Nanoparticle Cell Penetration - Aerosols: Measurement, Dosimetry, and Health Effects - page 219

interstitialized particles, either free or phagocytosed, may eventually be transported to the lymph
nodes (Lehnert, 1993). Current evidence suggests that the transportation of inhaled particles to the
lymph nodes is also size dependent (Oberdörster et al., 1988).
It has been demonstrated that a number of different entry mechanisms have been proposed to
explain the uptake of nanoparticles by a variety of cell types. However, it is also worth acknowledg-
ing that different types of endocytosis can operate simultaneously (Rejman et al., 2004) so that more
than one type of internalization pathway could contribute to their uptake and that if one uptake
pathway does not function, another can take over.
9.3 TRANSLOCATION ACROSS THE PULMONARY INTERSTITIUM: THE
INFLUENCE OF SIZE, SURFACE AREA, AND ASPECT RATIO
Although the lung consists of many cell types, the epithelial and macrophage cells play a pri-
mary role in controlling the entry of substances into the body via the lung. A number of studies
have demonstrated ability for nanoparticles to cross the lung epithelium. Probably the irst study
to do so was published by Oberdörster's group and demonstrated that following instillation or
inhalation of TiO 2 , nanoparticles (25 nm) to a greater extent than ine particles (250 nm), could
be found within the lung interstitium (Ferin et al., 1990a,b, 1992). Ferin et al. (1992) identiied
that “particles not phagocytosed by alveolar macrophages in the alveoli were taken up by alveo-
lar type I epithelial cells, which was probably the irst step for interstitial access of particles.”
This interstitialization and reduced clearance was associated with an elevated lung inlamma-
tory response, but it is worth noting that the exposure concentrations used in this study were
very high (500 μg instilled and up to 5 mg/rat lung burden following inhalation). A number of
in vivo studies conducted by different groups using different particles have also identiied simi-
lar results. For example, using electron microscopy (EM), it has been demonstrated that carbon
black particles instilled into the mouse lung were accumulated in gaps between the cytoplasmic
processes of alveolar epithelial cells, allowing transfer across the epithelial barrier into blood
(Shimada et al., 2006). In a study exposing rats to iridium-192 nanoparticles (Semmler-Behnke
et al., 2007), it was suggested that nanoparticles are much less actively phagocytosed by alveolar
macrophages than larger particles, but that they are effectively removed from the lung epithe-
lium into the interstitium. This is at odds with in vitro studies that suggest that nanoparticles are
taken up by macrophages (Clift et al., 2008), although it has been suggested that nanoparticles
decrease the subsequent phagocytosis of micron-sized particles (Renwick et al., 2001). Further
evidence is required before it is conirmed that diffusion is a viable uptake route for nanopar-
ticles into or across cells.
It is anticipated that modifying the speciic surface area and reactivity of the particles, such as
their ability to cause inlammation, would have effects on their ability to become interstitialized.
That is, the more inlammogenic the particles, the more they are likely to interstitialize. An inha-
lation study using differently sized poorly soluble particles, TiO 2 and BaSO 4 (Tran et al., 1999),
demonstrated a linear relationship between the particle burden expressed in units of surface area
and the number of neutrophil cells (indicative of inlammation) found in the bronchoalveolar lavage
(BAL) luid, suggesting that high speciic surface area of particles enhances inlammation (Stoeger
et al., 2006; Dufin et al., 2007), leading to interstitialization. Since surface area is inversely related
to size it could be hypothesized that increased surface area results in increased uptake into cells and
penetration of cell barriers, but it is not possible at this time to assess whether such an observation
is size or surface-area driven.
When considering the propensity of particles to enter the interstitium and potentially translocate,
a consideration not only of the primary particle size but also of the aggregation/agglomeration state
should be made. Indeed, it has been noted that in experimental systems of lung administration
with particles prone to agglomeration such as carbon nanotubes (CNTs), the degree of dispersion