Ricerc@Sapienza

Intracellular pathogens are a critical challenge for antimicrobial therapies. Staphylococcus aureus (S. aureus)
causes approximately 85% of all skin and soft tissue infections in humans worldwide and more than 30% of
patients develop chronic or recurrent infections within three months, even after appropriate antibacterial
therapies. S. aureus is also one of the most common bacteria found in chronic wounds. Recent evidences suggest
that S. aureus is able to persist within phagolysosomes of skin cells (i.e. keratinocytes, phagocytic cells), being
protected from both the immune system and a number of antimicrobials. To overcome these limits, nano-formulations
that enable targeted therapies against intracellular S. aureus might be developed. Herein, the biodistribution
and intracellular localisation of hyaluronan (HA) and HA-based nanoparticles (nanogels, NHs) are
investigated, both after intravenous (i.v.) injections (in mice) and topical administrations (in ex vivo human
skin). Results indicate HA and NHs accumulate especially in skin and liver of mice after i.v. injection. After
topical application on human skin explants, no penetration of both HA and NHs was detected in skin with intact
stratum corneum. By contrast, in barrier-disrupted human skin (with partial removal and loosening of stratum
corneum), HA and NHs penetrate to the viable epidermis and are taken up by keratinocytes. In mechanically
produced wounds (skin without epidermis) they accumulate in wound tissue and are taken up by dermis cells,
e.g. fibroblasts and phagocytic cells. Interestingly, in all cases, the cellular uptake is CD44-mediated. In vitro
studies confirmed that after CD44-mediated uptake, both HA and NHs accumulate in lysosomes of dermal fibroblasts
and macrophages, as previously reported for keratinocytes. Finally, the colocalisation between intracellular
S. aureus and HA or NHs is demonstrated, in macrophages. Altogether, for the first time, these results
strongly suggest that HA and HA-based NHs can provide a targeted therapy to intracellular S. aureus, in persistent
skin or wound infections.