(VCCEP) Tier 1 Pilot Submission for BENZENE - Tera

the measurements of doses are suspect, because the very large decreases in several blood
parameters are in contrast to most other studies, which found minimal or no response in bone
marrow cellularity at similar low exposure concentrations with 6 hours or more daily exposure.
Because benzene concentrations were not reported to have been measured after the first
3 days, the toxicity to bone marrow parameters suggests that actual benzene concentrations
were much higher than reported by the authors (IRIS, 1998).
Benzene-induced neurotoxic effects have been identified in humans and laboratory animals,
and demonstrated to diminish and/or disappear on termination of exposure. Effects usually
appeared at the same or higher doses than doses that induced hematopoietic toxicity (LOAEL
mice = 10 ppm; rats = 30 ppm) (ECB, 2003), and hematotoxic effects are routinely used to
establish LOAELs and NOAELs.
6.2.7 Immunotoxicity
Studies in which mice were exposed to benzene in drinking water have been performed by
several investigators to measure immunotoxic and hematotoxic effects. Hsieh et al. (1988b)
treated male CD-1 mice with 0, 31, 166, or 790 mg/L (0, 8, 40, or 180 mg/kg/day) benzene for
28 days. Statistically significant dose-related decreases in relative spleen weight, and nonstatistically
significant deceases in thymus weight were observed at all doses, while body
weight, liver weight, clinical signs, and necropsy results were comparable to controls. Doserelated
hematological effects (erythrocytopenia, leukocytopenia, lymphocytopenia, increased
motor conduction velocity) were observed at all exposure levels. Biphasic responses were
reported in immunological tests (mitogen-stimulated splenic lymphocyte proliferation, mixed
splenic lymphocyte culture response, and cytotoxic splenic T-lymphocyte response to allogenic
yeast artificial chromosome [YAC-1] cells), with significantly increased responses at 8 mg/kg
and significantly decreased responses at 40 and 180 mg/kg. Primary antibody response to
sheep red blood cells was significantly decreased at 40 and 180 mg/kg and similar to or higher
than controls at 8 mg/kg. A LOAEL of 8 mg/kg/day was designated by the authors. Hsieh et al.
(1991) later reported that, when supernatants from splenic T-lymphocyte cultures, stimulated
with ConA, were assayed for interleukin 2 (IL-2) content, splenic IL-2 production was
suppressed in the 40- and 180-mg/kg/day groups. The impact of benzene on cell-mediated
immunity and cytotoxic T-lymphocyte response in these studies support earlier work by
Rosenthal and Snyder (1987), in which reduced tumor lytic abilities of splenic cells were
demonstrated in C57Bl/6J mice exposed to 100 ppm (320 mg/m 3 ) benzene, 6 hours/day, 5
days/week for 10 days prior to tumor inoculation. Splenic T-cells from mice exposed to 10 ppm
and 100 ppm for 20 days showed delayed mixed lymphocyte reaction to alloantigens, possibly
due to benzene-induced impairment of functional abilities of alloreactive T-cells rather than
benzene-induced suppressor cells. Exposure to 100 ppm benzene for 20 days had not altered
relative proportions of splenic leukocytes, the percentage of splenic T-cell subsets, or the ratio
of splenic helper/suppressor cells.
Studies on humoral immune response in addition to Hsieh et al. (1988b) include work by
Aoyama (1986) in which male BALB/c mice, exposed to 50 or 200 ppm benzene vapor for
14 days showed reduced numbers of IgG- and IgM-plaque-forming cells/spleen in response to
sheep red blood cells (SRBC) in the plaque-forming assay. In contrast, male Sprague Dawley
rats showed no effect on humoral immune response measured in an ELISA [enzyme-linked
immunosorbent assay] of serum anti-SRBC IgM after 2–4 weeks exposure to 30, 200, or
400 ppm benzene, 6 hours/day, 5 days/week (Robinson et al., 1997). A reduction in the
number of B- and T-lymphocytes was observed at 400 ppm benzene for 2 weeks (B-cell only)
and 4 weeks.
Benzene VCCEP Submission
March 2006
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