Biomaterials are increasingly used in elderly patients, and clinical data show that women more often face complications after implantation. Around this work, research has already accumulated on how cells build the body, how a tumor creates its own family tree, and how T cells recognize the enemy. However, the influence of age and sex on the body's response to an implant remained poorly studied, especially in the context of chronic fibrosis.
C57BL6 mice aged 2 and 18 months had a defect created in the quadriceps femoris muscle and filled with polycaprolactone or polyethylene particles. After six weeks, the tissue was extracted and analyzed using RNA sequencing, flow cytometry, and histological stains, including immunofluorescence. Gene activity was assessed with gene set enrichment analysis, and fibrosis was quantified using QuPath software.
RNA sequencing revealed more than three thousand differentially expressed genes between the sexes at a single age, whereas between ages within one sex there were fewer than two hundred differences. In females, regardless of age, genes of inflammation and the extracellular matrix were elevated, including fibrillar collagens. However, histology showed no significant differences in the thickness of collagen fibers in females, while young males showed more mature collagen according to picrosirius red data. Flow cytometry showed that in old males the proportion of CD8+ T cells increases, while in females in material-induced fibrosis the proportion of CD4+ T cells grows. Macrophages in females in the presence of the implant expressed more CD86, a marker of the pro-inflammatory phenotype, whereas in males CD206, associated with matrix remodeling, was higher.
This work stands somewhat apart from neighboring studies that focus on cellular mechanisms or material properties. Here, for the first time, age and sex were systematically compared in a single fibrosis model with two different implants, which reveals context-dependent effects. Unlike studies examining only one factor, this research shows that predicting the response to an implant requires accounting for both variables and cannot rely on transcriptomic data alone.
The topic may develop toward clarifying the mechanisms underlying sex differences: the role of hormones and the chromosomal set. It would also be promising to study how age-related changes in stromal cells and the accumulation of senescent cells affect fibrosis around an implant. Perhaps materials will be developed with the patient's sex and age in mind, for example, with different degradation rates or immunomodulatory coatings.
The results will influence preclinical testing of biomaterials, forcing researchers to include both sexes and different age groups. This is also important for the development of immunomodulatory materials targeting macrophages, since their phenotype depends on sex and age.
The next step could be to study the role of sex hormones and the chromosomal set in the observed differences. It is also worth checking whether these patterns hold in other mouse strains and with the use of biodegradable materials.
The work is directly tied to an unsolved problem in the field of biomaterials: why some patients reject an implant and others do not. Age and sex are key variables that are still often ignored in preclinical studies, which can lead to erroneous conclusions about safety and efficacy.
The difference in gene expression between the sexes at a single age exceeded 3000 genes, whereas between ages within one sex there were fewer than 200 differences. This shows how much more strongly sex influences the transcriptome than age does in this model.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.