**4. Pathogenesis**

Pemphigus vulgaris is an autoimmune condition that is more likely to develop in patients with certain HLA types after triggers. Central to the pathogenesis of pemphigus is the presence of immunoglobulin (Ig) antibodies against proteins on the cell surface of keratinocytes. Immunochemical and molecular cloning studies have shown that antigenic targets in PV are desmogleins. Desmogleins (Dsg) are desmosomeassociated transmembrane glycoproteins that provide cell-cell adhesion in the epidermis [3]. The main antigen in PV is Dsg3 (130 kDa), but 50% of patients also have autoantibodies against Dsg1 (160 kDa). The ratio of Dsg1 and Dsg3 antibodies

appear to correlate with the clinical severity of PV; those with only Dsg3 antibodies have predominantly oral lesions. IgG antibodies against Dsgs impair the adhesive function of desmosomes and inhibit cell-cell adhesion. This results in epidermal acantholysis and drooping blister formation, which is the characteristic clinical feature of pemphigus diseases [3].

The localization of blister formation and involvement of mucosal surfaces varies with the pemphigus disease subtype and can be explained by the Dsg compensation theory. This theory states that on cutaneous surfaces, Dsg 1 is expressed in all layers of the epidermis, while Dsg 3 is expressed in deeper layers. Dsg 1 expression in the mucosa is minimal, while Dsg 3 is dominant. The interpretation of the Dsg compensation theory, as it relates to the clinical manifestations in pemphigus, can be summarized as follows: Only patients with antibodies to Dsg 3 should have mucosal dominant PV because Dsg 1 compensates for the loss of Dsg 3 in the skin. Mucous membranes predominantly contain Dsg 3; low levels of Dsg 1 in the mucosa cannot replace the lost Dsg 3 and lead to epithelial acantholysis and mucosal erosions. When antibodies against both Dsg 1 and 3 develop, epidermal acantholysis occurs in both the skin and mucous membranes. It is still a matter of debate whether the Dsg compensation theory can adequately explain the complex pathogenic mechanism of pemphigus [3, 12].

Although Dsgs are the main autoantigens targeted by autoantibodies in the vast majority of pemphigus patients, antibodies against other desmosomal cadherins (e.g., desmocollins), desmoplakin and acetylcholine receptors on keratinocytes have also been detected in patients with pemphigus vulgaris [13, 14]. In recent years, the blood of some PV patients has been found to show reactivity with Dsg4, which has similar properties to Dsg1 and Dsg3, but the pathogenic nature of anti-Dsg4 is unclear [14]. In the presence of both anti-Dsg 1 and anti-Dsg 3 antibodies in PV, the entire skin should have been completely lysed. However, such a clinic is not seen in any patient. Therefore, the separation of keratinocytes may also be due to a different subset of non-Dsg pemphigus antibodies that recognize and block keratinocyte acetylcholine receptors. In this light, the earliest sign of keratinocyte separation is maintained by autoantibodies to these acetylcholine receptors located on the keratinocyte membrane, while antiDsg antibodies will only then be activated, triggering cell-cell separation [8]. Pharmacological blockade of acetylcholine receptors with muscarinic or nicotinic antagonists, atropine and mecamylamine, respectively, resulted in pemphigus-like acantholysis in monolayers of human oral and epidermal keratinocytes in both conditions. These observations suggest that PV IgGs act antagonistically to acetylcholine receptors of keratinocytes, interrupting the stimulation of these receptors by acetylcholine, thereby, altering the normal pattern of keratinocyte adhesion via cholinergic signaling that can lead to acantholysis [15].

Caspases are involved in the pathogenesis of PV through apoptosis and acantholysis. Caspases are activated through various cellular pathways to affect programmed cell death. While apoptosis via caspases is a normal process in the body, the presence of PV-IgG causes pathological activation of caspases in keratinocytes. In all studies, inhibition of caspases has been shown to reduce apoptosis and acantholysis and thus have a positive effect on cell-cell adhesion [16].

The pathogenesis of pemphigus involves the production of activated B-cells and IgG with stimulation by IL-4 by T-helper 2 (Th2) cells. Excessive activation of Th2 cells causes the production of autoantibodies required for PV. Th2 cells secrete IL-4 and multiple interleukins (IL), which are known to play an important role in pemphigus. IL-4 promotes antibody production by primed B cells and an isotype switching

from IgG1 to IgG4 antibodies which are important in the active form of PV. IL-4 also causes naive CD4+ T cells to differentiate into Th2 cells, thus maintaining the disease. Production of autoantibodies and epitope binding are sufficient for the loss of adhesions directly between desmosomes [17].

Intracellular kinase signaling has been identified in the pathogenic process of PV. Intracellular kinase signaling has been identified in the pathogenic process of PV. The binding of autoantibodies in PV promotes phosphorylation of kinases. In the study in which patients with a diagnosis of PV were examined, kinases (e.g., phosphokinase C (PKC), p38 mitogen-activated protein kinase (p38MAPK), cyclin-dependent kinase (Cdk2), sarcoma-associated kinase (Src), extracellular signal-regulated kinase (ERK), Bruton tyrosine kinase (BTK), apoptosis signal regulatory kinase (ASK1), epidermal growth factor receptor kinase (EFGRK)), tyrosine kinase (TK)) were found to be phosphorylated in PV-induced models [16]. Inhibition of these pathways reduces acantholysis in vitro and in vivo. The role of p38MAPK has been highlighted in multiple studies. p38MAPK is activated after PVIgG-binding upstream of Rho kinase. In particular, p38MAPK inhibition prevents PV-IgG–induced redistribution of Dsg3 and results in PV-IgG and Dsg3 co-localizing at the cell membrane. As a result, p38MAPK inhibition has been shown to prevent both histologic and clinical blister formation in both in vivo and in vitro models. Therefore, p38MAPK plays an essential role in regulating Dsg3 internalization and must be considered as a key component in PV pathogenicity [16].

Proteases are enzymes that hydrolyze peptide bonds within proteins. Proteases have been blamed because the use of various protease inhibitors was found to be effective in inhibiting PV-associated acantholysis in studies [16]. Pemphigus antibodies (described as IgGp) may mediate acantholysis by the activation and release of non-lysosomal proteolytic enzymes from epidermal cells. Recent data indicate that the proteolytic enzyme is an activator of plasminogen. This has been demonstrated by the addition of a plasmin inhibitor such as aprotinin, which can inhibit the development of acantholysis when PV or PF IgG is added to the skin in organ cultures. Both plasminogen and IgG are required to cause acantholysis [15].

There are many mechanisms in the pathogenesis of PV that are still not understood and more studies are needed.
