Surface-dependent biodistribution and proinflammatory properties of semiconductor quantum dots

Methods based on nanotechnologies play a growing role in biomedical
research. Quantum dots (QDs) are a group of engineered fluorescent
nanoparticles suited for advanced imaging applications. The
substitution of the particle’s surface with defined molecular
structures could enable the adoption as targeted contrast agents or
therapeutic devices for a variety of clinical approaches. However,
important aspects such as the basic surfacedependent behavior of
non-targeted QDs in the organism and arising health effects upon
systemic administration remain incompletely understood. Acute
inflammatory effects for instance are often initiated on the
microcirculatory level and are probably relevant for cardiovascular
pathologies observed in epidemiologic and experimental studies of
certain nanoparticles. Most in vitro studies show that the surface
structures of QDs and other nanoparticles seem to be predominantly
accountable for different cytotoxic effects and a variable
potential to liberate proinflammatory cytokines. Currently, no
systematic in vivo studies have addressed surface-dependent
interactions of QDs on the level of the microcirculation and
assessed the resulting impact on biokinetics as well as on
proinflammatory parameters. Thus, this thesis aimed to i) analyze
the incidence of QDsurface- dependent acute microvascular
interactions and their influence on key biokinetic parameters and
ii) investigate acute immunomodulatory effects on the multistep
process of leukocyte recruitment in vivo. For this, three types of
commercially available QDs with different surface modifications:
carboxyl-QDs, amine- and polyethylene glycol-QDs (amine-QDs) and
polyethylene glycol-QDs (PEG-QDs) were used. The physicochemical
characterization was done by dynamic light scattering (DLS)
analysis and microscale thermophoresis. In a first set of
experiments, circulating half-lives, tissue distribution in
different organs, and hepatic as well as renal clearance were
measured. Ex vivo analysis of QD tissue distribution was performed
on selected tissue samples via transmission electron microscopy
(TEM) and two-photon microscopy. By combining reflected-light
oblique transillumination (RLOT) and fluorescence in vivo
microscopy of the murine M. cremaster, interactions of QDs with
components of the microcirculation as well as leukocyte migration
parameters were visualized and quantified. The extreme short
circulating half-life of anionic carboxyl-QDs was related to
pronounced clearance by the mononuclear phagocyte system. Beyond
this, further investigations showed, for the first time, that the
continuous capillary endothelium of skeletal and heart muscle
tissue has the capacity to directly extract carboxyl-QDs from the
circulation by means of caveolaemediated endocytosis. Carboxyl-QDs
were also taken up by perivascular macrophages in the surgically
exposed but not in the native M. cremaster and led to a significant
increase of adherent and (subsequently) transmigrated leukocytes in
this model. Further experiments provided evidence for a probable
involvement of mast cells in the intercellular adhesion molecule-1
(ICAM-1)- and endothelial (E)-selectinmediated modulation of
leukocyte recruitment. This process is most likely initiated by the
endocytosis of carboxyl-QDs through activated perivascular
macrophages. The primary activation of tissue-resident perivascular
macrophages seems to be the consequence of tissue damage related to
the surgical preparation of the cremaster muscle. This is
supposedly a prerequisite for the endocytosis of carboxyl-QDs
whereupon endothelial and mast cells seem to be secondarily
activated in a paracrine fashion that then leads to an increase in
leukocyte recruitment.