The Toxicologist - Society of Toxicology

for risk identification and characterization relative to allergic and autoimmune disease
(e.g., examining general vs. more susceptible subpopulations and disease induction
vs. disease exacerbation). This presentation will discuss allergic vs. autoimmune
responses, potential biomarkers for identifying chemical and drug hazards,
the importance of inflammation in these responses and options for revised safety
testing that could improve risk assessment for these chronic conditions.
2685 MECHANISMS UNDERLYING HEXACHLOROBENZENE-
INDUCED IMMUNOTOXICITY: COMPARISON WITH
IDIOSYNCRATIC DRUG REACTIONS.
J. Ezendam 1 and R. Pieters 2 . 1 National Institute for Public Health and the
Environment, Bilthoven, Netherlands and 2 Institute for Risk Assessment Sciences,
Utrecht, Netherlands.
The toxicity of hexachlorobenzene (HCB) was discovered in the 1950s after an accidental
poisoning in Turkey. Some symptoms, such as enlarged lymph nodes and
arthritis, were indicative for immunotoxicity. In rats, orally exposed to HCB several
immunological parameters were enhanced, including increased spleen and lymph
node weights, enhancement of humoral responses and induction of inflammatory
skin and lung lesions. The increased autoantibody levels and modulation of experimental
autoimmune diseases might indicate that HCB could induce autoimmunity,
similar to idiosyncratic drug reactions. Drugs like D-penicillamine and nevirapine
induce skin lesions and autoimmune phenomena as well. In the etiology of
these diseases T cells and macrophages are involved. The role of macrophages and T
cells in HCB-induced immunostimulation has been studied in Brown Norways
(BN) rats. After depletion of T cells with Cyclosporin A (CsA) in rats orally exposed
to HCB, the onset of skin lesion was delayed and splenomegaly was prevented
partly. The rise in IgE and autoantibody levels and lung eosinophilia were prevented
completely. However, adoptive transfer experiments showed that the disease
could not be transferred to naïve recipients, demonstrating that HCB does not induce
specific T cell sensitization. Depletion of macrophages with clodronate-liposomes
prevented of HCB-induced skin and lung lesions partly and inhibited elevation
of autoantibodies. The involvement of the innate immune system in
HCB-induced immune effects was further confirmed in a microarray study,
demonstrating that HCB activated several innate immune pathways. In conclusion,
HCB exposure induced a systemic inflammatory response, probably triggered by
macrophage activation. Subsequently, T cells are polyclonally activated and clinical
signs develop. In contrast, idiosyncratic reactions involve macrophages and T cells
as well, but this disease is induced after specific T cell sensitization.
2686 AUTOIMMUNITY IN IDIOSYNCRATIC DRUG
REACTIONS.
J. Uetrecht. University of Toronto, Toronto, ON, Canada.
Idiosyncratic drug reactions (IDRs) represent a major problem because at present
there is no way to accurately predict which drug candidates will cause such reactions
nor which patients are at risk. This unpredictability is largely due to a lack of
mechanistic understanding. Many IDRs are clearly immune-mediated but this is
not always the case, and some have characteristics that are atypical for an immune
response. Most drugs associated with a high incidence of IDRs can cause different
types of IDRs in different people, e.g. propylthiouracil can cause liver injury, agranulocytosis
or a lupus-like autoimmune syndrome. On a molecular level, drugs induce
a mixture of antibodies: some against various drug-modified proteins and also
autoantibodies against native proteins with a different pattern in each patient. This
is not surprising because each person has a different repertoire of major histocompatibility
antigens and T cell receptors, and it is expected that the highest affinity
recognition would be different in each individual. The typical characteristics of an
immune-mediated reaction include a delay between starting the drug and the onset
of the adverse event but a rapid onset with rechallenge. The time to onset varies
with the type of IDR with rashes typically occurring after a week or so, liver injury
after a month or so and autoimmune IDRs often requiring more than a year of
drug exposure before the onset of autoimmunity. This long delay in the onset of an
autoimmune reaction is presumably due to mechanisms that usually prevent autoimmunity.
In addition, autoimmune responses are not always associated with an
amnestic response, possibly for the same reason. Two animal models of drug induced
autoimmunity: penicillamine in rats and propylthiouracil in cats do not display
an amnestic response. Thus the variability in the nature of IDRs is most easily
explained by an immune mechanism, and features such as a long delay in onset and
a lack of amnestic response are consistent with an autoimmune component. Valid
animal models would greatly facilitate mechanistic studies but variability also
makes development of such models a challenge. Supported by grants from CIHR.
2687 TRICHLOROETHYLENE-INDUCED AUTOIMMUNITY;
DEPENDENCE ON METABOLISM AND GENETIC
SUSCEPTIBILITY.
K. Gilbert 1, 2 . 1 Microbiology and Immunology, Arkansas Children’s Hospital Research
Institute, Little Rock, AR and 2 University of Arkansas for Medical Sciences, Little
Rock, AR.
MRL +/+ mice exposed to occupationally relevant doses of the environmental contaminant
trichloroethylene (TCE) for 32 weeks developed T cell-mediated autoimmune
hepatitis, accompanied by the expansion of IFN-γ-producing and apoptosisresistant
(CD44 hi CD62L lo ) CD4 + T cells. TCE-induced alteration in CD4 + cell
activity required metabolism, and could be mimicked by exposure to the TCE
metabolite trichloroacetaldehyde hydrate (TCAH). However, TCAH did not induce
autoimmune hepatitis, but instead promoted skin inflammation and hair loss
similar to autoimmune alopecia in MRL+/+ mice. Unlike MRL+/+ mice, genetically-related
C3H/HeJ mice exposed to TCAH did not develop alopecia or altered
CD4 + T cell function. Interestingly, microarray analysis revealed that CD4 + T cells
from MRL+/+ mice had a much higher constitutive expression of the osteopontincoding
gene Spp1 than cells from C3H/HeJ mice. TCAH appears to further increase
expression of Spp1 as well as genes for other proinflammatory cytokines (e.g.
IFN-γ). Thus, it appears that altered constitutive as well as toxicant-induced expression
of certain CD4 + T cell genes may increase susceptibility to the immunotoxicity
of TCE and its metabolite TCAH.
2688 PBPK MODEL USE IN RISK ASSESSMENT: WHY BEING
PUBLISHED IS NOT ENOUGH.
E. D. McLanahan. ORD/NCEA, U.S. EPA, Research Triangle Park, NC.
While publication of a physiologically-based pharmacokinetic (PBPK) model in a
peer-review journal is a mark of good science, evaluation of published models and
the supporting computer code can identify issues or shortcomings that can be barriers
to their use in risk assessment. The quality of human health risk assessments
must be sustained, including use of appropriate PBPK models and other quantitative
tools for dose-response evaluation and interspecies extrapolation. A published
model can be modified to address assessment-specific issues, and having clear criteria
can reduce the number of review and revision iterations and hence the time
needed to bring a model to application. As probabilistic (population) PBPK models
are being implemented, parameter choices to fully characterize population variability
become critical. Thus a thorough but efficient process for the review and implementation
of PBPK models is necessary. While a typical process and set of criteria
were created specifically for PBPK models, the principles may be applied to other
types of computational models (e.g., dose-response) as they are developed. We have
convened a panel of experts in biologically-based modeling and relevant quantitative
methods to not only describe and discuss model evaluation and criteria, but
also issues and choices that arise in model application and assuring reasonable certainty,
as well as computational tools now available for improving model quality.
Addressing issues identified during the review and application of PBPK models for
human health risk assessments provides an opportunity to improve the science of
PBPK modeling as a whole. Attendees will obtain knowledge to improve their models
for use in risk assessment, evaluate when and how a model can be applied with a
fair degree of certainty, and better understand the constraints and requirements that
may otherwise limit use of a PBPK model. Finally, we will conclude with a panel
discussion and dialogue on the criteria, processes, methods, and choices that can
best assure the use of quality PBPK models in future risk assessments.
2689 A PROCESS FOR THE EVALUATION AND
IMPLEMENTATION OF PBPK MODELS IN HUMAN
HEALTH RISK ASSESSMENTS.
H. A. El-Masri. NHEERL, U.S. EPA, Research Triangle Park, NC.
An example process for the review and implementation of PBPK models in human
health risk assessments was developed. The process is intended to be rigorous and
comprehensive, yet efficient. A collaborative effort between the agency conducting
the risk assessment and its contractors is suggested, as well as engaging the original
authors of the models being evaluated. Several common errors identified in models
being evaluated for use in risk assessments will be discussed, with ways authors can
improve the potential for model use. Other models have not been implemented as
published for practical reasons that will be described. The ultimate goal of this presentation
is to inform and advise authors how to produce models that are more
likely to be usable “off-the-shelf” for assessment purposes.
SOT 2011 ANNUAL MEETING 575