Acting as fuel combustion catalysts to increase fuel economy, cerium dioxide (ceria, CeO2) nanoparticles have been used in
Europe as diesel fuel additives (EnviroxTM). We attempted to examine the effects of particles emitted from a diesel engine
burning either diesel (diesel exhaust particles, DEP) or diesel doped with various concentrations of CeO2 (DEP-Env) on innate
immune responses in THP-1 and primary human peripheral blood mononuclear cells (PBMC). Batches of DEP and DEP-Env
were obtained on three separate occasions using identical collection and extraction protocols with the aim of determining
the reproducibility of particles generated at different times. However, we observed significant differences in size and surface
charge (zeta potential) of the DEP and DEP-Env across the three batches. We also observed that exposure of THP-1 cells and
PBMC to identical concentrations of DEP and DEP-Env from the three batches resulted in statistically significant differences
in bioreactivity as determined by IL-1b, TNF-a, IL-6, IFN-c, and IL-12p40 mRNA (by qRT-PCR) and protein expression (by
ELISPOT assays). Importantly, bioreactivity was noted in very tight ranges of DEP size (60 to 120 nm) and zeta potential (237
to 241 mV). Thus, these physical properties of DEP and DEP-Env were found to be the primary determinants of the
bioreactivity measured in this study. Our findings also point to the potential risk of over- or under- estimation of expected
bioreactivity effects (and by inference of public health risks) from bulk DEP use without taking into account potential batchto-batch
variations in physical (and possibly chemical) properties.