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PTPN22 is a protein tyrosine phosphatase expressed by the majority of cells belonging to the innate and adaptive immune systems. Polymorphisms in PTPN22 are associated with several autoimmune diseases, such as systemic lupus erythematosus, rheumatoid arthritis and type 1 diabetes. This review discusses the role of PTPN22 in T and B cells, and its function in innate immune cells, such as monocytes, dendritic cells and NK cells. We focus particularly on the complexity that underlies the function of PTPN22 in the biological processes of the immune system; such complexity has led various research groups to produce rather conflicting data., (© 2013 Elsevier Inc. All rights reserved.)

To dissect the mechanisms of anti-TNFα-induced autoimmunity we examined the phenotype and function of B cells from anti-TNFα-treated patients. Levels of Lyn, Syk, SHP-1, tyrosine 348 phospho-Syk (Y348-Syk) and tyrosine phosphorylated (P-Y) proteins were evaluated and B-cell-surface CD20, CD21 and CD5 were also assessed in 29 patients treated with TNF-α blockers. Following treatment, Lyn, but not Syk or SHP-1, significantly increased particularly in patients with spondyloarthropathies. Increased Lyn levels following treatment correlated with increased Lyn activity as evidenced by a 2.9-fold increase of Y348-Syk (a Lyn target). Peripheral B-cells from 56.3% of the patients displayed a tendency towards increased P-Y levels without any BCR-initiated activation during treatment. CD20, but not CD21, significantly increased in patients with rheumatoid arthritis. Circulating CD5+ B-cells were also significantly expanded during treatment. Our findings suggest that B cells in anti-TNFα-treated patients display functional and phenotypical aberrations that may enhance our understanding of TNF-α blocker-induced autoimmunity., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Peripheral B cell depletion strategies have been employed recently in the treatment of systemic autoimmune diseases and the initial clinical results have been encouraging. Although the major target of rituximab-based treatments was to reduce the levels of circulating autoantibodies, additional mechanisms of action may operate. Recent studies have addressed the question of potential effects of transient B cell depletion on other, non-B cell populations. The data, albeit uncontrolled, suggest that anti-CD20 monoclonal antibody treatment is associated with significant effects in the T cell pool, whereas individual clinical responses do not always correlate with changes in autoantibody titers. More specifically, it has been reported that rituximab administration may decrease the activated phenotype of peripheral and tissue-resident T cells by abolishing antigen presentation by B cells, and may enhance the numbers and function of regulatory T cells. In this review we analyze and discuss available data emerging from B cell depletion studies in patients with systemic lupus erythematosus, rheumatoid arthritis and other autoimmune conditions. Further controlled studies are needed to confirm the role of B cell depletion in modifying T cell function in vivo.

CD40 ligand (CD40L, CD154) is overexpressed on T and B cells in systemic lupus erythematosus (SLE). Monocytes have been shown to contribute to immune-mediated pathology in SLE and to express CD40L under certain conditions. Therefore, we studied CD40L expression on lupus monocytes ex vivo, as well as after activation in vitro. A highly significant sevenfold increase in the frequency of CD40L-expressing peripheral monocytes from 23 SLE patients, compared to 16 healthy individuals (mean percentage of CD40L(+)CD14(+) among CD14(+) cells, 11.7 versus 1.6), was found by flow cytometry. Increased CD40L expression on monocytes correlated significantly with disease activity, elevated gamma-globulin serum levels, as well as increased CD40L expression on T cells. CD40L expression by lupus monocytes was verified at both the mRNA and protein levels, while LPS stimulation was found to upregulate CD40L mRNA accumulation and surface protein expression. CD40L expression on activated lupus monocytes within anti-CD3-stimulated, mononuclear cell cultures was also enhanced compared to control-derived monocytes. These novel findings underscore the multiplicity of pathways through which monocytes may contribute to SLE pathology and suggest that T cell-independent CD40L-mediated cell to cell interactions may be also involved in humoral immune activation in SLE.