐
31]. The IFN response is, among others, mediated via Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling [
30]. The aforementioned chemokines interact with (autoreactive) cytotoxic T cells via CXCR3-binding, also promoting keratinocyte cell death by recruitment of cytotoxic T cells [
32,
33]. Nucleic acid motifs also activate the inflammasome via melanoma 2 (AIM2) [
34]. Interestingly, IFN-k is upregulated and basal phospho-STAT (pSTAT) activity is higher even in healthy-appearing skin of CLE patients compared with skin of patients with other chronic inflammatory skin disease (psoriasis) [
30]. As outlined, the inflammatory interplay is complex and numerous inflammatory cells contribute to CLE pathology. Therefore, in the following sections, we outline recent findings considering specific types of cells in the orchestration of inflammation in SLE and CLE.
2.2.1 Dendritic Cells
After initial keratinocyte cell death, antigen-presenting cells (APCs) sense accumulating nucleic acids, among them dendritic cells (DCs) and pDCs. pDCs are recruited to the skin lesions via CXCL-chemokine interaction with CXCR3 [
24]. PDCs sense nucleic acids mostly via TLRs, especially TLR7 and TLR9 [
35]. Uptake of nucleic acids and immunecomplexes may be achieved by endocytosis via TLR9 and cluster of differentiation (CD) 32, at least in SLE [
36]. Upon PRR activation, pDCs produce large amounts of type I and type III IFNs, cytokines and ILs, further orchestrating the autoimmune circle [
37,
38]. The presence of type I IFN is necessary for pDC maturation and migration [
39].
PDC infiltrates are observed in a great proportion of skin biopsies and can form clusters in CLE skin lesions [
38,
40]; however, not all skin lesions harbor a pDC infiltrate [
41]. Recently, single-cell ribonucleic acid (RNA) and spatial RNA sequencing has shown that even healthy-appearing skin of CLE patients contains a type I IFN-rich environment and that CD16+ DCs undergo IFN priming in the skin, leading to proinflammatory subtypes [
42]. Because of their central role in CLE pathophysiology, pDCs are an attractive therapeutic target. One potential targeted pDC therapy is the blood DC antigen 2 (BDCA2) receptor, which is exclusively expressed on pDCs [
43]. BDCA2 suppresses IFN induction [
44].
2.2.2 T cells
Lesional inflammatory infiltrates mainly consist of T cells, B cells, DCs, natural killer (NK) cells, and, infrequently, neutrophils [
25,
45]. CXCR3-expressing T cells are recruited into skin lesions via CXCL10. Physiologically, T cells recognize antigens presented by APCs via T-cell receptor (TCR)/major histocompatibility complex (MHC) interaction. Upon TCR engagement, downstream signaling pathways are activated, leading to various T-cell functions. T cells have a lower threshold of activation in lupus patients [
46]. Due to defective CD3 chains, the spleen tyrosine kinase (SYK) and Fc receptor γ-chain (FcRγ) association results in higher phosphorylation of signaling molecules and an enhanced calcium influx, leading to enhanced TCR downstream signaling [
47]. Additionally, transcription factors lead to differential expression of numerous genes, including the CD40 ligand (CD40L) [
48], a co-stimulatory molecule engaged in B-cell interaction, promoting B-cell functions such as proliferation, differentiation, antibody production, and class switching [
46]. Increased CD40L does not only have an impact on B cells interacting with T cells but also on APCs. It leads to increased expression of co-stimulatory receptors on APCs, further intensifying the TCR signal [
49]. Several different pathways have been described as defective (such as cyclic adenosine monophosphate-dependent phosphorylation, protein kinase C) or increased (such as phosphatidylinositol-3 kinase (PI3K) [
46]. Apart from altered pathway signaling, lupus T cells display differential DNA methylation of several genes, leading to differential gene expression [
46]. Furthermore, SLE patients show an IL-2 deficiency [
50]. IL-2 is important for T-cell polarization, and decreased IL-2 expression enhances inflammatory T-helper 17 (Th17) cell formation [
51].
Upon activation, cytotoxic T cells target keratinocytes of the basal epidermal layer, histologically resulting in interface dermatitis [
52]. However, this applies for CLE subtypes with superficial involvement and plays a minor part in dermal or subcutaneous CLE subtypes such as LE profundus. Cytotoxic markers such as granzyme B expressed by CD8+ T cells are present in CLE skin lesions and are likely to be induced by IFN [
53,
54]. Interestingly, granzyme B expression is higher in scarring lesions of CDLE compared with non-scarring lesions of subacute CLE, suggesting a pathophysiologic role in scarring lesions in CLE [
54].
Initiation of cutaneous inflammation is likely to be triggered by Th2 cells, but fully established lesions shift to a Th1-dominated inflammation [
52,
55]). Th1 cells stimulate type I IFN production of cytotoxic T cells and macrophages [
52,
56]; not only cytotoxic T cells are responsible for keratinocytic apoptosis. Lesional CD4+ T cells can directly induce keratinocytic apoptosis via FAS/FAS ligand (FAS-L) interaction [
57]. T-helper cells produce IL-21, inducing the expression of granzyme B in pDCs, promoting NK cells to attack keratinocytes [
58,
59]. On the other hand, type I IFNs negatively regulate granzyme B production by pDCs [
58]. Th cells can react to nucleosomes released from dying cells and induce (anti-DNA-)antibody production of B cells in SLE [
60‐
62]. Th clones in lupus produce IL-2, IFNγ, and IL-4 [
63], and CD4+ T cells overexpress perforin, which is epigenetically regulated via DNA methylation [
64].
The number of CD4+, CD8+ regulatory, and γδ-T cells is significantly reduced in CLE compared with other inflammatory skin diseases or healthy individuals, and impairment of regulatory immunosuppressive function contributes to the autoimmune circle [
46,
65,
66]. Furthermore, emerging evidence points out that composition of the inflammatory infiltrate differs among CLE subtypes. CD4+ T cells and FOXP3+ T cells are significantly reduced in skin lesions of patients with subacute CLE compared with CDLE, as is the CD4/CD8 ratio [
67].
2.2.3 B Cells and Plasma cells
B cells harbor a central role in LE pathogenesis by the production of autoantibodies against nuclear components and their complex interplay with T cells [
18,
68‐
70]. The capacity of B cells to produce antibodies is enhanced by different IFNs, however, prolonged type I IFN exposure drives autoantibody production [
71]. A new mouse model established the role of IL-21 and TLR7/9 in the context of B-cell recruitment to inflammation sites in CLE lesions and localized antibody production [
72]. IL-17 recruits immune cells and augments antibody production of B cells in SLE [
73]. SLE patients frequently present with antinuclear antibodies (ANAs) but only a minority of CLE patients display detectable autoantibody levels in the serum [
74]. Similarly to SLE, autoantibodies against ribonucleoproteins (anti-Ro antibodies) and La are frequently found in SCLE but fewer in CDLE [
75]. Different studies reported a specificity of autoantibody presence and CLE subtype [
76]. The presence of antibodies are in accordance with the HLA-DR3 phenotype in SLE [
74], and the presence of different antibodies (e.g. Ro or LA) are associated with disease severity in SLE [
77]. IL-17 recruits immune cells and augments antibody production of B cells in SLE [
73]. Besides autoantibody production, B-cell migration, receptor engagement, antigen presentation, cytokine responsiveness and production, survival, differentiation and class-switching are IFN-dependent [
71].
Recently, the understanding of the pathophysiological role of B cells in LE has shifted, since strong B-cell signatures and lesional B-cell infiltrates have been described in patients with autoantibody-negative CLE [
41,
78]. Beside antibody production, B cells can contribute to the autoimmune reaction by different mechanisms. For example, emerging evidence points towards an antigen-presenting, T-cell activating function of B cells [
41]. Lesional B-cell infiltration varies among LE subtypes [
41,
79]. B cells can form clusters and arrange in lymphoid-like structures in the skin, called tertiary lymphoid organs/structures (TLO). In different subtypes of CLE, the formation of dense B-cell clusters or TLOs has been described, e.g. in LE profundus or CDLE [
41,
80]. TLOs are highly organized structures containing T and B cells, contributing to autoimmunity [
81], and have been described in detail in lupus nephritis [
82,
83]. B cells can harvest a regulatory function, as is described for SLE [
84], and can interact with keratinocytes via B-cell-activating factor (BAFF/Blys) and its receptor in both SLE and CLE, whereby BAFF is expressed by lesional keratinocytes and the associated receptors (BAFF-receptor [BAFF-r], transmembrane activator and CAML interactor [TACI], B-cell maturation antigen [BCMA]) by B cells [
41,
85‐
87]. BAFF is a membrane-bound or soluble factor necessary for B-cell maturation [
88]. BAFF expression in keratinocytes can be induced by immunostimulatory DNA motifs, highlighting its significance in CLE [
86]. B cells produce high levels of cytokines such as IL-6, which in turn is important for B-cell survival [
18,
89].
During the maturation process, B cells exhibit immunoglobulin class switching and somatic hypermutation to differentiate into antibody-secreting plasma cells; those processes can occur either in germinal centers or extrafollicular locations, and both have been described in SLE [
90]. Somatic hypermutation and isotype switching are dependent on CD40 and IL-21 [
18]. Plasma cell differentiation is supported by Th cells [
90]. After activation of naïve B cells, plasma cells are generated and persistently produce antibodies while receiving survival signals, mediated by the BAFF axis and IL-6, originating from adjacent cells [
81]. Plasma cells can reside and accumulate at the site of inflammation [
91]. Even in the absence of antigens, plasma cells can produce antibodies, as they receive survival signals via BAFF or IL-6 [
18]. Similar to naïve B cells, plasma cells are responsive to IFN [
92]. Different plasma cell subsets have been described to secrete autoantibodies against different structures in SLE [
93,
94].
2.2.5 Neutrophil Granulocytes
Neutrophil granulocytes are early responders in the course of tissue damage. Neutrophils produce antimicrobial peptides (AMPs; e.g. LL-37) and reactive oxygen species (ROS) [
98], and establish neutrophil extracellular traps (NETS), which are nets consisting of chromatin, histones and other intracellular content [
99,
100]. The process of NET formation, ‘NETosis’, can be a source of immunogenic materials, and along with release of AMPs, has been linked to autoimmunity. A recent study showed a high molecular heterogeneity in pathogenic neutrophil subsets in SLE (so-called low-density granulocytes [LGS]) with, among others, differences in NET formation and response to type I IFNs, displaying a high number of IFN-induced genes [
101]. LGS have been associated with increased vascular inflammation and arterial dysfunction in SLE [
102]. Not only are neutrophils prone to NETosis, but also impaired degradation of NETs due to enzymatic blockade or antibody formation can promote SLE activity [
103].
Upon NET formation and by release of other immune cells, complexes formed by double-stranded DNA (dsDNA) and LL-37 are taken up by pDCs via endocytosis and are recognized via TLR9, leading to activation and type I IFN production in SLE [
104]. LL-37/dsDNA complexes can serve as autoantigens [
98]. Higher levels of LL-37 and other AMPs have also been observed in CLE skin lesions, as well as skin lesions from SLE patients compared with healthy controls [
105,
106]. Furthermore, NETs are present in several CLE subtypes (panniculitis, ACLE, DLE) [
107]; however, it has yet to be elucidated if, and to what extent, those subsets of neutrophils and AMPs play a pathophysiologic role in CLE.
2.2.6 Macrophages
Monocytes and macrophages hold different biological functions, such as phagocytosis or cytokine production [
108]. Monocytes exert antigen-presenting properties in SLE [
109]. Contrary results have been published on whether differing monocyte and macrophage numbers in lupus patients versus healthy controls are found. Several studies describe an impairment in uptake of apoptotic material and prolonged phagocytosis, leading to accumulation of potential autoantigens and further immune stimulation lupus [
110,
111]. One study reported an impaired phagocytosis capacity of macrophages from SLE patients only in the presence of patients’ serum [
111]. Macrophages from SLE patients have an impaired adhesion capacity [
110]. Furthermore, macrophages can be classified into M1 macrophages, which harbor inflammatory and destructive properties and are induced by IFN, while M2 macrophages, which harbor regulatory properties, are involved in tissue repair and are induced by IL-4 or IL-13 [
112]. In SLE, polarization tends towards M1 macrophages, as M1 genes (e.g.
STAT1 and
SOCS3) were among differentially expressed genes in monocytes from SLE patients [
113,
114]. In a mouse model, adoptive M2 macrophage transfer led to decreased SLE severity [
115].
One study found FAS-L-expressing macrophages enriched around hair follicles in CLE patients, potentially being responsible for lupus-associated scarring alopecia via a direct FAS/FAS-L interaction with keratinocytes of hair follicles [
57]. More detail about the potential functions of macrophages in SLE is outlined elsewhere [
116]. Of note, a recent study suggested a pathophysiologic, inflammatory role for the microRNA (miRNA) miR-4512 in monocytes and macrophages in SLE via the TLR4-CXCL2 axis [
117].