Exposure to
diesel exhaust particles (
DEP) was shown to increase the susceptibility of the lung to
bacterial infection in rats. In this study, the effects of
DEP on alveolar macrophage (AM) phagocytic and bactericidal functions and
cytokine secretion by AM and lymphocytes in response to Listeria monocytogenes
infection were investigated in vitro and the roles of different
DEP components in these processes were compared. Exposure to
DEP or the organic extracts of
DEP (eDEP) significantly decreased the phagocytosis and killing of L. monocytogenes by AM obtained from normal rats. Washed
DEP (wDEP) also decreased AM phagocytosis and bacterial killing to a lesser extent, whereas
carbon black (CB) reduced AM phagocytosis but had no significant effect on AM bactericidal activity.
DEP or eDEP concentration-dependently suppressed L. monocytogenes-induced secretion of
tumor necrosis factor-alpha,
interleukin (IL)-1beta, and
IL-12 by AM and of
IL-2 and
interferon-gamma by lymphocytes obtained from L. monocytogenes-infected rats, but augmented the AM secretion of
IL-10. wDEP or CB, however, exerted little or no effect on these L. monocytogenes-induced
cytokines. These results provide direct evidence that
DEP, through the actions of organic components, suppresses AM phagocytic and bactericidal functions in vitro. Inhibition of AM phagocytic function and alterations of AM and lymphocyte
cytokine secretion by
DEP and
DEP organic compounds may be implicated in the diminished AM bactericidal activity and the lymphatic arm of the host immune system, thus resulting in an suppressed pulmonary clearance of L. monocytogenes and an increased susceptibility of the lung to
bacterial infection.