Sex-specific nanomedicine- and biomaterials-based therapies of chronic wounds

Abstract

Wound healing mechanisms differ depending on the sex, particularly in chronic wounds. Therefore, sex should be considered in the design of nanomedicine- and biomaterials-based wound healing therapies, both in preclinical and clinical testing.

Wound healing mechanisms, and, in particular, those associated with chronic wounds, exhibit sex-specific differences in the response to inflammation and infection, in terms of angiogenesis, matrix synthesis and cellular mechanisms as well as with regards to comorbidities associated with chronic wound disorders and ageing¹. From infectious disease progression and severity to vaccine efficacy and reactions, sex affects both innate and adaptive immune responses². For example, compared with macrophages from female individuals, macrophages from male individuals secrete more pro-inflammatory cytokines and express more Toll-like receptor 4 (TLR4) proteins². Moreover, in response to the release of reactive oxygen species and hydrolytic enzymes, which occurs in the inflammatory phase of wound healing, female macrophages have increased activation and phagocytic abilities compared with male macrophages². Furthermore, neutrophils in male individuals show higher expression of TLRs, whereas dendritic cells from female individuals demonstrate greater activity of type 1 interferon signalling and TLR7 expression². In addition to immune cells, keratinocytes and fibroblasts have indispensable roles in tissue repair and wound healing¹; however, it is not yet understood how keratinocytes and fibroblasts differ between sexes. The influence of sex on wound healing is also affected by age, as the levels of some components of the inflammatory response, typically higher in women than in men during adulthood, reverse at older age (>65), mainly owing to the hormonal effects². This shift correlates with the rapid decline in sex steroids in women and a more gradual decline in men, along with a general decline in immune system function associated with ageing. The microbiome composition in response to wounding also differs between healthy female and male individuals, with more abundant Streptococcus, Finegoldia and Anaerococcus in men, which may be related to the stronger immune reactions in women in comparison to men³. Interestingly, the same bacteria also play an important part in the chronicity of diabetic foot ulcers⁴.

These sex-specific differences in wound healing should be considered in the design of nanomedicine- and biomaterial-based treatment modalities for wound repair.

Potential effect of sex on biomaterial-based therapies

For the treatment of chronic wounds, a variety of nanomedicine- and biomaterial-based therapies have been explored. For example, multifunctional hydrogels and scaffolds have been developed to promote healing processes in chronic wounds through their anti-infective, angiogenic, anti-inflammatory, moisture- and exudate-regulating and/or pressure-relieving properties⁵. However, the influence of sex on the biological identity and effectiveness of nanomedicine- and biomaterial-based treatment modalities remains underexplored. Here, we outline scenarios of how sex may affect the function of (nano)biomaterials in the wound microenvironment.

Nanomedicine- and biomaterial-based products typically interact with biomolecules in the wound microenvironment (Fig. 1), where secreted biomolecules can adhere to their surface, resulting in the formation of a protein or biomolecule corona. Importantly, the composition and characteristics of this corona affects the interactions and fate of the (nano)biomaterials; for example, proteins (such as unfolded fibrinogen) that participate in the biomolecular corona can interact with immune cell receptors and promote inflammation. The formation of male and female biomolecular coronas has been demonstrated in various biological fluids (for example, plasma)⁶, and, therefore, the aforementioned sex-specific differences in the secretion of certain cytokines by immune cells in the wound microenvironment may lead to the formation of protein coronas that differ between men and women, thereby influencing the therapeutic effectiveness of the (nano)biomaterial.

Fig. 1 |. Nanomedicine and biomaterials design for wound healing should consider sex.

The molecular and cellular responses of the wound microenvironment to nanomedicine and biomaterials differ based on sex, which may affect the safety and therapeutic effectiveness of nanomedicine- and biomaterials-based wound dressings. DC, dendritic cell; TLR, Toll-like receptor.

For example, TLR4 has an essential role in pathogen recognition and promotes wound healing⁷; therefore, the high expression of TLR4 in male individuals may enhance the antibacterial properties of biomaterials, which may trigger rapid activation of innate immunity essential to combating bacteria invading the wound. Similarly, the high activity and phagocytic abilities of female macrophages and neutrophils may lead to rapid degradation and potentially rapid clearance of biomaterials in a wound¹, which may decrease their effectiveness in female patients (Fig. 1). Moreover, age should be factored in during the design of (nano)biomaterials; for example, age-related changes in immune system interactions, skin characteristics (in terms of mechanical properties and composition) and degradation processes should be considered. Finally, differences in the chronic wound microbiome may have to be accounted for, as bacterial metabolites may also contribute to sex-specific protein coronas.

Sex differences in animal models of wound healing

Tissue repair processes are evolutionarily conserved between animals and humans¹. Therefore, animal models are commonly used for preclinical wound healing research, particularly in mice, rats, pigs and rabbits. Importantly, animal sex should be considered in the testing of nanomedicine- and biomaterials-based therapeutic approaches for wound healing. For instance, a hydrogel-based wound dressing designed to support wound healing through antibiotic and growth factor release was tested in a mouse model, in which infection rate can be monitored; here, male mice showed a faster healing rate and a more favourable response to antibiotics compared to females⁸. Although in this case both male and female mice were used and the results were stratified by sex, such sex-specific responses are rarely assessed in animal models of wound healing, although they should be addressed in US National Institute of Health-funded studies.

Sex differences in clinical trials

Lower extremity ulcers, such as diabetic foot ulcers and venous leg ulcers, are increasingly treated with biomaterials-based wound dressings⁹. To ensure that the wound dressing is effective in all sexes and to test whether sex-specific safety concerns may arise, clinical trials should also take into account differences in wound healing and response to the particular biomaterial. However, male participants are often over-represented in clinical trials¹⁰, and, therefore, the efficacy and safety of the tested wound dressings may not be robustly established for female patients. Moreover, investigations into sex-¹⁰ or gender-specific¹¹ differences in terms of wound healing duration are often under-reported in clinical trials. Even in studies reporting the effects of sex on wound healing, the number of female participants is often low;^12,13 for example, both control and intervention groups of a clinical trial testing a human fibroblast-derived dermis comprised 93% men and only 7% women¹³.

Importantly, biological sex and self-reported gender should be disclosed in clinical studies, in addition to potential hormone replacement therapies. Gender and sex differences should be analysed and reported in clinical trials to assess potential differences in the effectiveness and safety of wound dressings and to allow the development of sex- and gender-specific wound care therapeutics.

Outlook

Accounting for sex differences is crucial for improving clinical outcomes of nanomedicine- and biomaterial-based treatment modalities for wound repair. In particular, personalized biomaterial designs can integrate sex- and gender-specific preclinical investigations. In addition, protein corona engineering through material surface modifications offers a promising strategy for creating (nano)biomaterials that accommodate sex- and gender-specific variations and to generate more effective and personalized medical interventions for all.