Approaches to assessing host resistance

Investigating the susceptibility of mice to a bacterial challenge after intravenous exposure to durable nanoparticles

AIM

The goal of this study was to determine whether bacterial clearance in a rodent model would be impaired upon exposure to gold, silver or silica nanoparticles (NPs).

MATERIALS & METHODS

Mice received weekly injections of NPs followed by a challenge of Listeria monocytogenes (LM). On days 3 and 10 after LM injections, the animals were sacrificed and their tissues were collected for elemental analysis, electron microscopy and LM count determination.

RESULTS

The untreated and NP-treated animals cleared LM at the same rate suggesting that bioaccumulation of NPs did not increase the animals' susceptibility to bacterial infection.

CONCLUSION

The data from this study indicate that the bioaccumulation of NPs does not significantly affect the ability to react to a bacterial challenge.

Effect of lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin on the susceptibility to Listeria infection

We studied the effect of lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the susceptibility to Listeria infection of offspring in C57BL /6NCji mice. The offspring were nursed by TCDD-treated dams and exposed to TCDD from birth to weaning via milk. The exposure had little effect on the weights of immune organs and the spleen or the thymus cell population in the dams and offspring, but it enhanced the production of tumor necrosis factor alpha (TNF-alpha) and interferon gamma (IFN-gamma) in the serum after Listeria infection. The clearance of Listeria monocytogenes from the spleen was impaired in the off-spring. These results suggest that the exposure to TCDD of the offspring via milk disrupted the host resistance of the offspring, even though the main immune parameters were unchanged.

Host resistance to Trypanosoma cruzi infection is enhanced in mice fed Fusarium verticillioides (=F. moniliforme) culture material containing fumonisins

Fumonisins, metabolites of Fusarium verticillioides (=F. moniliforme) and related fungi that occur naturally on corn, elicit various organ- and species-specific toxicities. However, immunologic effects of fumonisins are not well characterized. BALB/c mice were fed diets containing F. verticillioides culture material (CM) providing 50 (LD) or 150 (HD) ppm fumonisins (FB(1)+FB(2)) beginning 1 week before and continuing 5 weeks after challenge with the myotropic Brazil strain of T. cruzi. A control group (ZD) was fed a diet lacking CM. The LD and HD diets caused increases in tissue sphinganine/sphingosine ratios and minimum to mild hepatotoxicity, both of which are typically induced by fumonisins. Nitric oxide (NO) production by peritoneal macrophages from HD mice was significantly higher than by peritoneal macrophages from ZD mice on day 14 after challenge. NO production also was stimulated in macrophages from ZD mice, but the peak response did not occur until day 26 after challenge. Compared with ZD mice, LD and HD mice exhibited reduced parasitemia and decreased numbers of pseudocysts in cardiac muscle. Thus, the CM increased host resistance to T. cruzi by accelerating NO production by macrophages or otherwise enhancing the immune response. The findings provide additional evidence that fumonisins modulate immune function.

TNF-alpha blockade by a dimeric TNF type I receptor molecule selectively inhibits adaptive immune responses

Tumor necrosis factor-alpha (TNF-alpha) is a mediator of severe inflammatory processes, including rheumatoid arthritis. Suppression of TNF with a soluble type I or type II receptor molecule (TNF-RI or TNF-RII) has the potential to decrease cytokine levels and modulate inflammatory diseases in humans. However, it has recently been reported that treatment of mice with a TNF-RI:Fc immunoadhesin protein augmented Gram positive infections and subsequent mortality. To determine if TNF-alpha blockade with soluble TNF-alpha receptors might alter immune system function, assays were assessed in rodents treated with a dimeric form of the p55 TNF-RI, Tumor Necrosis Factor-binding protein (TNFbp). Administration of TNFbp resulted in suppression of primary and secondary IgG antibody responses and cell-mediated immune function. No treatment-related differences were detected in immune-enhancing assays or non-specific immune function parameters. Bacterial host resistance assays with Listeria monocytogenes, Staphylococcus aureus or Escherichia coli showed an increase in tissue colony counts only with L. monocytogenes challenged animals following TNFbp administration. These results suggest that TNFbp has the capacity to inhibit adaptive immune function in experimental animal models. Studies suggest that while reducing TNF-alpha is important in controlling cytokine-dependent disease states, maintenance of a threshold level may be critical for normal immune function.

Indicators of immunotoxicity in populations of cotton rats (Sigmodon hispidus) inhabiting an abandoned oil refinery

Wildlife species inhabiting contaminated sites are often exposed to complex mixtures of chemicals, many of which have known effects on physiological and biochemical function. Although sensitivity of the immune system to chemical exposure has been documented in laboratory animal and wildlife species, little work has been conducted on feral wildlife populations inhabiting contaminated sites. Immune function was measured in populations of wild cotton rats (Sigmodon hispidus) inhabiting replicated reference and contaminated study sites at an abandoned oil refinery in Oklahoma four times from 1991 to 1992. Several measures of immunocompetence were examined including immune organ mass and cellularity, hematology, in vivo hypersensitivity, macrophage function, killer cell activity, and lymphoproliferative responsiveness. In vitro proliferation of splenocytes, either spontaneous or induced with concanavalin A (Con A), was the most consistent and reliable indicator of immunotoxicity. Spontaneous proliferation of splenocytes was 48 and 24% higher for cotton rats collected from contaminated than reference sites in September 1991 and September 1992, respectively. Likewise, Con A-induced proliferation of splenocytes ranged form 20 to 53% higher in animals collected from contaminated than reference sites in three of four collection periods. The percentage of splenocytes (mean+/-SE) staining positive for Con A receptors was lower on contaminated sites (73.7+/-1.2%) than reference sites (77.0+/-1.4%) in September 1991. Other measures of immune function including macrophage metabolism, hypersensitivity, blood cellularity, and mass and cellularity of immune organs varied between contaminated and reference sites.

Single-organism model of host defense against infection: a novel immunotoxicologic approach to evaluate immunomodulatory drugs

The immunotoxicologic effects of drugs on host defense have been studied widely using various animal models of infection. Here we describe a new approach to testing host defense by using a single organism (Candida albicans) in CBA/J mice. The model is configured to test 3 effector systems via different routes of inoculation to stimulate different effector arms of the immune response. Nonspecific immunity was evaluated by C. albicans colony-forming unit (CFU) count from the spleen at 2 hr (uptake) and > or = 22 hr (clearance) following intravenous inoculation. Cell-mediated immunity was assessed by CFU count from an intramuscular injection site 6 days postinoculation. Humoral immunity was assessed by anti-Candida antibody titer, following multiple subcutaneous immunizations with C. albicans. Finally, overall immunity was evaluated following intravenous injection using survival as the endpoint. Histopathological, immunohistochemical, and electron microscopic evaluation of selected tissues revealed the involvement of the expected cell types in the different effector systems. Several immunomodulatory drugs--dexamethasone, cyclosporine, liposomal muramyltripeptide phosphatidylethanolamine, and SK&F 105685--were evaluated in the C. albicans model. Dexamethasone impaired host defense against C. albicans by suppressing all endpoints measured. Similarly, cyclosporine showed broad immunosuppressive activity, with the exception of yeast uptake from the spleen. In contrast, muramyl tripeptide-phosphatidylethanolamine enhanced all but cell-mediated immunity to C. albicans. SK&F 105685 displayed both stimulatory and inhibitory effects on immune responses to the infection. Our studies demonstrate that a single organism-based approach can be a useful method for evaluating the immunological hazards of drugs on host resistance to infection.

Approaches to the evaluation of chemical-induced immunotoxicity

The immune system plays a crucial role in maintaining health; however, accumulating evidence indicates that this system can be the target for immunotoxic effects caused by a variety of chemicals including the environmental pollutants of polychlorinated biphenyls, chlorinated dibenzo-p-dioxins, pesticides, and heavy metals. Adverse chemical-induced immunomodulation, which is studied within the discipline of immunotoxicology, may be expressed either as immunosuppression/immunodepression or immunoenhancement. The former may be manifested either as decreased resistance to opportunistic viral, bacterial, fungal, and other infectious agents or increased susceptibility to cancer. Immunoenhancement on the other hand may either increase the risk of autoimmune reactions or result in allergic reactions. This paper attempts to integrate several aspects of the immune system that are relevant to the assessment of potentially immunotoxic chemicals.

Host resistance assays as predictive models in styrene immunomodulation

Three infection models namely an oncogenic virus Encephalomyocarditis (EMCV), a rodent strain of malaria, Plasmodium berghei, and a rodent hookworm parasite, Nippostrongylus brasiliensis, were used to confirm the in vivo immunotoxic potential of styrene reported in our previous communication. The altered host resistance to these challenge infections was evaluated in rodents pre-treated with 0, 0.02, 0.03 or 0.05 x LD50 dose of styrene (5 days/week) for 4 weeks. Significantly increased mortality in mice was observed at the various tested dose levels of styrene when challenged with EMCV. Similarly the results obtained in the malaria infection model indicated increased blood parasitaemia as well as significantly enhanced mortality in styrene-treated animals. Also the rejection of N. brasiliensis was also found to be significantly impaired in animals treated with a higher dose of styrene. These results indicate that the exposure of rodents to styrene can markedly impair host resistance which may have biological significance.

Testing guidelines for evaluation of the immunotoxic potential of direct food additives

Immunotoxicity testing is a new addition to the safety assessment guidelines for direct food and color additives. The approaches and philosophy for this area of specialized testing are consistent with the case-by-case strategy applied in the regulatory approval process, which is based on structure-activity relationships, preexisting knowledge, and projected exposure estimates. Specialized testing such as immunotoxicity is not part of the basic testing requirements, but would be applied when indicators are positive. Concepts for immunotoxicity testing have evolved, in part, from research for evaluating various testing methods as well as specific study designs. This research, conducted over the last decade, has focused mainly on the rat as the rodent species of choice. The miniature swine was evaluated as a nonrodent model. Testing is defined by type 1 and type 2 tests, which differ in that type 1 tests are performed on the same animals used in the core study design. Sets of type 1 and type 2 tests, with reference to the indicators, define various testing levels. Retrospective testing, expansion of basic testing (such as histopathology and serum chemistry profiles), and alternative study designs, which include satellite groups for evaluation of the functional capacity of the immune system, can be considered in the evaluation of immunotoxic potential.

Activity of FCE 20696, a new synthetic immunomodulator, in models of viral and bacterial pathology

The dibenzopyran derivative FCE 20696 is an immunomodulator which protects mice infected with several viral agents and with Mycobacterium tuberculosis. The compound is able to decrease the severity of the lung lesions caused by influenza virus infection. Doses ranging from 12.5 to 100 mg/kg are effective even after a single administration. Activity is demonstrated by intraperitoneal, subcutaneous and oral administration of the drug. In systemic infection with herpes simplex virus type 1, the 100 mg/kg dosage, administered subcutaneously, is able to increase both the percentage and the mean survival time of mice. To have effect by oral administration it is necessary to give the compound twice in the day. Moreover, some activity has also been observed in mice infected by M. tuberculosis, when the compound is administered orally at doses of 1-2.5 mg/kg twice weekly for 5 weeks, during the course of the disease.

Identification of immunotoxic effects of chemicals and assessment of their relevance to man

Immunotoxicity is defined as the adverse effects of foreign substances (xenobiotics) on the immune system. Two types of effects are possible: immunosuppression (which may result in an increased susceptibility to infection or to the development of tumours) and immunopotentiation (which may manifest as an allergy or as autoimmunity). There is, as yet, little evidence that well controlled occupational exposure to industrial chemicals has led to clinically significant immunosuppression. In contrast, a number of industrial chemicals have been shown to cause immunopotentiation in exposed populations, producing occupational asthma and contact dermatitis and possibly autoimmunity. In experimental models, immunosuppression (usually assessed by in vivo or in vitro immune function tests) has been induced by a wide range of chemicals but there are a few reports of the immunosuppression leading directly to an increased susceptibility to infection or to the development of tumours. Predictive experimental models are available for type IV allergic reactions, but the identification of chemicals that have a potential to cause other types of allergy or autoimmune reactions requires further research and the development and validation of new animal models. It is considered that routine subacute and chronic toxicity studies should include a full gross and histopathological assessment of the lymphoid organs to more accurately detect the potential of a chemical to cause immunotoxicity. Should such studies indicate that a substance has affected the immune system directly, an assessment of overall immune competence and function tests may be necessary using dose levels below those which cause frank toxicity. However, precise interpretation of immune function tests in terms of their relevance to human health requires an improved understanding of the extent of the functional reserve of the immune system. A strategy for assessing immunotoxicity in exposed human populations demonstrates a need for reliable clinical assessment, accurate medical record-keeping, an environmental and biological monitoring for levels of contaminating chemicals and the judicious use of well-validated immune function tests.

Suppression of immune response in the B6C3F1 mouse after dietary exposure to the Fusarium mycotoxins deoxynivalenol (vomitoxin) and zearalenone

The effect that dietary exposure to the naturally-occurring Fusarium graminearum toxins deoxynivalenol (DON) and zearalenone (ZEA) may have on immune function was assessed in the B6C3F1 mouse. Dietary DON depressed the plaque-forming response to sheep red blood cells, the delayed hypersensitivity response to keyhole limpet haemocyanin and the ability to resist Listeria monocytogenes. Listerial resistance was similarly decreased in control mice fed restricted diets comparable to the dietary restriction caused by DON-induced feed refusal, whereas equivalent food restriction did not decrease the plaque or delayed hypersensitivity responses. ZEA ingestion decreased resistance to L. monocytogenes but did not affect splenic plaque-forming or delayed hypersensitivity responses. Resistance to Listeria was reduced to a greater extent by co-administration of DON and ZEA than by DON alone, whereas the ability of DON to inhibit the delayed hypersensitivity response was significantly lessened in the presence of ZEA. While effects on resistance to Listeria and delayed hypersensitivity were detectable in mice ingesting the mycotoxins for 2-3 wk, these effects disappeared upon extension of the feeding period to 8 wk. In contrast, some effect on the plaque-forming response was detectable with both the 2- and the 8-wk period of mycotoxin ingestion. Immunosuppression can thus result from ingestion of F. graminearum-infected agricultural staples, the suppression being attributable to interactions between direct immunotoxic effects of DON and ZEA and nutritional effects associated with DON-induced food refusal.

Modulation of serum complement levels following exposure to polychlorinated dibenzo-p-dioxins

Subchronic 14-day exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) suppressed serum total hemolytic complement activity (CH50) in female B6C3F1 mice at doses of 0.01, 0.05, 0.1, 0.5, 1.0, and 2.0 micrograms/kg. Serum levels of complement component C3 were also suppressed at doses of 0.5, 1.0, and 2.0 micrograms/kg. Another dioxin isomer, 1,2,3,6,7,8-hexachlorodibenzo-p-dioxin (HCDD), also produced dose-dependent suppression of complement activity at doses of 0.1, 1.0, and 10 micrograms/kg with decreased C3 levels at 10 micrograms/kg. Both TCDD and HCDD enhanced susceptibility to Streptococcus pneumoniae, a bacterial pathogen whose host defense is complement mediated. Recovery studies demonstrated that complement activity in TCDD (1 microgram/kg) and HCDD (10 micrograms/kg)-treated animals was suppressed until 50 days post-treatment, while low doses of HCDD (0.1 and 1.0 micrograms/kg) elevated CH50 levels. Acute exposure to TCDD (14 micrograms/kg) also suppressed complement CH50 and C3 levels. These studies demonstrate that the complement system and innate immunity represent potential target sites for polychlorinated dibenzo-p-dioxins.

Increased susceptibility to mouse hepatitis virus 3 of peritoneal macrophages exposed to dieldrin

Interaction of a single dose (36 mg/kg body wt) of the organochlorine pesticide dieldrin with mouse peritoneal macrophages was examined in C57Bl/6, (C57Bl/6 X A/J)F1, and A/J strains of different genetic resistance to mouse hepatitis virus 3 (MHV3) infection. In vivo studies showed increased susceptibility to MHV3 acute disease of C57Bl/6 and (C57Bl/6 X A/J)F1 animals challenged with the pesticide. Significant decrease of mean time of death in dieldrin-exposed, MHV3-infected susceptible C57Bl/6 mice was observed similarly upon po or ip administration of a single, sublethal dose of dieldrin. In addition, decrease of humoral response to the virus was quantified by determination of anti-MHV3 IgG antibodies in spleen cell supernatant fractions and in blood sera of dieldrin-exposed C57Bl/6 mice. A single dose of dieldrin did not alter the in vivo resistance of A/J animals to acute MHV3 disease. The resistant A/J mice, however, showed increased mortality upon two subsequent exposures to dieldrin followed by infection with high lethal doses of MHV3. Phagocytic activity, cell adherence capacity, and attachment and uptake of 3H-radiolabeled MHV3 by C57Bl/6 peritoneal macrophages were determined by in vitro studies. These affector activities of peritoneal macrophages were slightly decreased or unchanged in cells originating from animals exposed to the pesticide. However, the intrinsic activity of MHV3 restriction appeared to be affected in macrophages derived from dieldrin-treated animals: (i) peritoneal C57Bl/6 macrophages collected from the early phase of acute MHV3 disease contained increased MHV3 antigen and (ii) increased cytolysis was observed after in vitro MHV3 infection of macrophages originating from dieldrin-exposed C57Bl/6 mice.

Embryonic exposure to aflatoxin-B1: mutagenicity and influence on development and immunity

Chick embryos were mutagenized in ovo in order to study developmentally related alterations in immune functions in survivors of this prenatal toxicant insult. In this experimental system, a single exposure of 6-day chick embryos to 0.1 microgram aflatoxin-B1 (AF-B1) in 10 microliters of acetone was employed, and the control embryos received 10 microliters of solvent alone. This dosage of AF-B1 administered to 6-day embryos was found to increase the incidence of sister chromatid exchanges in blood cells approximately fivefold above the baseline observed in solvent controls. A second sham control, where no solvent was administered, was included in some experiments. The cell cycle times in blood increased slightly during the initial exposure to AF-B1. However, a majority of the AF-B1 and acetone exposed embryos survived and hatched without incident. Losses occurred mainly in the latter part of embryogenesis. After hatching, no significant differences were observed in body weight between different treatment groups up to 26 weeks of age and no change in primary humoral immunity was detected. In contrast, two parameters of cell-mediated immunity, graft vs host (GvH), and cutaneous basophil hypersensitivity (CBH) reactions were both depressed as a result of exposure to AF-B1. The AF-B1 treatment group was significantly reduced in the GvH reaction compared with sham-treated controls. In the CBH assay, AF-B1-exposed chicks showed reduced immunity compared with acetone controls. These results suggest that long-term selective immune depression can occur following embryonic exposure to AF-B1.