Interferon induced by Plasmodium berghei

Evaluation of the antimalarial and CD4+ T-cell modulatory effects of leaf methanol extract of Phyllanthus muellerianus (Kuntze) Exell (Phyllanthaceae) in Plasmodium berghei-infected mice

ETHNOPHARMACOLOGICAL RELEVANCE

Phyllanthus muellerianus (Kunze) Exell, a member of the Phyllanthaceae family, is a medicinal plant widely distributed in Africa. Decoctions from the leaves are used in Nigeria to treat fevers, convulsions, some neurological disorders and malaria.

AIM OF THE STUDY

This study is to evaluate the anti-malarial properties of methanol extract of Phyllanthus muellerianus (MEPM) leaves and its ethyl acetate fraction using a murine malaria model infected with Plasmodium berghei. Additionally, we seek to investigate the potential modulatory effects of this extract and fraction on CD4+ T-cell populations in the context of malaria infection.

MATERIALS AND METHODS

The anti-malarial effects of the leaf methanol extract of Phyllanthus muellerianus (MEPM) were screened using three established in vivo models of anti-plasmodial screening namely the curative, suppressive and prophylactic models. The methanol extract (MEPM) was afterwards fractionated into hexane (HFPM), ethyl acetate (EAFPM), and methanol (MFPM) fractions. In the pilot anti-malarial screening of the fractions, EAFPM exhibited the best antiparasitic activity. Subsequently, EAFPM was screened for anti-malarial activity using the three models above. The effects of the MEPM and EAFPM on haematological indices (Hb and PCV) of the inoculated animals were further screened and the mean survival time (MST) of the animals was monitored. CD4+ T cells of various groups were counted before and after treatment using a flow cytometer. The EAFPM was further subjected to HPLC analysis for identification of its major compounds.

RESULTS

The EAFPM (100 and 200 mg/kg) elicited 88% and 93% cure respectively in the curative model, while artesunate (5 mg/kg,- the positive control) gave 87% protection. The MEPM and EAFPM also gave significant suppression of parasitemia in the suppressive model. The treated groups survived beyond 28 days as against 11 days by the control group (infected but not treated). The treated groups also prevented anaemia seen in the negative control. The EAFPM group significantly modulated the CD4+ T cell. Compounds identified were Gallocatechin, Quercetin -3-O-gallate, Ellagic acid, and Methylellagic acid rhamnoside).

CONCLUSION

The study established that the leaf of Phyllanthus muellerianus possesses antimalarial activity, thus lending support to its use in the folkloric treatment of malaria.

Eosinophils Suppress the Migration of T Cells Into the Brain of Plasmodium berghei-Infected Ifnar1-/- Mice and Protect Them From Experimental Cerebral Malaria

Cerebral malaria is a potentially lethal disease, which is caused by excessive inflammatory responses to Plasmodium parasites. Here we use a newly developed transgenic Plasmodium berghei ANKA (PbA_Ama1OVA) parasite that can be used to study parasite-specific T cell responses. Our present study demonstrates that Ifnar1^-/- mice, which lack type I interferon receptor-dependent signaling, are protected from experimental cerebral malaria (ECM) when infected with this novel parasite. Although CD8⁺ T cell responses generated in the spleen are essential for the development of ECM, we measured comparable parasite-specific cytotoxic T cell responses in ECM-protected Ifnar1^-/- mice and wild type mice suffering from ECM. Importantly, CD8⁺ T cells were increased in the spleens of ECM-protected Ifnar1^-/- mice and the blood-brain-barrier remained intact. This was associated with elevated splenic levels of CCL5, a T cell and eosinophil chemotactic chemokine, which was mainly produced by eosinophils, and an increase in eosinophil numbers. Depletion of eosinophils enhanced CD8⁺ T cell infiltration into the brain and increased ECM induction in PbA_Ama1OVA-infected Ifnar1^-/- mice. However, eosinophil-depletion did not reduce the CD8⁺ T cell population in the spleen or reduce splenic CCL5 concentrations. Our study demonstrates that eosinophils impact CD8⁺ T cell migration and proliferation during PbA_Ama1OVA-infection in Ifnar1^-/- mice and thereby are contributing to the protection from ECM.

Type I Interferons and Malaria: A Double-Edge Sword Against a Complex Parasitic Disease

Type I interferons (IFN-Is) are important cytokines playing critical roles in various infections, autoimmune diseases, and cancer. Studies have also shown that IFN-Is exhibit 'conflicting' roles in malaria parasite infections. Malaria parasites have a complex life cycle with multiple developing stages in two hosts. Both the liver and blood stages of malaria parasites in a vertebrate host stimulate IFN-I responses. IFN-Is have been shown to inhibit liver and blood stage development, to suppress T cell activation and adaptive immune response, and to promote production of proinflammatory cytokines and chemokines in animal models. Different parasite species or strains trigger distinct IFN-I responses. For example, a Plasmodium yoelii strain can stimulate a strong IFN-I response during early infection, whereas its isogenetic strain does not. Host genetic background also greatly influences IFN-I production during malaria infections. Consequently, the effects of IFN-Is on parasitemia and disease symptoms are highly variable depending on the combination of parasite and host species or strains. Toll-like receptor (TLR) 7, TLR9, melanoma differentiation-associated protein 5 (MDA5), and cyclic GMP-AMP synthase (cGAS) coupled with stimulator of interferon genes (STING) are the major receptors for recognizing parasite nucleic acids (RNA/DNA) to trigger IFN-I responses. IFN-I levels in vivo are tightly regulated, and various novel molecules have been identified to regulate IFN-I responses during malaria infections. Here we review the major findings and progress in ligand recognition, signaling pathways, functions, and regulation of IFN-I responses during malaria infections.

T cell-mediated immunity to malaria

Immunity to malaria has been linked to the availability and function of helper CD4⁺ T cells, cytotoxic CD8⁺ T cells and γδ T cells that can respond to both the asymptomatic liver stage and the symptomatic blood stage of Plasmodium sp. infection. These T cell responses are also thought to be modulated by regulatory T cells. However, the precise mechanisms governing the development and function of Plasmodium-specific T cells and their capacity to form tissue-resident and long-lived memory populations are less well understood. The field has arrived at a point where the push for vaccines that exploit T cell-mediated immunity to malaria has made it imperative to define and reconcile the mechanisms that regulate the development and functions of Plasmodium-specific T cells. Here, we review our current understanding of the mechanisms by which T cell subsets orchestrate host resistance to Plasmodium infection on the basis of observational and mechanistic studies in humans, non-human primates and rodent models. We also examine the potential of new experimental strategies and human infection systems to inform a new generation of approaches to harness T cell responses against malaria.

CD4 T-cell subsets in malaria: TH1/TH2 revisited

CD4⁺ T-cells have been shown to play a central role in immune control of infection with Plasmodium parasites. At the erythrocytic stage of infection, IFN-γ production by CD4⁺ T-cells and CD4⁺ T-cell help for the B-cell response are required for control and elimination of infected red blood cells. CD4⁺ T-cells are also important for controlling Plasmodium pre-erythrocytic stages through the activation of parasite-specific CD8⁺ T-cells. However, excessive inflammatory responses triggered by the infection have been shown to drive pathology. Early classical experiments demonstrated a biphasic CD4⁺ T-cell response against erythrocytic stages in mice, in which T helper (Th)1 and antibody-helper CD4⁺ T-cells appear sequentially during a primary infection. While IFN-γ-producing Th1 cells do play a role in controlling acute infections, and they contribute to acute erythrocytic-stage pathology, it became apparent that a classical Th2 response producing IL-4 is not a critical feature of the CD4⁺ T-cell response during the chronic phase of infection. Rather, effective CD4⁺ T-cell help for B-cells, which can occur in the absence of IL-4, is required to control chronic parasitemia. IL-10, important to counterbalance inflammation and associated with protection from inflammatory-mediated severe malaria in both humans and experimental models, was originally considered be produced by CD4⁺ Th2 cells during infection. We review the interpretations of CD4⁺ T-cell responses during Plasmodium infection, proposed under the original Th1/Th2 paradigm, in light of more recent advances, including the identification of multifunctional T-cells such as Th1 cells co-expressing IFN-γ and IL-10, the identification of follicular helper T-cells (Tfh) as the predominant CD4⁺ T helper subset for B-cells, and the recognition of inherent plasticity in the fates of different CD4⁺ T-cells.

Experimental malaria infection triggers early expansion of natural killer cells

In order to gain a better understanding of gene expression during early malaria infection, we conducted microarray analysis of early blood responses in mice infected with erythrocytic-stage Plasmodium chabaudi. Immediately following infection, we observed coordinated and sequential waves of immune responses, with interferon-associated gene transcripts dominating by 16 h postinfection, followed by strong increases in natural killer (NK) cell-associated and major histocompatibility complex class I-related transcripts by 32 h postinfection. We showed by flow cytometry that the observed elevation in NK cell-associated transcripts was the result of a dramatic increase in the proportion of NK cells in the blood during infection. Subsequent microarray analysis of NK cells isolated from the peripheral blood of infected mice revealed a cell proliferation expression signature consistent with the observation that NK cells replicate in response to infection. Early proliferation of NK cells was directly observed in studies with adoptively transferred cells in infected mice. These data indicate that the early response to P. chabaudi infection of the blood is marked by a primary wave of interferon with a subsequent response by NK cells.

Expression microarray analysis implicates apoptosis and interferon-responsive mechanisms in susceptibility to experimental cerebral malaria

Specific local brain responses, influenced by parasite sequestration and host immune system activation, have been implicated in the development of cerebral malaria. This study assessed whole-brain transcriptional responses over the course of experimental cerebral malaria by comparing genetically resistant and susceptible inbred mouse strains infected with Plasmodium berghei ANKA. Computational methods were used to identify differential patterns of gene expression. Overall, genes that showed the most transcriptional activity were differentially expressed in susceptible mice 1 to 2 days before the onset of characteristic symptoms of cerebral malaria. Most of the differentially expressed genes identified were associated with immune-related gene ontology categories. Further analysis to identify interaction networks and to examine patterns of transcriptional regulation within the set of identified genes implicated a central role for both interferon-regulated processes and apoptosis in the pathogenesis of cerebral malaria. Biological relevance of these genes and pathways was confirmed using quantitative RT-PCR and histopathological examination of the brain for apoptosis. The application of computational biology tools to examine systematically the disease progression in cerebral malaria can identify important transcriptional programs activated during its pathogenesis and may serve as a methodological approach to identify novel targets for therapeutic intervention.

The role of type I interferons in non-viral infections

For a long time, the family of type I interferons (IFN-alpha/beta) has received little attention outside the fields of virology and tumor immunology. In recent years, IFN-alpha/beta regained the interest of immunologists, due to the phenotypic and functional characterization of IFN-alpha/beta-producing cells, the definition of novel immunomodulatory functions and signaling pathways of IFN-alpha/beta, and the observation that IFN-alpha/beta not only exerts antiviral effects but is also relevant for the pathogenesis or control of certain bacterial and protozoan infections. This review summarizes the current knowledge on the production and function of IFN-alpha/beta during non-viral infections in vitro and in vivo.

Gamma interferon production in endotoxin-responder and -nonresponder mice during infection

The production of gamma interferon (IFN-gamma) in response to infection and to a number of other agents was compared in Lpsn (C3H/HeN and C57BL/10ScSn) and Lpsd (C3H/HeJ and C57BL/10ScCr) mouse strains. Large differences in IFN-gamma production were observed between C57BL/10ScCr mice and the other mouse strains. With the exception of C57BL/10ScCr, all mouse strains, including C3H/HeJ, exhibited transient levels of IFN-gamma during infection with Salmonella typhimurium. Spleen cells of these mice, explanted on day 3 of infection, produced in vitro IFN-gamma spontaneously; this production was enhanced considerably by heat-killed S. typhimurium, heat-killed Propionibacterium acnes, concanavalin A (ConA), or lipopolysaccharide (LPS). These stimuli, except for LPS, also induced IFN-gamma production in cultures of normal spleen cells from noninfected animals. In contrast, C57BL/10ScCr mice produced no IFN-gamma following infection with S. typhimurium. Also, spleen cells of these mice, explanted on day 3 of infection, exhibited no spontaneous IFN-gamma production. A marginal response was obtained by additional stimulation of the cells with killed S. typhimurium, and a moderate response was obtained with ConA. Normal spleen cells from noninfected C57BL/10ScCr mice showed no IFN-gamma response to killed S. typhimurium, killed P. acnes, or LPS and only a low response to ConA. Impaired IFN-gamma production in C57BL/10ScCr mice was also evident during infection with Plasmodium chabaudi chabaudi, with which a low IFN-gamma response was seen only occasionally. Also, spleen cells from infected animals (days 2 to 8 after infection) exhibited only a very low level of IFN-gamma production in vitro; however, this production could be enhanced further by ConA. In comparison, C57BL/10ScSn mice infected with P. chabaudi chabaudi produced significant amounts of IFN-gamma. Spleen cells explanted from infected animals produced IFN-gamma spontaneously in vitro; this production was enhanced further by killed P. acnes and ConA. The results showed that in addition to the defect in LPS responsiveness, C57BL/10ScCr mice possess a defect in IFN-gamma production in response to different stimuli. During infection, IFN-gamma production and sensitization to LPS occurred in parallel. Infected Lpsn mice exhibited enhanced sensitivity and infected Lpsd C3H/HeJ mice exhibited reasonable sensitivity to the lethal effects of LPS. Lpsd C57BL/10ScCr mice remained resistant to LPS when infected with S. typhimurium and exhibited only marginal sensitivity when infected with P. chabaudi chabaudi.

Role of gamma interferon during infection with Plasmodium chabaudi chabaudi

A role has been proposed for inflammatory mediators such as gamma interferon (IFN-gamma) and reactive oxygen intermediates in the control of the blood stages of Plasmodium organisms. It was previously shown that IFN-gamma can be detected in the plasma of mice with a primary infection by Plasmodium chabaudi chabaudi (AS). We found that susceptible and other resistant mouse strains produced IFN-gamma, suggesting that susceptibility is not due to a defect in IFN-gamma production. Administration of IFN-gamma to intact C57BL/6 mice slightly decreased and partially delayed parasitemia, whereas in vivo depletion of IFN-gamma through injection of a "cocktail" of monoclonal antibodies against IFN-gamma exacerbated infection. Since CD4+ T cells are essential for the development of a protective immune response to P. chabaudi chabaudi, we tested whether CD4+ T cells are responsible for IFN-gamma production in vivo and whether exogenous IFN-gamma can replace the protective function of the CD4+ T cells. Mice depleted of CD4+ T cells were unable to produce IFN-gamma, but factors in addition to IFN-gamma may be important in parasite clearance.

Production of interferon-gamma during infection of mice with Plasmodium chabaudi chabaudi

An ELISA assay, designed to detect interferon-gamma (IFN-gamma) in the picogram range, was used to study the presence of IFN-gamma in serum and its production by T cells taken from C57BL/6 mice infected with Plasmodium chabaudi chabaudi. IFN-gamma was detectable in mouse plasma for two to three days before the peak of parasitaemia. Similarly, IFN-gamma production by T cells could be detected in vitro. In limiting dilution cultures, the production of IFN-gamma by as few as 1,000 T cells was detectable using this assay. The limiting dilution analysis revealed that a substantial IFN-gamma response by specific T cells occurs very early in a primary infection with P. chabaudi.

Reducing effects of rodent malaria on hepatic carcinogenesis induced by dietary aflatoxin B1

The present study was undertaken to evaluate the effects of Plasmodium berghei infection on the development of liver tumors induced in male Buffalo rats by aflatoxin B1 (AFB1). Intraperitoneal (i.p.) injection of 10(6) parasitized red blood cells (pRBC) into the rat 12 days prior to administration of 2 ppm dietary AFB1 for 10 weeks diminished hepatocellular carcinoma (HCC) induction compared to that observed in rats given AFB1 alone at weeks 60-82. No animals in a control group developed HCC lesions, while only 1 of 22 rats treated with P. berghei alone developed a neoplastic nodule at week 82. These data suggest a reducing effect of P. berghei on the development of liver tumors induced by AFB1 in male Buffalo rats.

Strain differences of interferon-generating capacity and resistance in toxoplasma-infected mice

To explore a possible correlation between susceptibility to Toxoplasma and interferon (IFN)-generating capacity in mice, we compared the levels of serum IFN induced by stimulation with Toxoplasma lysate antigen (TLA) in different strains of Toxoplasma-infected and uninfected mice. Injection of TLA into five strains of mice with chronic Toxoplasma infection resulted in the release of considerable amounts of IFN into the circulation. Most of these IFN activities were acid labile and not neutralized by sheep antiserum against mouse IFN-alpha/beta, indicating that IFN-gamma was the dominant form produced in this system. In contrast, the majority of IFN induced in uninfected mice was characterized as IFN-alpha/beta by their acid stability and antigenicity. The response of IFN production in Toxoplasma-infected and uninfected mice varied quantitatively depending on the mouse strains examined. C57BL/6 mice were found to be the best producers of both IFN-alpha/beta and IFN-gamma, while BALB/c mice were consistently poor producers of both IFN populations. A/J, DBA/2, and C3H/He mice could be roughly classified as intermediate producers of both IFN populations. C57BL/6 and C3H/He mice showed a significant prolongation of mean survival time following primary or secondary infection with Toxoplasma compared to that of BALB/c mice. However, there was no direct correlation between the susceptibility to Toxoplasma and the levels of serum IFN.

T-cell-mediated immune response in murine malaria: differential effects of antigen-specific Lyt T-cell subsets in recovery from Plasmodium yoelii infection in normal and T-cell-deficient mice

For analysis of the role of immune T cells in protective immunity against murine malaria, Plasmodium yoelii-immune Lyt T-cell subsets were functionally characterized in vitro and in vivo. Selected Lyt2- and Lyt2+ T cells from P. yoelii-immune C57BL/10 mice differed in their capability to proliferate in response to P. yoelii antigen in vitro. Only the Lyt2- T-cell population produced T-cell growth factor upon restimulation, and none of the selected T-cell subsets produced detectable amounts of macrophage activating factor. Lyt2- but not Lyt2+ lymphocytes were capable of transferring protection to normal C57BL/10 mice. When transferred into T-cell-deficient C57BL/6-nu/nu mice, adoptive resistance to P. yoelii by Lyt2- lymphocytes was only demonstrable after prior reconstitution of recipients with normal T cells. These results suggest an interaction between P. yoelii-immune Lyt2- T cells and normal T lymphocytes via T-cell growth factor in the development of protective immunity to malaria.

The interferon compartment of the immune response in human malaria: II. Presence of serum-interferon gamma following the acute attack

The present study concerns the monitoring of serum-interferon (serum-IFN) levels among 189 patients followed after and sometimes during an acute episode of malaria due mainly to Plasmodium falciparum (P. falciparum). Of these patients, 110 known to have no other parasitic or infectious disease were followed in France; 79 were from Thailand, among which 25 cases of neuromalaria were diagnosed. In a first four-month survey conducted in France, among 100 patients seen after the acute attack, serum-IFN-gamma was characterized among 87% cases for which at least two sera were controlled, whereas in a healthy population no serum-IFN was present. When efforts were concentrated on screening ten cases during the first 48 h of the febrile attack, serum-IFN-alpha was mainly characterized, whereas serum-IFN-gamma was present only once. Elevated leukocyte 2',5' oligoadenylate synthetase levels were found among several IFN-alpha positive patients of this study group. A peculiarity pertaining to the patients from Thailand was that one-third (25 cases) were cerebral malaria cases. Among these, 15 were followed under hospitalization during the first 96 h. In this study group, the onset of circulating immune interferon was found to be preceded or accompanied by that of IFN-alpha. Thus, if serum-IFN-gamma is largely characterized among malaria patients followed after the acute attack, it is possible that the onset of circulating immune interferon is generally preceded by that of IFN-alpha.

Antimalarial immunity in Saimiri monkeys. Immunization with surface components of asexual blood stages

Plasmodium falciparum polypeptides of 200 and 140 K mol wt exposed at the surface of merozoites and/or schizonts were purified by affinity chromatography and by electroelution from sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Monkeys were separated into three groups of four and immunized either with one of the two polypeptides or with saline (control). After intravenous challenge with 2.5 X 10(7) P. falciparum asexual blood stages, two monkeys of the control group had to be treated and two recovered spontaneously after peak parasitemia of 9 and 11%. The four monkeys immunized with the 140 K polypeptide recovered without treatment after peak parasitemia between 1.5 and 4.5%. Monkeys immunized with the 200 K polypeptide had similar peak parasitemia except one monkey who suffered from a large skin excoriation and who recovered spontaneously after a peak parasitemia of 11%. Prechallenge sera of the immunized monkeys reacted only with the polypeptide used for immunization except for one serum of the 140 K group, which precipitated an additional polypeptide of 39 K, and a polypeptide of 31 K weakly precipitated by the four sera of monkeys immunized with the 200 K polypeptide. The relatedness between the 200 and 140 K polypeptides was investigated using tryptic digestion and reverse phase chromatography. No clear analogy was found between the two polypeptides, which suggests that immunization with either of two independent surface components of P. falciparum asexual blood stages is able to induce at least a partial protective immunity in immunized hosts.

Interferons as macrophage-activating factors. II. Enhanced secretion of interleukin 1 by lipopolysaccharide-stimulated human monocytes

Human adherent monocytes were incubated with interferon (IFN) preparations, then washed and stimulated with endotoxins. The interleukin (IL1) activity in the supernatants of IFN-treated monocyte cultures was found to be increased as compared to control (not pretreated) cultures. This phenomenon was observed whether alpha or beta IFN was used, and was completely abolished by antiserum to the relevant IFN. IL1 secretion of IFN-treated monocyte cultures could be triggered by suboptimal dosage of endotoxins, unable to induce any IL1 activity in control medium-treated cultures. IFN was highly efficient in this system, since very low concentrations (of the order of 2 units) were effective. Since IFN alone, without endotoxin stimulation, was unable to induce IL1 secretion, the results indicate that IFN is able to activate monocytes, by inducing a potential secretory capable rather than an ongoing IL1 secretion function. The ability of IFN to enhance the IL1 secretory potential of human monocytes may be relevant to the known immunoenhancing effects of IFN preparations and to the pyrogenic effects of IFN administration in patients.

Cell-mediated killing of protozoa

Cell-mediated immunity represents an important host defence mechanism against protozoal infections. The effector cells directly involved are neutrophils, macrophages and, ultimately, activated macrophages. Within this simple scheme there are, however, considerable variations in activity. Effector cells from different animal species, and even from different strains of the same species, may be more or less effective in controlling a certain protozoal infection. Different protozoa differ in their susceptibility to cell-mediated killing according to genus, species, strain and morphological form. The most susceptible morphological form is that which occurs in the insect vector, and which has not yet adapted to protect itself from the vertebrate host. Epimastigotes of Trypanosoma and promastigotes of Leishmania are readily killed by phagocytic cells, while the corresponding trypomastigote and amastigote forms are considerably more resistant. Protozoa which live in macrophages, such as amastigotes of Leishmania, endozoites (tachyzoites) of Toxoplasma and amastigotes of reticulotropic strains of T. cruzi, have developed a remarkable resistance to the microbicidal activity of the host cell. Conversely, amastigotes of myotropic strains of T. cruzi, which live in muscle cells, have not developed this resistance to cell-mediated killing by macrophages. Readily accessible protozoa, such as T. brucei trypomastigotes and Plasmodium merozoites in the bloodstream, while they lack the marked resistance developed by reticulotropic protozoa, have a partial protection since they are attacked by phagocytic cells only when specific antibody is present. Granulocyte-mediated killing can be largely attributed to neutrophils. Eosinophils appear to play only a minor role and compete ineffectually when neutrophils are also present. The only group of protozoal species which may be significantly controlled by eosinophils are the stercorarian species of Trypanosoma. In vitro experiments show that antibody-coated trypomastigotes of T. cruzi can be killed by eosinophils, although there is little evidence that this occurs in vivo. Interestingly, this is the only species that has been reported to be susceptible to the major basic protein of eosinophils, a toxic component of the lysosomal granules which is very active against helminths. Neutrophils are not very active against endozoites of Toxoplasma gondii, Trypanosoma, trypomastigotes of salivarian Trypanosoma, free merozoites of Plasmodium, and promastigotes and amastigotes of Leishmania.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhanced resistance to acute infection with Trypanosoma cruzi in mice treated with an interferon inducer

For an exploration of the effects of interferon-inducible resistance mechanisms in acute American trypanosomiasis, the synthetic interferon inducer tilerone hydrochloride was administered to mice of the C57BL/6J strain, which is highly resistant to Trypanosoma cruzi, 18 to 24 h before infection with a potentially lethal dose of bloodstream trypomastigotes. Although all of the control mice died within 30 days of the acute infection, approximately 50% of the tilerone-treated animals were able to survive indefinitely (P less than 0.05). The tilerone-treated mice demonstrated significant levels of serum interferon and splenic natural killer cells at the time of infection. Macrophages isolated from the peritoneal cavities of tilerone-treated C57BL/6J mice appeared to kill significant numbers of trypanosomes during 2 to 3 days of in vitro culture, indicating that activated macrophages may contribute to the enhanced resistance to T. cruzi infection in these mice. Beige mice treated with tilerone did not survive T. cruzi infection as well as tilerone-treated heterozygotes did, suggesting a role for natural killer cells in interferon-induced resistance. These results suggest that interferon or effector mechanisms enhanced by interferon induction can play a significant role in influencing resistance to T. cruzi infection.

The effects of selective immunosuppression on resistance to Mesocestoides corti in strains of mice showing high and low initial susceptibility

Previous work has shown that C57BL/6 mice had the lowest initial susceptibility to Mesocestoides corti of six strains of mice examined. Parasite burdens in this strain and in CBA/H mice, a strain showing a higher initial susceptibility to M. corti, were compared following selective immunosuppressive treatments. Irradiation, splenectomy and the administration of cyclophosphamide and methyl prednisolone all resulted in higher parasite burdens in C57BL/6 mice. In contrast these treatments had a minimal effect on parasite burdens in CBA/H mice. In the light of these results the role of antibody in controlling parasite proliferation is discussed.

Differences in susceptibility of various mouse strains to haemoprotozoan infections: possible correlation with natural killer activity

Striking differences in the susceptibility to P. chabaudi and B. microti infections among different strains of mice are described. In most of the strains, both parasites induce a moderate, transient parasitaemia, followed by recovery. However, in the A strain P. chabaudi is rapidly lethal and B. microti induces a parasitaemia which persists for life in most animals. In contrast B10.A mice, which have the same H-2q haplotype as the A strain, recover from both infections. In first crosses (A X B10.A)F1 mice are uniformly resistant. Among other correlates, NK cells activity may be relevant. This activity is known to be high in strain of mice which are resistant to both of the parasites studied, such as C57B1 and CBA mice, and low in the susceptible A strain. We have examined the spleen weight, total cell number and NK activity at different times after infection. In the resistant strains there is a rapid increase in size and cell number which is still more marked during recovery. NK activity increases greatly during the infection; this is especially remarkable considering the dilution of nucleated cells with erythrocytes precursors. In contrast, in A mice cell numbers of NK activity remain almost unchanged. Thus, marked activation of NK cells occur, in resistant strains but not in susceptible ones. Possible mechanisms of activation of these cells and their effect in haemoprotozoan infections are discussed.

Protection of mice against Babesia microti with cord factor, COAM, zymosan, glucan, Salmonella and Listeria

Cord factor (trehalose 6-6' dimycolate). COAM (chlorite-oxidized oxyamylose), zymosan, glucan, Salmonella enteritidis 11RX and Listeria monocytogenes were found to protect mice against subsequent infection with Babesia microti, an intra-erythrocytic protozoan parasite. This protection was not observed after injection of Staphylococcus epidermidis, a viridans group Streptococcus, thioglycollate, or colloidal carbon. All the agents which protect against B. microti have also been reported to induce non-specific protection against experimental tumours. The parasites appear to die inside circulating red cells. This implies that these can exert non-specific protection against this parasite through the mediation of a soluble factor.

Viral interference and interferon

Viral interference is a phenomenon for which a cell infected by a virus becomes resistant toward a second outcoming infection by a superinfectant virus. Even though other mechanisms are known, it can be assumed that most cases of viral interference occurring in natural conditions are mediated by interferon, a low molecular weight protein produced by the infected cell in response to a stimulus provided by viral nucleic acid(s). The interferon produced by a cell can migrate to other cells not yet involved by the spreading infection, transmitting to them the antiviral-resistant state. Available evidence indicates that interferon acts by inducing the production of a second cellular protein, called antiviral protein, which is directly responsible for the antiviral state through some alterations of the cellular, virus-directed, proteosynthetic system. In addition to the antiviral activity, the interferon system can affect the growth of several nonviral organisms and that of tumour cells; rather controversial effects have been shown also on the immune responses; the mechanisms underlying these effects are still nuclear. However a relationship to the specific immune system is suggested also by the finding that interferon can be liberated by sensitized T-lymphocytes following antigenic stimulus. Activation of the interferon system can be operated in vitro and in vivo also by several non-viral substances of various nature, such as nucleic acids, polysaccharides, aromatic amines, etc. This fact, considering that interferon has been shown to play a critical role on the mechanisms of recovery from viral infections, may open new perspectives for their possible prophylactic and/or therapeutic use in viral diseases. This problem can be approached also by administering exogenous interferon. Encouraging preliminary results have so far been obtained either with interferon or its inducers. However, several problems of various nature have to be resolved before considering the actual use of interferon system as a wide range antiviral drug in natural viral diseases of man.

Antiviral activity and induction of interferon-like substance by quinacrine and acranil

Several drugs with certain structural similarities (tricyclic ring system with dialkylaminoalkyl side chains) to tilorone, a potent interferon inducer, were screened for antiviral activity in vivo. Two acridine drugs, Acranil and quinacrine, were found to be effective, the former being almost as protective as tilorone and the latter less so. Both agents induced an interferon-like substance which could be detected in the serum of treated mice. The concentration of the inhibitory factor in the serum was highest after exposure to tilorone, followed in turn by Acranil and quinacrine, based on the administration of equal weights of drugs. Both tilorone and Acranil induced lower levels of circulating interferon-like substance in Balb/c mice than in other strains of mice. The serum factor induced by Acranil was shown to be stable at pH 2.

Immunity to coccidiosis: protective effect of transferred serum in Eimeria maxima infections

Serum taken between 2 and 3 weeks after infection of the donors with E. maxima conferred a considerable degree of protection against infection with the homologous organism when injected daily into young recipients. Different pools of such sera had differing activities, but the majority gave protection of 50% or more whereas a few were inactive.

The degree of protection obtained was dose-dependent, varying directly with the volume of serum injected and inversely with the size of the challenge oocyst inoculum.

A course of injections started early in the life-cycle had a greater effect than those started in mid- or late cycle even when comparatively large volumes of serum were given.

Protection was species specific; serum obtained from birds infected with E. maxima did not protect against E. praecox or E. acervulina.

The assistance of Mrs Patricia Hesketh is gratefully acknowledged, as is the help of other members of the staff of the Parasitology Department. Mr P. Townsend, Senior Poultry Attendant, supervised the cleaning and sterilization of cages and the collection of faecal samples.