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Department of Internal Medicine, Justus-Liebig University, Giessen, Germany
Although cytokine synthesis in polymorphonuclear leukocytes (PMN)
was shown to be modulated by soluble mediators, the impact of
microenvironmental conditions has not been elucidated. In this study,
we investigated the effect of cell density on cytokine release from
human neutrophils. PMN were cultured at various cell densities (10
x 106 PMN/ml; 60 x 106 PMN/ml), and
LPS-induced release of cytokines was quantified by ELISA technique.
Upon an increase in PMN density, secretion of the CXC chemokine IL-8
was progressively reduced. This effect was paralleled by a decrease in
IL-8 mRNA. In contrast, TNF-
and IL-1
rose proportionally with
increasing cell density. The inhibition of IL-8 secretion was
reproduced by conditioned media of PMN at high cell density, but was
not affected by blocking
2 integrin-dependent adhesion.
When analyzing the supernatant of LPS-challenged neutrophils, large
amounts of soluble TNFRs p55 and p75 (sTNFRI, sTNFRII), and IL-1R
antagonist (IL-1RA), rising constantly with the cell density, were
detected. Interestingly, combined blocking of the bioactivities of
these mediators completely restored neutrophil IL-8 secretion at high
cell densities, with the anti-IL-1RA Ab being the more potent
agent. Moreover, combined application of exogenous IL-1RA and sTNFRs to
10 x 106 PMN/ml reproduced the suppression of IL-8
generation. We conclude that neutrophil IL-8 synthesis is
autoregulated, being suppressed under conditions of high cell density.
IL-1RA and sTNFRs, accumulating under these circumstances, seem to be
centrally involved in this regulatory mechanism by interfering with the
IL-1
- and TNF-
-dependent IL-8 generation. This feedback mechanism
may control further neutrophil recruitment and activation in a
neutrophil-rich environment, thereby preventing tissue
destruction.
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