Decitabine Enhances Lymphocyte Migration and Function and Synergizes with CTLA-4 Blockade in a Murine Ovarian Cancer Model.

The lack of second-line treatment for relapsed ovarian cancer necessitates the development of improved combination therapies. Targeted therapy and immunotherapy each confer clinical benefit, albeit limited as monotherapies. Ovarian cancer is not particularly responsive to immune checkpoint blockade, so combination with a complementary therapy may be beneficial. Recent studies have revealed that a DNA methyl transferase inhibitor, azacytidine, alters expression of immunoregulatory genes in ovarian cancer. In this study, the antitumor effects of a related DNA methyl transferase inhibitor, decitabine (DAC), were demonstrated in a syngeneic murine ovarian cancer model. Low-dose DAC treatment increases the expression of chemokines that recruit NK cells and CD8(+) T cells, promotes their production of IFNγ and TNFα, and extends the survival of mice bearing subcutaneous or orthotopic tumors. While neither DAC nor immune checkpoint blockade confers durable responses as a monotherapy in this model, the efficacy of anti-CTLA-4 was potentiated by combination with DAC. This combination promotes differentiation of naïve T cells into effector T cells and prolongs cytotoxic lymphocyte responses as well as mouse survival. These results suggest that this combination therapy may be worthy of further consideration for improved treatment of drug-resistant ovarian cancer.

Anti-inflammatory effect of triterpene 3ß, 6ß, 16ß-trihydroxylup-20(29)-ene obtained from Combretum leprosum Mart & Eich in mice.

ETHNOPHARMACOLOGICAL RELEVANCE: The 3ß, 6ß, 16ß-trihydroxylup-20(29)-ene (TTHL) is a pentacyclic triterpene obtained from a medicinal plant named Combretum leprosum. In folk medicine, this plant is used to treat several diseases associated with inflammation and pain. We previously demonstrated that TTHL presents a significant antinociceptive effect, suggesting the involvement of the glutamatergic system. AIM OF THE STUDY: This study was designed to investigate the effect of TTHL on nociception and vascular permeability induced by acetic acid. We also evaluated the effect of TTHL on carrageenan-induced peritonitis and the levels of cytokines (interleukin 1-ß [IL-1ß], tumor necrosis factor α [TNF-α] and interleukin 10 [IL-10]) on peritoneal fluid. MATERIALS AND METHODS: TTHL was administered orally by intra-gastric gavage (i.g.) 60 min prior to experimentation. Abdominal contractions and vascular permeability were induced by an intraperitoneal (i.p.) injection of acetic acid (0.6%). We also investigated whether TTHL decreases carrageenan-induced peritonitis (750 µg/cavity) by measuring leukocyte migration and vascular permeability. In addition, we evaluated the effects of TTHL on TNF-α, IL-1ß and IL-10 release induced by carrageenan on peritoneal fluid. The levels of these cytokines were measured by ELISA. RESULTS: TTHL (0.01-10 mg/kg) administered by intra-gastric (i.g.) gavage inhibited (69±3%) acetic acid-induced abdominal constrictions, with an ID50 of 0.15 (0.03-0.8) mg/kg. TTHL (10mg/kg) also reduced the leukocyte infiltration induced by acetic acid, with an inhibition of 59±9 but had no effect on abdominal vascular permeability. In addition, indomethacin (10 mg/kg, i.p.) reduced the nociceptive behavior (92±1%), total leukocyte migration (29±3%) and capillary permeability (71±3%) induced by acetic acid. While the glucocorticoid dexamethasone (2 mg/kg, s.c.) reduced partially but significantly the nociception (31±1%), besides to promote a marked reduction on total leukocyte migration (60±2%) to the peritoneal cavity caused by acetic acid. In a model of peritonitis induced by carrageenan, TTHL also reduced total leukocyte migration, mainly neutrophils (inhibition of 84±3% and 85±2% at 30 mg/kg and 100 mg/kg, respectively). Likewise, dexamethasone (0.5 mg/kg, i.p.) resulted in an inhibition of 93±3%. Nevertheless, carrageenan-induced abdominal vascular permeability was reduced by dexamethasone but was not altered by TTHL. Furthermore, dexamethasone and TTHL significantly reduced the TNF-α and IL-1ß levels in peritoneal fluid, whereas the IL-10 levels were unchanged. CONCLUSIONS: Altogether, our data confirm the antinociceptive effect of TTHL and demonstrate its effect in inflammatory animal models, providing novel data about this compound, which could be useful as an anti-inflammatory drug.

Effects of parenteral glutamine supplementation on modulating the immune response in rats undergoing a total gastrectomy.

The present study investigated the effect of parenteral glutamine (Gln) supplementation on cellular adhesion molecule expression and release of chemokines responsible for inflammatory cell recruitment in rats undergoing a total gastrectomy. Normal rats with internal jugular catheters were assigned to one control group and two experimental groups and received total parenteral nutrition (TPN). A total gastrectomy was performed in the experimental groups, whereas the control group received a sham operation (Sham). The TPN solutions were isonitrogenous and identical in nutrient composition except that the Sham group and one of the experimental group received conventional (Conv) TPN solution, whereas the other experimental group received 25% of the amino acid nitrogen as Gln. Half of the rats in each group were killed 1 or 3 d after surgery or the Sham to examine their immune response. The results showed that the surgery produced higher polymorphonuclear leucocyte CD11b/CD18 expressions, and Gln supplementation lowered CD11b/CD18 expressions compared with the Conv group post-operatively. The levels of monocyte chemotactic protein-1 and macrophage inflammatory protein-2 in peritoneal lavage fluid were higher in the Gln group than those in the Conv group 1 d post-operatively; these chemotactic proteins had returned to the levels comparable with those in the Sham group on post-operative day 3. These results suggest that Gln supplementation attenuated polymorphonuclear leucocyte integrin expression. In addition, Gln-enriched parenteral nutrition induced an earlier more intensive and rapid immune response to injury than the Conv parenteral nutrition after a total gastrectomy.

Rapid appearance of resolvin precursors in inflammatory exudates: novel mechanisms in resolution.

Resolution of inflammation is essential. Although supplementation of omega-3 fatty acids is widely used, their availability at sites of inflammation is not known. To this end, a multidisciplinary approach was taken to determine the relationship of circulating omega-3 to inflammatory exudates and the generation of resolution signals. In this study, we monitored resolvin precursors in evolving exudates, which initially paralleled increases in edema and infiltrating neutrophils. We also prepared novel microfluidic chambers to capture neutrophils from a drop of blood within minutes that permitted single-cell monitoring. In these, docosahexaenoic acid-derived resolvin D1 rapidly stopped neutrophil migration, whereas precursor docosahexaenoic acid did not. In second organ injury via ischemia-reperfusion, resolvin metabolically stable analogues were potent organ protectors reducing neutrophils. Together, these results indicate that circulating omega-3 fatty acids rapidly appear in inflammatory sites that require conversion to resolvins that control excessive neutrophil infiltration, protect organs, and foster resolution.

Dietary alpha-linked galacto-oligosaccharide suppresses ovalbumin-induced allergic peritonitis in BALB/c mice.

To determine whether alpha-linked galacto-oligosaccharide (alpha-GOS) prevents allergic peritonitis, BALB/c mice were fed a synthetic diet with and without alpha-GOS supplementation for 7 d, and were then subcutaneously immunized with ovalbumin on days 0 and 7. The mice were challenged by intraperitoneal injection with ovalbumin on day 14, followed by peritoneal lavage on day 15. The total number of peritoneal exudate cells was significantly lower in the mice fed the alpha-GOS diet than in those fed the control diet. Peritoneal lavage fluid from mice fed the alpha-GOS diet not only had less potency to attract peripheral blood leukocytes and peritoneal exudate cells ex vivo, but also had lower concentrations of monocyte chemoattractant protein-1 (MCP-1) and eotaxin. Preincubation of the cells with alpha-GOS failed to affect the migration to peritoneal lavage fluid. We propose that dietary alpha-GOS reduces cell infiltration in allergic peritonitis by reducing antigen-induced elicitation of MCP-1 and eotaxin in mice.

Translational control of inducible nitric oxide synthase by IL-13 and arginine availability in inflammatory macrophages.

Inducible NO synthase (iNOS) and its generation of NO from L-arginine are subject to transcriptional as well as posttranscriptional control by cytokines. In this study, we describe a novel, translational mechanism of iNOS regulation by arginine availability. Using mouse inflammatory peritoneal macrophages stimulated with IFN-gamma plus LPS, we demonstrate that the suppression of iNOS protein, which is observed after a 16-h (but not after a 6-h) pretreatment with IL-13, despite an unaltered iNOS mRNA level, results from arginine depletion by arginase. The addition of arginase inhibitors (in the pretreatment phase) or of arginine (in the stimulation phase) completely blocked the down-regulation of iNOS protein by IL-13. The rescuing effect of arginine supplementation was not due to a positive feedback regulation of iNOS expression via enhanced production of NO. A striking suppression of iNOS protein (but not of iNOS mRNA) was also seen, when IL-13 was replaced by purified arginase or when macrophages were stimulated with IFN-gamma/LPS in arginine-free medium. Arginine deficiency specifically impaired the de novo synthesis and the stability of iNOS protein, but did not affect the production of TNF and the overall protein synthesis of the macrophages. From these results, we conclude that arginine not only functions as a substrate for iNOS, but is also critical for maintaining normal levels of iNOS protein in cytokine-stimulated macrophages.

Immunostimulating activity of the hot water-soluble polysaccharide extracts of Anacyclus pyrethrum, Alpinia galanga and Citrullus colocynthis.

Hot water polysaccharide extracts of Anacyclus pyrethrum (L.) Link. (family Compositae) Citrullus colocynthis (L.) Schrad. (family Cucurbitaceae) and Alpinia galanga (L.) Willd. (family Zingiberaceae) were tested for their immunostimulating activity in mice. The fractions from Anacyclus pyrethrum and Alpinia galanga showed a marked stimulating effect on the reticulo-endothelial system (RES) and increased the number of peritoneal exudate cells (PEC), and spleen cells of mice. In this case, the optimum doses were 50 and 25 mg/kg for the two fractions, respectively. On the other hand, the polysaccharide extracts of both Anacyclus pyrethrum and Alpinia galanga markedly enhanced the proliferation of the murine spleen cells in vitro using two tests (in vitro and in vivo effect). The results of the in vivo effect at a doses of 50 and 25 mg/kg, showed a stimulation index better than obtained with the in vitro effect at 50 and 25 microg/ml for Anacyclus pyrethrum and Alpinia galanga, respectively. While the extract of Citrullus colocynthis showed much weaker and variable immunostimulating activity.

IL-5 production by NK cells contributes to eosinophil infiltration in a mouse model of allergic inflammation.

IL-5 production in vivo plays a unique role in the production, activation, and localization of eosinophils in a variety of allergic conditions. The current paradigm suggests that allergen-specific Th2 cells are the main source for the IL-5 production. The experiments outlined in this work, however, suggest that in vivo production of IL-5 by NK cells can separately influence eosinophil-associated inflammatory responses. Specifically, a mouse model of allergic inflammation was used in which C57BL/6 mice were immunized and challenged with a short ragweed Ag extract, known to induce a selective eosinophilia within the peritoneal cavity. Peritoneal lavage fluids from these mice also contained increased numbers of T cells and NK cells, as well as significantly elevated levels of IL-4, IL-5, and IFN-gamma. Flow-cytometric analysis of cytokine-producing cells in peritoneal lavage fluid revealed increased numbers of IL-5-producing cells in both T cell and NK cell populations following allergen exposure. Depletion of NK cells by treatment with NK1.1 Abs selectively reduced the number of infiltrating eosinophils by more than 50%. Moreover, the inhibition of the infiltration of eosinophils was accompanied by a complete loss of IL-5-producing NK cells and significantly reduced levels of peritoneal lavage fluid IL-5, whereas the number of IL-5-producing T cells was not affected. Thus, the results presented in this study provide clear evidence for a novel immunoregulatory function of NK cells in vivo, promoting allergen-induced eosinophilic inflammatory responses by the production of IL-5.

A complex of histamine/mouse gamma-globulin preferentially inhibits allergen-induced peritoneal accumulation of eosinophils, but not neutrophils, in mice.

A complex of histamine/human gamma-globulin (HhG) has been widely used in Japan for more than 25 years as a nonspecific hyposensitization drug in the treatment of allergic diseases. It has been reported that HhG decreases the number of eosinophils in the nasal secretions and peripheral blood of patients with allergy. In this study we used a mouse system to explore the possibility that HhG may actively inhibit the accumulation of eosinophils at inflammation sites. A complex of 0.15 microg of histamine dihydrochloride/12 mg of mouse gamma-globulin (HmG) was incubated for 2 hours in saline solution in the normal fashion for HhG. HmG at 50 to 150 mg/kg/day inhibited the peritoneal accumulation of eosinophils induced by ragweed pollen in BALB/c mice in a dose-dependent fashion when the HmG was administered subcutaneously six times during a 20-day sensitization period. The inhibitory effect of HmG on this eosinophil accumulation was significant at 24 and 48 hours after challenge, but HmG had no effect on neutrophil accumulation. Complexes of serotonin/mouse gamma-globulin (mgammaG), glutamine/mgammaG, and histamine dihydrochloride (His)/mouse albumin had no inhibitory effect when administered in the same way. The optimum combination ratio was between 0.15 microg of His/12 mg of mgammaG and 0.015 microg of His/12 mg of mgammaG for this eosinophil inhibition. Moreover, a 1- to 2-hour incubation period of His and mgammaG was needed to induce a plateau inhibition of the eosinophil accumulation. These results in mice suggest that HhG may actively inhibit allergen-induced eosinophil accumulation, which may be therapeutically useful in the treatment of allergic disease.

Soluble CD46 (membrane cofactor protein, MCP) in human reproductive tract fluids.

CD46 (membrane cofactor protein, MCP) is a cell surface complement regulatory protein which may have an additional role in human sperm-egg interaction. A soluble form (sCD46) has also been detected in a number of biological fluids, most notably seminal plasma. The present study has employed a monoclonal antibody-based ELISA to assay sCD46 in reproductive tract fluids in normal and pathological conditions. Large amounts of sCD46 were detected in seminal plasma of both fertile and infertile men (combined mean, 4859 ng/ml). Vasectomized men had lower levels (mean, 2421 ng/ml), indicating contributory sources both before and after the vas deferens ligation site. Pre-colostrum also contained relatively high quantities (mean, 445 ng/ml), whereas breast milk (mean, 117 ng/ml), peritoneal fluid (mean, 154 ng/ml) and follicular fluid (mean, 107 ng/ml), as well as uterine (mean, 208 ng/ml), umbilical (mean, 166 ng/ml) and peripheral (mean, 206 ng/ml) blood plasma, had sCD46 levels within a comparable range. Amniotic fluid had low sCD46 concentrations (mean, 22 ng/ml). In endometriosis, peritoneal fluid levels of sCD46 were significantly raised (mean, 199 mg/ml). These results indicate distinctive fluid compartmentalisation of sCD46 consistent with a biological function in human reproductive tract fluids.

Fish oil increases the release of tumour necrosis factor and interleukin-6, and has no effect on the incidence of multiple organ failure in rats with peritonitis.

OBJECTIVE: To find out if fish oil given intraperitoneally would cause a reduction in the release of tumour necrosis factor and interleukin-6 in abdominal exudate and blood (experiment A), and if it reduces the incidence of organ failure in rats with peritonitis (experiment B). DESIGN: Laboratory experiment. SETTING: University animal laboratory. MATERIAL: Thirty-six selectively decontaminated rats in each experiment. INTERVENTIONS: All rats were pretreated with 2 ml fish oil, lecithin, or saline, intraperitoneally for one or six weeks before intraperitoneal injection of zymosan. Experiment A: Samples of abdominal exudate and plasma were taken regularly for 24 hours after the zymosan had been given. Experiment B: Clinical, biochemical, and histological variables were measured over a 12-day period after the zymosan had been given. MAIN OUTCOME MEASURES: Experiment A: Concentrations of tumour necrosis factor and interleukin-6 in abdominal exudate and plasma. Experiment B: Incidence of multiple organ failure. RESULTS: Experiment A: Concentrations of tumour necrosis factor and interleukin-6 in abdominal exudate and plasma were significantly higher in rats pretreated with fish oil, compared with control rats. This effect was more pronounced after six weeks of pretreatment. Experiment B: There were no significant differences between the groups for any variable. CONCLUSION: Fish oil given intraperitoneally increased rather than reduced local and systemic release of tumour necrosis factor and interleukin-6, and did not reduce the incidence of organ failure in rats with sterile peritonitis.

Biased VH gene expression in murine CD5 B cells results from age-dependent cellular selection.

Flow cytometry-purified, peritoneal and splenic CD5+ and CD5- B cells from neonatal and adult C57BL/6 mice were studied for expression of VH and Vx gene families in RNA colony blot assays, and for frequencies of clones secreting antibodies to bromelain-treated mouse red blood cells (BrMRBC), single-stranded DNA, trimethyl ammonium and bovine gamma-globulin, by limiting dilution. The results show few overall differences between the two B cell subsets, which both manifest ontogenic D-proximal VH preferences that are lost with age. Biased VH11 expression in CD5 B cells is high in adult peritoneum and spleen but absent in newborns. It only partly correlates with the selection of anti-BrMRBC reactivity, which is considerably higher in peritoneum than in spleen. No particular Vx bias was observed in any of the populations studied with the possible exception of Vx22 in peritoneal CD5+ B cells. We conclude that the antibody repertoire expressed by peritoneal CD5+ B cells of adult mice is not the result of a genetic program, but rather the consequence of local, age-dependent cellular selection mechanisms.

Rat monoclonal antibodies. VII. Enhancement of ascites production and yield of monoclonal antibodies in rats following pretreatment with pristane and Freund's adjuvant.

The effect of intraperitoneal injections of pristane, incomplete Freund's adjuvant (IFA) and a v/v mixture of pristane and IFA (called PIFA) on ascites production and the yield of monoclonal antibodies has been studied in Louvain rats. The best results were obtained following injection of 2 ml PIFA at the moment of i.p. transfer of hybridoma or immunocytoma cells. Ascites production was increased by as much as 4.7 times and monoclonal antibody production by more than six times compared with untreated control rats.

Monoclonal antibodies to the major Lolium perenne (rye grass) pollen allergen Lol p I (Rye I).

Thirteen monoclonal antibodies (MAbs) were produced against Lol p I (Rye I), the major Lolium perenne (rye grass) pollen allergen. Spleen cells from A/J and SJL mice immunized with highly purified Lol p I (Lol I) were allowed to fuse with cells from the non-secreting Sp2/0-Ag14 myeloma cell line. Each MAb was analyzed for antigenic specificity by radioimmunoassay (RIA) using 125I-Lol I. The epitope specificities of seven of the MAbs were examined by competitive binding against a labelled standard MAb for the Lol I antigen (Ag). The dissociation constant, Kd, of one MAb (No. 3.2) that was studied most extensively was determined by double Ab RIA to be 3.5 X 10(-6) L/M. This MAb recognized the related 27,000-30,000 Group I glycoproteins found in the pollens of nine other species of grass pollens tested, including weak binding to Bermuda grass Group I (Cyn d I), which by conventional analysis using polyclonal anti-Lol I serum shows no detectable binding. Monoclonal antibody No. 3.2 was coupled covalently to Sepharose 4B and used to prepare highly purified Lol I from a partially purified rye pollen extract. Finally, an RIA was developed which permitted the analysis of the Group I components in rye grass and nine other grass pollen species. The latter assay is likely to prove useful in the standardization of grass pollen extracts according to their Group I contents.

[Effect of Streptomyces spheroides proteinases on the mechanism of antibacterial protection in an experiment]. / Vliianie proteinaz Streptomyces spheroides na mekhanizm antibakter-ial'noi zashchity v éksperimente.

The effect of Streptomyces spheroides proteinases on the process and character of the local reaction to intraperitoneal infection of mice with E. coli was studied experimentally. It was shown that administration of the proteinases promoted a decrease in the dissemination of the abdominal cavity and a more rapid elimination of the microorganisms from the infection foci. The proteinases potentiated the macrophagal component of the local reaction by accelerating migration of the macrophages and increasing their phagocytic activity and the activity of the lysosomal enzymes. Proteinases gave rise to a pronounced activation of the neutrophils, an increase in the phagocytic capacity of the young forms and a change in intracellular enzymes. It was demonstrated that proteinases changed interrelation between the cell elements in the infection foci, the character of interaction of the quantitative and functional parameters of the local reaction and interrelationship between the phagocytic activity and the enzymatic balance of the cells. They increased the effect of the macrophages on migration and the functional state of the neutrophiles.

Enhanced killing of Candida albicans by cultured peritoneal exudate cells treated with SM-1213, a synthetic immunomodulator.

The effect of SM-1213 [1,2-O-isopropylidene-3-O-3'-(N', N'-dimethylamino-n-propyl)-D-glucofuranose] on the candidastatic and candidacidal capacity of guinea pig peritoneal exudate cells was investigated. Phagocytes treated with SM-1213 demonstrated an enhanced ability to inhibit intracellular hyphal formation and elongation of Candida albicans cells as well as to reduce the percentage of viable cells remaining after phagocytosis. Significant drug-induced anticandidal effects were also observed when peritoneal exudate cells were preincubated with SM-1213 and washed before infection with C. albicans. SM-1213 had no direct anticandidal effect against the yeast or hyphal form of the fungus, suggesting that the observed candidastatic and candidacidal effect of this drug was due to the enhancement of antimicrobial functions of phagocytes.

Immunologically induced generation of tetraene and pentaene leukotrienes in the peritoneal cavities of menhaden-fed rats.

The generation of sulfidopeptide leukotrienes and leukotriene B (LTB) in response to an IgG-mediated immune complex reaction in the peritoneal cavities of rats fed either a menhaden oil-supplemented diet or a beef tallow-supplemented diet for 9 to 10 wk was determined with the combined techniques of radioimmunoassay (RIA) and reverse-phase high performance liquid chromatography. Rats on the fish fat diet (FFD) incorporated eicosapentaenoic acid (EPA) into pulmonary and splenic tissues with an EPA:arachidonic acid ratio of approximately 2:1, whereas rats on the beef fat diet (BFD) showed no detectable EPA. The estimated total quantities of immunoreactive sulfidopeptide leukotrienes generated by each group of rats were similar, ranging from 70 to 99 ng/ rat in the FFD groups and 65 to 109 ng/rat in the BFD groups; for rats on the FFD this total included the pentaene products LTC5, LTD5, and LTE5 in quantities ranging from 24 to 39 ng/rat. The total quantities of immunoreactive LTB generated in the two groups of rats were similar, being 6 to 29 ng LTB4/rat for the BFD groups and the sum of LTB4 and LTB5 of 8 to 36 ng/rat for the FFD groups. There was a two- to seven-fold preferential generation of immunoreactive LTB5 over LTB4 in the FFD rats. LTC5 was equipotent with LTC4 in contracting guinea pig pulmonary parenchymal strips and ileal tissues. In contrast, LTB5 was 1/30 to 1/60 as potent and did not reach the same maximum as LTB4 in eliciting neutrophil chemotaxis. The finding that FFD favors the immunologic generation of LTB5, which has attenuated biologic activity when compared to LTB4, suggests that EPA-enriched tissues may produce less pro-inflammatory activity than tissues that are EPA-poor.

Accessory cell response to rye grass pollen extract in mice, guinea pigs and man.

The ability of murine, guinea-pig and human accessory cells to function in antigen presentation has been assayed by a T-cell proliferative response to adherent cells pulsed with rye grass pollen extract. A population of phagocytic, esterase-positive, plastic-adherent splenocytes from non-immune Balb/c mice briefly incubated with solubilized rye antigen were capable of stimulating syngeneic T lymphocytes from immune mice in an antigen-specific manner. Accessory cell function was H-2 restricted and required Ia-positive cells. Treatment with a monoclonal anti-Ia antibody impaired proliferation, whereas the presence of polyclonal rabbit anti-rye antibody increased activity. Guinea-pig peritoneal and alveolar adherent cells pulsed with up to 1 mg/ml antigen increased the response of rye-immune lymph node T cells in vitro. Proliferation was dose-dependent, despite less than 0.1% of the available antigen being cell-associated, and could be inhibited by treating the accessory cells with trypsin, sodium iodoacetate and chloroquine. Adherent human peripheral blood cells treated with rye antigen supported the proliferation of non-adherent and nylon wool-enriched mononuclear cells obtained from both grass pollen-sensitive donors and from individuals lacking either detectable serum IgE antibody or a positive skin test to rye antigen.

Protection by polyI:polyC against infection with herpes simplex virus in mice pretreated with Corynebacterium parvum.

The effect of Corynebacterium parvum on the protection by polyinosinic-polycytidylic acid (polyI:polyC) against lethal infection with Herpes simplex virus type 1 (HSV) was studied in mice. Pretreatment with C. parvum resulted in prolonged survival times in all experiments. One third of the mice survived an infection with 100 LD50, whereas all mice died when treated with polyI:polyC alone. Increased protection was observed up to 6 weeks after pretreatment and only seen when both C. parvum and polyI:polyC were given at the same site of injection (intraperitoneally). Protection against HSV correlated with increased interferon (IFN) activities induced by polyI:polyC in the peritoneal cavity of C. parvum-pretreated mice. In these mice, natural killer cell activity of peritoneal exudate cells (PEC) was also augmented in response to polyI:polyC. Protection was markedly decreased by intraperitoneal injection of silica or of an antiserum against murine IFN. It appears that increased local levels of IFN presumably produced by macrophages in response to polyI:polyC in C. parvum-pretreated mice play the major role in the antiviral defence in our model and that activation of NK cells may be a secondary effect of IFN.