Selective alteration of the turnover of interferon beta mRNA in peritoneal macrophages from LPS-hyporesponsive mice and its role in the defective expression of spontaneous interferon

Role of endogenous interferon and LPS in the immunomodulatory effects of bovine lactoferrin in murine peritoneal macrophages

Lactoferrin (Lf) plays an important role in host defense against infection and excessive inflammation. Although the mechanisms underlying its immunomodulatory properties have not been fully elucidated yet, recent evidence suggests that some of these effects may be related to its capacity to form complexes with LPS. We report that the culture of resting mouse peritoneal macrophages (PM) with bovine Lf (bLf), prior to infection with the vesicular stomatitis virus (VSV), resulted in a significant reduction of virus yield with respect to control cultures. The antiviral activity of bLF was related to its capacity of inducing IFN-alpha/beta expression, which in turn inhibited VSV replication. Indeed, the accumulation of IFN-beta but not of IFNalpha(1-2) transcripts was up-modulated markedly early after bLf addition. Furthermore, bLf did not exert any antiviral activity in the presence of neutralizing antibodies to IFN-alpha/beta in PM from wild-type mice, as well as in PM from mice genetically defective for the response to IFN. The antiviral activity of bLf relied on its intrinsic capacity to bind LPS, as this protein did not induce IFN expression in PM from LPS-hyporesponsive mice. It is interesting that this LPS-binding property was dispensable for the production of TNF-alpha, which also occurred in LPS-hyporesponsive mice. Overall, these results indicate that some of the immunomodulatory effects ascribed to Lf may be related to its capacity to favor Type I IFN expression and argue in favor of an important role of the LPS-binding feature and TLR4 in some of the effects ascribed to this molecule.

Regulation of chemokine mRNA stability by lipopolysaccharide and IL-10

IL-10 has been reported to inhibit the expression of LPS-induced proinflammatory cytokines and chemokines by altering the rate of specific mRNA decay although the molecular target(s) for its action remain unknown. In the present study, using primary peritoneal exudate macrophages and a cell culture model in which a tetracycline-responsive promoter controls transcription of CXC ligand 1 (KC) mRNA, we demonstrate that LPS promotes a time-dependent increase in KC mRNA stability. Although IL-10 had no direct effect on mRNA decay, this treatment antagonized the stabilizing action of LPS. The mechanisms involved were further explored using a cell-free mRNA degradation system. A 5'-capped, polyadenylated in vitro transcript derived from the 3'-untranslated region of KC mRNA exhibited time-dependent decay in the presence of protein extracts prepared from untreated RAW264.7 macrophages. Extracts prepared from LPS-treated RAW264.7 cells had reduced decay activity and this change was antagonized if the cells were costimulated with IL-10. A substrate in which the AU-rich element motifs were mutated exhibited minimal decay that did not vary using extracts prepared from cells treated with LPS or LPS and IL-10. A nonadenylated RNA substrate was also degraded and that activity was diminished by LPS. In concert, these findings demonstrate that KC mRNA stability is regulated by LPS-induced alterations in activities that govern both deadenylation and degradation of the mRNA body. The effects of IL-10 on KC mRNA stability reflect antagonism of the response to LPS.

Inhibition of the constitutive and induced IFN-beta production by IL-4 and IL-10 in murine peritoneal macrophages

We had previously reported that freshly harvested peritoneal macrophages (PM) are in a type I IFN-mediated antiviral state, which is lost during in vitro culture of PM, concomitantly with a progressive decline in the expression of IFN-beta. We report herein that in vitro culture of PM in the presence of IL-4 or IL-10 results in an enhanced decay of the IFN-beta-mediated antiviral state to vesicular stomatitis virus (VSV). Moreover, IL-4 and IL-10 inhibited the production of type I IFN induced by LPS or NDV infection, as assessed by IFN production and induction of IFN-mediated antiviral state. The accumulation and physiological turnover of IFN-beta mRNA was not affected by IL-4 or IL-10. Finally, neither IL-10 nor IL-4 exerted any inhibitory effect on the antiviral activity induced by exogenous type-I IFN. These results suggest that Th2 cytokines, such as IL-4 and IL-10, act as negative regulators of the type I IFN-mediated antiviral response in PM and may represent stop signals for the constitutive or induced type I IFN expression in PM.

Inhibitory activity of constitutive nitric oxide on the expression of alpha/beta interferon genes in murine peritoneal macrophages

We investigated the role of the constitutive nitric oxide (NO) in the expression of interferon (IFN) genes in mouse peritoneal macrophages (PM). The treatment of PM with L-arginine-N(G)-amine (AA), a potent inhibitor of NO-producing enzymes, resulted in a marked accumulation of IFN-alpha4 mRNA and, to a minor extent, of IFN-beta mRNA. In contrast, the expression of IFN-gamma mRNA, as well as tumor necrosis factor alpha and interleukin-6 mRNA, was not affected. Furthermore, a remarkable increase in the expression of the IFN regulating factor 1 (IRF-1), but not of IRF-2, mRNA was detected in AA-treated PM. To investigate whether the AA-induced activation of the IFN system correlates with the production and antiviral activity of IFN, the extent of encephalomyocarditis virus (EMCV) replication was monitored in AA-treated PM with respect to control cultures. AA treatment strongly inhibited, in a dose-dependent manner, EMCV yields in PM. Likewise, similar results were obtained by the addition of the NO-scavenger carboxyphenyl-tetramethylimidazoline-oxyl-oxide. In addition, inhibition of NO synthesis by N(G)-mono-methyl-L-arginine in PM strongly decreased virus replication in coculture of PM and EMCV-infected L929 cells, whereas no antiviral effect was observed in L929 cells alone. Moreover, the AA-mediated antiviral activity was abrogated in the presence of antibody to IFN-alpha/beta, whereas antibody to IFN-gamma was completely ineffective. Taken together, these results indicate that low levels of NO, constitutively released by resting PM, negatively regulate the expression and activity of IFN-alpha/beta in PM. We suggest that NO acts as a homeostatic agent in the regulation of IFN pathway expression in macrophages.

IFN-gamma expression in macrophages and its possible biological significance

IFN-gamma is a pleiotropic cytokine endowed with potent immunomodulatory effects whose expression was long considered to be restricted to T and NK cells. Only recently, it became evident that IFN-gamma production can also occur in other cell types, including monocyte/macrophages. However, the biological relevance of macrophage IFN-gamma is still unclear. The purpose of this article is to provide an overview of the collected evidence demonstrating IFN-gamma expression in macrophages and to discuss the possible biological significance of this cytokine production in the early phase of host response to infectious agents.

Role of interferons and other cytokines in the regulation of the immune response

Cytokines represent the major factors involved in the communication between T cells, macrophages and other immune cells in the course of an immune response to antigens and infectious agents. A number of studies on mouse and human T helper (Th) clones have recently provided extensive evidence for the existence of different activities exhibited by Th cells (called Th1 and Th2), which was apparently inferred from the profile of cytokine secretion. The Th1-type immune response is generally associated with IgG2a production and the development of cellular immunity, the Th2-type response with IgE production, eosinophils and mast cell production. This review focuses on the role of different cytokines produced by macrophages (especially interferons (IFNs), TNF-alpha, IL-10 and IL-12) or T cells (IFNs, IL-2, IL-4, IL-10, IL-13 and TGF-beta) in macrophage-T cell interactions and the cytokine relevance in the differentiation of Th cells towards the Th1 or Th2 type of immune response. Th1-derived cytokines (IFN-gamma, IL-2, TNF-alpha) favor macrophage activation, whereas the Th2 cytokines (IL-4, IL-10, IL-13) exhibit suppressive activities on macrophage functions. A key role in the differentiation towards the Th1-type response is now attributed to IL-12, a recently described cytokine produced mainly by macrophages. Its production can be upregulated by IFN-gamma and is inhibited by IL-10 and IL-4. All this emphasizes the importance of macrophage-cytokine interactions in determining the type of immune response. This article also aims to review recent data concerning the roles of IFNs alpha/beta (type I) and IFN-gamma (type II) in the regulation of the immune response. While there is much information on the regulatory effects of IFN-gamma (also called "immune IFN") on the immune response, little is so far known of the role of type I IFNs. These cytokines, originally described as simple antiviral substances, are now taken to be important regulators of the immune response. Recent data indicate that these molecules (especially IFNs-alpha) specifically promote the differentiation towards the Th1-type response. The stimulatory effects of IFN-alpha on the generation of the Th1-type response may be involved in its therapeutic effects in some human diseases, including early AIDS, hypereosinophilia and certain tumors.(ABSTRACT TRUNCATED AT 400 WORDS)

Interferon gamma upregulates its own gene expression in mouse peritoneal macrophages

Interferon gamma (IFN-gamma) exerts a variety of immunoregulatory effects on several cell targets. It is generally assumed that IFN-gamma is specifically produced by T and large granular lymphocytes. In this study, we show that IFN-gamma is constitutively expressed in resting mouse peritoneal macrophages (PM). Treatment of PM with cycloheximide results in a significant accumulation of IFN-gamma mRNA, suggesting that a short-lived IFN-gamma mRNA accumulates when protein synthesis is inhibited. Moreover, treatment of PM with IFN-gamma also results in a clear-cut accumulation of this mRNA. This effect is not observed in murine lymphocytes from mesenteric lymph nodes (which instead produce IFN-gamma after phytohemagglutinin treatment) and in mouse cell lines. The treatment of PM with IFN-gamma also results in secretion of IFN-gamma after 24-48 h. The upregulation of IFN-gamma expression is also found in PM from anti-asialo GM1-treated nude mice. We suggest that the ability of PM to produce this IFN-gamma is indicative of an autocrine mechanism. The macrophage IFN-gamma may play a role in the regulation of cell differentiation and immune response.

Induction of cytoplasmic factors that bind to the 3' AU-rich region of human interferon beta mRNA during early development of Xenopus laevis

Certain endogenous Xenopus mRNAs, carrying a destabilizing 3' AU-rich sequence, are unusually very stable in oocytes and become unstable only after fertilization. In addition, heterologous short lived mRNA, containing 3' AU-rich sequences, appear to be very stable when injected into Xenopus oocytes. In the present study, a human interferon beta (hu-IFN beta) mRNA, carrying the destabilizing 3' AU-rich element, was used as a probe to identify Xenopus proteins that specifically bind to the 3' AU-rich element as well as to study their relative levels during early embryonic development. While three major proteins that specifically bind to the 3' AU-rich element were detected in human SV80 cells, that naturally express hu-IFN beta (proteins termed AU-F1, F2 and F3), only two proteins, migrating similarly to the SV80 AU-F1 and AU-F3, were detected in cytoplasmic extracts from Xenopus oocytes or eggs. Following fertilization, the intensity of the Xenopus AU-F1 and AU-F3 proteins increased considerably and a new protein, corresponding to SV80 AU-F2, was also detected. Cyclohexamide applied either at the morula or at the early blastula stages reduced the intensity of the AU-binding factors, while actinomycin D did not, indicating that the levels of these factors during these stages are regulated posttranscriptionally. In contrast, application of each of these metabolic inhibitors at the late blastula stage increased the intensity of the AU-binding proteins. The possible function of these AU-binding factors in regulating the expression and half life of AU-rich mRNAs is discussed.