(WHS-K2.06) DIVERGENT CONTRIBUTIONS OF SYSTEMIC IMMUNE CELLS AND LOCAL FIBROBLASTS TO WOUND CLOSURE AND FIBROSIS
Thursday, May 16, 2024
9:15 AM – 10:15 AM East Coast USA Time
Background: Tissue repair is a dynamic process requiring various cellular populations and non-cellular elements coalescing to restore functional tissue integrity. Chronological aging has been associated with impaired physiological wound healing via diminished inflammatory and fibroblast cells. While we know that impaired immune cells and dysfunctional fibroblasts impede wound healing within aged populations, limited evidence exists isolating the importance of one cell type over the other. To further characterize the age-related cellular mechanisms of wound healing, we developed a novel skin graft murine model to isolate the effects of impaired, aged systemic immune cells versus dysfunctional local fibroblasts.
Methods: We established a novel heterochronic full-thickness skin graft (FTSG) murine model utilizing both 8-week (young) and 80-week (old) C57BL/6J mice. We harvested FTSGs to transplant old skin (OS) onto the dorsum of young mice (Y+OS). Conversely, the harvested young skin (YS) was transplanted onto the backs of old mice (O+YS). Young mice receiving young mouse skin transplants were used as controls (CO). Additionally, unwounded skin also analyzed (US). Upon FTSG integration (40 days), we utilized our splinted excisional wounding model to create two cutaneous, dorsal wounds at the graft region. Gross wound size was measured to quantify wound closure rate, and healed wound tissue was explanted at postoperative day (POD) 20. Hematoxylin & Eosin (H&E), Masson’s Trichrome, and Picrosirius Red staining was performed to access epidermal and dermal thickness, dermal collagen density, and dermal integrity.
Results: Control and Y+OS mice healed at around POD ~15.5 while O+YS took up to POD 20 (p=0.0001). The rate of wound closure was significantly decreased in O+YS mice compared to Y+OS mice on PODs 6 (p=0.007), 8 (p=0.005), 10 (p=0.003), 12 (p < 0.0001), 14 (p=0.002), 16 (p=0.02), and 18 (p=0.03). Subsequently, the rate of wound closure was significantly decreased in O+YS mice compared to control mice on POD10 (p=0.005), 12 (p < 0.0001), 14 (p=0.0002), 16 (p=0.006), and 18 (p=0.02). Softwares CT-FIRE and CurveAlign were used to assess collagen fiber structure. We observed that CO and O+YS significantly increased collagen density (p < 0.001) compared to US while Y+OS did not demonstrate an increase.
Conclusions: Our results demonstrate an intriguing divergence of immune and fibroblast contributions during the process of wound healing and fibrosis. Regardless of the transplanted skin, young mice demonstrate accelerated wound closure, thus showing that the young circulating immune cells primarily contribute to the wound healing process as opposed to local cells. In contrast, regardless of the age of the mouse, mice receiving young skin demonstrated more fibrotic skin after injury, demonstrating that the old local fibroblasts dominate the fibrotic response over the systemic contributions.