Role of type I interferons in the activation of autoreactive B cells

Type I interferons (IFNs) are a family of cytokines involved in the defense against viral infections that play a key role in the activation of both the innate and adaptive immune system. IFNs both directly and indirectly enhance the capacity of B lymphocytes to respond to viral challenge and produce cytotoxic and neutralizing antibodies. However, prolonged type I IFN exposure is not always beneficial to the host. If not regulated properly IFN can drive autoantibody production as well as other parameters of systemic autoimmune disease. Type I IFNs impact B-cell function through a variety of mechanisms, including effects on receptor engagement, Toll-like receptor expression, cell migration, antigen presentation, cytokine responsiveness, cytokine production, survival, differentiation and class-switch recombination. Type I IFNs are also cytotoxic for a variety of cell types and thereby contribute to the accumulation of cell debris that serves as a potential source for autoantigens. Type I IFN engagement of a variety of accessory cells further promotes B-cell survival and activation, as exemplified by the capacity of type I IFNs to increase the level of B-cell survival factors, such as B lymphocyte stimulator, produced by dendritic cells. Therefore, it is not surprising that the loss of expression of the type I IFN receptor can have dramatic effects on the production of autoantibodies and on the clinical features of systemic autoimmune diseases such as systemic lupus erythematosus.     

Signal transduction by type I interferons

The two classes of interferons, type I (IFNalpha, IFNbeta, IFNomega, and IFNtau) and type II (IFNgamma) are pleiotropic cytokines that exhibit antiviral, antiproliferative, and immunomodulatory effects on their target cells. This article summarizes the advances made in elucidating the molecular events that mediate the biological responses to type I interferons.     

Structural linkage between ligand discrimination and receptor activation by type I interferons

Type I Interferons (IFNs) are important cytokines for innate immunity against viruses and cancer. Sixteen human type I IFN variants signal through the same cell-surface receptors, IFNAR1 and IFNAR2, yet they can evoke markedly different physiological effects. The crystal structures of two human type I IFN ternary signaling complexes containing IFNα2 and IFNω reveal recognition modes and heterotrimeric architectures that are unique among the cytokine receptor superfamily but conserved between different type I IFNs. Receptor-ligand cross-reactivity is enabled by conserved receptor-ligand "anchor points" interspersed among ligand-specific interactions that "tune" the relative IFN-binding affinities, in an apparent extracellular "ligand proofreading" mechanism that modulates biological activity. Functional differences between IFNs are linked to their respective receptor recognition chemistries, in concert with a ligand-induced conformational change in IFNAR1, that collectively control signal initiation and complex stability, ultimately regulating differential STAT phosphorylation profiles, receptor internalization rates, and downstream gene expression patterns.     

Induction of type I interferons by bacteria

Type I interferons (IFNs) are secreted cytokines that orchestrate diverse immune responses to infection. Although typically considered to be most important in the response to viruses, type I IFNs are also induced by most, if not all, bacterial pathogens. Although diverse mechanisms have been described, bacterial induction of type I IFNs occurs upon stimulation of two main pathways: (i) Toll‐like receptor (TLR) recognition of bacterial molecules such as lipopolysaccharide (LPS); (ii) TLR‐independent recognition of molecules delivered to the host cell cytosol. Cytosolic responses can be activated by two general mechanisms. First, viable bacteria can secrete stimulatory ligands into the cytosol via specialized bacterial secretion systems. Second, ligands can be released from bacteria that lyse or are degraded. The bacterial ligands that induce the cytosolic pathways remain uncertain in many cases, but appear to include various nucleic acids. In this review, we discuss recent advances in our understanding of how bacteria induce type I interferons and the roles type I IFNs play in host immunity.     

Structure-activity of type I interferons

Type I IFNs constitute a family of proteins exhibiting high homology in primary, secondary, and tertiary structures. They interact with the same receptor and transmit signals to cellular nucleus through a similar mechanism, eliciting roughly homogeneous biological activity. Nevertheless, the members of that family, IFNα species, IFNβ and IFNω, due to local differences in the structure sometime show distinct properties. From the reported data it results that even minute changes or differences in the primary sequences could be responsible for a significant variety of biological actions, thus inducing to the hypothesis that Type I IFNs, rather than to be the result of a redundant replication during the evolution, play definite roles in the defense of living organisms to foreign agents.     

Phagocytosis of Leishmania enhances macrophage activation by IFN-gamma and lipopolysaccharide

This investigation describes the ability of Leishmania promastigotes to enhance activation of bone marrow-derived murine macrophages in vitro if added together with rIFN-gamma in the presence or absence of LPS. Activation was defined as the capacity for arginine-derived NO2- production and the killing of intracellular Leishmania. Enhanced NO2- production was observed for either CBA or C3H/HeJ macrophages undergoing phagocytosis at the time of activation. Other phagocytic stimuli including inert polystyrene latex beads were as effective as Leishmania. No correlation could be demonstrated between the enhanced NO2- release and secretion of products of the respiratory burst or PGE2. However, TNF-alpha secretion was elevated in cultures undergoing phagocytosis and a relationship between hexosemonophosphate shunt activity and NO2- levels was evident. These studies confirm and extend previous reports that phagocytosis plays an important role in the regulation of macrophage physiology.     

Differential receptor subunit affinities of type I interferons govern differential signal activation

Type I interferons (IFNs) elicit antiviral, antiproliferative and immunmodulatory responses by binding to a shared cell surface receptor comprising the transmembrane proteins ifnar1 and ifnar2. Activation of differential response patterns by IFNs has been observed, suggesting that members of the family play different roles in innate immunity. The molecular basis for differential signaling has not been identified yet. Here, we have investigated the recognition of various IFNs including several human IFNalpha species, human IFNomega and human IFNbeta as well as ovine IFNtau2 by the receptor subunits in detail. Binding to the extracellular domains of ifnar1 (ifnar1-EC) and ifnar2 (ifnar2-EC) was monitored in real time by reflectance interference and total internal reflection fluorescence spectroscopy. For all IFNs investigated, competitive 1:1 interaction not only with ifnar2-EC but also with ifnar1-EC was shown. Furthermore, ternary complex formation was studied with ifnar1-EC and ifnar2-EC tethered onto solid-supported membranes. These analyses confirmed that the signaling complexes recruited by IFNs have very similar architectures. However, differences in rate and affinity constants over several orders of magnitude were observed for both the interactions with ifnar1-EC and ifnar2-EC. These data were correlated with the potencies of ISGF3 activation, antiviral and anti-proliferative activity on 2fTGH cells. The ISGF3 formation and antiviral activity correlated very well with the binding affinity towards ifnar2. In contrast, the affinity towards ifnar1 played a key role for antiproliferative activity. A striking correlation was observed for relative binding affinities towards ifnar1 and ifnar2 with the differential antiproliferative potency. This correlation was confirmed by systematically engineering IFNalpha2 mutants with very high differential antiproliferative potency.     

Adjuvant activity of type I interferons

Type I interferons (IFNs) produced primarily by plasmacytoid dendritic cells (pDCs) as part of the innate immune response to infectious agents induce the maturation of myeloid DCs and enhance antigen presentation. Type I IFNs also enhance apoptosis of virus-infected cells, stimulate cross priming and enhanced presentation of viral peptides. Type I IFNs are powerful polyclonal B-cell activators that induce a strong primary humoral immune response characterized by isotype switching and protection against virus challenge. Type I IFNs stimulate an IgG2a antibody response characteristic of Th1 immunity when ad-mixed with influenza virus vaccine and injected intramuscurarly (i.m.) or administered intranasally. The adjuvant activity of type I IFNs has been shown to involve direct effects of IFN on B-cells, effects on T-cells, as well as effects on antigen presentation. Oromucosal administration of type I IFNs concomitantly with i.m. injection of vaccine alone can also enhance the antibody response to influenza vaccination by enhancing trafficking of antigen-presenting cells towards the site of vaccination. Recombinant IFNs are potent adjuvants that may find application in both parenterally and mucosally administered vaccines.     

Regulation of immune cell homeostasis by type I interferons

Although initially identified and best characterized for their role in innate antiviral defence, type I interferons (IFN-I) are also known to have an important impact on the adaptive immune response. In part, this is linked to another long-recognised property of IFN-I, namely their ability to modify cellular proliferation and survival. Here, we review the influence of IFN-I on immune cell homeostasis, focusing on their effects on T cells and antigen-presenting cells.     

Immunomodulatory functions of type I interferons

Interferon-α (IFNα) and IFNβ, collectively known as type I IFNs, are the major effector cytokines of the host immune response against viral infections. However, the production of type I IFNs is also induced in response to bacterial ligands of innate immune receptors and/or bacterial infections, indicating a broader physiological role for these cytokines in host defence and homeostasis than was originally assumed. The main focus of this Review is the underappreciated immunomodulatory functions of type I IFNs in health and disease. We discuss their function in the regulation of innate and adaptive immune responses, the response to bacterial ligands, inflammasome activation, intestinal homeostasis and inflammatory and autoimmune diseases.     

Human primary immunodeficiencies of type I interferons

Type I interferons (IFN-alpha/beta and related molecules) are essential for protective immunity to experimental infection by numerous viruses in the mouse model. In recent years, human primary immunodeficiencies affecting either the production of (UNC-93B deficiency) or the response to (STAT1 and TYK2 deficiencies) these IFNs have been reported. Affected patients are highly susceptible to certain viruses. Patients with STAT1 or TYK2 deficiency are susceptible to multiple viruses, including herpes simplex virus-1 (HSV-1), whereas UNC-93B-deficient patients present isolated HSV-1 encephalitis. However, these immunological defects are not limited to type I IFN-mediated immunity. Impaired type II IFN (IFN-gamma)-mediated immunity plays no more than a minor role in the pathogenesis of viral diseases in these patients, but the contribution of impaired type III IFN (IFN-lambda)-mediated immunity remains to be determined. These novel inherited disorders strongly suggest that type I IFN-mediated immunity is essential for protection against natural infections caused by several viruses in humans.     

Overview of the biology of type I interferons

Type I interferons are pleiotropic cytokines with antiviral, antitumor and immunoregulatory functions. An aspect of their complex biology is the paradox that, depending on context, type I interferons can be anti-inflammatory and tissue protective or can be proinflammatory and promote autoimmunity. Along these lines, the activation of type I interferon pathways is effective in suppressing disease activity in patients with multiple sclerosis and in animal models of arthritis and colitis, while there is an expectation that blockade of the same pathways will be beneficial in the treatment of patients with systemic lupus erythematosus.     

A novel class of human type I interferons

The screening of a cDNA library prepared from mRNA of Sendai virus induced Namalwa (human Burkitt's lymphoma) cells, using a human IFN-alpha 2 DNA probe under conditions of low stringency, identified two weakly hybridizing clones containing sequences related to, but discernably different from those of the IFN-alpha class. Sequence and hybridization analysis of these cDNAs as well as expression in E. coli provided evidence that they encode proteins which have the characteristics of IFN type I but which are sufficiently diverged in sequence from both IFN-alpha s and IFN-beta to suggest that they are representatives of a new and distinct class of interferons named interferon-omega. Hybridization of these sequences to genomic DNA reveals that this class contains at least four members.     

Inhibition of angiogenesis by type I interferons in models of Kaposi's sarcoma

Kaposi's Sarcoma (KS) is a pathology which occurs with increased frequency and in a particularly aggressive form in AIDS patients. The HIV-1 Tat protein appears to be an important co-factor in the induction of the extensive neo-vascularization associated with AIDS-KS. Tat acts as a chemoattractant for endothelial cells in vitro, inducing both chemotactic and invasive responses. Several clinical trials have been performed testing the effectiveness of diverse biological agents in therapy of KS, among these the type I interferons. Type I IFNs have diverse biological functions besides their anti-viral activity, including anti-angiogenic properties. We have shown that IFN alpha and IFN beta are potent inhibitors of both primary and immortalized endothelial cell migration and morphogenesis in vitro as well as neo-angiogenesis induced by HIV-1 Tat in vivo. The inhibitory effect of IFN class I on HIV-Tat associated angiogenesis further supports its use as a therapy for epidemic Kaposi's sarcoma. The use of recombinant IFNs at the levels required to obtain a therapeutic effect are associated with side effects and toxicity, therefore we are now developing a gene therapy approach for constant and local delivery type I IFNs.     

Viral transformation for production of personalized type I interferons

Type I interferons (IFN) are cytokines with many functions and have been widely used to treat many human diseases such as hepatitis C virus infection. Using the viral transformation and priming properties of Epstein-Barr virus, we have developed a system that can produce high levels of “personalized” IFNs, which are produced from the cells of the patient to whom the IFNs are to be administrated. We demonstrate the feasibility of the system. This seems to be the first report for the establishment of a personalized IFN-production system. The personalized IFNs could have a longer circulation time, fewer side effects but higher efficacy. We anticipate that the system can provide an improved form of IFN for medical uses.     

Inhibition of ornithine decarboxylase in human fibroblast cells by type I and type II interferons

Dilution of human fibroblast GM2767 cell cultures into fresh serum-containing growth medium induces ornithine decarboxylase activity 45-fold over a six-hour interval. When the fibroblast cultures are supplemented with human fibroblast α-, β-, or γ-interferon at the time of dilution into fresh growth medium, the induction of ornithine decarboxylase is inhibited 61%, 90%, and 65%, respectively. β-Interferon is the most effective type of interferon to inhibit induction of ornithine decarboxylase.     

The role of type I interferons in TLR responses

Recent advances in unravelling the complexities of the signalling pathways that constitute innate immunity have highlighted type I interferon as a key component in the response to infection. Here we focus on the emerging field of pattern-recognition receptor signalling, specifically Toll-like receptors and retinoic acid inducible gene-like helicases, from the perspective of this 50-year-old cytokine. The type I interferon gene family encompasses more than 20 subtypes, whose nature and properties have been extensively studied during its relatively long history. In this review we update and integrate available data on the mechanics of activation of the interferon genes and the role of this cytokine family in the innate immune response.