L-arginine-dependent destruction of intrahepatic malaria parasites in response to tumor necrosis factor and/or interleukin 6 stimulation

Multiplication of Listeria monocytogenes in a murine hepatocyte cell line

Listeria monocytogenes was shown to invade and multiply in a murine hepatocyte cell line (ATCC TIB73). Hemolytic and nonhemolytic L. monocytogenes strains exhibited similar abilities to invade hepatocytes, but only hemolytic L. monocytogenes multiplied within this cell line. Microscopic evaluation of monolayers stained with Wright stain demonstrated focal necrosis (plaques) in the hepatocyte monolayers, with large numbers of intracellular listeriae visible within the hepatocytes that lined the margins of these plaques. Murine recombinant interleukin-1 alpha, human recombinant tumor necrosis factor alpha, and murine recombinant gamma interferon did not affect the multiplication of L. monocytogenes in the hepatocytes. These data confirm in vivo observations of the intracellular multiplication of L. monocytogenes in hepatic lesions in infected mice.

The role of TH1 and TH2 cells in a rodent malaria infection

CD4+ T cells play a major role in protective immunity against the blood stage of malaria, but the mechanism of protection is unclear. By adoptive transfer of cloned T cell lines, direct evidence is provided that both TH1 and TH2 subsets of CD4+ T cells can protect mice against Plasmodium chabaudi chabaudi infection. TH1 cells protect by a nitric oxide-dependent mechanism, whereas TH2 cells protect by the enhancement and accelerated production of specific immunoglobulin G1 antibody.

Kupffer cell elimination enhances development of liver schizonts of Plasmodium berghei in rats

We investigated the development of exoerythrocytic forms (EEF) of Plasmodium berghei in livers of normal and macrophage-depleted Brown Norway rats. Macrophages were depleted by use of liposome-encapsulated dichloromethylene diphosphonate. Upon inoculation of sporozoites, macrophage-depleted rats had significantly larger numbers of EEF than untreated rats. We also investigated the effect of macrophage impairment by silica treatment on the development of EEF and confirmed that silica induces a significant reduction of EEF development. Intravenous administration of silica induced high levels of interleukin-6 in plasma within a few hours. The seemingly contradictory results for EEF development may be explained by our previous observation that interleukin-6 strongly inhibits sporozoite penetration and EEF development in vivo. We conclude that in experimental infections with sporozoites, Kupffer cells inhibit rather than enhance EEF development.

In vivo induction of the nitric oxide pathway in hepatocytes after injection with irradiated malaria sporozoites, malaria blood parasites or adjuvants

The mechanisms responsible for malarial immunity induced by repetitive injections of X-irradiated sporozoites have not been fully established. We demonstrate here that a single injection of irradiated sporozoites induced, as soon as 24 h after, a non-permissive state to hepatocyte reinfection with sporozoites in vitro. The same effect was observed when malarial blood forms, irradiated promastigotes of Leishmania infantum, adjuvants (muramyl dipeptide, poly acidylic uridylic) or interferon-gamma was injected. Activation of the nitric oxide (NO) pathway in the hepatocyte by these factors was found to be responsible for hepatocyte refractory status. Additionally, this metabolic pathway is involved in protection given by repeated injections of irradiated sporozoites since protection could be reversed by treating mice at the time of sporozoite challenge with a competitive inhibitor (NG-monomethyl-L-arginine) of the NO pathway. These results suggest that, in view of an antisporozoite vaccine, further studies are needed to find out how to activate specifically a long-lasting nonspecific immune response.

Neutralization of gamma interferon and tumor necrosis factor alpha blocks in vivo synthesis of nitrogen oxides from L-arginine and protection against Francisella tularensis infection in Mycobacterium bovis BCG-treated mice

Peritoneal cells from Mycobacterium bovis BCG-infected C3H/HeN mice produced nitrite (NO2-, an oxidative end product of nitric oxide [NO] synthesis) and inhibited the growth of Francisella tularensis, a facultative intracellular bacterium. Both NO2- production and inhibition of bacterial growth were suppressed by NG-monomethyl-L-arginine, a substrate inhibitor of nitrogen oxidation of L-arginine, and monoclonal antibodies (MAbs) to gamma interferon (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha). Intraperitoneal injection of mice with BCG increased urinary nitrate (NO3-) excretion coincident with development of activated macrophages capable of secreting nitrogen oxides and inhibiting F. tularensis growth in vitro. Eight days after BCG inoculation, mice survived a normally lethal intraperitoneal challenge with F. tularensis. Treatment of these BCG-infected mice with MAbs to IFN-gamma or TNF-alpha at the time of BCG inoculation reduced urinary NO3- levels to those found in normal uninfected mice for up to 14 days. The same anticytokine antibody treatment abolished BCG-mediated protection against F. tularensis: mice died within 4 to 6 days. Intraperitoneal administration of anti-IFN-gamma or anti-TNF-alpha antibody 8 days after BCG infection also reduced urinary NO3- and abolished protection against F. tularensis. Isotype control (immunoglobulin G) or anti-interleukin 4 MAbs had little effect on these parameters at any time of treatment. IFN-gamma and TNF-alpha were clearly involved in the regulation of macrophage activation by BCG in vivo. Protection against F. tularensis challenge by BCG depended upon the physiological generation of reactive nitrogen oxides induced by these cytokines.

Gamma interferon-induced nitric oxide production reduces Chlamydia trachomatis infectivity in McCoy cells

McCoy cells, murine-derived cells commonly used for propagation of chlamydiae, were found to be efficient producers of nitric oxide (NO) when primed with murine gamma interferon (IFN-gamma) and then exposed to the second signals provided by Escherichia coli lipopolysaccharide, human interleukin-1 alpha, murine tumor necrosis factor alpha, or Chlamydia trachomatis type H. Murine recombinant IFN-gamma over a range of 0 to 50 U/ml inhibited infectivity of C. trachomatis type H in a dose-dependent fashion in McCoy cells while simultaneously inducing NO production. Quantitation of infectious chlamydia progeny remaining in McCoy cells 48 or 72 h postinfection revealed that IFN-gamma-primed McCoy cells reduced chlamydial inclusion-forming units (expressed as units per milliliter) by 4 log10 units at higher IFN-gamma concentrations (50 U/ml) compared with control values. The magnitude of this antichlamydial effect was directly related to increased synthesis of NO, the production of which was IFN-gamma dose dependent. The antichlamydial effects of IFN-gamma were blocked in a dose-dependent manner by the addition of N-guanidino-monomethyl L-arginine (MLA), an inhibitor of nitric oxide synthesis. These results suggest that although IFN-gamma priming of McCoy cells is required for antichlamydial activity, nitric oxide is a necessary effector molecule involved in the mechanism(s) of IFN-gamma-induced inhibition of chlamydial proliferation in this murine cell line. The ability to block the potent antichlamydial effects of IFN-gamma by inhibition of a specific enzyme, nitric oxide synthase, may give insights into mechanisms by which IFN-gamma and perhaps other cytokines are able to control proliferation of chlamydiae and other intracellular pathogens.

Cytokines, endotoxin, and glucocorticoids regulate the expression of inducible nitric oxide synthase in hepatocytes

Nitric oxide (NO.) is a short-lived mediator which can be induced in a variety of cell types and produces many physiologic and metabolic changes in target cells. The inducible or high-output NO. synthase (NOS) pathway was first characterized in macrophages activated by lipopolysaccharide (LPS) and interferon gamma (IFN-gamma). Hepatocytes also express an inducible NOS following exposure to the combination of endotoxin (LPS) and tumor necrosis factor (TNF), interleukin 1 (IL-1), and IFN-gamma. In this study, to identify which of these cytokines, if any, was acting to induce the gene expression for hepatocyte NOS, we measured the levels of rat hepatocyte NOS mRNA by Northern blot analysis after stimulation by various combinations of endotoxin and cytokines in vitro. We found the mRNA for hepatocyte NOS to be a single band at approximately 4.5 kilobases which was maximally up-regulated (approximately 70-fold) by the combination of TNF, IL-1, IFN-gamma, and LPS. Abundance of NOS mRNA peaked 6-8 hr after stimulation and then declined by 25% at 24 hr. Unstimulated hepatocytes in vitro showed only a trace mRNA band after prolonged autoradiographic exposure. As single agents, TNF and IL-1 were the most effective inducers of hepatocyte NOS mRNA. Combinations of two or three stimuli revealed strong synergy between TNF, IL-1, and IFN-gamma. The increased mRNA levels correlated with elevated nitrogen oxide release and cGMP levels in the culture supernatants. Dexamethasone and cycloheximide inhibited induction of mRNA for hepatocyte NOS in a dose-dependent fashion. The addition of NG-monomethyl-L-arginine had no effect on mRNA levels but effectively blocked NO. formation. The inducible hepatocyte NOS mRNA was also detected in rat hepatocytes following chronic hepatic inflammation triggered by Corynebacterium parvum injection in vivo. These data demonstrate that the inducible NOS is functional in rat hepatocytes both in vitro and in vivo and that this pathway is under complex control. Endotoxin and inflammatory cytokines act synergistically to up-regulate gene expression for hepatocyte NOS, whereas glucocorticoids down-regulate the mRNA.

The role of reactive nitrogen intermediates in modulation of gametocyte infectivity of rodent malaria parasites

Direct feeding of Anopheles stephensi mosquitoes on mice infected with Plasmodium vinckei petteri showed that, during the periods of schizogony in the blood, the infectivity of gametocytes was markedly reduced. This could be prevented by prior injection of the L-arginine analogue, Nw-nitro-L-arginine (NwNLA) showing that the altered infectivity was due to reactive nitrogen intermediates (RNI). Similar effects on transmission of P. yoelii nigeriensis were demonstrated in vitro by membrane feeding of the mosquitoes. The in vitro reduction in infectivity could be reversed by injecting the L-arginine analogue either into the infected mouse donor of serum, or into the membrane feeding chamber. Elevated levels of TNF and IL-6 were demonstrated during the course of infection but did not correlate well with nitrogen radical activity. Similarly, direct measurements of NO2- and NO3- did not reflect the nitrogen radical activity revealed by addition of the specific L-arginine analogue.

Enhancers of nonspecific immunity induce nitric oxide synthase: induction does not correlate with toxicity or adjuvancy

A range of bacterial products and related synthetic compounds, either alone or in combination, enhance nonspecific resistance to infection and tumors. These compounds, which vary in their other properties such as pyrogenicity, toxicity and adjuvancy, were used to assess the hypothesis that nonspecific resistance is mediated by induction of the L-arginine: nitric oxide (NO) pathway. The results obtained show that agents which stimulate nonspecific immunity, such as endotoxin, muramyl dipeptide (MDP) and combinations of monophosphoryl lipid A (MPL) with either trehalose dimycolate (TDM) or MDP cause a substantial induction of Ca(2+)-independent NO synthase in murine lung. In contrast, agents which do not stimulate nonspecific resistance, such as either MPL or TDM alone or threonyl MDP (ThrMDP), do not induce NO synthase. This difference in the ability of MDP and ThrMDP to induce NO synthase in the lung in vivo was also manifest in peritoneal macrophages in vivo as well as being evident in the greater than 100-fold greater potency of MDP in inducing the enzyme in vitro in lung slices. In contrast to the good correlation between induction of NO synthase and induction of nonspecific resistance, no correlation was observed with either the toxic effects of these agents or their adjuvancy.

Cytokines inhibit the development of liver schizonts of the malaria parasite Plasmodium berghei in vivo

The effect of induction of an acute-phase response and its mediators on the development of liver schizonts of the rodent malaria parasite Plasmodium berghei was investigated in Brown Norway rats. Subcutaneous injection of turpentine oil 24 h or 5 min before inoculation of sporozoites resulted in 80% and 35% reduction of schizont development, respectively. Turpentine oil induced high plasma levels of interleukin-6 (IL-6). Intraperitoneal administration of IL-1, IL-6 or both, significantly reduced liver schizont development. This reduction was also present if IL-6 had been administered 24 h after sporozoite inoculation. Inhibition induced by IL-1 could be prevented by simultaneous administration of polyclonal anti-IL-6. Administration of polyclonal anti-IL-6 without IL-1 resulted in a 40% increase of liver schizonts compared to control animals. We conclude that induction of an acute-phase response during experimental Plasmodium berghei infections in Brown Norway rats, strongly inhibits liver schizont development and that IL-6 is a key mediator in this process.

Serum cytokine profiles in experimental human malaria. Relationship to protection and disease course after challenge

Serum cytokine profiles were evaluated in immunized and nonimmunized human volunteers after challenge with infectious Plasmodium falciparum sporozoites. Three volunteers had been immunized with x-irradiated sporozoites and were fully protected from infection. Four nonimmune volunteers all developed symptomatic infection at which time they were treated. Sera from all volunteers were collected at approximately 20 time points during the 28-d challenge period; levels of IL-1 alpha, IL-1 beta, IL-2, IFN-gamma, tumor necrosis factor-alpha, IL-4, IL-6, granulocyte macrophage-colony-stimulating factor, and soluble CD4, CD8, and IL-2 receptor (sCD4, sCD8, and sIL-2R, respectively) were determined by ELISA. C-reactive protein (CRP) was assayed by radial immunodiffusion. Parasitemic subjects developed increases in CRP and IFN-gamma, with less marked increases in sIL-2R and sCD8; the other cytokines tested did not change. CRP increases were abrupt and occurred at the onset of fever (day 14 after challenge). IFN-gamma increases were also abrupt, preceding those of fever and CRP by one day. Increases in sIL-2R and sCD8 were more gradual. Increases in fever, CRP, IFN-gamma, and sCD8 were concordant in each volunteer. Early IL-6 increases were noted in the protected vaccinees. Thus, after challenge with virulent P. falciparum, unique systemic cytokine profiles were detectable both in immunized, nonparasitemic volunteers and in unvaccinated, parasitemic subjects. The contrasting cytokine profiles in the two groups may relate to mechanisms of protection and immunopathology in experimental human malaria.

Malaria: becoming more specific about non-specific immunity

For the hundreds of millions of people presently infected with malaria, survival may depend on relatively non-specific immune effector mechanisms. Progress has been made in understanding the anti-parasitic properties of tumor necrosis factor-alpha, interferon-gamma and nitric oxide, in defining the parasite toxins that induce tumor necrosis factor-alpha production, and in exploring the role of cytokines and adhesion molecules in the pathogenesis of cerebral malaria.

Activation of human macrophages for the killing of intracellular Trypanosoma cruzi by TNF-alpha and IFN-gamma through a nitric oxide-dependent mechanism

The protozoan parasite Trypanosoma cruzi is able to replicate in the cytoplasm of primary resident macrophages, but is killed by activated macrophages. Pretreatment of human macrophages with recombinant IFN-gamma and to a lesser extent with TNF-alpha, induced a significant trypanocidal activity. Furthermore, TNF-alpha had a synergistic effect with IFN-gamma on macrophage activation in T. cruzi killing. Similarly, IFN-gamma triggered the production of nitric oxide (NO) by macrophages, whereas TNF-alpha was less effective, although it was also synergistic with IFN-gamma. Both NO production and trypanocidal activity, but not superoxide (O2-) generation, induced in macrophages by TNF-alpha or IFN-gamma alone or in combination, were inhibited by N-monomethyl-L-arginine (N-MMLA), a competitive inhibitor of NO synthase activity. Furthermore, a strong correlation was found between the levels of NO production and trypanocidal activity induced by different lymphokine preparations. These results suggest that IFN-gamma and TNF-alpha are involved in the activation of the trypanocidal activity of human macrophages through a NO-dependent mechanism.

Synergism between tumor necrosis factor-alpha and interferon-gamma on macrophage activation for the killing of intracellular Trypanosoma cruzi through a nitric oxide-dependent mechanism

Intracellular replication of the protozoan parasite Trypanosoma cruzi inside macrophages is essential for the production of the disease and the development of the parasite. Two CD4+ T cell lines, A10 and A28, were established from T. cruzi-infected BALB/c mice which specifically proliferated to parasite antigens. The trypanocidal activity of BALB/c macrophages was induced upon culture with the A10, but not with the A28 T cell line. The cell-free supernatant from this A10 line, as well as from immune spleen cells stimulated with specific antigen or concanavalin A, but not from the A28 T cell line also activated the trypanocidal activity of peritoneal macrophages or of the J774 macrophage-like cell line. when the lymphokine content of the supernatants from both cell lines was analyzed, it was found that the A10 T cell line secreted interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha and interleukin 2, whereas the A28 line did not secrete IFN-gamma upon stimulation. Furthermore, the trypanocidal-inducing ability of A10 supernatant was completely abrogated by neutralizing anti-IFN-gamma antibodies and partially abrogated by neutralizing anti-TNF-alpha antibodies. When recombinant cytokines were added to J774 cells, IFN-gamma was able to induce significant trypanocidal activity whereas TNF-alpha was almost ineffective. However, TNF-alpha or lipopolysaccharide (LPS) showed a synergistic effect with IFN-gamma on macrophage activation. IFN-gamma triggered nitric oxide (NO) synthesis by J774 cells whereas TNF-alpha was almost ineffective. TNF-alpha and LPS were also synergistic with IFN-gamma in the NO production. Both the NO production and the trypanocidal activity in J774 cells induced by T cell supernatants or lymphokine combinations were inhibited by N-monomethyl-L-arginine, a competitive inhibitor of NO synthase activity. A good correlation between the levels of NO production and trypanocidal activity induced by different lymphokine preparations was found. Those results suggest that IFN-gamma and TNF-alpha, secreted by T. cruzi-immune T cells, are involved in the activation of the trypanocidal activity of mouse macrophages through an NO-dependent mechanism.

Iron depletion: a defense against intracellular infection and neoplasia

Macrophages and other host cells activated by interferon-gamma (IFN-gamma) can be induced to form a flavoprotein that converts L-arginine to nitric oxide+L-citrulline. Nitric oxide causes efflux of non-heme iron from neoplastic and infected host cells. In the absence of L-arginine, IFN-gamma-induced infected cells can lower their net uptake of iron. Cellular depletion of the metal via either mechanism suppresses DNA synthesis as well as the functioning of aerobic respiratory enzymes. Macrophage regulation of growth of other host cells during embryogenesis, immune responses, or immunosurveillance might involve iron depletion.

Immunomodulatory agents in the laboratory and clinic

This paper reviews naturally occurring and synthetic compounds that either enhance immune defences or lower both natural and acquired immunity. Immunomodulatory agents used both for laboratory study and clinically for the management of immunologically based diseases are considered.

Killing of Plasmodium falciparum in vitro by nitric oxide derivatives

We have investigated the in vitro susceptibility of the human malaria parasite Plasmodium falciparum to killing by nitric oxide and related molecules. A saturated solution of nitric oxide did not inhibit parasite growth, but two oxidation products of nitric oxide (nitrite and nitrate ions) were toxic to the parasite in millimolar concentrations. Nitrosothiol derivatives of cysteine and glutathione were found to be about a thousand times more active (50% growth inhibitory concentration, approximately 40 microM) than nitrite.

Mechanisms involved in mycobacterial growth inhibition by gamma interferon-activated bone marrow macrophages: role of reactive nitrogen intermediates

Murine bone marrow-derived macrophages are able to inhibit the growth of Mycobacterium bovis after stimulation with recombinant gamma interferon. This antimycobacterial activity was inhibited by NG-monomethyl-L-arginine, a specific inhibitor of nitrite and nitrate synthesis from L-arginine. Furthermore, there was a complete lack of mycobacterial growth inhibition in a medium deficient in L-arginine. Nitrite is generated by gamma interferon-activated bone marrow-derived macrophages after infection with M. bovis, and a correlation between mycobacterial growth inhibition and nitrite production was observed. These results indicate that reactive nitrogen intermediates derived from L-arginine are crucially involved in macrophage antimycobacterial activity.

The implications for malaria vaccine programs if memory T cells from non-exposed humans can respond to malaria antigens

Although the goal of current candidate vaccines is to expand a population of malaria antigen-specific lymphocytes, accumulating evidence suggests that peripheral blood of adult humans contains significant numbers of malaria-specific T cells prior to any exposure to vaccine or actual infection. The reason why such naive humans are susceptible to malaria infection may thus relate not to inadequate T-cell surveillance but to some other factor--possibly lack of suitable splenic modification. It is possible that current vaccine programs are misdirected because these other factors are not being addressed. The possibility of an attenuated vaccine should be re-examined.