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Infect Immun, May 1998, p. 1848-1854, Vol. 66, No. 5
Department of Pathobiology, University of
Guelph, Guelph, Ontario N1G 2W1, Canada
Received 11 September 1997/Returned for modification 21 January
1998/Accepted 4 February 1998
To look for a possible correlation between the virulence of
Rhodococcus equi and its cytokine-inducing capacity, we
evaluated intracellular survival and measured cytokine induction by
mouse macrophages infected with a virulent strain containing an 85-kb plasmid and expressing VapA (103+), its avirulent
plasmid-cured derivative (103
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Copyright © 1998, American Society for Microbiology. All rights reserved.
Cytokine Induction in Murine Macrophages Infected
with Virulent and Avirulent Rhodococcus equi
), and heat-killed
103+ (HK). After incubation with similar numbers of
bacteria, macrophages infected with 103
contained
significantly more organisms than those infected with 103+
or HK. The number of bacteria in the macrophages infected with 103
and HK decreased progressively, whereas the
103+ numbers remained constant over 48 h. Interleukin
1
(IL-1
), IL-6, IL-10, IL-12 p40, and tumor necrosis factor alpha
(TNF-
) mRNA induction peaked at 4 h and returned to baseline
between 12 and 48 h postinfection. IL-1
, IL-6, IL-10, and
TNF-
concentrations assessed by enzyme-linked immunosorbent assay
generally agreed well with mRNA expression; IL-12 could, however, not
be detected. For all the cytokines detected, mean concentrations in the
supernatants were consistently higher in the 103
-infected
monolayers than in those infected with 103+, although, with
the exception of IL-1
, the differences were not statistically
significant. R. equi HK was a poor inducer of cytokine
production. In conclusion, virulent and avirulent R. equi
strains induced similar levels of cytokine synthesis. The slightly
greater induction of most cytokines observed following infection with
103
is likely secondary to greater uptake by macrophages
rather than to a direct role of VapA or another plasmid-encoded product
in downregulating cytokine induction.
*
Corresponding author. Mailing address: Department of
Pathobiology, University of Guelph, Guelph, Ontario N1G 2W1, Canada. Phone: (519) 824-4120, ext. 4716. Fax: (519) 767-0809. E-mail: jprescott{at}ovcnet.uoguelph.ca.
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