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Objective: Plasmacytoid dendritic cells (PDCs) produce high concentrations of interferon-α (IFNα) following exposure to immune complexes containing nucleic acids. We previously reported that serum from healthy donors inhibits IFNα production by PDCs in response to systemic lupus erythematosus (SLE) immune complexes, and that inhibition is mediated, in part, by IgG. IgG is the major component of intravenous immunoglobulin and is well known to exert antiinflammatory properties. Although suppression of inflammation by the sialylated subfraction of IgG has been implicated in some models, the mechanism of IFNα inhibition by IgG and the importance of sialylation have not been studied., Methods: SLE immune complexes or synthetic Toll-like receptor (TLR) agonists were used to stimulate total or individual cell-depleted human mononuclear cell cultures in the presence or absence of IgG, Fc fragments, F(ab')2 fragments, and their sialylated or unsialylated subfractions. Cytokines were quantified by enzyme-linked immunosorbent assay., Results: We identified 2 distinct mechanisms by which IgG inhibits IFNα production. First, IgG Fc fragments inhibited SLE immune complex-stimulated IFNα production via a sialic acid-independent mechanism, by inhibiting immune complex binding to Fcγ receptor IIa on PDCs. In contrast, the F(ab')2 fragment of the sialylation-enriched fraction of IgG inhibited TLR-7 or TLR-9 agonist-induced IFNα production but did not require the sialic acid residue itself. The inhibitory activity of IgG on TLR agonist-induced IFNα required monocyte production of prostaglandin E2, a potent suppressor of IFNα production by PDCs., Conclusion: IgG attenuates IFNα production by PDCs by both cell surface receptor and intracellular pathways, depending on the nature of the inducing stimulus., (Copyright © 2013 by the American College of Rheumatology.)

Immune complexes (ICs) play a pivotal role in causing inflammation in systemic lupus erythematosus (SLE). Yet, it remains unclear what the dominant blood cell type(s) and inflammation-related gene programs stimulated by lupus ICs are. To address these questions, we exposed normal human PBMCs or CD14(+) isolated monocytes to SLE ICs in the presence or absence of C1q and performed microarray analysis and other tests for cell activation. By microarray analysis, we identified genes and pathways regulated by SLE ICs that are both type I IFN dependent and independent. We also found that C1q-containing ICs markedly reduced expression of the majority of IFN-response genes and also influenced the expression of multiple other genes induced by SLE ICs. Surprisingly, IC activation of isolated CD14(+) monocytes did not upregulate CD40 and CD86 and only modestly stimulated inflammatory gene expression. However, when monocyte subsets were purified and analyzed separately, the low-abundance CD14(dim) ("patrolling") subpopulation was more responsive to ICs. These observations demonstrate the importance of plasmacytoid dendritic cells, CD14(dim) monocytes, and C1q as key regulators of inflammatory properties of ICs and identify many pathways through which they act.

Almost all humans with homozygous deficiency of C1q develop systemic lupus erythematosus (SLE). The precise cellular mechanism(s) by which C1q prevents the development of SLE remains unclear. In this study, we tested the role of C1q in the regulation of IFN-α induced by immune complexes (ICs) in vitro, as well as the consequences of lack of C1q in vivo. Our experiments revealed that C1q preferentially promotes the binding of SLE ICs to monocytes rather than plasmacytoid dendritic cells, but this inhibition was not due to the induction of inhibitory soluble factors. The presence of C1q also altered the trafficking of ICs within monocytes such that ICs persisted in early endosomes. In patients with C1q deficiency, serum and cerebrospinal fluid levels of IFN-α and IFN-γ-inducible protein-10 levels were elevated and strongly correlated with Ro autoantibodies, demonstrating the clinical significance of these observations. These studies therefore associate C1q deficiency with defective regulation of IFN-α and provide a better understanding of the cellular mechanisms by which C1q prevents the development of IC-stimulated autoimmunity.

The complement pathway was implicated in the immunopathogenesis of lupus and other autoimmune disorders decades ago. The apparent paradox that early complement component (C1q, C2 and C4) deficiencies predispose to lupus has been explained by the beneficial roles of these proteins in promoting the clearance of immune complexes (ICs) and apoptotic cells. Recent findings demonstrate that, in the absence of C1q, instead of ICs binding to monocytes, they preferentially engage plasmacytoid dendritic cells (pDC) so generating interferon (IFN) alpha, the cytokine with potent immune adjuvant properties. C1q opsonized apoptotic cells also exert an immunosuppressive effect through cytokine regulation and the stimulation of additional opsonins by macrophages. C1q was recently reported to impede neutrophil extracellular trap (NET) degradation. NETs are known to promote type I IFN production in SLE by providing a source of antigen for the formation of ICs as well as through direct pDC activation by cathelicidin (LL37). Together, these findings provide both direct and indirect links between two key pathways implicated in lupus pathogenesis: complement and IFN. [ABSTRACT FROM AUTHOR]