Because the enriched environment (EE) increases the activity of T cells, contribute to
the immunopathogenesis of dengue virus infections (VDEN) we hypothesized that
animals maintained in an enriched environment (AE) compared with animals from
impoverished environment of standard laboratory cages (IE), would develop more
severe forms of the disease. Because older animals have less functional decline in
adaptive immunity T cells, we tested the hypothesis that AE old mice would show
higher number of deaths and more intense clinical signs than age-matched IE
animals, and this would be associated with greater expansion of T lymphocytes. To
test these hypotheses we established scheme of multiple inoculations in adult
animals of 9 and 18 months of age. Two regimens of inoculation were tested:
multiple injections of single serotype (VDEN3 genotype III) infected brain
homogenate (SS) or alternatively multiple injections with that infected brain
homogenate followed 24h later by inoculation of heterologous antibody (SSHA). In
both cases multiple i.p. inoculations were done. It was found significant differences in
the temporal progression of the disease in the animals submitted to one or another
scheme of inoculation: SSHA group (Kaplan -Meier log-rank test, p = 0.0025); IUS
(Kaplan -Meier log-rank test, p = 0.089). The survival curves of AE and AP under
SSHA regime were extended after a single injection of glucocorticoids, reducing the
symptoms and the number of deaths, and these effects were greater in the EE group
than in the IE (Kaplan-Meier log-rank test, p = 0.0162). In SSHA scheme, EE group
showed clinical signs more intense than the AP and those included dyspnea, tremor,
hunched posture, immobility, pre-terminal paralysis, shock and eventual death.
Compared to the IE group, the AE group regardless of age showed higher mortality
and more severe clinical signs. These more severe clinical signs in EE animals under
SSHA regime were associated with increased hyperplasia of T lymphocytes in the
spleen and increased infiltration of these cells in the liver, lungs and kidneys.
Although lymphocytic hyperplasia and infiltration have been more intense in older
than in younger animals, immunostaining for viral antigens in target organs was
higher in young than in the aged mice. The presence of the virus in various infected
organs were confirmed by real time PCR. Taken together the results suggest that the
enriched environment life style exacerbates the subsequent inflammatory response
to infection, and that is associated with more severe clinical symptoms, higher
mortality and increased T cell expansion. Behavioral and histopathological data
validate a new immunocompetent murine model for studies on dengue disease
allowing in vivo tests of a number of hypothesis raised by epidemiological and in vitro
studies.
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