1 Departments of Laboratory Medicine and Microbiology, University of Washington School of Medicine, Seattle, Washington 98195-7242; and 2 Department of Microbiology, University of Colorado Health Sciences Center, Denver, Colorado 80262
THE DISCOVERY OF NITRIC OXIDE (NO)
production from L-arginine by NO synthases has
revolutionized our understanding of many aspects of human physiology.
Since the seminal studies of Furchgott and Zawadzki (7),
tremendous progress has been made in understanding the regulation of
vascular tone by NO. Novel pharmacological agents based on the
circulatory actions of NO have already found widespread clinical
application. However, this contrasts with the role of NO in resistance
to infection, where clinical applications have as yet been largely
unrealized. The groundbreaking studies of Hibbs et al.
(8), demonstrating NO-dependent actions of murine macrophages, have proven more difficult to extrapolate to humans.
The inability to demonstrate NO production by human macrophages
initially posed a stumbling block. Humans unquestionably produce dramatically elevated quantities of NO during infection and other inflammatory conditions (4, 24). Nevertheless, in vitro
treatment of human peripheral blood mononuclear cells with
lipopolysaccharide and interferon- The answer to this question has emerged incrementally from more than
100 studies over the past decade. It is now indisputable that human
macrophages produce NO. NO synthase mRNA and protein have been
repeatedly demonstrated in activated human macrophages in a variety of
settings (10, 14, 25), along with biochemical evidence of
NO production (23). Inducible NO synthase (iNOS) protein
has been convincingly shown within infiltrating tissue macrophages of
infected patients (1). Importantly, NO-dependent biological actions of human macrophages have also been documented (13). Recent observations correlating iNOS promoter
polymorphisms with macrophage NO production and resistance to infection
(11) lend further support to an important role of NO in
human innate immunity. The key to demonstrating NO in human macrophages
is to allow stimulation to occur in vivo. Although NO generation can be
readily demonstrated in macrophages from patients with inflammatory
conditions such as tuberculosis (12), rheumatoid arthritis
(18), or malaria (2), in vitro conditions to
stimulate NO production by peripheral blood mononuclear cells of
healthy subjects remain incompletely characterized.
In this issue (Ref. 9, see p. L944),
Hickman-Davis et al. make an important addition to the list of
conditions found to promote NO production by human macrophages.
Alveolar macrophages from some healthy lung transplant patients
undergoing routine surveillance bronchoalveolar lavage, but not
macrophages from normal volunteers, were found to spontaneously
generate significant quantities of NO, which were further enhanced
following stimulation with surfactant protein A (SP-A) or live
Klebsiella pneumoniae. Notably, the SP-A-treated cells
exerted antimicrobial activity against Klebsiella that could
be abrogated by inhibition of NO production. This suggests that the
transplanted lung represents a stimulatory environment for host
defenses that promote resistance to bacterial infection and provides
further evidence that collectins such as SP-A promote innate immunity
in diverse ways. The demonstration of local NO synthesis in the setting
of lung transplantation corroborates earlier studies
showing iNOS mRNA in bronchoalveolar lavage fluid from lung allograft
recipients (16). Although bacterial pneumonia is common in
the initial period immediately following lung transplantation (3), alveolar macrophage activation may help to limit the
occurrence of pneumonia thereafter.
The attempts of Hickman-Davis et al. (9) to elucidate the
in vivo signals that precondition alveolar macrophages from certain transplanted patients to synthesize NO in response to SP-A, the mechanism by which this collectin augments phagocyte function and the
role of NO production in the antibacterial activity of human phagocytes
are less conclusive. Klebsiella alone elicits as much NO
production as SP-A, indicating that NO is contributory but not
sufficient to account for antimicrobial actions of SP-A. Similarly,
SP-A-induced rises in intracellular Ca2+ appear necessary
for these actions but occur in macrophages from both transplant
patients and normal controls. Hickman-Davis and colleagues show that a
combination of NO and reactive oxygen species produced by a
xanthine/xanthine oxidase system can rapidly kill Klebsiella
in vitro. However, it is unlikely that this explains the weak and
predominantly bacteriostatic effects of NO observed in alveolar
macrophages in vivo, and the investigators do not provide evidence for
a role of reactive oxygen species production by the macrophages. A
bacteriostatic effect of macrophage-derived NO would actually be
consistent with NO-dependent antimicrobial actions observed in murine
cells (22). The ability of NO to reversibly inhibit
essential cellular processes by nitrosylation of thiols and metals
provides a mechanistic rationale for bacteriostasis (17).
In addition to direct antimicrobial activity, it is conceivable that
NO-dependent actions of alveolar macrophages relate to pleiotropic gene
regulatory (5, 17) effects of NO. In support of this hypothesis, the protective actions of NO in a murine model of pulmonary
Klebsiella infection correlate with its role as a modulator of the expression of proinflammatory mediators (20).
The ability of NO to participate in diverse regulatory and cytotoxic
actions (6) creates a continuing challenge to elucidating its precise molecular mechanisms in complex biological systems like the
transplanted lung. Nevertheless, the observations of Hickman-Davis et
al. (9) reinforce the message that NO is an important
mediator in human innate immunity. Future therapeutic strategies for
infectious diseases may be designed to enhance NO production at sites
of infection or inhibit its deleterious collateral effects
(21). Although it has taken some time, researchers studying human macrophages are finally learning to take NO for an answer.
ARTICLE
TOP
ARTICLE
REFERENCES
fails to elicit NO production
(15), even though these stimuli are highly effective in
provoking murine macrophages to generate copious quantities of NO
(19). This led some investigators to question whether
human macrophages are capable of NO generation (15).
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FOOTNOTES |
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Address for reprint requests and other correspondence: F. C. Fang, 1959 NE Pacific St., Seattle, WA 98195-7242 (E-mail: fcfang{at}washington.edu).
10.1152/ajplung.00017.2002
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