ATTENUATED SALMONELLA VACCINE-INDUCED SUPPRESSION OF MURINE SPLEEN-CELL RESPONSES TO MITOGEN IS MEDIATED BY MACROPHAGE NITRIC-OXIDE - QUANTITATIVE ASPECTS
D. Huang et al., ATTENUATED SALMONELLA VACCINE-INDUCED SUPPRESSION OF MURINE SPLEEN-CELL RESPONSES TO MITOGEN IS MEDIATED BY MACROPHAGE NITRIC-OXIDE - QUANTITATIVE ASPECTS, Infection and immunity, 64(9), 1996, pp. 3786-3792
Previous reports from our laboratory have shown that 7 days after infe
ction of C3HeB/FeJ mice with an attenuated strain of Salmonella typhim
urium, there is profound suppression of responses to B- and T-cell mit
ogens and suppression of the capacity of spleen cells to mount a prima
ry, in vitro plaque-forming-cell (PFC) response to sheep erythrocytes.
Inhibition of the PFC response was shown to be mediated by nitric oxi
de (NO), as N-G-monomethyl-L-arginine (NMMA) gave complete reversal of
suppression, The experiments reported here examined the role of NO in
suppression of the response to the mitogen concanavalin A (ConA). In
contrast to the PFC system, it was found that addition of NMMA to ConA
-stimulated immune spleen cells: resulted in less than 20% reversal of
suppression, However, addition of NMMA resulted in a 50% reversal of
suppression in cocultures of immune and normal spleen cells at a ratio
of 1:4. A complete restoration of ConA-induced responses was achieved
in cocultures incubated in medium containing a reduced concentration
of L-arginine plus 1.25 mM NMMA. Investigation of why NMMA alone was n
ot 100% effective in reversing suppression showed that addition of Con
A significantly augmented production of nitrite and gamma interferon (
IFN-gamma) in cocultures containing immune cells. Addition of anti-IFN
-gamma reduced nitrite levels in the cultures, although results with t
he combination of anti-IFN-gamma and NMR IA were not significantly bet
ter than results with NMMA alone. These findings suggest that suppress
ion in cultures stimulated with ConA is difficult to reverse completel
y with NMMA alone because of an overproduction of NO, which can be off
set by either reducing the L-arginine concentration or blocking IFN-ga
mma. The quantitative relationship between nitrite levels and suppress
ion in cocultures was examined, It was found that suppression did not
correlate directly with the nitrite concentration but rather with the
log(10) of the nitrite concentration. Nitrite levels above 15 mu M gav
e almost complete suppression, and levels between 1 and 10 mu M gave a
wide range of suppression, These results strongly support NO as the s
uppressor factor in Salmonella-induced immunosuppression of responses
to ConA and, by inference, suppression of responses to mitogens induce
d by other microbes. The results show that involvement of NO cannot al
ways be demonstrated by simple addition of NMMA to suppressed mitogen-
stimulated spleen cell cultures.