**3. Antigen-specific cell-mediated immune response**

The antigen that serves as the trigger and/or driver of the immune responses seen in OLP is unknown. It is likely, in the majority of cases, to be an endogenous peptide, a protein sequence innate to the basal keratinocyte; therefore, OLP can be characterised as an auto-immune condition. It is also likely that supposed exogenous triggers for OLP, such as dental materials, certain drugs, viruses and even trauma serve to expose such self-antigens, or, alter the normal innate peptide sequences so that they are perceived by the immune-surveillance cells and system as being "non-self, that is "foreign". The immune responses to this, as yet, unidentified antigen develops in three stages: (1) T-cell migration into the epithelium, (2) T-cell activation, followed by (3) induction of basal keratinocyte apoptosis. [12]

**T-Cell Migration into the Epithelium:** Two hypotheses have been proposed to explain this occurrence. The "chance encounter" hypothesis suggests that normally circulating, antigenspecific CD8+ cytotoxic T-cells enter the epithelium for routine surveillance and by chance encounter the putative antigen when it is present in the epithelium. Alternatively, the kerati‐ nocytes direct the CD8+ cytotoxic T-cells to migrate into the epithelium by the release of cytokines that allow the lymphocytes to "home-in" on the antigen-bearing basal keratinocyte, the so called "directed migration" hypothesis. [12, 13]

CD95L) is a type-II transmembrane protein that belongs to the TNF family that on binding with its receptor induces apoptosis in the target cell. [17] Fas-induced apoptosis by the perforin pathway are the two main mechanisms by which cytotoxic T lymphocytes induce cell death in cells expressing foreign antigens. Thirdly, by the infusion of granzyme B by T cells into the keratinocytes. Granzymes are serine proteases that are released from cytoplasmic granules within cytotoxic T cells (and natural killer cells) and whose usual role is to induce apoptosis within virus-infected cells, thus destroying them. [18] Cytotoxic T-cells release a protein called perforin, which attacks the target cells forming multimeric complex (of granzyme B, perforin, and granulysin) that enters cells through the mannose 6-phosphate receptor. [19] Granzyme B is then released, to cause apoptosis by various pathways, including the cleaving of caspases (especially caspase-3), which in turn activates caspase-activated DNase and this enzyme

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degrades DNA, so inducing the apoptotic cascade culminating in cell death. [18, 19]

**of autoimmunity**

of "immune-privilege" in OLP. [14, 20]

function of regulatory T cells. [21, 22]

tion of IFN-γ by Th1 CD4+

chronicity of OLP.

**4. Development of autoantibodies against self-antigens and the promotion**

The autoimmune nature of OLP is evidenced by the chronic and protracted course of the disease, its later age of onset, its higher prevalence in women, its association with other autoimmune diseases, proven demonstration of the T-cell auto-reactivity and increased auto-cytoxicity against basal keratinocytes, its clinical, histological and immunological similarity to graft-versus host disease (GVHD) and the effectiveness of immunosuppres‐ sive therapies. Two theories have been advanced to explain the autoimmune nature of OLP, specifically the loss of self-tolerance of the basal keratinocytes: (1) impaired immunesuppression in OLP due to lack of Transforming Growth Factor-β1 (TGF-β1); and (2) loss

**Impaired Immune-Suppression in OLP (deficient TGF-β1):**TGF-β is believed to be important regulator of the immune system by inducing and increasing the differentiation of both Foxp3+ regulatory T cells (Tregs) (and Th17 cells). FOXP3 (forkhead box P3) also known as scurfin, is a protein involved in immune system responses. A member of the FOX protein family, FOXP3 appears to function as a master regulator (transcription factor) in the development and

TGF-β also appears to block the activation of lymphocytes and monocyte derived phagocytes. TGF-β1 levels and/or activity may be deficient for some of the following reasons: insufficient numbers of TGF-β1-secreting Th3 regulatory T cells; blockage of TGF-β1secretion; secretion of defective, non-functional TGF-β1; defective or inadequate TGF-β1receptor expression; or defective intracellular downstream signalling from the TGF-β1 receptors. Local overproduc‐

effect of TGF-β1and so up-regulate keratinocyte MHC class II expression and CD8+ cytotoxic T cell activity explaining the immune responses to antigens including self antigens and the

T cells in OLP lesions would down-regulate the immunosuppressive

**T-cell Activation:** The lymphocytic infiltration that characterises the OLP lesion histologically, is comprised predominantly of T-cells. The majority of the T-cells in proximity to the damaged and dying basal keratinocytes and within the epithelial layers are overwhelmingly activated cytotoxic CD8+ T-cells. [14] Cytotoxic CD8+ T-cells binding of antigen on the MHC Class I site of keratinocytes releases cytokines that attract other lymphocytes and immune-cells into the site of the developing OLP lesion. The cytotoxic CD8+ T-cells are also activated by the CD4+ helper cells found in the lamina propria. In OLP lesions, helper CD4+ T cells may be activated by antigen associated with Class II MHC presented by the professional antigen-presenting cells, the Langerhans cells, or, by the keratinocytes themselves, which are induced to present antigens on their Class II MHC sites. Langerhans cells are not only increased in number in OLP lesions but also have up-regulated Class II MHC expression. [14] Interleukin-12 (IL-12) is secreted by Class II MHC expressing Langerhans cells and keratinocytes which in turn promotes CD4+ T-cell secretion of interleukin-2 (IL-2) and interferon-γ (IFN- γ). These cytokines (IL-12, IL-2 and IFN- γ) and probably others, together with the presentation of an antigen associated with MHC class I on basal keratinocytes, promote cytotoxic CD8+ T-cell induction of keratinocytes apoptosis. So it would appear that within the OLP lesion there is a cycle of self-inducing and self-perpetuating T-lymphocyte activation.

**Basal Keratinocyte Apoptosis:** The apoptosis of the basal keratinocytes that characterises all forms of lichen planus, appears to be mediated predominantly by particularly active, cytotoxic CD8+ T-cells. [14] In one study of testing the reactivity of lesional versus non-lesional T-cell clones from LP patients against lesional and non-lesional autologous keratinocytes, the lesional T-cells lines derived from patients with LP were significantly more active and cytotoxic against autologous lesional keratinocytes than the T-cell lines obtained from clinically normal skin. [14] In this same study, it was also shown that the most cytotoxic T-cell clones were CD8+ and the least cytotoxic clones, were CD4+ . [15] However, in this same study, the cytotoxicity of some of these activated CD8+ T-cell clones was shown to be partially inhibited by anti-MHC class I monoclonal antibody (Dako clone W6/32). This antibody targets a monomorphic epitope on the 45 kDa polypeptide products of the HLA-A, B and C loci. [16] These findings indicated that the apoptosis of the basal keratinocytes so characteristic of LP (and OLP) is induced by the cytotoxic CD8+ T-lymphocytes activated by a putative basal keratinocyte antigen associ‐ ated with the MHC Class I.

The induction of keratinocytes apoptosis by CD8+ T-cells can occur by three established pathways: Firstly, secretion by T-cells of tumour necrosis factor-α (TNF-α), which binds TNFα receptor 1 on the keratinocytes surface [16] ; secondly, expression on the T-cell surface of CD95L (Fas ligand), which binds CD95 (Fas) on the keratinocytes surface. Fas ligand (FasL or CD95L) is a type-II transmembrane protein that belongs to the TNF family that on binding with its receptor induces apoptosis in the target cell. [17] Fas-induced apoptosis by the perforin pathway are the two main mechanisms by which cytotoxic T lymphocytes induce cell death in cells expressing foreign antigens. Thirdly, by the infusion of granzyme B by T cells into the keratinocytes. Granzymes are serine proteases that are released from cytoplasmic granules within cytotoxic T cells (and natural killer cells) and whose usual role is to induce apoptosis within virus-infected cells, thus destroying them. [18] Cytotoxic T-cells release a protein called perforin, which attacks the target cells forming multimeric complex (of granzyme B, perforin, and granulysin) that enters cells through the mannose 6-phosphate receptor. [19] Granzyme B is then released, to cause apoptosis by various pathways, including the cleaving of caspases (especially caspase-3), which in turn activates caspase-activated DNase and this enzyme degrades DNA, so inducing the apoptotic cascade culminating in cell death. [18, 19]

**T-Cell Migration into the Epithelium:** Two hypotheses have been proposed to explain this occurrence. The "chance encounter" hypothesis suggests that normally circulating, antigenspecific CD8+ cytotoxic T-cells enter the epithelium for routine surveillance and by chance encounter the putative antigen when it is present in the epithelium. Alternatively, the kerati‐ nocytes direct the CD8+ cytotoxic T-cells to migrate into the epithelium by the release of cytokines that allow the lymphocytes to "home-in" on the antigen-bearing basal keratinocyte,

**T-cell Activation:** The lymphocytic infiltration that characterises the OLP lesion histologically, is comprised predominantly of T-cells. The majority of the T-cells in proximity to the damaged and dying basal keratinocytes and within the epithelial layers are overwhelmingly activated

of keratinocytes releases cytokines that attract other lymphocytes and immune-cells into the site of the developing OLP lesion. The cytotoxic CD8+ T-cells are also activated by the CD4+ helper cells found in the lamina propria. In OLP lesions, helper CD4+ T cells may be activated by antigen associated with Class II MHC presented by the professional antigen-presenting cells, the Langerhans cells, or, by the keratinocytes themselves, which are induced to present antigens on their Class II MHC sites. Langerhans cells are not only increased in number in OLP lesions but also have up-regulated Class II MHC expression. [14] Interleukin-12 (IL-12) is secreted by Class II MHC expressing Langerhans cells and keratinocytes which in turn promotes CD4+ T-cell secretion of interleukin-2 (IL-2) and interferon-γ (IFN- γ). These cytokines (IL-12, IL-2 and IFN- γ) and probably others, together with the presentation of an antigen associated with MHC class I on basal keratinocytes, promote cytotoxic CD8+ T-cell induction of keratinocytes apoptosis. So it would appear that within the OLP lesion there is a

**Basal Keratinocyte Apoptosis:** The apoptosis of the basal keratinocytes that characterises all forms of lichen planus, appears to be mediated predominantly by particularly active, cytotoxic CD8+ T-cells. [14] In one study of testing the reactivity of lesional versus non-lesional T-cell clones from LP patients against lesional and non-lesional autologous keratinocytes, the lesional T-cells lines derived from patients with LP were significantly more active and cytotoxic against autologous lesional keratinocytes than the T-cell lines obtained from clinically normal skin. [14] In this same study, it was also shown that the most cytotoxic T-cell clones were CD8+

monoclonal antibody (Dako clone W6/32). This antibody targets a monomorphic epitope on the 45 kDa polypeptide products of the HLA-A, B and C loci. [16] These findings indicated that the apoptosis of the basal keratinocytes so characteristic of LP (and OLP) is induced by the cytotoxic CD8+ T-lymphocytes activated by a putative basal keratinocyte antigen associ‐

The induction of keratinocytes apoptosis by CD8+ T-cells can occur by three established pathways: Firstly, secretion by T-cells of tumour necrosis factor-α (TNF-α), which binds TNFα receptor 1 on the keratinocytes surface [16] ; secondly, expression on the T-cell surface of CD95L (Fas ligand), which binds CD95 (Fas) on the keratinocytes surface. Fas ligand (FasL or

. [15] However, in this same study, the cytotoxicity of some

T-cell clones was shown to be partially inhibited by anti-MHC class I

and

T-cells. [14] Cytotoxic CD8+ T-cells binding of antigen on the MHC Class I site

the so called "directed migration" hypothesis. [12, 13]

cycle of self-inducing and self-perpetuating T-lymphocyte activation.

the least cytotoxic clones, were CD4+

of these activated CD8+

ated with the MHC Class I.

cytotoxic CD8+

152 Skin Biopsy - Diagnosis and Treatment
