Interferons and the colony-stimulating factors IL-3 and GM-CSF enhance the IFN-alpha response in human blood leucocytes induced by herpes simplex virus

Plasmacytoid dendritic cells and RNA-containing immune complexes drive expansion of peripheral B cell subsets with an SLE-like phenotype

BACKGROUND

Hyperactive B cells and a continuous interferon (IFN)-α production by plasmacytoid dendritic cells (pDCs) play a key role in systemic lupus erythematosus (SLE). We asked whether the interaction between B cells and pDCs stimulated with RNA-containing immune complexes affects peripheral B cell subsets.

METHODS

B cells and pDCs were isolated from blood of healthy individuals and stimulated with immune complexes consisting of SLE-IgG and U1snRNP (RNA-IC). Expression of cell surface molecules as well as IL-6 and IL-10 production were determined by flow cytometry and immunoassays. Gene expression profiles were determined by a NanoString nCounter expression array.

RESULTS

We found a remarkable increase of double negative CD27-IgD- B cells, from 7% within fresh CD19+ B cells to 37% in the RNA-IC-stimulated co-cultures of B cells and pDCs, comparable to the frequency of double negative B cells in SLE patients. Gene expression analysis of the double negative CD27-IgD- and the CD27+IgD- memory B cells revealed that twenty-one genes were differentially expressed between the two B cell subsets (≥ 2-fold, p<0.001). The, IL21R, IL4R, CCL4, CCL3, CD83 and the IKAROS Family Zinc Finger 2 (IKZ2) showed higher expression in the double negative CD27-IgD- B cells.

CONCLUSION

The interactions between B cells and pDCs together with RNA-containing IC led to an expansion of B cells with similar phenotype as seen in SLE, suggesting that the pDC-B cell crosstalk contributes to the autoimmune feed-forward loop in SLE.

Activated T cells enhance interferon-α production by plasmacytoid dendritic cells stimulated with RNA-containing immune complexes

OBJECTIVES

Patients with systemic lupus erythematosus (SLE) have an ongoing interferon-α (IFN-α) production by plasmacytoid dendritic cells (pDCs). We investigated whether T cells can promote IFN-α production by pDCs.

METHODS

Human pDCs were stimulated with immune complexes (ICs) containing U1 small nuclear ribonucleic proteins particles and SLE-IgG (RNA-IC) in the presence of T cells or T cell supernatants. T cells were activated by anti-CD3/CD28 antibodies or in a mixed leucocyte reaction. IFN-α and other cytokines were determined in culture supernatants or patient sera with immunoassays. The effect of interleukin (IL) 3 and granulocyte-macrophage-colony-stimulating factor (GM-CSF) on pDCs was examined by the use of antibodies, and the expression of CD80/CD86 was determined using flow cytometry.

RESULTS

Activated T cells and supernatants from activated T cells increased IFN-α production by >20-fold. The stimulatory effect of T cell supernatants was reduced after depletion of GM-CSF (81%) or by blocking the GM-CSF receptor (55%-81%). Supernatant from activated T cells, furthermore, increased the frequency of CD80 and CD86 expressing pDCs stimulated with RNA-IC from 6% to 35% (p<0.05) and from 10% to 26% (p<0.01), respectively. Activated SLE T cells enhanced IFN-α production to the same extent as T cells from healthy individuals and a subset of patients with SLE had increased serum levels of GM-CSF.

CONCLUSIONS

Activated T cells enhance IFN-α production by RNA-IC stimulated pDCs via GM-CSF and induce pDC maturation. Given the increased serum levels of GM-CSF in a subset of patients with SLE, these findings suggest that activated T cells may upregulate type I IFN production in SLE.

Role of Natural Interferon-α Producing Cells (Plasmacytoid Dendritic Cells) in Autoimmunity

The type I interferons (IFNs) have antiviral, cytostatic and prominent immunomodulatory effects, which all are of great importance during viral infections. However, prolonged exposure of the immune system to type I IFN can break tolerance and initiate an autoimmune reaction, eventually leading to autoimmune disease. Recent observations in patients with systemic lupus erythematosus (SLE) have revealed that such individuals have endogenous IFN-alpha inducers, causing an ongoing IFN-alpha production and consequently a continuous stimulation of the immune system. These IFN-alpha inducers consist of small immune complexes (IC) containing DNA or RNA and act on the principal IFN-alpha producing cell, the natural IFN-alpha producing cell (NIPC), also termed the plasmacytoid dendritic cell (PDC). The NIPC/PDC is a key cell in both the innate and adaptive immune response but can also, either directly or via produced IFN-alpha, have a pivotal role in autoimmunity. In this review we summarize recent data concerning NIPC/PDC, including their activation, regulation, function and possible role in autoimmune diseases, especially SLE.

Anti-BDCA-4 (neuropilin-1) antibody can suppress virus-induced IFN-alpha production of plasmacytoid dendritic cells

Plasmacytoid dendritic cells (PDC) in human blood are the main source of virus-induced interferon (IFN)-alpha. They exhibit a lineage-negative phenotype but all express BDCA-4, which is homologous to the neuronal receptor neuropilin-1. Specific staining with anti-BDCA-4 antibody is used for positive isolation of PDC from blood by magnetic cells sorting. Here, it is demonstrated that these positively selected PDC showed reduced or completely abolished IFN-alpha release compared to unstained PDC, which were negatively selected by magnetic depletion of lineage-positive blood mononuclear cells. In addition, treatment of these unstained PDC with anti-BDCA-4 mAb also resulted in at least two-fold lower or reduced virus-induced IFN-alpha production. It is shown that the antibody not only affects cell survival or block virus attachment but also reduces IFN-alpha release induced by non-viral CpG oligodeoxynucleotides. In conclusion, data suggest an immunoregulatory role for BDCA-4 on PDC as demonstrated for IFN-alpha response to virus.

Herpes simplex virus type-1 induces IFN-alpha production via Toll-like receptor 9-dependent and -independent pathways

Type I IFN production in response to the DNA virus herpes simplex virus type-1 (HSV-1) is essential in controlling viral replication. We investigated whether plasmacytoid dendritic cells (pDC) were the major tissue source of IFN-alpha, and whether the production of IFN-alpha in response to HSV-1 depended on Toll-like receptor 9 (TLR9). Total spleen cells or bone marrow (BM) cells, or fractions thereof, including highly purified pDC, from WT, TLR9, and MyD88 knockout mice were stimulated with known ligands for TLR9 or active HSV-1. pDC freshly isolated from both spleen and BM were the major source of IFN-alpha in response to oligodeoxynucleotides containing CpG motifs, but in response to HSV-1 the majority of IFN-alpha was produced by other cell types. Moreover, IFN-alpha production by non-pDC was independent of TLR9. The tissue source determined whether pDC responded to HSV-1 in a strictly TLR9-dependent fashion. Freshly isolated BM pDC or pDC derived from culture of BM precursors with FMS-like tyrosine kinase-3 ligand, produced IFN-alpha in the absence of functional TLR9, whereas spleen pDC did not. Heat treatment of HSV-1 abolished maturation and IFN-alpha production from all TLR9-deficient DC but not WT DC. Thus pDC and non-pDC produce IFN-alpha in response to HSV-1 via both TLR9-independent and -dependent pathways.

Systemic lupus erythematosus and the type I interferon system

Patients with systemic lupus erythematosus (SLE) have ongoing interferon-alpha (IFN-alpha) production and serum IFN-alpha levels are correlated with both disease activity and severity. Recent studies of patients with SLE have demonstrated the presence of endogenous IFN-alpha inducers in such individuals, consisting of small immune complexes (ICs) containing IgG and DNA. These ICs act specifically on natural IFN-alpha-producing cells (NIPCs), often termed plasmacytoid dendritic cells (PDCs). Given the fact that the NIPC/PDC has a key role in both the innate and adaptive immune response, as well as the many immunoregulatory effects of IFN-alpha, these observations might be important for the understanding of the etiopathogenesis of SLE. In this review we briefly describe the biology of the type I IFN system, with emphasis on inducers, producing cells (especially NIPCs/PDCs), IFN-alpha actions and target immune cells that might be relevant in SLE. On the basis of this information and results from studies in SLE patients, we propose a hypothesis that explains how NIPCs/PDCs become activated and have a pivotal etiopathogenic role in SLE. This hypothesis also indicates new therapeutic targets in this autoimmune disease.

The natural interferon-alpha producing cells in systemic lupus erythematosus

Prolonged exposure of the immune system to type I interferons (IFN-alpha/beta/omega) in patients receiving IFN-alpha therapy frequently results in development of autoantibodies and autoimmune disease. This is attributed to the many immunostimulatory effects of these cytokines. Patients with the autoimmune disease systemic lupus erythematosus (SLE) have an ongoing IFN-alpha production. Recent studies of SLE demonstrated the presence of endogenous IFN-alpha inducers, acting specifically on natural IFN-alpha producing cells (NIPC), often termed plasmacytoid dendritic cells (PDC). These IFN-alpha inducers were potent, present at the blood level, and characterized as immune complexes that contained DNA and IgG as essential components. They were considered a likely reason for the activated IFN-alpha production in SLE, which, in turn, might be an important etiopathogenic factor. Here, we briefly review the biology of the type I IFN system, with emphasis on inducers, producing cells (especially NIPC/PDC), IFN-alpha actions, and target immune cells, which might be relevant in SLE. Based on such information and results from studies in SLE patients, we propose a hypothesis that explains how NIPC/PDC become activated and play a pivotal etiopathogenic role in SLE and perhaps also other autoimmune diseases. This hypothesis furthermore indicates new therapeutic targets.

The DNA vaccine vector pcDNA3 induces IFN-alpha production in pigs

The cytokine inducing capacity of the vaccine vector pcDNA3, a methylated form of the plasmid, and pcDNA3 encoding porcine interleukin (IL)-6 or granulocyte/macrophage colony-stimulating factor (GM-CSF) was studied in pigs, using a model with tissue chambers implanted subcutaneously. The production of interferon (IFN)-alpha, IFN-gamma, IL-6 and GM-CSF was studied at local (tissue chamber fluid (TCF)) and systemic (serum) levels during 3 days post-injection. All forms of the plasmid, except the methylated, induced a transient local production of IFN-alpha but no plasmid-induced production of IFN-gamma, GM-CSF or IL-6 could be detected after injection of the plasmids. The IFN-alpha response increased markedly at repeated injections of pcDNA3. This IFN-alpha inducing capacity of the plasmid is likely to affect immune responses at DNA vaccination of pigs.

Type I interferon production by plasmacytoid dendritic cells and monocytes is triggered by viruses, but the level of production is controlled by distinct cytokines

The principal interferon-alpha/beta (IFN-I)-producing cells are plasmacytoid dendritic cell (PDC) precursors belonging to the lymphoid lineage. Monocytes that can differentiate into dendritic cells (DC) also produce IFN-I, although much less than PDC, after interaction with infectious agents. We show that whereas viruses trigger these cells to produce IFN-I, the amount of IFN is tightly controlled by cytokines. Monocytes produced IFN-I in response to Sendai virus (SV) infection, and PDC responded to both SV and herpes simplex virus (HSV). All cytokines tested failed to induce production of IFN-I in the absence of infection. However, among 18 relevant cytokines, incubation of PDC with interleukin-4 (IL-4), IL-15, and IL-7 alone or in combination with IL-3 before infection, enhanced IFN-I secretion. At variance, IL-12 alone or in synergy with granulocyte-macrophage colony-stimulating factor (GM-CSF) was active on SV-infected but not on HSV-infected monocytes. Tumor necrosis factor-alpha (TNF-alpha) and IL-4 inhibited IFN-I production by PDC and monocytes, respectively, and IL-10 strongly inhibited IFN-I production in both cell lineages. The response of PDC to IL-7 and IL-15, which also activate natural killer (NK) cell maturation, further emphasizes the cooperation between these two cell subsets in the control of innate immunity.

An etiopathogenic role for the type I IFN system in SLE

The type I interferon (IFN) system plays a pivotal role in the etiopathogenesis of systemic lupus erythematosus (SLE). The initial appearance of autoantibody-producing B cells can be precipitated by infection-induced type I IFNs, but the further, significant generation of autoimmune T and B cells is caused by the prolonged production of IFN-alpha, which is maintained by a vicious circle mechanism. This involves the activation of immature dendritic cells, known as natural IFN-producing cells, by continuously formed endogenous IFN-alpha inducers. These IFN-alpha inducers consist of complexes of autoantibodies with nucleic-acid-containing autoantigens derived from apoptotic cells.

Activation of natural interferon-alpha producing cells by apoptotic U937 cells combined with lupus IgG and its regulation by cytokines

We recently demonstrated that IgG from patients with systemic lupus erythematosus (SLE) in combination with U937 cells made apoptotic by UV-irradiation, can induce interferon-alpha (IFN-alpha) production in normal peripheral blood mononuclear cells (PBMC). In the present study we show by flow cytometry that the actual IFN-alpha producing cells (IPC) among PBMC had the same phenotype (HLA-DR+, CD4+, CD11b-, CD11c-, CD14-, CD19-, CD32-, CD36+, CD40+, CD45RA+, CD68+, CD83+, CD86-, IL-3R+ and IL-10R-) and low frequency (approximately 2/10(4)PBMC) as the IPC activated by Herpes simplex virus type I. Consequently, these cells correspond to the natural IPC, also described as type 2 precursor dendritic cells. We also demonstrated that cytokines of possible importance in the pathogenesis in SLE had effects on the IFN-alpha production. Specifically, the IFN-alpha production was strongly increased by the type I IFNs, IFN-alpha and -beta, but markedly inhibited by IL-10 and also to some extent by TFN-alpha. In contrast, the cytokines IFN-gamma, IL-6, TGF-beta and GM-CSF had no clear effects. No production of IL-10 was detected in PBMC stimulated by apoptotic U937 cells and SLE IgG. These results may explain the cause of the ongoing IFN-alpha production in SLE patients and its relation to the autoimmune process.

The combination of apoptotic U937 cells and lupus IgG is a potent IFN-alpha inducer

Patients with active systemic lupus erythematosus (SLE) have signs of an ongoing IFN-alpha production, that may be of pathogenic significance in the disease. We previously showed that SLE patients have an IFN-alpha-inducing factor in blood, probably consisting of complexes containing anti-DNA Abs and immunostimulatory DNA. The DNA component could be derived from apoptotic cells, because SLE patients have been reported to have both increased apoptosis and reduced clearance of apoptotic cell material. In the present study, we therefore investigated whether apoptotic cells, together with IgG from SLE patients, could act as an IFN-alpha inducer in normal PBMC in vitro. We found that apoptotic cells of the myeloid leukemia cell line U937 as well as four other cell lines (MonoMac6, H9, Jurkat, U266) could induce IFN-alpha production in PBMC when combined with IgG from SLE patients. The IFN-alpha production by PBMC was much enhanced when PBMC were costimulated by IFN-alpha2b. The ability of IgG from different SLE patients to promote IFN-alpha induction by apoptotic U937 cells was associated with the presence of anti-ribonucleoprotein Abs, but not clearly with occurrence of anti-DNA Abs. These results suggest that apoptotic cells in the presence of autoantibodies can cause production of a clearly immunostimulatory cytokine, which is IFN-alpha. This mechanism for induction of IFN-alpha production could well be operative also in vivo, explain the IFN-alpha production seen in SLE patients, and be important in the pathogenesis of SLE.

Anti-Double-Stranded DNA Antibodies and Immunostimulatory Plasmid DNA in Combination Mimic the Endogenous IFN-α Inducer in Systemic Lupus Erythematosus

Patients with systemic lupus erythematosus (SLE) have increased blood levels of IFN-α, which correlate to disease activity. We previously identified an IFN-α-inducing factor (IIF) in the blood of SLE patients that activated the natural IFN-α-producing cells in cultures of normal PBMC. The SLE-IIF contained DNA and IgG, possibly as small immune complexes. In our study, we demonstrated that SLE-IIF correlated to the presence of anti-dsDNA Abs in patients and contained anti-dsDNA Abs as an essential component. Purified anti-DNA Abs or SLE-IgG caused only a weak IFN-α production in cultures of normal PBMC in the presence of costimulatory IFN-α2b. However, they converted the plasmid pcDNA3, which itself induced no IFN-α production in PBMC, into an efficient IFN-α inducer. A human monoclonal anti-ss/dsDNA Ab had the same effect. This IFN-α-inducing activity of the plasmid was abolished by methylation, suggesting that unmethylated CpG DNA motifs were important. Like IIF in SLE serum, the combination of SLE-IgG and pcDNA3 appeared to stimulate IFN-α production in natural IFN-α-producing cells, a unique cell population resembling immature dendritic cells. The IFN-α production was greatly enhanced by IFN-α2b and IFN-β, and for SLE-IIF it was also enhanced by GM-CSF but inhibited by IL-10. We have therefore identified a new function of DNA-anti-DNA Ab complexes, IFN-α induction, that might be important in the pathogenesis of SLE.

Patients with systemic lupus erythematosus (SLE) have a circulating inducer of interferon-alpha (IFN-alpha) production acting on leucocytes resembling immature dendritic cells

Patients with active SLE often have an ongoing production of IFN-alpha. We therefore searched for an endogenous IFN-alpha-inducing factor (IIF) in SLE patients and found that their sera frequently induced production of IFN-alpha in cultures of peripheral blood mononuclear cells (PBMC) from healthy blood donors, especially when the PBMC were costimulated with the cytokines IFN-alpha2b and granulocyte-macrophage colony-stimulating factor (GM-CSF). The phenotype of the IFN-alpha-producing cells (IPC) as determined by flow cytometry corresponded to that of the natural IPC, resembling immature dendritic cells. The IIF activity in SLE sera was sometimes as high as that of a virus and was present especially in patients with active disease and with measurable IFN-alpha levels in serum. The IIF had an apparent molecular weight of 300-1000 kD and appeared to consist of both immunoglobulin and DNA, possibly being immune complexes. This endogenous IFN-alpha inducer may be of pathogenic significance, since a reported occasional adverse effect of IFN-alpha therapy in patients with non-autoimmune disorders is development of anti-dsDNA antibodies and SLE.

Production of interleukin-4, interferon (IFN)-gamma and IFN-alpha in human immunodeficiency virus-1 infection: an imbalance of type 1 and type 2 cytokines may reduce the synthesis of IFN-alpha

Interferon-alpha (IFN-alpha) is an important molecule in the antiviral response, but cells from HIV-1-infected individuals show a reduced ability to secrete IFN-alpha. We investigated an association between an imbalance of type 1/type2 cytokines and the production of IFN-alpha in HIV-1 infection. We used whole blood culture to study the cytokine production profile, interferon-gamma (IFN-gamma) and interleukin-4 (IL-4), in response to HIV-1 antigens and to study the Sendai Virus and HSV-1-induced-production of IFN-alpha in seven HIV-1-infected patients. An impaired synthesis of IFN-alpha was obtained in patients with a predominant IL-4 production (IL-4 > IFN-gamma), and we found a positive correlation between the ex vivo production of IFN-alpha and the IFN-gamma/IL-4 ratio but not with the HIV RNA copy number in plasma. We investigated the role of T-cell-derived cytokines in the in vitro production of IFN-alpha by PBMC from eight healthy donors, activated with Sendai Virus or HSV-1. Whereas type 2 cytokines (IL-4, IL-13) inhibited virus-induced IFN-alpha synthesis, on the contrary, type 1 cytokines (IL-2, IFN-gamma) enhanced it. A disarray in the T-cell-derived cytokine response may play a role in the defect of IFN-alpha production in HIV-1-infected individuals. Further investigations are needed to explore this hypothesis.

The Mannose Receptor Mediates Induction of IFN-α in Peripheral Blood Dendritic Cells by Enveloped RNA and DNA Viruses

Peripheral blood dendritic cells (DC) produce IFN-α in response to challenge by many enveloped viruses including herpes simplex virus (HSV) and HIV, whereas Sendai virus predominantly stimulates IFN-α production by monocytes. Glycosylated viral envelope proteins are known to be important for the induction of IFN-α. In this study we demonstrate that stimulation of IFN-α synthesis by HSV is inhibited by a number of monosaccharides, including fucose, N-acetylglucosamine, and N-acetylgalactosamine as well as the yeast polysaccharide mannan, supporting a role for lectin(s) in the IFN-α stimulation pathway. Furthermore, antiserum to the mannose receptor (MR) also inhibited HSV, vesicular stomatitis virus, and HIV-induced IFN-α production, but failed to inhibit the IFN-α induced by Sendai virus. We further demonstrated that freshly isolated blood DC and IFN-α-producing cells responding to HSV stimulation express the MR. This study therefore implicates the MR as an important receptor for the nonspecific recognition of enveloped viruses by DC and the subsequent stimulation of IFN-α production by these viruses. Thus, the MR probably serves as a critical link between innate and adaptive immunity to viruses, especially given the role of the MR in Ag capture by DC and the importance of IFN-α in shaping immunity.

Exogenous and Endogenous IL-10 Regulate IFN-α Production by Peripheral Blood Mononuclear Cells in Response to Viral Stimulation

IL-10 is an important regulator of the production of proinflammatory cytokines. Its effect on IFN-α production, however, has not been reported. In this study, PBMC from healthy donors were stimulated with virus in the presence of IL-10. Human IL-10 (hIL-10) caused reductions in both the frequency of IFN-α-producing cells (IPC) and bulk IFN in response to herpes simplex virus type-1 (HSV-1), Sendai virus, Newcastle disease virus, and vesicular stomatitis virus. The inhibitory effect occurred when IL-10 was added 2 or 4 h before, or 2 h poststimulation with HSV or Sendai virus, but not when added 4 h postinduction. Unlike IL-10, IL-4 did not affect the IFN-α response to HSV. However, when PBMC were induced with Sendai virus, IFN-α production was also reduced by IL-4. IL-10 treatment of PBMC resulted in strong reductions in the steady state levels of both HSV- and Sendai virus-induced IFN-α1, -α2, and -β mRNA as determined by RT-PCR. IFN-α production to Sendai virus occurs predominantly by monocytes, whereas most enveloped viruses stimulate low frequency “natural IFN-producing cells (NIPC),” which are thought to be dendritic cells. Peripheral blood dendritic cells were found to express the IL-10 receptor, suggesting that IL-10 may directly act on the dendritic IPC. Addition of monoclonal anti-IL-10 to PBMC resulted in a significant increase in both the frequency of IPC and the amount of secreted IFN-α in response to HSV but not Sendai virus. We conclude that human IL-10 can serve as both an endogenous and exogenous regulator of IFN-α production.

Actinobacillus pleuropneumonia serotype 2--effects on the interferon-alpha production of porcine leukocytes in vivo and in vitro

Effects of a bacterial infection on the IFN-alpha production in vivo and in vitro were studied in eight specific pathogen free pigs experimentally infected with Actinobacillus pleuropneumoniae. Clinically, the experimental infection was manifested as a febrile stage which lasted approximately one week and by signs of respiratory disease. The Aujeszky's disease virus (ADV) induced IFN-alpha production, assessed in whole blood cultures, was increased for the infected pigs during the febrile stage. Potentiating effects on the IFN-alpha production could be transferred to cultures of purified peripheral blood mononuclear cells with sera collected from the infected pigs during this period of time. Although the experimental infection with A. pleuropneumoniae did not induce any detectable amounts of IFN-alpha in serum or nasal secretion, both a phenol-extract and a heat-inactivated preparation of the bacteria induced low levels of IFN-alpha in cultures of purified PBMC. The interferogenic structures of the bacteria were not identified but there were indications that the bacteria induced IFN-alpha production in the same cell type as ADV.

Tissue chambers--a useful model for in vivo studies of cytokine production in the pig

An in vivo tissue chamber model was developed to enable studies of local cytokine production and cellular events during inflammatory and immune reactions in the pig. Tissue chambers made of sialistic rubber tubing were surgically implanted in the subcutaneous tissue- and samples of tissue chamber fluid (TCF) and inflammatory cells were collected by aspiration with a syringe. To evaluate the model for local cytokine production, two cytokine inducers, polyribinosinic-polyribocytidylic acid (poly I:C) and fixed Aujeszky's disease virus infected PK15 cells (ADV-PK15), were injected into the tissue chambers and samples of TCF were collected 0, 4, 8, 12, 24 and 48 h post injection. Poly I:C injections induced local production of interferon-alpha (IFN-alpha) as well as tumor necrosis factor (TNF) in the TCF but kinetic differences in the production of the cytokines were noted. Poly I:C also induced an increase in cell numbers in the TCF, mainly due to increased neutrophil numbers. Injections of ADV-PK15 induced local IFN-alpha production in the TCF as long as the pigs were serologically negative to ADV. Immunofluorescence and in situ hybridization techniques could be applied for characterization of TCF cells. Moreover, cells recovered from the tissue chambers were viable and could be used in functional in vitro tests. Taken together, this tissue chamber model could prove very useful in in vivo studies of inflammatory/immune responses and cytokine production in the pig.

Repression of beta interferon gene expression in virus-infected cells is correlated with a poly(A) tail elongation

Expression of beta interferon (IFN-beta) is transiently induced when Namalwa B cells (Burkitt lymphoma cell line) are infected by Sendai virus. In this study, we found that an elongation of the IFN-beta mRNA could be detected in virus-infected cells and that such a modification was not observed when the IFN-beta transcript was induced by a nonviral agent, poly(I-C). Treatment of the cells with a transcriptional inhibitor (actinomycin D or 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole) resulted in further elongation of the transcript. Characterization of the elongated IFN-beta transcript by primer extension and RNase H treatment showed that the modification was a result of an elongated poly(A) tail of up to 400 nucleotides. We conclude that the poly(A) tail elongation of the IFN-beta transcript is associated with the viral infection. Furthermore, the presence of the elongated IFN-beta transcript correlated with a decrease of IFN-beta protein in the medium and in cell extracts. Sucrose gradient analysis of cytoplasmic extracts showed that IFN-beta transcripts with elongated poly(A) tails were found in the nonpolysomal fractions, whereas the shorter transcripts could be detected in both polysomal and nonpolysomal fractions. A longer form of the IFN-beta mRNA was also found in the nonpolysomal fractions of cells not treated with transcriptional inhibitors. Thus, the observed regulation of IFN-beta mRNA is not entirely dependent on the inhibition of transcription. To our knowledge, this study provides the first example of a poly(A) tail elongation in somatic cells that negatively influences gene expression in vivo.

Stimulation of natural interferon-alpha/beta-producing cells by Staphylococcus aureus

Human peripheral blood mononuclear cells (PBMCs) produced high levels of antiviral activity, as determined by bioassay, when stimulated by Staphylococcus aureus Cowan I (SAC) and E. coli. Specific immunoassays demonstrated the presence of both IFN-alpha and gamma and, for SAC, also low levels of IFN-beta. The frequencies of SAC-induced IF N-alpha-producing cells (IPCs) were up to 1-2 per 10(3) PBMCs. These IPCs expressed the HLA-DR and CD4 antigens but not CD3, CD14, or CD19, thus resembling the natural IFN-alpha-producing cells (NIPC). The SAC was more efficient as IFN inducer when heat killed than when streptomycin inhibited. The SAC was inhibitory to virally induced IFN-alpha responses, in particular when streptomycin inhibited. Both pronase treatment and mechanical disruption of SAC cells abolished their capacity to induce IFN-alpha production. Staphylococcal strains lacking or expressing low levels of protein A (SpA) showed a decreased ability to induce IFN-alpha production. However, purified SpA did not itself induce IFN-alpha. Possibly, SpA together with other bacterial surface proteins is important for the capacity of SAC to induce IFN-alpha production in NIPC.

Interferon-alpha generation in mice responding to challenge with UV-inactivated herpes simplex virus

In humans with advanced human immunodeficiency virus (HIV) infection, an interferon-alpha (IFN-alpha) response by a specialized blood mononuclear cell to herpes simplex virus (HSV) in vitro is associated with resistance to opportunistic infections. A cell type of unknown lineage, designated the natural IFN-producing cell (NIPC), has been identified preliminarily as the source of these IFNs and may have a role in other host defense functions. Earlier studies suggested the existence of analogous HSV-responsive cell populations in mice. The role specifically of IFN-alpha in the murine system, however, has not been characterized. Using IFN bioassay and neutralization with antisera against Type I IFNs and IFN-beta, we have defined the types and sources of IFNs produced by mice in response to in vivo and in vitro challenge with UV-inactivated HSV. After intraperitoneal inoculation with HSV, BALB/c and C57Bl/6 strains produced characteristically different levels of serum IFNs that appeared principally to be IFN-alpha. The response of mononuclear cells from these mice differed from that of the intact mouse. Isolated cells from bone marrow and spleen released detectable IFNs much later than did whole animals, and the IFNs produced by marrow, spleen, and peritoneal cells were usually neutralized by the anti-IFN-beta. Only bone marrow cells produced detectable amounts of IFN-alpha. Both intact mice and their cells became refractory to restimulation with similar kinetics.

Human natural interferon-alpha producing cells

Interferons (IFNs) are critical components of the host immune system, serving as antiviral agents, immunomodulators and inhibitors of cell growth. Among peripheral blood mononuclear cells, the primary IFN-alpha-producing cell is a light density, HLA-DR+ cell negative for cell surface markers typical for T cells, B cells, monocytes, natural-killer or progenitor cells and has been tentatively termed the 'natural IFN-producing cell' or NIPC. Although present in very low frequency (approximately 1:1000 among peripheral blood mononuclear cells), the NIPC are very potent, with an individual cell able to produce 1-2 IU of IFN. In this review, the characteristics, phenotype, regulation and relationship of NIPC to human disease are discussed.

The leukocyte function-associated antigen-1 (LFA-1) is involved in the interferon-alpha response induced by herpes simplex virus in blood leukocytes

The role of the leukocyte function-associated antigen-1 (LFA-1) family of integrins (beta 2 integrins) in the interferon-alpha (IFN-alpha) response was examined, using human peripheral blood mononuclear cells (PBMCs) stimulated in vitro by glutaraldehyde-fixed Herpes simplex virus-infected WISH amnion cells. Monoclonal antibodies (mAbs) to the beta 2 chain (CD18) and to the alpha chain of LFA-1 (CD11a) reduced the number of IFN-alpha-producing cells (IPCs) by 30-50%, but mAbs to CD11b or c caused no inhibition. The IB4 mAb to CD18 was inhibitory when added during the first 2 h of the IFN-alpha response, but did not alter its kinetic. In contrast, the IB4 prevented the early enhancement of the IFN-alpha response caused by addition of interleukin-3 (IL-3) or granulocyte-macrophage colony-stimulating factor (GM-CSF). However, a delayed down-regulation of the IPC response occurred in such PBMC cultures, and a paradoxical increase in the total production of IFN-alpha. The results suggest that LFA-1 (CD11a/CD18) participates in the early phase of the IFN-alpha response and may be activated by cytokines such as IL-3 and GM-CSF.

Enrichment of coronavirus-induced interferon-producing blood leukocytes increases the interferon yield per cell: a study with pig leukocytes

Porcine peripheral blood mononuclear cells, which secrete IFN alpha in response to a coronavirus, transmissible gastroenteritis virus, were detected by a filter immunoplaque assay (ELISPOT). IFN alpha-producing cells (IPC), which are present at a low frequency in the blood, could be enriched up to 100-fold by sequential depletion of plastic-adherent cells and cell fractionation on metrizamide density gradients. IPC were present in the non-adherent low-density cell subpopulation. Cell selection experiments using antibody (Ab)-coated immunomagnetic beads revealed that porcine IPC could be positively selected by anti-CD4 or -SLA-class-II Ab, but not by anti-CD2 or -CD8 Ab. The estimated IFN yield per IPC was found to increase when IPC were assayed at higher concentrations. These data suggest that IPC represent a unique and distinct cell population in the blood, which could secrete higher amounts of IFN following its accumulation at a site of viral infection.

Interleukin-4 down-regulates Sendai virus-induced production of interferon-alpha and -beta in human peripheral blood monocytes in vitro

Recombinant interleukin-4 (rIL-4) caused a 65-70% reduction of the interferon-alpha (IFN-alpha) and -beta production induced by Sendai virus (SV) in human peripheral blood monocytes in vitro. Significant inhibition was seen at concentrations as low as 0.1 ng/ml. The rIL-4 also reduced levels of IFN-alpha and -beta mRNA by 60% and 67%, respectively, as well as the frequency of IFN-alpha and -beta mRNA-containing cells by 65% and 54%, respectively. The frequency of IFN-alpha/beta mRNA-containing cells was inhibited by rIL-4 throughout the whole course of induction by SV. IL-4 caused a shift of the grain count distribution towards less heavily labelled cells, suggesting an inhibitory effect of rIL-4 on most IFN-alpha/beta mRNA-containing cells. Antibodies to rIL-4 did not influence the normal IFN-alpha/beta response induced by SV, but abolished the inhibitory effect of the rIL-4. When rIL-4 was added to cells 4 h after start of stimulation by SV, at which time much mRNA has accumulated but little IFN-alpha/beta has been secreted, no inhibition of the IFN-alpha/beta production by rIL-4 was seen. IFN-gamma had only a minor reversing effect on the rIL-4 inhibition, but if cells were precultivated in medium with or without IFN-gamma for 6 h before SV induction, rIL-4 paradoxically enhanced the IFN-alpha/beta response. Our results suggest that rIL-4 inhibits an early step of IFN-alpha/beta induction in monocytes, at the level either of transcription of IFN-alpha/beta genes or of the processing or stability of mRNA. The IL-4 effects may however depend on the state of activation of the monocytes.