Depressions in B- and T-lymphocyte mitogen-induced blastogenesis in mice exposed to low concentrations of benzene

ATSDR evaluation of health effects of benzene and relevance to public health

As part of its mandate, the Agency for Toxic Substances and Disease Registry (ATSDR) prepares toxicological profiles on hazardous chemicals found at Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) National Priorities List (NPL) sites that have the greatest public health impact. These profiles comprehensively summarize toxicological and environmental information. This article constitutes the release of portions of the Toxicological Profile for Benzene. The primary purpose of this article is to provide public health officials, physicians, toxicologists, and other interested individuals and groups with an overall perspective on the toxicology of benzene. It contains descriptions and evaluations of toxicological studies and epidemiological investigations and provides conclusions, where possible, on the relevance of toxicity and toxicokinetic data to public health.

ATSDR evaluation of potential for human exposure to benzene

As part of its mandate, the Agency for Toxic Substances and Disease Registry (ATSDR) prepares toxicological profiles on hazardous chemicals found at Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) National Priorities List (NPL) sites that have the greatest public health impact. These profiles comprehensively summarize toxicological and environmental information. This article constitutes the release of portions of the toxicological profile for benzene. The primary purpose of this article is to provide interested individuals with environmental information on benzene that includes production data, environmental fate, potential for human exposure, analytical methods, and a listing of regulations and advisories.

Effects of inhalation exposure to a binary mixture of benzene and toluene on vitamin a status and humoral and cell-mediated immunity in wild and captive American kestrels

Benzene and toluene are representative volatile organic compounds (VOC) released during production, storage, and transportation associated with the oil and gas industry and are chemicals of concern, as they are released in greater and possibly more biologically significant concentrations than other compounds. Most studies of air pollution in high oil and gas activity areas have neglected to consider risks to birds, including top-level predators. Birds can be used as highly sensitive monitors of air quality and since the avian respiratory tract is physiologically different from a rodent respiratory tract, effects of gases cannot be safely extrapolated from rodent studies. Wild and captive male American kestrels were exposed for approximately 1 h daily for 28 d to high (rodent lowest-observed-adverse-effect level [LOAEL] of 10 ppm and 80 ppm, respectively) or environmentally relevant (0.1 ppm and 0.8 ppm, respectively) levels of benzene and toluene. Altered immune responses characteristic of those seen in mammalian exposures were evident in kestrels. A decreased cell-mediated immunity, measured by delayed-type hypersensitivity testing, was evident in all exposed birds. There was no effect on humoral immunity. Plasma retinol levels as measured by high-performance liquid chromatography (HPLC) analysis were decreased in wild and captive kestrels exposed to the rodent LOAEL for combined benzene and toluene. This study indicates that American kestrels are sensitive to combined benzene and toluene. The study also illustrates the need for reference concentrations for airborne pollutants to be calculated, including sensitive endpoints specific to birds. Based on these findings, future studies need to include immune endpoints to determine the possible increased susceptibility of birds to inhaled toxicants.

Benzene's toxicity: a consolidated short review of human and animal studies

A large population of humans is exposed to benzene from various occupational and environmental sources. Benzene is an established human and animal carcinogen. Exposure to benzene has been associated with leukaemia in humans and several types of malignancies in animals. The exact mechanism of benzene-induced toxicity is poorly understood. It is believed that benzene exerts its adverse effects by metabolic activation to toxic metabolites. Certain benzene metabolites are genotoxic and mutagenic. This consolidated short-review is composed of human and animal studies to summarize the adverse effects of benzene with special reference to molecular mechanisms involved in benzene-induced toxicity.

Identification of human cell responses to benzene and benzene metabolites

Benzene is a common air pollutant and confirmed carcinogen, especially in reference to the hematopoietic system. In the present study we analyzed cytokine/chemokine production by, and gene expression induction in, human peripheral blood mononuclear cells upon their exposure to the benzene metabolites catechol, hydroquinone, 1,2,4-benzenetriol, and p-benzoquinone. Protein profiling showed that benzene metabolites can stimulate the production of chemokines, the proinflammatory cytokines TNF-alpha and IL-6, and the Th2 cytokines IL-4 and IL-5. Activated cells showed concurrent suppression of anti-inflammatory cytokine IL-10 expression. We also identified changes in global gene expression patterns in response to benzene metabolite challenges by using high-density oligonucleotide microarrays. Treatment with 1,2,4-benzenetriol resulted in the suppression of genes related to the regulation of protein expression and a concomitant activation of genes that encode heat shock proteins and cytochrome P450 family members. Protein and gene expression profiling identified unique human cellular responses upon exposure to benzene and benzene metabolites.

Health effects classification and its role in the derivation of minimal risk levels: Immunological effects

The Agency for Toxic Substances and Disease Registry (ATSDR) derives health-based guidance values known as minimal risk levels (MRLs). By definition, an MRL is a substance-specific estimate of the daily human exposure to a substance that is likely to be without an appreciable risk of adverse, noncancer effects over a specified duration of exposure. MRLs are preferentially derived from human studies, if available, or from the most sensitive animal species and the endpoint that is most relevant for humans. To date, the agency has derived 346 MRLs. Fifteen MRLs were derived for 11 different chemicals where the database has identified the immune system as the most sensitive target of toxicity. The chemicals include benzene, chlorfenvinphos, endosulfan, heptachlor, gamma-hexachlorocyclohexane, dibutyl tin, tributyl tin, PCBs, 2,3,4,7,8-pentachlorodibenzofuran, 2,3,7,8-tetrachlorodibenzo-p-dioxin, and 2,4-dichlorophenol. The agency's rationale for classification of immunological endpoints is discussed and a brief description given of the critical studies selected for MRL development using immune system endpoints.

Immunotoxic effects of chemicals: A matrix for occupational and environmental epidemiological studies

BACKGROUND

Many biological and chemical agents have the capacity to alter the way the immune system functions in human and animals. This study evaluates the immunotoxicity of 20 substances used widely in work environments.

METHODS

A systematic literature search on the immunotoxicity of 20 chemicals was performed. The first step was to review literature on immunotoxicity testing and testing schemes adopted for establishing immunotoxicity in humans. The second step consisted of providing a documentation on immunotoxicity of substances that are widely used in work environment, by building tables for each chemical of interest (benzene, trichloroethylene, PAHs, crystalline silica, diesel exhausts, welding fumes, asbestos, styrene, formaldehyde, toluene, vinyl chloride monomer, tetrachloroethylene, chlorophenols, 1,3-butadiene, mineral oils, P-dichlorobenzene, dichloromethane, xylene, 1,1,1-trichloroethane, ethylene oxide). The third step was the classification of substances; an index (strong, intermediate, weak, nil) was assigned on the basis of the evidence of toxicity and type of immunotoxic effects (immunosuppression, autoimmunity, hypersensitivity) on the basis of the immune responses. Finally substances were assigned a score of immunotoxic power.

RESULTS

Tables have been produced that include information for the 20 substances of interest, based on 227 animal studies and 94 human studies. Each substance was assigned an index of immunotoxic evidence, a score of immunotoxic power and type of immunotoxic effect.

CONCLUSIONS

This matrix can represent a tool to identify chemicals with similar properties concerning the toxicity for the immune system, and to interpret epidemiological studies on immune-related diseases.

DNA damage in lymphocytes of benzene exposed workers correlates with trans,trans-muconic acids and breath benzene levels

Benzene causes many kinds of blood disorders in workers employed in many different environments. These diseases include myelodisplastic syndrome and acute and chronic myelocytic leukemia. In the present study, five occupational work places, including six industrial process types, namely, printing, shoe-making, methylene di-aniline (MDA), nitrobenzene, carbomer, and benzene production were selected, and the levels of breath benzene, and trans,trans-muconic acids (t,t-MA) and phenol in urine were evaluated, as well as hematological changes and lymphocyte DNA damage. The concentration of benzene in breath was less than 3 ppm in the workplaces, and benzene exposure was found to be higher in work places where benzene is used, than in those where benzene is produced. At low levels of benzene exposure, urinary t,t-MA correlated strongly with benzene in air. Highest Olive tail moments were found in workers producing carbomer. Levels of breathzone benzene were found to be strongly correlated with Olive tail moment values in the lymphocytes of workers, but not with hematological data in the six workplaces types. In conclusion, the highest benzene exposures found occurred in workers at a company, which utilized benzene in the production of carbomer. In terms of low levels of exposure to benzene, urinary t,t-MA and DNA damage exhibited a strong correlation with breath benzene, but not with hematological data. We conclude that breath benzene, t,t-MA and lymphocytic DNA damage are satisfactory biomonitoring markers with respect to benzene exposure in the workplace.

Effects of volatile aromatics, aldehydes, and phenols in tobacco smoke on viability and proliferation of mouse lymphocytes

Thirteen chemicals present in tobacco smoke were assessed for their effect on viability and proliferation of mouse lymphocytes in vitro. Acetaldehyde, benzene, butyraldehyde, isoprene, styrene, and toluene produced no effect on either viability or proliferation after 3 h of exposure. Formaldehyde, catechol, acrylonitrile, propionaldehyde, and hydroquinone significantly inhibited T-lymphocyte and B-lymphocyte proliferation with IC50 values ranging from 1.19 x 10(-5) M to 8.20 x 10(-4) M after 3 h of exposure. Acrolein and crotonaldehyde not only inhibited T-cell and B-cell proliferation, but also acted on viability with IC50 values ranging from 2.06 x 10(5) M to 4.26 x 10(-5) M. Mixtures of acrolein, formaldehyde, and propionaldehyde or crotonaldehyde were tested and interactive effects at 0.5 and 1 x IC50 were observed. Two mixtures significantly inhibited T-cell proliferation when compared to the control at 0.1 x IC50 concentration. The present study shows that some chemicals known to be present in tobacco smoke exert an effect on lymphocyte viability and proliferation in vitro.

Single strand DNA breaks in T- and B-lymphocytes and granulocytes in workers exposed to benzene

Comet assays were carried out to evaluate DNA damage in T- and B-lymphocytes and granulocytes from 41 workers exposed to benzene in a printing company and 41 unexposed donors. In T-lymphocytes, DNA damage was slightly higher in exposed workers than in controls. The tail moments in the two groups were 1.75+/-0.29 and 1.47+/-0.41, respectively (P<0.0006). DNA damage of B-lymphocytes in the two groups showed the most significant difference among the three cell types. The tail moments were 3.86+/-0.71 and 1.51+/-0.39, respectively (P<0.0001). In granulocytes, DNA damage was also different, the tail moments being 3.61+/-0.75 and 2.60+/-0.59, respectively (P<0.0001). The comparison of DNA damage in both groups shows that B-lymphocytes could be a useful target in biomonitoring of human exposure to low levels of benzene.

Hydroquinone, a reactive metabolite of benzene, inhibits NF-kappa B in primary human CD4+ T lymphocytes

Hydroquinone (HQ), a reactive metabolite of benzene, is present in cigarette smoke and is known to inhibit mitogen-stimulated activation of both T and B lymphocytes. Despite extensive study, the underlying mechanism for HQ's immunotoxicity is not clear. NF-kappa B is a transcription factor known to regulate the expression of a number of genes critical for normal T cell activation. We therefore hypothesized that NF-kappa B might be involved in HQ-induced immunosuppression. In this study, we demonstrate that 1 microM HQ inhibits tumor necrosis factor alpha induced activation of NF-kappa B in primary human CD4+ T cells. This inhibition is not accompanied by a loss in viability, and HQ-treated T cells maintain other active signaling pathways throughout the exposure duration. Additionally, the inhibition of NF-kappa B is reversible as HQ-treated T cells regain normal functioning after 72 h in culture. HQ does not appear to alter NF-kappa B directly as preincubation of nuclear extracts with HQ does not diminish activity of this protein. We further demonstrate that 1 microM HQ inhibits intracellular IL-2 production in T cells stimulated with phorbol ester but does not alter surface expression of CD25 (the alpha-subunit of the IL-2 receptor). These data suggest that NF-kappa B may be an important molecular mediator of HQ's (and benzene's) immunotoxicity.

Benzene--a review of the literature from a health effects perspective

A literature review of the impact on human health of exposure to benzene was conducted. Special emphasis in this report is given to the health effects reported in excess of national norms by participants in the Benzene Subregistry of the National Exposure Registry--people having documented exposure to benzene through the use of benzene-contaminated water for domestic purposes. The health effects reported in excess (p < or = .01) by some or all of the sex and age groups studied were diabetes, kidney disease, respiratory allergies, skin rashes, and urinary tract disorders; anemia was also increased for females, but not significantly so.

Immunotoxicological effects of benzene inhalation in male Sprague-Dawley rats

The inhalation of benzene is toxic to various components of the immunologic system in rodents. Spleen and thymus weights, total spleen and femur marrow cell counts, enumeration of spleen B- and T-lymphocytes, and an assessment of humoral immunocompetence, were used to evaluate the immunotoxicity of benzene in male Sprague-Dawley rats. Rats were exposed to 0, 30, 200 or 400 ppm benzene for 6 h/day, 5 days/week for 2 or 4 weeks. An early indicator of immunotoxicity was a reduction in the number of B-lymphocytes after 2 weeks of 400 ppm. After 4 weeks of 400 ppm, there was a reduction in thymus weight and spleen B-, CD4+/CD5+ and CD5+ T-lymphocytes. Rats exposed to 30, 200 or 400 ppm benzene for 2 or 4 weeks and challenged with sheep red blood cells developed a humoral response comparable to that of the control (0 ppm) animals. Enumeration of spleen T- and B-lymphocytes in rats exposed to benzene and challenged with SRBC showed only a transient reduction in spleen B-lymphocytes after 2 weeks of exposure to 400 ppm. These data suggest that there are no immunotoxicological effects of exposure to 200 ppm benzene or less, in rats exposed for 6 h/day, 5 days/week for 2 or 4 weeks.

Effects of benzene on splenic, thymic, and femoral lymphocytes in mice

Chronic exposure to high concentrations of benzene, primarily by inhalation, can affect the function of the human immune system. Limited data are available on the immunotoxic effects of low concentrations of benzene. This study evaluated the effects of 1, 5, 10, 100, and 200 ppm benzene on lymphocytes in mice exposed by inhalation for up to 8 weeks. Exposure to 100 or 200 ppm benzene induced rapid and persistent reductions in femoral B-, splenic T- and B-, and thymic T-lymphocytes. The percentage of femoral B-lymphocytes and thymic T-lymphocytes in apoptosis was increased 6- to 15-fold by 200 ppm benzene compared to controls. Replication of femoral B-lymphocytes was increased during the exposure period in the bone marrow as a compensation for the lymphocyte loss induced by 100 and 200 ppm benzene. Exposure of mice to 10 ppm benzene or less did not have a statistically significant effect on numbers or replication of the lymphocyte populations evaluated. A reduced number of splenic B-lymphocytes after 2 weeks of exposure to benzene appeared to be the most sensitive end point and time point for evaluating benzene cytotoxicity in this study.

Aspirin-like drugs can protect human T lymphocytes against benzoquinone cytotoxicity: evidence for a NAD(P)H:quinone reductase-dependent mechanism

Benzene toxicity towards lymphocytes is thought to be mediated by metabolites of benzene including benzoquinone (BQ). NAD(P)H:quinone reductase (QR) is known to protect against BQ toxicity. The expression of the QR gene is regulated by the transcription factor AP-1. We had previously found that aspirin-like drugs (ALD) induce AP-1 in human T lymphocytes. It was therefore hypothesized that ALD would protect lymphocytes against BQ toxicity by inducing QR. Molt-4 cells (M4), a human T lymphocyte cell line, were incubated with different concentrations of two ALD, flurbiprofen and sodium diclofenac, and then exposed to BQ. Toxicity was measured by viability (trypan blue exclusion). Both drugs protected the cells against BQ cytotoxicity in a dose-dependent manner, e.g., sodium diclofenac at 15 microM reduced the fraction of BQ-treated dead cells by 70%. ALDs induced QR activity in the M4 cells in the same range of concentrations that protected the cells against BQ toxicity. The protective effect of ALD was significantly reduced by dicoumarol, a QR-specific inhibitor. Since human T cells and T cell lines do not metabolize arachidonic acid, our data suggest that ALD can protect human T lymphocytes against a metabolite of benzene by induction of QR activity.

Early effects of benzene exposure in mice. Hematological versus genotoxic effects

Female BDF1 mice were exposed to 100, 300 and 900 ppm benzene 6 h/day, 5 days/week, up to 8 weeks. Hematological studies included peripheral blood data, T4 and T8 lymphocyte counts in the blood and the spleen, hemopoietic stem and progenitor cell assays in the marrow (CFU-S, CFU-C, BFU-E, CFU-E). The single cell gel assay ("comet assay") was applied in parallel with cells from the peripheral blood, bone marrow, spleen and liver. The results showed minor changes in the stem and progenitor cells and the development of a slight anemia at 4 and 8 weeks, in agreement with reported data. New was the increase of the T4/T8 ratio in the peripheral blood (not in the spleen) at the end of the first week of exposure to 300 and 900 ppm. The results of the "comet assay" indicate a much higher sensitivity to this test system (strand breaks and alkali labile sites of DNA). The tail moment indicative of the damage to DNA increased as early as 3 days with 300 ppm in the peripheral blood cells. Furthermore, the liver cells did react to a much higher extent than the other cells tested. With 100 ppm significant changes were seen in the liver after 5 days, but not in the blood. The repair, studied 24 and 48 h after the end of the exposure, was almost complete after 5-day exposure period in the blood and the liver, but not after 4 weeks of exposure with 300 ppm in the blood, and 100 and 300 ppm in the liver.

Use of staphylococcal enterotoxin A-induced lymphoproliferation and interleukin 2 production as indicators of immunotoxicity

Suppression of mitogen-induced splenocyte lymphoproliferation and interleukin 2 (IL-2) production can be used as indicators of immunotoxicity. Staphylococcal enterotoxin A (SEA) is both a potent mitogen and the most potent in vitro inducer of IL-2 production that has been described. An in vitro system was used to measure impairment of SEA-induced lymphoproliferation and IL-2 production using splenocytes from female C57BL/6 mice dosed with either cyclosporin A (30 mg/kg/day, 14 days), benzene (220, 440, or 880 mg/kg/day, 14 days), or vehicle. Splenocytes were stimulated with either concanavalin A (con A) or SEA. Benzene- and cyclosporin A-treated mice demonstrated significant decreases in splenocyte proliferation. IL-2 production was determined by incubating splenocyte culture supernatants with IL-2 dependent cytotoxic T-cells (CTLL-2), pulsing with 3H-thymidine, and determining amount of incorporated label. Cell proliferation and IL-2 production were inhibited by both benzene and cyclosporin A, effects more clearly demonstrated using SEA than con A. SEA was a superior mitogen compared to con A in the assays evaluated here.

Modulation of benzene induced toxicity by protein A

Administration of benzene (i.p. 1.0 mL/kg body weight) for 3 consecutive days produced leucopenia and lymphocytopenia in female albino rats. In addition, the total iron content, lipid peroxidation and superoxide dismutase activity of the liver and bone marrow were significantly (P < 0.001) increased. Low molecular weight (LMW) bleomycin-detectable iron accumulated only in bone marrow. Prior administration of Protein A (PA), a multipotent immunostimulant and interferon inducer (60 micrograms/kg body weight, i.v. twice weekly for 2 weeks), ameliorated most of the adverse effects of benzene. PA restored the changes in hepatic histological architecture, reversed leucopenia and superoxide dismutase activity, lipid peroxidation, total iron content and LMW iron content of bone marrow were normalized. Isozymes of glutathione-S-transferase (alpha, pi, mu) which decreased following benzene exposure increased in PA pretreated benzene exposed rats. This study suggests that pretreatment with PA modulates the toxicity of benzene.

Human immune toxicity

This review examines xenobiotic toxicity to the immune system, stressing in particular those aspects of most relevance to humans. Immunotoxicity is examined especially from three points of view: by what immunological component is affected, by classes of foreign agents that adversely affect the human immune system and by critical evaluation of human case reports and epidemics. Mechanisms by which xenobiotics interrupt cytokine networks are emphasized. The concept that microbial agents, both environmental as well as infectious, may act as immunotoxicants, either alone or in synergism with conventional agents is introduced. Instances of human immunotoxicology are critically evaluated in terms of clinical relevance, i.e. whether increased susceptibility to opportunistic infections or tumor emergence takes place in the affected individuals.

A prospective study in the Australian petroleum industry. II. Incidence of cancer

This paper reports incidence of cancer in employees of the Australian petroleum industry from 1981 to 1989. Two surveys by personal interview incorporated more than 15,000 employees, representing 92% of the eligible population. Subjects were included in the analysis after completing five years of service in the industry. At the time of this report the cohort did not include sufficiently large numbers of women for useful analysis; results presented are restricted to the men. On 31 December 1989, 50,254 person-years of observation had accumulated in the men with 152 incident cancers reported. The standardised incidence ratio (SIR) analysis showed overall cancer rates close to those of the national population. Whereas deficits were seen in some cancer sites, notably lung cancer (SIR 0.5, 95% confidence internal (95% CI) 0.3-0.9), incidence rates for some other cancer sites suggested increased risk. An excess of observed over expected cases was present in all subcategories of lymphohaematopoietic cancer except Hodgkin's disease (no cases), and was most apparent in myeloid leukaemia (SIR 4.0, 95% CI 1.6-8.2). The other major site with a raised number of cases observed over expected was melanoma (SIR 1.4, 95% CI 0.8-2.1).

Subclinical effects of groundwater contaminants. III. Effects of repeated oral exposure to combinations of benzene and toluene on immunologic responses in mice

Toxicity of environmental pollutants may be expressed as combined effects of a chemicals. Benzene, a proven hematotoxic agent, frequently occurs with toluene in cocontaminated groundwater. Groups of CD-1 male mice were exposed continuously for 4 weeks to benzene (166 mg/l), toluene (80 and 325 mg/l), and combinations of benzene (166 mg/l) + toluene (80 mg/l or 325 mg/l) in drinking water. Benzene-induced anemia was alleviated by simultaneous toluene treatment. Leukopenia and lymphopenia were observed in the case of benzene only and benzene + toluene (80 mg/l)-treated mice. The cytopenia, however, was less severe in the benzene + toluene (325 mg/l)-treated group. Immunotoxicity induced by benzene treatment alone was characterized by involution of thymic mass and suppressions of both B- and T-cell mitogeneses, mixed lymphocyte culture response to alloantigens, the tumor lytic ability of cytotoxic T-lymphocytes as determined by 51Cr-release assay, and antibody production response to T-dependent antigen (sheep red blood cells). IL-2 secretion by Con A-stimulated mouse T-cells was decreased in the benzene-treated group. Toluene (325 mg/l) completely inhibited these adverse effects when it was coadministered with benzene, while the low dose of toluene (80 mg/l) did not protect against benzene-induced depressions of immune functions. Toluene administered alone at levels up to 325 mg/l showed no obvious immunotoxic effects. Results of this study demonstrated that toluene, in sufficient amounts, has an antagonistic effect on benzene immunotoxicity.

Modulation of benzene toxicity by polyinosinic-polycytidilic acid, an interferon inducer

Repeated intraperitoneal administration of benzene (1.0 ml/kg body wt.) for 3 days produced leucopenia, lymphocytopenia and significantly decreased body wt. (P less than 0.001) and organ weights of thymus (P less than 0.001) and spleen (P less than 0.001) in female albino rats. Total iron content, lipid peroxidation and superoxide dismutase activity of the liver and bone marrow were significantly increased as a result of benzene exposure. Low molecular weight (LMW) bleomycin detectable iron content was accumulated in bone marrow, whereas hepatic LMW iron was not detectable after benzene intoxication to rats. Prior administration of single dose (250 micrograms/100 g body wt.) of Poly IC, an interferon inducer with immunomodulating potential was found to be ameliorate some of the adverse effects of benzene as well as restoration of hepatic architecture histologically. Superoxide dismutase activity, lipid peroxidation, total iron content and LMW iron content (bone marrow) were normalised. Pretreatment of animals with Poly IC was able to enhance the SRBC antibody titre in benzene-treated animals. This study suggests that the beneficial effects of Poly IC in the amelioration of the acute toxicity of benzene has clinical significance.

A combined testing protocol for assessing genotoxicity in individual animals: application to environmental toxicology

A multiple end-point approach to assessing genetic toxicity (the combined testing protocol, CTP) was evaluated in male and female CD-1 mice exposed subacutely (3 and 6 weeks) to low levels of a custom-blended gas mixture (epichlorohydrin, benzene, chloroprene and xylene, at 50, 100, 100, and 100 ppb, respectively, as the low dose, with concentration levels 10-fold and 100-fold higher as the intermediate and high doses, or 0.1, 1 and 10 ppm of benzene). Urine mutagenicity was tested in the Salmonella/microsome assay, chromosome aberrations were examined in bone marrow and spleen lymphocytes, micronuclei were measured in bone marrow and peripheral erythrocytes, and cytochrome P450 and glutathione S-transferases were measured in the liver. Structural aberrations in alveolar macrophages and spermatocytes, and thioguanine resistance in spleen lymphocytes were examined for their suitability for incorporation into the overall protocol. Spleen lymphocytes were the most sensitive indicator cells, and showed a dose-related increase (P less than 0.01) in structural chromosome aberrations and in cytotoxicity after 6 weeks of exposure. Analysis of micronucleus formation and metaphase aberrations in the bone marrow, and micronuclei in peripheral erythrocytes showed an overall statistically non-significant but positive trend at the high dose. No mutagenicity was detected in pooled urine samples. Liver microsomal cytochrome P450 was not increased, but cytosolic glutathione S-transferases were significantly increased in a dose-related manner. Since the probability of detecting a genotoxic effect increases with the number of endpoints and tissues examined, this approach should be applicable to many situations without having to perform separate experiments for each tissue examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Hematologic effects of benzene: a thirty-five year longitudinal study of rubber workers

We studied over 17,000 peripheral blood counts, accumulated during surveillance from 1940 through 1975, from a cohort of 459 benzene-exposed workers. Linear regressions demonstrated significant decreases in white and red cell counts, as well as hemoglobin, for workers exposed during the 1940's, without persistent trends over the ensuing 25 years. Strongly positive correlations were observed between these blood count fluctuations and fluctuations in retrospective estimates of benzene exposures for this plant for the earlier periods of surveillance (mean estimated exposure 1940 to 1948, 75 ppm), but not for later years (mean estimated exposure 1948 to 1975, 15 to 20 ppm). These data suggest substantial limitations of hematologic examination of populations to detect abnormalities in populations currently exposed to benzene. The data also demonstrate a novel approach to the biological validation of exposure estimates based upon limited industrial hygiene and historical record data.

Genotoxic effects of a sub-acute low-level inhalation exposure to a mixture of carcinogenic chemicals

A study was conducted using a combined testing protocol (CTP), to determine whether short-term biological end-points, singly or in combination, are sufficiently sensitive to identify damage induced by exposure to ambient levels of industrial chemicals. A small-scale inhalation set-up which is both economical and easy to assemble was designed. Mice were exposed to 4 concentrations of a custom-blend mixture of benzene, chloroprene, epichlorohydrin and xylene in a ratio of 2:2:1:2, respectively. The concentrations for benzene, chloroprene and xylene were 0, 0.1, 1.0 and 10 ppm each. Concentrations for epichlorohydrin were half those for the other components. Groups of 22 males and 22 female mice were exposed to each concentration of the mixture for 3 and 6 weeks. Selected biological end-points including urine mutagenesis, bone marrow cell aberrations and micronuclei, spleen lymphocyte aberrations and liver enzyme induction were monitored. The spleen lymphocyte aberrations and liver enzyme induction were the most sensitive end-points. The lymphocytes showed a significant induction of chromosome aberrations from exposure for 3 weeks to all 3 concentrations of the mixtures. After 6 weeks of exposure, significant induction of aberrations was observed after exposure to low and medium concentrations but not to the high concentration. This lack of response at the high concentration after 6 weeks exposure, appeared to correlate with a significant induction of glutathione S-transferase in the liver. Since this enzyme is known to detoxify 3 of the 4 chemicals in our mixture, it may indicate a detoxification mechanism after enzyme induction. The present study indicates that the CTP is sufficiently sensitive to identify toxicological effects after exposure to ambient levels of a gas mixture.

Inhaled benzene reduces aspects of cell-mediated tumor surveillance in mice

Benzene is a potent bone marrow toxicant with particular activity against lymphocytes. Despite the recognized effects of benzene on lymphocyte populations, few data exist concerning the effects of benzene on in vivo immune responses. We have been conducting a series of studies concerning the effects of inhaled benzene on murine cell-mediated immune responses. The studies in this report involve the interaction of inhaled benzene with some of those cell-mediated immune responses associated with tumor surveillance. Exposures to 100 ppm benzene (5 days/week X 20 weeks) induced lethal tumor growth in 9/10 C57Bl/6 mice inoculated with 10(4) viable PYB6 tumor cells. Lethal tumor incidences in air controls and mice exposed to lower benzene concentrations were 3/10 or less. Exposures to 100 ppm benzene (5 days/week X 4 weeks) also reduced the tumor lytic abilities of cytotoxic T lymphocytes as determined by 51Cr-release assays. In addition, splenocytes taken from mice exposed to 10 or 100 ppm benzene (5 days/week X 4 weeks) exhibited delays in peak mixed leukocyte responses. Coculture experiments demonstrated that these delays were not due to an induction of suppressor cell activity by benzene. There were no alterations in the relative percentages of B cells, T cells, or T-cell subsets among splenocytes from animals exposed to any concentration of benzene tested. These results demonstrate that inhaled benzene can inhibit some of the processes associated with tumor surveillance, and that this inhibition is due, at least in part, to impairments of the functional abilities of some of the cells responsible for tumor surveillance.

Recent advances in the metabolism and toxicity of benzene

Benzene is a heavily used industrial chemical, a petroleum byproduct, an additive in unleaded gas, and a ubiquitous environmental pollutant. Benzene is also a genotoxin, hematotoxin, and carcinogen. Chronic exposure causes aplastic anemia in humans and animals and is associated with increased incidence of leukemia in humans and lymphomas and certain solid tumors in rodents. Bioactivation of benzene is required for toxicity. In the liver, the major site of benzene metabolism, benzene is converted by a cytochrome P-450-mediated pathway to phenol, the major metabolite, and the secondary metabolites, hydroquinone and catechol. The target organ of benzene toxicity, the hematopoietically active bone marrow, metabolizes benzene to a very limited extent. Phenol is metabolized in the marrow cells by a peroxidase-mediated pathway to hydroquinone and catechol, and ultimately to quinones, the putative toxic metabolites. Benzene and its metabolites appear to be nonmutagenic, but they cause myeloclastogenic effects such as micronuclei, chromosome aberrations, and sister chromatid exchange. It is unknown whether these genomic changes, or the ability of the quinone metabolites to form adducts with DNA, are involved in benzene carcinogenicity. Benzene, through its active metabolites, appears to exert its hematological effects on the bone marrow stromal microenvironment by preventing stromal cells from supporting hemopoiesis of the various progenitor cells. Recent advances in our understanding of the mechanisms by which benzene exerts its genotoxic, hematotoxic, and carcinogenic effects are detailed in this review.

Induction of cytogenetic damage in rodents after short-term inhalation of benzene

Experiments were designed to investigate both the induction of sister chromatid exchanges (SCEs) in peripheral blood lymphocytes (PBLs) and micronuclei (MN) in bone marrow polychromatic erythrocytes (PCEs) of mice and rats after inhalation of benzene (BZ). Male DBA/2 mice (17-19 weeks old) were exposed to target concentrations of either 0, 10, 100, or 1,000 ppm BZ for 6 hr. Male Sprague-Dawley rats (11-14 weeks old) were exposed to target concentrations of either 0, 0.1, 0.3, 1, 3, 10, or 30 ppm BZ for 6 hr. Blood was obtained by cardiac puncture 18 hr after exposure, and PBLs were cultured in the presence of lipopolysaccharide (mouse B cells, 60 micrograms/ml) or concanavalin A (rat T cells, 30 micrograms/ml) to stimulate blastogenesis for SCE analysis. Femoral bone marrow smears from both species were analyzed for MN in PCEs 18 hr after BZ exposure. Mouse PBLs revealed a significant concentration-related increase in the SCE frequency over controls at 10, 100, or 1,000 ppm BZ. Mouse bone marrow showed a significant concentration-dependent increase in MN over controls after exposure to 10, 100, or 1,000 ppm BZ. Rat PBLs showed a significant increase in the SCE frequency after exposure to 3, 10, or 30 ppm BZ. The statistical significance of the 1 ppm BZ result was borderline and dependent on the statistical test chosen. Rat cells revealed a significant concentration-related increase in MN after inhalation of either 1, 3, 10, or 30 ppm BZ. PBLs from treated mice showed significant concentration-dependent decreases in mitotic indices; however, cell cycle kinetics and leucocyte counts remained unaffected. Rat PBLs showed significant decreases in mitotic activity only after exposure to 3 and 30 ppm BZ, whereas cell cycle kinetics and leucocyte counts were unaffected. These results show that BZ can induce statistically significant cytogenetic effects in PBLs and PCEs of both mice and rats after a 6-hr inhalation of BZ at low concentrations.

Protracted exposure of C57BL/6 mice to 300 ppm benzene depresses B- and T-lymphocyte numbers and mitogen responses. Evidence for thymic and bone marrow proliferation in response to the exposures

Groups of C57BL/6J, male mice were exposed to 300 ppm benzene via inhalation for 115 exposures (6 h/day, 5 days/week), a regimen known to cause thymic lymphoma in these animals. The effects of these exposures on lymphoid parameters were determined by measuring the numbers of B- and T-lymphocytes and mitogen-induced proliferation of B- and T-lymphocytes in bone marrow, spleen, and thymus after 6, 30, and 115 exposures. The numbers of B-lymphocytes in bone marrow and spleen and the numbers of T-lymphocytes in thymus and spleen were found to be markedly reduced after all 3 periods. Mitogen-induced proliferation of bone marrow and splenic B-lymphocytes exhibited a progressive depression throughout the exposure period reaching a point of no observable response after 115 exposures. Splenic T-cell mitogen-induced proliferation was also markedly depressed throughout the exposures, but there was no evidence of a progressive decline in this response during the exposures. Bone marrow cellularity increased 3-fold and the numbers of thymic T-cells increased 15-fold in benzene-exposed mice between the 6th and 30th exposure. No corresponding increase in splenic cells was observed in benzene-exposed mice during this interval. The marked increases in the numbers of cells in bone marrow and thymus are interpreted as arising from compensatory proliferation of a subpopulation of cells in response to the exposures. The absence of increases in cell number in the spleen is interpreted as reflecting the lack of lymphoid restorative capacity in this organ. The marked increases of thymic and bone marrow cellularity are discussed relative to the known ability of this benzene exposure regimen to produce thymic lymphoma in these animals.

Modulation of the immune response to Listeria monocytogenes by benzene inhalation

Benzene is a potent bone marrow toxicant. While all blood cell types are targets for benzene poisoning, lymphocytes are particularly sensitive. The immunotoxic consequences of benzene or its metabolites have been demonstrated in a number of in vitro studies; however, little data exist regarding the effects of benzene on host resistance to infectious agents. This investigation examined the effects of benzene on murine resistance to an infectious agent, Listeria monocytogenes. Four concentrations of benzene were employed, 10, 30, 100, and 300 ppm. To determine recovery from the effects of benzene, two exposure regimens were employed: 5 days prior to infection (preexposure), or 5 days prior to and 7 days during infection (continuous exposure). Appropriate air controls were maintained. Splenic bacterial counts and immune responsive cell populations were determined from mice killed at Days 1, 4, and 7 of infection. Preexposure to benzene produced increased bacterial numbers at Day 4 of the infection only at the highest benzene concentration (300 ppm). In contrast, continuous exposure produced increased bacterial numbers at Day 4 of infection at all but the lowest benzene concentration (10 ppm). Bacteria counts were not increased in any benzene-treated group at Day 1 or Day 7 of infection. The increased bacterial numbers at Day 4 suggest an effect on cell-mediated immune responses. Both T and B lymphocytes were particularly sensitive to benzene exhibiting reductions at all concentrations greater than or equal to 30 ppm for both exposure regimens. Esterase-positive cells, however, were relatively resistant to benzenes effects. The results point to a benzene-induced delay in the immune response to L. monocytogenes.

Projections of leukemia risk associated with occupational exposure to benzene

In 1982, White et al published an assessment of quantitative leukemia risk associated with lifetime occupational exposure to benzene. At about the same time, IARC (1982) published estimates of quantitative cancer risk associated with industrial chemicals. Benzene was one of the two chemicals selected by IARC for its risk estimation. This paper presents a summary of these assessments along with new study results demonstrating adverse effects on bone marrow and peripheral blood cells as a result of low-level benzene exposure. Mathematical extrapolations based on epidemiologic studies are consistent with a finding of significant risk of dying from leukemia under the current occupational permissible exposure limit of 10 ppm. Although a significant reduction of risk could be expected to be achieved by reducing exposure to 1 ppm, a significant risk may still remain. The uncertainty of the dose-response projections rests on the underlying estimates of relative risk of death from leukemia, the estimates of benzene exposure (dose), and the appropriateness of the mathematical model. Recent findings in experimental animals demonstrate chromosomal damage to bone marrow cells, significant depression of the bone marrow, and disturbances of immune system function as a result of less than 1 week of exposure to the current permissible benzene exposure limit of 10 ppm. This was the lowest dose tested. These experimental findings provide further evidence of a potentially significant risk of bone marrow proliferative cancer (leukemia) as a result of low-dose benzene exposure.