Immune sensing of nucleic acids in inflammatory skin diseases

Autoimmunity in vitiligo: Therapeutic implications and opportunities

Vitiligo is an acquired chronic pigmentary disorder affecting the melanocytes, mainly in the skin and mucosae. It occurs due to the dynamic interaction between genetic and environmental factors leading to autoimmune destruction of melanocytes. Defects in melanocyte adhesion and increased oxidative stress further augment the immune response in vitiligo. It is a cosmetically disfiguring condition with a substantial psychological burden. Its autoimmune nature with resultant chronicity, variable responses to therapeutic modalities, and frequent recurrences have further diminished the quality of life. Hence, treatment should aim to provide more extended remission periods, prevent recurrences, provide good cosmetic outcomes and ensure patient satisfaction. These treatment goals seem plausible with the recent progress in our understanding of the complex pathogenic mechanisms underlying vitiligo at a molecular and genetic level. We provide a literature review of the pathogenic mechanisms and the therapies targeting these mechanisms.

The Role of Cutaneous Type I IFNs in Autoimmune and Autoinflammatory Diseases

IFNs are well known as mediators of the antimicrobial response but also serve as important immunomodulatory cytokines in autoimmune and autoinflammatory diseases. An increasingly critical role for IFNs in evolution of skin inflammation in these patients has been recognized. IFNs are produced not only by infiltrating immune but also resident skin cells, with increased baseline IFN production priming for inflammatory cell activation, immune response amplification, and development of skin lesions. The IFN response differs by cell type and host factors and may be modified by other inflammatory pathway activation specific to individual diseases, leading to differing clinical phenotypes. Understanding the contribution of IFNs to skin and systemic disease pathogenesis is key to development of new therapeutics and improved patient outcomes. In this review, we summarize the immunomodulatory role of IFNs in skin, with a focus on type I, and provide insight into IFN dysregulation in autoimmune and autoinflammatory diseases.

Vitiligo: Focus on Clinical Aspects, Immunopathogenesis, and Therapy

Vitiligo is an acquired chronic depigmenting disorder of the skin, with an estimated prevalence of 0.5% of the general population, characterized by the development of white macules resulting from a loss of epidermal melanocytes. The nomenclature has been revised after an extensive international work within the vitiligo global issues consensus conference, and vitiligo (formerly non-segmental vitiligo) is now a consensus umbrella term for all forms of generalized vitiligo. Two other subsets of vitiligo are segmental vitiligo and unclassified/undetermined vitiligo, which corresponds to focal disease and rare variants. A series of hypopigmented disorders may masquerade as vitiligo, and some of them need to be ruled out by specific procedures including a skin biopsy. Multiple mechanisms are involved in melanocyte disappearance, namely genetic predisposition, environmental triggers, metabolic abnormalities, impaired renewal, and altered inflammatory and immune responses. The auto-immune/inflammatory theory is the leading hypothesis because (1) vitiligo is often associated with autoimmune diseases; (2) most vitiligo susceptibility loci identified through genome-wide association studies encode immunomodulatory proteins; and (3) prominent immune cell infiltrates are found in the perilesional margin of actively depigmenting skin. However, other studies support melanocyte intrinsic abnormalities with poor adaptation of melanocytes to stressors leading to melanocyte instability in the basal layer, and release of danger signals important for the activation of the immune system. Recent progress in the understanding of immune pathomechanisms opens interesting perspectives for innovative treatment strategies. The proof of concept in humans of targeting of the IFNγ /Th1 pathway is much awaited. The interplay between oxidative stress and altered immune responses suggests that additional strategies aiming at limiting type I interferon activation pathway as background stabilizing therapies could be an interesting approach in vitiligo. This review covers classification and clinical aspects, pathophysiology with emphasis on immunopathogenesis, and promising therapeutic approaches.

Heat shock protein 70 potentiates interferon alpha production by plasmacytoid dendritic cells: relevance for cutaneous lupus and vitiligo pathogenesis

BACKGROUND

Plasmacytoid dendritic cells (pDCs) are a subset of dendritic cells specialized in the production of type I interferon (IFN-α/β) and involved in various cutaneous inflammatory and autoimmune disorders, such as cutaneous lupus erythematosus (CLE) and vitiligo. Heat shock proteins (HSPs) are molecular chaperones essential for maintaining cellular functions, but they can act as a danger signal during inflammation.

OBJECTIVES

To decipher the role of HSP70 in the production of IFN-α by pDCs in CLE and vitiligo.

METHODS

Expression of HSP70 and CD123⁺ pDCs was analysed by immunohistochemistry or immunofluorescence in CLE and vitiligo skin samples. Flow cytometry was performed to analyse expression of HSP70 receptors, activation markers on pDCs and DNA uptake by pDCs in the presence of HSP70. The impact of HSP70 on DNA-induced IFN-α secretion by pDCs was evaluated by enzyme-linked immunosorbent assay (ELISA). The effect of IFN-α on chemokine (C-X-C motif) ligand 9 (CXCL9)/10 gene and protein expression by keratinocytes was determined by real-time polymerase chain reaction and ELISA.

RESULTS

Infiltration of pDCs in CLE and progressive vitiligo was primarily located in the epidermis, close to keratinocytes expressing HSP70. In vitro experiments revealed that the pDCs expressing HSP70 receptor Lox-1 (lectin-like oxidized low-density lipoprotein-receptor-1) were able to aggregate HSP70. Exogenous HSP70 induced activation of pDCs and increased the uptake of exogenous DNA. Furthermore, HSP70 potentiated DNA-induced IFN-α production by pDCs. Finally, IFN-α induced expression of CXCL9 and CXCL10 by keratinocytes.

CONCLUSIONS

These data demonstrate that interaction between HSP70 and pDCs in CLE and vitiligo is a prerequisite for the enhancement of IFN-α production, and could be an interesting target.

Inactivation of DNase1L2 and DNase2 in keratinocytes suppresses DNA degradation during epidermal cornification and results in constitutive parakeratosis

The stratum corneum of the epidermis constitutes the mammalian skin barrier to the environment. It is formed by cornification of keratinocytes, a process which involves the removal of nuclear DNA. Here, we investigated the mechanism of cornification-associated DNA degradation by generating mouse models deficient of candidate DNA-degrading enzymes and characterizing their epidermal phenotypes. In contrast to Dnase1l2^−/− mice and keratinocyte-specific DNase2 knockout mice (Dnase2^Δep), Dnase1l2^−/−Dnase2^Δep mice aberrantly retained nuclear DNA in the stratum corneum, a phenomenon commonly referred to as parakeratosis. The DNA within DNase1L2/DNase2-deficient corneocytes was partially degraded in a DNase1-independent manner. Isolation of corneocytes, i.e. the cornified cell components of the stratum corneum, and labelling of DNA demonstrated that corneocytes of Dnase1l2^−/−Dnase2^Δep mice contained DNA in a nucleus-shaped compartment that also contained nucleosomal histones but lacked the nuclear intermediate filament protein lamin A/C. Parakeratosis was not associated with altered corneocyte resistance to mechanical stress, changes in transepidermal water loss, or inflammatory infiltrates in Dnase1l2^−/−Dnase2^Δep mice. The results of this study suggest that cornification of epidermal keratinocytes depends on the cooperation of DNase1L2 and DNase2 and indicate that parakeratosis per se does not suffice to cause skin pathologies.

Cytosolic RNA:DNA Duplexes Generated by Endogenous Reverse Transcriptase Activity as Autonomous Inducers of Skin Inflammation in Psoriasis

Psoriasis is a chronic skin disease of unknown ætiology. Recent studies suggested that a large amount of cytosolic DNA (cyDNA) in keratinocytes is breaking keratinocytes DNA tolerance and promotes self-sustained inflammation in the psoriatic lesion. We investigated the origin of this cyDNA. We show that, amongst all the possible DNA structures, the cyDNA could be present as RNA:DNA duplexes in keratinocytes. We further show that endogenous reverse transcriptase activities generate such duplexes and consequently activate the production of Th1-inflammatory cytokines. These observations open a new research avenue related to endogenous retroelements for the aetiology of psoriasis and probably of other human chronic inflammatory diseases.