Functional Role of Dimerization of Human Peptidylarginine Deiminase 4 (PAD4)

Peptidylarginine deiminase 4 (PAD4) is a homodimeric enzyme that catalyzes Ca2+-dependent protein citrullination, which results in the conversion of arginine to citrulline. This paper demonstrates the functional role of dimerization in the regulation of PAD4 activity. To address this question, we created a series of dimer interface mutants of PAD4. The residues Arg8, Tyr237, Asp273, Glu281, Tyr435, Arg544 and Asp547, which are located at the dimer interface, were mutated to disturb the dimer organization of PAD4. Sedimentation velocity experiments were performed to investigate the changes in the quaternary structures and the dissociation constants (Kd) between wild-type and mutant PAD4 monomers and dimers. The kinetic data indicated that disrupting the dimer interface of the enzyme decreases its enzymatic activity and calcium-binding cooperativity. The Kd values of some PAD4 mutants were much higher than that of the wild-type (WT) protein (0.45 µM) and were concomitant with lower kcat values than that of WT (13.4 s−1). The Kd values of the monomeric PAD4 mutants ranged from 16.8 to 45.6 µM, and the kcat values of the monomeric mutants ranged from 3.3 to 7.3 s−1. The kcat values of these interface mutants decreased as the Kd values increased, which suggests that the dissociation of dimers to monomers considerably influences the activity of the enzyme. Although dissociation of the enzyme reduces the activity of the enzyme, monomeric PAD4 is still active but does not display cooperative calcium binding. The ionic interaction between Arg8 and Asp547 and the Tyr435-mediated hydrophobic interaction are determinants of PAD4 dimer formation

Expression of Lectin-Like Transcript 1, the Ligand for CD161, in Rheumatoid Arthritis

Precursor Th17 lineage cells expressing CD161 are implicated in Rheumatoid Arthritis (RA) pathogenesis. CD4+CD161+ T-cells accumulate in RA joints and may acquire a non classical Th1 phenotype. The endogenous ligand for CD161 is lectin-like transcript 1 (LLT1). CD161/LLT1 ligation may co-stimulate T-cell IFN-γ production. We investigated the presence and identity of LLT1-expressing cells in RA synovial fluid (SF) and synovial tissue (ST). We also assessed levels of soluble LLT1 (sLLT1) in different phases of RA development.Paired samples of peripheral blood mononuclear cells (MC) and SFMC (n = 14), digested ST cells (n = 4) and ST paraffin sections (n = 6) from late-stage RA were analyzed for LLT1 expression by flow cytometry and immunohistochemistry. sLLT1 was measured using a sandwich ELISA. Sera and SF from late-stage RA (n = 26), recently diagnosed RA patients (n = 39), seropositive arthralgia patients (SAP, n = 31), spondyloarthropathy patients (SpA, n = 26) and healthy controls (HC, n = 31) were assayed.In RA SF, LLT1 was expressed by a small proportion of monocytes. In RA ST, LLT1-expressing cells were detected in the lining, sublining layer and in areas with infiltrates. The LLT1 staining pattern overlapped with the CD68 staining pattern. FACS analysis of digested ST confirmed LLT1 expression by CD68+ cells. Elevated systemic sLLT1 was found in all patient groups.In RA joints, LLT1 is expressed by cells of the monocyte/macrophage lineage. Serum levels of sLLT1 were increased in all patient groups (patients with early- and late-stage RA, seropositive arthralgia and spondyloarthropathy) when compared to healthy subjects

Tumor necrosis factor-α blockade treatment decreased CD154 (CD40-ligand) expression in rheumatoid arthritis

CD154 (commonly referred to as CD40-ligand) is a critical T cell factor that participates in the pathogenesis of autoimmune and is over-expressed in rheumatoid arthritis (RA). TNF-α blockade treatment had dramatic efficacy in RA.To investigate whether TNF-α blockade treatment can inhibit CD154 expression in RA.Blood samples were collected from 33 patients with rheumatoid arthritis before and 3 months after TNF-α blockade treatment. Clinical serological data determined by standard assays and T cell CD154 expression levels determined by flow cytometry were statistically analyzed for these two time points.The percentage of CD154 expression on gated CD4+ T cells of PBMCs from RA patients after 3 months TNF-α blockade treatment was significantly lower than before treatment (2.94 ± 3.21% vs. 7.21 ± 5.64%; p = 0.0001). The disease activity and anti-CCP antibody levels were also significantly reduced after TNF-α blockade treatment. The CD154 expression levels were positively correlated with disease activity index DAS28, and CRP. The post-stimulated CD154 expression percentage of purified CD4+ T cells between baseline and after TNF-α blockade treatment was not significantly different (p = 0.221). Baseline CD154 levels were positively correlated with treatment-induced changes in DAS28 (p = 0.014; r2 = 0.187).TNF-α blockade treatment significantly decreased the CD154 expression on CD4+ T cells, disease activity and anti-CCP antibody simultaneously in RA patients. However TNF-α blockade did not impair T cell capacity to express CD154 after stimulation. These results suggest that decreased CD154 expression after TNF-α blockade may be due to decreased RA disease activity but not direct inhibition of CD154 responsiveness of T cells