Nanoparticles reveal that human cervicovaginal mucus is riddled with pores larger than viruses

Abstract

The mechanisms by which mucus helps prevent viruses from infecting mucosal surfaces are not well understood. We engineered non-mucoadhesive nanoparticles of various sizes and used them as probes to determine the spacing between mucin fibers (pore sizes) in fresh undiluted human cervicovaginal mucus (CVM) obtained from volunteers with healthy vaginal microflora. We found that most pores in CVM have diameters significantly larger than human viruses (average pore size 340 ± 70 nm; range approximately 50–1800 nm). This mesh structure is substantially more open than the 15–100-nm spacing expected assuming mucus consists primarily of a random array of individual mucin fibers. Addition of a nonionic detergent to CVM caused the average pore size to decrease to 130 ± 50 nm. This suggests hydrophobic interactions between lipid-coated “naked” protein regions on mucins normally cause mucin fibers to self-condense and/or bundle with other fibers, creating mucin “cables” at least three times thicker than individual mucin fibers. Although the native mesh structure is not tight enough to trap most viruses, we found that herpes simplex virus (approximately 180 nm) was strongly trapped in CVM, moving at least 8,000-fold slower than non-mucoadhesive 200-nm nanoparticles. This work provides an accurate measurement of the pore structure of fresh, hydrated ex vivo CVM and demonstrates that mucoadhesion, rather than steric obstruction, may be a critical protective mechanism against a major sexually transmitted virus and perhaps other viruses.

Lamellar Structures of MUC2-Rich Mucin: A Potential Role in Governing the Barrier and Lubricating Functions of Intestinal Mucus

“…Olmsted found, using macromolecules, viruses, and polystyrene nanoparticles, an apparent pore size of 100−110 nm (agreeing with electron micrographs showing pore sizes of 20−200 nm reported in the same work), while Lai found that the diffusion rates they had recorded for poly(ethylene glycol) (PEG)-coated nanoparticles of sizes up to 500 nm required a pore size of at least 340 ± 70 nm. 8 Although the mucus used in both these works was cervicovaginal and did not contain MUC2, Macierzanka et al 6 observed that nonmucoadhesive nanoparticles as large as 2 μm were able to diffuse through porcine jejunal mucus containing MUC2 mucin at different rates depending on the local microviscosity of the mucus, including some at similar rates to water, implying an open-structured gel mesh similar to that calculated by Lai et al It is important to note that Macierzanka et al reported similar distributions of Stokes viscosities for the 500 nm nonmucoadhesive nanoparticles in both a mucin preparation similar to the one studied in this work and in an ex-vivo mucus sample that had not been purified (and thus reflects the behavior of the mucin in its native mucosal environment). The authors concluded from this that the obstruction to particle diffusivity they observed was due to the mucin and not, to a significant degree, to the presence of other mucus components.…”

“…A model is required to fit the diffusion rates and determine a mesh size for the network that will allow passage of the particles. Olmsted et al 4 and Lai et al 8 used a model describing homogeneous hydrogels, 45,46 a model appropriate for a physically entangled linear polymer system. Olmsted found, using macromolecules, viruses, and polystyrene nanoparticles, an apparent pore size of 100−110 nm (agreeing with electron micrographs showing pore sizes of 20−200 nm reported in the same work), while Lai found that the diffusion rates they had recorded for poly(ethylene glycol) (PEG)-coated nanoparticles of sizes up to 500 nm required a pore size of at least 340 ± 70 nm.…”

Delivery strategies to enhance oral vaccination against enteric infections

“…Although, few studies have conclusively addressed the role of size in dictating mucosal immunity following oral delivery, there is a consensus that particles with a diameter smaller than 1 μm are more readily taken up by M cells [190]. In addition to the role of size in regulating the potential for trans-epithelial uptake, the ability of particles to migrate through the pores in the mucus is dependent on both size and surface electrochemical properties [33,191]. While M cells remain a key target for particulate oral vaccines, it was recently shown that particles of 1-5 μm in size were taken up across the jejunum and ileum via non phagocytic processes, suggesting that particle uptake is not solely via M cells [192].…”

Correlating gastric emptying of amphotericin B and paracetamol solid lipid nanoparticles with changes in particle surface chemistry

“…Particles with sizes below 10 µm are well taken up in the GI tract but uptake occurs more favourably when sizes are below 1000 nm (Jani et al, 1989;Kreuter, 1991;Shakweh et al, 2005). In addition, the mesh-pore spacing of the intestinal 295 mucosal barrier is 50-1800 nm (Lai et al, 2010;Primard et al, 2010).…”