Morphological aspects of interactions between microparticles and mammalian cells: intestinal uptake and onward movement.

Uptake of ingested microparticles into small intestinal tissues and on to secondary organs has moved from being an anecdotal phenomenon to a recognised and quantifiable process, which is relevant to risk assessment of accidental exposure, treatment of multi-organ dysfunction syndrome and therapeutic uses of encapsulated drug or vaccine delivery. This review puts in context with the literature the findings of a morphological study of microparticle uptake, using two approaches. The first is a rat in vivo in situ model, appropriate to a study rooted in the exposure of human populations to microparticles. Latex microspheres 2 µm in diameter are the principal particle type used, although others are also investigated. Most data are based on microscopy, but analysis of macerated bulk tissue is also useful. Uptake occurs at early time points after a single dose and is shown to take place almost entirely at villous rather than Peyer's patch sites: however, multiple feeding and therefore a longer time-span produces a higher proportion of particles associated with Peyer's patches, albeit for very small total uptake at those later time points. Uptake is less affected by species, fasting and immunological competence than by age and reproductive status. The second approach uses in vitro methods to confirm the role of intercellular junctions in particle uptake. Particle-associated tight junction opening, in a Caco-2 monolayer, is reflected in changes in transepithelial resistance and particle uptake across the epithelial monolayer: Tight junction opening and particle uptake are both increased further by external irradiation, ethanol and sub-epithelial macrophages, but reduced by exposure to ice. An M cell model has looser tight junctions than Caco-2 cells, but a similar level of particle uptake. These results, along with the changes seen in junctional proteins after particle addition, confirm the role of tight junctions in uptake but suggest that adhering junctions are also important.

Bystander-mediated genomic instability after high LET radiation in murine primary haemopoietic stem cells.

Communication between irradiated and unirradiated (bystander) cells can result in responses in unirradiated cells that are similar to responses in their irradiated counterparts. The purpose of the current experiment was to test the hypothesis that bystander responses will be similarly induced in primary murine stem cells under different cell culture conditions. The experimental systems used here, co-culture and media transfer, are similar in that they both restrict communication between irradiated and bystander cells to media borne factors, but are distinct in that with the media transfer technique, cells can only communicate after irradiation, and with co-culture, cells can communication before, during and after irradiation. In this set of parallel experiments, cell type, biological endpoint, and radiation quality and dose, were kept constant. In both experimental systems, clonogenic survival was significantly decreased in all groups, whether irradiated or bystander, suggesting a substantial contribution of bystander effects (BE) to cell killing. Genomic instability (GI) was induced under all radiation and bystander conditions in both experiments, including a situation where unirradiated cells were incubated with media that had been conditioned for 24h with irradiated cells. The appearance of delayed aberrations (genomic instability) 10-13 population doublings after irradiation was similar to the level of initial chromosomal damage, suggesting that the bystander factor is able to induce chromosomal alterations soon after irradiation. Whether these early alterations are related to those observed at later timepoints remains unknown. These results suggest that genomic instability may be significantly induced in a bystander cell population whether or not cells communicate during irradiation.

Effect of reproductive status on uptake of latex microparticles in rat small intestine.

This study investigates whether pregnancy or lactation affects microparticle uptake across the small intestinal mucosal barrier, since aspects of gastrointestinal physiology such as motility may be altered in these conditions. It also reports on validation of the model by several methods and discusses the findings in relation to possible mechanisms. Anaesthetised, pregnant, lactating, virgin female or male adult rats were gavaged with fluorescent latex microparticles. The small intestine was removed and fixed either 5 or 30 min later and successive segments of equal length were examined with fluorescence microscopy. Minor adjustments were made to experimental methods to explore details of the uptake mechanism. Control sections contained no particles. All experimental samples showed luminal and surface particles and also contained particles within the tissue, most associated with villous absorptive enterocytes. Particle uptake was greatest at the 30-min time-point, when maximum uptake was usually in the proximal jejunum; although in the early lactating group, this was shifted distally. Total tissue uptake was increased in pregnant and early lactating groups, mainly at villous absorptive and mucus-secreting cells. Accumulation and progression of particles was reflected in increased numbers in the lamina propria. These data were validated by several methods, including particle detection in the blood and mesenteric lymph nodes in some groups. At both time-points, uptake profiles for pregnancy and early lactation differed from those of other groups, implying possible links between particle uptake and hormone levels, surface mucus and tight junction patency.