Abstract

Rheumatoid arthritis (RA) is an autoimmune disease characterized by dysregulated and chronic systemic inflammatory responses that affect the synovium, bone, and cartilage causing damage to extra-articular tissue. Innate immunity is the first line of defense against invading pathogens and assists in the initiation of adaptive immune responses. Polymorphonuclear cells (PMNs), which include neutrophils, are the largest population of white blood cells in peripheral blood and functionally produce their inflammatory effect through phagocytosis, cytokine production and natural killer-like cytotoxic activity. TREM1 (triggering receptor expressed by myeloid cells) is an inflammatory receptor in PMNs that signals through the use of the intracellular activating adaptor DAP12 to induce downstream signaling. After TREM crosslinking, DAP12’s tyrosines in its ITAM motif get phosphorylated inducing the recruitment of Syk tyrosine kinases and eventual activation of PI3 kinases and ERK signaling pathways. While both TREM1 and DAP12 have been shown to be important activators of RA pathogenesis, their activity in PMNs or the importance of DAP12 as a possible therapeutic target have not been shown. Here we corroborate, using primary RA specimens, that isolated PMNs have an increased proportion of both TREM1 and DAP12 compared to normal healthy control isolated PMNs both at the protein and gene expression levels. This increased expression is highly functional with increased activation of ERK and MAPKs, secretion of IL-8 and RANTES and cytotoxicity of target cells. Importantly, based on our hypothesis of an imbalance of activating and inhibitory signaling in the pathogenesis of RA we demonstrate that inhibition of the DAP12 signaling pathway inactivates these important inflammatory cells.

Highlights

  • Rheumatoid arthritis (RA) is a relatively common autoimmune disease affecting over two million people in US and characterized by dysregulalted and chronic systemic inflammatory responses that affect the synovium, bone, and cartilage causing damage to extra-articular tissue [1, 2]

  • We started by examining the expression of triggering receptor expressed by myeloid cells (TREM)-1 in the polymorphonuclear leukocytes (PMNs) of RA primary specimens, after isolating granulocytes by Ficoll gradient centrifugation and using CD66 as a specific flow cytometric marker for these cells (Fig. 1A)

  • While this increase was matched with an increase in TREM1 gene expression from the sorted PMNs of RA patients by real-time RT

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Summary

Introduction

Rheumatoid arthritis (RA) is a relatively common autoimmune disease affecting over two million people in US and characterized by dysregulalted and chronic systemic inflammatory responses that affect the synovium, bone, and cartilage causing damage to extra-articular tissue [1, 2]. The last group, neutrophils, are major players in innate immunity and the first line of defense against invading pathogens by assisting in the initiation of adaptive immune responses (Reviewed in [4]). Polymorphonuclear leukocytes (PMNs) due to the appearance of multiple nuclei, they comprise the largest population of white blood cells in the peripheral blood as it accounts for about 40–70% of the total white blood cell population during inflammatory stimulation [4, 5]. They perform their main innate immune function through phagocytosis, cytokine production and their NK-like activity

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