Induction of interferon-alpha production in plasmacytoid dendritic cells by immune complexes containing nucleic acid released by necrotic or late apoptotic cells and lupus IgG

CTLA4Ig prevents initiation but not evolution of anti-phospholipid syndrome in NZW/BXSB mice

NZW "x" BXSB F1 mice develop SLE that is associated with an anti-phospholipid syndrome characterized by anti-cardiolipin antibodies, thrombocytopenia and small coronary artery thrombosis. This syndrome is immune mediated and, dependent, on CD4+T cells. To determine whether disease in these mice can be treated with blockade of T cell costimulation we treated them with the CD28 antagonist CTLA4Ig at 9 or 12 weeks of age. CTLA4Ig completely prevented both SLE nephritis and myocardial infarcts if it was given at 9 weeks of age, before anti-cardiolipin antibodies could be detected in the serum and prevented both B cell expansion and activation and the development of peripheral monocytosis. If treatment was delayed until 12 weeks of age after cardiolipin antibodies had arisen but before the onset of clinical disease, CTLA4Ig had very little effect on disease progression. These findings indicate that CD4+T cell activation through CD28 is critical for disease initiation in this model but plays little role in disease progression or tissue damage. These findings have relevance to the treatment of anti-phospholipid syndrome in humans.

Activation of the type I interferon system in primary Sjögren's syndrome: a possible etiopathogenic mechanism

OBJECTIVE

The etiopathogenesis of primary Sjögren'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 IFNalpha-producing cells and measurable IFNalpha levels, respectively. The ability of primary SS sera together with apoptotic or necrotic cells to induce IFNalpha production in normal peripheral blood mononuclear cells was examined. The IFNalpha inducer was characterized, and IFNalpha-producing cells were identified. Clinical data were correlated with the IFNalpha-inducing capacity of primary SS sera.

RESULTS

Numerous IFNalpha-producing cells were detected in salivary gland biopsy specimens, despite low serum IFNalpha levels. Autoantibodies to RNA-binding proteins, combined with material released by necrotic or late apoptotic cells, were potent inducers of IFNalpha 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 Fcgamma receptor IIa. The IFNalpha-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 IFNalpha synthesis is caused by RNA-containing immune complexes that activate PDCs to prolong IFNalpha production at the tissue level. This IFNalpha promotes the autoimmune process by a vicious circle-like mechanism, with increased autoantibody production and formation of more endogenous IFNalpha inducers.

Ultraviolet radiation-induced injury, chemokines, and leukocyte recruitment: An amplification cycle triggering cutaneous lupus erythematosus

OBJECTIVE

To investigate the activation and recruitment pathways of relevant leukocyte subsets during the initiation and amplification of cutaneous lupus erythematosus (LE).

METHODS

Quantitative real-time polymerase chain reaction was used to perform a comprehensive analysis of all known chemokines and their receptors in cutaneous LE lesions, and the cellular origin of these chemokines and receptors was determined using immunohistochemistry. Furthermore, cytokine- and ultraviolet (UV) light-mediated activation pathways of relevant chemokines were investigated in vitro and in vivo.

RESULTS

In the present study, we identified the CXCR3 ligands CXCL9 (interferon-gamma [IFNgamma]-induced monokine), CXCL10 (IFNgamma-inducible protein 10), and CXCL11 (IFN-inducible T cell alpha chemoattractant) as being the most abundantly expressed chemokine family members in cutaneous LE. Expression of these ligands corresponded with the presence of a marked inflammatory infiltrate consisting of mainly CXCR3-expressing cells, including skin-homing lymphocytes and blood dendritic cell antigen 2-positive plasmacytoid dendritic cells (PDCs). Within cutaneous LE lesions, PDCs accumulated within the dermis and were activated to produce type I IFN, as detected by the expression of the IFNalpha-inducible genes IRF7 and MxA. IFNalpha, in turn, was a potent and rapid inducer of CXCR3 ligands in cellular constituents of the skin. Furthermore, we demonstrated that the inflammatory CXCR3 ligands cooperate with the homeostatic chemokine CXCL12 (stromal cell-derived factor 1) during the recruitment of pathogenically relevant leukocyte subsets. Moreover, we showed that UVB irradiation induces the release of CCL27 (cutaneous T cell-attracting chemokine) from epidermal compartments into dermal compartments and up-regulates the expression of a distinct set of chemokines in keratinocytes.

CONCLUSION

Taken together, our data suggest an amplification cycle in which UV light-induced injury induces apoptosis, necrosis, and chemokine production. These mechanisms, in turn, mediate the recruitment and activation of autoimmune T cells and IFNalpha-producing PDCs, which subsequently release more effector cytokines, thus amplifying chemokine production and leukocyte recruitment, finally leading to the development of a cutaneous LE phenotype.

Interferons as pathogenic effectors in autoimmunity

Interferons (IFNs) type-1 (IFN alpha/beta) and type-II (IFN-gamma) are the most pleiotropic molecules in the intricate cytokine network. This dominance arises from three crucial factors: (i) initiation of IFN-alpha/beta and IFN-gamma production at the inception of most innate immune responses, which primes for the ensuing adaptive immune responses, primarily through the sine qua non upregulation of major histocompatibility complex and costimulatory molecules; (ii) magnification of their production and signaling by cross-talk between themselves, and synergistic or antagonistic effects on other cytokines; and (iii) direct or indirect initiation of transcription of hundreds of immunologically relevant genes. Considering that aberrant immune responses against self-molecules seem to depend on the same constituents and pathways as those against exogenous antigens, it follows that IFNs are also major effectors in the pathogenesis of autoimmunity. Here, we review the diverse biological effects of IFNs on the immune system, discuss findings pertaining to the nature of exogenous and endogenous stimuli that might induce IFN production through the engagement of Toll-like receptors, and summarize the detrimental and, in some instances, beneficial effects of IFNs in systemic and organ-specific autoimmune diseases.

The role of innate immunity in autoimmunity

During the 2004 International Congress of Immunology in Montreal, a panel of experts gathered for an “Ideashop” discussion on the potential role of innate immunity in autoimmunity and the ways in which this might be targeted in future therapies.

Type I interferons (alpha/beta) in immunity and autoimmunity

The significance of type I interferons (IFN-alpha/beta) in biology and medicine renders research on their activities continuously relevant to our understanding of normal and abnormal (auto) immune responses. This relevance is bolstered by discoveries that unambiguously establish IFN-alpha/beta, among the multitude of cytokines, as dominant in defining qualitative and quantitative characteristics of innate and adaptive immune processes. Recent advances elucidating the biology of these key cytokines include better definition of their complex signaling pathways, determination of their importance in modifying the effects of other cytokines, the role of Toll-like receptors in their induction, their major cellular producers, and their broad and diverse impact on both cellular and humoral immune responses. Consequently, the role of IFN-alpha/beta in the pathogenesis of autoimmunity remains at the forefront of scientific inquiry and has begun to illuminate the mechanisms by which these molecules promote or inhibit systemic and organ-specific autoimmune diseases.

The emerging role of interferon in human systemic lupus erythematosus

Recent studies of patients with systemic lupus erythematosus, together with data from lupus-prone mice, suggest that inappropriate activation of type I interferon might have a role in disease pathogenesis. Serum levels of IFN-alpha are elevated in SLE patients, and gene expression profiling of peripheral blood cells shows that most lupus cases demonstrate an upregulation of IFN-responsive genes. Of interest, the IFN gene 'signature' correlates with more severe disease. The available data support a model whereby chromatin-containing immune complexes circulating in the blood of lupus patients stimulate leukocytes to produce IFN, which perpetuates disease. These emerging insights into the connection between IFN and lupus provide a host of new diagnostic and therapeutic opportunities.

Polymorphisms in the tyrosine kinase 2 and interferon regulatory factor 5 genes are associated with systemic lupus erythematosus

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.

Requirement of type I interferon signaling for arthritis triggered by double-stranded RNA

OBJECTIVE

Arthralgias and overt arthritides are often associated with viral infections. Viral infections expose the infected host to proinflammatory double-stranded RNA (dsRNA), which can cause joint inflammation and is a potent activator of interferon-alpha (IFNalpha). The aim of this study was to determine the role of IFNalpha and dsRNA-related signaling molecules in the onset of joint inflammation induced by viral dsRNA.

METHODS

IFNalpha and different forms of RNA were injected into the knee joints of wild-type mice, mice lacking the type I interferon receptor (IFNAR(-/-)), and mice deficient in dsRNA-dependent protein kinase (PKR(-/-)). Histologic evidence of joint damage and the ability of splenocytes to produce cytokines in response to dsRNA or IFNalpha were assessed.

RESULTS

Viral dsRNA, but not short single-stranded RNA, induced arthritis. The arthritis was aggravated by intracellular delivery of dsRNA. The expression of PKR was not mandatory for dsRNA-induced joint inflammation. In contrast, IFNalpha/beta signaling was important for dsRNA-induced joint inflammation because IFNAR(-/-) mice did not develop arthritis. Furthermore, intraarticular deposition of IFNalpha induced arthritis in PKR(-/-) and control mice, whereas IFNAR(-/-) mice were protected. The arthritogenic effect of IFNalpha was attenuated by in vivo depletion of monocyte/macrophages.

CONCLUSION

Arthritis triggered by dsRNA is not dependent on the expression of the dsRNA-signaling molecule PKR (or Toll-like receptor 3, as previously shown), but is associated with the ability to produce type I IFN and is critically dependent on type I IFN receptor signaling. The intrinsic arthritogenic properties of IFNalpha implicate a role of this cytokine in joint manifestations triggered by various interferogenic stimuli.

The effect of inflammation on the generation of plasma DNA from dead and dying cells in the peritoneum

To assess the effects of inflammation on the generation of circulating DNA from dead and dying cells, plasma DNA levels were determined in BALB/c mice, administered apoptotic or necrotic Jurkat cells following induction of peritonitis by treatment with thioglycollate (TG), peptone (PT), or sodium periodate (NaIO(4)). In mice receiving TG or NaIO(4), plasma DNA levels following intraperitoneal administration of Jurkat cells were significantly reduced compared with controls, whereas they were not affected in mice receiving PT. To determine the basis of these differences, the cellular composition of peritoneal fluids prior to the administration of the dead cells was analyzed. Among agents tested, TG administration led to the largest increase in cells, both neutrophils and monocytes. As shown by flow cytometry, the exudates contained apoptotic neutrophils and macrophages, with the highest levels in the TG-induced exudates. Analysis of DNA and caspase 3 in the fluids also showed differences. TG exudates showed increases in DNA and caspase 3, while NaIO(4)-induced exudates had an increase only in DNA. Fluid from PT-treated mice did not have increases in DNA or caspase 3. Together, these results indicate that prior inflammation can affect the generation of blood DNA from apoptotic or necrotic cells, although this effect may vary depending on the composition of the exudates with respect to cells as well as DNA.

The immune response to cell death in SLE

Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease characterized by the production of antinuclear antibodies (ANA). These antibodies target a wide variety of antigens whose presence in an immunologically active form may result from cell death processes that cause their translocation and release from cells. As indicated by in vivo model systems, the release of DNA from cells may not be a simple consequence of cell death but rather may require the intervention of other cell types including macrophages. Thus, in mice, administration of either apoptotic or necrotic cells produces a blood DNA response, whereas mice lacking macrophages fail to show blood DNA under the same conditions. Furthermore, the circulating DNA arising from apoptotic and necrotic cells displays a similar pattern with respect to size distribution, with both showing DNA laddering, a pattern indicating enzymatic cleavage. Since circulating DNA in the form of immune complexes can play a role in lupus pathogenesis, these findings suggest that the generation and clearance of dead cells are important events that may underlie autoimmunity in this disease and may be targeted for therapy.