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Lichen planus is a chronic disease affecting the skin, appendages, and mucous membranes. A cutaneous lichen planus is a rare disease occurring in less than 1% of the general population, while oral illness is up to five times more prevalent; still, both forms equally impair the patient’s quality of life. The etiology of lichen planus is not entirely understood. Yet, immune-mediated mechanisms have been recognized since environmental factors such as hepatitis virus infection, mechanical trauma, psychological stress, or microbiome changes can trigger the disease in genetically susceptible individuals. According to current understanding, lichen planus immunopathogenesis is caused by cell-mediated cytotoxicity, particularly cytotoxic T lymphocytes, whose activity is further influenced by Th1 and IL-23/Th-17 axis. However, other immunocytes and inflammatory pathways complement these mechanisms. This paper presents a comprehensive insight into the actual knowledge about lichen planus, with the causal genetic and environmental factors being discussed, the immunopathogenesis described, and the principal effectors of its inflammatory circuits identified.

Lichen planus is a chronic inflammatory mucocutaneous disorder commonly affecting the skin, nails, hair, and mucous membranes (1). Although its main cause is still unclear, lichen planus is defined as a T cell- mediated inflammatory disease (2). CD8+ T lymphocytes activate their cytotoxic armamentarium and induce basal keratinocyte apoptosis (2). The susceptibility of basal keratinocytes to CD8+ T lymphocytes is enhanced through MHC class I expression on keratinocytes triggered by interferon (IFN)- gamma (3). The cytotoxic process is mainly mediated by the release of lytic molecules, such as perforin, granzyme B, and granulysin (GNLY) (4–8). The upregulation of perforin and granzyme B has been previously demonstrated in the peripheral blood and lesional skin of patients with lichen planus (4–8). GNLY is a member of the saposin-like lipid-binding protein (SAPLIP) family and has a cytotoxic effect on tumour cells and a variety of microorganisms (9, 10). It is located in the cytotoxic granules of T cells, which are released upon antigen stimulation, and in the cytotoxic granules of natural killer (NK) cells. GNLY presence has only recently been observed in lichen planus lesions, but the expression level of this molecule in peripheral blood and subsequent determination of cells that express this molecule in blood and lesions remains unknown (8). In this study we examined the dynamics of GNLY expression in different cytotoxic subtypes of peripheral blood lymphocytes and in the skin lesions of patients with lichen planus.

Psoriasis is a recurrent, chronic, immune-mediated, systemic inflammatory disease of the skin, joints, and other organic systems. After atopic dermatitis, chronic stationary psoriasis is the most common inflammatory skin disease, affecting an average of 2–4% of the world’s population. The disease carries a significant burden due to its numerous comorbidities and the major impact on patients’ social and emotional aspects of life. According to current knowledge, psoriasis is a multifactorial disease that occurs in genetically predisposed individuals under various environmental factors, which trigger an immune response disorder with a series of complex inflammatory cascades. The disease is initiated and maintained by mutual interaction of the innate and adaptive immune cells, primarily dendritic cells, T lymphocytes, and keratinocytes, whose leading role alternates at different stages of the disease, consisting mainly in the IL-23/Th17 pathway. Inflammatory events result in consequent epidermal and dermal changes and evolution of the characteristic psoriatic phenotype, respectively. This paper aims to present a comprehensive overview of current knowledge on psoriasis genetic and environmental etiological factors, immunopathogenesis, and the leading cellular and cytokine participants in the inflammatory pathways of this disease.

The important role of CD8(+) T-cells in the pathogenesis of psoriasis is well-determined. However, besides type 1 cytokines that were formerly known, it was recently found that these cells secrete type 17 and type 22 cytokines. The majority of IL-17A(+)CD8(+) T- cells in the blood belong to a subset of innate T-cells named mucosa-associated invariant T- cells (MAIT). However, the majority of IL- 17A(+)CD8(+) T-cells in psoriatic epidermis are conventional T-cells and are up-regulated in psoriasis. In contrast to Th17 cells that secrete only IL-17, Tc17 cells secrete IFN- gamma, TNF-alpha, CCL20, IL-22, and granzyme B as well. The key cytokine is IL-17A, which promotes keratinocyte hyperproliferation and stimulates them to produce other proinflammatory cytokines. These activities initiate and propagate the inflammation and architectural changes in the skin that clinically manifest as psoriatic lesions. However, a relatively novel cell subtype named Tc22 has been discovered in psoriasis that could secrete IL-22 in the absence of IL-17 and IFN-gamma. IL-22 stimulates proliferation and de-differentiation of keratinocytes, subsequently leading to epidermal acanthosis. As the understanding of the pathogenesis of psoriasis increases, the new selective therapies may offer an optimal balance between increased clinical benefit and reduced risk of side-effects.

Psoriasis is considered to be a cytokine-driven immune-mediated disease, although the cell cytotoxicity mechanisms involved remain unrecognized. Herein, we analyzed granulysin expression in different lymphocyte subsets of peripheral blood of 40 psoriatic patients (20 with severe and 20 with mild psoriasis) and seven sample of psoriatic skin. The simultaneous detection of intracellular granulysin and cell surface antigens was performed using flow cytometry in peripheral blood and immunohistochemistry in skin lesions. The frequency of granulysin+ cells, mean fluorescence intensity for granulysin, and the frequency of CD8+ T lymphocytes, NK cells, and NKT cells expressing granulysin molecules in peripheral blood were significantly higher in patients with severe psoriasis compared to mild disease and healthy individuals. These were also correlated with disease severity. Furthermore, granulysin+ cells, CD8+granulysin+ T lymphocytes, and CD56+granulysin+ NK cells were present in a higher frequency in the epidermal basal cell layer and in the dermal infiltrate of lesional skin as compared to non-lesional and healthy skin. In conclusion, granulysin+ cytotoxic cells are upregulated in blood and lesions of patients with psoriasis suggesting the involvement of granulysin mediated cytotoxicity in psoriasis pathogenesis.

Lichen planus is a chronic mucocutanous disorder histopathologically characterized with a keratinocytes apoptosis, subsequent basal cell layer liquefaction and accumulation of the inflammatory infiltrate in papillary dermis. A formation of apoptotic bodies in basal cell layer is due to a cytotoxic lymphocyte attack to the basal keratinocytes. It has been demonstrated that the cytotoxic molecules included in this attack are perforin and granzyme B. Both molecules are found upregulated in CD8+ lymphocytes that are in close contact to keratinocytes. However, their amount is lower in lichen planus than in other skin disease characterized by liquefaction and vacuolar degeneration of the basal epidermal layer. This could speculate about other cytotoxic molecule such as granulysin that could mediate keratinocyte apoptosis. Therefore, in this article we hypothesize about the crucial role of granulysin molecule in keratinocytes killing that could contribute to a lichen planus pathogenesis.

Psoriasis is a chronic papulosquamous skin disease, histologically characterized by epidermal hyperproliferation and dermal infiltration of inflammatory cells. The majority of T lymphocytes infiltrating dermis are CD4+ T lymphocytes secreting type 1 and type 17 cytokines. These cytokines are responsible for triggering keratinocyte proliferation as well as chemokine secretion and subsequent migration of other inflammatory cells in the skin. Contrarily, lymphocytes that accumulate in epidermis are mainly CD8+ T lymphocytes. According to the recent findings, these cells can also secrete type 1 and type 17 cytokines. However, it is demonstrated so far that epidermal CD8+ T lymphocytes contain higher amounts of cytolytic molecules, such as perforin, granzyme B and granulysin whose role in psoriasis pathogenesis is still unknown. Therefore, in this article we hypothesize the active involvement of cell mediated cytotoxicity in killing the proliferating keratinocytes as a mechanism of potential self-defense and possible brake in psoriatic plaque formation, maintaining skin homeostasis.