-KL7 than 2308 while the alternative immune control
mechanisms that operate in BALB/c mice during the plateau
phase of infection are (The complete mechanisms operating
in BALB/c mice are unclear but include TNFa ). In sup-
port of this, while bacA
-KL7 was more vulnerable to kill-
ing by C57BL/6 macrophages than was 2308 the two strains
showed similar sensitivity to control by IFNg-activated macro-
phages. Thus, any differences seen with non-IFNg-activated
macrophages would be expected to be minimized in C57BL/6
mice since they continually produce IFNg during the in-
fection. Presumably this occurs in vivo since we found
-KL7 is indeed vulnerable to IFNg-based mechanisms
of control shown by the higher bacA
-KL7 CFU recovered
from the C57BL/6 IFNg-deﬁcient mice than from C57BL/6
wild-type mice. In contrast, sensitivity to complement-based
killing, reported here, would be expected to contribute to de-
-KL7 recovery in all stains of mice.
The surprising result was the greater virulence of bacA
KL7 in immune-deﬁcient mice. Gene deletion resulting in
increased virulence is not without precedence as it has been
reported for Shigella, E. coli and Brucella that have lost their
ability to acquire iron . Here the increased inﬂammation in
the IFNg-deﬁcient mice infected with bacA
-KL7 is consis-
tent with the increase in pro-inﬂammatory cytokines induced
-KL7 and its LPS. While incr eased ability to stim-
ulate TNFa may be particularly germane to its attenuation in
BALB/c mice whose immunity to brucellosis is known to de-
pend upon TNFa as noted above . The increased produc-
tion of pro-inﬂammatory ctyokines is likely to increase
pathogenesis in IFNg-deﬁcient mice since the increase in
IL-12 would not result in an increase in the protective IFNg
in those mice.
The increase in pro-inﬂammatory cytokines also favors the
hypothesis that reduced very-long-chain fatty acid modiﬁca-
tion of the lipid A in bacA
-KL7 engenders the LPS with
greater capacity to interact with host TLR , albeit puriﬁed
-KL7 LPS was still less potent than Salmonella LPS.
In addition, the altered LPS of bacA
-KL7 may enable it
to interact with additional types of TLR  although both
are speculative. Finally, it cannot be ruled out that unidentiﬁed
alterations in addition to the lipid A effect could result from
the bacA gene deletion and contribute to the increased patho-
logy induced by bacA
-KL7. These possibilities provide av-
enues for future investigations.
In conclusion, these data show that the degree of attenua-
tion of the bacA
-KL7 is depend ent upon the host and the
nature of its immune response. Loss of bacA resulted in less
attenuation relative to 2308 when the mutant was evaluated
in C57BL/6 mice compared to that in BALB/c mice. More-
-KL7 caused unique pathology in immune-
deﬁcient mice characterized by abscesses and wasting. These
observations illustrate important considerations for designing
live vaccines and are in keeping with the observation that
while the live vaccine strain B. abortus S19 is attenuated in
cattle, it is not attenuated in humans. They also indicate that
a bacA mutation in a potential vaccine strain may be particu-
larly inappropriate for hosts with immune deﬁciencies.
This work was supported by USDA-CSREES competitive
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