Laser Treatment of Hypertrophic Scar in a Porcine Model Induces Change to Epidermal Histoarchitecture That Correlate to Improved Epidermal Barrier Function.

Mechanisms and timing of hypertrophic scar (HTS) improvement with laser therapy are incompletely understood. Epidermal keratinocytes influence HTS through paracrine signaling, yet they are understudied compared to fibroblasts. It was hypothesized that fractional ablative CO2 laser scar revision (FLSR) would change the fibrotic histoarchitecture of the epidermis in HTS. Duroc pigs (n=4 FLSR and n=4 controls) were injured and allowed to form HTS. HTS and normal skin (NS) were assessed weekly by non-invasive skin probes measuring trans-epidermal water loss (TEWL) and biopsy collection. There were 4 weekly FLSR treatments. Immediate laser treatment began on day 49 post-injury (just after re-epithelialization), and early treatment began on day 77 post-injury. Punch biopsies from NS and HTS were processed and stained with H&E. Epidermal thickness and rete ridge ratios (RRR) were measured. Gene and protein expression of involucrin (IVL) and filaggrin (FIL) were examined through qRT-PCR and immunofluorescent (IF) staining. After treatment, peeling sheets of stratum corneum were apparent which were not present in the controls. TEWL was increased in HTS vs. NS at day 49 indicating decreased barrier function (p=0.05). In the immediate group, TEWL was significantly decreased at week 4 (p<0.05). The early group was not significantly different from NS at the pre-laser timepoint. After 4 sessions, epidermal thickness was significantly increased in treated scars in both FLSR groups (immediate: p<0.01 and early: p<0.001, n=8 scars). Early intervention significantly increased RRR (p<0.05), and immediate treatment trended towards an increase. There was no increase in either epidermal thickness nor RRR in the controls. In the immediate intervention group, there was increased IVL gene expression in HTS vs. NS that decreased after FLSR. Eight scars had up-regulated gene expression of IVL vs. NS levels pre-treatment (FC>1.5) compared to 4 scars at week 4. This was confirmed by IF where IVL staining decreased after FLSR. FIL gene expression trended towards a decrease in both interventions after treatment. Changes in epidermal HTS histoarchitecture and expression levels of epidermal differentiation markers were induced by FLSR. The timing of laser intervention contributed to differences in TEWL, epidermal thickness, and RRR. These data shed light on the putative mechanisms of improvement seen after FLSR treatment. Resolution of timing must be further explored to enhance efficacy. An increased understanding of the difference between the natural history of HTS improvement over time and interventional-induced changes will be critical to justifying the continued approved usage of this treatment.