U1 small nuclear ribonucleoprotein immune complexes induce type I interferon in plasmacytoid dendritic cells through TLR7

Abstract

Plasmacytoid dendritic cells (PDCs), which produce IFN-␣ in response to autoimmune complexes containing nuclear antigens, are thought to be critically involved in the pathogenesis of systemic lupus erythematosus (SLE). One of the immunostimulatory components of SLE immune complexes (SLE-ICs) is self DNA, which is recognized through Tlr9 in PDCs and B cells. Small nuclear ribonucleoproteins (snRNPs) are another major component of SLE-ICs in 30% to 40% of patients.In this study, we show that murine PDCs are activated by purified U1snRNP/ anti-Sm ICs to produce IFN-␣ and proinflammatory cytokines and to up-regulate costimulatory molecules. The induction of IFN-␣ and IL-6 by U1snRNPs in murine bone marrow-derived PDCs required the presence of intact U1RNA and was largely dependent on Tlr7 but independent of Tlr3. Intracellularly delivered isolated U1snRNA and oligoribonucleotides derived from the stem loop regions and the Sm IntroductionSystemic lupus erythematosus (SLE) is a prototypic autoimmune disorder affecting approximately 1 in 2000 people. The etiology of the disease is unknown. Genetic susceptibility and environmental factors play a role. Defects in tolerance, apoptosis, clearance of immune complexes (ICs), or generation of regulatory cells lead to uncontrolled hyperactivity of self-reactive T-and B-lymphocytes and to the sustained production of tissue-damaging autoantibodies and ICs. The presence of high levels of circulating autoantibodies against nuclear antigens is the immunologic hallmark of SLE. Anti-dsDNA antibodies are found in 70% of patients, and high titers correlate with disease severity. Thirty percent of patients with SLE also have circulating anti-Smith (Sm) antibodies recognizing the 7 Sm proteins (B, D1, D2, D3, E, F, G), which are common to all small nuclear ribonucleoproteins (snRNPs) and which associate with U snRNA (U1, 2, 4, 5). In addition, 40% of SLE patients have anti-RNP antibodies, which specifically bind to the A and C and the 70-kDa proteins within U1snRNPs. 1 It has been demonstrated that ICs containing nucleic acids can directly activate murine B lymphocytes and conventional DCs as well as human plasmacytoid DCs, thus contributing to the development of full-blown SLE. [2][3][4] Type I interferon (IFN-␣/␤), which is induced by SLE ICs in vitro, is found at high levels in the sera of most patients with active SLE. Microarray analysis in peripheral blood mononuclear cells of patients with SLE revealed the typical "IFN signature" of gene expression, which correlates with disease activity. 5,6 There is strong evidence for a central role of IFN-␣/␤ in SLE pathogenesis from studies in the New Zealand Black (NZB)/New Zealand White (NZW) mouse model of lupus, 7,8 whereas conflicting results were obtained in the Fas lpr mouse model of lupus-like disease. 9,10 The SLE-promoting role of IFN-␣/␤ may be attributed to the support of B-cell differentiation and antibody production 11,12 and to the induction of dendritic cell (DC) differentiation and maturation. 13 Plasmacytoid DCs (PDCs), whi...

In vivo adjuvant activity of the RNA component of the Sm/RNP lupus autoantigen

“…Moreover, when C3H/HeJ (TLR-4-mutant) mice were immunized with NP-KLH plus U1 RNA, antigen-specific antibody responses remained intact (results not shown), which excludes the possibility that the adjuvant effect was mediated by contaminating endotoxin. Consistent with previous in vitro study findings suggesting that U1 RNA is likely to exert its immunostimulatory effects via TLR-7 and protein kinase R (6,21,22), the immunostimulatory activity of U1 RNA could be blocked using a specific inhibitor of TLR-7 (IRS-661).…”

“…The Y1-5 small cytoplasmic RNPs, which are reactive with anti-Ro 60 autoantibodies, also contain extensive duplex RNA as well as single-stranded RNA (18). Although there is evidence that U1 RNA and other cellular RNAs can activate immune cells in vitro (6)(7)(8)(19)(20)(21)(22), it has not been shown that they can initiate innate immune responses or augment adaptive immunity in vivo.…”

Viral RNA Induces Type I Interferon-Dependent Cytokine Release and Cell Death in Mesangial Cells via Melanoma-Differentiation-Associated Gene-5

“…RNA can be protected from RNase digestion when being complexed to proteins such as Igs or nucleoproteins. [17][18][19] In glomerulonephritis, such immune complexes are often deposited in the glomerular mesangium where they are taken up by glomerular mesangial cells. 20 Studies from our group showed that poly I:C RNA activates human and murine mesangial cells to produce Il-6 and Ccl2 in vitro and in vivo, which we confirmed here at the mRNA level for Il-6.…”

Molecular mimicry in innate immunity? The viral RNA recognition receptor TLR7 accelerates murine lupus

“…Distinct endogenous RNA molecules and RNA-associated lupus autoantigens, like U1snRNP, have been shown to activate autoreactive B lymphocytes and DC via Tlr7 [11][12][13][14]. A prerequisite for Tlr7 activation is the uptake of RNA-containing autoantigens into intracellular endosomes.…”

“…In B cells this process involves the B cell receptor, which is engaged either directly or indirectly via rheumatoid factor [11]. In myeloid and plasmacytoid DC RNA autoantigen-containing immune complexes are internalized via Fc receptors [12,14,15]. U1snRNP-containing immune complexes trigger plasmacytoid DC to secrete large amounts of type I interferon, which enhances the response of B cells and DC to RNA autoantigens by enhancing TLR7 expression up to 20-fold [11,12,[14][15][16].…”