Systemic lupus erythematosus (SLE) is a complex systemic autoimmune disease caused by both genetic and environmental factors. Genome scans in families with SLE point to multiple potential chromosomal regions that harbor SLE susceptibility genes, and association studies in different populations have suggested several susceptibility alleles for SLE. Increased production of type I interferon (IFN) and expression of IFN-inducible genes is commonly observed in SLE and may be pivotal in the molecular pathogenesis of the disease. We analyzed 44 single-nucleotide polymorphisms (SNPs) in 13 genes from the type I IFN pathway in 679 Swedish, Finnish, and Icelandic patients with SLE, in 798 unaffected family members, and in 438 unrelated control individuals for joint linkage and association with SLE. In two of the genes--the tyrosine kinase 2 (TYK2) and IFN regulatory factor 5 (IRF5) genes--we identified SNPs that displayed strong signals in joint analysis of linkage and association (unadjusted P<10(-7)) with SLE. TYK2 binds to the type I IFN receptor complex and IRF5 is a regulator of type I IFN gene expression. Thus, our results support a disease mechanism in SLE that involves key components of the type I IFN system.

Objective. To investigate the release of interferon-␣ (IFN␣)-inducing material by necrotic or apoptotic cells, its properties, and the necessity of autoantibodies from systemic lupus erythematosus (SLE) patients for the interferogenic activity.Methods. U937 monocytic leukemia cells or peripheral blood mononuclear cells (PBMCs) were rendered necrotic by freeze-thawing or apoptotic by treatment with ultraviolet light. Cell culture supernatants from these cells and IgG from SLE patients (SLE IgG) were added to cultures of normal PBMCs or purified plasmacytoid dendritic cells (PDCs). The importance of nucleic acids for IFN␣ induction was investigated by RNase and DNase treatment. The IFN␣ levels were measured by immunoassay.Results. Both necrotic and apoptotic U937 cells released material that, combined with SLE IgG, induced IFN␣ production in PDCs. The release from apoptotic cells occurred with a 16-hour delay, in late apoptosis. Also, normal PBMCs released IFN␣-inducing material, but only during necrosis. The interferogenic activity of the necrotic material required the presence of RNA, while both RNA and DNA were important in the apoptotic material. In both cases, the presence of SLE IgG was necessary, and its activity correlated with the presence of antibodies to RNA-binding proteins, but not anti-DNA antibodies.Conclusion. Necrotic and late apoptotic cells release material that, combined with SLE IgG, induces production of IFN␣ in PDCs. The IFN␣ inducers probably consist of immune complexes (ICs) containing RNA and possibly DNA as essential interferogenic components. The presence of such interferogenic ICs could explain the ongoing production of IFN␣ in SLE and could be of etiopathogenic importance.

The objective was to investigate the relation between serum levels of interferon-alpha (IFN-alpha), the activity of an endogenous IFN-alpha inducing factor (SLE-IIF), clinical and immunological disease activity as well as serum levels of antiretroviral antibodies in SLE. Serum levels of IFN-alpha were measured in serial sera from 30 patients sampled at different stages of disease activity (SLEDAI score). The SLE-IIF activity was measured by its ability to induce IFN-alpha production in cultures of normal peripheral blood mononuclear cells. Both serum IFN-alpha and SLE-IIF increased markedly at flare in serially followed patients. The SLEDAI score, levels of anti-dsDNA antibodies and IL-10 correlated positively, and complement components Clq, C3 and leukocytes correlated inversely with serum concentrations of IFN-alpha. The extent of multiple organ involvement correlated with serum IFN-alpha. No relation between concentrations of retroviral peptide binding antibodies and IFN-alpha or SLE-IIF activity was found. The close relationship between disease activity in SLE patients and IFN-alpha serum levels suggests that activation of the type 1 IFN system might be of importance in the disease process.

Objective. The etiopathogenesis of primary Sjö-gren's syndrome (SS) is largely unknown. In other autoimmune diseases, type I interferon (IFN) may play a pivotal role by triggering and sustaining the disease process. We therefore aimed to determine whether patients with primary SS had an activated type I IFN system.Methods. Salivary gland biopsy specimens and sera from patients with primary SS were investigated for the occurrence of IFN␣-producing cells and measurable IFN␣ levels, respectively. The ability of primary SS sera together with apoptotic or necrotic cells to induce IFN␣ production in normal peripheral blood mononuclear cells was examined. The IFN␣ inducer was characterized, and IFN␣-producing cells were identified. Clinical data were correlated with the IFN␣-inducing capacity of primary SS sera.Results. Numerous IFN␣-producing cells were detected in salivary gland biopsy specimens, despite low serum IFN␣ levels. Autoantibodies to RNA-binding proteins, combined with material released by necrotic or late apoptotic cells, were potent inducers of IFN␣ production in plasmacytoid dendritic cells (PDCs). This appeared to be attributable to RNA-containing immune complexes triggering PDCs by means of RNA and interaction with Fc␥ receptor IIa. The IFN␣-inducing capacity of sera was associated with positive results of a labial salivary gland biopsy (focus score >1) and with dermatologic, hematologic, and pulmonary manifestations.Conclusion. Patients with primary SS have an activated type I IFN system. Although virus may initiate the production of IFN, the continued IFN␣ synthesis is caused by RNA-containing immune complexes that activate PDCs to prolong IFN␣ production at the tissue level. This IFN␣ promotes the autoimmune process by a vicious circle-like mechanism, with increased autoantibody production and formation of more endogenous IFN␣ inducers.

An ongoing production of IFN-α may be of etiopathogenic significance in systemic lupus erythematosus (SLE). It may be due to the natural IFN-producing cells (NIPC), also termed plasmacytoid dendritic cells (PDC), activated by immune complexes that contain nucleic acids derived from apoptotic cells. We here examined the role of FcγR in the IFN-α production in vitro by PBMC induced by the combination of apoptotic U937 cells and autoantibody-containing IgG from SLE patients (SLE-IgG). The Fc portion of the SLE-IgG was essential to induce IFN-α production, because Fab fragments or F(ab′)2 were ineffective. Normal, especially heat-aggregated, IgG inhibited the IFN-α production, suggesting a role for FcγR on PBMC. Using blocking anti-FcγR Abs, the FcγRIIa,c (CD32) but not FcγRI or FcγRIII were shown to be involved in the IFN-α induction by apoptotic cells combined with SLE-IgG, but not by HSV or CpG DNA. In contrast, the action of all of these inducers was inhibited by the anti-FcγRIIa,b,c mAb AT10 or heat-aggregated IgG. Flow cytometric analysis revealed that ∼50% of the BDCA-2-positive PBMC, i.e., NIPC/PDC, expressed low but significant levels of FcγRII, as did most of the actual IFN-α producers activated by HSV. RT-PCR applied to NIPC/PDC purified by FACS demonstrated expression of FcγRIIa, but not of FcγRIIb or FcγRIIc. We conclude that FcγRIIa on NIPC/PDC is involved in the activation of IFN-α production by interferogenic immune complexes, but may also mediate inhibitory signals. The FcγRIIa could therefore have a key function in NIPC/PDC and be a potential therapeutic target in SLE.