Combustion-derived carbon nanoparticles cause delayed apoptosis in neutrophil-like HL-60 cells in vitro and in primed human neutrophilic granulocytes ex vivo.

Background: Inhalation of combustion-derived nanoparticles may contribute to the development or exacerbation of inflammatory lung diseases by direct interaction with neutrophilic granulocytes. Earlier studies have shown that exposure of human neutrophils to carbon nanoparticles ex vivo causes a prolongation of cellular life by the reduction of apoptosis rates. Accordingly, reduced neutrophil apoptosis rates were observed in neutrophils from bronchoalveolar lavages from carbon nanoparticle-exposed animals. The current study describes molecular and cellular modes of action responsible for this proinflammatory effect.

Results: Experiments with human blood neutrophils or neutrophil-like differentiated HL-60 cells exposed to carbon nanoparticles revealed dose dependent reduction of apoptosis rates. In both experimental systems, intracellular reactive oxygen species proved to be causally linked to this endpoint. Among the human samples, only primed cells from donors with slightly elevated proinflammatory plasma factors responded by delayed apoptosis. These neutrophils are characterized by an immunophenotype (CD16 CD62L) which is also observed in inflammatory lung diseases. Upon exposure to carbon nanoparticles these cells are further activated in an oxidant dependent manner. This activation appears to be linked to reduced apoptosis as samples with unchanged apoptosis rates were also not responding at this level. As reactive oxygen species triggered by carbon nanoparticles are known to cause membrane rearrangements, lipid raft structures were investigated by ganglioside M1 staining. Exposure of neutrophils resulted in a reduction of raft structures which could be prevented by an antioxidant strategy. The destruction of lipid rafts by depleting cholesterol also caused an activated immunophenotype and delayed apoptosis, indicating that membrane rearrangements after carbon nanoparticle exposure in primed neutrophils are responsible for cell activation and delayed apoptosis.

Conclusions: The antiapoptotic reactions observed in two independent experimental systems, differentiated neutrophil-like HL-60 cells and primed neutrophils, may be considered as additional proinflammatory effect of inhaled combustion-derived nanoparticles. Particularly in chronic diseases, which are characterized by neutrophilic lung inflammation, this effect can be expected to contribute to the deterioration of the health status. The data describe a mode of action in which intracellular reactive oxygen species cause membrane rearrangements that are responsible for neutrophil activation and delayed apoptosis.