Objective. Autoantibodies to citrullinated proteins (ACPAs) are specific for rheumatoid arthritis (RA) and probably are involved in its pathophysiology. Citrullyl residues, posttranslationally generated by peptidyl arginine deiminase (PAD), are indispensable components of ACPA-targeted epitopes. The aim of this study was to identify which PAD isotypes are expressed in the synovial tissue (ST) of patients with RA and are involved in the citrullination of fibrin, the major synovial target of ACPAs.Methods. Expression of all PAD isotypes, including the recently described PAD type 6 (PAD-6), was explored by reverse transcription-polymerase chain reaction and immunoblotting, first in blood-derived mononuclear leukocytes from healthy donors, then in ST samples from 16 patients with RA and 11 control patients (4 with other arthritides and 7 with osteoarthritis [OA]). In ST samples from patients with RA, PADs were localized by immunohistochemistry.Results. In lymphocytic and monocytic cells and, similarly, in ST samples from patients with RA, the PAD-2, PAD-4, and PAD-6 genes were found to be transcribed, but only PAD-2 and PAD-4 enzymes were detected. PAD-2 was also expressed in ST from control patients, including those with OA, while PAD-4 was preferentially expressed in ST from patients with other arthritides. In RA, the expression levels of PAD-2 and PAD-4 were correlated with the intensity of inflammation (cell infiltration, hypervascularization, and synovial lining hyperplasia), and both enzymes were demonstrable within or in the vicinity of citrullinated fibrin deposits.Conclusion. PAD-2 and PAD-4 are the only PAD isotypes expressed in the ST of patients with RA and those with other arthritides. Inflammatory cells are a major source, but PAD-4 also comes from hyperplastic synoviocytes. Both isotypes are probably involved in the citrullination of fibrin.

No abstract

Atopic dermatitis is a chronic inflammatory skin disorder characterized by defects in the epidermal barrier and keratinocyte differentiation. The expression of filaggrin, a protein thought to have a major role in the function of the epidermis, is downregulated. However, the impact of this deficiency on keratinocytes is not really known. This was investigated using lentivirus-mediated small-hairpin RNA interference in a three-dimensional reconstructed human epidermis (RHE) model, in the absence of other cell types than keratinocytes. Similar to what is known for atopic skin, the experimental filaggrin downregulation resulted in hypogranulosis, a disturbed corneocyte intracellular matrix, reduced amounts of natural moisturizing factor components, increased permeability and UV-B sensitivity of the RHE, and impaired keratinocyte differentiation at the messenger RNA and protein levels. In particular, the amounts of two filaggrin-related proteins and one protease involved in the degradation of filaggrin, bleomycin hydrolase, were lower. In addition, caspase-14 activation was reduced. These results demonstrate the importance of filaggrin for the stratum corneum properties/functions. They indicate that filaggrin downregulation in the epidermis of atopic patients, either acquired or innate, may be directly responsible for some of the disease-related alterations in the epidermal differentiation program and epidermal barrier function.