The neutrophil is the first haemopoetic cell to arrive at the site of infection. In acute
respiratory distress syndrome (ARDS), dense neutrophilic infiltrates are found in the
lung in response to bacterial infection as well as generalised inflammatory stimuli,
such as pancreatitis. At sites of infection, phagocytosis of bacteria by neutrophils
enhances their subsequent apoptosis and clearance by macrophages however at
inflammatory sites, the lifespan of the neutrophil is influenced by both pro- and antiapoptotic
factors in the inflammatory milieu. Furthermore subsequent macrophage
phagocytosis of apoptotic neutrophils induces the macrophage to switch to an antiinflammatory
phenotype thereby hastening resolution of inflammation.
The Fas death receptor pathway is important in T lymphocyte apoptosis but its role
in neutrophil apoptosis is controversial. We have shown that neutrophils express the
Fas receptor (CD95) on their surface but there is no evidence of expression of its
natural ligand (FasL). An agonistic anti-Fas monoclonal antibody (CH-11)
accelerated neutrophil apoptosis under certain culture conditions.
Lipopolysaccharide (LPS) originating from Gram-negative bacteria is often found at
sites of inflammation. We have shown that LPS attenuated CH-11 - induced
neutrophil apoptosis unless the Fas/FasL death receptor pathway was activated prior
to the LPS signalling pathway. This LPS-mediated attenuation did not involve the
p42/44 ERK, protein kinase C or phosphatidylinositol 3-kinase signalling pathway
however the p38 MAPK and NF-κB pathway appeared to be partially involved. We
have shown that neutrophils express the protein cFLIPs and that CH-11 and
inflammatory mediators altered its expression.
Although macrophages are principally phagocytes, they are also important in
determining the composition of the milieu at an inflammatory site. Macrophages
have been shown to express FasL which can be shed and may contribute to the pools
of sFasL found in the bronchoalveolar lavage fluid (BALF) in ARDS patients. We
have shown that the conditioned supernatants from LPS-activated macrophages
induced neutrophil apoptosis at early time points. The pro-apoptotic activity was
mediated by TNF-α and was found in the fraction containing proteins with molecular weights greater than 50kD. Macrophage phagocytosis of apoptotic neutrophils
suppressed TNF-α production by LPS-activated macrophages and this was associated
with loss of the pro-apoptotic activity.
In summary, our data suggest that Fas/FasL fratricide does not appear to be involved
in spontaneous neutrophil apoptosis. However LPS attenuates Fas-induced apoptosis
unless the Fas/FasL death receptor pathway is activated prior to LPS signalling
pathways. The signalling pathways involved in this attenuation are not clear but may
involve cellular FLIP. Furthermore, activated macrophages secrete inflammatory
mediators and at early time points, TNF-α appears to be the most important in
inducing neutrophil apoptosis
